A Living Work of Art: The Life and Science of Hendrik Antoon Lorentz 0198870507, 9780198870500

Hendrik Antoon Lorentz was one of the greatest physicists and mathematicians the Netherlands has ever known. Einstein ca

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l i v i ng wor k of a rt ”

the l ife a n d science of hen dr ik a n toon lor en tz

“A Living Work of Art” The Life and Science of Hendrik Antoon Lorentz A. J. Kox H. F. Schatz Institute of Physics, University of Amsterdam


1 Great Clarendon Street, Oxford, OX2 6DP, United Kingdom Oxford University Press is a department of the University of Oxford. It furthers the University’s objective of excellence in research, scholarship, and education by publishing worldwide. Oxford is a registered trade mark of Oxford University Press in the UK and in certain other countries © A. J. Kox and H. F. Schatz 2021 The moral rights of the authors have been asserted First Edition published in 2021 Impression: 1 All rights reserved. No part of this publication may be reproduced, stored in a retrieval system, or transmitted, in any form or by any means, without the prior permission in writing of Oxford University Press, or as expressly permitted by law, by licence or under terms agreed with the appropriate reprographics rights organization. Enquiries concerning reproduction outside the scope of the above should be sent to the Rights Department, Oxford University Press, at the address above You must not circulate this work in any other form and you must impose this same condition on any acquirer Published in the United States of America by Oxford University Press 198 Madison Avenue, New York, NY 10016, United States of America British Library Cataloguing in Publication Data Data available Library of Congress Control Number: 2020952678 ISBN 978–0–19–887050–0 DOI: 10.1093/oso/9780198870500.001.0001 Printed and bound in the UK by TJ Books Limited


This book is a revised and expanded edition of a Dutch-­language biography of Lorentz by one of us (AJK) that appeared in 2019 (Kox 2019). Parts of the book that deal with specifically Dutch situations or contexts were explained or extended to make them more understandable for an international readership. Also, a new chapter was added about Lorentz’s wife, whose activities and sharp observations offer a more personal perspective on the life of the Lorentz family. In addition, many editorial and stylistic changes were made, and some parts were completely rewritten, in a truly joint effort to bring Lorentz to life as a man of flesh and blood, in spite of the scarcity of personal material. Writing the biography of a scientist is not easy under any circumstances. In the case of Lorentz, the difficulties were particularly daunting, because virtually no personal correspondence, diaries, or other personal material remain to shed light on his personal life, his emotions, his motives, his triumphs, or his disappointments. It is not known whether such materials ever even existed. What is known, though, is that after Lorentz’s death his widow, on his explicit instructions, proceeded to burn three sealed packages of letters “before anyone would succumb to the temptation to open them,” as she wrote to Lorentz’s successor, Paul Ehrenfest. What was inside the packages is anyone’s guess. Like anyone writing a biography, the authors have had moments of doubt about the ultimate success of their project. In the case of Lorentz the project was especially difficult, because the essence of Lorentz’s personality continuously seemed to evade us. The authors do not intend to go so far as Newton’s biographer, Richard Westfall, who wrote in his masterful Never at Rest: “The more I have studied him, the more Newton has receded from me.” Yet, we must admit that his words have often been in our thoughts. Now it is up to the reader to decide whether we have been successful in the task we set out to achieve. A. J. Kox H. F. Schatz Amsterdam and Torrazza May 2020


Over the years, many people have been kind enough to provide moral or practical support that was instrumental for the publication of this biography. To the editors and staff at the Einstein Papers Project at the California Institute of Technology, in particular General Editor Diana Buchwald, we owe a debt of gratitude. The staff at the Noord-­Hollands Archief in Haarlem, especially Godelieve Bolten, the archivist in charge of scientific archives, have provided essential help and advice. We also extend our heartfelt thanks to Jed Buchwald, Margriet van de Heijden, Michel Janssen, Laura Kox, Jan Lepeltak, Ad Maas, Jonathan van der Meer, Jos van der Meer, Abel Streefland, and Dalila Wallé for various forms of assistance and support. Finally, we thank the four anonymous referees who carefully reviewed this book for their useful and constructive comments.



Archief Staatscommissie Zuiderzee, Archief Dienst Zuiderzeewerken, Nieuw Land Erfgoedcentrum, Lelystad CICI Commission internationale de coopération intellectuelle CIR Conseil international de recherches CT Collection  J.  Th. Thijsse, Archief Koninklijke Hollandsche Maatschappij der Wetenschappen, NHA. (Documents concerning Thijsse’s work in the Zuiderzee Commission.) FAL Documents concerning the Zuiderzee Commission, Family archive of Cornelis Lely, Nationaal Archief, Den Haag FC Family Correspondence 1908–28 (NHA) LA Archief H. A. Lorentz, NHA NHA Noord-­Hollands Archief, Haarlem PC Material from private collection of Lorentz heirs (NHA) RB Rijksmuseum Boerhaave, Leiden UAI Union académique internationale ZA Archief P. Zeeman, NHA

Aller seelische Wagemut liegt heute in den exakten Wissenschaften. Nicht von Göthe, Hebbel, Hölderlin werden wir lernen, sondern von Mach, Lorentz, Einstein, Minkowski, von Couturat, Russell, Peano. Robert Musil, 1912. (Gesammelte Werke, II, p. 1318.) All intellectual daring nowadays lies in the exact sciences. We will learn not from Göthe, Hebbel, Hölderlin, but from Mach, Lorentz, Einstein, Minkowski, from Couturat, Russell, Peano. Robert Musil, 1912. (Gesammelte Werke, II, p. 1318.)

Chapter 1 Childhood and student years

On the 13th of November 1852, thirty-­year-­old Gerrit Fredrik Lorentz married his young bride Geertruida van Ginkel in Arnhem, a city roughly in the geographical middle of the Netherlands.1 Geertruida, reputedly a pretty and intelligent woman, had been widowed two years earlier when she was only twenty-­four. She was left with Hendrik Jan Jacob, or Jan, her two-­year-­old son from her marriage to Jan Jacob Janssen. Gerrit Lorentz, a prosperous commercial gardener, owned a large market garden on Musschenberg, just outside Arnhem. His grandfather, Friedrich Gottlob Lorentz, had started a commercial garden on Steenstraat, or Velpersteenstraat, the main road from Arnhem to Zutphen. This gardening business, with its reputation for high-­quality cauliflower, was subsequently passed on to his son Tobias, Gerrit’s father. Grandfather Friedrich originated from the German town of Bautzen, sixty kilometers north of Dresden, and had arrived in Arnhem in the second half of the eighteenth century. Possibly he served as a soldier in the Prussian army that was called in by Stadtholder2 Willem V in 1787 to suppress a patriotic uprising in the Netherlands. The patriots in the Republic of the Seven United Netherlands rose up against the absolutist administration of the Stadtholder and demanded administrative reforms for its citizens. In 1786 and 1878 fighting broke out between the patriots and the supporters of the Stadtholder. With the help of the Prussians, Willem V managed to put an end to the fighting in October of 1887. In the years around 1850, Arnhem, which had obtained city rights as early as 1233, was rapidly changing from a relatively sleepy provincial town to a flourishing mid-­size city. This period of growth had started twenty years earlier, after the 1   Unless specified otherwise, all particulars about Lorentz’s childhood in this chapter were drawn from Haas-­Lorentz  1957 (a biographical article by Lorentz’s eldest daughter, Geertruida Luberta [mostly denoted as “Berta”]), biographical notes by his wife, Aletta Lorentz-­Kaiser (RB), reminiscences by Lorentz’s friend and colleague Herman Haga (PC), and documentation that was used for the booklet De Arnhemse jaren van Hendrik Antoon Lorentz, edited by Jos Diender, Tineke Seebach, and Wout van Ast and published by Bezoekerscentrum Sonsbeek, Arnhem. 2   In the Low Countries, a Stadtholder (place holder) was appointed to represent a feudal lord. From the sixteenth to the eighteenth century, in the early Dutch Republic, the role had become that of a hereditary head of state. Willem V, the last Stadtholder of the Republic, fled to England when the French revolutionary forces established the Batavian Republic in 1795. In 1815, after the defeat of the French army, his son was crowned King Willem I of the United Netherlands.

“A Living Work of Art”: The Life and Science of Hendrik Antoon Lorentz. A. J. Kox and H. F. Schatz, Oxford University Press (2021). © A. J. Kox and H. F. Schatz. DOI: 10.1093/oso/9780198870500.003.0002

Childhood and student years


city’s classic fortifications had been demolished. City walls were torn down, city gates were removed, and moats were filled in. Townhouses and mansions for the newly wealthy were constructed along widened canals and city parks. A railroad to connect the city with the western part of the country was built in 1845, and a few years later a concert hall, Musis Sacrum, was the first cultural venue to be constructed. This development continued in the following decades. New and better opportunities for secondary education opened up, a public library was established, and more cultural opportunities arose after a theater opened its doors. As a result, Arnhem became more cosmopolitan, with a lively arts scene, and increasingly attractive as a city to settle and do business. Industry and trade also flourished, although the number of industrial enterprises remained relatively small. The number of inhabitants grew quickly: from 20,000 in 1850 to 50,000 by the end of the century.3 Less than a year after their wedding, on July 18, 1853, a son was born to the Lorentz couple. He was named Hendrik Antoon but went by the name of “Hentje.” Five years later they had another son, Alexander Cornelis. Hendrik was rather short for his age4 and did not start to talk until late, but that was hardly a sign of his in­fer­ ior intelligence. On the contrary, he did quite well in school and was so popular in class that at one point his fellow students gave him a beautiful pencil—a gift that was spoiled somewhat for Hendrik when his classmates wanted him to confirm over and over how much he loved it. He clearly liked reading: He was so pleased with a book his parents gave him that he wrote his initials H.A.L. on every page. An early memory that stayed with him throughout his life shows that he was a curious child. Young Hendrik was fascinated by the comet of 1860 which, by the end of June, was visible to the naked eye in the night sky over the Netherlands.5 Of course, one needed to have an unobstructed view to see the comet, as it appeared low on the northern horizon, but living on the outskirts of Arnhem he must have had plenty of opportunity to see it. Though it does not appear in any of the other available sources on Lorentz, the following story was recounted by his student Adriaan Fokker, to attest that Lorentz did not like to get into a fight but that he was not a pushover either, even at a young age. He calls Lorentz “already in his boyhood years a peace-­loving, but at the same time logically thinking person.”6 The story has it that one day, when Hendrik was still a young boy, he had to go and fetch a kettle of hot water at a  See Arnhem 1933 for more about the history of Arnhem.   His daughter Berta suggests that at the time when he went to university, Lorentz was still of short stature, because she referred to him as a “dark little person” (Haas-­Lorentz 1957, 24). Today it is difficult to determine how tall Lorentz really was. The passport that has been preserved unfortunately does not state his exact height. Some indication may be gleaned from a photograph showing Einstein and Lorentz standing right next to each other. Einstein was over 1.70 meters (5 ft., 7 in.) tall and in the picture, Lorentz is clearly quite a bit taller. It can be concluded that Lorentz was not particularly short, and certainly not for those days, considering that around 1900 the average height for men was 1.69 meters (5 ft., 6.5 in.). 5   Probably the non-­periodical comet C/1860 M1. 6  See Fokker 1946. In terms of approach and tone, this article is completely in line with the title of the book in which it appeared: Nederlandsche Helden der Wetenschap (Dutch Heroes of Science). 3 4


Childhood and student years

“water and fire woman’s shop,” since it was not always so easy in those days to heat up water at home. On his way home, he was accosted by an alley-­kid looking for a fight. He managed to avoid the fight by pointing out to his attacker that ­hot-­water burns are not only very painful but also leave nasty scars. Apparently, they ended up parting company as good friends. In 1861, Hendrik’s life changed course fundamentally. On December 1 his mother passed away, only thirty-­five years old. A few weeks earlier, his little brother Alexander had also died, at the tender age of three-­and-­a-­half. These events, in such a short time-­span, must have been traumatic for eight-­year-­old Hendrik and must have stayed with him for the rest of his life. His daughter Berta recounts in her reminiscences that he used to visit his mother’s grave every time he went to Arnhem, until the cemetery eventually closed. At the time the family lived at Steenstraat, where Gerrit had inherited some property and a few simple little houses, the “Lorentz houses.” As he had a crippled hand and foot, the garden work became increasingly difficult for him, so a small grocery store was set up in one of the houses. This was the reason why Gerrit’s occupation on Geertruida’s death certificate was listed as “shopkeeper.” Having been left with two young children, Gerrit did not wait long to look for a new wife: Six months later he married the forty-­two-­year-­old widow Lubberta Hupkes, so the children would have a stepmother—for Hendrik’s stepbrother Jan, already his second. Lubberta, who had no children of her own, was apparently a good mother to the two boys. At least, Hendrik was so fond of her that he named the elder of his two daughters after her. In the years that followed Gerrit had to close down the grocery store for lack of customers. He sold the nearby property and had the “Lorentz houses” torn down to replace them with three large, modern houses. The Lorentz family itself moved to the ground floor of the middle house, while the second floor and the houses on either side were rented out.

Primary school When his mother died in 1861 Hendrik was still in primary school, in Master Swaters’ class, having started school two years earlier at the age of six. After his first year in school, in 1860, the first photograph of young Hendrik was taken at the country fair he was visiting with his mother. A serious young boy is looking into the lens, hair neatly combed, in a dapper little suit. As a young child he had broken his nose, falling off a hand cart while playing. Although his nose had become somewhat misshapen as a result, this does not really show in the picture. No particular meaning should be attached to his serious look. In those days sitting for a portrait photograph was something quite special and a serious business. The ubiquitous smile of present-­day photos did not become fashionable until much later, in the second half of the twentieth century.

Primary school


After a few years with Master Swaters, Hendrik transferred to the French School of Master Geurt Kornelis Timmer. This school, consisting of six grades for extended primary education, offered classes in the morning and in the afternoon and—if students wished—also in the evening. The school was expensive: tuition amounted to around forty guilders per year. As the average pay of a blue-­collar worker was no more than a few hundred guilders per year in those days, not every­one could afford to send their children to such an expensive school. The fact that Hendrik’s father was able to pay this kind of tuition was clear proof of his relative prosperity. Apart from Master Timmer, the school had two other teachers who taught foreign languages, mathematics, physics, draftsmanship, and agriculture. The school provided a kind of Dalton education, aimed at stimulating a student’s individual development. In Hendrik’s case this education was more than successful. When he was nine or ten years old, he went out and bought a logarithm table, paid for with his own pocket money, and taught himself how to use it. Hendrik himself remembered later that “in the evening hours everybody worked on arithmetic as much as they pleased. In this way, we were able to learn quite a bit of lower mathematics.”7 Master Timmer was the one who introduced his pupils to the field of physics. He was an enthusiastic teacher and an active member—later even chairman—of the Arnhem Natuurkundig Genootschap onder de Zinspreuk tot Nut en Genoegen (Physics Society under the Motto for Benefit and Pleasure). The society’s members—who changed its name to Natuurkundig Genootschap Wessel Knoops (Physics Society Wessel Knoops) in 1879 to honor its founder—gave lectures, studied scientific literature together, and, by the end of the nineteenth century, even had their own building.8 It was founded in 1824 by Wessel Knoops, who was a pharmacist in Arnhem, and in the first years of the society its members held their meetings in his pharmacy. The Society can be viewed as a typical product of the Enlightenment and its culture of scientific societies. Especially in the latter decades of the eighteenth century, many such societies were established to encourage the spread of ideas generated by natural science. Apart from being an inspiring teacher in the classroom, Timmer also authored popular science textbooks, like his Handleiding tot Algemeene Kennis van den Aardbol. Een Volksleesboek (Textbook for General Knowledge of the Earth: A Popular Reader) published in 1840 by the Maatschappij tot Nut van ’t Algemeen (Society for Public Welfare). In the nineteenth and early twentieth century, this very influential society—still in existence today—aimed to promote the well-­being of the people through higher levels of cultivation and civilization and specifically through better education. In connection with Lorentz’s own activities, its role will be further discussed later on. Lorentz always spoke of Timmer with great warmth and appreciation, even many years later. Not only was he a gifted and inspiring teacher, he 7   “in de avonduren ieder rekende naar zijn lust meebracht. Zoo konden wij heel wat van de lagere wiskunde leren.” Physica 6 (1926): 24. 8  See Ven 1998 for the history of the Society.


Childhood and student years

was also able to create an especially attractive classroom atmosphere by doing small experiments with the students. Lorentz himself also became a member of the Physics Society Wessel Knoops and in 1876 and 1880 he gave lectures there about “Magnets” and “The Essence of Electricity” respectively. Obviously, at the Society’s 100th anniversary, in 1924, Lorentz was the guest of honor. Before addressing the members in a lecture with demonstrations about “old and new physics” he made a point of praising the ­science education he had received from Master Timmer.9

Secondary school With the support and encouragement of Master Timmer, in December 1866 Lorentz took the entrance examination for the third grade of the Arnhem Hoogere Burgerschool (HBS) that had opened a few months earlier. This new type of secondary education was introduced by Johan Rudolph Thorbecke, one of the most influential Dutch politicians of the nineteenth century. Influenced by the 1848 liberal uprisings throughout Europe, Thorbecke was instrumental in revising the Dutch constitution and established many features of Dutch parliamentary democracy that remain in place to this day. As Prime Minister, he presided over the passage of a raft of new laws and reforms, including election reform, tax reform, and healthcare legislation. In his 1862 Secondary Education Act, Thorbecke provided for a new type of secondary school, geared toward educating students for managerial positions in trade and industry. Contrary to the already existing Gymnasium, a type of grammar school where students were instructed in the classics as well as the sciences, it was intentionally not designed to prepare students for university. Master Timmer apparently made every effort to prepare his students as best he could for the very important entrance examination to their new school. He even used to get up very early every morning to improve his own English before the start of the school day, in order to be able to convey his newly acquired knowledge to his pupils later in the day. During the entrance examination Hendrik met Herman Haga, the son of a local minister, who was a year older. They would remain friends until Lorentz’s death. The HBS in Arnhem was a municipal school rather than a school run by the state, like many HBS schools elsewhere in the country. It started the school year in 1866 with 77 students, divided across four grades.10 The fourth grade had just two students and the fifth, final grade would only come later, once the ­fourth-­grade students had reached that level. Tuition was sixty guilders per year, quite

 See Arnhemsche Courant, March 14, 1924.   Information about the HBS in Arnhem is taken from Gedenkboek 1966 and from archival material in the Gelders Archief in Arnhem. 9


Secondary school


a ­substantial fee in comparison to the salaries of the teachers, who earned between 1,200 and 1,800 guilders per year. At the HBS Hendrik van de Stadt, who taught physics, was another teacher who was an important influence on young Lorentz’s intellectual development. In his anniversary lecture at the Physics Society Wessel Knoops, Lorentz mentioned him in one breath with Master Timmer. Much later, at the celebration of his golden doctorate, he again emphasized Van de Stadt’s important role in his ­academic development and in his choice to study physics. In his speech on that occasion he said: Whether physics or mathematics was the most beautiful science, that was a question about which I was in doubt at the time, but which was decided in favor of physics when, once admitted to the hoogere burgerschool, we enjoyed the lively teaching of van de Stadt. Van de Stadt, who had recently received his doctorate in Leiden, transplanted in us the enthusiasm which, in himself, had been aroused in the first place by Kaiser.11

Despite the high esteem in which he held Van de Stadt, Lorentz still attached more value to Timmer’s teaching, at least according to his university friend Gerrit Jan Michaelis.12 Like Timmer, Van de Stadt did not limit himself to teaching. He was also the author of a well-­known physics textbook, Beknopt Leerboek der Natuurkunde (Concise Textbook of Physics), first published in 1879 and reprinted many times since. Another teacher who taught Lorentz a great deal, both in and outside the classroom, was Jacob Maarten van Bemmelen, the school’s director, who taught chemistry and later became a professor in Leiden. Under his guidance, Lorentz carried out his first science experiments: I had to trot around a concert hall with greater or lesser speed, holding up a wind gauge tied to a long wooden slat, and then, as much as my knowledge of mathematics allowed at the time, I had to express in a formula the relationship between the indications on the instrument and the speed at which I had been running.13

Experiments were not only performed in a school context. In his speech on the occasion of the fiftieth anniversary of his doctorate, Lorentz remembered the 11   “Of nu natuurkunde dan wel wiskunde het mooiste vak was, dat was een vraag waaromtrent ik toen twijfelde, maar die ten gunste der natuurkunde beslist werd toen wij, op de hoogere burgerschool gekomen, het levendige onderwijs van Van de Stadt genoten. Van de Stadt, pas te Leiden gepromoveerd, plantte op ons de geestdrift over, die bij hem zelf, in de eerste plaats wel door Kaiser, gewekt was.” See Physica 6 (1926): 21–29 for the speech. Frederik Kaiser was Professor of Astronomy in Leiden and doctoral dissertation advisor to Van de Stadt. 12   See Michaelis to Aletta Lorentz-­Kaiser, May 1, 1928 (LA 725). Michaelis was a slightly older peer of Lorentz, with whom he liked to played chess. After his doctorate in 1872, Michaelis took a position as a teacher of mathematics and mechanics at the Arnhem HBS in 1873. 13   “Ik moest met een windmeter die aan een lange omhoog gehouden lat was gebonden, met grooter of kleiner snelheid in een concertzaal ronddraven, en vervolgens, zoo goed en zoo kwaad als mijn toen­ malige wiskunde het toeliet, het verband tusschen de aanwijzingen van het instrument en de snelheid waarmee ik geloopen had, in een formule uitdrukken.” Physica 6 (1926): 25.


Childhood and student years

lively discussions with Haga on walks in the woods around Arnhem, “the often impossible experiments we devised and how at some point, rather to our satisfaction, we decided what the essence of electricity was supposed to be.”14 According to Haga, walking was actually the only “sport” that Lorentz practiced with enthusiasm: “fencing, ice-­skating, swimming, or gymnastics” were clearly not Lorentz’s favorite pastimes. Riding a bicycle was a skill Lorentz did not master until 1890, when it became more commonplace to do so, but he later spent many a day on bicycle trips with his own children.15 Young Lorentz’s school performance at the HBS was legendary, and justifiably so. On all his report cards he scored a five—the highest grade at the time—for almost all subjects, even theology, a subject that was taught in the final (fifth) year. His all-­round talent was especially remarkable: neither the exact sciences nor the modern languages gave him any trouble at all. He also turned out to have an exceptionally good memory, something that would stand him in good stead throughout his life. He effortlessly gave long lectures and speeches by heart, without using notes. A half hour of preparation time was enough for him.16 In later life he also made good use of his exceptional mastery of English, French, and German at the many international scientific meetings he attended and in his prolific contacts with ­foreign colleagues. In order to boost his mastery of French, Hendrik often attended Sunday services in the Walloon Church, and to improve his English he read English authors like Charles Dickens.17 This choice of reading gave cause for criticism from his English teacher, who thought that his English usage in compositions resembled the language of Dickens too much. Apparently, Lorentz’s good memory was ­getting in the way here. As far as academic performance and intellect were concerned, Lorentz was far and away better than average. He was awarded prizes for both mathematics and literary subjects when he was moved up from grade three to four and from grade four to five. According to an anecdote, Van de Stadt had recruited his pupils to put together a booklet of answers to the problems in one of his textbooks. In as little as two weeks Lorentz had solved all the problems, including the material that had not yet been discussed in class. In Hendrik’s final school year, 1868–1869, there were two other students in his class: his friend Haga and W. H. de Jong.18 There is a well-­known photo of the 14   “de veelal onmogelijke proeven die wij beraamden en hoe wij eens, nog al tot onze tevredenheid uitmaakten wat de electriciteit eigenlijk wel zou zijn.” Ibid. 15   See, for example, Lorentz to Woldemar Voigt, July 20, 1899, in which he writes that he must end the letter because he must go bicycling with his children. (Kox 2008, 62). At the time the family was spending the summer in a rented cottage in the seashore village of Noordwijkerhout, not far from Leiden. 16   According to daughter Berta, Hendrik’s grandfather Lorentz also boasted a very good memory. It enabled him to write down the Sunday sermons verbatim, and recycling of any sermon was something he noticed immediately, even many years later. (Haas-­Lorentz 1957, 21). 17   According to Haga, he had read Dickens’s A Child’s History of England. 18   The name is spelled as in the HBS archive. Haas-­Lorentz 1957 uses the spelling De Jongh.

Secondary school


three boys, taken by Van de Stadt who was an avid amateur photographer. They pose by a ground-­floor window in the janitor’s house. The janitor had quickly managed to insert a rifle in the picture, leaning it close to the boys. Having a rifle in the picture at all was probably connected with the fact that military drills and the “treatment of the rifle” had been introduced by Director Van Bemmelen as part of the HBS curriculum. According to Haga, the three boys were discussing their “military readiness” when they were photographed. Haga also mentions a previous photo where Lorentz’s head was not in focus—because it had absorbed all the ultraviolet rays, according to his classmates. The three classmates were good friends and during long walks in the surroundings of Arnhem they had discussions about all sorts of things and speculated about what the future would hold for each of them. For Lorentz they envisaged a professorship in the German city of Jena—Jena because their German teacher had suggested that they visit the city. Hendrik’s final examination also became legendary. He was the number one student in the entire province of Gelderland and, so the story goes, his exam was consulted whenever the correctors were not quite sure about the correctness of a solution given by another student.19 In a farewell speech to the final year’s class, director Van Bemmelen praised Lorentz’s exceptional talents and especially impressed upon him, “considering his well-­known humility and goodness, to take care that others would not abuse his knowledge, that he himself would not lose himself in it, but that he would bring science a step forward.”20 Yet, in spite of his brilliant final examination, Lorentz was not allowed to go to university immediately. Since the HBS was not designed to prepare students for university, the school’s final diploma did not grant automatic access to any degree course: it still required a Gymnasium diploma. In practice, this meant that those who had already graduated from the HBS still needed to take an additional state examination in Greek and Latin to be allowed to enter university. Together with Haga, Hendrik spent the following year studying the classics under the guidance of T. T. Kroon, the deputy director of the Arnhem Gymnasium. In his recollections, Haga describes how for Latin they had focused primarily on the work of the Roman historian Livy, because in the preceding years the candidates for the state examination had had to translate texts by this author. Unfortunately, no such luck in 1870. On the program this time was Ovid, a poet with a completely different vocabulary. Haga failed the exam miserably, but Hendrik powered through. The words that he did not know from Livy were not a problem, he said, “as one could more or less guess those.”21

  See J. F. van Bemmelen (son of Jacob Maarten) to Aletta, May 13, 1928 (LA 725).   “bij zijn bekenden nederigheid en goedheid te zorgen, dat anderen geen misbruik maken van zijn kennis, hijzelf daar niet in opga, maar de wetenschap ene schrede voorwaarts brenge.” Quoted in Gedenkboek 1966, 39. 21   “die kon men wel zoowat raden.” 19 20


Childhood and student years

Studies in Leiden Hendrik was the only one in his class to go to university that year. He had chosen to study physics in Leiden, the oldest and most prestigious university in the Netherlands, which counted 600 students at the time.22 His friend Haga could not start university until a year later, when he picked physics as well, while De Jong opted for a training course to become a civil servant in the Dutch East Indies. As he did not want to isolate himself, Hendrik became a member of the traditional Leiden student association the Leidsch Studenten Corps, which boasted a lively fraternity life. During the groentijd, the generally rough hazing period in the freshman year, he showed off his exceptional memory when he had to introduce himself to a roomful of older students. As Haga tells it, at the end of the meeting one student asked Lorentz “with all the usual expletives, whether he did not believe it was worth the trouble to write down their names, to which Lorentz answered that he believed he had already memorized all the names and proceeded to reel them off one by one.”23 Student life was apparently something that Lorentz had mixed feelings about. His school friend Becking, who had taken his final examination two years after Lorentz and had gone to study at the Delft Polytechnic, reminisced in 1871: “You have often wished the whole studentencorps [Leidsch Studenten Corps] to damnation [. . .] you have often come home feeling wretched.”24 When Lorentz began his studies, Leiden was a mid-­size city recovering slowly from serious economic decline. After obtaining city rights in 1277, the city had become the flourishing hub of the Dutch textile industry during the sixteenth and seventeenth century. By the end of the seventeenth century—the Dutch “Golden Age”—Leiden had grown to no fewer than 70,000 inhabitants. It had become the second largest city in the Netherlands after Amsterdam. Economic decline set in early in the eighteenth century, resulting in a steady decrease in the number of inhabitants to as few as 30,000 by around 1800. In spite of the newly constructed railroads connecting the city to Haarlem, Amsterdam, and The Hague, it was not until the last quarter of the nineteenth century that Leiden showed visible signs of recovery. The number of inhabitants began to grow again and eventually reached about 40,000 in 1870. Four years prior to Lorentz’s move to Leiden, the city had suffered another setback, as a serious cholera epidemic took its toll. It stands to reason that the poor were especially hard hit. Much like in other Dutch cities, the Leiden factory workers lived in dire poverty and were housed in grimy slums beyond the waterways and canals surrounding the elegant seventeenth-­century city center. 22   The university was founded in 1675. Lorentz’s choice was probably also influenced by both Van de Stadt and Van Bemmelen having received their doctorates there. 23   “met de gewone krachtuitdrukkingen of hij het niet de moeite waard vond de namen op te schrij­ ven, waarop Lorentz antwoordde dat hij de namen wel meende te kennen en ze alle achter elkander noemde.” 24   “Gij hebt het geheele studentencorps vaak naar de maan gewenscht [. . .] ge zijt vaak beroerd thuis gekomen.” S.W. Becking to Lorentz, October 8, 1871 (LA 651).

Studies in Leiden


Even before his arrival in Leiden, Lorentz was known among the professors as a very promising student. One such professor was Frederik Kaiser, who had heard much about the talented Lorentz from his doctoral student Van de Stadt. Kaiser received Lorentz warmly and welcomed him into his home. Lorentz’s presence was an opportunity for Kaiser to resume his lectures on theoretical astronomy, classes he had suspended earlier due to a lack of interest on the part of the students. Lorentz quickly became the only student in the class, and at the end of the year Kaiser decided that for Lorentz to sit a formal exam would be nothing less than “an unnecessary sham fight.” Lorentz also had a good relationship with Petrus Leonardus (Pieter) Rijke, Leiden’s only professor of physics. Later, on the occasion of his golden doctorate in 1925, Lorentz praised Rijke’s “good and warm heart,” even though the man might have looked somewhat forbidding at first sight. The student years were characterized by hard work and frugality. Lunch was sometimes replaced by a long walk with a student friend and later Lorentz remembered “how ‘pitiful’ it was to come home like that in a city where the others had just finished their midday meal.”25 As early as November 6, 1871, a little over a year after arriving, Lorentz had already passed his kandidaatsexamen26 with the highest honors, magna cum laude. This was exceptionally fast but, according to Berta, the mathematician Van Geer was still slightly disappointed with the candidate’s performance. That disappointment quickly faded when it turned out that Van Geer had believed Lorentz was taking the doctoraal examination instead of the kandidaatsexamen. For Lorentz there was no question of disappointment; he was more likely very relieved, according to his remark in a letter to Van Bemmelen the following day: “the word ‘magna’ has been more of a compensation for my examination anxiety than I had dared to expect.”27 After the exam, Lorentz let his membership of the Leidsch Studenten Corps lapse and in February 1872 he returned home to Arnhem. Student and fraternity life were clearly not something he yearned for and, moreover, he was far too busy with his studies. He had decided that he could also prepare for the doctoraal exam­in­ ation at home in Arnhem, all by himself. That decision worked out very well indeed. On June 14 and 15, 1873 he sat for the examination, once again passing with the highest honors.

25   “hoe ‘zielig’ het was zoo thuis te komen in een stad waar de anderen juist hun middagmaal geëindigd hadden.” Reminiscences of Aletta Lorentz-­Kaiser (RB). 26   In those days there were two university examinations: the kandidaatsexamen, or candidate’s exam, an exam that was taken more or less midway through the curriculum, and the final doctoraalexamen, or doctoral examination. This terminology is a bit confusing. The doctoraalexamen does not confer the degree of doctor, a degree that is achieved only after the defense of a doctoral dissertation. In some faculties, such as medicine, students also had to take a comprehensive examination after their freshman year, the propaedeutic examination. 27   “het woord ‘magna’ heeft mij beter voor de examenkoorts schadeloos gesteld dan ik had durven verwachten.” Lorentz to Van Bemmelen, November 7, 1871 (RB).


Childhood and student years

Lorentz as a schoolteacher Meanwhile, in order to support himself, Lorentz had taken a position as a teacher—for a yearly salary of 1400 guilders—at the Burgeravondschool, the evening school that was part of the Arnhem HBS. At first, he did not find the work easy at all. Teaching boys of his own age who worked during the day as carpenters, bricklayers, housepainters, and the like, and who were not particularly engrossed in their studies, turned out to be very difficult. Lorentz’s struggles to keep order in class sometimes led to unpleasant situations: an attempt to remove one of the students from the class even ended up in fisticuffs and resulted in a torn garment. Yet Lorentz improved, slowly but steadily, and in the annual report for the year 1873 the school’s director Van Bemmelen was able—fortunately—to report good progress: Although Mr. Lorentz, who has been hired by the School on 1 October of last year as a teacher of Mathematics, Chemistry, and Natural History, teaches in a way that in form as well as content is excellent for the class of young men who attend evening school, order in his classes has sorely been lacking during the academic year 1872/1873. The cause can only lie in the inexperience of Mr. Lorentz, who was as yet unable to develop the necessary authority and decisiveness with regard to his students. The deputy director, Dr. H. van de Stadt, together with the undersigned, has tried in all sorts of ways to work towards an improvement. I am happy to report that Mr. Lorentz—who is not in the least lacking in good will and diligence—has made good progress in the art of managing a class since the summer vacation. An improvement with regard to order in his class is noticeable.28

Perhaps these problems of order in the classroom were what motivated Lorentz in the summer of 1873 to apply for a position as a teacher of mathematics at the HBS in the town of Alkmaar, 45 kilometers north of Amsterdam. He had not yet completed his doctorate, which was probably one of the reasons he did not get the job.29 In contrast to the present-­day situation, it was quite normal in those days for teachers in secondary schools to have doctorates, regardless of whether they were teaching at a Gymnasium or a HBS. Be this as it may, Lorentz’s improvement apparently continued, for the next annual report stated that “irregularities of any im­port­ ance” had not occurred that year. Obviously, he had become more experienced 28   “Ofschoon de Heer Lorentz, sinds 1 Oct des vorigen jaars aan de School geplaatst als leeraar in Wiskunde, Scheikunde en Nat. historie, een onderwijs geeft, dat zoowel in vorm en inhoud uitmuntend is voor de klasse van jongelieden, die de avondschool bezoeken, heeft evenwel aan de orde op zijne lessen in den Cursus 1872/73 nog al wat ontbroken. De oorzaak kan alleen gelegen zijn in de onerva­ renheid van den Heer Lorentz, die nog niet de noodige kracht en vastheid tegenover zijne leerlingen wist te ontwikkelen. De onderdirecteur Dr. H. van de Stadt heeft met den ondergeteekende getracht op allerlei wijze aan eene verbetering mede te werken. Het is mij aangenaam mede te kunnen deelen dat de heer Lorentz—dien het allerminst aan goeden wil en ijverig streven ontbreekt—sedert de groote vacantie goede vorderingen heeft gemaakt in de kunst van eene klasse te besturen. Eene verbetering wat de orde betreft is merkbaar.” Archive HBS, Gelders Archief, Arnhem. 29   See Lorentz to Van Bemmelen, July 25, 1873 (RB).

Physics in theory and practice


and had gained some authority. What may have helped was that Lorentz grew a full beard around that time to look a little older. He also took the old adage “never turn your back on the class” to an extreme; his friend Haga remembers how Lorentz managed to discuss extremely complicated figures with his back to the blackboard. Since teaching came more easily to Lorentz now, he allowed himself more time for relaxation. Occasionally he was even able to afford a trip abroad, together with friends or with his parents. In the summer of 1879, for example, Lorentz and his parents traveled to Paris for a two-­week vacation. In a little notebook he kept a day-­to-­day account of their sightseeing, the attractions they visited, and even the restaurants where they had meals.30 In the Folies-­Bergère they saw “trained dogs, Negroes caterwauling, strong girl with weights, ballet with electric lights.” A visit to the morgue of Paris—a major attraction in those days—he found disappointing, because there were no bodies on display.31 In the diary he jotted down quite a few dryly comical observations: “lady with parrot,” “taken a bath and stolen the fragrant soap,” or “lost Baedeker [guidebook], great consternation.” On the way back on the train he noticed “farm women speaking Walloon that no Christian soul could understand,” and in Antwerp he “especially admired: the handsome waiters and the politeness of a Belgian when he is asked for directions.”32 When Lorentz left the Arnhem HBS, a “parting gift accompanied by a fitting letter,” presented to Lorentz by students and teachers together, shows that his teaching had eventually worked out just fine. In later years, Lorentz’s teaching would be more than just fine. Newspaper reviews of his public lectures often commended him for his crystal-­clear presentation, while his Leiden students also highly appreciated his lectures; his Monday morning lectures in particular became legendary.

Physics in theory and practice Apart from teaching evening school in those Arnhem years, Lorentz was busy with his own physics experiments. Gerrit Jan Michaelis, his fellow student from Leiden, wrote about this: In 1873 I saw him again in Arnhem. He was full of new ideas about physics then and assumed the existence of phenomena that he wanted to try and see. But there was   The notebook is in RB.   Unidentified bodies that had been found, for instance, in the Seine were displayed behind glass windows to facilitate identifications. As an unintended consequence the morgue became a public attraction. 32   “gedresseerde honden, Negers met kattenmuziek, sterke meid met gewichten, ballet met electrisch licht,” “Dame met de papagaai,” “een bad genomen en de welriekende zeep gestolen,” “Baedeker verloren, grote ontsteltenis,” “boerinnen die Waalsch praatten en voor geen christenmensch te verstaan waren,” “vooral bewonderd: de mooie kellners en de beleefdheid van een Belg als men hem de weg vraagt.” 30 31


Childhood and student years little opportunity in Arnhem to do so. In his father’s house a primitive laboratory was set up then, equipped in part with his own handmade little instruments, or sometimes those made with the assistance of Otto Lincker, the instrument-­maker and assistant for physics at the HBS. Sometimes we also went to the school building on Sunday mornings, to work in the small cabinet of Van de Stadt, a teacher at the time, but later the director of the HBS. We did this without asking his permission, but we took care that everything was put back in place afterwards. Light experiments were done primarily with a telescope I owned. In the cover of the brass tube that could be slid around the glass, a round hole was drilled to allow a thin beam of light to shine through. Thus, diffraction phenomena were investigated. Lorentz was convinced of the existence of electrical waves and tried to observe them through discharge phenomena in a Leyden jar. Sometimes he believed he did see something, but then he declared the next day that it could also be explained by means of the old theory. It did not work because the tools were too deficient. [. . .] During all these cozy meetings at his house, Lorentz’s father was present as well, thoroughly enjoying himself. He was an easy-­going, kind man, who later behaved like a hero when his leg had to be amputated without the possibility of being anaesthetized.33

After his doctoraal examination Lorentz continued to work stubbornly in the seclusion of his father’s home to complete a doctoral dissertation. He read the works of the great men of the theory of electromagnetism, like the German Hermann von Helmholtz and the Scotsman James Clerk Maxwell. He actually began his study of Maxwell in his second year at university, when he happened to find a still-­unopened copy in the university library of Maxwell’s difficult but most important work: A Treatise on Electricity and Magnetism. His great admiration for the Frenchman Augustin Fresnel as a pioneer in the theory of light also dates back to those days. A portrait of Fresnel always hung in Lorentz’s study, and toward the end of his life his admiration appeared to be as great as ever, judging by what he said at the 100th anniversary celebration of Fresnel’s birth:

33   “In 1873 zag ik hem in Arnhem weder. Hij was toen vol van nieuwe ideeën op natuurkundig gebied en vermoedde verschijnselen, die hij wilde trachten te zien. Maar daartoe was in Arnhem weinig gelegenheid. In het huis van zijn vader werd toen een primitief laboratorium ingericht, ten deele met zijn eigen gemaakte instrumentjes, ook wel met behulp van Otto Lincker, instrumentmaker en amanuensis voor natuurkunde aan de H.B.S. Soms gingen wij ook wel op een Zondag ochtend naar de school, om in het kabinet van Van de Stadt, toen leeraar, later directeur van de H.B.S., te werken. Wij deden dat zonder hem permissie te vragen, maar zorgden, dat alles weer netjes op zijn plaats kwam. Lichtproeven werden voornamelijk gedaan met een verrekijker van mij. In den deksel van de koperen buis die om het glas kon geschoven worden, werd een ronde opening geboord om een dunnen lichtbundel door te laten. Zoo werden buigingsverschijnselen onderzocht. Lorentz was overtuigd van het bestaan van electrische golven en trachtte daarvan iets waar te nemen door ontladingsverschijnselen van een Leidsche flesch. Soms meende hij zoo iets te zien, maar dan vertelde hij den volgenden dag, dat het ook door de oude theorie te verklaren was. Het gelukte niet, omdat de hulpmiddelen te gebrekkig waren. [ . . . ] Bij al die gezellige bijeenkomsten in zijn huis was ook de vader van Lorentz aanwezig, die er veel pleizier in had. Hij was een gemoedelijke vriendelijke man, die zich later als een held gedragen heeft, toen zijn been geamputeerd moest worden zonder dat hij onder narcose kon gebracht worden.” Michaelis to Aletta Lorentz-­Kaiser, May 1, 1928 (LA 725). The amputation was performed in 1887, in Lorentz’s presence (see E.  van Dieren to Lorentz, undated, LA 17).

Physics in theory and practice


Fresnel has been one of those masters to whom I owe the most, and I still remember, more than half a century ago, when my resources allowed me to buy a somewhat more extensive book on physics than the usual textbooks, that I procured for myself the publication of the Complete Works of Augustin Fresnel. [. . .] My admiration and my respect had blended with my love and affection.34

On December 11, 1875, Hendrik obtained his doctorate, once again magna cum laude, with a doctoral dissertation entitled Over de Theorie der Terugkaatsing en Breking van het Licht (“On the Theory of Reflection and Refraction of Light”). His dissertation advisor was Rijke. The content of this groundbreaking work will be discussed in more depth in Chapter 4. Having a doctorate did not bring any great changes to Lorentz’s lifestyle, at least not yet. He continued to live in Arnhem and he also continued to study physics. Earlier in the year 1875, his first publication had already appeared: his solution to a mathematical problem.35 During that period, Lorentz also reworked his dissertation for publication in a German-­ ­ language journal. Additionally, he worked on an article about the propagation of light which was so important that Rijke presented it for publication at the Academy of Sciences in September 1877.36 Teaching at secondary schools no longer presented any difficulties to him: in the meantime he had taken on another hour of teaching more advanced students and substituted now and again at the HBS. At the end of 1877, though, this quiet little life came to a rather abrupt end.

34   “Fresnel a été un des maitres auxquels je dois le plus, et je me rappelle encore que lorsque, il y a plus d’un demi-­siècle, mes ressources me permirent d’acheter un livre de physique un peu plus étendu que les manuels ordinaires, je me suis procuré la publication des Œuvres Complètes d’Augustin Fresnel. [. . .] Mon admiration et mon respect s’étaient mêlés d’amour et d’affection.” Lorentz 1927b, 515. 35  See Nieuw Archief voor Wiskunde 1 (1875): 189–193. 36   Lorentz 1877a, Lorentz 1877b, Lorentz 1878b, Lorentz 1879a, and Lorentz 1879a.

Chapter 2 Professorship and family life

Changes in higher education In the spring of 1876, the Dutch parliament passed a law involving a fundamental reform of higher education. It went into effect in October of 1877.1 Following the lead of developments abroad, in particular in Germany, the law entailed that, in addition to intellectual and academic development, the independent pursuit of science became an explicit objective of a university education. At the same time the old Latin School, or Athenaeum Illustre, in Amsterdam was elevated to the level of a full-­ fledged university, thereby creating the fourth university in the Netherlands.2 Beside the traditional professorships, the new position of privaatdocent was created at universities. A privaatdocent had the right to lecture in classes but was not employed by the university and was paid by the students. Words were followed by deeds, as the government also provided ample financial means to support the far-­reaching changes. By 1880 the total size of the budget for universities had been doubled, compared to that of 1876, and the number of professorships had risen rapidly. In the faculties of mathematics and physics the number of chairs was increased from twenty in 1877 to twenty-­eight in 1879, and Amsterdam and Groningen, for example, built new, state-­of-­the-­art laboratories. A fifty-­percent raise in salary was also part of the package. In Leiden, Lorentz’s dissertation advisor Rijke, the only professor of physics at the time, argued for the institution of a chair specifically for mathematical physics. Whether or not this drive would be a success was doubtful. It was feared that the minister of education would view this expansion of the number of mathematics professors from two to three as unnecessary. Be this as it may, there was an obvious candidate for the chair: Johannes Diderik van der Waals, who had obtained his PhD in 1873 under Rijke. Van der Waals, a self-­made man, was director of the HBS in The Hague at the time. His work had become world famous all at once when his dissertation was published in 1873. As early as 1874, the prominent physicist James Clerk Maxwell had already written a laudatory review, predicting that “there can be no doubt that the name of  Van der Waals will soon be among  See Berkel 1985, chaps. 4 and 5 for the complicated history of the law and its aftermath.  The Athenaeum educated physicians and lawyers and did not have the right to confer doctorates.

1 2

“A Living Work of Art”: The Life and Science of Hendrik Antoon Lorentz. A. J. Kox and H. F. Schatz, Oxford University Press (2021). © A. J. Kox and H. F. Schatz. DOI: 10.1093/oso/9780198870500.003.0003

Appointment as professor


the foremost in molecular science.”3 During a lecture a short while later, he expressed himself again in very complimentary terms about Van der Waals’ work. It was the same lecture in which he made the often-­quoted pronouncement: “It has certainly directed the attention of more than one inquirer to the low-­Dutch language in which it is written.”4 At any rate, Maxwell himself had found the work a good reason to study the Dutch language.

Changes for Lorentz All these developments caused a drastic change in Lorentz’s quiet life. In the summer of 1877, he hatched a plan to apply for a position to teach physics at the HBS in the predominantly Catholic town of Roermond, in the province of Limburg. Rijke got wind of this and sprang into action. In a letter to Lorentz dated August 23, 1877, he laid out his plan to get Lorentz to move to Leiden.5 A job in Roermond would be difficult to get, he wrote, as Lorentz was not a Roman Catholic and he had a competitor who was of that religion. However, through the grapevine Rijke had heard that a mathematics and physics teaching position was about to open up at the Leiden Gymnasium. He suggested that Lorentz apply for the job and, at the same time, become a privaatdocent at the university. Rijke also wrote that it was not at all certain that Van der Waals would come to Leiden, since there was also an offer from Amsterdam. Should he not be coming, then Lorentz would be the right man in the right place, according to Rijke. Van der Waals did indeed accept the offer from Amsterdam. On September 19 he was already appointed professor there.6 Rijke viewed Lorentz’s application to teach at the Gymnasium and a subsequent position as lecturer as the first steps on the way to a full-­fledged professorship. It would at least bring him back to Leiden, where he could then make an extra effort to distinguish himself with his lectures. Lorentz followed Rijke’s advice and applied for the job. On November 7 he was nominated as the only candidate and the Municipal Council of Leiden passed a proposal to appoint Lorentz as the third mathematics teacher at the Gymnasium.7 Nevertheless, an unexpected course of events prevented his appointment.

Appointment as professor On November 3, Lorentz received a letter from Rijke. He was told, without much ado, that the curators of the university8 had decided during their meeting that day 4   Maxwell 1874.  See Maxwell 1875.   All letters from Rijke to Lorentz are in LA 101. 6  See Nederlandsche Staats-­Courant, September 20, 1877. 7  See Handelingen van de Gemeenteraad van Leiden, October 31, 1877. 8  At Dutch universities the Board of Curators was responsible for academic appointments and budgetary matters. 3 5


Professorship and family life

to write to the Minister of the Interior “to request with insistence that you be appointed immediately as Professor of Mathematical Physics.” Rijke added somewhat dryly: “This news will surely mystify you.”9 He then went on to explain what had happened. In Utrecht, a second chair of mathematics needed to be filled. The candidate for the job was George Baehr, professor at the polytechnic school—which later became a technical university—in Delft.When it turned out, though, that Cornelius Grinwis, the occupant of the existing chair, wanted to lecture on all the important topics, Baehr declined. Meanwhile, Grinwis had reviewed the manuscript by Lorentz that Rijke had submitted to the Amsterdam Academy a few months earl­ ier and in a report, written together with Van der Waals, he had commended it highly. He was so impressed by the article that he even wanted to offer the chair of mathematics to Lorentz. This turn of events caused a slight panic in Leiden, but some fast footwork by Rijke enabled the curators and the Faculty to send off their letter to the Minister on November 3. As little as two weeks later, on November 17, Lorentz was appointed by Royal Decree to the position of professor in the faculty of math­em­ at­ics and physics.10 It is interesting to note how Rijke emphasized in a later letter that Lorentz’s assignment of mathematical physics was only meant to meet a temporary demand. None of the mathematicians was willing to lecture on the subject at the time, so a redistribution of tasks was expected.11 In the end, this redistribution never came to pass. Another interesting detail is that Lorentz was the first professor in the Netherlands, and one of the first in Europe, to occupy himself specifically with the subject of mathematical—or theoretical—physics. In the years to follow, this field would quickly develop internationally into an important independent discipline.12

Inaugural lecture On January 25, 1878 Lorentz accepted his professorial duties by delivering his inaugural lecture on molecular theories in physics, entitled De Moleculaire Theoriën

9   “met aandrang te verzoeken U dadelijk tot Professor in de Mathematische Physica te benoemen.” “Die tijding zal U zeker wel bevreemden.” 10  See Nederlandsche Staats-­Courant, November 18 and 19, 1877. Obviously some information had been leaked, as the daily Het Nieuws van den Dag of November 8 already mentioned the proposal of the Leiden curators and the Utrecht efforts. 11   Rijke to Lorentz, November 7, 1877 (LA 101). In a later note (LA 192) Lorentz phrases it as follows: “17 November 1877. Appointed to teach mathematics and physics, assigned for the time being to teaching mathematical physics, with the prospect of a later, perhaps more desirable distribution of the mathematics teaching load among the professors.” (17 November 1877. Benoemd tot het geven van onderwijs in de wis- en natuurkunde met de bedoeling vooralsnog onderwijs te geven in de wiskundige natuurkunde met het oog op een latere wellicht meer gewenschte verdeeling van het onderwijs in de wiskunde onder hoogleeraren.) 12  See Jungnickel and McCormmach 1986 for more about this development.

Inaugural lecture


in de Natuurkunde.13 It is an interesting and important piece of work that deserves more detailed attention, especially because his observations are in part still relevant today. After welcoming his audience with the unusual form of address “Zeer gewenschte toehoorders” (Much desired audience) Lorentz began his lecture by observing something that will sound familiar to a great many physicists from ­conversations at birthday parties or other social events: When someone who has never seriously practiced any part of physical science glances at our physics journals and sees what subject matter experimental research involves, he is likely inclined to view a large part of those investigations as completely useless.14

Lorentz continued by giving a few examples of arcane experimental work, such as the determination of specific heat of the rare element cerium, but then went on to point out that the purpose of the science of physics was not to collect as many arbitrary facts as possible: On the contrary, they only truly acquire their proper meaning if one succeeds to uncover the relation between them, and it must be the ultimate goal of the investigation to derive the countless phenomena of nature as necessary consequences of some basic principles.15

Lorentz then pointed out that several branches of natural science had been quite successful in summarizing all kinds of phenomena in this scientific way and that quantitative research in physics had made especially great advances. For this quantitative research, the language of mathematics was indispensable, so he asserted, and therefore mathematical—nowadays theoretical—physics had ­developed side by side with experimental physics. To illustrate his point, Lorentz then elaborated on an important and very promising concept: molecular theory. The words he used to introduce this subject are remarkable: For nobody it will be unfamiliar nowadays that physicists imagine any physical body as a system of very small particles, the so-­called molecules, each of which can be composed, as chemistry teaches us, of a number of still smaller particles, the atoms.16   “Molecular Theories in Physics.” Lorentz 1878a.   “Wanneer iemand, die nog nooit eenig deel der natuurwetenschap ernstig heeft beoefend, een blik slaat in onze natuurkundige tijdschriften en ziet, met welke onderwerpen zich het experimenteel onderzoek bezig houdt, is hij allicht geneigd, een groot deel dier onderzoekingen voor volstrekt nutteloos te houden.” 15   “Integendeel verkrijgen deze eerst hunne rechte beteekenis, als het gelukt, het verband er tusschen op te sporen en het einddoel van het onderzoek moet het zijn, de talloze verschijnselen der natuur als noodzakelijke gevolgen uit enkele grondbeginselen af te leiden.” 16   “Wel niemand zal het tegenwoordig onbekend zijn, dat de natuurkundigen zich elk lichaam voorstellen als een stelsel van zeer kleine deeltjes, de zogenaamde moleculen, waarvan elke, zoals de scheikunde ons leert, uit een aantal nog kleinere deeltjes, de atomen, kan zijn opgebouwd.” 13 14


Professorship and family life

His statement was particularly remarkable because of its certainty. Actually, to most people in the nineteenth century the existence of atoms and molecules was not at all self-­evident. Some, like the positivistic Austrian physicist and philosopher Ernst Mach, had trouble with the assumption of entities that were not ac­cess­ ible to direct observation. There was also a movement, which was gaining traction in the latter decades of the nineteenth century, toward explaining natural phenomena not by means of atomism but by assigning a central position to the concept of energy. In the end, it was not until the beginning of the twentieth century that even the final skeptics wholly accepted the idea of atomism.17 The atomistic world view, so Lorentz contended, went back to antiquity and was further developed in the seventeenth century by assuming forces were at work between the smallest particles, either attracting or repelling them. He emphasized, though, that these forces cannot be understood by analogy, for example, to muscular force. They occur in the mathematical equations describing the motions of particles without the possibility of indicating a specific mechanism. As Lorentz said, “one starts from a basic principle which itself escapes any further explanation.”18 One should beware of an excess of illustration, he admonished. The point of view that Lorentz expressed here was very clear and showed that he had given serious thought to the basic principles of his discipline. He then went on to discuss some explanatory mechanisms in which particles and forces did not play a role and reached the conclusion that, even though room should be left for alternative approaches, the “representation according to which the atoms attract and repel each other is still way ahead of its competitors.”19 The rest of his lecture he devoted to the results that had been achieved by means of the molecular concept of matter. In rapid succession he discussed fluids, gases, and solid bodies and explained clearly what advances had been made and what problems there still were. In particular he dwelled on the dissertation by Van der Waals, who derived an equation—the “Van der Waals equation of state”—that provided a much better description of the relation between pressure, density, and temperature of gases than the old gas laws. Lorentz also mentioned efforts to draw conclusions from the optical properties of transparent bodies about the molecular structure of such bodies—precisely the subject of the paper he had written at the end of 1877. The discussion of Van der Waals’ work, in particular a footnote in the published version of his inaugural lecture where Lorentz stated that he had found an add­ ition to Van der Waals’ calculations, drew Van der Waals’ attention and led to a note 17   The experimental confirmation of Albert Einstein’s molecular theory for the so-­called Brownian motion played an important role in this acceptance process. Brownian motion is the phenomenon that observable microscopic particles, such as pollen, are in chaotic motion when placed in suspension in a fluid. Einstein explained this motion as a result of collisions between the particles and the molecules of which the fluid consists. His theory was quickly confirmed experimentally by the French physicist Jean Perrin. See also Nye 1976. 18   “gaat men van een grondbeginsel uit dat zelf aan verdere verklaring ontsnapt.” 19   “de voorstellingswijze, volgens welke de atomen elkaar aantrekken of afstooten, hare mededingsters nog ver vooruit is.”

Lorentz’s methodology


from him thanking Lorentz for his kind words and showing curiosity about Lorentz’s new result.20 Lorentz’s addition was only published in 1881,21 but there can be no doubt that he shared it with Van der Waals well before that time.

Lorentz’s methodology Lorentz’s ideas about the structure of matter and his views on the field of physics were further elaborated and clarified in a number of his lectures in the following decades.22 Two topics were particularly important in this context: the role of hypotheses and the central position of the concept of determinism in natural ­science. In a lecture in 1891, Lorentz discussed the choice of hypotheses.23 Prefacing the actual topic of the lecture entitled “Electriciteit en Ether” (Electricity and Ether), Lorentz reflected on the method a theoretical physicist employed to acquire knowledge about phenomena in nature: With regard to any natural phenomenon we are more or less in the position of someone who, without being familiar with the working of a clock, can only observe the hands and the pendulum. Such a person would also draw up his hypotheses and perhaps he might be able to find more than one that would satisfy him. The same is possible with respect to the phenomena offered to us by nature, and thus we arrive at the difficulty of making a choice.24

In this choice of hypothesis, so Lorentz asserted, a physicist must be guided by the question of “which hypothesis is the most satisfying because of its simplicity,”25 though he admitted that this criterion of simplicity also involves a matter of taste. In two lectures, one in 1900 and one in 1905, Lorentz further elaborated on this problem of choice.26 He now made an important distinction between two kinds of theories: the “careful” or “phenomenological” theories on the one hand, and the more “adventurous” ones on the other:27 In the first kind, one respects as much as possible what has been directly observed; [. . .] in the theories of the second group, on the other hand, we imagine, beyond what   Johannes D. van der Waals to Lorentz, March 8, 1979 (Kox 2018, 1). 22  See Lorentz 1881a.   See also Theunissen 2000, chap. 8, for Lorentz’s concept of science. 23   Lorentz 1891b. 24   “Tegenover elk natuurverschijnsel verkeeren wij min of meer in het geval van iemand die, zonder met de inrichting van een uurwerk bekend te zijn, alleen de beweging van de wijzers en de slinger kan waarnemen. Zoo iemand zou ook zijne hypothesen stellen en hij zou er wellicht meer dan één kunnen vinden, die hem bevredigde. Hetzelfde is mogelijk bij de verschijnselen, die de natuur ons aanbiedt en zoo geraken wij in de moeilijkheid, eene keus te doen.” 25   “welke hypothese door haren eenvoud het meest bevredigt.” 26  See Lorentz 1900a and Lorentz 1905c. 27   “Voorzichtige of fenomenologische.” “gewaagde.” Lorentz 1905c. A similar distinction was made in Lorentz 1900a. 20 21


Professorship and family life we observe, a world of invisible and hidden particles, whose expressions are the effects that are accessible to us.28

Examples of the second kind were the theory of hydrodynamics and Maxwell’s theory of electromagnetism. Hydrodynamics was led by the macroscopic equations of motion, while Maxwell’s theory was led by the so-­called Maxwell equations for the electromagnetic field. Implicitly, Lorentz made it clear that the second kind of theory was to be preferred, although in drawing up such a theory the physicist should, of course, beware of being carried away too much by his ­imagination. In his 1900 lecture, Lorentz had been more explicit in stating his preference: The satisfaction that the general laws do not grant us, we seek, and also find in part, in the special theories about the mechanism of the phenomena; these are the ones that give us a vivid and clear, though limited, image of the cohesion and the essence of things. While they teach us to recognize as necessary what has already been found, they cause us to identify what is still hidden, and they lead us to new experiments, perhaps to new discoveries.29

By introducing these two kinds of theories, Lorentz anticipated a similar distinction that was later made by Einstein between “theories of principle” and “constructive theories.” According to Einstein, thermodynamics and special relativity were examples of the first kind, and kinetic theory, from which thermodynamics can be derived, an example of the second, which he declared preferable.30 In his 1905 lecture, while discussing determinism, Lorentz very clearly presented his viewpoint. He asserted that natural science should be strictly deterministic in order to function properly: We must assume that from the state of the physical world at one moment the state at a following moment must follow by necessity.31

Lorentz also stated that mathematics was indispensable for drawing up theories of physics, since the deterministic laws of physics expressed quantitative relations. A few years later he further elaborated his views on determinism in a series of three 28   “In de eerste houdt men zich zooveel mogelijk aan het rechtstreeks waargenomene; [. . .] in de theorieën der tweede groep daarentegen stellen wij ons achter hetgeen wij waarnemen een wereld van onzichtbare en verborgen deeltjes voor, waarvan de voor ons toegankelijke werkingen de uitingen zijn.” Lorentz 1905c. 29   “De bevrediging, die de algemeene wetten ons niet schenken, zoeken, en vinden wij ook ten deele, in de bijzondere theorieën over het mechanisme der verschijnselen; deze zijn het, die ons van den samenhang en het wezen der dingen eene wel is waar gebrekkige, maar toch levendige en heldere voorstelling geven. Terwijl zij ons het reeds gevondene als noodzakelijk leeren erkennen, doen zij ons het verborgene op het spoor komen, en leiden tot nieuwe proefnemingen, misschien tot nieuwe ontdekkingen.” 30   That Lorentz anticipated Einstein was stated earlier, in Frisch 2005. 31   “Wij moeten wel aannemen dat, uit den toestand der stoffelijke wereld op één oogenblik, de toestand op een volgend oogenblik met noodzakelijkheid voortvloeit.”

Lorentz’s methodology


more or less similar lectures on prediction in natural science. One of them was published and reports about the other two appeared in some of the Dutch newspapers.32 On the basis of a few examples—like the return of Halley’s comet and the prediction that the coefficient of heat conductivity of a gas is independent of its density—Lorentz concluded that the laws of science permit the prediction of many phenomena with great certainty: One thing is certain, however, and that is that we can often predict the course of the physical and chemical phenomena in a very satisfactory manner.33

Lorentz then proceeded to venture into the realm of speculation, though. He posed himself the question whether this deterministic viewpoint was also valid for the world of psychology and decided that there is a close relationship between the material and the psychological world: “Everything leads to the belief that for each function of our spirit there is a corresponding state that is determined by the brain.”34 The corollary of this relationship was obvious, he believed: If one admits that every psychical phenomenon is by necessity accompanied by something that occurs in the material world, something that, furthermore, leads us nat­ur­al­ly to admitting also the inverse proposition, then it is clear that one cannot be a determinist in physics without also being one in psychology.35

This immediately confronted Lorentz with a well-­known problem: the apparent contradiction between free will and a world ruled by deterministic laws. Lorentz had to admit defeat here, and the only option left open to him was to simply accept this unsatisfactory state of affairs. His resulting conclusion was rather lame: If, on the one hand, we cannot evade the idea of determinism and if, on the other hand, we are conscious of being able to act freely, that proves a lack of unity within our inner being.36

Later on, in 1915, Lorentz went still further in his speculations about the relationship between the material and the non-­material world, writing to the theologian 32   The lectures were held in Leiden on March 23, 1911 (reported in newspaper Het Vaderland of the following day), in Groningen on February 20, 1912 (reported in Nieuwsblad van het Noorden of the following day), and in Brussels, in French, on March 28, 1914 (published as Lorentz 1921c). 33   “Une chose est certaine cependant, c’est que nous pouvons souvent prévoir le cours des phénomènes physiques et chimiques d’une façon très satisfaisante.” 34  “Tout porte à croire qu’à chaque fonction de notre esprit  correspond un état déterminé du ­cerveau.” 35   “Si on admet que tout phénomène psychique est nécessairement accompagné de quelque chose qui se passe dans le monde matériel, ce qui, du reste, nous conduit naturellement à admettre aussi la proposition inverse, alors il est clair qu’on ne peut être déterministe en physique sans l’être aussi dans la psychologie.” 36   “Si d’une part nous ne pouvons nous soustraire à l’idée du déterminisme et si d’autre part nous avons la conscience de pouvoir agir librement, cela prouve un manque d’unité dans notre être intime.”


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H.  Y.  Groenewegen, who had asked him for his opinion about a book by the German philosopher Gustav Theodor Fechner.37 It is worth quoting this letter because it provides a deeper perspective into Lorentz’s thinking, reaching well beyond his image as a sober-­minded physicist: The simple observation that the spirits of different people can understand each other, that the emotions and opinions of one are not indifferent to another, must lead us to the assumption of a connection between them, which many of us would like best to imagine in such a form that all these individual spirits are part of one great whole, a world spirit or deity. [. . .] But if the spirit is part of a great whole, just like the body is part of the entire material world, then naturally one arrives at the generalization that, as a rule, every event in the psychic realm responds to a change in the material realm. [. . .] The concept one arrives at in this way is that the spiritual and the material are inextricably connected, that they are two sides of the same thing, that the material world is a representation of the world spirit.38

In none of his writings, apart from a letter to Einstein in which he also discussed this “world spirit,”39 did Lorentz venture so far outside the realm of natural science.

Work in quiet seclusion Lorentz’s appointment and his move to Leiden may have been a life-­changing experience for him, but in one respect his life did not change all that much. He continued to work hard and publish regularly, but apart from his lectures at the university he did not venture much outside his study. An illustration of this voluntary isolation is a story remembered by his daughter Berta. Around 1892, the family was discussing at lunch that a stranger had been spotted in town that day. Lorentz’s immediate reaction was telling: “As long as it is not a physicist.”40 The subjects of his dissertation on electromagnetics and optics and his in­aug­ ural lecture on molecular theory would continue to be the leading topics in much of Lorentz’s scholarly work during the first decades of his professorate. In this 37   Lorentz to Groenewegen, April 5, 1915 (copy in ZA). Fechner’s book is: Ueber die physikalische und philosophische Atomenlehre. 38   “De eenvoudige waarneming dat de geesten van verschillende personen elkaar kunnen verstaan, dat de gevoelens en meeningen van den een den ander niet onverschillig zijn, moet ons tot het aannemen van een onderling verband leiden, dat menigeen zich zeker liefst in dien vorm zal voorstellen dat al de individueele geesten deelen van een groot geheel, een wereldgeest of godheid uitmaken. [. . .] Maar is nu de geest een deel van een groot geheel, evenals ook het lichaam deel van de geheele stoffelijke wereld is, dan komt men van zelf tot deze generalisatie, dat in het algemeen aan elk gebeuren op psychisch gebied een verandering op materieel gebied beantwoordt. [. . .] De opvatting waartoe men aldus komt is dat het geestelijke en het stoffelijke onverbrekelijk met elkaar verbonden zijn, dat het twee kanten van dezelfde zaak zijn, dat de materieele wereld een verschijningsvorm van den wereldgeest is.” 39   Lorentz to Einstein before January 23, 1915 (Kox  2008, 278). In this letter he introduced the Weltgeist to show that simultaneity could have an absolute character. 40  See Haas-­Lorentz 1957. Unless stated otherwise, the biographical information in the remainder of this chapter is derived from this publication.

Work in quiet seclusion


way, Lorentz developed a world view that created a clear separation between matter, consisting of small particles, on the one hand, and the ether, the medium functioning as carrier of electromagnetic action, on the other. In Chapter 4 this topic will be discussed in more detail. Lorentz also wrote a number of more general publications on molecular theory as such, separate from electrodynamics. During his first years in Leiden works on a range of topics were published, including hydrodynamics, thermodynamics, and the theory of sound. Soon after his appointment as professor, Lorentz was officially inducted into the Dutch scientific community. In May of 1881 he was elected a member of the Royal Academy of Sciences.41 The Academy, established in 1851, was the successor of the Royal Institute founded by King Louis Napoleon in 1808. Over time, the Academy had developed into the leading institution of scientific endeavor in the Netherlands.42 Since 1885 the Academy was—and still is—headquartered in the famous and magnificent Trippenhuis on one of Amsterdam’s canals. Until 1885 the building also housed the Rijksmuseum, at which point the museum’s collections were moved to their current location. The Academy’s two sections, one for the sciences and one for the humanities, counted a total of fifty ordinary members and twenty-­five foreign members, as well as fifteen corresponding members. The departments each had their own chairman and secretary, with the chairmen taking turns to serve as chairman of the entire Academy for two years. Both departments held monthly meetings; for the sciences, these were on the last Saturday of the month. One of the Academy’s stated duties was to provide so­li­ cit­ed or unsolicited advisory opinions to the Dutch government. Some such opinions were concerned with arcane questions like putting lightning rods on the cathedral in ‘s  Hertogenbosch or how to combat excessive noise in prisons. In reports on these questions in the Academy’s proceedings, the name Lorentz appears frequently.43 Throughout his life, Lorentz was an active member of the Academy. He faithfully attended the meetings, published a large part of his scientific production in both the Dutch and the English versions of its proceedings, and in the period between 1910 and 1921 he served as his section’s chairman. This was quite a burdensome task that took up much of his time, especially during World War One and its aftermath. In 1881, Lorentz also had his first doctoral student: Eduard Otto, who defended a dissertation on the theory of sound. He was the first of a total of twenty-­five doctoral students. The last one, Willem van den Berg, presented his dissertation in 1921, on Einstein’s gravitational theory. According to daughter Berta, Lorentz coached his students intensively and sometimes, if things were not moving along

  In 1938, its name was changed to the Royal Netherlands Academy of Sciences.  See Berkel 2008 and Berkel 2011 for a comprehensive account of the Academy’s history. 43  See Berkel 2008, chap. 9, for more on the reports, including accounts of the friction they sometimes caused between the Academy and the government. 41 42


Professorship and family life

fast enough for his liking, took the bull by the horns and wrote parts of the dissertation himself.44 In the list of Lorentz’s doctoral students a few details are noteworthy. In the first place, there is an enormous variation in dissertation topics. Apart from the two topics already mentioned, his students worked on theses about optics, radiation phenomena, hydrodynamics, electromagnetism, electron theory, and kinetic gas theory. Secondly, not many of his students made much of a career in physics. Only three of them became university professors—Leonard Ornstein, Johannes Droste, and Adriaan Fokker—while some others became secondary school ­teachers. In quite a few cases their careers were so insignificant that no further information about them can even be found. It is also interesting that no fewer than four of his final eight doctoral students were women—highly unusual in those days in the male-­dominated field of science, but not surprising in view of his wife Aletta’s strong commitment to the cause of women’s rights, which will be discussed in more detail in Chapter  3. Finally, it is worth noting that Lorentz’s daughter Berta was also one of his PhD students, something that is frowned upon nowadays as an undesirable conflict of interest.45 Three years after becoming a member of the Academy, in May of 1884, Lorentz was also elected to membership in a second prominent scientific society, the Hollandsche Maatschappij der Wetenschappen (Holland Society of Sciences), headquartered in the town of Haarlem.46 This Society, dating back to 1752, is a typical product of the Enlightenment. Its objective was—and still is—the advancement of science in the broadest sense. For many years its most important activity was organizing contests and awarding prizes. These days the Society organizes lectures and gives out stipends and achievement awards. Its membership consists of persons interested in science (“directors”) and active scientists (“members”). Starting in 1920, Lorentz became its secretary and, at the same time, the representative for science in the Society’s board of directors. Through his membership of the Hollandsche Maatschappij, Lorentz became involved in an extraordinary editorial project: the publication, undertaken by the Maatschappij, of the Œuvres Complètes of Christiaan Huygens, the most celebrated scientist in the Netherlands of the seventeenth century. This ambitious edition did not only include Huygens’s published works but also his cor­res­pond­ ence, all carefully annotated. It was a costly and time-­consuming project: While the first volume appeared in 1888, the twenty-­second and final volume did not come off the press until 1950. All volumes were printed in a specially designed type font on paper watermarked with the names “Christiaan” and “Huygens” on adjacent pages. Since the volumes were published without reference to the authors of the various annotations, it is not possible to determine precisely what Lorentz’s contributed. Judging by his correspondence with the mathematician   Haas-­Lorentz 1957, 36.   Berta obtained her doctorate in 1912, with a dissertation on Brownian motion. The other women were Johanna Reudler (1912), Eva Dina Bruins (1918), and Hendrika Johanna van Leeuwen (1919). 46   Royal Holland Society of Sciences since 2002. 44 45

Lorentz as experimentalist


D. J. Korteweg, who directed the project for many years, the conclusion is justified that Lorentz worked on the topics of optics and mechanics and that he invested much time in the work.47

Lorentz as experimentalist A special event in this period was the discovery by Pieter Zeeman—in 1896—of a new effect that was later named after its discoverer: the magnetic separation of spectral lines. In Chapter  4 the discovery and the explanation of this Zeeman effect is presented in more detail. Zeeman, who had obtained his doctorate a few years earlier, made his im­port­ ant discovery in the Leiden physics laboratory where he was working as an as­sist­ ant at the time. Only a few days after Zeeman had made his discovery public, Lorentz already came up with an explanation that naturally followed from his earlier ideas about the existence of small charged particles. Based on his theory, he also made an experimental prediction that was subsequently confirmed by Zeeman, an excellent example of successful cooperation between an experimentalist and a theoretician. A few months after his discovery, Zeeman was appointed lecturer in Amsterdam, and this prompted a frequent and extensive correspondence between Lorentz and Zeeman that has largely been preserved. In his letters, Lorentz showed a side of himself that has received rather less attention in many descriptions of his activities: his work in experimental physics. Lorentz was extremely interested in the experimental side of Zeeman’s discovery and in everything connected with it, so he started experimenting himself. These experimental activities are documented in detail in their correspondence. There is even a kind of division of labor between the two men. Lorentz devises an experiment, but is often not sure of himself with regard to the more technical aspects of the matter. After asking Zeeman for advice, and even sometimes borrowing Zeeman’s instruments, Lorentz then presents his experimental results and his theoretical interpretation to Zeeman. The other way around, Zeeman also performs experiments, and Lorentz guides him with regard to the theoretical interpretation of his results. Of course, for this intensive exchange of ideas it was helpful that in those days mail was delivered four times a day. All of this makes clear how great Lorentz’s enthusiasm for experimental work really was.The difference between the two scientists also emerges clearly: Zeeman’s professional approach and the extensive possibilities available to him in the Amsterdam laboratory, as opposed to the primitive and more improvised approach by Lorentz. In short, a classic case of cooperation between an experimentalist and

47   The letters from Korteweg are in LA 43, and those from Lorentz are in the library of the University of Amsterdam.


Professorship and family life

a theoretician, with the added twist that the theoretician also performed ex­peri­ ments himself.

Colleagues and friends In 1892, Lorentz was joined by a new colleague of the same age, the enthusiastic and ambitious experimentalist Heike Kamerlingh Onnes, who had succeeded Rijke as professor of experimental physics.48 Lorentz and Onnes soon became close friends. Lorentz already knew Kamerlingh Onnes before his arrival in Leiden. They had met at the end of their HBS years through their chemistry teacher, Van Bemmelen, who had taught Onnes in Groningen and, subsequently, Lorentz in Arnhem. For Onnes his chemistry teacher had played a similarly inspiring role as Van de Stadt had done for Lorentz. After having passed the kandidaatsexamen at the university in Groningen, Onnes went on to study in Heidelberg, where he started working on the dissertation he eventually finished in Groningen in 1879. A year earlier, he had become the assistant to Johannes Bosscha at the Polytechnic School in Delft. Under Onnes, a physics laboratory was established in Leiden where two im­port­ ant lines of investigation were started. One line of research was a series of ex­peri­ ments on magneto-­optics, the influence of magnetism on light. It became known as the “Lorentz series.” The field of magneto-­optics included effects like the modi­fi­ca­tion of light refracted by a magnetized mirror, known as the Kerr effect. As the name of the series implied, Lorentz was closely involved in these experiments, which were based for the most part on his theoretical considerations. A second line of inquiry was the field of low-­temperature physics. In close cooperation with Van der Waals, who took care of the theoretical aspects, Kamerlingh Onnes started an ambitious program to achieve ever-­lower tem­per­at­ ures, with the eventual goal of liquefying the “permanent” gases. Nobody had been able to liquefy these gases until that time, despite a number of prior efforts. The program’s crowning glory, in 1908, was Onnes’ successful liquefaction of helium. From that moment on, the emphasis of the Leiden research program changed somewhat. Since it had now become possible to reach these low tem­per­ at­ures, it was felt that this opportunity should be used to systematically investigate all sorts of physical properties at extremely low temperatures. This approach led to a number of new findings, among which was the discovery of the phenomenon of superconductivity.49 Aside from Onnes, there was another colleague who played an important role in Lorentz’s life, both professionally and personally. The professor of astronomy Hendricus Gerardus van de Sande Bakhuyzen, who had succeeded Frederik  See Delft 2007, chap. 9, for the complicated history of the appointment.  See Delft 2007 for a biography of Kamerlingh Onnes and a detailed overview of the experimental research in Leiden. 48 49

Lorentz’s first foreign contacts


Kaiser when he had passed away in 1872, became a close friend to Lorentz. In a eulogy at his friend’s funeral in 1923, Lorentz remembered how at first he only had a master–student relationship with Bakhuyzen, who was fifteen-­years his se­nior, but it gradually evolved into an increasingly personal connection.50 He called him a “counselor to whom I could always turn, full of confidence in his insight and interest, [. . .] also when I needed to take some difficult decision that was important to me.”51 He praised the hospitality of Bakhuyzen and his wife and recounted how he met a group of men in their house who excelled in all kinds of different fields and who, like himself, cherished “delightful memories” of the times they spent there.

Lorentz’s first foreign contacts Completely in line with his apparent desire for seclusion, Lorentz did not travel abroad much until well into his career, and his contacts with foreign colleagues during the period between 1987 and 1900 were mainly in writing. The Lorentz cor­res­ pond­ence archive for that period contains only some twenty letters by physicists from outside the Netherlands. The better part of this correspondence is with the German physicist Woldemar Voigt and the Austrian physicist Ludwig Boltzmann.52 From 1883 onwards, Voigt became a fixture in Lorentz’s correspondence, especially after the discovery and the theoretical explanation of the Zeeman effect in 1896. Voigt had carried out many experiments in this field and had also developed a theory about the phenomenon. Unlike Lorentz, he based his theory to a much lesser extent on an explicit particle model.53 Despite their professional differences, Lorentz and Voigt got on well together. Not surprisingly, Voigt was the first foreign colleague who was received by Lorentz in Leiden. Actually, this visit did not take place until 1899, which speaks volumes about the seclusion Lorentz had initially chosen for himself. Voigt’s visit to Leiden was preceded by a visit to Voigt by Lorentz. During a vacation in the German town of Holzhausen (in Lippe) in the summer of 1897, Lorentz made plans for an outing to Göttingen. Voigt held a professorship there and wanted to meet Lorentz. The visit was a resounding success. Not only did he meet Voigt himself but he also made the acquaintance of important colleagues like the mathematician Felix Klein and the physicist Walther Nernst. After Voigt’s 1899 return visit to Leiden, the two men began to use the German salutation Verehrter Freund (Esteemed Friend) in their correspondence. Voigt was   Nieuwe Rotterdamsche Courant, January 11, 1923.   “raadsman, tot wien ik mij steeds vol vertrouwen op zijn inzicht en belangstelling kon wenden, [. . .] ook als ik een of ander moeilijke en voor mij belangrijke beslissing had te nemen.” 52   Their correspondence to and from Lorentz has been published in Kox 2008. 53  See Voigt  1908 for a detailed explanation. Voigt calls his approach “neutral” because it is not based on an explicit atomistic model that might be inadequate in more complicated cases (see Voigt to Lorentz, November 15, 1899, Kox 2008, 71). 50 51


Professorship and family life

one of very few of Lorentz’s correspondents whom he continued to address like this, and this unusual occurrence in Lorentz’s letters speaks to the friendship between the two men.54 Lorentz being awarded an honorary doctorate—his first— at the University of Göttingen on June 17, 1899 may also have had something to do with the cordial relationship between the two men.55 Unfortunately, he was unable to attend the award ceremony in Göttingen. Lorentz’s correspondence with the famous Austrian physicist Ludwig Boltzmann is less personal and mostly of interest for its scientific content. Boltzmann, like Lorentz, was a strong proponent of the atomistic view of matter. He was well known because of his work on kinetic gas theory, the molecular theory of gases and liquids, and this theory was the main topic of their correspondence. Without going into the matter too deeply here, it is fair to say that Lorentz made a few important improvements and additions to Boltzmann’s work.56 Boltzmann clearly did not take Lorentz’s corrections the wrong way, judging by the beginning of one of his letters: Already by the postmark and the handwriting I recognized that the letter came from you and I was delighted. It is true that any letter from you means a mistake made by me; only, I always learn so much from them that I would almost wish that I made more mistakes, in order to receive still more letters from you.57

As Boltzmann felt that his work was underappreciated in his own country, he also expressed his pleasure in Lorentz’s pertinent comments: “I am very happy that I have found in you someone who works on the further elaboration of my ideas on gas theory. In Germany there is almost nobody who properly understands the matter.”58 In his letters Lorentz, on his part, appeared to be pleased with Boltzmann’s willingness to enter into a discussion with him, and it must have been satisfying that Boltzmann, a figure he much admired, frankly acknowledged his own mistakes. In the eulogy Lorentz gave in 1907 in Berlin, after Boltzmann’s dramatic suicide, he called him a “leader in our science and a groundbreaker in many fields” and he remembered “the kindness he has shown me and the excitement I found 54   Another, and possibly the only other, correspondent who was addressed as Verehrter Freund by Lorentz was the German physicist Eilhard Wiedemann. A more common form of address in Lorentz’s correspondence was Verehrter Herr College (Esteemed Colleague) or variations on that theme. To address Dutch colleagues Lorentz generally used the Latin phrase Amice (my friend), a relatively informal form of address that was commonly used among Dutch academics and professionals at the time. 55   Voigt was Dean of the Philosophische Fakultät and signatory of the official letter of the faculty awarding him the honorary doctorate (Fakultät to Lorentz, June 17, 1899, LA 200). 56  See Kox 1990b and Kox 1993b for more detailed discussions. 57   “Schon an dem Poststempel und der Handschrift erkannte ich, dass der Brief von Ihnen stammt und hatte eine Freude. Freilich bedeutet jeder Brief von Ihnen einen von mir gemachten Fehler; allein ich lerne dabei immer so viel, dass ich fast wünschen möchte noch mehr Fehler zu machen, um von Ihnen noch mehr Briefe zu erhalten.” Ludwig Boltzmann to Lorentz, December 21, 1890 (Kox 2008, 17). 58   “Ich freue mich sehr, dass ich in Ihnen jemand gefunden hat, welcher an dem Weiterbau meiner Ideen über Gastheorie arbeitet. In Deutschland ist fast niemand, welcher die Sache ordentlich verstünde.” Boltzmann to Lorentz, December 11, 1886 (Kox 2008, 3).

Lectures in Leiden


in conversations with him.”59 In her reply to Lorentz’s condolence note, Boltzmann’s widow confirmed that the feeling was mutual: “The communication with you, Esteemed Professor, was among the most wonderful and happiest mem­ or­ies of his life.”60 One of Boltzmann’s letters contains an invitation that would mark the end of Lorentz’s self-­imposed seclusion during his first years in Leiden.61 It was a request to participate in the 70th annual congress of the Gesellschaft Deutscher Naturforscher und Ärzte, the national society of German natural scientists, mathematicians, and physicians. This Naturforscherversammlung took place in Düsseldorf in September of 1898. The invitation was prompted by the desire of the Gesellschaft’s board of governors to increase the involvement of Dutch natural scientists in their yearly meetings.62 The organizing committee had chosen a special topic, about which Wilhelm Wien would be giving the keynote address. Wien was privatdozent in Aachen, a position equivalent to that of a Dutch privaatdocent. Lorentz was asked to present a supplementary lecture, followed by a discussion. The topic in question was the problem of motion of material bodies through the ether, a theme that aligned closely with Lorentz’s recent work. In Chapter 4 this work will be presented in more detail. Lorentz accepted the invitation and made his presentation. He met Boltzmann for the first time and was also introduced to Max Planck and other well-­known German colleagues. According to his daughter Berta, he thoroughly enjoyed the visit. As she wrote in her biographical notes, “Seldom have I seen my father in such good spirits as after his return from the congress.” Lorentz had now definitively become part of the international world of physics, and his self-­imposed seclusion had finally come to an end. In the years to come he would make regular journeys abroad to attend congresses and conferences, visit universities, and give lectures.

Lectures in Leiden Immediately after giving his inaugural address, Lorentz had started his regular lectures in Leiden. His daughter Berta provided an appealing description of the relaxed atmosphere in Lorentz’s classes. Before the actual lecture began, he would engage the students in some small talk while they were all warming themselves by the coal stove. After a joke here and there and a glance at the wall-­clock, he would invite the students, most of whom were barely younger than he was, to sit down so 59   “Führer unserer Wissenschaft, ein Bahnbrecher in manchen Richtungen [...] das Wohlwollen das er mir zeigte und die Anregung, die ich im Gespräch mit ihm fand.” Lorentz 1907c. 60   “der Verkehr mit Ihnen hochgeehrter Herr Professor gehörte zu den schönsten und glücklichsten Erinnerungen seines Lebens.” Henriette Boltzmann to Lorentz, January 19, 1907 (LA 8). 61   Boltzmann to Lorentz, October 13, 1897 (Kox 2008, 38). 62   See Felix Klein to Lorentz, October 20, 1897 (LA 41), repeating the invitation and requesting Lorentz to pass on the invitation to his colleagues.


Professorship and family life

he could begin the class. Afterwards he would walk along Rapenburg—where the university was located—to Turfmarkt on his way to his rented rooms on the floor above a tobacconist’s shop. In his first academic year, capillarity and kinetic gas theory were the topics of Lorentz’s lectures on mathematical physics.63 He taught four one-­hour classes a week for a small group of only seven physics students. In the following year he did not only teach physics to physicists (optics this time); he also taught theoretical physics to chemistry students and basic geometry to medical students. In all, he was responsible for seven hours of lecturing every week. Right from the start, Lorentz’s lectures were very well received by the students and highly praised in the yearly Almanak, the yearbook of the Leidsch Studenten Corps. Much later, in 1925, his former collaborator Balthasar van der Pol showed himself equally taken with Lorentz’s skills as a lecturer: “A lecture by Professor Lorentz is a scientific and esthetic pleasure.” Adding a word of caution, he con­tinued: “The extremely clear way of treating the problems may cause the less initiated to ­underestimate the difficulty of the questions now and then, difficulties that only come to the fore if one later attempts to reproduce what one has read or heard.”64 The well-­known Dutch writer and psychiatrist Frederik van Eeden also provided some insight into Lorentz’s appearance and teaching style with a few sharp observations after attending one of his lectures for a general audience in 1911:65 Especially interesting was the insight I gained into the psychological circumstances of these new discoveries, what happens inside a head like that, when it jumps from calculation to sense of reality, and from feeling once again to calculation [. . .] Curious in this scholar is the childlike aspect. There is a naïve, jokingly pleasant sense of fun in the calculation and verification of the results. Lorentz kept speaking of “those things” when he made his energy elements vibrate and collide and crisscross. [. . .] Lorentz, with his bald, chunky head, his thick black eyebrows, his dark eyes, large flat nose, and long beard, is more reminiscent of an old philosopher, sometimes also of a rabbi. But from this head, between all these other scholarly heads, the genius, the

63   Information about Lorentz’s teaching is derived from the annual reports on the state of education in the Netherlands (Verslag van den Staat der Hooge-, Middelbare en Lagere Scholen in het Koninkrijk der Nederlanden). 64   “Een college van Professor Lorentz is een wetenschap­pelijk en aesthetisch genot.” “De uitermate heldere wijze van behandelen van de problemen doen de minder ingewijden misschien de moeilijk­ heden van de vraagstukken wel eens onderschatten, moeilijkheden die eerst recht naar voren komen als later geprobeerd wordt het gelezene of gehoorde te reproduceren.” Pol 1925, 323. A similar observation was made by Hendrik Casimir in a personal communication with one of the authors (AJK). 65  Van Eeden was keenly interested in natural science. In a public lecture in 1904 he praised Lorentz’s work in physics while—unbeknownst to him—Lorentz happened to be in the audience. They met afterwards and became friendly (see Haas-­Lorentz 1957, 86). In later years, Van Eeden met with Lorentz several times and attended many of his public lectures. He regularly commented on them in his diary (Eeden 1971), for example in the entry of March 3, 1913: “I attended Lorentz’s lecture about the principle of relativity. It was good and clear, but did not learn anything new.” (Ik was bij Lorentz’ reede oover het relativiteits beginsel. Het was goed en duidelijk, maar leerde niets nieuws.) The lecture Van Eeden described here was one of a series of three in November 1911, entitled “De Theorie van Energie-­Elementen” (The Theory of Energy-­Elements), at Teylers Museum in Haarlem.

Lectures in Leiden


extraordinariness is immediately noticeable—just like one immediately recognizes the hero, the great actor, among actors in a drama.66

Apart from the topics of his lectures, of course, Lorentz’s teaching remained essentially unchanged until the academic year 1883–1884. In that year, three hours per week were added to his course load in the form of an elementary physics course for a large class of forty medical students. In the following year, these elementary classes were complemented by another six hours of “practical exercises,” or practicum. Including his other lectures, Lorentz’s teaching by then amounted to no less than thirteen hours a week: indeed, a heavy teaching load that increasingly weighed on Lorentz over time. The cause of all this was Kamerlingh Onnes. He had opted out of teaching the medical students, even though it was officially part of his duties. For a while, Onnes had health problems and was unable to combine the time-­consuming classes for medical students with his activities in the laboratory. For this reason, Onnes had foisted this job on Lorentz. Even after having recovered completely, Onnes was more than happy to leave the situation unchanged. In 1900 he even argued that his directorship of the laboratory placed such strenuous demands on him that teaching medical students was out of the question.67 Lorentz continued the classes for the medical students and the practicum. Lorentz’s wife was not altogether happy about this situation, and later on she clearly expressed her own viewpoint about Onnes’ role. In a letter to her daughter she wrote: Onnes did not have a heart for the medical students, he took the physicists separately, but did not really lecture to them, also because of his health. Pa cared about the medical students, that is why he took over the class and did not want to let it go before he could get a good substitute. The practicum has been Pa’s idea and started very simply at first, but not in the first year.68

In Chapter 6 a more elaborate description shows how Lorentz was unable to put an end to this unfortunate situation until much later. The course for medical 66   “Vooral interessant was het inzicht dat ik kreeg in de psychologische toedracht van deze nieuwe ontdekkingen, hoe het in zoo’n hoofd toegaat, als het van berekening op realiteitsgevoel, en van gevoel weer op berekening overspringt [. . .] Eigenaardig is bij den geleerde het kinderlijke. Er is een naïeve, schertsend gemoedelijke pret in het berekenen en het toetsen der uitkomsten. Lorentz sprak aldoor van “die dingen” als hij zijn energie-­elementen liet vibreeren en botsen en door elkaar heengaan. [. . .] Lorentz, met zijn kaal, bonkig hoofd, zijn dikke zwarte wenkbrauwen, zijn donkere oogen, groote platte neus, en langen baard doet meer aan een oude wijsgeer denken, soms ook aan een rabbi. Maar onmiddellijk is aan zijn kop, tusschen al die andere geleerdenhoofden, het geniale, bizondere te bemerken— zooals men onder acteurs in een drama onmiddellijk den held herkent, den grooten speler.” Eeden 1971, entry of November 26, 1911. 67  See Delft 2007, 354. 68   “Onnes had geen hart voor de medici, nam de natuurkundigen afzonderlijk maar gaf eigenlijk geen college, ook al om zijn gezondheid. De medici gingen Pa ter harte daarom nam hij het college over en wilde het ook niet afstaan voor hij een goede plaatsvervanger kon krijgen. Het practicum is een denkbeeld van Pa geweest en eerst heel eenvoudig begonnen maar niet het eerste jaar.” Aletta Lorentz-­ Kaiser to Berta de Haas-­Lorentz, July 6, 1928 (FC).


Professorship and family life

s­ tudents did produce something of permanence, though. It resulted in a ­much-­used two-­volume textbook entitled Beginselen der natuurkunde. Leiddraad bij de lessen aan de Universiteit te Leiden (Principles of Physics: Handbook to the Lectures at the University of Leiden). The book was reprinted and revised nine times and was translated into German, Russian, and even Japanese.69

Marriage and family life After Frederik Kaiser’s death in 1872, Lorentz continued to be friendly with his sons. Kaiser’s son Pieter Jan introduced Lorentz to his cousin Aletta Catharina (informally called “Et” or “Ettie”) in the summer of 1880. She was the youngest daughter of one of Frederik’s brothers, Johan Wilhelm Kaiser, and five years Lorentz’s junior. Their first meeting was rather unconventional, as Aletta’s younger brother Rudolf remembered: “When she saw him for the first time, as far as I have heard, he was sitting in a merry-­go-­round.”70 Not long afterwards they became engaged. Aletta came from a large family: She had four brothers and three sisters. Their father, a well-­known engraver—he designed the first Dutch postage stamp—was a professor at the National Academy of Fine Arts in Amsterdam and also the director of the Rijks Museum van schilderijen en penningen (National Museum of Paintings and Coins), the predecessor of the present-­day Rijksmuseum. As was noted earlier, until 1885, when it moved to the building it still occupies today, the museum, together with the Academy of Sciences, was housed on one of Amsterdam’s canals in the famous Trippenhuis, the elegant building the Kaiser family also called their home. Lorentz’s student Adriaan Fokker later described Aletta as a “reserved woman, somewhat severe, perhaps, in her ideas,”71 but in family correspondence that has been preserved, she comes across as a warm and involved mother. She was also a sharp and witty observer of the world around her, as is shown in the travel diaries she kept during their many trips, which have been put to good use for this book. Rudolf reported that Aletta’s sister Elisabeth (“Betsy”)—who will also be mentioned in later sections—was quite taken with Hendrik because of his “warm simplicity.” Only his misshapen nose, which had never healed properly after having been broken when he was a young child, did not meet with her approval, but she believed she would surely get used to that. 69   The first edition appeared in 1888. From the fourth edition onwards (1904–1905), Lodewijk Siertsema was co-­author. Siertsema was lecturer in physics at Leiden University until 1905, when he became professor at the Delft Technical University. Earlier, in 1882, Lorentz had published a textbook entitled Leerboek der differentiaal- en integraalrekening en van de eerste beginselen der analytische meetkunde met het oog op de toepassingen in de natuurwetenschap (Textbook of Differential and Integral Calculus and Analytic Geometry with a View on Applications in Physics) which had resulted from his lectures to physics students. It was also translated into German. 70   “Toen ze hem voor het eerst zag zat hij, zoals ik vernomen heb, in een draaimolen.” See Rudolf Kaiser’s recollections of Lorentz in a letter to Berta of January 28, 1940 (PB). 71   “een ingetogen vrouw, ietwat streng misschien in haar opvattingen.” Fokker 1946, 62.

Marriage and family life


Lorentz grew very fond of his father-­in-­law, Johan Kaiser. After having been pensioned off, in 1883, a year before the inauguration of the new Rijksmuseum, he had moved first to Wassenaar, to the country estate Zuidwijk, and five years later, to Leiden. Because of his deafness, he increasingly withdrew into the privacy of his home and Lorentz made a habit of visiting him every week. Lorentz brought him the books that he wanted for his studies from the university library and always managed to find a discussion topic to distract him. Rudolf Kaiser, remembering his own regular visits to his father and noting how he had little to contribute in the way of conversation, remarked: “[I was] often struck by my father’s radiant recovery when Lorentz came to visit.”72 Lorentz and Rudolf quickly became friendly. For that reason, his recollections offer interesting information about Lorentz. Of course, they provide factual information, which is always important, but they are particularly valuable because Rudolf writes about Lorentz as his friend and discloses all kinds of homely details that are almost completely lacking elsewhere. For example, Rudolf remembered his first visit to Lorentz’s rooms, “where he received me in slippers, without his collar and smoking a pipe. [. . .] We really hit it off, we took a walk to ‘De Vink,’ where he treated me to Boerenjongens.”73 Lorentz’s warmth, in particular, which he had experienced on many occasions, had stuck in Rudolf’s memory: “To be agreeable or of service to someone, nothing was too much for him. That is the way Lorentz was.”74 With gusto he also described the parties in the summer of 1881, during the bridal days preceding Hendrik and Aletta’s wedding. The highlight was the festive evening in Zomerzorg75 where the guests gave all sorts of performances. The Kaisers participated wholeheartedly. Brothers Willem and Louis and sister Betsy performed a play, preceded by an aria from Mozart’s Die Zauberflöte. Willem and Rudolf sang a comical duet, and Rudolf also sang a well-­known French song with lyrics adapted for the occasion.76 There was also a group outing to Vogelenzang77 where they ate pancakes and drank hypogras, also known as hypocras, a popular alcoholic concoction for weddings made out of white or red wine, honey, herbs, and spices. On the train home, at the station in the town of Halfweg, Lorentz apparently had a taste for more: He switched to the compartment that seated Rudolf and his company and joined them to enjoy another glass of the festive beverage. On July 15, 1881, Hendrik and Aletta got married. After the ceremony in the Amsterdam City Hall, the church wedding was performed by minister Petrus   “[Ik was] menig maal getroffen door de stralende opleving van mijn vader als Lorentz kwam.”   “waar hij op pantoffels, zonder boord en met een pijp mij ontving. [. . .] Het boterde bijzonder goed, wij maakten een wandeling naar ‘De Vink’ waar ik op boerenjongens getracteerd werd.” “De Oude Vink” was a well-­known inn just outside Leiden. Boerenjongens (Farm boys) is a traditional alcoholic drink of brandied raisins, while another such drink, Boerenmeisjes (Farm girls) contains dried apricots. 74   “Om iemand aangenaam of van dienst te kunnen zijn was niets hem te veel. Zoo was Lorentz.” 75   A café with several banquet halls near the Leiden railway station. 76   The lyrics and the program are in LA 655. 77   A village near the coastal town of Bloemendaal. 72 73


Professorship and family life

Hugenholtz in the church of the Vrije Gemeente.78 Rudolf recounted how he went to collect the nervous Lorentz at Hotel De Keizerskroon, where he was spending the night, and how he helped Lorentz by lending him a pair of white gloves, since he had apparently forgotten to bring his own. After the wedding ceremony there was a banquet at the Trippenhuis. For Rudolf, the wedding would forever be connected to the image of Lorentz’s “radiantly happy jet-­black eyes behind the glinting glasses.”79 In spite of the church’s role at the wedding, it does not appear that Lorentz was very religious. Although he was raised in an orthodox Protestant environment, his preferences eventually veered toward a more liberal concept of religion. As a secondary school student he did attend services of the (liberal) Walloon Church, but this was mainly to improve his knowledge of French. Berta recounted how, later on in Arnhem, her father went to listen to the sermons by minister De Keyzer, but even his brand of liberal faith did not satisfy her father in the end.80 Actually, she failed to mention that Lorentz formally terminated his membership of the Dutch Reformed Church after a conflict about church taxes in 1895.81 After a honeymoon in Germany and Belgium82 the newlyweds moved into a house at Hooigracht 60, where their first child was born on November 20, 1885. It was a daughter, who was named Geertruida Luberta after both Lorentz’s mother and his stepmother, and went by the name of Berta, or Ber. The name Luberta may have been a spelling error, or perhaps the young parents found Lubberta less suitable. Six months later, the Lorentz family bought a house a few doors down, at Hooigracht 48, where they continued to live for over thirty years until their move to Haarlem in 1912. It was an expensive purchase, costing as much as 11,200 guilders, which was paid for in part by a 5,000-­guilder mortgage. The building, with its double-­stepped gable, consisted of two identical seventeenth-­century houses joined together. Its interior was noted for its magnificent salon in Louis Seize style with striking murals. Now designated a national historic landmark, the building’s facade boasts a stone with the name LORENTZ chiseled in capital letters. In the rear of what used to be the rightmost house Lorentz had his study, a quiet room with a view of the large garden. There were seven steps leading down to the garden level. One of the study’s walls was completely covered by a bookcase and there was a roll-­top desk near the window, as well as a stove and a simple easy chair. In later years the desk was replaced by a large writing table. If he was not sitting at his desk or pacing back and forth to put his thoughts in order, Lorentz 78  The Vrije Gemeente was an offshoot of the Dutch Reformed Church, founded by the brothers Philip Reinhardt and Petrus Hermannus Hugenholtz. The church building on Weteringschans is now a well-­known venue for rock and popular music known as Paradiso. 79   “glunder stralende gitzwarte oogen achter de glimmende brillenglazen.” 80   She even devoted close to two pages in Haas-­Lorentz 1957 to a lament that her father had foregone the comfort of faith, writing how she failed to understand how he had been able to live a harmonious life without “guidance from above.” 81 82   See correspondence with J. H. Goudsmit in LA 27.   See travel diary in LA 656.

Lorentz’s first public activities


liked to sit and think in his easy chair, smoking a cigar. His pacing back and forth could easily be heard in the basement immediately below the study, according to Berta. She also remembered being intrigued by the row of portraits of “clever men” hanging over Lorentz’s desk and how her father answered her by pointing to Augustin Fresnel and Heinrich Hertz when she asked him who was the cleverest.83 Lorentz generally spent the better part of the day in his study, and in the evening hours Aletta came in to keep him company. Sometimes she also assisted him if he had to go to the laboratory to prepare experiments for the next day’s practicum. On April 29, 1889, a second daughter was born, who was named Johanna Wilhelmina after her grandfather Kaiser and went by the name Hannie or Han. Four years later, on May 6, 1893, a son arrived. He was called Gerrit, after the other grandfather. The little boy did not live long: on February 14, 1894 he died of one of the fatal illnesses that so many children succumbed to in those days. It was a difficult time for the young parents—and no doubt also for the children. As Berta remembered it, her father had the “serenity of mind” to cope with the loss, and he also found distraction in his work, but for Aletta the loss was much more difficult to bear. Withdrawal into her grief, which she tried to keep to herself as much as possible, turned her into an even more reserved personality than she already was. A year after Gerrit’s death, another son was born. He was named Rudolf, after Aletta’s brother. Rudolf was a sickly child and required much care, but he did survive. In this period, Aletta increasingly began to devote much effort to enabling her husband to immerse himself completely in his work without needing to worry about daily distractions. As was mentioned earlier, his teaching load in those days was very heavy, and owing to Aletta’s support he was able to continue his scientific work at a steady pace. Aletta made sure that the children did not disturb their father while he was at work. Although they loved to play at the bottom of the steps leading to his study, they were as quiet as possible when they played there. In spite of his hard work, Lorentz also spent time with his children. Berta described him as a devoted father—and a faithful husband—who took pleasure in taking walks in the woods with his children and teaching them the names of wildflowers. From him Berta also learned the basics of arithmetic.

Lorentz’s first public activities In the first decades after his appointment in Leiden, Lorentz’s public and social activities were mostly confined to giving lectures for a general audience. He did so 83   Fresnel was already mentioned in Chapter 1. Hertz was the man who had experimentally demonstrated the existence of electromagnetic waves for the first time. These waves had been predicted by Maxwell. According to Maxwell’s theory, light was also an electromagnetic wave phenomenon. As is explained further in Chapter 4, the insight that light is electromagnetic in nature plays a central role in Lorentz’s dissertation.


Professorship and family life

frequently and with great commitment. In the period 1881–1901, he gave no fewer than nine lectures for the renowned Haagsche Maatschappij voor Natuurkunde Diligentia (The Hague Society for Physics Diligentia), often in two consecutive sessions, about topics like electricity, light, and ether, or the world of molecules. During all of these lectures he also performed experiments, sometimes requiring many hours of preparation, for which he was praised elaborately in newspaper reports.84 From December 1880 onwards, he was also a member, and later the chairman, of the Municipal Oversight Commission for Secondary Education in Leiden. Another educational appointment was his membership of the board of the Genootschap der Beschouwende en Werkdaadige Wiskunde onder het motto Mathesis Scientiarum Genitrix (Society of Theoretical and Practical Mathematics Under the Motto Mathesis Scientiarum Genitrix), “Mathesis” for short. The goal of this society, which dated back to the eighteenth century, was to further national prosperity through lessons in mathematics. From its inception, the society administered an evening school that offered a technical education and taught some mathematics as well. In the nineteenth century this had become its main activity.85 Lorentz was chairman during several terms, and in this capacity, according to the extensive newspaper coverage, he played an important role in the yearly awarding of prizes to students who had performed exceptionally well. His involvement in Mathesis did not last long, though. Having joined the society’s board in 1882, he gave up his seat in 1888. He did not fulfill his duties as a board member wholeheartedly anyhow. When he discovered that he had been elected chairman during a meeting where he had been absent, he attempted to pass the job off to Van Bemmelen, claiming that he was too busy.86 Through all these activities, Lorentz took his first steps on the path toward greater social engagement. Yet it was not until after World War One that Lorentz would become more outgoing and more noticeably active in social and political issues.

84   On December 6 and 20, 1889, Lorentz lectured about “the electrical current as a phenomenon of motion.” For the first lecture he had been busy the evening before, from five o’clock to midnight, “positioning and fine-­tuning his instruments.” The second lecture required two days of preparation in his laboratory and then another several hours “to position the apparatus and put it in order” (Haagsche Courant, December 10, 1889). 85  See Peeperkorn 1985 for more about the history of Mathesis. 86   See Lorentz to Van Bemmelen, July 30, 1884 (RB). Lorentz’s attempt failed.

Chapter 3 Aletta, woman in her own right

In most biographies of men of science, the work of the subject and the chronology of his life take precedence. This biography is no exception. As a consequence, the activities of wives are often skimmed over. If a wife does not have an independent career or an important social position in her own right, her activities are generally given short shrift and she is limited to being described as the backbone of her husband’s and her family’s home life. On the face of things, Aletta’s activities fall in the same category and might not deserve much separate mention. Nevertheless, an entire chapter of this book is devoted to Aletta. She is placed here in the context of the position of Dutch women of her station in life in the late nineteenth and early twentieth centuries. Aletta had a sharp eye for her surroundings. In later chapters her keen and sometimes witty observations while attending official functions, or during the many travels with her husband, provide unique views behind the scenes, both in the Netherlands of her day and abroad. Her sharp assessments of others in her diaries and letters provide a personal perspective on life in the upper strata of Dutch society and especially on the place and the behavior of women.1 On a private note, her activities outside the home show how ambitious a person she was and how she was able to make the most of her limited possibilities as a professor’s wife in the rather staid and provincial town of Leiden. In spite of the social limitations placed on women of her status at the time, Aletta managed to make her mark as a progressive woman and a diplomatic but tenacious feminist, not only in Leiden, but in Dutch society at large. Her various involvements in the upper echelons of the women’s movement provide an insight into the obstacles women still needed to overcome and the changing fabric of Dutch society in the last decades of the nineteenth and the first decades of the twentieth century. Even sleepy Leiden was thrust forward—albeit only modestly so—on the wings of the general sense of social awakening that characterized the period.

1  Unless stated otherwise, the information on Aletta’s role in the Dutch women’s movement is derived from the groundbreaking paper Steen 2011, the first publication to highlight Aletta’s role as an early feminist.

“A Living Work of Art”: The Life and Science of Hendrik Antoon Lorentz. A. J. Kox and H. F. Schatz, Oxford University Press (2021). © A. J. Kox and H. F. Schatz. DOI: 10.1093/oso/9780198870500.003.0004


Aletta, woman in her own right

Professor’s wife Aletta’s position as a professor’s wife in small-­town Leiden was not much different from that of any other scientist’s wife in her day. She created the environment in which her husband was able to do his important scientific work and become the famous scientist he was. Aletta never did any paid work outside the home: Lorentz commanded a substantial salary, so the family hardly needed the extra cash and, besides, in her day and age paid work was not fitting, or even allowed, for married women of the upper classes. With regard to the outside world, her task as a professor’s wife consisted mainly of keeping the house presentable for visitors and being prepared to appear—well-­mannered, properly attired, but unobtrusive—by the side of her husband at formal occasions and functions, at home or abroad. As a good mother and a successful “wife of,” she was also instrumental in shielding her husband from demanding children or overly intrusive outsiders. As Adriaan Fokker phrased it: “She always faithfully stood by Lorentz’s side. Who will say how often she has shielded Lorentz from being duped by people who would have been able to abuse his good faith?”2

Charity and care for the needy Taking good care of her husband and children did not stop Aletta from being engaged in social issues. At first, she mostly showed her concern for the needy in the form of charitable work, the traditional realm of proper ladies. She offered practical assistance to individual indigent families wherever she could, for ex­ample by preparing daily bottles of milk for a poverty-­stricken family with newborn twins. Lorentz readily participated as well, making sure that the poor family’s oldest son could train to become a skilled bricklayer instead of having to find a backbreaking job in one of Leiden’s many filthy factories.3 Aletta did not limit herself to charity for individual families. She extended her range by joining an initiative by textile entrepreneur C. H. Krantz and his wife E. M. H. Krantz-­van Dijk to open a crèche for children between six weeks and three years old. The crèche was meant to accommodate children of working and sick mothers, among them workers from the Krantzes’ own factory. To this end a  society was established, the Vereeniging tot Verzorging van Kleine Kinderen (Society for the Care of Small Children). The crèche was to offer a safe and clean environment as well as a contribution to the children’s welfare by providing good nutrition, clothing, healthcare, and developmental stimulation.

2   “Zij heeft Lorentz altijd trouw ter zijde gestaan. Wie zal zeggen hoe vaak zij Lorentz ervoor behoed heeft, dupe te worden van mensen, die van zijn goed vertrouwen misbruik hadden kunnen maken?” Fokker 1946, 63. 3  See Haas-­Lorentz 1957, 45–46.

Working women


Aletta became the secretary of its board of governors. Money was raised through membership fees and donations and sometimes from other sources, such as bene­ fit performances. In addition, a very small fee was required from the parents themselves, in order to spare them the indignity of having to accept charity. On November 11, 1885, the crèche festively opened its doors in an airy building with a large garden at Lange Scheistraat 5. Aletta left the board in 1893. Her son Gerrit, her third child, was born and died in that same year, so it is not unlikely that her stepping down was connected to the birth or the illness of her son. In spite of the need to care for her own three small children—her fourth child, her sickly son Rudolf, was born in 1895—Aletta continued to show an interest in the welfare of children growing up in a less privileged environment. At the end of 1904, for example, she was instrumental in a new initiative, this time to open a playground accessible to children from schools for the poor of all denominations.

Working women Aletta’s social engagement did not end with charity work. During the latter half of the nineteenth century, the disadvantaged position of women in the Netherlands gradually began to chafe, especially among more educated women like Aletta. At the time, unmarried women had to leave all legally binding decisions to parents or brothers until they either reached the age of thirty, or widowhood. Married women were on a par with children and the mentally ill, in legal terms at least—a legal position which, for that matter, did not change until 1956. Decent unmarried women had few possibilities to earn their own keep. Work outside the home was limited to respectable environments like teaching, childcare, or dispensing medicine; hence many women chose to become pharmacists. Unless they were over the age of forty-­five, unmarried women who had a civil service job were dismissed on the day after they married. Presumably at forty-five they were beyond childbearing age.4 Higher education did not open to women until the last decades of the nineteenth century, and women were not allowed to vote until 1919. The first initiatives to break the taboo on “civilized” women working for a living were two General Women’s Societies: Arbeid Adelt (Work Ennobles) and Tesselschade,5 both established in 1871 to provide aid to women during the aftermath of the Franco-­German War of 1870 and still in existence today. Operating under the protection of the Dutch royal family—until the present day royal patrons actively support the activities by buying their craft products—they aspired to improve the lot of indigent women from the “civilized classes” by encouraging diligence and artistic sense and providing opportunities for respectable women to   Dismissal of pregnant women did not become illegal until 1970.   Tesselschade was named after the well-­known woman poet, singer, and glass engraver Maria Tesselschade Roemers Visscher (1594–1649), whose middle name memorized a shipping disaster near the island of Texel (or Tessel) in which her father lost forty-­four fully loaded cargo ships. 4 5


Aletta, woman in her own right

earn money in a socially acceptable manner. The societies did so by selling crafts like sewing, knitting, embroidery, or crochet work, made by poor craftswomen in their own homes. In spite of a conflict about retaining the anonymity of the contributing craftswomen—to spare them the public shame of needing to work for a living—the societies joined forces in organizing crafts training for women at the hitherto all-­male Amsterdamsche Rijksschool voor Kunstnijverheid (Amsterdam State School for Arts and Crafts), leading to the awarding of a diploma to the first three women students in 1883. Aletta enthusiastically participated in Tesselschade. Both she and her cousin and namesake Aletta Rebecca Maria Kaiser, daughter of the astronomer Frederik Kaiser, as well as Maria Cornelia Fabius, the wife of her cousin Pieter Jan, joined the five-­member board of Tesselschade Leiden. The chapter bought the crafts and sold them at fairs or sales parties in the homes of unmarried or widowed women. Advertisements in the newspapers promised a wide selection of products, ranging from artistic gifts and underwear for children and adults to special items for “maids and housekeepers.” Apart from organizing the sale of crafts, the Leiden chapter also encouraged and paid for the training of several young women to work as a nannies, nurses, biology teachers, directors of nursery schools, florists, or housekeepers. To provide jobs for these and other single women, Tesselschade Leiden even opened a job placement bureau as early as 1893.

Women’s suffrage In the 1880s, the call for women’s suffrage became ever louder, led, among others, by another namesake: Aletta Jacobs. She was the first woman to be admitted to a Dutch university—in 1872—and the first woman to attend medical school. Jacobs was an ardent advocate for women’s rights and not only established a practice catering specifically to poor women but also founded the first birth control clinic in the world. Since women were not explicitly barred from doing so by law, she pursued a position on the 1883 electoral roll for the Amsterdam City Council and took the cause all the way to the Supreme Court—to no avail. After the court’s negative decision, the law was changed to exclude women explicitly. Jacobs’s public appeal did lead to the foundation of several organizations to promote women’s suffrage and women’s equality. The first one, the nationwide Vereeniging Voor Vrouwenkiesrecht (Society for Women’s Suffrage), was established in 1894 as a non-­partisan organization, open to all those—men as well as women—who could pay the Society’s membership-fee. It aimed to achieve the women’s vote, as well as the general emancipation of women, by holding public meetings, offering lectures, publishing magazines and other writings, and using any other “legal means” to sway public opinion. From the beginning, Aletta was active in the Society, and in 1900 she joined the board of the newly established Leiden chapter as its treasurer. Aletta put a great deal of energy into increasing the Society’s membership among the Leiden

Women’s suffrage


University professors, making good use of her position as Lorentz’s wife. Success in this respect did not always come easily, as feminism was often viewed as “ultra” or extremist, sometimes bordering on socialism: hardly a recommendation in university circles that were, for the most part, quite conservative. One of Aletta’s letters to her friend and fellow board member Lizzy van Dorp6 shows how Aletta had to juggle the duties of a young mother and the demands of her suffragist activities and how some of the more conservative elements in Leiden looked askance at her feminist efforts and those of her—supportive—husband: I am nothing but women’s suffrage nowadays, o my dear child, what a job it is with all these little ones. But the chapter is flourishing. Even old man D’Aumerie has become a member. By and by we are pervading the entire university with our ideas. [. . .] It is fortunate that my husband heard that he is a feminist à outrance [extremist] and even against their will wants to give women the right to vote.7

In February 1907 an unresolved dispute on whether or not men should be allowed to sit on the board of governors gave rise to a schism, so a more moderate offshoot of the Society was born: the Nederlandsche Bond voor Vrouwenkiesrecht (Dutch Union for Women’s Suffrage). Aletta had played an active role in the dispute and did not shy away from once again using her husband’s position at the university to tap various professors and other acquaintances and drum up support for membership of the new organization. Both Aletta and Lorentz were among the ­sixty-­three signatories of the founding charter, as well as a considerable number of the professors and professors’ wives whom Aletta had approached. Later, Aletta even managed to recruit her own daughter, Berta, who had meanwhile started to study physics at the university.8 Somewhat surprisingly, Berta makes no mention of this in her biographical notes on Lorentz, nor does she refer to her mother’s other feminist activities, for that matter.9 Because of her academic education, Berta may have identified less with the feminist ideals of her mother than with the men’s world of physics that her father and her husband Wander de Haas inhabited. Besides, in the fifties, when Berta wrote down her notes, women’s rights were not much of an issue. Little had become of the feminist ideals of the twenties, and the ardor of the early twentieth-­century women’s movement had all  but disappeared. Apart from having won the right to vote and the right to

6   Lizzy van Dorp was the first woman to study law in Leiden (1897), the first woman to obtain a law degree (1901), and the second woman to obtain a law doctorate (1903). She was elected to the Dutch parliament in 1922. Just before World War Two, concerned about the rise of National Socialism, she left for her mother’s birthplace, the Dutch Indies, and died in a Japanese internment camp weeks after the Japanese capitulation. 7   “Ik ben tegenwoordig enkel vrouwenkiesrecht, o lieve kind, wat een werk heeft men met allerlei kleintjes. Maar de afdeling bloeit. Zelfs de oude heer D’Aumerie is lid geworden. Zoetjes aan doortrekken we met onze denkbeelden de heele universiteit. [...] Gelukkig dat mijn man hoorde dat hij een feminist à outrance is en zelfs de vrouwen tegen hun wil het kiesrecht wil geven.” Aletta to Lizzy van Dorp, May 1, 1902, quoted in Steen 2008, 189. Martinus d’Aumerie was a retired judge in Leiden. 8 9  See Steen 2011, 148.   Haas-­Lorentz 1957.


Aletta, woman in her own right

r­ epresent themselves legally, women in post-­war Dutch society had a position that had not changed fundamentally. Aletta clearly showed her ambition to play a leading role in the Union. In another letter to Lizzy van Dorp—meanwhile chairperson of the Union—recounting her efforts to recruit potential board members, Aletta is enthusiastically angling for a governing board position for herself. She points out her own drive and ardor for the women’s cause—as opposed to that of various others—as well as the availability of her husband to lend his support: If I check and see the various women here, I myself must confess that with my thinking I am the one that is most involved of any of them, without taking account of education or intellect, you know me enough to know what I am worth, if you believe that I can be of help in the committee, then I would want to be on it, especially because I always have my husband to back me up and discuss things.10

By displaying her strenuous efforts to find candidates for the governing board, she appears to further showcase her own suitability for the new Union’s board position: I immediately wanted to start on my hunt for profs, but they all lecture just at eleven, so I will put that off until this afternoon. [. . .] but if you think it is better that I look around some more, I will do my best, but perhaps there will also be a man available, e.g. A. Fokker,11 but I do not find him very clever.12

Spreading the word Soon Aletta did indeed become a member of the new Union’s central board. As vice president, she took on general responsibility for propaganda. Propaganda for the Union chiefly consisted of organizing public meetings, afternoon teas at members’ homes—including Aletta’s—and home visits to potential members throughout the country, especially in parts of the Netherlands that had not yet been exposed to the movement to promote women’s suffrage. Aletta became so heavily involved in the propaganda efforts to expand the Union’s activities and membership that she herself went out of her way to spread

10   “Als ik hier de vrouwen zo eens naga, dan moet ik zelf bekennen dat ik er nog het meest van allen inzit met mijn gedachten, zonder daarbij op ontwikkeling of intellect te letten, je kent me genoeg om te weten wat je aan mij hebt, vindt (sic) je dat ik iets kan helpen in het comitee dan wil ik er wel inkomen, vooral omdat ik steeds mijn man achter den rug heb om mee te overleggen.” Aletta to Lizzy van Dorp, February 5, 1907, quoted in Steen 2011, 142. 11   Adriaan Fokker, who was still a student of physics at the time. 12   “Ik wilde dadelijk op de proffenjacht gaan, maar zij hebben juist allen om elf uur college, dat stel ik dus tot vanmiddag uit [...] maar vind ge het beter dat ik nog eens verder rondkijk, ik zal mijn best doen, maar misschien zal er nog wel een man te vinden zijn, bijv. A. Fokker, maar ik vind hem niet erg snugger.” Ibid.

Spreading the word


the word and establish chapters in other parts of the country. She made home visits in various small towns around Leiden, like Bloemendaal, Boskoop, or Hillegom, to establish new local chapters—efforts that were generally successful. She did not succeed in drumming up support in her native city of Amsterdam, in spite of attempting to involve her husband’s Amsterdam friend and colleague Pieter Zeeman when he visited the Lorentz home for coffee. All this activity was meant to energize as many people as possible, introduce them to the Union’s ideas, and increase the reach of the women’s suffrage movement. Successfully so, it appears, as membership rose to no fewer than ninety-­three in the first year and more than doubled to 191 in the second. Debating clubs and educational courses, for example on economics, parliamentary history, or constitutional law, were also part of the Union’s membership drive, as well as the performance and publication of “suitable” plays. In its propaganda, the Union assiduously tried to project an image of moderation and avoid the impression of supporting a radical “man-­hating” struggle between the sexes. However, opinions differed on what was to be considered suitable, at times leading to controversy and fierce criticism. In an angry letter to Aletta after the Union’s Utrecht chapter had published a few new plays in which men were portrayed quite unfavorably, one member vented her opinion in no uncertain terms: “I am flabbergasted. How could the central board—you were still a member, weren’t you—have given its permission for these disgusting things.”13 Aletta sprang into action, writing immediately to the Utrecht chapter that she herself had not liked some of the spoofs on the role of men. She had found some of the content “in­deli­ cate” and “coarse” and proceeded to take full responsibility for the central board’s oversight in not barring them. After initial hesitation, possibly about having to combine the function with her duties as a mother and her work on the Union’s central governing board, Aletta eventually became the Leiden chairwoman as well. In spite of their local character, her activities in Leiden attracted substantial public attention, even nationally. She chaired the reception committee for the participants of the National Women’s Council’s eleventh annual meeting, which was held in Leiden in 1910.14 It was an international, three-­day event during which Aletta also delivered the apparently witty opening address to the large plenary meeting at the final festivities.15 A year later, Aletta made another highly visible public appearance at a meeting of the Leiden chapter. In a fiery address she elaborated the standpoint of the Union in relation to its counterpart, the Society. Emphatically she presented the 13   “Ik ben paf. Hoe heeft het H.B. [hoofdbestuur]—u zat er toch nog in—z’n toestemming tot die walgelijke dingen kunnen geven.” Quoted in Steen 2011, 201–205. 14   The National Women’s Council (Nationale Vrouwen Raad) was established on October 29, 1898 to improve the contacts between women’s organizations in the Netherlands and serve as a point of contact for the international women’s movement. Its successor, the Nederlandse Vrouwen Raad, is still in existence today. 15  See Leidsch Dagblad, April 7, 1910. After the formal address, the “real festive mood” did not arrive until after some “mischievous remarks” by Aletta.


Aletta, woman in her own right

Union as an addition to the Society, not as its competitor. Its objective, as she put it, was not only to achieve the vote for women, but also the development of women to become good citizens, since not all women were naturally suited for the right to vote. Incidentally, the title of the Union’s magazine, De Ploeger (The Plougher), also referred to this objective and was by no means accidental: It clearly indicated the need to “plough”—to educate women to be able to use their future citizens’ rights properly—before the “seed” of the vote could be sown. In her speech, Aletta drew particular attention to: the orderly nature of the Union, which focuses mainly on developing women for work in society and for insight in the meaning of the right to participate in the business of State and Municipality, convinced that by doing so, the conviction will take hold among the governing powers that suffrage should not be withheld on the basis of difference in sex.16

Clear language, even though the right to vote could not actually be exercised by women until 1919.

A social club for university women As a matter of course, most first-­year university students in Leiden joined the venerable Leidsch Studenten Corps as soon as they arrived. It supported a lively fraternity life in individual fraternities and at the club building in the city center, including serious drinking and yearly—often rather rough—hazing ceremonies. Being part of the Corps was de rigueur for students and, under considerable social pressure, the better part of the student population joined, albeit not always with great enthusiasm, as Lorentz’s experiences during his own hazing period show clearly. As the Corps did not admit women, the still small group of women ­students—only since 1871 had women been admitted to universities at all— wanted to have its own social club. At first, getting women students to socialize turned out to be difficult, as many of them were not allowed out at night. Often they were still living at home with their parents, as it was not considered decorous for a young woman to live on her own. Besides, it was hardly viewed as ladylike to spend evenings drinking at a social club. Proper women, especially those who had managed to enter university, were only supposed to engage in “useful” pastimes. To meet the need for a place for women students to socialize, a few of them started a “reading circle” in 1900, which eventually evolved into the full-­fledged women’s counterpart of the Corps, the Vereeniging voor Vrouwelijke Studenten te 16   “het ordelijk karakter van den Bond die het er vooral op toelegt de vrouwen te ontwikkelen voor het maatschappelijk werk en inzicht in de beteekenis van het recht om mee te spreken in zaken van Staat en gemeente, overtuigd, dat daardoor ook bij de regeerende macht de overtuiging zich vestigen zal dat het kiesrecht niet mag worden onthouden op grond van sekse-­verschil.” Leidsch Dagblad, April 7, 1911.

A social club for university women


Leiden (VVSL) (Society for Women Students at Leiden) that exists to this day. That Aletta and her husband immediately stepped up when it came to providing practical support for the initiative is clear. The new social club, the Leesgezelschap van Vrouwelijke Studenten (Reading Circle of Women Students) circulated a portfolio of literary and feminist periodicals, most of which were initially “provided willingly by Prof. Lorentz and Prof. Drucker who have, from the outset, shown a great interest in our club.”17 This, by the way, as well as various quotes from the correspondence between Aletta and her friend Lizzy van Dorp, shows that Lorentz was wholeheartedly supportive of his wife’s social and feminist activities. Lorentz’s was clearly committed to the movement for women’s suffrage, even though he stayed somewhat in the background at meetings where Aletta made her public appearances. Moreover, his support for the women’s cause was by no means only theoretical, not only in view of his early involvement in the “reading circle,” but especially judging by the unusually ample opportunities for advancement he offered to women students, including his own daughters. Both Berta and Hannie were admitted to study physics—though Hannie dropped out after a few years. Berta even obtained a doctorate under the guidance of her father. Reading could hardly be considered an activity to be reserved for women only, so several members wanted to introduce male students, while others wanted to preserve their “cozy” club life devoid of male interference. As Berta had meanwhile joined the reading circle, her mother Aletta got wind of the conflict and called on her old friend Lizzy, the reading circle’s first chairwoman, to scupper that plan: On behalf of some women students I am kindly asking you to come to the next club meeting, as there are problems with the drafting of the by-­laws [. . .] it has been proposed to allow the members the freedom to always introduce male students [. . .] it seems to me that will not serve to increase conviviality.18

Aletta, an intelligent woman with keen powers of observation, may very well have regretted not having been able to enjoy higher education herself, but she more than compensated for this by stimulating the younger generation of women,

17   “welwillend waren afge­staan door Prof. Lorentz en Prof. Drucker, die van den aanvang af een groote belangstelling in onze club hebben getoond.” Een wereldcorrespondentie van meisjes-­studenten (Rotterdam: Brusse, 1910), 5. Hendrik Lodewijk Ducker was Professor Emeritus of Law and a member of Dutch parliament. His illegitimate half-­sister Wilhelmina Drucker was a feminist and socialist politician who, like Aletta, was a member of the “Vereeniging Onderlinge Vrouwenbescherming.” She ­successfully sued her half-­brother for half of his banker father’s inheritance to become financially ­independent. 18   “Uit naam van eenige vrouwelijke studenten kom ik je vriendelijk verzoeken om op de eerstvolgende clubvergadering te komen want er zijn moeilijkheden bij het vaststellen van het reglement [...] men heeft voorgesteld om de leden vrijheid te geven om steeds heeren studenten te introduceren. [...] mij dunkt het zal ook niet strekken om de gezelligheid te vermeerderen.” Aletta to Lizzy van Dorp, May 1, 1902, quoted in Steen 2011, 150.


Aletta, woman in her own right

including her own two daughters, to do what she had been unable to do. In the same letter, she shows an avid interest in Lizzy’s progress with her dissertation in the faculty of law, writing: “I hear with pleasure that you are coming along nicely with your studies. I am burning with desire to read your dissertation.”19 In a later letter to Lizzy, who was meanwhile getting discouraged by her difficult position as an unmarried woman academic, Aletta, fourteen years her senior, counseled her to be an example for later generations, implying that young women now in university were a pioneering generation that had to make good on opportunities never offered to their mothers: [. . .] that I ask you as a mother, offer a good example, vanquish yourself and do not tire of the struggle, not only for your own mother, but also for all those girls that come after and unwittingly will tread the path of their predecessors. Your example will be of great influence and after years and years it will provide peace of mind for yourself if you vanquish now, but to do so requires power and good health. So, make sure that you obtain good help in your work. A healthy mind has more power if the body is more powerful. Think about the many who will suffer after you, those you can and must support and show the right way.20

Women’s rights Women’s suffrage, women’s right to paid work, and women’s legal and political position were not the only issues Aletta pursued actively. She was also keenly interested in more generally improving the social position of women. There was one group of women who suffered particular hardship: unmarried mothers. Children born out of wedlock were by no means exceptional in Leiden at the turn of the century, as the percentage of illegitimate children hovered at around ten percent at the time.21 Unwed pregnant women often found themselves in dire financial straits and were frequently forced into prostitution—brothels had been legalized in Leiden since 1853—to be able to provide for their children. Often, (married) presumptive fathers were unwilling to formally acknowledge paternity or legitimize illegitimate children, much less support them and their mothers financially. To help men avoid financial liability and allay fears of being forced by surprise paternity claims to acknowledge “moral missteps” and disgrace

19   “Ik hoor met genoegen dat je prettig opschiet met je studie. Ik brand van verlangen je proefschrift te lezen.” Ibid. 20   “[. . .] dat ik je als moeder vraag, geef een goed voorbeeld, overwin je en wordt het strijden niet moe, niet alleen voor je eigen moeder, maar ook voor al die meisjes die na komen en onbewust enigszins het pad van hun voorgangsters zullen gaan. Je voorbeeld zal van zoo grooten invloed zijn en na jaren en jaren zal het je zelf rust geven als je nu overwint, maar daarvoor heb je kracht en gezondheid nodig. Maak dus dat je goede hulp bij je werk krijgt. Een gezonde geest heeft meer kracht naarmate het lichaam krachtiger is. Denk om de velen die na je zullen lijden, hen kan en moet je helpen en een goeden weg wijzen.” Aletta to Lizzy van Dorp, December 18, 1907, quoted in Steen 2011, 151. 21  See Steen 2011, 154.

Winding down


“respectable” families, investigations into paternity were forbidden. This left women without the wherewithal to claim child support that was rightfully theirs. A number of initiatives to improve the lot of these women made an immediate connection between single motherhood, prostitution, and sin. Although charitable organizations provided food and shelter to “fallen women” who had become prey to prostitution, their first priority was to institutionalize them and save their souls under “Christian discipline and Evangelical leadership.”22 Liberal initiatives to abolish the legal protection of prostitution and the drive to promote birth­ control—which presumably led straight to prostitution—were severely frowned upon in these circles. In 1896, a widely circulated petition to the Queen-­Regent and the Minister of Justice called for abolition of the ban on paternity investigations. Inspired by the petition, members of the Society for Women’s Suffrage began a new charity in 1897 to care for “young mothers and their children,” as it was worded carefully: the Vereeniging Onderlinge Vrouwenbescherming (VOV) (Association for the Mutual Protection of Women). Clearly born out of dissatisfaction with the paternalistic approach of existing charitable institutions for unwed mothers, the VOV no longer viewed single mother­hood in terms of sin and redemption, but as a social ill. It went to bat against double standards and aimed to lend “moral and material support” to these young women in their “difficult struggle for survival.” In doing so, it paid particular attention to the precarious position of servant girls. Maids were particularly vulnerable, as they often lived with their employers—and their employers’ sons— who were known to sometimes take advantage. The VOV believed it was important to preserve the dignity of single mothers by providing them with individual financial support instead of patronizing them in charitable institutions. Leiden established a VOV chapter in 1902 with the same general objectives as the national VOV, and Aletta became its first chairwoman. Birth control was high on the agenda, as was the struggle for legal equality of legitimate and illegitimate children and the fight against double standards—exemplified by legalized prostitution. The most important issue, however, remained the abolition of the ban on paternity investigations. Eventually, in 1909, single mothers were at long last allowed to obtain a legal judgement to require a contribution by presumptive fathers to the costs of childbearing and childrearing. Yet, in practice, such judgements turned out to be difficult to enforce if the father remained unwilling to pay up.

Winding down By the end of 1911, Lorentz had decided to give up his chair in Leiden and move to the town of Haarlem to take up the position of Curator of the Physisch Kabinet   Steen 2011, 155.



Aletta, woman in her own right

of Teylers Foundation, a move that will be further elaborated in Chapter  7. A house was found in Haarlem and the Lorentz family moved in July of 1912. Now Aletta took on a more traditional woman’s role: She and daughter Hannie were in charge of planning and executing the move. Lorentz and son Rudolf went on a trip and only came back after everything had been nicely arranged in the new home. While Aletta was preparing for the impending move to Haarlem she wound down many of her activities in Leiden, and in 1911 she formally withdrew from the governing board of the Leiden Union for Women’s Suffrage. That did not prevent her from remaining active in the national women’s movement: In 1913, for instance, she attended a national feminist event and even treated participants of an international women’s conference to a tour of her new hometown: Ma went to Amsterdam for the annual meeting of the Union for Women’s Suffrage and then to some evening events on the occasion of the meeting in The Hague of the Board of the International Women’s Council. There, much could be heard that was interesting and stimulating, and it has always done Ma a great deal of good. Today she is receiving, together with a number of Haarlem ladies, some twenty ladies of the International Women’s Council, who are coming to see the city.23

In the same letter, Lorentz, an active participant in such feminist events, was quite complimentary of the women advocating for the cause of voting rights for women: I also went to The Hague one evening and I heard several woman speakers from the peace movement, some very good. The last one who spoke was Clasine Kluyver, who is on the board of “Vrede door Recht.” She did quite well. It was quite a job for her, for she had only been commissioned to speak the previous day and it had to be in  English. I also took part in the dinner of the Union for Women’s Suffrage in Amsterdam (together with your friend Van Reedt Dortland and some others of my own sex); it was quite pleasant.24

23   “Moe was naar Amsterdam voor de jaarvergadering van de Bond voor Vrouwenkiesrecht en toen naar enige avondmeetings bij gelegenheid van de vergadering in den Haag van het Bestuur van den Internationalen Vrouwenraad. Daar was veel interessants en opwekkends te hooren, en het is Moe altijd goed bekomen. Vandaag ontvangt zij met eenige Haarlemsche dames een twintigtal dames van den I. Vr. R. die de stad komen zien.” Lorentz to Berta, May 29, 1913 (FC). 24   “Ik ben ook een avond naar den Haag gegaan en heb daar verschillende spreeksters van de vredebeweging gehoord, sommige heel mooi. De laatste die sprak was Clasine Kluyver, die in het bestuur van Vr. d. Recht is. Zij heeft het er wel goed afgebracht. Het was een heele toer voor haar, want men had het haar pas den vorigen dag opgedragen en het moest in het Engelsch. Ook heb ik deelgenomen aan het diner van den B.  voor Vr. K.  te Amsterdam (met je vriend van Reedt Dortland en eenige anderen van mijn kunne); het was wel aardig.” Ibid. Clasina Kluyver, daughter of a Leiden mathematics professor, was active in the Dutch peace movement and later became a member of the Dutch dele­ ga­tion at the League of Nations. See Chapter 9 for more on the organization Vrede door Recht (Peace through Justice). Pieter Hendrik van Reedt Dortland was librarian at Leiden University and the husband of physics teacher Annie Sillevis, who was active in the women’s movement at the time.

Winding down


The move to Haarlem may well have been an important motive for Aletta’s gradual withdrawal from her feminist activities in Leiden, but another influencing factor may have been a sentiment that was felt more generally by many of the women of the first hour. Despite their tireless efforts in the years before World War One, the movement had made little concrete progress in achieving its goals, and initial enthusiasm for pursuing women’s suffrage appeared to be waning. After the outbreak of the war, many women in the country focused their efforts elsewhere, on issues like refugee relief or the mounting difficulties with the food supply, and felt less inclined to spend their time on women’s rights issues. Still, Aletta remained a convinced and often vocal advocate for women’s rights. In spite of her, sometimes lengthy, travels with her husband as he became more and more famous at home and abroad, she continued to take an interest in changing the social and political position of Dutch women. Judging by what he wrote to his daughter in the letter quoted earlier, Lorentz, on his part, continued to be among the few men who were openly supportive of the feminist movement.

Chapter 4 Early work and the Theory of Electrons

It is impossible within the framework of this biography to do justice to all of Lorentz’s scientific work, and, besides, doing so would not necessarily serve the reader well. His bibliography counts over 350 entries. Even when popular articles and lectures, textbooks, reprints, and translations are omitted, 150 scientific contributions still remain, of which as many as fifty date from the first twenty-­five years of his career.1 Not only is the sheer quantity of his publications impressive, but also the diversity of their subjects. They span practically the entire field of physics: classical mechanics, electrodynamics, optics, the theories of gases, liquids and solid matter, special and general relativity theory, radiation theory, and quantum theory. Inevitably, a selection must be made here. Of all Lorentz’s scientific achievements, his theory of electromagnetic phenomena, later named the electron theory, is without a doubt the most important one. His contemporaries were also convinced of its importance, considering that he was awarded the Nobel Prize for this work.2 In 1953, in a short discussion of Lorentz’s work and his personality written for an exhibition in Leiden on the occasion of the one hundredth anniversary of Lorentz’s birth, Albert Einstein specifically emphasized Lorentz’s work on electromagnetism and remarked that younger physicists are seldom aware of how decisive a role Lorentz played in the development of contemporary theoretical physics. The reason is, in Einstein’s striking formulation, that Lorentz’s fundamental ideas have so passed into their flesh and blood that they are barely capable of becoming fully aware of the boldness of these ideas and the simplification they have brought about in the foundations of physics.3

  See the bibliography of Lorentz’s work in Kox 2008, 709–753.   See Chapter 6 for more about the background of Lorentz being awarded this prize. 3   “dass die Lorentz’schen grundsätzlichen Ideen ihnen so in Fleisch und Blut übergegangen sind, dass sie kaum noch imstande sind sich der Kühnheit dieser Ideen und der durch sie erzeugten Vereinfachung des physikalischen Fundamentes voll bewusst zu werden.” Einstein 1953, 3. 1 2

“A Living Work of Art”: The Life and Science of Hendrik Antoon Lorentz. A. J. Kox and H. F. Schatz, Oxford University Press (2021). © A. J. Kox and H. F. Schatz. DOI: 10.1093/oso/9780198870500.003.0005

Physics in the early nineteenth century


This is a striking observation, for few of today’s physicists are fully aware that the form in which the theory of electromagnetism is presented in textbooks actually goes back directly to Lorentz’s work. In fact, there is another reason why the ­theory of electrons has been forgotten to such an extent, in addition to the one Einstein mentioned in Leiden. Ironically enough, it is Einstein himself who played an important role here. In the opinion of many physicists, even today, Einstein’s 1905 special theory of relativity made the electron theory obsolete, and so, in one fell swoop, Lorentz’s ideas became history. This chapter is intended to show that this view of Lorentz’s role requires, to say the least, a somewhat more qualified approach.

Physics in the early nineteenth century To gain a full understanding of the electron theory, it is necessary to return to the beginning of the nineteenth century.4 The development of physics in that period is characterized by—among other things—a great increase in the use of math­em­ at­ics, so that parts of physics that could initially be discussed only in qualitative terms were made accessible for strictly quantitative observations. One of these areas was the field of electricity and magnetism. Another typical ­nineteenth-­century development is also apparent here: the pursuit of unification, of connecting seemingly totally different phenomena.5 In the beginning of the ­nineteenth century, electricity and magnetism were still viewed as two separate phenomena, but this gradually changed over time. First, the Danish physicist Hans Christian Oersted discovered, in 1819, that an electrical wire and a magnet appear to have the same effect on a compass needle. Both wire and magnet can influence the position of the needle.6 This induced the Frenchman André-­Marie Ampère to carry out a series of follow-­up experiments in which he discovered, among other things, that two parallel electrical wires exerted a force on one another.7 This discovery led Ampère to the hypothesis that all magnetic phenomena could be understood as the effects of electrical currents. He imagined that in permanent magnets, ­internally, there are a great number of microscopic currents that run in closed circular circuits (“circular currents”) and cause the magnetic force.

4   The development of nineteenth-­century ideas about electricity and magnetism can, of course, only be described here in broad strokes. See Harman 1982 and Purrington 1997 for more detailed overviews. More technical discussions of the development of nineteenth-­century electromagnetic theories can be found in Whittaker 1951–1953, vol. 1, and Darrigol 2000. 5   Especially under the influence of the prominent Naturphilosophie movement in Germany at the beginning of the nineteenth century, scientists were searching for unity in the diversity of natural ­phenomena. 6   The year 1819 is generally viewed as the year in which electromagnetism was discovered. 7   A repulsive force if the current in the wires have the same direction and an attractive force in the other case.


Early work and the Theory of Electrons

Faraday and Maxwell The thought that electricity and magnetism are two aspects of the same phenomenon gained further acceptance through the work of the Englishman Michael Faraday. He discovered the phenomenon of induction: In a wire moved in the vicinity of a magnet an electrical current appears to be generated spontaneously. The experimentally gifted but mathematically poorly educated Faraday systematically investigated this phenomenon and other, related ones in a number of experiments and eventually he conceived of the idea of electrical force. In Faraday’s idea of the mechanism responsible for the transfer of electrical and magnetic forces, the concept of the “tension” plays a prominent role. Faraday assumed, for example, that when two charged particles exerted a force on one another, the electrical charge caused what he called a “state of tension” between them. Presumably, this state was then propagated to reach the second particle in one way or another. The second particle would then experience the state of tension as a force originating from the first particle. The phenomenon of induction could be described in a similar fashion. By introducing the notion of state of tension, Faraday provided the first basis for the modern concept of an electric field. In his view, an electrical charge at any point in space is connected with a quantity that indicates how great a force is experienced by another charge present at that particular point. As was mentioned already, Faraday was poorly educated in mathematics, so his work is mainly qualitative. That did not apply to the Scotsman James Clerk Maxwell, who further expanded the work of Faraday and provided a strictly mathematical formulation for it. Maxwell managed to draw up a completely unified theory of electromagnetism in which magnetism and electricity are treated on an equal footing. The “Maxwell equations,” the four equations that form the core of his theory, treat electric and magnetic fields in a correlated manner.8 In Maxwell’s work the concept of the ether plays an important role. In nineteenth-­century physics in general, the ether was an important ingredient. It was the substance that filled the totality of space and it served, for example, as the carrier of light waves.9 The properties of the ether were the subject of a great deal of debate. Were its properties comparable to those of ordinary matter or were they completely different? Were there different kinds of ether? Was the light ether, the medium through which light was propagated, a different ether than the ether associated with electrical and mechanical phenomena? One ether model after another was proposed without any agreement being reached. Yet the idea that such a thing as ether existed was not at all in doubt.

8   Only the empirical phenomenon that no independent magnetic charges exist, causes a certain degree of asymmetry. 9   It was thought that light waves were carried by the ether in the same way as water is needed to carry water waves and air is necessary for the propagation of sound waves.

Faraday and Maxwell


One of the great successes of Maxwell’s theory was its prediction of the existence of electromagnetic waves. Since the propagation speed of these waves turned out to be equal to the speed of light, Maxwell drew the conclusion that light was nothing but an electromagnetic wave phenomenon in an electromagnetic ether.10 This constituted a further step on the road to unification. Light, which had initially been viewed as an oscillation phenomenon in the light ether, had now been included in the theory of electromagnetism, and eventually the light ether and the electromagnetic ether could be identified as one and the same. It is worthwhile to pause here and consider the question of whether Maxwell and his predecessors and contemporaries viewed this theory of electromagnetism as something fundamentally new, or rather as a theory that would fit in with existing notions. The answer is that the theory would explicitly fit in with existing notions. Maxwell tried to understand his equations by drawing up mechanical analogies: mechanical models described by the same equations. It is not always clear whether he believed that the mechanical models represented reality, but he was certainly convinced that these kinds of models were necessary for a true understanding of the theory. Maxwell, and many others with him, believed that the laws of mechanics constituted the basic laws of physics and that new phenomena should be understood in mechanical terms. Of course, this raises the question of why exactly mechanics was assigned this role of basic science. The answer is that the notions of matter and motion on which mechanics was based were conceptually simple, and this made it an appealing candidate. Besides, it was the most developed branch of physics. Newton had given mechanics a new, mathematical foundation at the end of the  seventeenth century. After Newton, the theories of mechanics had been further developed and had become even more mathematical during the eighteenth century, chiefly through the work by French scholars like Legendre, Maupertuis, D’Alembert, and Laplace. So, in this respect, mechanics was already much more advanced than any other area of physics. This is important for what follows. Maxwell’s theory is characterized by a combination of complicated and advanced mathematical treatment and a lack of conceptual clarity. How he ­understood fundamental concepts like charge and current is obscure. In 1923, the physicist Paul Ehrenfest expressed vividly how Lorentz’s generation struggled with the work of Maxwell: If one has a chance to ask one of those men of the older generation how they felt at the time—around 1873—about the writings of Maxwell, there appears something in their eyes like the glint of the love of their youth, but at the same time they betray to

10  In the eighties of the nineteenth century, the German physicist Heinrich Hertz succeeded in ­actually producing electromagnetic waves and studying their properties.


Early work and the Theory of Electrons us that especially Maxwell’s “Treatise” (published in 1873) was a kind of intellectual jungle, almost impenetrable in its uncultivated fertility.11

Because Maxwell’s work was so inaccessible to many of his contemporaries, they resorted to reformulations in different, more understandable terms. The German Hermann von Helmholtz had, for example, formulated a theory in which Maxwell’s theory constituted a special case. Lorentz had also relied on Helmholtz at first, and it was not until years later, in  the nineties of the nineteenth century, that he based his work directly on Maxwell. By way of justification, Lorentz remarked, in an address given in the spring of 1891, that “it is not always easy to fathom Maxwell’s thought processes.”12 Much later he gave as the reason for this: “one feels a want of unity in his book, due to the fact that it faithfully reproduces his gradual transition from old to new ideas.”13 It is characteristic of this phase of the development of electromagnetism that the theory was primarily macroscopic in nature. It had no clear underlying concept of  the structure of matter, and there was no mention of a possible connection between the microscopic world of atoms and molecules, on the one hand, and the phenomena observed in our macroscopic world, on the other. As yet, Maxwell saw no need for an atomistic foundation of electromagnetic theory, even though he showed himself a dedicated atomist in his work on kinetic gas theory.

Lorentz’s dissertation Lorentz’ work embodied a radical change in the view of the structure of matter. Starting with his dissertation of 1875,14 Lorentz undertook a research program that would keep him occupied for many years. He focused on Maxwell’s and Faraday’s electromagnetism and attempted to base it on a systematic foundation. Two concepts were central to his efforts, and both of these already appeared in his dissertation.

11   “Indien men bij gelegenheid aan een van de mannen dier oudere generatie vraagt, hoe het hun destijds—omstreeks 1873—bij de geschriften van Maxwell te moede was, dan komt er in hun oogen zoo iets als de weerglans van een jeugdliefde, maar tezelfdertijd laten zij ons merken, dat vooral Maxwells “Treatise” (in 1873 verschenen) een soort intellektueel oerwoud was, bijna ondoordringbaar in zijn onontgonnen vruchtbaarheid.” Nieuwe Rotterdamsche Courant, July 17, 1923. Ehrenfest referred to Maxwell’s A Treatise on Electricity and Magnetism. The article was written on occasion of Lorentz’s seventieth birthday on July 18. 12   “het niet altijd gemakkelijk is, zich in Maxwells gedachtegang te verplaatsen.” Lorentz 1891b. 13   “men voelt een gebrek aan eenheid in zijn boek, vanwege het feit dat het trouw zijn geleidelijke overgang reproduceert van oude naar nieuwe ideeën.” Lorentz  1923f. The book to which Lorentz refers is A Treatise on Electricity and Magnetism. 14   Lorentz 1875b.

Lorentz’s dissertation


The first was the idea that the world is made up of small particles—atoms— some of which carry an electrical charge.15 The second was that the ether, the medium in which electromagnetic phenomena occur, should be viewed as distinct from matter. Matter and ether were different entities, according to Lorentz. They coexisted and were independent of one another, obeying different natural laws. This was an important conceptual clarification of Maxwell’s ideas. An electrical current, for example, could now simply be viewed as a current of material, charged particles. The mechanism for electrical or magnetic action could now also be ­conceptualized differently: Matter is the source of the electromagnetic action, and the ether is its carrier. In Lorentz’s view, charged particles caused the ether to be  brought in a state of tension which propagated and, when it reached other particles, expressed itself as the action of force. In his dissertation, Lorentz aimed to apply Maxwell’s theory to some phenomena connected to the reflection and refraction of light that were as yet unexplained. He took the standpoint, following Maxwell, that light is an electromagnetic wave phenomenon. Yet, for the treatment of the phenomena he was studying, Lorentz resorted to the reformulation of Maxwell’s theory by Helmholtz, as mentioned earlier. However, there was something strange about Helmholtz’s reformulation. Completely in accordance with Faraday, Maxwell assumed that electromagnetic action was propagated from place to place, mediated by what he called fields of force. In this respect, Maxwell diverged from the way of thinking that was customary on the European continent, which presupposed that electromagnetic action without mediation of an ether works instantaneously—in other words, without any loss of time. This principle of action at a distance played a central role in thinking about electromagnetic phenomena at the time. Helmholtz also adhered to this principle and, curiously enough, in his reformulation of Maxwell’s theory he contrived to reintroduce the notion of action at a distance. Of course, this treatment did not at all do justice to Maxwell’s ideas, and it is completely justified that Helmholtz’s reformulation was later judged to be unconvincing: Not only was this kind of approach completely alien to the spirit [of Maxwell’s ­theory], it also tended to obscure its characteristic properties.16

Yet Lorentz justified his choice for the reformulation by Helmholtz in his dissertation by writing: In deriving the equations of motion for electricity I will follow Helmholtz for the most part. Similar to this physicist, I will base myself here on the immediate action at

15  See also Chapter  2 for a discussion of Lorentz’s atomistic ideas and the place of atomism in nineteenth-­century physics. 16   Rosenfeld 1956.


Early work and the Theory of Electrons a distance; thus, we do have the advantage that the theory is based on the most direct conception of the facts.17

This latter sentence in particular may seem somewhat curious to the modern reader, as since then action at a distance has become quite an unnatural concept. Lorentz’s justification of his chosen method did not only show the extent to which action at a distance had its natural place in physics in Lorentz’s day, it also showed how revolutionary Maxwell’s diverging ideas really were, also in this regard.18 Still, action at a distance was not the be-­all and end-­all for Lorentz. It was a choice that showed a certain degree of pragmatism, as can be seen in the continuation of the earlier quote: However, I do not want to view this actio in distans as an unwavering dogma, as some others have done; rather, the obtained differential equations, not the actions, are the actual basis of the theory. I believe it is not unlikely, once these equations have been drawn up in their most likely form, and once they have been checked against ex­peri­ ments in a large number of cases, that one should also be able to derive them successfully from the examination of molecular forces.19

This quote is important, in the first place because it illustrates so clearly Lorentz’s pragmatic attitude—an attitude, for that matter, that can be observed often throughout his career. Another reason for the quote’s importance is expressed in the last sentence. It outlines a research program to which Lorentz would devote the better part of his life’s work: the derivation of phenomena like elec­tro­mag­net­ ism from assumptions about the microscopic structure of matter, its constituting particles, and the forces between them. In this sense, the quote can be viewed as the key to a large part of Lorentz’s oeuvre. At the end of his dissertation, having successfully managed to describe the refraction and reflection of light, Lorentz listed a number of phenomena that were 17   “Bij de afleiding van de bewegingsvergelijkingen der elektriciteit zal ik grootendeels Helmholtz volgen. Evenals deze natuurkundige zal ik daarbij uitgaan van de onmiddellijke werking op een afstand; aldus toch hebben wij het voordeel, dat aan de theorie de meest rechtstreekse opvatting der feiten ten grondslag ligt.” Lorentz 1875b, 28. 18   In an unpublished overview of his own work (LA 264) Lorentz offers as additional motivation for his choice for Helmholtz that “the work of Helmholtz had become much clearer to me than the book by Maxwell.” (“Ce que me portait à cette manière de voir, c’est que le travail de Helmholtz m’était devenu beaucoup plus clair que le livre de Maxwell.”) The (unfinished) manuscript is written in French and contains 121 pages. According to a note by Aletta on the cover, it is a “draft of an overview of his own work sent to Prof. Mittag-­Leffler 1902.” Gösta Mittag-­Leffler was Professor of Mathematics at the University of Stockholm. See Chapter  6 for more about the document and the reason why Mittag-­Leffler had requested it. 19   “Ik wil daarmeê echter niet, zooals men dit soms gedaan heeft, die actio in distans als een onwankelbaar dogma beschouwen; veeleer zijn de verkregen differentiaalvergelijkingen, niet de werking, het eigenlijke uitgangspunt der theorie. Ik acht het niet onwaarschijnlijk, dat men, wanneer eens die verge­ lijkingen in haar meest waarschijnlijken vorm zijn opgesteld en in een groot aantal gevallen aan de ervaring zijn getoetst, er in zal kunnen slagen, ze ook uit de beschouwing van moleculaire krachten af te leiden.” Lorentz 1875b, 28.

Lorentz’s research program


as yet unexplained. Among them were dispersion, emission and absorption of light, and heat radiation. In connection with these phenomena he speculated, in  view of Maxwell’s successful hypothesis, that light consisted of electrical ­oscillations and that perhaps this kind of oscillation also occurred within atoms. In the final paragraph of his dissertation Lorentz allowed the reader a glimpse of the future: Far from having taken on a definitive form, the theory of Maxwell requires clarification of many more questions for which an explanation cannot be given as yet, or only in broad strokes. But precisely the usefulness of any expansion of our knowledge of  nature partly lies therein, that it brings to the fore more clearly that which still remains to be done and the direction in which one might successfully move in ­further investigations.20

Not long after Lorentz defended his doctoral dissertation, Maxwell obtained a copy and studied it.21 Unfortunately, in the three years Maxwell had left to live, the two scholars never had a chance to communicate with one another.

Lorentz’s research program This statement at the end of Lorentz’s dissertation was, in fact, the program for the research that Lorentz felt he still needed to accomplish. In the years that followed, most of the topics he mentioned here did indeed become the subjects of his publications. One of the most important ones was an extensive and very ambitious article that appeared as early as 1878.22 It was devoted to the propagation of light in transparent media like glass. The article, which can be characterized with good reason as a mathematical tour de force, began with some very general assumptions on how particles of matter undergo the influence of electromagnetic waves. Already, this led to important results about the relationship between the propagation speed of light and the properties of matter. But Lorentz wanted more. He went a step further and introduced a specific hypothesis about what happens inside atoms. This hypothesis would become very fruitful in his later work as well, and together with the concept of atomism and the distinction between ether and matter it was to become characteristic for his

20   “Wel verre dus van een definitieven vorm te hebben aangenomen, eischt de theorie van Maxwell nog opheldering van vele zaken, waarvan thans de verklaring niet, of slechts in ruwe trekken te geven is. Maar juist hierin is voor een deel het nut van elke uitbreiding onzer natuurkennis gelegen, dat zij ons duidelijker voor oogen stelt, wat er nog te doen overblijft, en de richting aangeeft, waarin men zich met goed gevolg bij verder onderzoek zal kunnen bewegen.” Lorentz 1875b, 177. 21   See Maxwell to Peter Guthrie Tait, February 14, 1876, and Maxwell to George Gabriel Stokes, July 7, 1876 (Harman 2002, 586 and 612). 22   Lorentz 1879a.


Early work and the Theory of Electrons

approach. Under discussion here was what Lorentz himself later called the “hypothesis of a single oscillating particle.”23 This hypothesis came down to the idea that, inside atoms, there are still other charged particles.24 Lorentz later called them “ions,” in analogy to the electrically charged particles known in chemistry. The central assumption here was that one of these particles could begin to oscillate;25 for example, when a light wave would hit the atom. On account of it being in oscillation—in other words, in accelerated motion—the ion would also begin to emit light. One of the basic principles of classical electrodynamics is, after all, the fact that accelerated charges emit radiation. By using this mechanism to explain the interaction between light and matter, Lorentz was also able to provide a good description of how light propagates through—transparent—matter.26 One of the results presented in his article was the derivation of a relationship between the refraction index of a substance and its density. This relationship was later dubbed the Lorentz–Lorenz formula, named additionally after the Danish physicist Ludvig Lorenz, who had also derived the formula, albeit on the basis of different considerations.

The Zeeman effect and the electron Another extremely important success of the oscillating ion hypothesis was Lorentz’s explanation, not much later, of the phenomenon that would become known as the Zeeman effect. He was awarded the 1902 Nobel Prize for this work, together with the experimental physicist Pieter Zeeman, his former assistant.27 Zeeman was the man who discovered the effect, in the Leiden laboratory in the autumn of 1896. He had become interested in an intriguing problem: the possible influence of magnetic forces on the position of the spectral lines emitted by various substances. What this means can be described as follows. When heated sufficiently, every substance emits light of a very specific composition. That is to say that the light emitted by each substance consists of a unique combination of various colors. When the light is analyzed through a prism, or another instrument that has the same effect, a spectrum is observed that consists of colored lines and dark bands. When, for example, sodium is heated, it emits a yellow light that breaks up into two lines. The yellow color can easily be observed by sprinkling some kitchen salt

  Lorentz uses this term in the overview of his own work that was mentioned in footnote 18.   Because atoms are electrically neutral, they must contain equal numbers of positively and negatively charged particles. 25   More precisely: The ion performs an oscillation around an equilibrium position, in step with the periodic electric force exerted by the incoming light wave. 26   In particular, Lorentz was able to provide an explanation for the phenomenon of “dispersion,” the property that the refraction index of a substance depends on the frequency of light. 27  See Kox 1997 and Kox and Troelstra 1998 for the history of the discovery and its aftermath based on previously unknown archival material. 23 24

The Zeeman effect and the electron


in a gas flame, or by simply letting one’s potatoes boil over and watching the salted water pour into the gas flame. Zeeman speculated that the color of the emitted light—in other words, the pos­ ition of the spectral lines—could be influenced by a magnetic field. For this ex­peri­ ment, he was inspired by the knowledge that, more than thirty years earlier, Michael Faraday, whom he much admired, had already been searching—in vain— for such an influence. In the months of September and October of 1896, Zeeman carried out a series of experiments showing definitively that the influence he was looking for does indeed exist. The effect, which would later be named the Zeeman effect, comes down to a splitting of the individual spectral lines of a substance into two or three lines under the influence of a magnetic field. Three years later Lorentz vividly described the discovery: In Prof. Kamerlingh Onnes’ laboratory, this jewel of our University, not because of its outer form, but because of its inner organization, we find, now roughly three years ago, Dr. Zeeman at work with a sodium flame between the poles of a strong electromagnet. From the yellow light of the flame a spectrum is cast [. . .] Zeeman tried, armed with the modern tools, to track down a phenomenon which, it seems, Faraday had already been searching for, without success. Would the forces emanating from the poles of the magnet, whose effect on some light phenomena was known, not also be able to cause some change in the radiation of the light? Zeeman found a slight widening of the spectral lines and careful observation and control experiments taught him that indeed these had to be attributed to a direct influence of the ­magnetic forces.28

On Zeeman’s behalf, his boss Heike Kamerlingh Onnes, the director of the physics laboratory of Leiden University, presented the discovery during a meeting of the Section of Sciences of the Royal Academy of Sciences on Saturday October 31, 1896.29 As early as the following Monday, Lorentz asked Zeeman to come and see him and presented to him a complete theoretical explanation for the observed phenomenon.30 Essential to this explanation was the existence of the oscillating ion, and Zeeman’s discovery provided powerful support for the presence, within atoms, of these charged particles. The only strange thing was that the mass of the

28   “In Prof. Kamerlingh Onnes’ laboratorium, dat sieraad onzer Universiteit, niet door den uiterlijken vorm, maar door de inwendige organisatie, vinden wij, nu ruim 3 jaren geleden, Dr. Zeeman aan het werk met eene natriumvlam tusschen de polen van een sterken electromagneet. Van het gele licht der vlam wordt een spectrum ontworpen [. . .] Zeeman beproefde, gewapend met de moderne hulpmiddelen, een verschijnsel op het spoor te komen, waarnaar, zooals het schijnt, Faraday reeds, doch zonder te slagen gezocht had. Zouden niet de van de magneetpolen uitgaande krachten, waarvan de werking op sommige lichtverschijnselen bekend was, ook in de uitstraling van het licht eenige verande­ ring kunnen brengen? Zeeman vond eene geringe verbreeding der spectraallijnen, en nauwgezette overweging en controleproeven leerden hem, dat deze inderdaad aan een rechtstreeksen invloed der magnetische krachten moest worden toegeschreven.” Lorentz 1900a. 29  See Zeeman 1896. Ironically, Zeeman had started his experiments while Onnes was away on vac­ ation, because he knew that Onnes would not have given him permission to work on them. 30  See Zeeman 1928, 111.


Early work and the Theory of Electrons

light ions turned out to be much smaller than the mass of the chemical ions that were known so far.31 Initially, Lorentz called this “a bad thing,”32 but soon he realized that the Zeeman effect could be observed precisely because of this very small mass. This gave rise to the assumption that the light ion might be an altogether new kind of particle. A greater mass would cause the splitting of spectral lines to be invisible. That the mass of the light ion turned out to be so small gave rise to the assumption that a new kind of particle might have been observed. This assumption was proven correct when, less than a year after Zeeman’s discovery, the German Emil Wiechert and the Englishman Joseph John Thomson discovered the same charged particles outside atoms, in the so-­called cathode rays. They were given the name electron, and it is justified, in a certain sense, to say that Zeeman and Lorentz participated in the discovery of the electron. Because of Lorentz’s theoretical explanation on the basis of a microscopic atomistic model, the Zeeman effect became the method par excellence to gain insight into the structure of atoms. For this reason, the importance for modern physics of Zeeman’s discovery can hardly be underestimated. Now to return to Lorentz. In a series of publications after 1878 he further elaborated his theory. This turned out to be possible without the need to make specific assumptions about the nature of the ether. Initially, Lorentz still treated the ether as a mechanical system and tried to derive the Maxwell equations from the mechanical properties of the ether, but later on he abandoned this idea and simply took the Maxwell equations as his starting point. In an article published in 1892, Lorentz went even further and presented a formula for the force exerted on a charged particle in an electromagnetic field.33 It is important to note here that this force did not directly follow from the Maxwell equations, but was added separately. Even now, this force, which was later named the Lorentz force, is still a central and indispensable part of electromagnetic theory.

The role of the ether Over time, the ether slowly began to lose its mechanical properties for Lorentz, until only one property remained: its immobility. This means that the ether as a whole is completely at rest, or, expressed more precisely, that the different parts of the ether are at rest with respect to one another.

31   Strictly speaking, this concerned e/m, the ratio of charge and mass, which was some 1,000 times greater for light ions than for the lightest chemical (electrolytic) ion, hydrogen. Because it was unlikely that the light ion had a very large charge, the difference was ascribed to the difference in mass. It is interesting, actually, and also somewhat surprising, that neither Zeeman nor Lorentz drew much attention to this great difference in their published writings. There is only one letter by Zeeman, to his English colleague Oliver Lodge, in which he agrees that the value of e/m is large and suggests that light ions probably differ from electrolytic ions. 32 33   “kwaad ding.” Zeeman 1928, 111.  See Lorentz 1892f.

Michelson and Morley


Given the idea of an ether at rest, combined with the insight that it would be highly unlikely for our earth to be at rest with respect to the ether, an interesting problem presented itself: the problem of describing phenomena in bodies that are in motion in relation to the ether. Why should this be considered a problem? For the simple reason that Maxwell’s theory and the Maxwell equations are valid in systems that are at rest with respect to the ether. Since it would have been very unlikely that the earth is at rest in relation to the ether, Maxwell’s theory had to be expanded to include systems in motion if it was to describe terrestrial phenomena correctly.34 Lorentz was very much aware of this necessity. This is shown in his publications, but also in the overview of his own work that was mentioned earlier. He writes there: One of the reasons why I have made a distinction between the role of the ether and that of ponderable matter in light phenomena was the hope of thus being able to approach the problem of the propagation of light in a moving ponderable body.35

One of the most notable consequences of the Maxwell equations being valid only in the ether system was that the value measured for the velocity of light would have to depend on the velocity of the system in question with respect to the ether. After all, the velocity of light was conceived as its velocity through the ether. An observer moving in relation to the ether would, therefore, measure a different value for the speed of light than an observer at rest with regard to the ether. In this way, it should be possible to determine at what speed an object like the earth was moving through the ether by measuring the speed of light. The situation could be compared to a passenger in a moving train, walking through the train at a speed, for example, of five kilometers per hour. With respect to his fellow passengers, he is indeed moving at the indicated speed, but for someone standing outside by the train track, his speed is much greater, as the passenger’s own velocity of five kilometers per hour must be augmented by the speed of the train.

Michelson and Morley Determining whether it was indeed possible to find the speed with which the earth moved through the ether by measuring the speed of light was an experimental

34   A remark is in order here. Although, strictly speaking, Maxwell’s theory should not simply be applied to terrestrial phenomena, in practice this is permissible in most cases because the corrections that need to be made are extremely small. At most, they are in the same order of magnitude as the ratio between the velocity of the earth and the velocity of light. 35   “Une des raisons pour lesquelles j’avais distingué les rôles que jouent l’éther et la matière pondérable dans les phénomènes lumineux était l’espoir de pouvoir traiter ainsi le problème de la propagation dans un corps pondérable qui est en mouvement sans entraîner l’éther qu’il contient.” LA 264. By “ponderable” matter, Lorentz means “ordinary” matter as distinct from the ether.


Early work and the Theory of Electrons

challenge that was met by the American physicist Albert A. Michelson. He devised a clever experiment. It was not designed to measure the absolute speed of light— that would have been too difficult—but it compared velocities of light in two directions perpendicular to one another. In this way, it was possible to infer how fast the earth moved through the ether. The surprising result of the experiment was that Michelson did not measure any difference. That was rather curious, since it was extremely unlikely that the earth would be at rest in relation to the ether, as was noted earlier. Was Maxwell’s theory incorrect? That was also very unlikely, given the many successes of the theory. Perhaps the hypothesis of the ether being at rest was no longer tenable. Perhaps the earth dragged the ether along with it, at least on its surface. That would be quite a blow to Lorentz’s theory, in which the hypothesis of the ether being immobile played such a central role. Lorentz went to work on a new analysis of the experiment and discovered that Michelson had overestimated the magnitude of the expected effect by a factor of two.36 His apparatus had not been precise enough to measure the true effect. For the time being, Lorentz was saved by the bell. But not for long. A few years later, in 1887, Michelson, meanwhile assisted by his colleague Edward Morley, repeated his experiment with an improved and much more precise experimental setup. The result was still the same: no effect. Now a problem had cropped up, and Lorentz was very well aware of it. He wrote to a colleague: “I am really at a loss how to clear away this contradiction.”37 An expedient was needed. This expedient consisted of the following assumption: A body moving through the ether is shortened slightly in the direction of its motion.38 The size of this contraction provided the compensatory effect to explain away the null-­ result of the Michelson–Morley experiment. Nowadays, this assumption or hypothesis is known as the “Lorentz–FitzGerald contraction.” The name of the Irish physicist George Francis FitzGerald was added because, completely independently, he had arrived at the same hypothesis.39 The contraction hypothesis did appear to be nothing but a ploy, so Lorentz was accused by the French mathematician Henri Poincaré, among others, and later also by Einstein, of introducing an ad hoc hypothesis designed purely to explain the negative result of the Michelson and Morley experiment.40 This accusation is not fully justified. Lorentz did have a justification for his assumption, albeit one that could not convince everyone. Without going into too much detail, his assumption was based on an argument about the stability of bodies made up of molecules, in

37  See Lorentz 1886a.   Lorentz to Lord Rayleigh, August 18, 1892 (Kox 2008, 22).  See Lorentz 1892c. 39   In a letter to Lorentz of November 14, 1894 (Kox 2008, 25) FitzGerald expressed his pleasure with Lorentz’s hypothesis: “I am particularly delighted to hear that you agree with me, for I have been rather laughed at for my view over here [. . .] but now that I have you as an advocate and authority I shall begin to jeer at others for holding any other view.” 40   See, e.g., Poincaré 1902, 536, and Einstein 1915. 36 38

The theory of electrons


which the molecules exert forces on one another that are comparable to electromagnetic forces.

The theory of electrons Further elaboration of Lorentz’s theory in a number of articles, a monograph, and a review article eventually led to his formulation of the final version of the theory of electrons in 1904.41 Two aspects of this final version deserve special attention. The first is the relationship between microscopic and macroscopic phenomena. Lorentz’s work was based wholly on a world of molecules and atoms, charged and uncharged, all moving in the ether. In that ether there were two fields: an electrical field generated by electrical charges and a magnetic field generated by currents, or more precisely, moving electrical charges. In the ether between the particles, these fields could change considerably in terms of magnitude. Now if one wanted to know what the values of these fields were in the familiar macroscopic world, one had to calculate average values for groups consisting of many atoms and mol­ ecules. One had to, as it were, focus a little less sharply. Using this method to demonstrate the existence of macroscopic fields—of which there turned out to be four in all, in accordance with Maxwell’s theory—was an idea that was later characterized by Einstein as “an act of intellectual liberation.”42 The second important aspect of the electron theory was the role of the so-­called “theorem of corresponding states.” This theorem, formulated for the first time in 1895, followed directly from Lorentz’s endeavors to provide a consistent treatment of electromagnetic phenomena in moving bodies. It offered the possibility of reducing this kind of phenomenon to corresponding ones in bodies at rest—hence the term corresponding states—by means of mathematical transformations. These were later named the Lorentz transformations. They involved transformed spatial coordinates and introduced a new time coordinate, the local time. These new coordinates were mathematical auxiliary quantities and were explicitly devoid of physical meaning. The Lorentz transformations also appeared in Einstein’s theory of relativity, but with a different interpretation.43 A proper understanding of electromagnetic phenomena in bodies at rest meant, in effect, that at this point everything was known that was required to fathom what

41  The most important ones are the almost 200-­ page-­ long Lorentz  1892f, the monograph Lorentz 1895b, the articles Lorentz 1899d and Lorentz 1904c, and the review article Lorentz 1904f. In Lorentz 1895b, the Maxwell equations appear for the first time in the form in which they are generally presented today. Still well known is the comprehensive presentation of the theory in the monograph The Theory of Electrons (Lorentz  1909i, second edition 1916), based on lectures given at Columbia University in New York in 1906 (see also Chapter 6). 42   “Erlösende Tat.” Einstein  1953. He was referring in particular to the assumption of only two microscopic fields. 43  See Janssen 2019 and Janssen and Kox 2011 for further discussions of the theorem of corresponding states.


Early work and the Theory of Electrons

goes on in bodies in motion. Interestingly enough, Lorentz was able to show in his 1904 publication that all experimental attempts to demonstrate the motion of the earth through the ether, for instance by electromagnetic or optical experiments, were doomed to fail. The theory of electrons was now complete, since the difficulty presented by the negative result of the Michelson–Morley experiment—and by all similar experiments—had been cleared up definitively.44

The anomalous Zeeman effect Despite the electron theory’s successes in explaining a great many electromagnetic phenomena, one tough problem remained unsolved. It had to do with the more complicated forms of the Zeeman effect. Soon after the discovery of the Zeeman effect, physicists had found that not all spectral lines split into two or three components under the influence of a magnetic field. In some cases more lines appeared, sometimes even six or seven. For this so-­called “anomalous Zeeman effect” no satisfactory explanation could be found, despite many efforts by Lorentz and others to achieve results on the basis of ever  more complicated variations on the simple model of one single oscillating particle.45 Yet the failure of the theory of electrons in the special case of the anomalous Zeeman effect was not a reason to lose faith in the theory. On the contrary, the successes of nineteenth-­century theories of electromagnetism gave rise, around 1900, to an interesting development that fit in well with the nineteenth-­century efforts toward unification. Physicists were, by then, beginning to hope that all of physics could be reduced to electromagnetism. Lorentz’s own confidence in the possibilities of his electron theory also increased.

The electromagnetic world view As early as 1895, Lorentz speculated that material particles might be nothing but singularities in the ether.46 Later, in February 1900, in an address entitled “Electromagnetic Theories of Phenomena of Physics,”47 he even suggested that electromagnetism could become the basic theory of physics. Reviewing some of the more notable achievements of the past decades, such as the prediction of the 44   Lorentz’s theory did have competitors, like the theory of the Englishman Joseph Larmor, but there were no differences of a truly fundamental nature between these theories. 45   For example, Lorentz investigated the behavior in a magnetic field of a pulsating sphere charged on the surface. A satisfactory explanation could not be found until the framework of quantum mechanics provided the hypothesis of “electron spin” in 1925. 46   Here singularities are points in the ether with properties that fundamentally differ from those of the surrounding ether. 47   “Electromagnetische theorieën van natuurkundige verschijnselen.” Lorentz 1900a.

The electromagnetic world view


electromagnetic nature of light waves and the explanation of the Zeeman effect, Lorentz’s speech exuded nothing but optimism about the promising possibilities for an electromagnetic world view. However, there were two developments that Lorentz failed to discuss in his address. In the first place, he did not mention his introduction of the concept of electromagnetic mass. From Maxwell’s theory, it followed that the interaction between a charged particle and its own field manifested itself as if, apart from its material mass, the particle had an additional mass: the electromagnetic mass.48 As soon as the insight had taken hold that such a thing as electromagnetic mass existed, the question arose as to whether perhaps all mass might be electromagnetic in nature. In particular the work of the German physicist Walter Kaufmann caused this idea to take root. Kaufmann carried out experiments that showed that, if the electron were to have a material mass at all, it would be small—in comparison with the electromagnetic mass, at any rate. Kaufmann—and with him a number of others, like Lorentz—concluded from this that electrons did not have any ordinary, material mass at all. Even in 1909, Lorentz still called this conclusion “one of the most important results of modern physics.”49 The second topic that Lorentz did not touch upon in his speech was the ­gravitational force. This was the only natural force that, until then, had escaped an electromagnetic explanation. Later that year Lorentz did attempt to deal with this issue.50 He assumed that the attractive force between two equally large charges of opposing signs was not exactly equal to the repulsive force between two equally large charges of the same sign. In addition, he assumed that atoms, though elec­ tric­al­ly neutral, contain charges: obviously an equal positive and negative charge. Based on this model, it turned out that material particles attracted one another with a force that had the same distance dependence for bodies at rest as the ­gravitational force. For bodies in motion, additional velocity-­dependent terms needed to be added. As was described earlier, Lorentz’s address showed great optimism about the possibilities for an electromagnetic world view, an optimism that was shared by others in the world of physics. One of them was the German physicist Wilhelm Wien. In an article in a volume published on the occasion of the twenty-­fifth anniversary of Lorentz’s doctorate, in 1900, Wien set out a research program to base mechanics on an electromagnetic foundation.51 He was one of the first physicists to explicitly formulate such a program.52

48   One can imagine this as follows: An electron in motion has a greater electromagnetic energy than an electron at rest because of the interaction between its charge and its own field. This means that more work needs to be performed to put an electron in motion than is the case for an uncharged particle with the same material mass. Or rather, it is as if the charged particle is heavier than the uncharged particle. This extra “mass” is the electromagnetic mass. 49 50 51   Lorentz 1909i, 43.   Lorentz 1900b.   Wien 1900 (reprinted as Wien 1901). 52   See also McCormmach 1970a for a detailed historical review of the role of the electromagnetic world view in the work of Lorentz and his contemporaries.


Early work and the Theory of Electrons

Yet, the electromagnetic world view did not stand the test of time, in spite of the initial optimism about the theory. Of course, the interesting question is why. Two factors have contributed to its obsolescence: the rise of quantum theory and the formulation of the special theory of relativity.

The special theory of relativity In 1905, the year in which Einstein presented his special theory of relativity, the state of affairs in physics was not generally viewed as unsatisfactory. The existing theory for electromagnetism was considered perfectly serviceable for the ex­plan­ ation of experimental results. There were still some problems in a few areas,53 but, on the whole, physicists agreed that the theory of electrons was generally the right approach. It was not surprising, then, that Lorentz was awarded the 1902 Nobel Prize for his work. The prize, which he shared with Pieter Zeeman, was officially awarded for the work, by both of them, on the relationship between optical and electromagnetic phenomena. However, it has meanwhile been established from documents in the Nobel Committee archives that his sponsors had something else in mind: They wanted to award the prize to Lorentz alone, for his entire body of work.54 All of this shows that, at the beginning of the twentieth century, theoretical physics was not in a state of crisis—at least not in the minds of most scientists.55 It is fair to say that, contrary to what is often suggested, Einstein’s theory did not arrive as a savior in times of dire need or help physics emerge from a critical situation. Modern physicists often view the fruitless attempts to demonstrate the motion of the earth with respect to the ether—like the Michelson–Morley ­ex­peri­ment—as great problems. Nothing is farther from the truth. This is already crystal clear in the first pages of Einstein’s famous article, where he unfolds his special theory of relativity.56 In the first paragraph Einstein presents the arguments that led him to formulate his theory. He points out two issues he finds intriguing. The first one had to do with the principle of relativity. Looking at mechanics, for example, it was clear that one was always dealing only with relative velocities of bodies or observers. In a moving train, for example, billiard players would not be able to see from the motion of the billiard balls that the train was in motion—as long as the train’s motion was uniform along a straight line; that is, non-­accelerated.   In particular, with regard to the anomalous Zeeman effect mentioned earlier.   See Chapter 6 for more on Lorentz and Zeeman being awarded the Nobel Prize. 55   A notable exception was the French mathematician and mathematical physicist Henri Poincaré. See Poincaré 1905, chap. 8; see also Darrigol 1995. 56  See Einstein 1905. The addition “special” refers to the fact that the theory only applies to systems in uniform rectilinear motion. It became popular after the publication, in 1915, of the “General Theory of Relativity,” Einstein’s theory for gravitational force, which can be considered an extension of the 1905 theory to arbitrary motions. 53 54

The special theory of relativity


For their game it would not make any difference whether the train was moving or standing still, as only the relative motion of the billiard balls was decisive for their game. In electrodynamics the situation was different. Because of the presence of the ether, the speed of an object with respect to this medium was also important. For the sake of convenience this speed was called absolute velocity. According to Einstein, this state of affairs was unsatisfactory. Einstein’s second issue was that electrodynamics showed a curious asymmetry. Many phenomena for which only the relative velocity of two objects was of any importance were described differently, depending on whether they were observed from the standpoint of one object or that of another object. As an example, Einstein cited the phenomenon of electromagnetic induction. If a wire and a magnet moved relative to one another, an electromotive force occurred in the wire, resulting in a current. The magnitude of the force was dependent on the relative velocity of wire and magnet, but the way in which that difference was calculated depended on one’s standpoint. Taking first the case of a moving wire and a magnet at rest, the explanation proceeded as follows. Charges moving in a magnetic field were subject to the Lorentz force. The electrons in the moving wire felt this force, and this caused them to move. In the second case, the magnet moved and the conducting wire was at rest. Now there was no Lorentz force, but there was a changing magnetic field in the proximity of the wire. The equations from Maxwell’s theory show that such a changing magnetic field generated an electrical field. This field, in turn, caused the electrons to be subjected to a force, so, as a consequence, an electric current arose. As mentioned earlier, these two completely different arguments led to the same result, in which only the relative velocity of wire and magnet played a role. Einstein viewed this asymmetry as a fundamental shortcoming of the theory of elec­tro­ mag­net­ism and made it his mission to develop an alternative approach. He did so in a very interesting way. He simply postulated that the principle of relativity holds for all of physics, including electrodynamics, and he additionally assumed that the velocity of light is independent of the velocity of the light source.57 These were the basic ingredients for Einstein’s new theory. After stating them very clearly, he presented a thorough analysis of the way in which distances and time intervals are measured in practice, by means of light signals. This led him to the surprising conclusion that the simultaneity is a relative concept: Two events that are separated in space and take place simultaneously for one observer would not be simultaneous for a second observer who is moving with respect to the first one. For the measurement of lengths, a similar situation applied. The rest of the article Einstein devoted to the further development of his relativistic mechanics and electrodynamics.

57   Note that the speed of light is not postulated to be independent of the speed of the observer, as is often asserted incorrectly.


Early work and the Theory of Electrons

Three points are important to note here. In the first place—and this is very important—accelerated motion was not taken into consideration. Einstein only looked at observers in systems that moved with respect to one another at constant rectilinear speeds. Secondly, the theory was completely relativistic. This means that only relative velocities played a role. Thirdly, in Einstein’s theory the laws of physics were the same for all observers, irrespective of their state of motion. This meant that Einstein no longer allowed for a privileged system like the ether. The absolute component that characterized the theory of electrons had thereby disappeared. As a result, it was an obvious step to declare the concept of the ether superfluous and, therefore, to drop it completely. This is exactly what happened.58

Electron theory or relativity theory? Now the question arises as to whether Einstein had refuted Lorentz’s theory.59 Curiously enough, that was not the case. Their two theories, the theory of relativity and the theory of electrons, were completely equivalent in terms of formalism and they generated exactly the same predictions.60 Of course, it is clear from the previous paragraphs that the foundations for each of the two theories were completely different. Lorentz himself formulated this in a very striking way in his book The Theory of Electrons, in which he summarized his main achievements. At the end of the book, he discussed Einstein’s theory and drew the following conclusion:61 Einstein simply postulates what we have deduced, with some difficulty and not al­together satisfactorily, from the fundamental equations of the electromagnetic fields.

Clearly, no crucial experiments could be devised to force a decision between the two theories. In spite of his great and often-­expressed admiration for Einstein’s work, Lorentz was able, until the end of his life, to maintain his adherence to the concept of an ether, even though its existence could not be demonstrated, not even indirectly. His argument was that electromagnetic waves required a medium in order to propagate. For the electromagnetic field energy to be present, for example, a medium would also be indispensable, in exactly the same way as it would only be possible to hang up one’s coat if an object like a coat rack were available.

58   Though the concept of the ether was dropped completely in modern physics, it survives to this day in popular non-­scientific usage, for example to indicate the medium through which radio broadcasts propagate. 59   See also Janssen 2002 and Janssen 2009 for a discussion of the relation between special relativity and Lorentz’s theory. 60   To be precise, this is only true for the final 1904 version of Lorentz’s theory, in which he had made corrections and also concluded that his transformed “local time” variable is in fact the time measured by observers. 61   Lorentz 1909i, 230.

Electron theory or relativity theory?


The most modern physicists, like Einstein and Minkowski, prefer not to discuss the ether at all. It becomes a question of taste, though, and a question of words. Whether or not the ether may be there, electromagnetic fields do exist and, similarly, the energy of electrical oscillations. Now if we do not want to mention the ether any longer, we need to make use of a different word, in order for us to hang all these things onto it, like on a coat rack.62

In and of itself, this standpoint was not incomprehensible for a man like Lorentz, certainly in light of how his work developed over time. The ether had obviously served him well during the development of his theories and had played such a central role in his thinking that it was difficult for him to let go of it.63 Many years after Lorentz had passed away, Einstein reflected some more on Lorentz’s point of view with respect to the ether. He concluded “that it was psychologically impossible for him to let go of the reality of the ether as a material thing (carrier of the electromagnetic field). Whoever has lived through those times, understands this.”64 Although his colleagues remained respectful of his ideas, Lorentz stood increasingly alone in his thinking. After some initial hesitations, the theory of relativity was embraced by most of the prominent physicists. An important role in the adoption of Einstein’s theory was played by the influential German physicist Max Planck, who was one of the first to be aware of its importance and who made a number of vital contributions to it. Another contributing factor to the theory’s acceptance was the result of a series of experiments in which the velocity dependence of the (electromagnetic) mass of electrons was determined. Actually, in this context it is worth noting an interesting philosophical phenomenon. The theories that were compared in these ex­peri­ ments were, on the one hand, the theories of Einstein and Lorentz—after all, these two made the same predictions—and, on the other hand, a theory for the structure of the electron advanced by the German physicist Max Abraham. The first results appeared to point in the direction of Abraham. The difference between Lorentz and Einstein in the way they reacted to this experimental evidence was very telling. Lorentz reacted as expected: “It seems very likely that we shall have to relinquish [the theory] altogether.”65 Einstein, on the contrary, discussed the experiments in a widely read review article about the

62   “De modernste physici, zooals Einstein en Minkowski, spreken liefst in het geheel niet meer over den aether. Het wordt echter een kwestie van smaak en een kwestie van woorden. Want de aether moge er zijn of niet, de elektromagnetische velden zijn er wel, en evenzoo het arbeidsvermogen der elektrische trillingen. Wanneer wij nu den aether niet willen noemen, dan moeten wij gebruik maken van een ander woord, om er, als aan een kapstok, al deze dingen aan op te hangen.” Lorentz 1922j, 35. This is the published version of a lecture by Lorentz on special relativity. 63   See also Kox 1988. 64   “Dass es für ihn psychologisch unmöglich war auf die Realität des Aethers als eines materiellen Dinges (Träger des elektromagnetischen Feldes) zu verzichten. Wer diese Zeit miterlebt hat, begreift es.” Einstein to G. J. B. Bremer, July 15, 1954 (RB). Reproduced in Bremer 1955. 65   Lorentz 1909i, 213.


Early work and the Theory of Electrons

special theory of relativity66 and concluded that the differences between ex­peri­ ment and theory were really not all that great. He admitted that Abraham’s theory was indeed more congruous with the experiments, but added immediately that this theory was not very probable, because its predictions about the velocity dependence of the electron were not part of a more comprehensive theory that also explained all sorts of other phenomena—a theory, in other words, like his own. These remarks also showed, by the way, that Einstein was certainly not lacking in self-­confidence. Anyhow, around 1910 the situation had gradually changed to the point that for the description of electromagnetic phenomena in moving systems the special theory of relativity had eclipsed Lorentz’s electron theory, in spite of its prior successes. For this reason, and, as was mentioned earlier, also because of the rise of quantum theory, the electromagnetic world view had, by then, been almost completely abandoned. This had happened surprisingly quickly, especially taking into consideration how the electron theory had at first raised the highest expectations. A new world view of relativity and discontinuity had taken its place, and the ­unification of physics that many had so ardently sought seemed further away than ever.

Conclusion It is worthwhile to look back and see what parts of Lorentz’s work were the subject of particular attention here. As was stated at the beginning of this chapter, Lorentz’s most important and most influential work was the theory of elec­tro­mag­ net­ism. Based on the groundbreaking work by Maxwell, Lorentz steadily worked to formulate this theory in a much more accessible form, a form which is, in fact, still used in present-­day physics. Very soon after finishing his doctorate, Lorentz set himself a clear goal for future investigations in this area; namely, to provide a satisfactory description of electromagnetic phenomena in bodies that were in motion with respect to the ether. Two fundamental and extraordinarily fruitful assumptions were his guiding principles in these efforts. On the one hand, he made a sharp distinction between ether and matter, and on the other he incorporated the notion that matter is atomistic in nature. In addition, he formulated the exceptionally fruitful hypothesis of the oscillating ions. Lorentz’s atomistic concept of matter was also the foundation for other publications in his early years that were not discussed in more detail here. This applies in particular to a number of articles in the field of kinetic gas theory, some of which were mentioned earlier. In this work he made additions and corrections to the

  Einstein 1907.




work of Van der Waals and Boltzmann.67 Furthermore, he wrote an article on the propagation of sound in gases, also based on the kinetic theory.68 Aside from his work on gas theory, his work on the concept of entropy is also worth mentioning here,69 as well as the articles on thermodynamics—especially those on ­thermo-­electricity, the phenomenon that an electrical current is generated by a difference in temperature.70 Given the limitations of this biography, much of Lorentz’s work must, by necessity, be omitted here.

  Lorentz 1881a, Lorentz 1887a, and Lorentz 1891a.    68  Lorentz 1880a.   Lorentz 1896d.   70  Lorentz 1885a, Lorentz 1885a, and Lorentz 1889a.

67 69

Chapter 5 The new century: Prospect and retrospect

The dawning of a new century was an occasion for many to articulate prospects for the future and for retrospection upon the previous century.1 Physicists in the Netherlands were no exception. In the year 1900—on February 8, March 12, and September 17—three professors of physics at three universities in the Netherlands each addressed the public, expounding their visions for the future of physics. Two of them, Lorentz and Herman Haga, both happened to be Rector Magnificus2 of their respective universities in the academic year 1899–1900. Their rectors’ speeches, delivered in front of large audiences and subsequently published, offer an interesting insight into their assessments of the nineteenth century and their expectations for the century to come. The third professor who gave a public speech was Pieter Zeeman, who was already discussed at length in Chapters 2 and 4. He gave his address on the occasion of his inauguration as an extraordinary professor at the University of Amsterdam, and he too presented his views on the future of physics. In order to place these speeches in their proper perspective, it is necessary to give some thought to the developments in Dutch physics in the last quarter of the nineteenth century and the state of affairs around 1900. Much had changed in the previous twenty-­five years.

The rise of physics On Wednesday November 18, 1908, the Amsterdam professor Johannes Diderik van der Waals—already mentioned in Chapter 2—was awarded a well-­deserved decoration. He received the honorary gold medal of the Genootschap ter Bevordering van Natuur-, Genees- en Heelkunde (Society for the Advancement of Science and

  This chapter is based in part on Kox 2012.   At the time, the Rector Magnificus was elected from among the faculty to serve as the university’s president for an academic year. 1 2

“A Living Work of Art”: The Life and Science of Hendrik Antoon Lorentz. A. J. Kox and H. F. Schatz, Oxford University Press (2021). © A. J. Kox and H. F. Schatz. DOI: 10.1093/oso/9780198870500.003.0006

The rise of physics


Medicine), the highest distinction to be awarded by this Society.3 To mark the occasion, Zeeman, the Society’s chairman, gave an address in which he said the following: Only rarely have the physical sciences (we may even say the natural sciences) flourished in our country like they are flourishing now. Surely, during the first half of the previous century physics was practiced here with diligence, but the truly new, the great discoveries were expected from Paris, Berlin, London, or Göttingen. It was believed that results should be published there, and a great discovery, so it was almost expected, could not be made in our country. Nowadays, what is offered by Dutch physics may be a match for the best that is offered by other nations.4

A similar observation, this time from Germany, so the source is above suspicion, was made by the Göttingen physicist Woldemar Voigt, who wrote in 1913, when Kamerlingh Onnes had been awarded the Nobel Prize: During the first three quarters of the nineteenth century, the Netherlands have participated little in the mighty movement in physics that unfolded during that period. [. . .] The turning point in the rise of the Netherlands to become a superpower in the field of physics (which is unquestionably the case today) is the year 1875, because of the publication of the brilliant dissertation by Van der Waals.5

Zeeman and Voigt were not exaggerating. As recently as 1885, the French-­Swiss botanist Alphonse de Candolle had passed a very negative judgment on Dutch natural science when he wrote in his overview of the history of science: “[. . .] as far as the scientific value is concerned, in the current century its [the Netherlands’] position is modest, or even rather faded.”6 Since then, though, the quality of Dutch natural science had improved enormously and had gained an international reputation. This period of success has meanwhile become known as the “Second Golden Age of Dutch Science.”7

3   This Society organized bi-­yearly meetings of Dutch scientists and physicians that were the counterpart of the meetings of the German Naturforschergesellschaft. 4   “Slechts zelden heeft in ons land de natuurkunde (wij mogen zelfs wel zeggen de natuurweten­ schap) een bloei getoond, als zij thans beleeft. Wel werd ook in de eerste helft der vorige eeuw de na­tuurkunde hier met vlijt beoefend, maar het werkelijk nieuwe, de groote vondsten verwachtte men van Parijs, Berlijn, Londen of Göttingen. Daar, meende men, moesten resultaten gepubliceerd worden en een groote ontdekking, dacht men bijna, kon in ons land niet gedaan worden. Thans mag wat de Nederlandse natuurkunde geeft zich met het beste meten wat andere volken geven.” Zeeman 1909. 5   “Während der ersten drei Viertel des neunzehnten Jahrhunderts haben sich die Niederlande an der mächtigen Bewegung in der Physik, die sich in dieser Zeit entfaltete, nur wenig beteiligt. [. . .] Der Wendepunkt zu dem Aufsteigen der Niederlande zu einer Großmacht im Gebiete der Physik (die sie jetzt fraglos darstellen) bezeichnet das Jahr 1875 durch das Erscheinen der genialen Dissertation von Van der Waals.” Chemiker-­Zeitung 38 (1913): 1518. 6   “[. . .] quant à la valeur scientifique, dans le siècle actuel sa position est modeste, ou même assez effacée.” Candolle 1885, 425. 7   The term was coined by the Dutch sociologist Bastiaan Willink.


The new century: Prospect and retrospect

The great reputation of Dutch physics in particular is all the more notable if one considers the extent of the field in the Netherlands in terms of personnel. In 1900, all the physics departments in Dutch universities together counted a total of no more than eighteen scientific staff positions. Amsterdam had three professors and two assistants, and in Leiden there were two professors, one privaatdocent, and four assistants. Leiden and Amsterdam had by far the largest staff: Utrecht had to make do with two professors and two assistants, while Groningen had only one professor, supported by one lone assistant.8 When comparing the Dutch figures with similar data from abroad, two aspects are worth noting. On the one hand, the Dutch effort is quite modest in size, at least in terms of absolute numbers. By way of comparison, the university physics departments in Germany employed some one hundred professors and privatdozenten. On the other hand, it is striking how great the Dutch effort was in a relative sense. The Netherlands had 4.1 university positions per million inhabitants, as opposed to 2.9 in Germany. This German figure was more or less the norm for all of Europe at the time, except for Switzerland, which really stood out with its 8.1 per million.9 It is clear, anyhow, that something extraordinary was happening in the Netherlands. To be able to elaborate further on these developments, it is useful to  first discuss the achievements of Dutch physics in the quarter century that preceded the year 1900.

Physics around 1900 The state of physics in Leiden around the turn of the century already came up in Chapter 2. Lorentz and Kamerlingh Onnes were the towering figures in Leiden physics, while in Amsterdam Van der Waals and Zeeman were calling the shots. By then, Van der Waals, at 63, had already become the grand old man of Dutch physics. In the preceding period, working together with his students, he had extended and generalized the work on the equation of state that was initially part of his dissertation. Three of his important results are the law of corresponding states, the theory of capillary phenomena, and the theory of binary compounds, none of which will be discussed in more detail here, as they have relatively little relevance to the work of Lorentz. The third professor in Amsterdam was Remmelt Sissingh. Like Zeeman, he had obtained his doctorate in Leiden after writing a dissertation on m ­ agneto-­optical phenomena. Besides experimental work, especially in the field of optics, Sissingh

8   For an even better understanding of the enormous expansion of physics in the decades after 1900, it is useful to focus on the increased numbers in Amsterdam. By 1935, Zeeman’s laboratory alone had six assistants, one conservator (managing director), five doctoral candidates, and a technical staff of six, all working under his direction. 9  See Forman et al. 1975.

Lorentz’s rectoral address


carried a large teaching load, teaching physics students as well as medical students. In Utrecht around the turn of the century, things were not quite as lively as in Leiden and Amsterdam. There were no great names that captivated the im­agin­ ation, but nonetheless there were some interesting physicists connected to the faculty. Physics was somewhat of a family affair in Utrecht: The two professors were Victor August Julius and his cousin Willem Henri Julius. The latter Julius was an interesting and original man who had just started to become interested in physical phenomena in the solar atmosphere. His interest resulted in a long series of publications—and later in a book—in which he tried to use the refraction, reflection, and anomalous dispersion of sunlight to account for all kinds of phenomena, such as the shift and widening of spectral lines in the solar atmosphere—a highly original and very interesting, but controversial theory that was completely forgotten after Julius’s death in 1925. For a while, Julius’s theory piqued Albert Einstein’s curiosity, as is demonstrated by the extensive cor­res­ pond­ence between the two scientists in the years between 1910 and 1911.10 This correspondence, by the way, also dealt with a second interesting topic; namely the possible appointment of Einstein as professor in Utrecht. More about the Utrecht vacancy is discussed in Chapter 7. In Groningen, physics made less of a splash around 1900. Herman Haga, Lorentz’s former classmate and good friend, was the only professor there. He had made a name for himself with his experimental research, together with Cornelis Wind, on the nature of the mysterious X-­rays that had been discovered by Wilhelm Röntgen in 1895. They had succeeded in demonstrating that X-­rays showed diffraction phenomena, which provided strong support for the wave theory for these rays. Their publications were frequently cited in later research.

Lorentz’s rectoral address Back to the three academic speeches that were discussed earlier. In the academic year 1899–1900, Lorentz was Rector Magnificus of Leiden University, as was mentioned earlier. His rectoral address on February 8, 1900, on the occasion of the 325th anniversary of the university, was about a subject, as he formulated it himself, “derived from the cycle of my daily investigations and reflections.”11 The title of the address was: “Electromagnetische theorieën van natuurkundige verschijnselen” (Electromagnetic Theories of Physical Phenomena).12 After presenting an overview of some of the achievements of n ­ ineteenth-­century physics, like the law on the conservation of energy, the second law of

  The correspondence is published in CPAE-­5.   “ontleend aan den kring mijner dagelijkschen onderzoekingen en overdenkingen.”   Lorentz 1900a.

10 11 12


The new century: Prospect and retrospect

t­ hermodynamics, and the molecular theories of matter, Lorentz broached the real subject of his speech: Among the theories of present-­day physics there is one group—one could call them the electromagnetic theories of physical phenomena—that appear to me to be so promising for the near future that I believe I might discuss them at some more length.13

Moving on from there, he discussed the experimental work by Michael Faraday and the electromagnetic theory of James Clerk Maxwell. Maxwell’s great merit was, according to Lorentz, that he had successfully managed to unify the phenomena of electricity and magnetism in one and the same theory. Lorentz also noted an important prediction of this theory; namely the existence of electromagnetic waves, as well as the experimental confirmation of their existence by Heinrich Hertz. Lorentz then went on to discuss the progress in the microscopic theory of light emission. This theory assumed that small charged particles would be present within the atoms and molecules that make up matter and that they would oscillate and, as a result, emit light. It is characteristic that Lorentz alluded nowhere to the fundamental contributions that he himself had made to this theory. Lorentz continued his speech by addressing the subject of spectroscopy—the study of the light emitted by atoms—as a tool to reveal the internal structure of atoms. In particular, he discussed the discovery and the theoretical explanation of the Zeeman effect, again without mentioning that he himself had drawn up this theory as well. Lorentz expected a great deal of the theory of electromagnetism, not only for revealing the structure of atoms, but also for a more general insight into the structure of spectra. the regularities that were found and the strong resemblance between the spectra of some elements [offer] much hope that a light will shine on all this at some point, that perhaps with one single fortunate idea much will be achieved. And, without a doubt, as far as can be judged now, the electromagnetic theory offers the best chance to resolve the problem.14

Lorentz ended his speech with some remarks about a possible electromagnetic origin of molecular forces and even of gravitational force. About this latter possibility Lorentz also wrote an article, which appeared later that year.15 It is clear

13   “Onder de theorieën der hedendaagsche physica is er ééne groep—men kan ze de electromagnetische theorieën van natuurkundige verschijnselen noemen—die mij voor de naaste toekomst zoveel schijnen te beloven, dat ik meen ze iets uitvoeriger te mogen bespreken.” 14  “[ . . .] de gevonden regelmatigheden en de sterke gelijkenis tusschen de spectra van sommige elementen [geven] alle hoop dat over dit alles eenmaal licht zal worden ontstoken, dat misschien met een enkelen gelukkigen inval veel zal worden bereikt. En ongetwijfeld heeft men, voor zoover dat nu te beoordelen is, met de elektromagnetische theorie de meeste kans het vraagstuk op te lossen.” 15   Lorentz 1900b. More on this paper is found in Chapter 4.

The speeches by Haga and Zeeman


from all this that Lorentz had a great deal of confidence in the further expansion of the theory of electromagnetism.

The speeches by Haga and Zeeman Lorentz was not alone in his confidence in the theory of electromagnetism. The second academic speech, this one delivered by the Groningen Rector Magnificus Haga, shows equal confidence. Six months after Lorentz, Haga gave an address about his own field of interest when he ceded his rectorship to his successor on September 17, 1900. His speech, entitled “De ontwikkeling der natuurkunde in de 19de eeuw” (The Development of Physics in the 19th Century),16 showed a striking resemblance to the one by Lorentz, judging by the following quote: What achievements can it [physics] point to in this century that is rushing towards its final days? It seems to me that the answer to this question is not too difficult; as a few prominent parts in the line of development they stand out: the recognition of the conservation of energy and the newer insights regarding the electrical phenomena.17

In his speech, Haga first discussed the role of the law of conservation of energy and then moved on to electromagnetism, placing the work by Faraday, Maxwell, and Hertz in a central position—just like Lorentz did in his speech. Contrary to Lorentz himself, Haga also explicitly discussed Lorentz’s work, especially in connection with the explanation of the Zeeman effect and the electromagnetic theory of gravity. Haga ended his speech with a number of questions about the future: [it] will surely have become obvious to you that, even though our insights with regard to electricity have totally changed in nature, the arbitrary boundary between cen­tur­ ies cannot be traced in the development of physics. What is electricity? What is the ether? What is the mechanism of the changes of state, to which so much is ascribed? These are the questions forcing themselves upon us immediately, as it were, and through whose resolution alone the phenomena discussed here would be forged into a well-­rounded whole. They are the questions that, in view of what this nineteenth century has brought about, we can pose with confidence to the twentieth century.18

  Haga 1900.   “Waarop kan zij [de natuurkunde] wijzen in deze ten einde spoedende eeuw? Me dunkt dat het antwoord op deze vraag niet al te lastig is; als een paar uitstekende deelen in de lijn der ontwikkeling staan daar: de erkenning van het behoud van arbeidsvermogen en de nieuwere inzichten aangaande de electrische verschijnselen.” 18   “[het] zal u wel duidelijk gebleken zijn, dat al zijn onze inzichten op electrisch gebied geheel van aard veranderd, de willekeurige grens van een eeuw niet teruggevonden wordt in den ontwikkelingsgang der natuurkunde. Wat is electriciteit? Wat is de aether? Wat is het mechanisme der toestandsveranderingen waaraan men zooveel toeschrijft? Dat zijn de vragen, die als het ware onmiddellijk zich aan ons opdringen en door wier beantwoording de besproken verschijnselen eerst tot een goed afgerond geheel zouden worden. Het zijn de vragen, die wij, lettende op hetgeen deze 19de eeuw tot stand heeft gebracht, met vertrouwen stellen aan de twintigste eeuw.” 16 17


The new century: Prospect and retrospect

The final academic speech is the address by Zeeman. On January 17 he was promoted, at the young age of thirty-­four, from the position of lecturer to that of extraordinary professor. Two months later, on March 12, he gave his inaugural lecture, entitled: “Experimenteele onderzoekingen over deelen kleiner dan atomen” (Experimental Investigations about Particles Smaller than Atoms).19 He spoke about the new insights in the structure of atoms and molecules which had led to the assumption that even smaller particles exist within atoms, an idea that found powerful support in the discovery of the Zeeman effect. In Zeeman’s own words: We further know that the atoms may only be called “indivisible” insofar as we are unable to subdivide them any further at this time. The complex spectra of even the simplest gases have led us to decide with certainty that the structure of atoms must still be very complex and that within them still other particles must thus be distinguished. Yet, only the investigations of the most recent date have proven the independent existence of particles smaller than atoms beyond a doubt and have even left open the possibility that the alchemists’ dream of a transmutation of elements ­contains some truth.20

Zeeman’s speech offered an overview of numerous experimental indications for the existence of small charged particles, varying from discharge phenomena like cathode rays and canal rays to the photo-­electric effect. But pride of place was, almost self-­evidently, given to the Zeeman effect. After a crystal-­clear exposé about the effect itself and about Lorentz’s theoretical explanation by assuming the  presence of oscillating electrically charged particles—negative ions—within atoms, Zeeman reached the following conclusion: We surely cannot doubt that this negative ion must play a fundamental role in all electrical theories. Perhaps it is the fundamental magnitude in which all electrical processes can be expressed, since its mass and charge appear to be unchangeable and also independent of the electrical processes through which and the matter from which it originates. [. . .] The experimental study of radiation phenomena under a variety of circumstances will, so it appears to me, offer important building blocks, in more than one direction, for our knowledge of nature.21

  Zeeman 1900.   “We weten verder dat de atomen slechts in zooverre ‘ondeelbaar’ mogen heeten, dat we op ‘t ogenblik ze niet verder kunnen verdeelen. De samengestelde spectra zelfs der meest eenvoudige gassen doen ons echter reeds met zekerheid besluiten dat de structuur der atomen nog zeer samengesteld moet zijn en dat er dus nog deelen in onderscheiden moeten worden. Het zijn echter onderzoekingen van den allerlaatsten tijd die het zelfstandig bestaan van deelen kleiner dan atomen buiten twijfel hebben gesteld en die zelfs de mogelijkheid laten dat de droom der alchimisten over eene transmutatie der elementen waarheid bevat.” 21   “We kunnen er dus wel niet aan twijfelen of dit negatieve ion moet eene fundamenteele rol spelen in alle electrische theorieën. Misschien is het wel de fundamenteele grootheid waarin alle electrische processen uitgedrukt kunnen worden, want zijn massa en lading schijnen onveranderlijk te zijn en ook onafhankelijk van de electrische processen, waardoor en van de stoffen waaruit het ontstaat. [. . .] De 19 20

The electromagnetic world view


At the end of his lecture, in a striking formulation, Zeeman unfolded his ambitious plans for the future: It will be my ambition in the Physics Laboratory to encourage these investigations, which are so closely connected to the ultimate foundations on which the world is built. Or let us rather not assume the appearance of believing that in the direction of the small such foundations can be found. In the direction of the great and of the small the world is infinite and, in the end, one always reaches that hazy perspective where first our senses and then our thoughts abandon us.22

In other words, Zeeman’s lecture also placed great emphasis on the importance of electromagnetism to gain a deeper insight in the structure of matter and, in his case, it was complemented by a draft of a research program designed to achieve that goal. In 1908 Zeeman succeeded Van der Waals as ordinary professor and director of the laboratory. This provided him the opportunity to expand the research on thermodynamics in the broadest sense of the word—the original research field in Amsterdam—by including spectroscopy. These two lines of investigation were very fruitful and continued to coexist for decades.

The electromagnetic world view In spite of the important contributions made by Dutch physicists to thermo­ dynam­ics and kinetic gas theory in the preceding decades, the theory of electromagnetism was clearly the center of attention in 1900. Much was expected of the further development of this theory. Its role was believed to be so important that, for a while, electromagnetism was even expected to replace mechanics as the basic science. In the Netherlands, Lorentz was the chief advocate of this electromagnetic world view, and in Germany Wilhelm Wien was an explicit supporter of the idea, as was already discussed extensively in Chapter 4. The question arises, of course, of whether the three physicists were correct in the pronouncements they made in their academic speeches. It is still true today that the theory of electromagnetism was one of the most important research achievements in physics at the end of the nineteenth century, perhaps even the

experimenteele studie der stralingsverschijnselen, onder eene verscheidenheid van omstandigheden, schijnt mij toe in meer dan eene richting belangrijke bouwsteenen te zullen leveren voor onze ­natuurkennis.” 22  “Het zal mijn streven zijn in het Natuurkundig Laboratorium op te wekken tot deze onderzoekingen, die zoo nauw in verband staan met de laatste fundamenten waarop de wereld gebouwd is. Of laat ons liever niet den schijn aannemen te meenen dat in de richting van het kleine dergelijke fundamenten gevonden kunnen worden. In de richting van het groote en van het kleine is de wereld oneindig, en ten slotte komt men steeds in dat nevelachtig verschiet, waar eerst onze zintuigen en dan onze gedachten ons in den steek laten.”


The new century: Prospect and retrospect

most important one. Still, Lorentz, Haga, and Zeeman eventually turned out to be mistaken in their optimistic expectations for the twentieth century. Even around the turn of the century, not all physicists were equally optimistic about the role of electromagnetism. The English physicist Lord Kelvin (William Thomson)—by no means the least of the critics—had identified two clouds in the clear sky of theoretical physics, in an address delivered in April 1900.23 The ­Irish-­Scottish Kelvin was by far the most famous and influential British physicist of the nineteenth century. Lorentz had met the venerable seventy-­four-­year-­old in Rotterdam in June 1899, where he discussed physics with him at length. He was much impressed: “I felt the powerful attraction of the unpretentious and yet so profound way in which he spoke about certain problems, in particular about the secrets of the ether and of gravitation. Lord Kelvin is still very spry.”24 Lord Kelvin’s first cloud had to do with the explanation of atomic spectra. The great number of spectral lines emitted by various substances appeared to indicate that atoms and molecules would have an extremely complicated internal structure. That would need to have experimental consequences; for example, for the specific heat of these substances. However, such consequences did not occur. That was a sticky problem, and not even a beginning of a solution was available. The other cloud was the problem Kelvin had encountered with every attempt to experimentally demonstrate the motion of the earth through the light ether. They had all come to naught. In the end, revolutionary changes in physics were necessary to dissolve Kelvin’s clouds. The first cloud disappeared when the development of quantum theory led to fundamentally new insights about the atomic and the sub-­atomic world. The other cloud was removed by Einstein’s theory of relativity, as it completely changed the view of space and time. In the end, the electromagnetic world view turned out to be untenable, and the theory of electromagnetism became subordinate to the new theoretical developments. Once more, predicting future developments had proven to be a precarious endeavor.

The “Second Golden Age” As was set out earlier, around 1900 Dutch physics was held in high esteem outside the Netherlands. Not only can this be concluded from the three academic speeches quoted here, but there are also other, more concrete indications. In the short period between 1901 and 1913, for example, no fewer than five Dutch natural scientists were honored with the Nobel Prize. Lorentz and Zeeman received it in

23   The address was delivered on April 27, 1900 at the Royal Institution in London. See Kelvin 1901 for the text. 24   “ich fühlte den mächtigen Reiz der anspruchslosen und dennoch so tiefsinnigen Weise, in der er über manche Probleme, speciell über die Geheimnisse des Aethers und der Gravitation sprach. Lord Kelvin ist noch sehr rüstig.” Lorentz to Woldemar Voigt, June 18, 1899 (Kox 2008, 60).

The “Second Golden Age”


1902, Van der Waals in 1910, and Kamerlingh Onnes was awarded the prize in 1913. Though the fifth Nobel laureate was not a physicist but a chemist, its re­cipi­ ent, Jacobus van ’t Hoff, was the very first Nobel laureate for chemistry, receiving the prize in 1901. Also, beyond the fields of physics and chemistry, the Netherlands produced scientists of world renown. Examples were the medical scientists Christiaan Eijkman, who was awarded the Nobel Prize in 1929 for the discovery of vitamin B, and Willem Einthoven, winner of the Nobel Prize in 1924 for the invention of the electrocardiograph, as well as the botanist Hugo de Vries, whose groundbreaking work on plant genetics was fundamental for genetic theory. There are also other objective indicators to show that Dutch natural science was doing very well. In his dissertation and in later publications, the Dutch sociologist Bas Willink was the first to analyze how Dutch science flourished at the end of the nineteenth century.25 In his quantitative research he showed that the reputation of Dutch natural science did indeed grow considerably during that period. Among other indicators, Willink considered the amount of space allotted to Dutch scientists in the well-­known biographical dictionary by Poggendorff. The notable increase after 1880 is significant. Now, of course, it is justified to question whether this development was autonomous and completely accidental, or whether stimulating factors can be identified that were instrumental in this fast ascendance of Dutch science. One might be of the opinion that it was just a fortuitous accident that a unique concentration of talent happened to be present in the Netherlands, enabling Dutch science to be driven to great heights. Still, the mere presence of great talent is not a sufficient condition. Talent must be nurtured and given the opportunity to blossom. In Willink’s study of what he called the Second Golden Age, he concluded that two important developments were instrumental in promoting this abundance of talent at the turn of the century. The establishment of the HBS as a new type of secondary school was the first factor that contributed to the success of natural science in the Netherlands. Education at the HBS was more modern and, in many respects, its curriculum provided a more suitable preparation for the study of one of the exact sciences at the university than the traditional Gymnasium curriculum. It also drew a new group of students, especially from the middle class. Before the existence of the HBS they would not even have considered attending secondary school, given the elite status of the Gymnasium. Even so, admission to a university still required a Gymnasium diploma, and it was not until 1918 that a diploma from the HBS allowed graduates equal access to university studies. Curiously enough, a significant number of HBS graduates still ended up studying at a university. They went to the trouble of taking the state Gymnasium examination, often at the expense of an additional year of study of the classical

 See Willink 1988, Willink 1998, and Willink 1991.



The new century: Prospect and retrospect

languages, and were subsequently registered as regular students, especially in one of the exact sciences. Willink’s figures are interesting in this regard. In 1863, no fewer than fifteen HBS schools were opened throughout the country. As early as the period between 1864 and 1876, more than half of the HBS graduates went on to study at a university. Between 1879 and 1884, twelve new members were elected to the physics department of the Royal Academy of Science. Half of these were HBS graduates. Sixty years later, in the academic year 1937–1938, the majority of professors in natural science had been educated at the HBS (202), as opposed to eighty-­five with a Gymnasium diploma and thirty-­one with some other type of educational background. Incidentally, these figures also clearly show the enormous expansion of Dutch natural science during those sixty years. The importance of the HBS for the development of natural science in the Netherlands is also illustrated indisputably by the array of Dutch Nobel laureates in the first decades of the twentieth century. Of the five winning scientists, only Van der Waals was not HBS-­educated. He had taken the long educational route by first finishing the MULO, a type of middle school, and then acquiring teaching qualifications, which also allowed him to go to university. Yet, he had experience with HBS education, too, as he taught HBS ­students for many years. Apart from the establishment of the HBS, there was another important development that promoted the emergence of natural science in the Netherlands. The new law regulating university education, already discussed in Chapter 2, represented a great stimulus for the quality of an academic education. Since the law’s enactment, the universities offered a much better, modernized curriculum that was especially attractive to HBS graduates. An additional advantage was the availability of new teaching positions at the growing number of HBS schools all over the country. They offered an attractive prospective career for university graduates, and in many cases teachers of physics and mathematics even held doctorates. The influence of these talented teachers on young and impressionable students should not be underestimated, judging, for example, by the quote from Lorentz in Chapter 1 about the importance of his physics teacher for his later career. A third and final factor behind the rise of Dutch science was advanced by the historian of science Ad Maas.26 He argued that the new generation of scientists that became ascendant in the final decades of the nineteenth century had a new, more individualistic view of science. They were far less inclined to pursue science in the interest of society. Instead, they practiced science purely for its own sake. Maas characterized this as “la science pour la science,” analogous to the qualification “l’art pour l’art,” which was the credo of the “Movement of the Eighties.” This new artistic movement, which had become prominent in the 1880s in the Netherlands, also served, according to Maas, as an inspiration to science.

 See Maas 2001.


Lorentz at the turn of the century


End of the golden era After looking back to the nineteenth century, it is also appropriate to briefly look ahead. Everything must come to an end, and so it was with the golden age of Dutch natural science around the turn of the century. After World War One, the role of the Netherlands gradually diminished. Leiden, as the center for theoretical physics, had to start competing with places like Göttingen and Copenhagen. This does not mean, though, that the work in the Netherlands was no longer of outstanding quality. Lorentz remained, until his death in 1928, the undisputed ­éminence grise of international theoretical physics. Zeeman continued his very high-­quality precision research in the field of spectroscopy and, later, in nuclear physics. A foreign visitor called his laboratory a “pearl.” Leiden continued to be an international center for low temperature physics, and Paul Ehrenfest, Lorentz’s successor since 1912, gathered a band of faithful and sharp-­witted young students around himself in a unique atmosphere of scientific enthusiasm. Similar remarks can be made about physics in Utrecht and Groningen, but nevertheless things had changed. Nobel prizes failed to materialize, and many young physicists went abroad for shorter or longer periods. Dutch physics stabilized at a high level, but no longer the very highest level. About the causes of this stagnation there is little to say that does not amount to conjecture. Perhaps the exceptional talents were lacking, or perhaps the education or the facilities at the universities were no longer sufficient. At any rate, it is certain that the facilities in Germany were better, and that the United States in particular were catching up fast because of their well-­financed research efforts. As a result, many Americans came to Europe and many European scientists visited the United States in the 1920s and 1930s. Without a doubt, further historical analysis will bring to light other, material factors that may have contributed to this decline of the Netherlands in the world of physics.

Lorentz at the turn of the century For Lorentz personally, the beginning of the new century was auspicious and unfortunate at the same time. In the beginning of August, he traveled to Paris for a large international conference where he presented a paper on the theory of magneto-­optical phenomena.27 The conference was part of a great world fair for the general public, the Exposition universelle, which showed the achievements of the past century as well as an array of all sorts of new inventions. Lorentz was one of three Dutch scientists who made an appearance. The other two were Van der Waals and the chemist Van ‘t Hoff. Incidentally, it is interesting to note how most of the conference presentations—which were published in three fat volumes—pay   Lorentz 1900e.



The new century: Prospect and retrospect

hardly any attention to a historical review of the nineteenth century; certainly not in the way Lorentz and Haga had done in their rectors’ speeches of the same year. On December 11, it would be the twenty-­fifth anniversary of Lorentz’s doctorate. A committee of colleagues, led by Kamerlingh Onnes and Haga, wanted to celebrate this event and took the initiative to put together a Festschrift, a volume of tributes. In July 1900, a call for papers went out to a large number of colleagues in the Netherlands and abroad, asking for contributions to this celebratory volume. The response was overwhelming. A special issue of the journal Archives néerlandaises des sciences exactes et naturelles, which counted as many as 687 pages, contained no fewer than fifty-­eight articles by physicists from all over Europe. Among them were luminaries and friends like Pieter Zeeman, Johannes van der Waals, Ludwig Boltzmann, Max Planck, Henri Poincaré, Lord Rayleigh, J. J. Thomson, and Woldemar Voigt. On the festive day itself, Haga and Kamerlingh Onnes went to see Lorentz to present him with the finished volume, the “surprise of which I cannot tell you how deeply it has moved me,” as Lorentz wrote to Zeeman in a note of thanks for his contribution.28 As Lorentz himself would have it, he would otherwise have forgotten the anniversary. Unfortunately, the joyous day was overshadowed by the death, on November 29, of Lorentz’s 87-­year-­old father-­in-­law, Johan Wilhelm Kaiser. As a consequence, the celebration was kept “very modest,” Lorentz wrote to Zeeman. As was already described in Chapter 2, Lorentz was very fond of Kaiser. He called him “one of the best and dearest people that I have ever known.”29 The celebratory volume was accompanied by a dedication, written by Kamerlingh Onnes. It clearly showed the versatility that characterized Lorentz’s scientific work of the previous twenty-­five years: To H. A. Lorentz, The eminent scholar, of great versatility, philosophical insight, mathematical talent, Who has enriched the chapters of hydrodynamics, sound, heat, and ­thermo-­electricity, Who has clarified the connection between ether and matter, reflected in the facts of dispersion, absorption, radiation, inside and outside the magnetic field, The trailblazer, who, in the line of Faraday and Maxwell, has summarized the theory of light and the electrodynamics of moving bodies into one undivided whole, The master, unsurpassed in the clear representation and untiring in lending ­assistance, This volume is dedicated by colleagues, friends, students.30

28   “verrassing waarvan ik u niet kan zeggen hoe diep zij mij heeft getroffen.” Lorentz to Zeeman, December 18, 1900 (Kox 2018, 42). 29   “een der beste en beminnelijkste menschen die ik ooit gekend heb.” Lorentz to Willem Julius, December 19, 1900 (Kox 2018, 43). 30  “Aan  H.A.  Lorentz, den voortreffelijken geleerde, van groote veelzijdigheid, philosophisch inzicht, mathematisch talent, die de hoofdstukken van hydrodynamica, geluid, warmte en thermoelectriciteit verrijkt heeft, die het verband tusschen aether en stof, afgespiegeld in de feiten der dispersie, absorptie, straling in en buiten het magneetveld heeft opgehelderd, den baanbreker, die in de lijn van Faraday en Maxwell, de leer van het licht en de electrodynamica van bewegende lichamen in één geheel

Lorentz at the turn of the century


There was official recognition as well. By Royal Decree of December 5, Lorentz was awarded the Orde van de Nederlandse Leeuw, a very prestigious decoration, and the first one in an array of Dutch royal decorations he was to receive during his lifetime. The general public was able to share in the celebration as well, as art­ icles appeared in several newspapers. The daily Algemeen Handelsblad, in its issue of December 12, described it as a “particular, significant tribute” and praised the “very high vantage point that Professor Lorentz occupies in the scientific world.”31 The newspaper even listed all the authors of the celebratory volume by name and ordered by country. For Lorentz the anniversary was once again an opportunity to look back on his chosen profession, this time not publicly, but privately, and consider how he had experienced the past twenty-­five years. To Voigt he wrote: With intense gratitude I now think back on the years that have passed. I was allowed to work under exceptionally favorable circumstances, in a period during which our science flourished, while a profusion of new, exciting thoughts germinated, and while a vivid scientific exchange kept awake the conviction among every investigator that all of us together are working on a great and beautiful task. Then our work becomes a joy and a delight.32

The sentiment that Lorentz expressed here, he would repeat in similar words in 1925, on the occasion of the fiftieth anniversary of his doctorate. This celebration of Lorentz’s golden doctorate will be discussed further in Chapter 12. Meanwhile, in the new century, life went on and it was business as usual. Lorentz’s workload increased steadily and his correspondence grew accordingly. From the period between 1900 and 1910 more than 1,200 letters to and from Lorentz have been preserved. More and more often it took him months to find the time to reply to letters he had received, and his replies often contained a standard sentence to apologize for the delayed response. Apart from his substantial teaching load, Lorentz spent his time producing an impressive series of scientific articles and also taking on all sorts of extra obligations. For example, he accepted the invitation to write two extensive articles for the Encyklopädie der mathematischen Wissenschaften, an ambitious enterprise to

heeft samengevat, den leermeester, onovertroffen in de heldere voorstelling en onvermoeid in ’t verleenen van hulp, wordt deze bundel opgedragen door vakgenooten, vrienden, leerlingen.” 31   “eigenaardige, veelbeteekenende hulde,” “zeer hooge standpunt, dat Prof. Lorentz in de wetenschappelijke wereld inneemt.” 32   “Mit inniger Dankbarkeit denke ich jetzt an die verflossenen Jahre zurück. Es war mir vergönnt, unter seltsam günstigen Umständen zu arbeiten, in einer Blütheperiode unserer Wissenschaft, wo eine Fülle neuer anregender Gedanken emporkeimte, und wo ein lebhafter wissenschaftlicher Verkehr die Ueberzeugung bei jedem Forscher wach erhielt, dass wir alle zusammen an einer grossen und schönen Aufgabe arbeiten. Da wird die Arbeit zur Lust und Freude.” Lorentz to Woldemar Voigt, December 26, 1900 (Kox 2008, 80).


The new century: Prospect and retrospect

summarize the state of the mathematical sciences in the broadest sense of the word in a series of review articles.33 Lorentz’s progress in the work for this encyclopedia is well documented in his correspondence with Arnold Sommerfeld, the editor of the volumes about physics. A charming detail is Sommerfeld’s suggestion to Lorentz not to send the manu­script to him in Aachen, across the Dutch border in Germany, but instead to send the package by General Delivery to the town of Vaals, just on the Dutch side of the border. Sommerfeld could easily ride his bike across the border to pick up the mail at the Vaals post office. That way Lorentz would not need to spend his hard-­earned Nobel Prize cash to buy stamps.34 Another sizeable project Lorentz embarked on was the publication of his own collected papers, in two volumes. The contract with the publisher dates from 1900,35 but the first volume—in two parts—did not appear until 1906 and 1907.36 No wonder, since all twenty-­one chapters were either reworked versions of articles that had already been published, or work that had never been published before. Not surprisingly, Lorentz never got around to publishing a second volume. Lorentz also continued his public lectures for general audiences. On February 4, 1900, Lorentz began a series of six weekly lectures entitled “Zichtbare en onzichtbare bewegingen” (Visible and Invisible Motions), under the auspices of the Maatschappij tot Nut van ’t Algemeen, the Society for Public Welfare discussed earlier in Chapter 1. The lectures were a huge success. They were published in a booklet37 and even translated into German, Russian, and Polish. In the meantime, Lorentz also found the time to give a presentation about gravity to the Physics Society of Rotterdam, the Natuurkundig Genootschap.38 In all, it appears that Lorentz spent almost all his time working, even when he went on vacation with his family.39 In the following chapter it will become even clearer that this growing number of commitments was getting on top of him, and that Lorentz, as he grew older and his international reputation reached its pinnacle, found it harder and harder to continue to meet all his obligations.

33   Besides mathematics, the papers covered physics, mechanics, geophysics, and astronomy. In the period between 1898 and 1935, twenty-­three thick volumes appeared eventually, three of which were devoted to physics. Lorentz wrote about the theory of Maxwell (Lorentz 1904e) and the electron theory (Lorentz 1904f  ). In addition, he commented on articles by others. 34 35   See Sommerfeld to Lorentz, January 6, 1903 (Kox 2008, 99).   See LA 205. 36   Lorentz 1906g and Lorentz 1907i. 37   Lorentz 1901d. Even now, the lectures are still very informative and clear. 38  See Haagsche Courant, March 13, 1901. 39   See, for example, Lorentz to Woldemar Voigt, July 20, 1899 (Kox 2008, 62), a letter on scientific matters that he wrote from Noordwijkerhout, a village on the North Sea not far from Leiden, where the family spent a few summers in a rented house. This was not the only scientific correspondence he carried on from his vacation home.

Chapter 6 Nobel Prize and international recognition

On the morning of November 12, 1902, Lorentz received a telegram from Stockholm reading: “nobel prize awarded to you and zeeman request secrecy ­further information by letter.”1 Lorentz’s friend and colleague Pieter Zeeman, professor of physics in Amsterdam, received a similar telegram.2 The telegram was signed by Per Olov Aurivillius, Secretary of the Swedish Academy of Sciences and Director of the Museum of Natural History in Stockholm. The most striking aspect of the telegram is the request for secrecy. Contrary to the present-­day procedure of first notifying the Nobel laureate by telephone and immediately afterwards informing the entire world press, it was customary in the first years of the Nobel Prize that the names of the laureates were not announced until December 9, the day before the official award ceremony. In the case of Lorentz and Zeeman, this custom caused a certain amount of concern among the two laureates, as will be seen in what follows. The Nobel Prize requires little explanation nowadays, as this most prestigious prize in the sciences is also by far the most well known. In 1902, only the second year the prizes were awarded, the situation was still quite different. The prize was not yet as famous among the general public as it is nowadays, but it appears— judging from the newspaper reports back in the day—that it was already con­ sidered an awe-­inspiring achievement. There are several reasons for this. To begin with, the prize was awarded by a respected foreign institution, the Swedish Academy of Sciences. Also, science and the professors who represented it were still held in high esteem at the time. Perhaps the most important reason, though, was the large amount of money connected to the prize: this especially captured the imagination.3

  “nobelpreis für physik ihnen und zeeman zuerkannt bitte geheim zu halten näheres brieflich.”   The actual telegram sent to Lorentz is not part of the Lorentz Archive, but it was obviously almost identical to the telegram received by Zeeman (ZA 855). 3   In her history of first years of the Nobel prizes, Crawford argues that the science community took longer to begin to appreciate the prizes than the general public. See Crawford 1984, chap. 7. 1 2

“A Living Work of Art”: The Life and Science of Hendrik Antoon Lorentz. A. J. Kox and H. F. Schatz, Oxford University Press (2021). © A. J. Kox and H. F. Schatz. DOI: 10.1093/oso/9780198870500.003.0007


Nobel Prize and international recognition

From notes that have been preserved, it is known that Lorentz and Zeeman each received an amount of almost 50,000 Dutch guilders.4 The true magnitude of this amount can be gauged by comparison with the annual salaries of the two laureates. As special professor, Zeeman earned 4,000 guilders a year, while Lorentz received the maximum salary for an ordinary professor, amounting to 6,000 guilders. It should also be noted that professorial salaries were not exactly low in comparison to the average income in the Netherlands. In relative terms, the value of the Nobel Prize has devalued considerably since then, as the present-­day prize is worth a great deal less than in 1902. In 2016 it was equivalent to around 800,000 euros, while in comparison a professor’s annual salary in the Netherlands amounted to something in the region of 100,000 euros. It can safely be assumed that in Amsterdam, at the Zeeman home, the surprise about the prize must have been a good deal greater than it was for Lorentz. The decision was preceded by quite some back and forth, of which Lorentz was well aware, but about which Zeeman had no idea. In December 1901, not long after the presentation of the very first Nobel prizes, the Swedish mathematician Gösta Mittag-­Leffler had started a campaign to award the 1902 prize to Lorentz for his entire body of work on the theory of electromagnetism.5 To achieve this, he called on the world-­famous French mathematician Henri Poincaré to write a report on Lorentz’s work.6 Because Poincaré was also active in mathematical physics, he was able to provide a well-­rounded judgment of Lorentz’s merits. The report was finished in January of 1902, was signed by some other physicists and mathematicians, and served subsequently as the official nomination for the prize.7 In his report, Poincaré mainly described Lorentz’s work in the field of  electrodynamics—not surprisingly so, because he knew this work well.8 He emphasized that Lorentz’s work did not only offer an important clarification of the work by Maxwell, but that it also filled two important gaps in Maxwell’s the­ or­ies: the explanation of the phenomenon of aberration,9 and a satisfactory treatment of electromagnetic phenomena occurring in bodies in motion. Lorentz’s theory showed, according to Poincaré, that the failure of all experimental attempts to demonstrate the earth’s motion through the ether is not accidental, but based on a general principle. Poincaré also discussed the Zeeman effect, saying that

4  To be exact, Lorentz received 48,948.95 guilders (Stockholms Enskilda Bank to Lorentz, December 11, 1902, RB). Zeeman received 47,282.26 guilders (note in ZA 314). In Stockholm Lorentz had already received roughly 500 Swedish kronor in cash, hence the difference. 5  See Crawford 1984, chap. 5. 6  See Mittag-­ Leffler to Poincaré, draft, December 14, 1901 and Poincaré to Mittag-­ Leffler, December 19, 1901 (Poincaré 1999, 181 and 183). Mittag-­Leffler’s ulterior motive was to use the prize for Lorentz in clearing the way to award Poincaré the prize as well. 7  See Poincaré 2007, 62.7, for the text. Apart from Poincaré, the report was signed by thirteen other mathematicians and physicists, among whom were Max Planck, Wilhelm Röntgen, the 1901 physics laureate, and, of course, Mittag-­Leffler himself. 8   See also Chapters 4 and 8 for more about Lorentz’s work. 9   A difference between the direction in which a star is observed and the actual direction of that star, as a result of the relative motion of the observer and the star.

Nobel Prize and international recognition


Lorentz had predicted this effect and that Zeeman had subsequently confirmed his prediction. He further elaborated that, even though the discovery of the anomalous Zeeman effect appeared to put Lorentz’s theory in doubt, a simple adaptation of the theory helped to account also for the splitting of spectral lines in more than three components.10 In spite of the positive tone of the report, Mittag-­Leffler still had some misgivings, so he decided to involve Lorentz himself in the process. Nowadays, such a move would be viewed as extremely undesirable, but in the early days of the Nobel Prize it was apparently not completely clear what the rules were, or should be. Be this as it may, in a letter of April 25, 1902 Mittag-­Leffler approached Lorentz, asking him to forward a list of publications, offprints of his most im­port­ ant publications, and preferably also an annotated overview of his entire body of work.11 It is hard to believe that Lorentz did not have an inkling that something was being prepared in his honor. On April 30, Lorentz had already mailed off the list and the offprints, and in early July the annotated overview of his work was put in the mail as well.12 He had obviously taken the request very seriously, as he apologized for the length of the overview in the accompanying letter. He did so with good reason. The overview that Mittag-­Leffler received can no longer be found in any archive, but a draft version is preserved among Lorentz’s papers and, even though the last few pages are missing, this version counts no fewer than 112 pages.13 There is no further correspondence between Lorentz and Mittag-­Leffler until the day of the telegram. Immediately afterwards, in a letter of November 12, Mittag-­Leffler expressed his regret that he had not managed to ensure that Lorentz would receive the prize on his own. He called the award “only ­half-­satisfactory”14 and indignantly decried the lack of knowledge of theoretical physics among the members of the Nobel Committee, who “understand ­absolutely

10   The passage about the Zeeman effect has given rise to the curious misunderstanding that Poincaré believed that the anomalous effect had destroyed Lorentz’s theory and that, by 1902, the electron theory had become untenable. This misunderstanding was created by Elizabeth Crawford in her book about the first years of the Nobel prizes (Crawford 1984). She writes there that, according to Poincaré, the discovery of the anomalous effect had not confirmed Lorentz’s theory, but “had led to its ruin” (p. 138). She goes on to state, on p. 139, that Poincaré “tried to paper over these difficulties” by writing that, although the new phenomena were not predicted by the theory, they could nonetheless be explained by making changes to the original hypothesis. This interpretation misses the point of what Poincaré actually wrote: “On a pu croire un instant que ce déplacement des raies spectrales, que M. Lorentz avait prévu et qui avait semblé une confirmation éclatante de sa théorie, allait au contraire en déterminer la ruine. Ce que M. Lorentz avait annoncé en effet, c’était la formation de trois raies: au lieu de cela on voyait quatre ou davantage. Mais la théorie de Lorentz s’est au contraire très facilement adaptée à ces particularités qu’elle n’avait pas prévues. De légères modifications aux hypothèses ­pri­mi­tives en ont facilement rendu compte.” So Poincaré only mentions the possible destruction of the the­ory, which, as he stated, had been easily prevented by a simple adaptation of the theoretical basis. 11   Mittag-­Leffler to Lorentz, draft, April 25, 1902 (Institut Mittag-­Leffler, Djursholm, Sweden). 12  Lorentz to Mittag-­ Leffler, April 30 and July 12, 1902 (Institut Mittag-­ Leffler, Djursholm, Sweden). 13 14   LA 264.   “A moitié satisfaisant.”


Nobel Prize and international recognition

nothing of the theory and cannot even appreciate the value of ­mathematical-­physical investigations as deep as yours.”15 He added that Zeeman could very well have waited his turn to receive the prize in a later year. It is clear that the joint award with Zeeman represented a compromise reached at the last moment and that Mittag-­Leffler found it barely palatable. Now the question arises as to why the prize was not awarded to Lorentz alone and why Zeeman was included. There are several good reasons for doing so. Most importantly, Zeeman had made a significant discovery by observing a completely new physical phenomenon that would soon be named after him: the Zeeman effect. The link with Lorentz was that he had provided the theoretical explanation, immediately after Zeeman’s discovery of the effect. Zeeman’s discovery and the theories by Lorentz were already discussed in more detail in Chapter 4. Of course, Zeeman’s discovery was of great importance and besides, it should also be remembered that the statutes of the Nobel Foundation stipulated explicitly that the prize for physics should be awarded for the most important discovery or invention of the past year. In fact, it turned out that the stipulation “in the past year” was already disregarded almost immediately in the case of Zeeman, as he had already made his discovery in 1896. In later years, the Nobel Committee also stretched the concept “discovery” quite a bit further. Now, of course, Zeeman needed to have been nominated and, interestingly enough, a nomination had indeed been submitted by the physical chemist Svante Arrhenius, a member of the committee.16 It is unclear whether this nomination was written at the last minute. Whatever the situation may have been, Zeeman ended up being Lorentz’s fellow-­laureate, and the prize was awarded to the two physicists together “for their groundbreaking investigations of the influence of magnetism on radiation phenomena.”17 Lorentz’s reaction to the letter by Mittag-­Leffler was very gracious, as could be expected from him. About Zeeman he wrote: “Zeeman is a scholar to whom we may refer with pride as one of our compatriots and I have deep admiration for his great talents, his tireless efforts and the beauty of his discovery.” He was proud to share the prize with him, but the award was for him, above all, “a symbol of the close connection that always needs to exist between experimentalists and theoreticians, and has always existed between him and myself.”18 This remark was not a cliché, but completely in accordance with reality.

15   “[qui] ne comprennent absolument rien de la théorie et qui ne sachent pas même apprécier la valeur de recherches mathématico-­physiques de la profondeur des vôtres.” A few months earlier, in a letter to the French mathematician Paul Painlevé, Mittag-­Leffler had called the members of the Nobel Committee “nonentities” (“nullités;” see Mittag-­Leffler to Painlevé, August 29, 1902, Institut Mittag-­ Leffler, Djursholm, Sweden). 16  See Crawford et al. 1987, 22. 17   “[. . .] für ihre bahnbrechenden Arbeiten über den Zusammenhang zwischen den optischen und den elektromagnetischen Erscheinungen.” Nobel 1902. 18   “M. Zeeman est un savant que nous sommes fiers de compter parmi nos compatriotes, et j’admire profondément ses grands talents, son zèle infatigable et la beauté de sa découverte. [...] Je suis donc très

Nobel Prize and international recognition


As Lorentz and Zeeman lived in different cities, they carried on a very lively correspondence, and in their letters it is possible to follow in quite some detail how they made plans and preparations for the journey to Stockholm to receive the prize.19 After their exchange of congratulations—they were obviously not held back by the imposed secrecy, which they had taken quite seriously otherwise— they agreed to travel together, in the company of their wives. A journey like that was quite an enterprise in 1902 and needed to be prepared thoroughly. Interestingly enough, Lorentz now resolutely took the lead. He did present his decisions to Zeeman for consultation and approval, but he was clearly in charge. Nevertheless, he also expressed mixed feelings about the whole enterprise and he readily admitted that he anticipated the coming days in Stockholm with some degree of dread, writing “but we will have to muddle through.”20 To inform himself about the train connections, Lorentz ordered a European train atlas at his booksellers’ so he could reserve the correct train tickets well enough in advance, something that was necessary in those days. He also carefully consulted the ­ well-­ known Baedeker travel guide to find hotels during the journey and in Stockholm. Not the least of Lorentz’s and Zeeman’s worries was the public address each of the two laureates was obliged to make a day or two after the award ceremony.21 They had to carefully coordinate the content and, here too, Lorentz proceeded energetically to offer numerous suggestions. Zeeman’s somewhat passive attitude also must have had to do with some nasty health problems that had arisen during the month of November. Zeeman was unwell, recovered at first, but then fell ill again. At the end of the month, things really took a turn for the worse. Zeeman’s wife was suffering from bronchitis and she became so seriously ill that the doctor forbade her to travel. Not long afterwards, Zeeman himself contracted influenza. He, too, was unable to travel. His journey was cancelled, as was his hotel reservation, and Lorentz and his wife traveled to Stockholm by themselves, without the Zeeman couple. Before leaving, Lorentz wrote notes to his closest colleagues, to be delivered on Tuesday, December 9, the day before the award ceremony, so that they would not have to learn the good news from the newspaper. On the same day, Lorentz’s daughters would inform his mother-­in-­law. Unfortunately, that was already too late: The Amsterdam newspaper Algemeen Handelsblad had already proudly carried the news in its evening edition of December 8. Lorentz, meanwhile, tried to keep his fellow-­laureate abreast of his experiences in Sweden. In a note from Stockholm, two days after the ceremony, Lorentz described some of his adventures and, in another letter, Aletta wrote to the ­children

heureux de partager le prix avec lui, partage qui est pour moi le symbole du lien étroit qui doit toujours exister entre les expérimentateurs et les théoréticiens et qui a toujours existé entre lui et moi.” Lorentz to Mittag-­Leffler, November 15, 1902 (Institut Mittag-­Leffler, Djursholm, Sweden). 19 20 21

  The letters have been published in Kox 2018.   “we zullen ons er doorheen moeten slaan.”  See Lorentz 1905v for Lorentz’s Nobel address.


Nobel Prize and international recognition

how things went in Stockholm.22 She recounted how the days were full of activity with only a few hours of free time. Together with the other laureates, they visited the Swedish King Oscar II. “The King was quite easygoing and showed the gentle­men around a few rooms, pointing out to them all his beautiful baubles and trinkets, but he did not discuss physics.”23 The couple stayed in the Grand Hȏtel, which she found very luxurious: “We have hot water in our washbasins and we only need to flick a switch to turn on all manner of lights, near the bed, the desk, the washbasin, or the big chandelier.”24 She also noted that “people eat all the time here, at all hours of the day, and in the morning they already start with steak, chops, etc. etc.”25 After the award ceremony, there was a stately supper in the Grand Hȏtel and the next day Lorentz gave his Nobel address. He was also expected to propose toasts on several occasions, once on behalf of Zeeman. Visits to colleagues were also part of the busy schedule. On December 22, Lorentz and his wife returned to Leiden, tired but satisfied. Two days later, he went to Bank Lippman en Rosenthal in Amsterdam to collect the prize money. For Lorentz this marked the end of a few stressful weeks. In the spring of 1903, Zeeman finally made the journey to Stockholm to meet his obligation to the Nobel Committee and make a public address. Unfortunately, he had to travel alone, as his wife had fallen ill again.

Reactions in the Netherlands In the Netherlands, the award of the Nobel Prize to Lorentz and Zeeman did not go unnoticed. Two days before Lorentz’s return, the daily newspaper De Telegraaf devoted a large section on the front page of its Sunday supplement to Lorentz and Zeeman. With glowing pride about the Nobel Prizes having been awarded to the two scientists, the newspaper characterized Lorentz as [a] hero of science, whose reputation shores up our nation’s honor and whose great merits in the peaceful struggle for the well-­being of mankind have recently been recognized and honored in the land of the midnight sun.26

  Aletta to the children, December 13, 1902 (LA 743).   “De koning was heel gemoedelijk en liet de heeren eenige kamers zien en wees hen al zijn mooie potjes en pannetjes aan, maar sprak niet over natuurkunde.” 24   “We hebben warm water in onze waschtafels en behoeven maar aan knopjes te draaien om allerlei lichtjes bij bed, op een schrijftafel bij de waschtafel of de groote kroon aan te steken.” 25   “Men eet hier altijd door op alle tijden van de dag en begint ’s morgens al met biefstuk, karbonade enz. enz.” 26   “[. . .] held der wetenschap, die de eer van ons vaderland hoog houdt en wiens groote verdiensten in den vreedzamen strijd voor ’t welzijn der menschheid onlangs in het land der middernachtszon werden erkend en gehuldigd.” 22 23

Growing recognition


The article briefly summarized the work by Lorentz and Zeeman and was accompanied by two short biographies and illustrated with two portrait drawings. It ended with this striking characterization of the two laureates: It goes without saying that Zeeman and Lorentz, both members of the Royal Academy of Sciences, have attracted public attention at the Academy’s meetings more than once, the former by his youthful appearance and the latter by his powerful bearded countenance.27

In spite of these laudatory reviews, Zeeman was not convinced that everybody in the Netherlands was equally proud of the Nobel laureates. After all, for him the story did not end there. He still had to fight the Dutch internal revenue service, which—to Zeeman’s great indignation—wanted to tax his prize money. This must have been a private initiative of the Amsterdam office, since Lorentz had not been taxed at all in Leiden. It was not until the fall of 1904, after an appeal to the tax inspector general, that the tax assessment was retracted.28 In a letter to Lorentz in 1904, Zeeman characterized this rogue action as one more proof that the authorities were not impressed by the award. He complained that the university curators in Amsterdam had not deigned to give any recognition to their success and brought his point home by drawing a comparison with the great enthusiasm in France when three French physicists, Pierre and Marie Curie and Henri Becquerel, were awarded the Nobel Prize in 1903.29 Be this as it may, Zeeman put the prize money to good use, spending part of it on building a country home in the village of Huis ter Heide, not very far from Utrecht.

Growing recognition Professionally, the new century had started propitiously for Lorentz. On the occasion of the twenty-­fifth anniversary of his doctorate, in 1900, he had received a royal decoration, as was mentioned in Chapter 5. The list of academies and other important scientific organizations that Lorentz joined continued to grow steadily after 1900.30 There were also more tangible tokens of recognition, like the honorary doctorates he received in Dublin (1902) and Aberdeen and Philadelphia (both in 1906).

27   “Zooals vanzelf spreekt, zijn Zeeman en Lorentz lid der Koninklijke Academie van Wetenschappen, in wier vergaderingen de eerste door zijn jeugdig uiterlijk, de laatste door zijn krachtigen gebaarden kop, meermalen de aandacht trokken van ‘t publiek.” 28  See Zeeman to Lorentz, September 5, 1904 (Kox  2018, 75). See also the correspondence of Zeeman and the Dutch internal revenue service in the Zeeman Archive (ZA 318). 29   Zeeman to Lorentz, draft, February 15, 1904 (Kox 2018, 72). 30   See LA 192 for a (probably incomplete) overview; see also Chapter 11.


Nobel Prize and international recognition

In April 1905 Lorentz made an important three-­day trip to Paris to attend the Easter meeting of the Société Française de Physique (French Physics Society). The trip was important, not so much because Lorentz gave a lecture at the meeting,31 but rather because for the first time he met many of his prominent French colleagues, with some of whom he had been in correspondence already. In a letter to Pieter Zeeman he mentions Henri Abraham, Marcel Brillouin, Pierre and Marie Curie, Paul Langevin, and Jean Perrin.32 He had lunch at the Curie home and visited several laboratories. Unfortunately, Henri Poincaré was out of town. Lorentz came back with “very pleasant memories.”33 There were more honors, too. At the end of November 1908, Lorentz and Aletta traveled to England to receive the Rumford Medal of the British Royal Society. Even today, this prestigious medal is still awarded every two years to a physicist who has distinguished himself in the field of heat and light. For Lorentz the citation was “On the ground of his investigations in optical and electrical science.”34 They first went to Cambridge, where they were the guests of J. J. Thomson, the physicist who discovered the electron. Aletta, as it turned out, got along famously with Mrs. Thomson and her twin sister, who happened to be visiting. “We discussed all kinds of intimate things, they are both women who keep busy and have a broad view of current affairs and give assistance wherever they can.”35 Having made the acquaintance of these modern and progressive women must have warmed Aletta’s feminist heart. The Lorentz couple also went to church during the visit, with mixed feelings it appears from Aletta’s description: “that is quite a physical exercise, now sitting down, then standing up, then turning left, so that Pa and I thought we were about to go marching, then all of us kneeling again, then again stand up, sit, turn, kneel, etc.”36 As was noted earlier, Lorentz and Aletta were not exactly churchgoing ­people, something that is also quite clear from this quote. Lord Rayleigh, the President of the Royal Society, conferred the medal to Lorentz on November 30, during the Society’s annual meeting in London. At the dinner that followed, Lorentz pronounced the first toast, of course after the customary toast to the King. In his short speech he expressed his admiration for the operation of the Royal Society as a completely independent organization and for  See Lorentz 1905h.   See Lorentz to Zeeman, May 3, 1905 (Kox 2018, 76). Henri Abraham was Chargé de Cours in Physics at the Sorbonne, Marcel Brillouin was professor of physics at the Collège de France, Pierre Curie was professor of physics at the Sorbonne, Marie Skłodowska Curie was laboratory chief in the laboratory of her husband, Paul Langevin was professor of physics at the Collège de France, and Jean Perrin was Chargé de Cours in Physical Chemistry at the Sorbonne. 33 34   “zeer aangename herinneringen.”   Nature 79 (1908): 15. 35   “Wij spraken over allerlei intieme zaken, het zijn allebei vrouwen die wat uitvoeren en een ruime blik hebben op de tegenwoordige toestanden en overal helpen zooveel zij kunnen.” Aletta to the children, November 30, 1911 (LA 743). 36   “dat is een hele lichaamsoefening, nu eens zitten dan staan dan linksom, zoodat Pa en ik dachten dat we gingen marcheeren dan weer knielden allemaal, dan weer staan, zitten, draaien, knielen enz.” 31 32

Job offers from abroad


its great achievements, mentioning in particular the intensive contacts between members of the Society and the Dutch seventeenth-­century scientist Christiaan Huygens.37

Socially oriented activities As far as Lorentz’s more socially oriented activities are concerned, little changed in the next few years. He did make an effort to achieve things he considered important, like the establishment of a public library in Leiden.38 As one of the initiators of a preparatory committee, he wrote an article in the daily Leidsch Dagblad to emphasize the importance of the reading room and generate financial support.39 Eventually, the initiative was successful. After its official establishment on January 3, 1909, the library opened its doors to the public in 1910.40 Lorentz also continued his series of public lectures for general audiences, albeit less frequently than before 1900. He delivered two lectures at Diligentia in The Hague, in addition to the earlier lecture series on visible and invisible motions that was mentioned in Chapter 5. Less public, but interesting, because it offers an insight into Lorentz’s political ideas, is his membership of the Vrijzinnig Democratische Bond (Liberal Democratic Union), a left-­leaning liberal political party that had detached itself in 1901 from the more conservative Liberale Unie (Liberal Union). In a letter of condolence to  Aletta, its chairman called Lorentz a “faithful party-­member” who took the liberal-­democratic principles very much to heart and who regularly attended meetings of the Unie.41

Job offers from abroad Because of his growing international fame, Lorentz also received three invitations for professorships at foreign universities. At least three, that is, for it is possible that he received other offers from other quarters that cannot be retrieved in the archive.

37  See Nature 79 (1908): 137. The Leiden newspaper Leidsch Dagblad covered Lorentz’s toast extensively in its edition of December 2. 38  There was a need in Leiden for a library that was accessible to everybody. The Volksleeszaal (People’s Reading Room) had been closed in 1908 and the library of the Maatschappij tot Nut van ’t Algemeen (see Chapter  1 for information on this organization) did not attract much interest (see Roozen 1983). 39  See Leidsch Dagblad, January 23, 1909. 40   It was named “Reuven’s Library” after Mrs. Reuven, the donor of two buildings and a yearly allowance. 41   R. Kranenburg to Aletta, February 8, 1928 (LA 674).


Nobel Prize and international recognition

The first one arrived just before he was awarded the Nobel Prize. On June 19, 1902, Otto Wiener, professor of physics at the University of Leipzig, wrote that a vacancy had opened up in theoretical physics because Ludwig Boltzmann had been appointed in Vienna and that the selection committee could not wish for a  better successor than Lorentz.42 Boltzmann had only spent a short while in Leipzig. He had arrived from Vienna in 1900, but he was unhappy in Leipzig, and when the possibility to return to Vienna presented itself, he seized the opportunity with both hands. In his letter Wiener asked whether Lorentz would, by any chance, be willing to come to Leipzig. He tried to make his offer more attractive by emphasizing the advantages of working at a large German university. He added that an institute for theoretical physics was being built that would provide ample facilities for the new professor to do experiments. Lorentz needed some time to think it over, but, in the end he declined the offer, although he added: “at your great university I could do perhaps do more useful work than here.”43 This may be a reference to the possibilities for experiments that were on offer in Leipzig, possibilities that Lorentz lacked so painfully in Leiden. In 1905 a new offer came along, and this time Lorentz reacted differently. In  January of that year, Wilhelm Conrad Röntgen, professor of physics at the University of Munich and the very first Nobel laureate in his field, traveled to Leiden with an attractive proposition.44 Lorentz was offered a professorship of theoretical physics in Munich, where he would be occupying the chair of—once again—Ludwig Boltzmann, which had been vacant since 1894. Actually, at Boltzmann’s recommendation, the Munich faculty had proposed Lorentz for the position immediately after his departure, but at the time this had not resulted in a formal invitation.45 This time it had. From the letter written by Röntgen after his return from Leiden, it can be concluded that the offer was discussed in detail with Lorentz.46 Lorentz announced the proposal and gave some clarification in the faculty meeting of January 29.47 The following day he wrote a letter to the faculty, putting in writing what had been   Otto Wiener to Lorentz, June 19, 1902. All letters from and to Wiener are in LA 89.   “Vielleicht an Ihrer grossen Universität nützlicher als hier wirken könnte.” Lorentz to Wiener, undated draft. The reply by Wiener is dated July 4, 1902. In December Wiener tried again (see Wiener to Lorentz, December 3, 1902), after two later candidates had declined the offer, but at that point the Nobel Prize put a spanner in the works. 44   The Lorentz Archive contains a copy in Lorentz’s handwriting of a letter to Röntgen from Anton von Wehner, the Bavarian minister charged with university matters, dated January 5, 1905 (LA 87). On behalf of the government, he requested that Röntgen travel to Leiden in order to ask Lorentz to accept an offer from Munich. Röntgen was authorized to offer Lorentz a salary of 12,000 German marks. Röntgen, son of a German father and a Dutch mother, had lived in the Netherlands until age sixteen and had an excellent command of spoken and written Dutch. 45  See Jungnickel and McCormmach 1986, 157–158, and Lorentz to Boltzmann, after December 12, 1895 (Kox 2008, 27), in which he thanks Boltzmann for his efforts. 46   Röntgen to Lorentz, January 30, 1905 (LA 65). 47   Minutes, faculty meeting, January 29, 1905 (Archief Faculteit Wis- en Natuurkunde, Universiteit Leiden). 42 43

Job offers from abroad


discussed at the meeting the day before.48 He made clear that it was time for a change in his teaching duties. Owing to his heavy teaching load, he had “repeatedly been forced to drop a research project or leave scientific research unfinished.”49 Lorentz was referring here in particular to his classes for first-­year medical students, which he had been teaching dutifully since 1883. These lectures and the accompanying laboratory practicum now required twelve to fifteen hours of work each week: quite a substantial teaching load. Between 1901 and 1904, Lorentz had been relieved for a while from teaching the medical students, as Lodewijk H. Siertsema had taken on the classes after his appointment as lector of physics. In 1904, though, Siertsema was appointed to a teaching position at the Polytechnic in Delft so, from then on, Lorentz was left to his own devices once again and had to resume the lectures. It was not surprising, then, that by 1905 he had really had quite enough and he was ready to use the Munich offer as leverage to put some pressure on the faculty. In his letter to the faculty, Lorentz stated that he would feel obliged to go to Munich if the circumstances under which he had to work in Leiden continued to differ from what had been offered to him in Munich. However, if two ­requirements—which he politely called “wishes”—were met, he would be willing to stay in Leiden. First of all, he had to be relieved of the lectures for medical students. Additionally, he wanted to have an assistant who could help him ­prepare his lectures in theoretical physics. He generously offered to continue to  teach the medical students until a permanent solution was found, but he did  insist on having the assistant at his disposal already in the current academic year. Not surprisingly, the faculty was in a panic. A letter to curators speaks of “a heavy blow, yes a true disaster” if Lorentz should leave.50 Curators sprang into action and were soon able to show results. They managed to convince the Education Minister of the seriousness of the situation and, as early as February 17, they were able to write to the faculty that the Minister had conceded to Lorentz’s wishes. In 1906 the faculty budget was to include an item for a third professor of physics, and the required assistant could be appointed immediately.51 The Chairman of the College of Curators, Cornelis Fock, apprized Lorentz of the minister’s decision in a personal letter.52 Nevertheless, he was not completely reassured that Lorentz would in fact decline the offer from Munich, as is clear from this personal appeal. “[. . .] I call out to you in all seriousness and with the

  Lorentz to Faculteit Wis- en Natuurkunde, draft, January 30, 1905 (LA 106).  “herhaaldelijk een wetenschappelijk onderzoek moest achterwege laten of onvoltooid moest laten rusten.” 50   “een zware slag, ja een ware ramp.” Faculty to Curators, March 5, 1906 (Archief Faculteit Wis- en Natuurkunde, Universiteit Leiden). 51   Curators to Faculty, February 17, 1906 (Archief Faculteit Wis- en Natuurkunde, Universiteit Leiden). 52   Cornelis Fock to Lorentz, February 17, 1905 (LA 23). 48 49


Nobel Prize and international recognition

utmost urgency: stay in Leiden and persevere as a Dutchman in the advancement of science with the great gifts of your mind and heart.”53 For Lorentz it was sufficient. On February 21 he wrote to the faculty that he had decided to stay in Leiden.54 He clearly did so with some regret, as it was not until six days later that he formally declined the Munich offer in a letter to Röntgen.55 He explained that the promised changes in his work situation had prompted his decision, but that he had found the decision difficult, because the “idea to go to Munich had much that attracted me.”56 The following month, the faculty—much relieved—invited Lorentz to dinner in restaurant In Den Vergulden Turk, a dinner, so they wrote, “which—in keeping with your spirit—will have to be of a simple and confidential nature.”57 The long-­awaited assistant was eventually appointed toward the end of the ­academic year, on April 1, 1906,58 but it would not be until December, well into the next academic year, that the third professor made his appearance. The new arrival was Johannes Kuenen, professor in Dundee since 1895, who had already spent time in the laboratory in Leiden after obtaining his doctorate there a few years earlier. Meanwhile, Lorentz’s material working environment also improved con­sid­er­ ably. After a renovation of the laboratory in 1906, Lorentz was given his own theoretical physics institute for the first time. It consisted of a small lecture room and two offices, one for Lorentz and one for his assistant.59 In light of these new developments, it is not surprising that, in 1906, Lorentz quickly declined the next job offer as well. He was invited to fill a very important vacancy in Vienna, once again a chair held by Ludwig Boltzmann, for the third time in a row. This time the chair had become vacant because of Boltzmann’s tragic suicide earlier that year. The invitation came in the form of a letter from Vienna, from Professor Franz Exner, inquiring on behalf of the faculty whether Lorentz was available.60 Ten days later Lorentz declined the offer.61 In his reply he wrote that his position in Leiden was now completely in accordance with his wishes and that it offered him

53  “[. . .] roep ik U met ernst en met de meesten aandrang toe: blijf in Leiden en volhard als Nederlander met uw groote gaven van geest en hart de wetenschap te bevorderen.” 54   Lorentz to Faculteit Wis- en Natuurkunde, February 21, 1905 (LA 106). 55   Lorentz to Röntgen, February 27, 1895, quoted in Wylick 1966, 124. 56   “[. . .] er was in het denkbeeld van naar München te gaan veel dat mij lief was.” 57   “die—overeenkomstig uw geest—een eenvoudig en vertrouwelijk karakter moet dragen.” Faculteit Wis- en Natuurkunde to Lorentz, March 14, 1905 (LA 106). The dinner was to take place on March 24. 58   The assistant was Hermann B. A. Bockwinkel, who had just completed his studies at the end of February (see Leidsch Dagblad February 25 and March 25, 1905). He obtained his doctorate under Lorentz’s supervision on March 21, 1907. 59  See Delft 2007, 355–356. 60   Franz Exner to Lorentz, December 4 and 20, 1906 (LA 23). 61  See Lorentz to Victor von Lang and Franz Exner, December 14, 1906 (Österreichische Nationalbibliothek, Vienna).

Lectures at Columbia University


such favorable circumstances to do his research that there was no question of his exchanging Leiden for Vienna, even though he had found the offer very flattering. Under different circumstances he would have considered it a wonderful op­por­ tun­ity to continue the work of Boltzmann to the best of his ability.

Lectures at Columbia University Lorentz’s growing fame also resulted in an increasing number of invitations to lecture and teach abroad. A special invitation arrived early in 1905.62 He was asked to give a series of lectures in New York City the following year, at Columbia University. Lorentz was the second foreign guest to be invited: only the Norwegian physicist Vilhelm Bjerknes had preceded him, in December 1905.63 For travel and daily expenses Lorentz was to receive 1,200 dollars,64 equivalent to approximately 3,000 Dutch guilders. It was certainly a tidy sum, amounting to half of his yearly salary as a professor. From correspondence with William Hallock, Dean of the Columbia Physics Department,65 and from announcements in the university daily the Columbia Spectator, it is known that Lorentz began lecturing on March 23 and gave his tenth and final lecture on April 27. The question of what moved Lorentz to accept the invitation to Columbia University is justified. In those years, the United States did not play a role of much importance in physics,66 and even though Columbia was one of the better universities in the United States, it was not very likely that Lorentz would be finding much inspiration in the contacts with his American colleagues. Nonetheless, he undertook the journey, perhaps attracted by the opportunity to explain his theory of electrons to a new audience, as well as the chance to travel to America. A journey to this faraway country, relatively unknown to most Dutchmen, was no trifle in those days, and for Lorentz it must have been quite an expedition. Whatever his reasons, it was an adventurous decision that Lorentz would not regret. On March 10, 1906, Lorentz set sail from Rotterdam on the steamship Statendam,67 together with twenty-­year-­old Berta, arriving in NewYork on March 21.

62  In a letter to Lorentz of February 13, 1905 (LA 62) Michael Pupin, professor at Columbia University, announced that Nicholas Butler, President of the University, would send him an invitation soon. Lorentz’s acceptance of the invitation was announced officially at a meeting of the Trustees of Columbia University on April 3, 1905 (see Columbia Spectator, April 4, 1905). 63   Later visitors were Max Planck (1909) and Wilhelm Wien (1913). 64   The amount is mentioned in the invitation letter to Albert Einstein to come to Columbia in the academic year 1912–1913. The letter says that this amount was also paid to previous guest speakers and that it is “somewhat more than sufficient to defray the expenses of the eight or ten weeks visit to America.” George Pegram to Einstein, January 9, 1912 (CPAE-­5, 337). Einstein, by the way, did not accept the invitation. 65   The letters are preserved in the Columbia University Archives. 66   See for example Kevles 1978, chap. 6. 67   See the list of passengers of the outbound journey in Het Nieuws van den Dag of March 15, 1906; they sailed on May 2 for the return journey, also on the Statendam, and arrived in Rotterdam on


Nobel Prize and international recognition

Berta accompanied her father instead of Aletta, who had health problems, according to her daughter. Clearly, the need to care for the two younger children, who were still of school age, must also have contributed to Aletta’s reluctance to come with her husband on the long trip. Berta and Lorentz did not arrive back in Leiden until mid-­May, and a two-­month absence may have been longer than Aletta was willing to stay away from home. There is no written account by Lorentz himself of his experiences during the visit and the preceding journey. For more details about their stay the only source is the volume of Berta’s reminiscences—written in English—that was published in 1957. This clearly shows how exciting all the experiences and impressions were for the two travelers. According to her reports, the trip was indeed a great adventure for both father and daughter.68 The excitement already began on their departure, so Berta writes, when a group of students showed up for a farewell for the long and unusual journey into unknown territory. The fifty-­two-­year-­old Lorentz showed “a degree of enthusiasm which one would expect to see in a young man.” Most of the fellow passengers on the ship were “young people who were going to the United States to try their luck [. . .] Some of them went with the assurance of finding kind help and support in the new country, others with the knowledge that they would have to fight their own way through. All of them were looking forward, with hope and expectation, to life in the new world.” Lorentz and Berta were put up in a three-­room apartment on 118th Street West, “without cooking, only water.” Safe drinking water in New York was a problem in those days. In restaurants, patrons could be seen dissolving disinfectant tablets in the water they were being served and the Lorentzes were warned immediately to drink only water that had been boiled. They faithfully followed this advice and, as a result, were lucky enough not to fall ill, even for one day, “something, our hosts told us, that had never happened to other European guests.” Lorentz thoroughly enjoyed the stay and for Berta this made it doubly pleasant. We both had the feeling that we were on holiday, although my father had a great deal of work to do. [. . .] We certainly enjoyed everything and had many a good laugh. Our lighter moods were most evident when we were on trips away from New York and during such hours as my father could devote to me.

Lorentz was physically strong and was equal to the task of having to lecture in a foreign language, two hours at a time, on two successive days. There was a great deal of interest in his lectures and, according to Berta, some students traveled eight hours back and forth to New York each week in order to attend.

May 12 (see Lorentz to William Hallock, February 16, 1906 [Columbia University Archives] for the ­departure date and Dutch national newspapers for the arrival).  See Haas-­Lorentz 1957, 91–96. All quotes in the following paragraphs are taken from this booklet.


Lectures at Columbia University


Lorentz also became acquainted with the most prominent American physicists, and he was continuously peppered with physics questions and problems. Sometimes this caused precarious situations: In case my father could not find the time to receive those who laid siege to him, they would find some way of accompanying him while he was walking from one building to another; and would then pose their questions to him. I have often been afraid by the danger to which those “shop talking” physicists exposed themselves in the midst of the crowded New York streets.

Traffic in New York was also a source of great wonder and sometimes of considerable anxiety: [. . .] no automobiles at that time, only electric trams, one after the other, and then a never ending stream of horse drawn vehicles which filled the entire width of the streets. If the traffic was stopped, in order to give the pedestrians a chance to cross, these were forced to make their way across, literally underneath the horses’ heads.

An especially exciting event for father and daughter was a trip through the city in an automobile. Apparently, automobiles were still such a rare phenomenon in New York’s city traffic at the time that daughter Berta even forgot to mention them earlier on in her reminiscences. Back in the Netherlands, when Lorentz was asked if he had not been afraid, he answered, according to Berta: “[N]ot at all, I had the impression that the driver had better control of his automobile than the coachmen of their horses.” In her reminiscences Berta mentions their excursions to other cities only in passing. One of their most important trips was a visit to Philadelphia, where Lorentz, as one of the two representatives of the Dutch Academy of Sciences— the second one was the well-­known biologist Hugo de Vries—attended the celebration from 17 to 20 April of the 200th birthday of the famous American scientist Benjamin Franklin. On the occasion they were also both awarded an honorary doctorate (in jurisprudence!) from Pennsylvania State University.69 In addition, Lorentz gave an address to the American Philosophical Society, which was founded by Franklin and had participated in organizing the celebration.70

69  See Algemeen Handelsblad, April 21, 1906. The award ceremony for the honorary doctorates took place on April 19 (see C.  C.  Harrison to Lorentz, April 7, 1906, [LA 31], for the awarding of the ­honorary doctorate). Lorentz made further visits to Boston (see F. M. Gilley to Lorentz, April 3, 1906 [LA 26]), New Haven (Yale University; see H. A. Bumstead to Lorentz, January 3, 1907 [LA 13]), Washington, D.C., where he addressed the American Physical Society (probably on April 21) about Gibbs’s statistical mechanics (see C.  W.  Chamberlain to Lorentz, October 29, 1907 [LA 13] and S. B. Rosa to Lorentz, March 30, 1906 [LA 65]), Ithaca, NY (Cornell University; see E. Merritt to Lorentz, January 19, 1922 [LA 54]), and Baltimore (see Lorentz to W. Hallock, February 26, 1906 [Columbia University Archives]). 70   Lorentz 1906c.


Nobel Prize and international recognition

One day after the visit to Philadelphia, San Francisco was struck by the great earthquake that destroyed the better part of the city and caused disastrous fires and much loss of life. Curiously enough, Berta completely ignores this traumatic event in her written reminiscences, even though American newspapers were full of this calamity, which also received much attention in the Dutch press. Back in the Netherlands, Lorentz reported on the Franklin celebration in the Academy meeting of May 26. He recounted how the congratulatory message from the Academy was one of the few that was read aloud—there were no fewer than 250 messages—and he provoked some laughter when he said that the congratulations could be considered very successful, since the text was all in Dutch and the audience had listened with extreme care in order to catch at least some of its meaning.71 The lectures at Columbia University were published in a book, entitled The Theory of Electrons,72 and this may well be the most important permanent result of Lorentz’s stay in the United States. In some 300 pages the book provides an extremely clear overview for non-­specialists of the state of affairs in the theory of electrons. It also includes an appendix in which a long series of notes provides more in-­depth mathematical detail for a number of topics. The book appeared in  print in 1909 and was an immediate success. In 1916 a revised edition was published with additional notes, which is still available as a reprint today.73

Congress in Rome Three years after his visit to New York, Lorentz went on another journey, this time not quite as far-­flung, though it was still quite a distance in those days. He traveled to Rome together with Aletta, who was fortunately able to accompany him this time. He had been invited to speak at the Fourth International Congress of Mathematicians, which was held there from April 6 through April 11, 1908. It was an enormous conference with 535 participants from twenty-­two different countries.74 Lorentz’s presentation at this mathematical congress was purely physical in nature. On April 8 he addressed a plenary meeting on the topic of radiation the­ ory.75 Actually, he was not the only one who discussed a topic that could only with some effort be viewed as applied mathematics. The same applied to the papers about the motion of the Moon and about the northern lights. The content of Lorentz’s address, and the sharp criticism it encountered, will be discussed further in Chapter 8. Suffice it here to say that, by means of a mathematical tour de force, Lorentz succeeded in demonstrating unambiguously that

72  See Algemeen Handelsblad, May 26, 1906.   Published as Lorentz 1909i.   See also Kox 2005 for more about the importance of the book. 74   There were two other participants from the Netherlands: L.  E.  J.  Brouwer (Amsterdam) and W. A. Versluys (Delft). See also the conference proceedings Atti 1909. 75   Lorentz 1908b, Lorentz 1908e, Lorentz 1909g, Lorentz 1909h. 71 73

Congress in Rome


classical mechanics and classical electromagnetic theory inevitably led to a result—Jeans’s law of radiation—that turned out to be experimentally incorrect. Yet, Lorentz did not conclude from this that the classical theory needed to be revised. It is worthwhile to dwell some more here on the trip to Rome, because there are some amusing personal impressions by Lorentz and Aletta that have been preserved in two letters to their children.76 The two travelers came well prepared for the trip, having learned some Italian beforehand, which stood them in good stead, as Lorentz recounts to his children: The room is on the fourth floor, though, but there is an elevator and we now have the opportunity to practice in Italian with Enrico, the boy who operates the elevator, which works out pleasantly on both sides. Mother has been complimented by a ­similar youngster in Florence: “La signora parla ben l’Italiano.”77

Apparently, they had stopped over in Bologna and Florence on their way to Rome, as Lorentz remarked in his account of a trip to Tivoli, organized as part of the Congress, that the weather was fine, albeit not as beautiful as in Bologna and Florence. In Rome, they were very happy with their hotel, so Lorentz writes: “For 10 lire, so almost 5 guilders per person we have everything, and everything really good.”78 Aletta’s letter showed how impressed she was by the spaces where the conference was held: You should see how solemnly I am sitting here in the reception parlor for the ladies of the congress, a gigantic room with magnificent paintings and furnishings and endless adjoining libraries also leading to a great gallery along which all the meeting rooms are situated, and there is also a table full of delicacies. I am having a cup of tea and a glazed chestnut, which is delicious.79

Their stay was physically demanding, as “everywhere in the houses and in the streets there are stairs to climb.”80 Still, they crisscrossed the hilly city on foot. “If I had added up all that I have climbed here, I would have been on top of the

  Aletta to the children, April 10, 1908, and Lorentz to the children, April 11, 1908 (FC).   “Wel is de kamer op de vierde verdieping, maar er is een lift en wij hebben nu gelegenheid om met Enrico, den jongen die de lift bedient, in het Italiaansch te oefenen, wat van weerskanten heel genoegelijk gaat. Moe heeft van een dergelijk jongmensch in Florence eens een compliment gehad: ‘La signora parla ben l’Italiano.’ ” 78   “Voor 10 lire, dus bijna f 5 per persoon hebben we alles, en alles heel goed.” 79   “Jelui moest eens zien hoe plechtig ik hier zit in het ontvangsalon voor dames van het congres, een reusachtig vertrek met prachtige schilderijen en meubels, grenzend aan eindelooze boekenkamers en ook uitkomend op een groote omloop waarop ook de vergaderzalen zijn, er is ook een tafel met heer­lijkheden. Ik heb een kop thee en een geconfijte kastanje, die heerlijk is.” The plenary sessions took place in the magnificent Sala degli Orazi e Curiazi in the Palazzo dei Conservatori on Piazza del Campidoglio. 80   “het is hier overal in de huizen en in de straten trappen klimmen.” 76 77


Nobel Prize and international recognition

Pic du Midi by now,” Aletta wrote.81 Eventually, they had resorted to taking “horse-­drawn cabs, to spare the legs a bit.”82 The couple was thoroughly impressed by their visit to the Pantheon, with its “gigantic dome with an opening in the middle through which light, sun and rain come in.”83 In his letter, Lorentz also described a visit to what he called “Peter’s Church,” as well as an excursion to the Via Appia. That visit was somewhat disappointing, he recounts, “as the road is dreary because of all the ruins and in between one sees nothing but dismal taverns as soon as one reaches the outskirts of the city,” but “it does not matter, now that we have seen so much beauty.”84 On April 15 or 16, they hoped to be back in Leiden.

Visits to Göttingen and Paris In 1910 Lorentz traveled to the German city of Göttingen, on his own this time. The original idea had been that, after Berta, it was now the turn of t­ wenty-­one-­year old Hannie to accompany Lorentz, but at the last moment this plan fell through because she was ill.85 Again, Lorentz had accepted a prestigious invitation. In October of that year he would give the “Wolfskehl lectures” at the University of Göttingen.86 The lecture series was named for Paul Wolfskehl, the man who funded it. On his death in 1906, his testament provided for a prize of 100,000 German marks to be given to the scientist who would be able to prove a mathematical theorem that many generations of mathematicians had so far been unable to crack.87 Until the moment the prize could be awarded, the interest accrued on the prize money could be used by the Göttingen Academy of Sciences to promote the mathematical sciences. In 1908, an Academy committee decided to use part of the Wolfskehl proceeds for guest lecturers. The first lecturer to be invited, in 1909, was the famous French mathematician Henri Poincaré. Lorentz was the second guest professor. In later years, other luminaries, like Albert Einstein, Arnold Sommerfeld, Max Planck,

  “Als ik alles bij elkaar had wat ik hier al geklommen ben dan zat ik wel op de Pic du Midi.”   “om rijtuigjes te nemen om de beenen wat te sparen.” 83   “reusachtigen koepel met een open gat in het midden waardoor licht, zon en regen binnenkomt.” 84   “want de weg is triest wegens al de bouwvallen en daartusschen ziet men zoodra men een eindje buiten de stad is, niets dan ellendige herbergen, maar het hindert niet, nu wij zooveel moois gezien hebben.” 85   See Voigt to Lorentz, March 20, 1910 (Kox 2018, 199) and Lorentz to Voigt, April 10, October 6, and November 1, 1910 (Kox 2018, 200, 207, and 212). 86  Lorentz had been invited by the famous mathematician David Hilbert, who offered him an ­honor­ar­ium of 2,500 German marks (see Hilbert to Lorentz, April 27, 1910 [LA 33]; see also Lorentz’s reply to Hilbert of May 9, 1910 [Niedersächsische Staats- und Universitätsbibliothek, Göttingen], in which he accepts the invitation). 87   The theorem in question is Fermat’s Last Theorem, which was proven in 1995 by Andrew Wiles. He received the prize in 1997. 81 82

Visits to Göttingen and Paris


and Niels Bohr, also came to Göttingen to give these lectures. In all, Lorentz gave no fewer than six lectures in Göttingen in the five days between October 24 and 29, under the general title “Alte und neue Fragen der Physik” (Old and New Questions in Physics). In his lectures he successively explored ideas about the ether, the principle of relativity, and the theory of radiation. The lectures were written down by the Göttingen privatdozent Max Born and were subsequently published in the leading German journal Physikalische Zeitschrift.88 Born actually had mixed feelings about the material covered by Lorentz. In particular, Lorentz’s assertions about the ether could find no favor in the eyes of Born. He called them “absurd and reactionary.”89 In his lectures about the theory of relativity, Lorentz called it a matter of taste whether one chooses his theory of electrons, based on the existence of an ether, or Einstein’s theory of relativity that has no place for the ether. In Chapter 4 this question was already discussed in more detail. During his stay in Göttingen, Lorentz was the guest of Woldemar Voigt, with whom he had carried on a correspondence since 1883.They had meanwhile become friends. Lorentz had obviously had a good time at the Voigt home, since, on his return to Leiden, Lorentz wrote to thank him in a letter describing the “tender motherly and fatherly care” the Voigt family had shown him as their “eldest son.”90 In April 1913 Lorentz was in Göttingen again, this time together with Aletta, for a small conference financed by the Wolfskehl fund about kinetic theories for matter and electricity. On this occasion he was again invited to stay with the Voigt family, but he declined the offer. The plan was that Berta, her husband the physicist Wander de Haas, and their eighteen-­month-­old son Albert, who were living in Berlin, would come to Göttingen and that all of them would stay in the same hotel. Aletta would then be able, if necessary, to look after her grandson occasionally.91 A few months earlier, Lorentz had made a visit to Paris. At the end of November 1912, he had given the prestigious Michonis lectures at the Collège de France, a series of six lectures about statistical physics and thermodynamics. It is important to mention these lectures not only because the published version prompted Einstein’s admiration, but also because the booklet is still very readable and informative today.92 In February 1914 Lorentz was invited to Göttingen once again. The math­em­ at­ician David Hilbert, who also had an interest in physics, asked him to come and lecture for two months during the summer of 1914. He would be the first occupant of a newly established guest professorship.

89  See Lorentz 1910g.   “absurd und reaktionär.” Born 1969, 72.   “zärtlichen mütterlichen und väterlichen Sorge;” “ältesten Sohn.” Lorentz to Voigt, November 1, 1910 (Kox 2008, 212). 91  See Voigt to Lorentz, February 20, 1913 (Kox  2018, 249) and Lorentz to Woldemar Voigt, March 2, 1913 (Kox 2018, 250). 92   Lorentz 1916e. See Die Naturwissenschaften 4 (1916): 480–481 for Einstein’s laudatory review. 88 90


Nobel Prize and international recognition

Lorentz declined the invitation because he was too busy and would not have time to prepare properly. He would like to come at a later date, though: for example, in the summer of 1915, he wrote to Hilbert.93 No decision was reached and the start of World War One put a final stop to all such plans. The war also marked the end of Lorentz’s ties to Göttingen: He never visited the university again. What is more, he is not known to have even set foot in Germany again after the war.

93   See David Hilbert to Lorentz, January 22, 1920 (LA 33) and Lorentz to David Hilbert, February 1, 1914 (Niedersächsische Staats- und Universitätsbibliothek, Göttingen).

Chapter 7 Haarlem, Einstein, Ehrenfest, and Solvay

In the fall of 1909, Lorentz had an unexpected opportunity to move his career in  a  different direction. He was invited to head the Physisch Kabinet of Teylers Foundation in the town of Haarlem. This development requires some clarification.1 Teylers Foundation was established in 1778, based on a provision in the last will of the very wealthy merchant Pieter Teyler van der Hulst. He bequeathed the colossal sum of two million Dutch guilders2 for the management, after his death, of his valuable collection of coins, books, prints, and naturalia. He had appointed five of his friends, who were given the title of director, to oversee the execution of his testament and the establishment of the foundation, which, to this day, is still governed by a directorate consisting of five directors. The foundation’s objectives were “the cultivation of religion, the encouragement of arts and sciences, and the promotion of general welfare.”3 The will also provided for the establishment of two societies, the First or Religious Society and the Second Society, institutions that were to bring about these objectives in practice.4 In ­addition, a museum was founded to house the collection, which was named Teylers Museum. The city of Haarlem, twenty kilometers due west of Amsterdam, was granted city rights as early as 1245 and has a rich history. As was the case for many cities in Holland—the western part of the present-­day Netherlands—the seventeenth century was a very prosperous time for Haarlem. It was the third largest city in Holland, after Amsterdam and Leiden. The city’s central square, the Grote Markt, is dominated by the Gothic Great, or St. Bavo, Church, completed in 1520, and the monumental city hall with its seventeenth-­ century façade. Like Leiden,

1   Unless stated otherwise, the information about Lorentz’s appointment at Teylers Foundation that follows here is derived from Weiss 2013. 2   The amount is equivalent to eighty million euros in 2018. 3   “bevorderingen van Godsdienst, aenmoedigingen van Kunsten en Weetenschappen, en ’t nut van ’t algemeen.” 4   Since their establishment, the societies have occupied themselves primarily with the arrangement of competitions and the organization of lectures.

“A Living Work of Art”: The Life and Science of Hendrik Antoon Lorentz. A. J. Kox and H. F. Schatz, Oxford University Press (2021). © A. J. Kox and H. F. Schatz. DOI: 10.1093/oso/9780198870500.003.0008


Haarlem, Einstein, Ehrenfest, and Solvay

Haarlem had also experienced an economic downturn during the eighteenth century, and it was not until 1880 that a slow recovery had set in. By 1910, Haarlem’s economy had become more robust and the city had grown to 80,000 inhabitants from a low point of about 20,000 around 1800. A variety of industries, like printing and en­gin­eer­ing, provided jobs for the population and, besides, the city was the center for the trade in flower bulbs.

Lorentz and Teylers Lorentz’s involvement with Teylers Foundation began on the death of Elisa van der Ven, who, as curator, had been managing the collection of physical instruments in the Physisch Kabinet since 1878 and had been very actively involved in popularizing science. As they were looking for a successor, the directors invited Lorentz for a meeting, asking him to elaborate his views on the future of the Physisch Kabinet. They were clearly hoping that they might be able to tempt Lorentz to fill the newly vacant position himself. The background to their thinking was probably the wish that Teylers Museum would also begin to make a name for itself as a research institution, and this required the hiring of a prominent physicist. In early August 1909, Lorentz met with two of the Teylers Foundation’s directors. Their hopes became reality: From that moment on, Lorentz was the prospective successor of Van der Ven. That Lorentz shared the ambitions of the directors is already clear from his own account of the meeting, as he wrote that it would be his task at Teylers “to make it a place where work of some importance is carried out and from which emanates a certain degree of influence.”5 After the meeting in early August, a process of negotiations got underway, leading eventually to Lorentz’s appointment, on December 10, 1909, as curator of the physics laboratory of the Teylers Foundation.6 In addition—and this was stipulated by Lorentz—a conservator was appointed to run the laboratory on a day­to-­day basis. This position was given to Gerhard Joan Elias, who, incidentally, could not start the job until April 1910, because until that time he still held a pos­ ition in Berlin.7 Lorentz’s only obligations were to give lectures to general audiences and provide training courses for teachers at secondary schools. He did so with much enthusiasm.8 5   “tot een plaats te maken, waar werk van eenige beteekenis gedaan wordt en vanwaar een zekere invloed uitgaat.” H. A. Lorentz to J. Bosscha, August 17, 1909, quoted in Weiss 2013, 271, n. 131. 6   At some point, the notion was entertained that Lorentz would be given the title of “Professor at Teylers Foundation,” but the idea was rejected because the government might object to the use of the professorial title by an institution that was not a university (see Lorentz to Johannes D. van der Waals, November 12, 1909 [Kox 2018, 98]). Lorentz also became a member of Teylers Second Society. 7  Elias had obtained his doctorate cum laude in Utrecht, in 1909, on the basis of experimental research in the field of magneto-­optics. He worked in Berlin under the guidance of the Dutch professor Henri du Bois, who had been professor in Utrecht before his appointment in Berlin in 1904. 8   In the period 1910–1912, he gave thirteen lectures, some of them in several installments. Several of these lectures were also published.

Lorentz and Teylers


Both appointments went into effect on January 8, 1910. This did not mean, however, that Lorentz was leaving Leiden on that date to fill his new position. He had negotiated a three-­to-­four-­year transition period, during which time he would remain professor in Leiden while awaiting the right moment to make the transfer to Haarlem. After all, following his departure—and his succession—he wanted to retain a connection with Leiden as an extraordinary professor and, in order for this to happen, additional government funding was needed. Besides, he wanted to wait until there was enough financial security for his succession, as well as a sufficient number of suitable succession candidates to make a good choice. Financially, Lorentz certainly had no reason to complain: The directors of Teylers were def­in­ ite­ly not tight-­fisted. As long as he remained in Leiden, Lorentz was to receive a yearly salary of 2,000 Dutch guilders from Teylers, on top of his already substantial professor’s salary. His total yearly remuneration would then add up to no less than 8,000 guilders—a considerable sum in those days. After the transition to Teylers, once he would be appointed special professor in Leiden, Lorentz’s professor’s salary would inevitably be reduced. Even then, Teylers would supplement his remuneration to the total amount of 8,000 guilders. Considering that Lorentz’s Leiden salary would then be reduced to 3,000 guilders, this implied an increase in Teylers’ contribution of no less than 5,000 guilders.9 Apart from this generous salary, the directors made available a budget of 10,000 guilders for other expenses, like the purchase of instruments and the salaries for the conservator and an as­sist­ ant in the laboratory. Meanwhile, in November, Lorentz had presented his plans to the Leiden faculty10 and to a number of his colleagues, including Johannes  D.  van der Waals, Pieter Zeeman, and Herman Haga. He addressed Van der Waals mainly in his role as curator of Leiden University. Although he did not need official permission from curators, he did want to notify him personally. In his reply, Van der Waals’ reaction was favorable.11 On November 17, Lorentz visited Zeeman in Amsterdam. In connection with “the vacancy at Teylers Foundation,” he wanted Zeeman’s opinion about “some

9   In a letter to the curators of Leiden University, Lorentz asserted that, in 1912, he would have achieved forty years of service, since he had been appointed as a secondary school teacher at the Arnhem Burgeravondschool in 1872. On this basis he might be able to claim a salary of 3,000 guilders for his extraordinary professorship, as this was the amount of the pension he would be receiving after his sixty-­fifth birthday (see Lorentz to Directors Teyler, November 24, 1911 [AT 37] and Lorentz to Curators Leiden, undated draft [LA 107]). In the end, the minister did not want to go beyond 2,000 guilders, the usual salary of an extraordinary professor (see Lorentz to Director Thöne, December 6, 1911 [AT 37] and Royal Decree appointment to extraordinary professor, September 25, 1912 [copy in LA 108]). The contribution by Teyler was subsequently increased to 6,000 guilders, in conformity with the agreement to supplement to the total amount of 8,000 guilders. 10   Lorentz to Faculteit Wis- en Natuurkunde, draft, November 9, 1909 (LA 106). 11   See Lorentz to Johannes Diderik van der Waals, November 12, 1909 (Van der Waals Archive, NHA) and Van der Waals to Lorentz, November 20, 1909 (AT 183). In his letter to Van der Waals Lorentz announced that he had spoken earlier to President-­Curator Cornelis Fock. Haga’s reaction was also positive (see Herman Haga to Lorentz, November 22, 1909 [AT 183]).


Haarlem, Einstein, Ehrenfest, and Solvay

young physicists who have studied in Amsterdam.”12 After the visit he wrote about “the solution you have held out to me as previously possible, and which I have declared the most beautiful of all,” although “I hardly dare to imagine that this solution could still become reality today.”13 From Zeeman’s reply it becomes clear what this solution entailed. I must admit that the possibility to work together with you and then especially in a direction like the one that is now becoming a reality at Teylers, appears to me to be the very best I could imagine for a young physicist. Under the circumstances of a few years ago, I would not have hesitated for a moment and I would have accepted immediately if you had made me an offer of this kind.14

He added that he was unable to leave Amsterdam, because of his appointment, earlier that year, to become director of the Physics Laboratory and ordinary professor, as one of the two successors of Van der Waals.15 He was looking forward, though, to being able to maintain closer contact with Lorentz in the future, because Haarlem was so much closer to Amsterdam than Leiden. Obviously, the period of intensive experimental and theoretical contact in the years following the discovery of the Zeeman effect had created happy mem­or­ ies for both men.

The choice for Lorentz All this raises the question of why the directors approached Lorentz, of all people, since he was, first and foremost, a theoretical physicist. Another question is why he accepted the offer from Teylers so eagerly. An important factor appears to be  Lorentz’s dissatisfaction with his situation in Leiden. As was already noted here, after 1902 Lorentz had been approached several times for professorships abroad—Leipzig in 1902, Vienna in 1906, and Munich in 1905. After the Munich offer, his professorial teaching duties were reduced as a result of the appointment of Johannes Kuenen. Clearly, this had provided little real relief to Lorentz, according to a letter to Johannes Bosscha in which he unburdened himself:

12   “de vacature aan Teyler’s Stichting.” “eenige jonge physici die in Amsterdam gestudeerd hebben.” Lorentz to Zeeman, November 15, 1909 (ZA 108). 13   “de oplossing die gij mij als vroeger mogelijk hebt voorgespiegeld, en die ik voor de mooiste van alle verklaarde” hoewel “ik mij nauwelijks durf voor te stellen dat die oplossing nu nog verwezenlijkt zou kunnen worden.” Lorentz to Zeeman, November 18, 1909 (ZA 108). 14   “Ik moet bekennen dat de mogelijkheid om met U samen te werken en dan in het bijzonder in een richting als nu in Teylers zal verwezenlijkt worden, mij het allermooiste schijnt, wat ik mij voor een jong physicus kan denken. Onder de omstandigheden van enkele jaren geleden, zou ik dan ook, indien u aan mij een aanbod in bovengenoemde zin had gedaan, geen ogenblik hebben geaarzeld en onmiddellijk dat hebben aanvaard.” Zeeman to Lorentz, November 21, 1909 (AT 183). 15   The second one was Van der Waals’ son, Johannes Diderik Junior.

The choice for Lorentz


I may, I believe, view it this way, that only in the interest of the University, and not at all for me personally, a change has been achieved.16

In the same letter Lorentz also indicated what made the position in Haarlem so attractive to him: it is also tempting, before it becomes too late to do so, to take a somewhat different direction, in which I might be able to work more fruitfully than in [...] my current position.17

Lorentz is obviously referring here to his ambition to do experiments himself, something that had been virtually impossible for him since his cooperation with Zeeman, even after Kuenen had been appointed in 1906. It is true that more of Lorentz’s time had been freed up for experimental work since then because Kuenen had taken over the time-­consuming teaching of the medical students. Yet, Lorentz lacked the space for experiments in Leiden and it did not look as though that would change any time soon. In an extension of the Leiden laboratory, which was finished in 1907, a “room for experiments” was to be provided for Lorentz, but the process of setting up laboratory facilities was not moving anywhere. Even in 1913, long after Lorentz had departed to Haarlem, all kinds of equipment were still missing.18 The prospect of being able to experiment by himself and, more importantly, to do so independently, must have been irresistible to Lorentz. At the same time, he was aware that he was, first and foremost, a theoretician and, since he intended to remain one, he required the support of an able and experienced experimenter. Hence his wish to appoint an assistant, the conservator. The close cooperation between experimental and theoretical physics envisioned in Haarlem was im­port­ ant to Lorentz: He had already advocated this approach in his inaugural lecture in Leiden. Even though Elias’s appointment had already been formalized on April 8, 1910, it was not until September of that year that he actually started work. Until that time he stayed in Berlin to finish a series of experiments he had recently initiated there. In the meantime, large-­scale—and costly—remodeling and renovations had begun in the Haarlem laboratory, supervised by Lorentz, who traveled up and 16   “Ik mag het, dunkt mij, zoo beschouwen dat er alleen ten behoeve van de Universiteit en geens­ zins voor mij persoonlijk eene verandering is gekomen.” Lorentz to Bosscha, ca. August 17, 1909 (ATS 183). Bosscha was the Secretary of the Hollandsche Maatschappij der Wetenschappen, which trad­ ition­al­ly had close ties to Teylers Foundation. Probably, Bosscha had alerted Teylers’ directors to Lorentz’s dissatisfaction with his position (see Weiss 2013, 272). 17   “[...] ook is het verlokkend, vóór het daartoe te laat wordt, nog eens een ietwat andere richting in te slaan, waarin ik misschien met meer vrucht zou kunnen werkzaam zijn dan in [...] mijne tegenwoordige positie.” 18  See Delft  2007, 352–357. There Van Delft also refutes the reason given by Berta Lorentz for Lorentz’s departure for Haarlem, namely that two rooms that had been promised to him earlier were taken over by Kamerlingh Onnes at the last moment (see Haas-­Lorentz 1957, 97–98).


Haarlem, Einstein, Ehrenfest, and Solvay

down from Leiden regularly to oversee the work. In the fall, a proper beginning was finally made with the laboratory work: a series of magneto-­optical experiments.19 From a scientific point of view, the results of the Teylers laboratory were disappointing, despite the enthusiasm of all parties involved. There was not much ­continuity in the research, as numerous different conservators, all with different research interests, followed one another in rapid succession.20 At the same time, Lorentz was increasingly involved in his work on the Zuiderzee project, described in Chapter 10, and in the international scientific community (see the relevant later chapters). Especially after World War One, these activities increasingly kept him away from Teylers and left him with little time for oversight of his assistants or for his own experimental work. In a report to the directors of Teylers Foundation in 1926, Lorentz aptly summarized the less-­than-­stellar results of the laboratory with the dry qualification: “It didn’t go badly.”21

Lorentz and Einstein In February 1911, Lorentz had a meeting that would have a lasting influence on the rest of his life. He became personally acquainted with Albert Einstein. At the time, Einstein was a rising star in theoretical physics. He had made a great impression with a series of publications during what would later be termed his “miracle year,” 1905, followed by other important articles with regard to relativity theory and quantum theory. After having spent seven years in the ­ Swiss  patent office in Berne, he had become professor at the University of Zürich in 1909. At the invitation of the Leiden students he came to Leiden at the beginning of February to give a lecture. Even though it was, according to Einstein, “a curious undertaking for me to be carrying theoretical physics to Leiden,”22 he had been eager to accept the invitation because it offered him the opportunity to be introduced in person to Lorentz and Kamerlingh Onnes.23 Lorentz’s invitation to Einstein and his wife Mileva Marić to stay with the family made the visit even  more attractive.24 Actually, this invitation was quite unusual, certainly

19  See Weiss  2013, 276–278, and Hoorn  1998 for more details about the renovations and about Elias’s work. 20   Elias was succeeded by Wander de Haas in 1916, followed in 1918 by Jan Burgers, who left after less than a year to be succeeded by Balthasar van der Pol (1919–1922), Dirk Coster (1923–1924), and finally Adriaan Fokker (appointed in 1927). Haarlem not being a university town was a factor in their short tenures, as it created a certain sense of academic isolation for the conservators. 21   “Het is wel aardig gegaan.” Quoted in Weiss 2013, 286. 22   “ein kurioses Unterfangen für mich ist, theoretische Physik nach Leyden zu tragen”. Einstein to Lorentz, January 27, 1911 (Kox 2008, 218). 23   With the latter, Einstein had applied in vain for a position as an assistant in 1901. Kamerlingh Onnes had not even taken the trouble to respond (see Delft 2005, 316). 24   See Einstein to Lorentz, January 27, 1911 (Kox 2008, 218; CPAE-­5, 250), in which he thanked Lorentz for his invitation to stay in his house. He also wrote that he had accepted the invitation to give a lecture because this would enable him to meet Lorentz.

Lorentz and Einstein


c­ onsidering that the two scientists had only had written contact with one another until then.25 Einstein gave his lecture on Friday, February 10, and, according to the nationally circulating daily Algemeen Handelsblad, he had “an attentive audience that included numerous well-­known professors.”26 His lecture was apparently con­ sidered of such importance that both the local newspaper, Leidsch Dagblad, and the Algemeen Handelsblad carried an extensive summary the next day.27 The visit was a resounding success, judging by Einstein’s letter of thanks, in which he not only commends Lorentz on his hospitality, but also recalls the interesting discussions about physics with him, Kamerlingh Onnes, and Willem Keesom, the conservator who managed the Leiden laboratory.28 Besides Leiden, the Einstein couple had also visited Haarlem, where Lorentz had shown them Teylers Museum and the laboratory.29 Prior to their meeting, Lorentz and Einstein had already been in regular written contact for almost two years. The two men had started a correspondence in March of 1909, when Einstein sent Lorentz a copy of his recent article about radiation theory with a short cover note. Shortly thereafter he sent Lorentz a postcard to compliment him, having just read the paper on radiation theory that Lorentz had given in Rome in the spring of 1908.30 He wrote: “Reading your paper is an event for me.”31 After another month, a long and detailed letter by Lorentz followed, in reply to a letter by Einstein which has been lost. The subject was, once again, the theory of radiation and its relationship with the new quantum theory. The handwritten letter counted as many as twenty-­two pages and is one of the longest letters by Lorentz that is known.32 Einstein’s reply was almost equally elaborate: seventeen pages.33 These two letters are of extreme importance for the history of science, as they provide an insight into the thinking by two scholars from two totally different worlds. On the one hand, there is Lorentz, the classical physicist, describing his problems in reconciling the new quantum theory with observations—for example of the propagation of light—which could, until then, be accounted for very well 25   Max Planck and Woldemar Voigt had also stayed with Lorentz when they visited Leiden, in 1908 and 1909 respectively, but earlier they had already met Lorentz in person. See Planck to Lorentz, November 21, 1908 and Lorentz to Voigt, March 12, 1909 (Kox 2008, 178 and 181). 26  “een aandachtig gehoor, waaronder talrijke bekende hoogleeraren.” Algemeen Handelsblad, February 11, 1911. 27   The two (anonymous) summaries are identical. The Handelsblad also adds the title “Über das Boltzmannsche Prinzip und einige unmittelbar aus demselben fliessenden Folgerungen” (“About the Boltzmann Principle and some results immediately following from it”). Einstein is described as “the well-­known Professor of Theoretical Physics in Zürich” (“de bekende hoogleeerar in der theoretische natuurkunde te Zürich”). Einstein’s notes have been published in CPAE-­3, 19. 28 29   Einstein to Lorentz, February 15, 1911 (Kox 2008, 220).  See Weiss 2013, 288. 30   Einstein to Lorentz, March 30 and April 13, 1909 (Kox  2008, 183 and 185). The article was Einstein 1909. 31   “Die Lektüre Ihrer Abhandlung ist für mich ein Ereignis.” 32   Lorentz to Einstein, May 6, 1909 (Kox 2008, 189; CPAE-­5, 153). 33   Einstein to Lorentz, May 23, 1909 (Kox 2008, 190; CPAE-­5, 163).


Haarlem, Einstein, Ehrenfest, and Solvay

with the old, classical physics. On the other hand, there is the revolutionary Einstein, creator of relativity theory and one of the founders of quantum theory, doing his utmost to make the new ideas plausible.34 Einstein was clearly very pleased with Lorentz’s long reply: not only with its scholarly content, but, without a doubt, also with a personal remark at the end. Lorentz wrote there that he had long been an admirer of Einstein’s work and he expressed his satisfaction that he had now established personal contact with him. Einstein also expressed his feelings about Lorentz with abundant clarity in a letter written to a good friend, even before he replied to Lorentz himself. He wrote that he had carried on an extremely interesting correspondence with Lorentz about the problem of radiation, adding: “I admire this man like no other, I would like to say, I love him.”35 This quote is typical for the tone of Einstein’s a­ d­mir­ation— yes, even adoration—of Lorentz, not only as a physicist, but also as a human being. He would continue to express himself in such superlative terms about Lorentz on many different occasions throughout his life. The admiration was mutual. In spite of the age difference of twenty-­six years and a fundamentally different view of physics—Lorentz was never able to accept the theory of relativity completely, as is explained in Chapter 4—Lorentz admired Einstein for his cre­ ativ­ity and vision and Einstein respected Lorentz because of his fundamental ­contributions to physics and his extensive scientific knowledge.36 After Lorentz’s death in 1928, Paul Ehrenfest, Lorentz’s successor in Leiden, described a scientific discussion between Lorentz and Einstein that evoked a vivid image of the relationship between the two physicists. The conversation took place in the autumn of 1916, almost one year after Einstein had completed his General Theory of Relativity. An intensive written exchange about his theory had taken place between Lorentz, Einstein, and Ehrenfest in the early months of 1916, and Einstein wanted to continue the exchange in person, in Leiden. The war made this difficult. Einstein had been living in Berlin since the summer of 1914 as a remunerated member of the Prussian Academy of Science—the remuneration was an exceptional honor—as well as professor without teaching duties at the University of Berlin. A journey from warring Germany to the neutral Netherlands required special permission. In part because he had had an official invitation from Lorentz, Einstein eventually managed to come to the Netherlands for two weeks at the end of September. As Ehrenfest recalls: Already on the day of his arrival, he was with Lorentz in Haarlem. – Lorentz in his typical way had first taken care at dinner that Einstein felt enveloped by purely human sympathy in a good atmosphere of quiet and warmth. Without being rushed one was later conducted to Lorentz’s familiarly simple study. Having carefully

  Chapter 8 further elaborates the physics contained in the correspondence.   “Ich bewundere diesen Mann wie keinen andern, ich möchte sagen, ich liebe ihn.” Einstein to Jakob Laub, May 19, 1909 (CPAE-­5, 161). 36   See also Kox 1993a for an analysis of the relationship between Lorentz and Einstein. 34 35

Lorentz and Einstein


straightened the best easy chair next to the work table for the dear guest, to everyone’s impatience having first put him at ease by properly caring for the guest’s cigar, and only now Lorentz, calmly explaining, began to formulate a finely painted question about Einstein’s theory. Comfortably smoking in the easy chair, Einstein listened to the exposé, nodding happily, happily enjoying how Lorentz, in his mastery, had traced, while studying the articles, all those enormous difficulties, all of which Einstein had first had to bring under control, before he could, as he did in the publication, lead the reader more directly in a so much more effortless way to the goal. But, as Lorentz continued to speak on and on, Einstein’s cigar began to smoke slower and slower and his body became more tense in the easy chair. And when Lorentz was done, Einstein already sat bent over the sheet of paper on which, while he was speaking, Lorentz had accompanied his words with mathematical formulas. From the cigar no more smoke curled up and, while thinking, Einstein’s fingers twirled the locks over his right ear. Lorentz, however looked with a smile at Einstein, completely lost in thought. Really the way a father looks at a particularly dear son. Full of certain trust that the younger man will crack the nut, but still longing to know how. It took a while, but suddenly Einstein’s head flew up, he “had it!” Some more brief back and forth; interrupting each other, with parallel objections; solutions leaping up and happily understanding each other and with radiant eyes both of them now flew through all this delight of the new theory.37

A few days later Lorentz and Aletta went on a short vacation to hike on the heath in the Gooi area, south of Amsterdam, where Einstein came to visit for a day to take a hike with them. Of course, the men took the opportunity to have another discussion. In her travel diary Aletta describes vividly how Lorentz and Einstein totally forgot about their surroundings.

37   “Schon am Tag seiner Ankunft war er bei Lorentz in Haarlem. – Lorentz hatte erst in der ihm eigenthuemlichen Art bei Abendessen dafuer gesorgt dass Einstein sich durch eine wohlige Atmosfaere von Ruhe und warmer rein menschlicher Sympatie umhuellt fuehlte. Ohne Hast wurde man spaeter hinauf in Lorentzes traulich einfaches Arbeitszimmer gefuehrt. Sorgfaltig der beste Lehnstuhl fuer den lieben Gast neben dem grossen Arbeitstisch zurechtgerueckt, aller Ungeduld zur Beruhigung noch erst gut die Zigarre des Gastes versorgt und nun erst begann Lorentz ruhig exponierend eine fein geschilderte Frage die Einsteinsche Theorie betreffend zu formulieren. Behaglich im Lehnstuhl rauchend hoerte Einstein der Exposition zu, freudig nickend, freudig davon geniessend, wie Lorentz in seiner Meisterhaft beim Studium der Arbeiten alle die enormen Schwierigkeiten zurueckgefunden hatte die Einstein alle erst hatte bezwingen muessen, bevor er, wie er das in der Publikation that, den Leser mehr direkt auf soviel mueheloserem Weg zum Ziele fuehren konnte. Aber wie so Lorentz weiter und weiter sprach, da begann Einstein’s Zigarre allmaelich langsamer zu rauchen und sein Koerper richtete sich gespannter im Lehnstuhl auf. Und als Lorentz geendet hatte da sass Einstein schon gebeugt ueber dem Papierblatt auf das Lorentz waehrend des Sprechens seine Worte mit mathematischen Formeln begleitet hatte. Aus der Zigarre kam kein Rauch mehr und Einsteins Finger drehten nachdenklich die Haarlocken ueber seinem rechten Ohr. Lorentz aber sah laechelnd auf den voellig in Nachdenken verlorenen Einstein. Echt wie ein Vater auf einen ihm besonders lieben Sohn. Voll sicherem Vertrauen, dass der Juengere die Nuss knacken wird, aber doch begierig zu wissen, wie. Es dauerte eine Zeit, aber ploetzlich flog Einsteins Kopf freudig in die Hoehe, er ‘hatte es!’ Noch ein kurzes Hin und her; Ein einander in die Rede fallen, mit partiellem Widerspruch; sprunghafte Klaerung und freudiges einander Verstehen und mit strahlenden Augen durchflogen nun die Beiden all die Herrlichkeit der neuen Theorie.” Ehrenfest Archive (RB); spelling errors in the original.


Haarlem, Einstein, Ehrenfest, and Solvay

I went ahead to give the gentlemen the opportunity to talk at leisure about gravity, they would follow me, first it all went fine but once I looked back at a fork in the road, in order to wait for the gentlemen but I had lost my little sheep and I had to go back quite a ways. Finally, I found Einstein, Hendrik had gone scouting, they had been writing in the sand and that is why I got ahead of them.38

At times Lorentz was also somewhat surprised by Einstein’s eccentricity, in spite of his admiration and sympathy, as becomes clear from a letter to his son Rudolf, in which he reports on a meeting in Geneva. Between our two meetings I went to have coffee with Einstein in the Jardin Anglais, continuously talking about the Germans and the relativity theory. But he was quite sociable and dressed more like a normal human being than before. He told me quite normally that he would be visiting his first wife in Zürich after the meetings.39

The Solvay Conference and the Solvay Institute Later in 1911, Lorentz and Einstein met again, this time in Brussels. The occasion was a unique gathering where a number of leading physicists of the day, especially theoreticians, discussed urgent and often still obscure physics problems.40 This Conseil de Physique, held between October 30 and November 11, had been convened by the Belgian industrialist Ernest Solvay, who had amassed great wealth by developing a process to produce carbonate of soda.41 Solvay was very interested in the natural sciences and was quite willing to lend generous financial support to scientific endeavors.42 An added advantage of the Conseil was the chance it offered him to expound his own somewhat curious ideas about physics in front of an audience of top-­notch scientists.43 Solvay paid all participants a compensation for

38   “Ik liep vooruit om de heeren gelegenheid te geven rustig samen over de zwaartekracht te praten, zij zouden mij volgen, eerst ging alles goed maar eens keek ik bij een tweesprong om ten einde op de heeren te wachten maar ik had mijn schaapjes verloren en moest een heel eind terug. Eindelijk vond ik Einstein, Hendrik was op verkenning uit, zij hadden in het zand staan schrijven, zoo kwam ik hen vooruit” (LA 728). 39   “Tusschen onze twee vergaderingen in ging ik met Einstein in den Jardin Anglais koffiedrinken, altijd over de Duitschers en de relativiteitstheorie pratende. Maar hij was heel gezellig en meer als een gewoon mensch gekleed dan vroeger. Hij vertelde mij heel gewoon dat hij na afloop der vergaderingen zijne eerste vrouw in Zürich zou opzoeken.” Lorentz to Rudolf Lorentz, July 28, 1925 (LA 742). Lorentz was in Geneva for a session of the CICI (see Chapter 11). 40  Unless referenced otherwise, the information in the following paragraphs about the Brussels meeting (the Solvay Conference), its preparation, and its aftermath, as well as Lorentz’s role, is taken from Mehra 1975, Barkan 1999, and Berends 2015. See also the conference proceedings Solvay 1911 and the updated German translation published in 1914 (Solvay 1914). 41  See Coupain 2015 for more about Solvay and his scientific, industrial, and social activities. 42   He had already established a physiological institute in 1895 and a sociological institute in 1909. Both institutes were housed in the Brussels Parc Léopold. 43   He did so at the beginning of the Conseil. See Solvay  1911, 1–5, for his “gravito-­materialistic study” about the structure of matter, space, and energy. Solvay had also sent the participants a printed

The Solvay Conference and the Solvay Institute


travel and expenses of 1,000 Belgian francs, at the time amounting to approximately 500 guilders. Lorentz was quite taken with Solvay’s generosity toward the sciences and later called him “a noble man” and “an idealist who has the firm conviction that it is the progress of science, especially of natural science, that will make mankind happier.”44 The Conseil took place in the elegant Hotel Métropole, where eighteen participants, chaired by Lorentz, carried on scholarly discussions about topics like radiation theory, the quantum hypothesis, specific heat, and kinetic gas theory.45 Lorentz was quite impressed by the hotel’s luxury. In a letter to his daughter he told her that he had had a “royal” stay in a “magnificent room with toilet and bathroom.” Ernest Rutherford, a physicist from New Zealand who worked in England, had also gladly accepted the invitation: “Some wealthy man in Brussels pays a thousand francs each for our expenses. This is the sort of Congress I have no objection to attending.”46 Not everybody was so enthused about Solvay’s hospitality, though. Lorentz’s German colleague Arnold Sommerfeld wrote, ­ somewhat sourly, to his wife: “We are in a hotel of downright stupendous ostentatiousness. Everyone has bathroom, W.C.  of his own in his room. I bathe every morning. Also at all lunches, dinners etc. we are the guests of Solvay. A lunch of no fewer than 5 courses! Silly!”47 The meeting was organized at the instigation of the Berlin physical chemist Walther Nernst. He had become interested in the quantum hypothesis through his experimental research on specific heat, in particular at low temperatures, when specific heat showed behavior that could not be explained by the classical theory. Einstein, in an article in 1907, had drawn up a theory based on the quantum hypothesis that described this deviant behavior much better. Nernst was intrigued by this theory and had been very impressed by Einstein when he first met him in the spring of 1910. As early as the summer of 1910, he tried, through a mutual acquaintance,48 to contact Solvay, who was quite taken with the idea of financing a conference.

version of his theory. It appears that Solvay’s ideas were, for the most part, ignored by the participants. That Lorentz had serious misgivings about Solvay’s ideas becomes clear from a critical discussion of them in an undated draft letter to the Solvay family, written after Solvay’s death in 1922 (LA 73). 44   “een nobel man.” “een idealist, die het vaste vertrouwen heeft dat dat het de vooruitgang der wetenschap, in het bijzonder van de natuurwetenschap is, die de menschheid gelukkiger zal maken.” Lorentz to Berta, November 28, 1911 (FC). 45   The sessions were also attended by three scientific secretaries. The afternoon session of the third day took place in the Solvay Institute for Physiology, perhaps because the blackboard in the hotel ­meeting room was not large enough for the presentations being given that afternoon (see Lambert 2015, 2032). 46   Ernest Rutherford to Bertram Boltwood, October 21, 1911 (Badash 1969, 255). 47   “Wir sind in einem Hotel von geradezu stupender Protzigkeit. Jeder hat eigenes Bad, W.C. in seinem Zimmer. Ich bade jeden Morgen. Auch zu allen Lunches, Diners etz sind wir Gäste von Solvay. Ein Lunch nicht unter 5 Gängen! Blöde!” Arnold Sommerfeld to Johanna Sommerfeld, October 13, 1913, quoted in Eckert 2013. 48   Robert Goldschmidt, collaborator of Nernst and a friend of Solvay.


Haarlem, Einstein, Ehrenfest, and Solvay

Earlier that year, Nernst had already discussed his plan with Planck and Lorentz. Planck was hesitant and argued for a two-­year postponement, but Lorentz was all in favor. Nernst then proceeded to send Solvay a more detailed proposal with a list of topics and of scientists to be invited and suggested the spring of 1911 as a good time for the gathering. At the suggestion of Solvay, the end of October 1911 was eventually agreed upon. In his invitation letter—actually drafted by Nernst—Solvay listed the topics that could be discussed during the meeting. He mentioned new, important developments regarding the principles on which the molecular concept of matter is based.49 He also referred to problems in the field of radiation theory and the theory of specific heat and their possible solution by means of the new quantum hypothesis, and he expressed the hope that a written and oral exchange of ideas about these matters could create more clarity. A number of the invitees would be asked to write a paper about a specific topic. These papers would then be circulated among the participants in ­preparation for the conference. As has already been mentioned, Solvay also promised all participants a generous compensation for travel and expenses of 1,000 Belgian francs.50 In reply to Solvay, Lorentz thanked him for the invitation and expressed high hopes for what the Conseil could accomplish with regard to the important problems that were on the program: “[the meeting] will undoubtedly serve largely to elucidate them, to formulate the difficulties more precisely and to prepare the way to their solution.”51 Lorentz’s having been asked to chair the Conseil de Physique was not only evidence of his status in the scientific community, but also a reflection on him personally, as he was obviously considered to be eminently suited to leading the discussions.52 His knowledge of foreign languages and his tact were indispensable for this role, as not all French participants spoke German, for ­example, and not all Germans were proficient in English. In his opening address to the conference, Lorentz was more cautious than in his letter to Solvay. He did state that physics had reached a stalemate, “as the old theories have shown themselves more and more to be powerless in piercing the obscurity surrounding us on all sides.”53 He emphasized, however, that he was not

49  The invitation to Lorentz is dated June 15, 1911 (LA 73). See also CPAE-­5, 269, for the ­published version of the invitation to Einstein (dated July 9). 50   An invitation was extended to twenty-­three physicists, eighteen of whom were present in Brussels. Lorentz and Kamerlingh Onnes represented the Netherlands. Van der Waals was also invited, but declined. During the discussions, probably the most important part of the Conseil, careful notes were taken and, together with the French translations of the papers, the proofs were presented for correction to the discussion participants in December 1912. 51   “[la réunion] servira sans doute largement à les élucider, à préciser les difficultés et à préparer la voie à leur solution.” Lorentz to Solvay, July 3, 1911 (Archive Solvay Institute, Brussels). 52   Lorentz was actually not the first choice. Nernst had first suggested Lord Rayleigh and later Max Planck had also been mentioned, but Solvay opted for Lorentz. 53   “les anciennes théories s’étant montrées de plus en plus impuissantes à percer les ténèbres qui nous entourent de tous côtés.”

The Solvay Conference and the Solvay Institute


expecting any surprises and that the Conseil would probably not lead to any breakthroughs. Progress, according to Lorentz, would be more likely made by individuals than by a conference like this one, but the individual participants might well be inspired by the discussions. As chairman, Lorentz was commended highly by the conference participants. Prior to the meeting, Einstein had called it a Hexensabbat (witches’ sabbath), and in a letter to a friend he concluded afterwards that the gathering would have been “a delight for diabolic Jesuit priests.”54 For Lorentz in his role as chairman, he had nothing but praise. As he wrote to another friend, “H.A. Lorentz presided with incomparable tact and unbelievable virtuosity. He speaks all three languages equally well and has a uniquely keen scientific acumen.”55 In a later letter he even went so far as to characterize Lorentz as “a living work of art!”56 Even Kamerlingh Onnes, who already knew Lorentz very well, was duly impressed: It was a pleasure to see how all of them could not praise enough that extreme lucidity, ease and friendliness, with which he was able to lead people and managed to generate and maintain a pleasant, friendly and comfortable, yet serious tone, in spite of all the differences of opinion.57

Fourteen years later, on the fiftieth anniversary of Lorentz’s doctorate in December 1925, another participant, Marcel Brillouin, drew a vivid image of the meetings and of Lorentz’s leading role: [. . .] since it was to be feared that the small room in the Hotel Métropole would become a veritable Babel! But Mr. Lorentz followed everything, interrupted the speaker if his exposé, being somewhat difficult to follow, appeared to elude some of us and reproduced the essence, filtered by his clear intellect, in the two national languages of the other conference participants.58

54   “ein Delicium für diabolische Jesuitenpatres.” See Einstein to Michele Besso, October 21, 1911 (CPAE-­5, 296). 55   “H.A. Lorentz präsidierte mit unvergleichlichem Takt und unglaublicher Virtuosität. Er spricht alle drei Sprachen gleich gut und ist von einzigem wissenschaftlichem Scharfsinn.” Einstein to Heinrich Zangger, November 7, 1911 (CPAE-­5, 303). 56   “Ein lebendiges Kunstwerk!” Einstein to Heinrich Zangger, November 15, 1911 (CPAE-­5, 305). In a short speech at Lorentz’s funeral in February of 1928, Einstein used the same simile: “His life he has formed like an exquisite work of art, down to the smallest detail” (“Sein Leben hat er gestaltet wie ein köstliches Kunstwerk bis ins kleinste”) (Einstein 1934, 27). 57   “ ’t Was een genot te zien hoe niemand lof genoeg wist te vinden voor die buitengewone helderheid, gemakkelijkheid en vriendelijkheid, waarmee hij de mensen wist te leiden en bij alle meningsverschil een prettige, vriendschappelijke, genoeglijke en toch ernstige toon wist te doen ontstaan en te onderhouden.” Heike Kamerlingh Onnes to C.  A.  Crommelin, November 4, 1911, quoted in Delft 2005, 441. 58   “[. . .] comme il était à craindre que la petite salle de l’Hotel Métropole devînt une véritable Babel! Mais Mr. Lorentz suivait tout, arrêtait le conférencier quand son exposé, un peu difficile à saisir paraissait échapper à quelqu’un d’entre nous, et en reproduisait l’essentiel, filtré par sa claire intelligence, dans les deux langues nationales des autres congressistes.” Brillouin 1926, 31.


Haarlem, Einstein, Ehrenfest, and Solvay

A few weeks after the meeting Lorentz shared his impressions with his daughter Berta. He looked back on the meeting with satisfaction:59 On Sunday I arrived at half past six together with Onnes. [. . .] Reunion at half past eight and the following day at 10 o’clock the first conference. [. . .] We had in turn clarified our reports (beginning with my report about the old radiation theory) and then had a discussion about them. And lively too, in all (i.e. 3) languages, for the Germans and the English preferred to speak their own language. It was really quite exhilarating and everyone was satisfied, even though we have not resolved any questions. That cannot be done with twenty people, but all this does give much food for  thought. In the evenings, too, there was a great deal of argumentation. [. . .] Mrs. Curie was indefatigable and excellently informed about everything, Rutherford was delightful and joking, Wien very agreeable and friendly, like all of them, for that matter, and Einstein perceptive as ever. He does see the furthest of any of them. He participated much in the discussions and could squarely contradict someone in such an amiable fashion that one could never be upset about it. I had a magnificent room with toilet and bathroom, in the immediate proximity of the meeting room; [. . .] when there was a quarter of an hour’s break, I could retire to an easy chair to collect my thoughts. After my return I was almost sorry to have participated so much in the discussions, for I had to write down all those remarks and send them to the secretaries.60

The importance of the Solvay Conference The Conseil de Physique, which soon began to be called Solvay Conference, has taken on almost mythical proportions in the historical literature.61 There are a number of reasons for this great reputation. First of all, the informal and small-­scale form of the conference was a novelty. International conferences were thin on the ground in those days and, if they took

  Lorentz to Berta, November 28, 1911 (FC).   Zondags kwam ik er te half zeven met Onnes aan. [. . .] Te half negen reunie en den volgenden dag te 10 uur de eerste bijeenkomst. [. . .] Wij hebben op onze beurt onze rapporten toegelicht (te beginnen met het mijne over de oude stralingstheorie) en daar dan over gediscussieerd. En druk ook, in alle (nl.  3) talen, want de Duitschers en de Engelschen spraken toch het liefst hun eigen taal. Het was ­werkelijk heel opwekkend en ieder was er voldaan over, al hebben wij natuurlijk geen quaesties opgelost. Dat kan men niet met zijn twintigen, maar wel geeft dit alles veel te denken. Ook des avonds werd er nog heel wat geredeneerd. [. . .] Mevr. Curie was onvermoeid en van alles voortreffelijk op de hoogte, Rutherford genoegelijk en joking, Wien heel aangenaam en vriendschappelijk, zooals trouwens allen en Einstein scherpzinnig als altijd. Die ziet toch van allen het verst. Hij nam veel aan de discussies deel en kon daarbij iemand op zoo’n beminnelijke wijze vierkant tegenspreken dat men er nooit ontstemd over kon zijn. Ik had een prachtige kamer met toilet en badkamer, in de onmiddellijke nabijheid van het vergaderlokaal; [. . .] als er een pauze van een kwartier was, kon ik dus in een luie stoel mijn gedachten gaan verzamelen. Na mijn terugkomst had ik haast berouw zooveel aan de discussies deelgenomen te hebben, want ik moest al die opmerkingen nog opschrijven en aan de secretarissen zenden. 61   See, for example, Barkan 1999 and Schirrmacher 2015. The name Solvay Conference is in accordance with the usage in the international literature on the history of science. 59 60

The importance of the Solvay Conference


place at all, the number of participants was large. An example is the international physics conference in Paris in 1900, which counted as many as 836 participants from fifteen different countries and lasted for a whole week. Apart from these enormous international gatherings, there were regular and well-­attended national conferences, like the yearly meeting of scientists and physicians in Germany, the Naturforscherversammlung, or the Meetings of the British Association for the Advancement of Science. In the Netherlands, there was the bi-­yearly Natuur- en Geneeskundige Congres, the Dutch National Congress of Scientists and Physicians, but this type of meeting was seldom attended by foreign scientists. Furthermore, the impressive group of participants in the Conseil captured the imagination. Among the eighteen scientists present there, no fewer than nine were (future) Nobel laureates in physics. The official photo of the conference has become a much-­used symbol of the revolutionary developments in physics during the first decades of the twentieth century: a group of world-­famous physicists wrapped up in work to unveil the mysteries of quantum theory. Marie Curie and Henri Poincaré do not even have the time to pose for the photograph, but continue their discussion, while Jean Perrin is deeply concentrating on an article he is reading. There is, incidentally, something amusing about the picture. Solvay was absent when it was taken, so someone else had to take his place and, later on, through photomontage, a picture of Solvay’s head was substituted for that of the stand-­in. Unfortunately, Solvay’s head was too large, so the photo looks decidedly odd. The conference clearly satisfied a demand at the time, because of its novel form. The question remains, though, whether it was also successful in the sense of achieving concrete scientific results. Opinions differed in this respect. Einstein, for example, was not undividedly positive about the meeting’s outcome. In a letter to his friend Michele Besso he wrote: “The conference actually looked like a lament on the ruins of Jerusalem. Nothing positive was brought about.”62 For Einstein personally, the congress was a resounding success, as he became acquainted with all his prominent colleagues and made a great impression on all of them. Consequently, in his letter of thanks to Solvay he was very complimentary about the conference.63 Another slightly more charitable observation was made by one of the secretaries of the conference, Frederick Lindemann. He wrote to his father that, although the discussions were very interesting, “the result is that we seem to be getting deeper into the mire than ever. On every side there seem to be contradictions.”64 Bringing to light contradictions can, of course, also be viewed as the first step on the way toward a solution, and this seems to be the predominant opinion among most of the participants. In any event, there was at least one attendee who

62   “Der dortige Kongress sah überhaupt einer Wehklage auf den Trümmern Jerusalems ähnlich. Positives kam nicht zustande.” Einstein to Besso, December 12, 1911 (CPAE-­5, 331). 63  “Der Solvay-­Kongress wird stets eine der schönsten Erinnerungen meines Lebens bleiben.” Einstein to Solvay, November 22, 1911 (CPAE-­5, 312). 64   F. A. Lindemann to his father, November 4, 1911, quoted in Barkan 1999,192.


Haarlem, Einstein, Ehrenfest, and Solvay

put Lorentz’s words about inspired individual participants into practice: the French mathematician Henri Poincaré. He published an article a few months after the conference, in which he created clarity about the role of the quantum hypothesis in the theory of radiation. Be this as it may, the conference needed a follow-­up. On the last day, Solvay and eight other participants sat down together to make plans for the future. Lorentz was asked to write a proposal for the establishment of a foundation. The possibilities were probably discussed further during a visit by Solvay to Leiden, a week after the conference. On January 4, 1912, Lorentz sent off a first draft of a plan and on February 2, after the discussions with Solvay and one of his collaborators, he presented a definitive proposal for the statutes of the foundation that was to be established. On May 1 the Institut International de Physique Solvay was formally established, for an initial period of thirty years. Solvay put up the enormous initial capital of one million Belgian francs, which was invested in such a way that the foundation could dispose of a budget, each year, of about 55,000 Belgian francs.65 The foundation’s statutes stipulated that the institute’s mission was to stimulate research that would increase, and especially deepen, the knowledge of natural phenomena, particularly in the area of physics.66 This was to be achieved by ­offering scholarships and subsidies to scientists from Belgium and abroad in order to stimulate and promote their work, and by organizing further Solvay Conferences. Two commissions would administer the institute: the Commission administrative, bearing the financial responsibility, and the Commission scientifique, deciding on scientific matters, such as requests for subsidies and Solvay Conferences. Lorentz became the chairman of the scientific committee and remained so until his death. During his long tenure the committee would generate much work for Lorentz. In his lifetime, another four Solvay Conferences were held, all chaired by Lorentz, in 1913, 1921, 1924, and 1927.67 Lorentz’s commitment to the establishment of the Solvay Conferences and his hard work were instrumental in creating

65   Comparable to 4.7 million euros (rate of 2011; Berends 2015, 2098). The Lorentz Archive contains calculations and notes by Lorentz about the yearly return on the capital for different interest percentages and different times of operation of the foundation (LA 153). The money was invested with the insurance company Compagnie Belge d’Assurances Générales sur la Vie, which had committed to disburse an annuity of 55,332 Belgian francs for a period of thirty years. 66  Article 2 of the foundation’s statutes, as published in the proceedings of the second Solvay Conference (Solvay  1913), reads: “The objective of the Institute is to encourage research that, by nature, expands and especially deepens the knowledge of the natural phenomena in which Monsieur Solvay continues to have an interest. The Institute has as its main focus the advancement of Physics [. . .].” (“Le but de l’Institut est d’encourager des recherches qui soient de nature à étendre et surtout à approfondir la connaissance des phénomènes naturels, à laquelle M. Solvay ne cesse de s’intéresser. L’Institut a principalement en vue les progrès de la Physique [. . .].”) 67  See Solvay 1913, Solvay 1921, Solvay 1924, and Solvay 1927 for the proceedings. The topics were respectively: the structure of matter, atoms and electrons, electrical conductivity, and electrons and photons. See also Mehra 1975 for a historical overview of the meetings.

The Utrecht vacancy


the beginning of a long tradition. The Institut International de Physique Solvay exists to this day, and even now Solvay Conferences are still organized regularly.

The Utrecht vacancy Apart from scientific matters, Lorentz and Einstein also discussed a delicate question in Brussels, a question that had already been on their minds for a few weeks. At Utrecht University, the chair of mathematical physics had become vacant after the death of its occupant, Cornelis Harm Wind, on August 7, 1911. As early as August 20, the experimentalist Willem Henri Julius, the other professor of physics in Utrecht, wrote a confidential letter to Einstein, inquiring on behalf of the faculty whether he would accept a possible appointment in Utrecht.68 Einstein had not yet reached the pinnacle of his fame in 1911, but he already enjoyed a great reputation in scientific circles. Einstein, who had left Zurich in mid-­April for a professorship at the German University in Prague, turned down the idea. He was unwilling to change employers again so soon.69 He did take the opportunity to ask Julius’s opinion about a  scientific subject. It concerned the shift of lines in the solar spectrum, which he had predicted on the basis of his theory of relativity. Chapter 8 provides further details about this gravitational red shift. Einstein was hoping that Julius, who had been working on the physics of the solar atmosphere for some time, could provide a decisive answer about the possibility of observing this red shift. This is how they started a correspondence about solar physics—and about the succession question. A month after Einstein’s refusal, Julius wrote to him again.70 The faculty, not taking no for an answer and not taking Einstein’s arguments seriously, had decided to ask him again whether he was willing to accept an appointment. Now Einstein was beginning to doubt his earlier decision. In a letter to his Zürich friend Heinrich Zangger71 he called it an attractive thought to have Julius as his close colleague and to have Lorentz and Kamerlingh Onnes close by. Also, apparently, he had immediately been offered the full professor’s salary of 6,000 guilders, instead of the starting salary of 4,000 guilders. However, there turned out to be a complication, and that was the reason Einstein was writing to Zangger. He would prefer to return to Zürich; not to the University, but to the Eidgenössische Technische Hochschule (ETH), his alma mater. For quite some time, Zangger had been making strenuous efforts to have Einstein appointed at the ETH, and Einstein wanted to find out from him what the current situation

  Julius to Einstein, August 20, 1911 (CPAE-­5, 277).   Einstein to Julius, August 24, 1911 (CPAE-­5, 278). 70   Julius to Einstein, September 17, 1911 (CPAE-­5, 284). 71   Einstein to Heinrich Zangger, September 20, 1911 (CPAE-­5, 286). 68 69


Haarlem, Einstein, Ehrenfest, and Solvay

was.72 Whatever the case may be, he wrote, Utrecht or Zürich, he would be leaving behind halb barbarisch (half barbaric) Prague without any regrets. Einstein then proceeded to write to Julius that he was tempted to accept the offer, but he mentioned at the same time that there was a good chance that Zürich would be prompted to act, now that Utrecht was showing an interest in him.73 This was the beginning of a period of uncertainty. Einstein was waiting for a decision from Zürich and had meanwhile asked Utrecht for a postponement of his decision.74 When Einstein traveled to Brussels for the Solvay Conference at the end of October, the situation was still unchanged. However, when he visited Julius in Utrecht after the conference, on November 4,75 he had made his decision. He would not come to Utrecht, even though nothing had been decided yet in Zürich. Back in Prague, he wrote to Julius to thank him for his hospitality and looked back on their conversation.76 He repeated the more material reasons why he was staying in Prague: a spacious institute, a beautiful library, and no linguistic difficulties. Added to all this was another important consideration: “personal scruples, which we have discussed together and which I cannot dismiss.”77 Actually, Einstein had gathered from a conversation in Brussels that Lorentz would prefer to see a Dutchman appointed to the Utrecht chair. His impression was based on an unfortunate misunderstanding. This became clear when Einstein admitted in an apologetic letter to Lorentz that he had mainly taken the difficult decision not to come to Utrecht because he did not know whether Lorentz agreed with the appointment of a foreigner in the position.78 Einstein’s choice of words in the letter is interesting in light of later analyses of the relationship between Einstein and Lorentz as a father–son relationship. He wrote: “I am writing this letter with a heavy heart, as someone who has somehow done his father wrong.”79 Lorentz forwarded Einstein’s letter to Julius,80 surprised and disappointed. He “was very sorry” that he had unwittingly been the cause of Einstein’s refusal, while he had “said nothing other than that it would be a great pleasure for him.”81 In a later letter to Julius he once again expressed his regrets about the course of events in no uncertain terms: “[. . .] it saddens and oppresses me very much that

  It concerned the chair of the mathematician Hermann Minkowski, vacant since 1902.   Einstein to Julius, September 22, 1911 (CPAE-­5, 288). 74   In a letter to Julius of October 18, 1911 (CPAE-­5, 295) he asks for a few weeks of extra time to reach a decision. 75   See Einstein to Julius, November 1, 1911 (CPAE-­5, 302), in which Einstein announces his visit. 76   Einstein to Julius, November 15, 1911 (CPAE-­5, 304). 77   “persönliche Skrupel, die wir zusammen erwogen haben, und die ich nicht los werden kann.” 78   Einstein to Lorentz, November 23, 1911 (CPAE-­5, 313). 79  “Ich schreibe Ihnen diesen Brief mit schwerem Herzen, wie einer, die seinem Vater eine Art Unrecht zugefügt hat.” 80   Lorentz to Julius, November 25, 1911 (Kox 2018, 106). 81   “het doet mij zeer leed.” “niet anders had gezegd dan dat het mij zeer aangenaam zou zijn.” 72 73

The Utrecht vacancy


Einstein has attached such great weight to a supposed opinion of mine.”82 If only Einstein had asked Julius to discuss the question with Lorentz! Now it was too late. In his reply to Einstein, Lorentz also showed his great unhappiness with the course of events: if only he had expressed himself more clearly, or if he had only known earlier about Einstein’s scruples.83 Nevertheless, in their letters both Einstein and Lorentz expressed the hope that this question had not harmed their good relationship. Meanwhile, on November 16—one day after his letter of thanks to Julius— Einstein had already received a letter from his friend Marcel Grossman, Professor of Mathematics at the ETH, asking him on behalf of the ETH whether he was willing to accept an appointment.84 Of course, his reaction was affirmative—but it was not until January 30, 1912 that he was formally appointed. Once again, the question of the succession in Utrecht was completely open. Lorentz continued to be involved, and in letters to the faculty he gave his ­opinion about a number of candidates. Eventually, the choice was made to appoint the Dutchman Peter Debye, Einstein’s successor at the University of Zürich, whom Einstein had strongly recommended. His formal appointment took place on February 3, 1912.85 Looking back on the events described here, the question arises as to how ser­ ious Einstein’s interest in the Utrecht position really was. It is well known that he was not happy in Prague and that he wanted to return to Zürich, to the ETH. For this reason, it is not impossible that he realized, after having initially turned down the position on an impulse, that he could use the offer from Utrecht to his advantage. He could use it to put pressure on the Zürich authorities, while at the same time he knew for certain that he would never accept the offer. Once he was no longer able to keep Utrecht in the dark, even though there was still no decision from Zürich, he mainly used Lorentz’s presumed objection to turn down Utrecht. The question is whether Lorentz had really been so unclear in Brussels. Or was Einstein just using the conversation with Lorentz as an excuse? That he did not ask Lorentz or possibly Julius for clarification is food for thought, but at this point there is no way to know what exactly motivated Einstein. Anyhow, it is certain that this incident did not influence Einstein’s cordial relationship with Lorentz. The Utrecht affair even brought him a new friend: Willem Henri Julius. On his regular visits to Leiden, later on, Einstein always tried to see the Julius family as well and, if it was at all possible, he made sure to play music with them.86

82   “[. . .] het verdriet en drukt mij toch zeer dat Einstein eene onderstelde opvatting van mij zoo zwaar heeft laten wegen.” Lorentz to Julius, November 28, 1911 (Kox 2018, 108). 83   Lorentz to Einstein, December 6, 1911 (Kox 2018, 316). 84  See Einstein to Julius, November 16, 1911 (CPAE-­5, 306) and Einstein’s reply to Marcel Grossmann of November 18, 1911 (CPAE-­5, 307). 85  See Snelders 1987 for more on Lorentz’s opinion about the candidates and the appointment of Debye. 86   See also Einstein’s obituary of Julius (Einstein 1926), in which he calls him an “old friend” and a “clear-­sighted, artistically fine-­spirited man.”


Haarlem, Einstein, Ehrenfest, and Solvay

Lorentz’s succession At the end of 1911 Lorentz took an important decision: The time had come to make definitive plans for his departure from Leiden. The beginning of the academic year 1912–1913 seemed to him a good time to give up his professorship. As he wrote in a letter to Van der Waals, then curator of Leiden University, he foresaw that the Solvay Foundation would be making considerable demands on his time, time that he did not have at his disposal as an ordinary professor. In add­ ition, he had become chairman of the Academy’s Section of Sciences, a position that took up a fair amount of time as well. Besides, he was optimistic about the funding of both his special professorship and the position of his successor. In a meeting of curators on November 22, 1911, Lorentz’s intentions were officially announced and were discussed at length.87 Initially, the curators, fearing that the minister would raise financial objections, suggested that he would continue to teach his full course load after his appointment as an extraordinary professor, so that his succession could be postponed until the next year.88 Lorentz rejected this plan out of hand. Instead, he put additional pressure on the curators: “My ac­tiv­ ities are really accumulating too much, and especially the work that is overdue [. . .] is growing too much in volume.”89 In a letter to curators, dated December 7, 1911, Lorentz further elaborated his plans.90 He intended to continue his lectures for advanced students about special topics, as long as he was still extraordinary professor in Leiden. He did assume that, after his retirement at age 70, these lectures would be continued by the new ordinary professor, who would have a few years of experience by then. After all, Lorentz himself, as extraordinary professor, was not meant to have a successor. The curators ended up supporting Lorentz’s plans, and in mid-­December the Minister also decided favorably.91 At this point a delicate question arose: Who was to succeed Lorentz as ordinary professor? From correspondence that has been preserved, it is clear that Lorentz wanted to arrange his succession himself. In February of 1912, Lorentz consulted Van der Waals—again in his capacity of curator of the university—about the ­candidate he wished to be appointed. Like his colleague Julius in Utrecht, Lorentz wanted nothing less than to appoint Albert Einstein. As was noted earlier, Lorentz and Einstein had meanwhile become well acquainted. He wrote to Van der Waals:

87  See Lorentz to Van der Waals, November 21, 1911 (Kox  2018, 105). Lorentz wrote that he wanted to inform Van der Waals what the order of business would be in the meeting of curators of the following day. He also mentioned that he had already spoken about the matter to the president of cur­ ators, Witius Hendrik de Savornin Lohman. 88   See De Savornin Lohman to Lorentz, November 25, 1911 (LA 68). 89   “Mijne bezigheden hoopen zich werkelijk te zeer op, en vooral neemt het achterstallige werk [. . .] een te groten omvang aan.” Lorentz to De Savornin Lohman, November 27, 1911 (copy in LA 68). 90   Archive Curators, Leiden University. 91   See Minister to Curators, December 15, 1911 (Archive Curators, Leiden University).

Lorentz’s succession


Indeed, I consider him as one of the very foremost, who sees deeper and farther than others, while I also value highly his personal qualities and his clear and simple presentation. I hold him in such high esteem that I consider it my duty to attempt to win him for Leiden.92

In his reply to Lorentz, Van der Waals seemed pleased with Einstein’s candidacy, although he was disappointed that there were no Dutch physicists available of the desired high academic stature.93 On February 13, 1912, a day after the reply by Van der Waals to his earlier letter, Lorentz wrote a letter to Einstein on behalf of the Faculty of Mathematics and Physics of Leiden University, inviting him to become his successor.94 From the following quote it is abundantly clear how eager Lorentz was to have Einstein ­succeed him: As far as I am personally concerned, I can hardly tell you how attractive the prospect would be for me to be able to work with you in a continuous exchange. If I were to have the chance to welcome you here as my successor and at the same time as my colleague, this would fulfill a wish I have already entertained in silence for a long time, but have never dared to express before. When one grows older and the power to create diminishes by and by, one admires all the more the joyful, enthusiastic urge to create of a younger man, and for this reason it would have great appeal for me to hear much about your work and thoughts, all the more so as I am certain that the scientific exchange between us, just as it has been thus far, would, at the same time, be of a heartfelt friendliness.95

This attempt, as late as February 1912, to still draw Einstein to Leiden was somewhat curious. Lorentz knew, after all, that Einstein had been asked to come to Zürich already in November of the previous year and that he had meanwhile accepted. The invitation from Zürich had been repeated formally in December, in

92   “Inderdaad beschouw ik hem als een der allereersten, die dieper en verder ziet dan anderen terwijl ik ook zijn persoonlijke eigenschappen, zijn heldere en eenvoudige voordracht, zeer waardeer. Ik stel hem zoo hoog dat ik mij verplicht reken, eene poging te doen om hem voor Leiden te winnen.” Lorentz to Van der Waals, February 10, 1912 (Kox 2018, 109). Lorentz’s words are reminiscent of the first part of the well-­known statement by Isaac Newton: “If I have seen further, it is by standing on the shoulders of giants.” 93   See Van der Waals to Lorentz, February 12, 1912 (Kox 2018, 110). 94   The decision to make a formal offer to Einstein was made in the faculty meeting of February 12, at Lorentz’s proposal. (Archive Faculty, Leiden University.) 95   “Was aber mich persönlich betrifft, so kann ich Ihnen kaum sagen, wie verlockend für mich die Aussicht wäre, in stetem Verkehr mit Ihnen arbeiten zu können. Wäre es mir vergönnt, Sie hier als Nachfolger und zu gleicher Zeit als Collegen zu begrüssen, so wäre damit ein Wunsch erfüllt, den ich im Stillen lange gehegt habe, aber leider nicht früher habe aussprechen dürfen. Wenn man älter wird und die Kraft zum Schaffen allmählich versagt, so bewundert man um so mehr den frohen, begeisterten Schaffensdrang eines Jüngeren, und deshalb hätte es für mich hohen Reiz, viel von Ihren Arbeiten und Gedanken zu hören, um so mehr als ich gewiss bin, dass der wissenschaftliche Verkehr zwischen uns, ebenso wie bis jetzt, zugleicherzeit ein herzlich freundschaftlicher sein würde.” Kox 2008, 236.


Haarlem, Einstein, Ehrenfest, and Solvay

a letter from the President of the Swiss Schulrat.96 Also, Einstein had explained once more, at Lorentz’s request, that he had already accepted in principle and that  he felt obligated to accept the offer, even though negotiations were still to ­follow.97 Had Lorentz forgotten all this, or was he hoping against hope that absence of further news meant that the appointment might not come through? If he had known that Einstein had meanwhile been to Zürich to discuss further details and  that the eventual appointment would now be nothing but a formality, he would have known that Einstein was no longer available. For Einstein, the offer from Leiden came as a complete surprise, but it was clear that he was unable to accept it because, in the meantime, he had officially accepted the long-­awaited appointment in Zürich.98 As he wrote in his reply to Lorentz, he had already committed himself to start his work there as a professor of theoretical physics in the fall of 1912.99 Actually, his job in Zürich would end up being of short duration, as he moved to Berlin in the summer of 1914. Einstein’s reply to Lorentz showed his mixed feelings about the Leiden offer. He began by expressing how attractive he found the idea of working together with Lorentz. Something more wonderful than to experience in conversations with you the problems and events of our science full of secrets I cannot imagine. My feeling of mental inferiority with regard to you cannot tarnish the great joy of such conversations, especially since the fatherly goodness that you show to all people, does not allow a feeling of distress to arise.100

But then he voiced his ambivalence. “To occupy your chair would have something unspeakably onerous for me, however.”101 In a letter to a good friend he wrote in considerably clearer terms that he felt relieved to have had a good reason to refuse.102 Later that year he confessed that his heart sank at the thought of succeeding Lorentz when he read his letter:

  Robert Gnehm to Einstein, December 8, 1911 (CPAE-­5, 317).  See Lorentz to Einstein, December 8, 1911 and Einstein to Lorentz, December 12, 1911 (Kox 2008, 233 and 234). 98   On February 7, 1912 the letter of appointment was sent to Einstein; on February 12, one day before the date of Lorentz’s letter, he officially accepted the Zürich appointment (see CPAE-­5, 355 and 358). 99   Einstein to Lorentz, February 18, 1912 (Kox 2008, 238). 100   “Etwas Schöneres als in Gesprächen mit Ihnen die Probleme und Ereignisse unserer geheimnisvollen Wissenschaft zu erleben kann ich mir nicht denken. Mein Gefühl geistiger Inferiorität Ihnen gegenüber kann die grosse Freude an solchen Gesprächen nicht trüben, zumal die väterliche Güte, die Sie allen Menschen entgegenbringen, ein Gefühl des Gedrücktseins nicht aufkommen lässt.” 101   “Auf Ihrem Lehrstuhl zu sitzen, hätte allerdings etwas unsagbar Drückendes für mich.” 102   See Einstein to Heinrich Zangger, before February 29, 1912 (CPAE-­5, 366): “Es ist gut, dass ich in Zürich schon verpflichtet war; denn sonst hätte ich unbedingt dorthin gehen müssen.” (It is good that I had already committed myself to Zürich; or else I would certainly have had to go there.) 96 97

Lorentz’s succession


“When Lorentz called on me at that time, I experienced an undeniable shiver.”103 Apparently, all these negative feelings eventually got the better of him. Einstein’s refusal opened up the succession question all over again, but Lorentz wasted no time in finding a new candidate. Soon after receiving Einstein’s refusal letter, Lorentz inquired with his German colleague Arnold Sommerfeld104 about the address of the physicist Paul Ehrenfest. Who was this Ehrenfest?105 An Austrian by birth, Ehrenfest came from a Viennese middle-­class Jewish background, studied in Vienna, and received his doctorate in physics there in 1904 under the famous Ludwig Boltzmann, whose reputation was already discussed earlier. From then on, Ehrenfest’s career path was rather unorthodox. After two years of fruitlessly trying to find a position at a university in Austria or Germany, Ehrenfest and his wife, the Russian mathematician Tatiana Afanassjewa, established themselves in St. Petersburg. Tatiana had found a job there as a teacher of mathematics at a secondary school for girls. Initially, Ehrenfest was unsuccessful in finding a position for himself.106 He only had some income from incidental courses at the local university. He also organized private seminars and at St. Petersburg University he set up an informal discussion group with other physicists. It was not until 1909 that he managed to obtain a full-­fledged position at the Polytechnic Institute of St. Petersburg, but that ended unexpectedly after the first year. Severely disappointed, Ehrenfest con­ tinued his attempts to land a job at a university in Germany or Austria, but partly due to his indecisiveness, partly because of a lack of openings, none of these attempts were successful.107 Meanwhile, he had been able to attract the attention of some of his colleagues with a number of publications in which he critically reviewed some new developments in theoretical physics.108

103  “Als mich Lorentz damals rief, empfand ich ein unleugbares Gruseln.” Einstein to Paul Ehrenfest, December 20–24, 1912 (CPAE-­5, 425). Einstein and Ehrenfest had met each other in February 1912 during a visit by Ehrenfest to Prague, and they had immediately struck up a close friendship. (See Klein 1970, chap. 12.) 104   The letter by Lorentz has been lost, but it cannot have been written much later than February 18, because Sommerfeld’s reply is dated February 25, 1912 (Kox 2008, 239). 105  For biographical information about Ehrenfest, see Klein  1970, Huijnen and Kox  2007, and Hollestelle 2011. Lunteren en Hollestelle 2013 present an interesting analysis of Ehrenfest’s attitude with regard to contemporary developments in science and society. The exchange of letters between Lorentz and Ehrenfest has been published almost in its entirety in Kox 2018. See also Kox 2010 for an earlier discussion of the relation between Lorentz and Ehrenfest. 106   Ehrenfest’s being a foreigner and not having a Russian degree were important contributing factors here. 107   His search for a position was especially difficult, because Ehrenfest needed to “habilitate” himself first, meaning that he needed to acquire the right to be appointed as a professor at a university in a German-­ speaking country by working his way through a serious second dissertation, or Habilitationsschrift. For further details about Ehrenfest’s attempts to find a position, see Huijnen and Kox 2007. 108   For example, an article from 1906 in which he offers a critical analysis of Max Planck’s theory of radiation (Ehrenfest 1906). See also Klein 1970 for a detailed discussion of this article and other articles by Ehrenfest from the years after 1904.


Haarlem, Einstein, Ehrenfest, and Solvay

Two months later, at Lorentz’s request, Sommerfeld gave his opinion about the scientific qualities of Ehrenfest.109 He praised him as someone who was always in search of the deepest foundations of physics and had a strongly critical attitude. He also commended him for his didactic qualities: “He lectures masterfully. Hardly ever have I heard any man speak so grippingly and so brilliantly.”110 However, Sommerfeld also added that Ehrenfest had sometimes been inhibited in his work by psychological problems. Around the end of May, Lorentz also asked the Viennese physics professor Fritz Hasenöhrl for references.111 Hasenöhrl, student and successor of Boltzmann, who had known Ehrenfest since his student days, answered immediately.112 Like Sommerfeld, he emphasized Ehrenfest’s didactic qualities and, besides, he characterized him as an idealist. As Sommerfeld had done before him, Hasenöhrl also noted how Ehrenfest refused, as a matter of principle, to declare any religious denomination. Apparently, that was the reason why he had missed the chance to succeed Einstein at the German University in Prague. In the Austro-­Hungarian Empire someone without a religious affiliation could not be appointed as a public servant. Einstein would have liked Ehrenfest to succeed him in Prague and showed little understanding for Ehrenfest’s “confessionslosigkeit” (lack of a religious affiliation), which he called “fanatisch” (fanatical).113 He himself had not even thought twice about listing his faith as mosaisch (mosaic) for his appointment in Prague. In Einstein’s formulation: “to return to Abraham’s lap—that was really nothing at all. A signed piece of paper.”114 Hasenöhrl characterized Ehrenfest as perhaps a trifle too much of an idealist when he wrote: I have always regretted very much that that, because of his lack of a religious af­fili­ ation, he was beyond consideration for Austria; I have also told him to follow the example of Henry IV, who considered the French crown worth a mass. He said, though, that he would never do that. That is a great deal of idealism, as I believe, almost too much.115

  Sommerfeld to Lorentz, April 24, 1912 (Kox 2008, 244).   “Er trägt meisterhaft vor. Ich habe noch kaum einen Menschen so fesselnd und glänzend reden hören.” 111   Lorentz to Hasenöhrl, draft, May 30, 1912 (LA 106). Lorentz had recently seen him in Brussels during the Solvay Conference, which Hasenöhrl had attended. 112   Hasenöhrl to Lorentz, June 1, 1912 (LA 5). 113   Einstein to Heinrich Zangger, before February 29, 1912 (CPAE-­5, 366). Einstein to Jacob Laub, August 27, 1910 (CPAE-­5, 224, n. 3). The principled refusal by Ehrenfest and his wife to join any religion is also discussed in Ehrenfest to Lorentz, May 19, 1912 (Kox 2018, 114). 114   “In den schoss Abrahams zurück zu kehren—dass war gar nichts. Ein unterschriebenes Papier.” Paul Ehrenfest to Tatiana Ehrenfest, February 25, 1912 (Ehrenfest Archive, RB). 115   “Ich habe immer sehr bedauert, dass er infolge seiner Confessionslosigkeit für Österreich nicht in Betracht kommt; ich habe ihm auch zugeredet, nach dem Beispiele von Henri IV vorzugehen, der die Krone von Frankreich einer Messe wert hielt. Er sagte aber, er wurde das nie tun. Das ist sehr viel, wie ich glaube, fast zu viel, Idealismus.” 109 110

Lorentz’s succession


Meanwhile, in a letter of April 20, 1912, Lorentz had carefully sounded out Ehrenfest about his current position and his experiences since they had first met.116 This first meeting had taken place in the spring of 1903, when Ehrenfest had spent six weeks in Leiden and had even spent an evening at home with Lorentz. About what happened in the two months between Einstein’s refusal and the first letter to Ehrenfest, little is known. Probably, Lorentz spent time familiarizing himself further with Ehrenfest’s work. In his letter he referred to Ehrenfest’s publications, mentioning in particular the recent review article on statistical mechanics written together with his wife.117 It stands to reason that Lorentz also consulted with his colleagues in Leiden, and perhaps also with Van der Waals, as he had done in February with regard to Einstein’s candidacy.118 In any event, Lorentz had garnered enough support to approach Ehrenfest for the position. It is clear that Ehrenfest had no inkling of the real reason for Lorentz’s letter. He replied succinctly to Lorentz’s request for information and promised to write in more detail later.119 In his answer to Ehrenfest, Lorentz eventually got down to business.120 Ehrenfest was his choice for the chair in Leiden. Though he did keep his options open, as it was still possible that a young Dutchman would be chosen, he clearly expressed his preference for Ehrenfest, writing: “Since I value your work highly, because of the thoroughness, clarity, and incisiveness it shows, I have thought of you.”121 Ehrenfest was simply stunned, and he answered Lorentz in a long and rather emotional letter, explaining his current situation in great detail, as well as his possible prospects in Germany and his general plans for the future.122 Without restraint he described his personal feelings; for example, about the places where he did or did not feel at home. In his native city of Vienna, he felt like a stranger, but in St. Petersburg he would be able to feel at home, if only there was a future

  Kox 2018, 111.   Ehrenfest, P. and T. 1911. This article is of lasting importance because of its critical discussion of the foundations of statistical mechanics. That Lorentz was familiar with the article is clear from his letter to Ehrenfest of April 20, 1912 (see previous note), in which he thanked Ehrenfest for sending him an offprint. In this letter, Lorentz also mentioned a number of other publications by Ehrenfest. 118   There are no grounds whatsoever for the malicious story, brought into the world by Peter Debye, that Lorentz had chosen Ehrenfest at the recommendation of Einstein and that Einstein had been motivated to do so by the “question of race” (“Rassenfrage”) (see Peter Debye to Arnold Sommerfeld, November 3, 1912 [Sommerfeld Archive, Deutsches Museum, Munich]). Debye would have liked to become Lorentz’s successor and felt that he had been passed over. Debye was no longer under consideration, though, because of his candidacy for the vacant chair in Utrecht. The story about Einstein’s recommendation was stirred up again twenty-­one years later by Lorentz’s son-­in-­law Wander de Haas in an obituary about Ehrenfest in the daily Het Vaderland of September 27, 1933. 119   Ehrenfest to Lorentz, April 24, 1912 (Kox 2018, 112). In order to avoid confusion, all dates are presented here according to the Western (Gregorian) calendar, although in Russia the Julian calendar was still in use, which was thirteen days behind the Gregorian calendar. 120   Lorentz to Ehrenfest, May 13, 1912 (Kox 2018, 113). 121   “Da ich Ihre Arbeiten wegen der Gründlichkeit, der Klarheit und des Scharfsinns, wovon sie zeugen, sehr hoch schätze, so habe ich auch an Sie gedacht.” Lorentz to Ehrenfest, May 13, 1912 (Kox 2018, 113). 122   Ehrenfest to Lorentz, May 19, 1912 (Kox 2018, 114). 116 117


Haarlem, Einstein, Ehrenfest, and Solvay

for him there.123 He also noted that he and Tatiana were making preparations to leave Russia permanently. Unless he were to receive an offer from a ­German-­language university, wrote Ehrenfest, he was very willing to come to Leiden. This letter was Lorentz’s first introduction to the unrestrained and outspoken style of Ehrenfest’s correspondence. It is difficult to gauge what kind of impression this letter made on Lorentz. From his answer it cannot be deduced, but there is a later recollection by Lorentz’s son-­in-­law, Wander de Haas, who wrote how Lorentz “would read aloud with visible pleasure from the letters by Ehrenfest.”124 Lorentz’s reply was short.125 He asked to be sent a list of publications, which he received shortly thereafter.126 The next step was Ehrenfest’s formal nomination by the faculty to the curators. This nomination was made on June 10, at the recommendation of a commission consisting of Lorentz, Kamerlingh Onnes, and Kuenen.127 In the nomination, Ehrenfest was characterized as a “young physicist with a very sharp mind, who has done outstanding work and is heartily recommended by Sommerfeld and others.”128 The faculty commission also passed judgment on Willem Keesom and Leonard Ornstein, who appeared—more or less for the sake of formality—as second and third candidates on the nomination. Ornstein, who had obtained his doctorate with Lorentz in 1908 and had become lector in Groningen, was characterized, in Lorentz’s own words, as “only somewhat wild,” while Keesom, Kamerlingh Onnes’s collaborator, was “not wild but extremely solid and reliable.”129 However, Ehrenfest was “superior,” and the Faculty con­ sidered it “a privilege to be able to find someone like Ehrenfest to succeed Professor Lorentz.”130 In a lengthy letter to the curators of June 20, 1920, Lorentz further elaborated on the nomination.131 He predicted that Ehrenfest would “be counted among the first, one day” and characterized Ornstein as “the most exceptional” of his students.132 This was the start of a nerve-­wracking period for Ehrenfest, during which he did not receive any news from the Netherlands. By the end of June, he was so ­worried that he inquired whether he might have failed to receive any further ­correspondence that had been sent to him because of the unreliable Russian mail 123  He also wrote: “Yes even—however laughable it may sound—during a stay of several weeks among the fishing population of Schiermonnikoog, I quickly felt more at home than I could in Vienna.” (  Ja selbst—so lächerlich es klingen mag—bei einem mehrwöchentlichen Aufenthalt unter der Fischerbevölkerung von Schiermonnikoog fühlte ich mich sehr rasch mehr heimisch, als ich es in Wien könnte.) Schiermonnikoog is a small island off the northern coast of the Netherlands. 124   W. J. de Haas, “Prof. Dr. P. Ehrenfest †,” Het Vaderland, September 27, 1933. 125   Lorentz to Ehrenfest, May 30, 1912 (Kox 2018, 115). 126   See Ehrenfest to Lorentz, June 4, 1912 (Kox 2018, 116). 127   Archive Faculty of Mathematics and Physics, Leiden University. 128  “een jong physicus van grote scherpzinnigheid die voortreffelijk werk geleverd heeft en door Sommerfeld en anderen warm wordt aanbevolen.” 129   “alleen een beetje wild” “niet wild maar uitermate solide en degelijk.” 130   “een voorrecht iemand als Ehrenfest te kunnen krijgen om de opvolger te worden van de Heer Lorentz.” 131   Archive Curators, Leiden University. 132   “eenmaal tot de eersten zal worden gerekend.” “meest uitmuntende.”

Lorentz’s succession


service.133 This turned out not to be the case.134 The matter dragged on as the summer vacation caused further delay because administrative activities had come to a virtual standstill among the faculty and the curators. At the end of September, Ehrenfest sent a somewhat panicked telegram, since he had not heard anything since the end of July.135 Finally, on September 29, deliverance arrived in the form of a telegram:136 “Ehrenfest appointed professor leiden heartfelt congratulations, letter follows Lorentz.” The telegram was followed by a letter containing detailed information.137 Ehrenfest was so delirious with joy that he needed to calm down for a few days before he was able to answer Lorentz.138 In his reply he immediately unfolded plans for his teaching and for his inaugural lecture. He promised that he would do his utmost to be a worthy successor to Lorentz: “I will really use all my powers to justify at least to some extent the trust of those men who deemed it possible to appoint me to your chair.”139 In a characteristic expression of his insecurity, Ehrenfest also apologized in advance for any missteps he might commit: The risk that—particularly at first—I will show very nasty failings is, unfortunately, rather great: my unfortunately all too lively temperament connects here with lack of experience, lack of knowledge of the special situation in Leiden, and fear with respect to the special position where I will be ending up.140

The role he had in mind for Lorentz in his Leiden life also became clear now: Against irreversible failings I only know of one protection: at first to remain absolutely passive and leave myself completely to your guidance; later, though, when spontaneous activity is expected from me, to ask you for energetic censorial and veto-­interventions.141

  Ehrenfest to Lorentz, June 30, 1912 (Kox 2018, 117).   See Lorentz to Ehrenfest, July 7, 1912 (Kox 2018, 118). 135   Ehrenfest to Lorentz, September 22, 1912 (Kox 2018, 122). Lorentz’s most recent letter was dated July 25, 1912 (Kox 2018, 120). 136   “Ehrenfest professor leiden ernannt herzliche glueckwuensche brief folgt Lorentz.” Lorentz to Ehrenfest, telegram, September 29, 1912 (Kox 2018, 125). 137  Lorentz to Ehrenfest, September 29, 1912 (Kox  2018, 126). The Royal Decree appointing Ehrenfest to the chair of theoretical physics is dated September 25, 1912. In the same Royal Decree, Lorentz was honorably discharged as ordinary professor and was appointed extraordinary professor (see Nederlandsche Staatscourant, September 30, 1912). 138   Ehrenfest to Lorentz, October 5, 1912 (Kox 2018, 127). 139  “Ich will wirklich alle meine Kräfte anspannen um wenigstens einigermassen das Vertrauen ­derjenigen Männer zu rechtfertigen, die es für möglich fanden, mich auf Ihre Lehrkanzel zu berufen.” 140   “Die Gefahr, dass ich—besonders in der ersten Zeit—sehr üble Missgriffe begehe ist leider sehr gross: mein leider allzu lebhaftes Temperament tritt hier in Verbindung mit mangelhafter Erfahrung, Unkenntnis der specifischen Verhältnisse in Leiden und Befangenheit in Anbetracht der speciellen Stelle an die ich gelange.” 141   “Vor irreversiblen Missgriffen weiss ich nur einen Schutz: mich zunächst absolut passiv zu verhalten und mich ganz Ihrer Führung zu überlassen; später aber, wo spontane Activität von mir verlangt wird, Sie um energische Censur-­und Veto-­Eingriffe zu bitten.” 133 134


Haarlem, Einstein, Ehrenfest, and Solvay

Lorentz obviously did not much feel like taking on this role, at least judging by his reaction: “Such an important and even ‘severe’ role as you ascribe to me in your letter I will certainly not be able to play.”142 The reason why Lorentz decided to opt for Ehrenfest so quickly, once Einstein had declined the offer, has never become quite clear. Perhaps suitable successors were thin on the ground in the Netherlands, though in countries like Germany, France, or England good candidates were certainly more plentiful. Lorentz may have been attracted by emotional qualities he saw in Ehrenfest, which he admired and which he felt he was lacking himself. The question is whether, at the time, he was sufficiently aware of the problematic sides of Ehrenfest’s personality, which would clearly manifest themselves in their later correspondence. At any rate, it was a very conscious choice by Lorentz. As he wrote to his Utrecht colleague Willem Julius, after Ehrenfest’s official appointment, “It has made me sad that I was un­able to bring a Dutchman to the forefront, but I believed that I needed to elicit the decision that has now been taken.”143

Ehrenfest in Leiden, Lorentz in Haarlem In March of 1912, Lorentz and Aletta began to look around in Haarlem to find a new place to live. Soon they found a spacious house, on Zijlweg 76, renting for 900 guilders per year. As the place had been vacant for some years, the family could move in quickly and the move was planned for the end of July. Before leaving Leiden for good, they celebrated one final festive occasion in the Hooigracht house: the wedding of their second daughter Hannie. On May 28 she married Hendrik Carel (“Henri”) Leemhorst, who was Secretary of the Board at the Hollandsche Lloyd shipping company in Amsterdam. Berta had already preceded her sister, marrying Wander Johannes de Haas, Assistent in Kamerlingh Onnes’s laboratory, in December 1910. Rudolf moved to Haarlem with his parents and did not marry until 1926. Incidentally, Lorentz himself had little involvement with the preparations to move to Haarlem. The actual move took place while he was taking a break for a few weeks in Belgium and Luxemburg, together with Rudolf. In the meantime, Aletta and the two daughters had taken care of all the packing and unpacking and had made the family comfortable in their new home. On his return from the trip on August 2, Lorentz found his new house fully furnished and almost completely in tip-­top order: “Only in my room there was a chaos of books; when I started arranging the bookcases I was already happy when I had finally reached the table-­top

142   “Eine so wichtige und sogar “strenge” Rolle, wie Sie mir in Ihrem Briefe zuschreiben, werde ich gar nicht zu spielen haben.” Lorentz to Ehrenfest, October 10, 1912 (Kox 2018, 128). 143   Lorentz to Willem Julius, September 29, 1912 (Archive Julius, Universiteitsmuseum Utrecht). The letter has the same date as the telegram to Ehrenfest announcing his appointment.

Ehrenfest in Leiden, Lorentz in Haarlem


in one spot, after having cleared away the stack.”144 He was quite pleased with his new home: “the house is really very attractive and welcoming, even though we still have to get used to it all, and the garden also turned out better than I expected; one can sit out there quite pleasantly.”145 On October 17, 1912, the Ehrenfest family arrived in Leiden.146 On the day of their arrival, the Ehrenfests wasted no time in visiting Lorentz in Haarlem to pay their respects. Meanwhile, during their search for a suitable rental home, they stayed in a boarding house. On December 4, Ehrenfest formally accepted his professorial duties by giving his inaugural lecture.147 According to the plan, this was also the moment for him to take over the regular teaching load, with the exception of Lorentz’s weekly lectures for advanced students. Lorentz continued to give these “Monday morning lectures,” and the classes became legendary, both among students and faculty. In a letter to Berta of November 10, Lorentz, somewhat awe-­struck, reports on all the activities Ehrenfest had already deployed since his arrival. Every week there is a meeting in the evening at his house, for which the number of participants has now grown to 15; [...] The journals are divided up and reviews about them are presented, for which the requirements are not high; then a simple “lecture.” [...] So he really tries hard and spreads quite a bit of enthusiasm. [. . .] It is very good that he manages to get the participants to speak up in this way. So, I believe that it is all going quite well. [. . .] So far, he has spent an evening here every week, with or without his wife, and we have gefachsimpelt [talked shop] together. He is really very sharp-­witted and full of enthusiasm, though sometimes speaking perhaps a little too fast.148

And in a letter of November 19, he adds: E. has succeeded, something that I have never managed to achieve at the colloquium, in getting the participants in these meetings to start speaking up. He must be exuding

144   “Alleen op mijn kamer was het een chaos van boeken; toen ik den volgenden dag aan het schikken in de kasten ging was ik al blij toen ik eindelijk bij het wegnemen van den stapel op één plek tot het blad der tafel was doorgedrongen.” Lorentz to Berta, August 7, 1912 (FC). 145   “Het huis is, ofschoon wij nog aan een en ander wennen moeten, werkelijk heel prettig en vriendelijk en ook de tuin viel mij zeer mee; men kan er met plezier in zitten.” Ibid. 146   Paul and his wife Tatiana were accompanied by their daughters Tatiana and Anna (Galinka), a nanny, and Tatiana’s aunt Sonja (Baba Sonja). 147   Ehrenfest 1912. 148   “Wekelijks is er ’s avonds bij hem een bijeenkomst, waarvan het aantal deelnemers nu tot 15 is aangegroeid; [. . .] De tijdschriften worden verdeeld en daarover referaten gehouden, waaraan geen hooge eischen gesteld worden; daarna een eenvoudige ‘voordracht.’ [. . .] Hij doet dus wel zijn best en verspreidt heel wat opwekking. [. . .] Het is wel mooi dat hij de deelnemers zoo aan het praten weet te krijgen. Ik geloof dus dat dit alles best gaat. [. . .]. Tot nog toe is hij elke week met of zonder zijn vrouw een avond hier geweest en hebben wij samen ‘gefachsimpelt.’ Hij is werkelijk heel scherpzinnig en vol enthousiasme, soms misschien wat al te snel pratende.” Lorentz to Berta, November 10, 1912 (FC).


Haarlem, Einstein, Ehrenfest, and Solvay

quite a lot of enthusiasm and for that reason, and also for other reasons, it is good that he has come.149

After Ehrenfest’s arrival in Leiden, his correspondence with Lorentz really took off. Mail was delivered four times a day at the time, so they had every opportunity for a frequent and extensive exchange of ideas. Apart from continued further emotional outbursts by Ehrenfest, their correspondence is characterized by intensive discussions about a great number of widely varying scientific subjects. A magnificent example of the way in which a scientific discussion used to take place in this pre-­electronic era is to be found in an exchange of letters in the months between December 1915 and January 1916. The—in this case three— participants in the discussion were Einstein, Ehrenfest, and Lorentz, in Berlin, Leiden, and Haarlem respectively.150 The subject of their correspondence was the brand-­new General Theory of Relativity by Einstein, recently published in a series of articles in the months of November and December of 1915. Lorentz and Ehrenfest were fascinated by the theory and tried to fathom its depths, keeping each other abreast of their progress. Soon Ehrenfest encountered a problem which he presented to Lorentz. Lorentz was unable to resolve it and, besides, he came up with a new problem that appeared to be so serious that it threatened to destroy the foundations of the new theory. They both wrote to Einstein and forwarded his replies to each other. After a fast and furious back and forth, everything fell into place and the three scholars reached agreement: The General Theory of Relativity was sound. Lorentz concluded the discussion by congratulating Einstein. At the end of a letter to Ehrenfest he wrote: “I have congratulated Einstein on his brilliant results.”151 Apart from its content, this exchange of letters is also interesting because of its form. It completely reflects the personalities of Lorentz and Ehrenfest, and this is visible also throughout the rest of their correspondence. The letters by Ehrenfest, full of exclamation marks, underscored text, thick question marks, and heavily framed formulas, are the unrestrained reflection of a scientific stream of consciousness, a struggle with unresolved problems. Besides, his insecurity about being a worthy successor to Lorentz and his doubts about his own performance

149   “E. is er in geslaagd, wat mij op het colloquium nooit gelukt is, de deelnemers aan die bijeenkomsten aan het praten te krijgen. Er zal heel wat opwekking van hem uitgaan en het is daarom, en om andere redenen, goed dat hij gekomen is.” Ibid. 150   The discussion takes up a total of twenty letters: three by Einstein to Lorentz, dated January 1, 17, and 19, 1916 (Kox 2008, 304, 305, and 306; CPAE-8, 177, 183, and 184); four by Ehrenfest to Lorentz, dated December 23 and 24, 1915 and January 9 and 12/13, 1916 (Kox 2018, 175, 176, 180, and 183); six by Lorentz to Ehrenfest, dated December 23 and 26, 1915 and January 6, 9, 10/11, and 12, 1916 (Kox 2018, 174, 177, 178, 179, 181, and 182); six by Einstein to Ehrenfest, dated December 26 and 29, 1915 and January 3, 5, 17, and 24, 1916 (CPAE-­8, 173, 174, 179, 180, 182, and 185); and one letter by Ehrenfest to Einstein, dated January 1, 1916 (CPAE-­13, Vol. 8, 177a). Although only one letter to Einstein has been preserved, the entire correspondence can be reconstructed quite well on the basis of the extant letters. See also Kox 1988 for an analysis. 151   Lorentz to Ehrenfest, January 10/11, 1916 (Kox 2018, 181).

Ehrenfest in Leiden, Lorentz in Haarlem


are also constantly recurring themes in Ehrenfest’s letters. In comparison, the letters by Lorentz are a trifle dull. They are focused on the scientific process, clearly written, well ordered, and without any superfluous punctuation. Here, the thought process has already been completed and the reader is given a crystal-­clear ex­pos­ ition of the results. In the expression of personal feelings Ehrenfest did not exactly exercise restraint, as has already been noted earlier. A characteristic example illustrates this clearly. On March 14, 1913, more than six months after his arrival, Ehrenfest looked back on his activities as a professor so far. He was not satisfied. He felt guilty about his shortcomings, real or imagined, and in a letter to his great predecessor, perhaps in search of absolution, he bared his soul: I am asking your forgiveness for the following things: That I have behaved unseemly during your colloquium. I can assure you that something like this will never happen again [. . .] And I also want to take this opportunity to ask for your forgiveness for this: that I am still behind with respect to learning the Dutch language. That I can neither speak English, nor French [. . .] That I have not had or followed up a single thought since the summer.152

From Lorentz’s reaction it can be concluded that he did not really know what to make of this emotional outburst. In his answer, a day later, he wrote: Let me begin by saying that there is really nothing for which you should ask me to excuse you. [. . .] You have entered your new sphere of activity with most admirable dedication and have in this short time won the affection and trust of your students and of many other people. [. . .] And if with all this, and with the great changes that have taken place in your surroundings and your way of life, you have not done much scientific work, that is surely very natural. Indeed, it would have been miraculous if you had been able to do that too. [. . .] Now what the future brings is out of our hands; but as long as we strive towards a good and worthy goal we have done our duty.153

152   “Ich bitte Sie um Verzeihung für folgende Dinge: Dass ich mich auf Ihrem Colloquium ungezogen benommen habe. Ich kann Sie versichern, dass niemals wieder etwas Ähnliches geschehen wird. [. . .] Und so will ich Sie auch bei dieser selben Gelegenheit um Verzeihung bitten dafür: Dass ich noch im Rückstand mit dem Erlernen der holländischen Sprache bin. Dass ich weder französisch noch englisch sprechen kann. [. . .] Dass ich seit dem Sommer keinen einzigen Gedanken mehr gehabt oder verfolgt habe.” Ehrenfest to Lorentz, March 14, 1913 (Kox 2018, 141). 153   “Laat ik beginnen met te zeggen dat er werkelijk niets is, waarvoor gij mij om verontschuldiging zoudt moeten vragen. [. . .] Gij hebt U met een zeer te waardeeren toewijding aan Uw nieuwen werkkring gegeven, en hebt in dien korten tijd de genegenheid en het vertrouwen van Uwe studenten en van vele anderen gewonnen. [. . .] En dat gij nu bij dit alles, bij al de groote veranderingen die Uwe omge­ ving en Uwe levenswijze ondergaan hebben, niet veel wetenschappelijk werk hebt geleverd, dat is toch zeer natuurlijk. Het zou wel wonder zijn, zoo gij het wél hadt kunnen doen. [. . .] Wat nu de toekomst brengt, ligt niet in onze hand; maar zoo wij naar een mooi en goed doel streven, hebben wij onzen plicht gedaan.” Lorentz to Ehrenfest, March 15, 1913 (Kox 2018, 142).


Haarlem, Einstein, Ehrenfest, and Solvay

Warm words, but would this well-­meaning attempt to reassure Ehrenfest really have been the reaction Ehrenfest was hoping for? Would he not rather have read that Lorentz had unlimited confidence in his abilities and viewed him as his ideal successor? Or would he have expected a more personal pronouncement, showing that Lorentz himself had also struggled at times with doubts about his own achievements and dissatisfaction with his shortcomings? It hardly seems likely that the impersonal, somewhat obligatory final sentence of this quote truly served to boost Ehrenfest’s self-­confidence. Nonetheless, the entire correspondence between Ehrenfest and Lorentz shows that, at least in writing, Lorentz was not capable of much more empathy. In a letter of December 8, 1917, for example, he wrote soberly: In any case you can have the feeling that you have done your very best. That we should do this, each of us in the way that best suits his nature and personality, is in fact all that can be demanded of us.154

Incidentally, Ehrenfest’s emotional outpourings toward Lorentz were not limited to his letters. In a written account to Lorentz’s daughter and son-­in-­law, for ex­ample, Lorentz’s student Adriaan Fokker describes Lorentz’s last lecture in May of 1923, just before his official retirement, which was also attended by Ehrenfest.155 At the end of the lecture Lorentz made the rounds and shook hands with all those present. In Fokker’s own words: “Then Ehrenfest said: professor Lorentz, I wish you a good journey, he grasped his hand, kissed it at length, and then, sobbing, absconded through the side door.” Even then, Lorentz controlled himself, albeit with difficulty, according to Fokker: “Lorentz’s eyes also filled with tears for a moment, but he remained in control of his emotions.”156

Ehrenfest as professor All of this should not lead to the conclusion that Ehrenfest’s professorship was a failure. As a professor he was completely different than Lorentz, but in his own way Ehrenfest was very successful.157

154   “In elk geval kunt gij het gevoel hebben, zeer Uw best gedaan te hebben. Dat wij dit doen, ieder op de wijze die aan zijn aard en persoonlijkheid het best beantwoordt, is eigenlijk alles wat van ons verlangd kan worden.” Lorentz to Ehrenfest, December 8, 1917 (Kox 2018, 215). 155   See Fokker to Wander and Berta de Haas, May 10, 1923 (Archief Fokker, RB). 156   “Toen zei Ehrenfest: mijnheer Lorentz, ik wensch u goede reis, hij greep zijn hand, zoende die lang, en maakte snikkende dat hij door de zijdeur wegkwam.” “Lorentz’ ogen schoten ook een ogen­blik vol, maar hij bleef zichzelf meester.” 157   This is not the place for a detailed evaluation of theoretical physics in Leiden in the Ehrenfest era. For the first years, see Klein 1970; for the later years only a partial analysis is available (Hollestelle 2011). See also the personal recollections of Ehrenfest’s student Hendrik Casimir in Casimir 1983, chap. 3.

Ehrenfest as professor


Ehrenfest involved himself with his students and his doctoral candidates intensively, something that Lorentz had never done in such a personal way. He took all kinds of initiatives to turn the group of students, especially those in theoretical physics, into a tight-­knit body of disciples. Among other things, he organized weekly colloquia (seminars) and revived the association of physics students that was named after the famous Dutch physicist and mathematician Christiaan Huygens. Perhaps most importantly, he set up a reading room, the Bosscha reading room, where students could study and have the most important books and journals at their fingertips. Because of his many contacts, his visits to foreign colleagues, and his enthusiastic participation in conferences at home and abroad, Ehrenfest managed to lure many prominent physicists to Leiden to visit the department. One of these foreign visitors was Einstein. At the initiative of Lorentz and supported by Ehrenfest and Kamerlingh Onnes, he was appointed special professor on behalf of the Leidsch Universiteits-­Fonds (Leiden University Fund).158 His obligations were not substantial. In return for a yearly fee of 2,000 guilders he had to come to Leiden for a few weeks every year and preferably also give some lectures. This is how Einstein became a regular and welcome guest, finding the peace and quiet in Leiden that he was often lacking in busy Berlin. Because of his international contacts, Ehrenfest was also able to ensure that his most promising students had a chance to gain some foreign experience; for example, by visiting Niels Bohr in Copenhagen. In short, under Ehrenfest the nature of theoretical physics in Leiden changed to become more open and outward-­looking. For Ehrenfest personally, the situation was more complex. His organizational activities and the successes of his students gave him much satisfaction. At the same time, he had an increasing sense of becoming isolated in the physics community, especially because he believed that he was unable to keep up with modern developments. As early as 1924, while traveling in the United States, he entertained a plan to move there, as he thought he would be able to make more of a contribution there in terms of organization and teaching. For Leiden, he believed, he no longer had the appropriate level of knowledge.159 He continued to feel the  pressure of his position as the successor of the famous Lorentz, and con­ tinued to suffer from feelings of inadequacy in that role. It probably did not help that  Lorentz had remained active as special professor and still came to Leiden ­regularly.

158   The appointment procedure was lengthy and plagued by all sorts of bureaucratic troubles and delays. Among other things, the Ministry of Education confused Einstein with a Belgian namesake with communist sympathies, and this held up his appointment for months. See Dongen  2012 for the ­complete story. 159  See Ehrenfest to Carel de Ridder, n.d. (Collection Van Aardenne-­Ehrenfest, NHA), a letter ­written from the United States, where Ehrenfest spent the first few months of 1924.


Haarlem, Einstein, Ehrenfest, and Solvay

Ehrenfest’s decline The death of Lorentz, in 1928, caused Ehrenfest to descend into a deep depression. At the graveside he gave a very emotional eulogy, and he claimed that he suffered from serious headaches for weeks.160 Ehrenfest’s state of mind only con­ tinued to go downhill after 1928. Even though the man Lorentz was no longer physically present, Ehrenfest did not feel any less pressure as his successor, and he experienced a growing feeling of not measuring up. Eventually, these anxieties appeared to become an unbearable burden to him. In August of 1932 he wrote two letters in which he announced his suicide.161 One year later, he put his plan into effect. The circumstances of his suicide were especially dramatic. In the morning of the 25th of September Ehrenfest went to visit the physics laboratory of the University of Amsterdam—as became apparent later, for a farewell to his Amsterdam colleagues.162 In the course of the afternoon, Ehrenfest left the laboratory for the pedagogical and psychological institute run by Professor Jan Waterink at the Vrije Universiteit.163 This institute, located immediately on the edge of Amsterdam’s Vondelpark, was caring for his fifteen-­year-­old son Wassily (“Wassik”), who had Down’s syndrome. In the institute’s waiting room Ehrenfest pulled out a

  See Chapter 12 for Ehrenfest’s eulogy.   The first letter, which was actually never sent, is dated August 14, 1932 and was written to five colleagues, among whom were Bohr and Einstein. The first sentence is: “I absolutely do not know any more, how I must continue to drag, even just for the next months, the burden of my life that has become unbearable for me.” (Ich weiss absolut nicht mehr, wie ich auch nur die nächsten Monate die unerträglich gewordene Last meines Lebens weiterschleppen soll.) Further down in the same letter, Ehrenfest wrote: “Apart from the primitive animal abhorrence of the physical act of dying I have a very positive, joyful longing for not being.” (Abgesehen von der primitiev-­animalischen Abscheu vor dem physischen Act des Sterbens habe ich eine ganz positieve, freudige Sehnsucht nach dem Nichtsein.) Copies of this letter are to be found, among other places, in the Einstein Archive (Hebrew University of Jerusalem) and the Bohr Archive (Niels Bohr Institute, Copenhagen). See also Pais 1991, 409–410, for a partial English translation. The second letter, dated August 15, 1932, was meant for Ehrenfest’s former students, among whom were his former doctoral students Jan Burgers, Hendrik Casimir, Arend Rutgers, and Jan Tinbergen. Tinbergen and Casimir never received the letter; Burgers probably did. In this letter Ehrenfest wrote: “Each of you has been to me during a period of your life something like my own child. [. . .] And now it is so that to me [. . .] no other way out will remain but to kill myself.” ( Jeder von Euch ist mir während einer Strecke Eures Lebens so etwas, wie mein eigenes Kind gewesen. [. . .] Und nun ist es so, dass mir [. . .] kein anderer Ausweg übrigbleiben wird, als mich zu töten.) A copy of this second letter is preserved in RB; see Delft 2008 for more information. 162  Arend Rutgers, who had taken his doctorate with Ehrenfest and was then working in the Amsterdam laboratory, recounted this visit in a letter to professor Pieter Zeeman (who was in Paris at the time), dated September 27, 1933 (ZA 131). That Ehrenfest’s suicide did not come as a complete surprise is confirmed by what Rutgers wrote: “As you know, he had already been carrying around this notion for a long time, and eventually he had completely familiarized himself with the thought of not-­ being, yes this thought must have strongly attracted him in the end, so that finally he was no longer able to resist it.” (Zoals U weet, droeg hij reeds lang de plannen met zich om, en had hij zich ten slotte geheel met de gedachte van niet-­zijn vertrouwd gemaakt, ja, moet deze hem ten laatste sterk aangetrokken hebben, zoodat hij ten slotte geen weerstand meer heeft kunnen bieden.) 163   The “Vrije Universiteit” (Free University) is the Protestant University of the Netherlands. It is one of the two universities in Amsterdam. 160 161

Ehrenfest’s decline


handgun, firing first at Wassik, who was injured, and then shooting himself. Wassik died twelve hours later.164 Already in one of the letters he had written a year earlier, Ehrenfest had mentioned the autumn of 1933 as the ultimate time for him to give up his chair in Leiden. In the same letter he had also announced that he would kill Wassik so as not to burden his wife and his other children with the boy’s care.165 It is abundantly clear that Ehrenfest’s despondency about his shortcomings as a physicist and as Lorentz’s successor played an important part in his decision to take his own life. Yet, there were other factors that also contributed to his depression. He had serious marital problems and a divorce seemed inevitable. He was also increasingly concerned about the situation in Germany, where Nazism and anti-­Semitism were steadily on the rise. He was very worried about these developments, especially about the recent violence against Jews and Jewish institutions and the possible consequences of the rise of the Nazis for the rest of Europe.166 In hindsight, one may wonder what Ehrenfest’s life would have been like without the enormous pressure that occupying Lorentz’s chair represented for him. At the same time, it is clearly visible how these two personalities who were so different, even in the ways their lives ended, have both been fundamental for theoretical physics in Leiden. Because of his multi-­faceted and influential work, Lorentz invested Leiden with prestige. Building on this prestige, Ehrenfest introduced the human factor. Under him, Leiden became a lively center for theoretical physics that was highly attractive to many.

  See for example the report in the daily Het Vaderland of September 25 and 26, 1933.   This is the letter of August 14, 1932. 166  In an obituary, Einstein ascribes Ehrenfest’s suicide mainly to his sense of insufficiency as a physicist: “Whoever knew him as well as I was allowed to know him is aware that this pure personality has fallen victim to a conflict of conscience, a conflict of conscience from which, in some form or other, no university professor is spared who has lived beyond the age of fifty [...] In truth, however, he felt unhappier than all the others that have become close to me. That was because he did not feel up to the high demands that were placed on him.” (Wer ihn so gut kannte, wie es mir vergönnt war, der weiss, dass diese reine Persönlichkeit einem Gewissenskonflikt zum Opfer gefallen ist, einem Gewissenskonflikt, der in irgend einer Form keinem Universitäts-­Lehrer erspart bleibt, der etwa das fünfzigste Lebensjahr überschreitet. [...] In Wahrheit fühlte er sich jedoch unglücklicher als alle andern, die mir näher getreten sind. Dies kam daher, dass er sich der hohen, ihm gestellten Aufgabe nicht gewachsen fühlte. [Einstein 1934]). Rutgers refers explicitly to the “divorce difficulties” in his letter (see n. 162). See also Casimir 1983, 175–177, for Casimir’s comments. 164


Chapter 8 Quantum theory and the General Theory of Relativity

The most important achievements of modern physics are the quantum theory and the theory of relativity, both developed in the first decades of the twentieth century.1 These theories have not only become essential elements of physics, they have also become part and parcel of daily life. The fundamentals of modern med­ ical imaging techniques like MRI, for example, are based completely on quantum effects. Also, the technology of navigation by GPS—meanwhile almost in­dis­pens­ able in modern-­day traffic—would not be nearly as precise if its software did not take into account the corrections that are based on the theory of relativity. Lorentz made substantial contributions to both these theories. Perhaps less so to quantum theory than to relativity theory, but in either case enough to devote a full chapter to these two theories. The classical physicist Lorentz, as it will turn out, approached the two new theories with critical detachment. He was able to make the most of this critical attitude and provided further clarification, in ­particular conceptually, in a number of important contributions.2

From intuitive to abstract world view Sophisticated scientific developments like MRI or GPS are now generally accepted as perfectly mundane technologies, but they find their basis in extremely abstract theoretical concepts. They are an illustration of the premise that, at this point, modern physics has become so abstract that it can no longer be understood in­tui­ tive­ly. Over the course of the past four or five centuries, the world view of natural science has developed from a purely demonstrative view, closely connected to day-­to-­day experience, to a science so abstract and so far removed from daily experience that people no longer have an intuitive understanding of it.

 See Kragh 1999 for a general overview of twentieth-­century physics.   For a more detailed treatment of Lorentz’s work than can be given here, see Kox 2013a (quantum theory) and Kox 1988 (relativity theory). 1 2

“A Living Work of Art”: The Life and Science of Hendrik Antoon Lorentz. A. J. Kox and H. F. Schatz, Oxford University Press (2021). © A. J. Kox and H. F. Schatz. DOI: 10.1093/oso/9780198870500.003.0009

Radiation theory


In the Aristotelian world view of the late Middle Ages, the concept of motion was intuitively understandable. If one decides to move a table, it is necessary to push or pull the table. If the table is no longer pushed or pulled, then it stops mov­ ing. As Aristotle formulated it, a “motor” is required for motion to occur. With Newton’s Second Law of Motion, this intuitive view changed fundamentally. Newton asserted that an object, on which no forces operate, is either at rest—still understandable on the basis of daily experience—or is in “uniform motion.” This means that the object moves at a constant speed along a straight line—and this is something that is not nearly as easy to understand on the basis of real-­life ex­peri­ ence. In everyday life, an object on which no forces are exerted does not exist, since even the farthest corners of the seemingly empty universe are not devoid of forces. Gravity works always and everywhere, as long as there is matter. So, with Newton’s law, a new level of abstraction was reached in physics. Modern physics has meanwhile become still more abstract. It is now concerned chiefly with minuscule particles, particles that are so small that they can never be observed directly. They have properties that are totally alien to daily life. When they collide, for example, they can transform into other particles for no apparent reason, and—among many other strange phenomena—their energy can only take on certain values. To be able to describe the behavior of these minute particles, intuition is no longer sufficient. Complicated mathematics is required to do so. It even seems sometimes as though the concrete concept of “particle” has become subordinate to such mathematical formalisms. In order to understand somewhat more clearly how modern physics has arrived at this juncture, some more background will be provided here, as well as some explanation about the nature of quantum theory and relativity theory.

Radiation theory In popular books and articles, the moment of origin of quantum theory is gener­ ally placed in the year 1900, because of the work on radiation theory done in that year by the German physicist Max Planck.3 This is true and not true at the same time. Based on a desire to invest factual developments with a certain degree of logic, it happens regularly that the origin of a theory is placed in a historical ­context that does not really do justice to the actual development. This applies to quantum theory, but also to relativity theory, as was already explained earlier in Chapter 4. In 1900, Planck did indeed introduce a hypothesis that can be considered the first step on the road to quantum theory, but his hypothesis was not the one that 3  See Kuhn 1978 for the early history of quantum theory; see also Duncan and Janssen 2019 for a very readable account of the history of quantum theory up to the year 1920. The six volumes of Mehra and Rechenberg 1982–2001 provide a comprehensive, very technical history of twentieth-­century quan­ tum theory.


Quantum theory and the General Theory of Relativity

is now known as the quantum hypothesis. The quantum hypothesis actually says that energy, of whatever nature, can only occur in specific quantities. However, the meaning of Planck’s assumption was much more limited. To understand this, it is necessary to spend some time on Planck’s motives and the exact context of his hypothesis. According to popular history, it was mainly the problem of the so-­ called “­black-­body radiation” that kept Planck up at night. Now, of course, the question arises what exactly black-­body radiation is. Everyone is familiar with the phenom­ enon that an object changes color if it is heated: A heated piece of iron will change color slowly but steadily, turning red first, then violet, and eventually white: “white-­hot.” All substances behave in this way under the influence of heat. They change color as they become hotter. In other words, the wavelength of the light emitted by a substance—the emitted radiation—is subject to change under the influence of heat. After all, color and wavelength are related. Red light, for ex­ample, has a longer wavelength than violet light. This change in wavelength is not only visible, but can also be tracked very pre­ cisely. For each separate wavelength it is possible to determine exactly how much radiation—or, more precisely, how much radiation energy—is emitted by a body that has been heated to a certain temperature. Subsequently, this can be deter­ mined for a whole series of temperatures. As it turns out, the way in which the emitted radiation energy is distributed across the various wavelengths is different for each substance. In turn, this distribution across wavelengths depends on the temperature of the substance. To be able to more easily explain these radiation phenomena theoretically, nineteenth-­century physicists came up with the idea that it would be useful to introduce a standard source of radiation. This should be a body with properties that would easily lend themselves to theoretical treatment. An “ideal black body” was selected for this purpose, an object with the characteristic that it completely absorbs any radiation that is directed at it. An example of such a body would be an object that is totally covered in pitch-­black soot. In 1860, the German physicist Gustav Kirchhoff had been able to show the­or­ et­ic­al­ly that such an ideal black body emitted radiation whose composition—the distribution of the emitted radiation energy across various wavelengths—no longer depended on the material composition of the black body itself. It also turned out to be possible to actually produce such black bodies in practice. This is how it was also possible, by means of experiments, to carefully study the radiation emitted by these bodies: the so-­called black-­body radiation. Around 1900, the experimental resources had reached such a degree of sophistication that it was possible to carry out precision measurements on black-­body radiation. This work was done mainly in Berlin. The names of the physicists Otto Lummer and Ernst Pringsheim are forever connected to these measurements. Not only was it possible to carry out ever more refined and precise experiments, substantial theoretical advances had also been made. The German physicist Wilhelm Wien, for example, and the Englishman Lord Rayleigh had derived two

Max Planck


different laws of radiation. In doing so, they had provided a theoretical description of the distribution across all kinds of wavelengths of the radiation, or rather the energy, that was emitted by a black body. Although these laws initially seemed to provide a good description of the experimental results, discrepancies between the theory and the experiments gradually came to light. Wien’s law turned out to be adequate only for short wavelengths, while Rayleigh’s law, on the other hand, gave the correct theoretical description only for long wavelengths.

Max Planck Now back to Planck’s work. Contrary to what is often written, Planck was not primarily interested in the theoretical description of black radiation. He was mainly preoccupied with the phenomenon that is known as irreversible processes. Such processes are always characterized by a temporal direction; in other words, by the fact that countless natural processes only run in one direction of time. An example is heat, which always runs from high to low temperatures, and never the other way around. This temporal direction manifests itself not only in physics, but also in daily life. Everyone only gets older, never younger, and in human ex­peri­ ence there is a past and a future that are not interchangeable. Curiously enough, though, at the microscopic level this temporal direction is missing. Mathematical laws for the theoretical description of the motions and collisions of atoms in a gas, for example, are not subject to a temporal direction. The same is true for the Maxwell equations, the fundamental laws that provide a theoretical description of electromagnetism. The famous Austrian physicist Ludwig Boltzmann—he already appeared ­earl­ier—explained the irreversibility problem on a macroscopic level on the basis of probability. He postulated, for example, that heat running from high to low temperature is so much more probable than the reverse, that this latter possibility is never observed in actual practice. In the same way, it cannot be ruled out that, for instance, all gas molecules that are present in a room would, all of a sudden, find themselves in a thin layer, right under the ceiling. Fortunately, the probability of such an event is negligibly small, even in a time-­span that is many times longer than the current age of the universe. Max Planck had difficulty with Boltzmann’s reasoning and wanted to replace his probability considerations with a more precise mathematical derivation of irre­ versibility based on underlying time-­reversible equations. He saw a possibility to achieve his goal by considering black-­body radiation in a hollow space whose walls are a perfect mirror. He understood these walls as containing so-­called re­son­ators, charged vibrators that can emit and absorb radiation. Without going into much more detail here, Planck’s irreversibility project eventually foundered. As a side effect of his theoretical work, though, something else happened. Planck was actually able to derive a radiation law that was identical to Wien’s law. As was already noted before, Wien’s law only corresponded to the experimental results for small wavelengths. Now Planck tried to adjust his theoretical derivation


Quantum theory and the General Theory of Relativity

in such a way that he would also be able to describe large wavelengths sat­is­fac­tor­ ily. He only succeeded in doing so by introducing a curious hypothesis. He postu­ lated that the average energy of the resonators could be calculated by assuming that a large number of small, equal units of energy had been randomly distributed among them. The size of this unit, which he dubbed the energy element, was the  product of the frequency of the oscillator and a constant, introduced by Planck. This constant, which was later named Planck’s constant, was denoted by the symbol h. In this way, Planck was able to derive a new radiation law that was valid for all wavelengths and corresponded to what had been demonstrated experimentally. Besides, Planck’s law agreed with Wien’s law for small wavelengths, and for large wavelengths it was in accordance with Rayleigh’s law. To this day, Planck’s law for radiation still holds. From the preceding paragraphs it will be clear that Planck’s hypothesis had a  far more limited scope than the modern-­day quantum hypothesis. As was explained earlier, this hypothesis postulates that, always and in any form, energy can occur only in discrete quantities, or energy quanta. Planck’s hypothesis, how­ ever, only referred to his statistical method of distributing energy over resonators. Initially, neither Planck nor the other physicists had a clear idea of how his new hypothesis was to be implemented. Did the energy element serve only to achieve the desired result, or had a totally new property of nature come to light? If this was the case, the current ideas about nature would have to be drastically revised. For the time being, Planck tended toward the first idea. He later even called the introduction of quanta an “act of desperation.”4 In this attitude he was certainly not alone. With a few exceptions—more about this later—Planck’s colleagues were of the opinion that a basis for his quantum hypothesis would eventually have to be found in existing, classical physics.

Lorentz Quite soon after the appearance of Planck’s work, Lorentz commented for the first time on his mysterious new ideas. He devoted an article to the radiation prob­ lem, in which he attempted to derive a radiation law on the basis of a specific model.5 In his article, Lorentz considered a metal in which electrons were able to move freely to collide with each other and with the metal atoms. As was already stated earlier, it is a cornerstone of classical electromagnetic theory that acceler­ ated charges emit radiation. In the metal that Lorentz was considering, electrons were accelerated and slowed down by the collisions and, as a result, they were emitting radiation. Now Lorentz wanted to determine what the composition of this radiation was. That is to say, he wanted to derive a radiation law for this special case. The   “Akt der Verzweiflung.” See Planck to Robert Wood, October 7, 1931, quoted in Hermann 1969, 31.   Lorentz 1903c.

4 5



calculation that was required to do so turned out to be extremely complicated, and even Lorentz did not quite manage to pull it off. He had to resort to introducing approximations and, as a consequence, the radiation law he derived in this way was only valid for long wavelengths. Curiously enough, Lorentz’s result turned out to correspond with the form of Planck’s law for the case of long wavelengths and, therefore, also with Rayleigh’s law. Lorentz called this a remarkable result, because his derivation and the one by Planck were fundamentally different: Lorentz’s derivation was lacking the energy element postulated by Planck. Lorentz could not get the radiation problem out of his mind, and an invitation to give a paper at a large conference in Rome in 1908 gave him an opportunity to tackle the problem anew, in a more systematic and fundamental manner. His point of departure was the most general model he could imagine: a space filled with radiation and particles, both charged and uncharged. By applying the most fundamental laws of classical mechanics and electromagnetic theory, he pro­ ceeded to derive a radiation law.6 Although this time Lorentz had not needed to introduce approximations, he still reached the same result as before: Rayleigh’s law. Incidentally, this law had meanwhile become known as the law of Rayleigh–Jeans, or Jeans’ law for short, since the Englishman James Jeans had corrected a numerical error in Rayleigh’s derivation of the law. Now Lorentz concluded, on the basis of his general and compelling derivation, that only Jeans’ law could be the correct one. According to him, something had to be wrong with the experimental results that appeared to corroborate the validity of Planck’s law. Immediately, the experimental physicists were up in arms about his conclusion, and their comments were scathing.7 Lorentz realized very quickly that they were right and admitted that he was wrong.8 From that moment on, he accepted Planck’s law as the only tenable law of radiation. However, he was 6  See Lorentz 1908b, Lorentz 1908e, Lorentz 1909g, and Lorentz 1909h, the various published ver­ sions of Lorentz’s paper. 7   See, for example, the correspondence with Wilhelm Wien from 1908 in Kox 2008. 8   An important insight that brought him to this realization was that the Rayleigh–Jeans law predicted that a bar of silver should be visible in the dark at room temperatures. In a letter to Pieter Zeeman of August 20, 1908 (Kox 2018, 87) Lorentz confessed to his embarrassment: “I have to tell you that I have been very unfortunate in a certain respect with my Rome lecture. [. . .] I spoke of the well-­known way in which Jeans wants to explain the contradiction between his theory and the observations, but unfortunately I overlooked (I don’t quite understand how) that one cannot enter that road without getting into conflict with phenomena that are very well known and simple. Thus, instead of concluding ‘only Planck’s theory is feasible,’ I expressed myself at the end as if a decision between this theory and Jeans’s should still be sought. [. . .] I am, of course, extremely sorry that I was wrong, but I could do nothing else but admit it, because spoken words, too, cannot be undone.” (Ik moet U eindelijk schrij­ ven dat ik met mijne voordracht te Rome in zeker opzicht ongelukkig ben geweest. [. . .] Ik heb gespro­ ken van de bekende wijze waarop Jeans de tegenspraak tusschen zijn theorie en de waarnemingen wil verklaren, maar ongelukkigerwijze heb ik over het hoofd gezien (ik begrijp zelf niet hoe) dat men dien uitweg niet kan inslaan zonder met zeer bekende en eenvoudige verschijnselen in strijd te komen. Dientengevolge heb ik, in plaats van te concludeeren: ‘alleen de theorie van Planck is houdbaar,’ mij aan het slot van de voordracht uitgelaten alsof er nog een beslissing tusschen deze en die van Jeans zou gezocht moeten worden. [. . .] Het spijt mij natuurlijk zeer dat ik mij vergist heb, maar ik kon niet anders doen dan het erkennen, want ook gezegde woorden nemen geen keer.)


Quantum theory and the General Theory of Relativity

very much aware that a strange, non-­classical element had insinuated itself into physics. Despite his incorrect conclusion, Lorentz’s calculation was important, because its result had shown, once and for all, that the classical theory was unable to account for the radiation phenomena that had been observed experimentally. In the following years, Lorenz spent a considerable amount of time on the search for the correct interpretation of the quantum hypothesis, especially in his cor­res­pond­ ence with colleagues.9 Was the energy of Planck’s oscillators quantized as well? Was it only the exchange of energy between oscillators and the electromagnetic field—or, in Lorentz’s terminology, the ether—that took place in discrete quan­ tities? About these questions the physicists racked their brains, but a clear answer still eluded them.

Einstein Albert Einstein was one of the few who realized very quickly that Planck’s hypoth­ esis might have much farther-­reaching consequences than was originally antici­ pated.10 He made this clear in a pioneering publication in 1905.11 As he did so often in his work, Einstein looked with completely different eyes at one aspect of nature that was generally taken for granted. He noted a curious difference in the ways in which matter and electromagnetic phenomena were treated in physics. Matter is supposed to be constituted of all kinds of particles. In other words, matter is discrete. However, in order to explain electromagnetic phenomena, physicists make use of fields: electrical and magnetic fields. One characteristic of fields is that electrical and magnetic forces are present at every point in space. Unlike matter, fields are, therefore, not discrete but continuous. Now Einstein wondered whether an electromagnetic phenomenon like light might also have a discrete aspect. In a conceptually difficult, but mathematically relatively simple treatment, he then went on to show that light—and consequently all electromagnetic radiation—can, in some situations, also take on a discrete char­ acter. A light wave can behave as if it were made up of particles: the so-­called light quanta. Besides, Einstein was able to prove that the energy of such a light quan­ tum had the same value as Planck’s energy element: the product of the light fre­ quency and the constant h.12

9   See in particular the correspondence between Lorentz and Max Planck and Albert Einstein in Kox 2008. 10   One of the others, for that matter, was Lorentz’s successor, Paul Ehrenfest. For his contributions to the quantum theory, see the biography Klein  1970, which has meanwhile become a classic, and Duncan and Janssen 2019. 11   It was one of the four revolutionary articles from Einstein’s annus mirabilis (“miracle year”) 1905. The topics of the other three were the special theory of relativity, the Brownian motion, and the relation between mass and energy (E = mc2). 12   See also Darrigol 2014 for the history of the light quantum.



In his article Einstein also showed that his hypothesis could serve to explain experimental results that had not been properly understood until then. This is how he wanted to make his result credible to the physics community. One of those poorly understood experimental results was the so-­called photo-­electric effect, the phenomenon that a beam of light hitting a metal plate can free electrons from that plate. Now Einstein’s idea was that a light quantum colliding with an atom trans­ fers all its energy to an electron within that atom. This electron then uses the energy to escape from the metal and fly away. In his explanation, Einstein also supposed that the light quantum did not take up much space, or else it would be difficult for the quantum to hit an electron with precision. Incidentally, it was the explanation of the photo-­electric effect for which Einstein was awarded the 1921 Nobel Prize.13 He did not receive the prize, as is often thought, for his special theory of relativity or for his general relativity theory. That remains a permanent stain on the reputation of the Nobel Committee, dominated as it was at the time by conservative forces. Not surprisingly, Einstein’s hypothesis of the light quantum was met with a great deal of skepticism. Well-­known phenomena, like deflection of light and inter­ ference of light waves, could, until then, be explained only by assuming that light was a wave phenomenon. For that reason, it was difficult to fathom how a wave could, at the same time, consist of particles; particles that, on top of everything else, also took up very little space. Even the experimental evidence that Einstein had initially put forward was not viewed as convincing, and resistance against Einstein’s hypothesis did not disappear until more compelling experimental evi­ dence was assembled to support it.14 How controversial the quantum hypothesis really was, at the time, is also clear from the following. At the end of 1913, Einstein was nominated—by Planck, among others—for membership of the Prussian Academy in Berlin. In the ­nomination Einstein was lavished with praise for all his work, except for his light quantum hypothesis, which was characterized in so many words as a “stumble.” Nevertheless, the Academy said generously: “That, in his speculations, he may also sometimes have overshot his mark, like for example in his hypothesis of the light quanta, should not be held against him too much.”15 Lorentz also got involved in the discussion about the light quantum. In a lengthy letter to Einstein he clearly explained what he found problematic.16 The ­photo-­electric effect, he wrote, could only be explained by assuming that light

13   To be precise: He received the 1921 prize in 1922 because the prize had not been awarded in 1921 for lack of suitable candidates. 14   In particular, through experiments by the American physicists Robert Millikan (the photo-­electric effect) and Arthur Compton (the so-­ called “Compton effect”). See Styewer  2014 for historical ­discussions. 15   “Daß er in seinen Spekulationen gelegentlich auch einmal über das Ziel hinausgeschossen haben mag, wie z. B.  in seiner Hypothese der Lichtquanten, wird man ihm nicht allzuschwer anrechnen ­dürfen.” CPAE-­5, 445. 16   Lorentz to Einstein, May 6, 1909 (Kox 2008, 189).


Quantum theory and the General Theory of Relativity

quanta are small.Yet he deduced from a concrete example, namely the formation of the image in a telescope, that quanta would have to spread across several meters. How could this contradiction be eliminated?17 In later publications, Lorentz repeated this line of reasoning, without being able to reach a conclusion. Eventually, in a paper given in June of 1923, he could only express the hope for a future synthesis that would allow particles and waves to exist side by side: “We cannot help thinking that the solution will be found in some happy combination of extended waves and concentrated quanta, the waves being made responsible for interference and the quanta for photo-­electricity.”18

Bohr An exceptionally important step forward in the development of quantum theory was the work by the Danish physicist Niels Bohr, in 1913. He presented an atomic model that made mincemeat of the usual classical concepts. Besides, it provided a further extension of the application of the quantum hypothesis. Bohr’s model built on the ideas of Ernest Rutherford, a physicist who was born in New Zealand but worked in England. On the basis of experiments, Rutherford had concluded that atoms consisted of a small, hard, positively charged nucleus surrounded by negatively charged electrons. Now Bohr modified Rutherford’s model and postu­ lated that the electrons could move around the nucleus like planets in the solar system, but only in certain stable orbits and with a constant energy. Bohr also indicated what the distance of these orbits from the nucleus was and to what amounts of energy they corresponded. The further away from the nucleus, the higher the energy of the electrons in that orbit. Bohr now asserted that electrons were able to jump from a higher to a lower orbit and that, as a result, the atom would lose energy through radiation. Furthermore, he postulated that the transition was determined by the so-­called frequency rule: The frequency of the radiated light was equal to the difference in energy between the two orbits, divided by Planck’s constant.19 In order to reach a higher orbit electrons had to absorb radiation, according to the same principle. The extraordinary aspect of this atomic model was that it could not possibly be stable, at least not according to classical electromagnetic theory. As was stated before, according to classical electromagnetic theory accelerated charged particles emit radiation continuously. Since a circular motion is also an accelerated motion, 17   Modern quantum theory says nothing about this other than that light can behave sometimes like waves, as in the formation of an image in a telescope, and under other circumstances it can behave like particles, as in the photo-­electric effect. The two aspects are “complementary.” 18   “We kunnen alleen maar denken dat de oplossing zal worden gevonden in de een of andere geluk­ kige combinatie van uitgebreide golven en geconcentreerde quanta, waarbij de golven verantwoordelijk worden gemaakt voor interferentie en de quanta voor foto-­elektriciteit.” Lorentz 1924a, 611. 19   For those who can handle some mathematics: E = h𝜈, with 𝜈 the frequency of the radiation and E the energy difference.

The end of classical physics


the electrons in Bohr’s model would, by necessity, have to lose their energy very quickly in the form of radiation and would then eventually have to fall on the nucleus. One curious, if not revolutionary, element of Bohr’s model was that he proceeded to completely ignore this classical principle and postulated simply that “his” electrons were circulating without emitting radiation and were only radiat­ ing when they “jumped” to a lower orbit. Bohr’s model trampled on the tenets of classical electromagnetic theory. Yet, it was accepted rather quickly, because its further elaboration generated spectacular successes. Bohr was eminently able to explain which spectral lines, for example, would be emitted by a substance like hydrogen, and he could theoretically predict the value of an experimentally demonstrated constant without any trouble. Once  again, quantum theory had been successful where classical theory had fallen short.

The end of classical physics There is not one single moment that defines when the physics community accepted that the physics of classical theories had to be replaced by the physics of quantum theory. The theory’s acceptance was a gradual process, a chain of the­or­ et­ic­al and experimental developments. In this way, classical physics gradually made room for what is now called the old quantum theory and, in the years after 1920, for two still newer theories: the theories of wave mechanics and matrix mechanics. It would lead the reader too far afield to elaborate further on these develop­ ments. Suffice it to remark here that Lorentz did not have any trouble with the mathematical formalisms of these new theories, but that he did indeed struggle with the conceptual problems involved. This applied in particular to the very abstract theory of matrix mechanics.20 Wave mechanics, on the other hand, the theory that describes atomic matter in terms of waves in a multidimensional space, clearly was closer to Lorentz’s classical way of thinking. In an extensive cor­res­ pond­ence about this new theory he managed to impress the theory’s author, the Austrian physicist Erwin Schrödinger, with his deep knowledge and thorough understanding of his publications.21 Despite the conceptual difficulties that made it hard for Lorentz to understand the new theories, he lectured about them in his classes. Both in Leiden, in his Monday morning lectures, and at the California Institute of Technology in Pasadena—where he spent a few months teaching in 1922 and 1927—he devoted time to the latest theoretical developments and insights. Yet his attitude toward quantum theory remained ambivalent until the very end. In Chapter 12, this will

  See, for example, the correspondence with Ehrenfest about this subject in Kox 2018.   The letters have been published in Kox 2008.

20 21


Quantum theory and the General Theory of Relativity

be discussed in more detail in connection with his last publication, a paper that he presented at a conference in the fall of 1927.

General Theory of Relativity Einstein’s special theory of relativity was already discussed at length in Chapter 4. As was explained there, Einstein published his application of the relativity prin­ ciple to electromagnetic phenomena in 1905. He also highlighted the curious ­consequences of his theory for the interpretation of space and time. After a seven-­year struggle and after being sidetracked a number of times, Einstein followed up on his special relativity theory with the publication of his General Theory of Relativity in 1915. This theory, as the name indicates, is an extension of the special theory of relativity and, at the same time, a theory for gravity.22 It is useful to briefly outline the development history of Einstein’s new theory here. This provides the proper context to be able to delve into the work by Lorentz with regard to Einstein’s theories. In the decade following the 1915 publication, Lorentz was actually quite actively involved in Einstein’s work, and made im­port­ ant contributions to the General Theory of Relativity. The great limitation of the special relativity theory was that it was only valid for systems (or observers) that moved at a constant velocity in a straight line: the uniform motion mentioned earlier. In accelerated systems, such as planets revolv­ ing around the sun, or free-­falling elevators, the theory cannot be used. For this reason it is not surprising that, soon after 1905, Einstein started searching for a more general theory. Of course, such a theory had to be a generalization of the special relativity theory. In other words, the special theory had to be a special case of the general theory. A brilliant idea, which he later called “the most fortunate thought of his life,”23 delivered to Einstein the key to the desired generalization. This thought was gen­ erated by a so-­called thought experiment, an experiment that is not carried out in reality, but is only thought through in the mind of the “experimenter” and is based on solid physical principles.24 The thought experiment ran as follows: Consider the situation of a man in an elevator which is being pulled up by means of a con­ stant force. The elevator is put into accelerated motion and, as a result, the occu­ pant feels that he is pushed against the elevator floor. However, if the elevator’s occupant would be unable to look out, he might very well imagine that the force

 See Janssen 2014 for a historical overview of the development of the General Theory of Relativity.   “Der Glücklichste Gedanke meines Lebens.” CPAE-­7, 31, p. 265. 24   Thought experiments can be used to think through new hypotheses. For Einstein, thought ex­peri­ ments also played an important role in other situations, but this is not the place to go into this more extensively. See, for example, Norton 1991 for more details. 22


General Theory of Relativity


pushing him toward the elevator floor would not be caused by the accelerated motion, but by gravity. Now consider the case that the elevator cable is cut loose. The elevator will then go into free fall; it will fall down, in other words, in an accelerated fall. Now something interesting happens: The man in the falling elevator has become weightless because he is falling with a speed equal to the elevator itself. In effect he no longer feels the earth’s gravity, even though he is still subjected to it, of course. The phenomenon of weightlessness is well known from images showing ­free-­floating astronauts in their capsule revolving around the earth. Many people are under the impression that their weightlessness is caused by the astronauts being so far removed from the earth that they can no longer feel the earth’s grav­ ity. This is not the case. The astronauts are, in effect, experiencing a form of free fall, as the rotation of both the capsule and astronauts around the earth is caused entirely by the earth’s field of gravity—which actually extends much further than the orbit of the astronauts. The moon is also subjected to it, for example, so it is also responsible for the moon’s rotation around the earth. The conclusion Einstein drew from his thought experiment was that acceler­ ated motion and gravity were equivalent. In today’s physics this principle is referred to as Einstein’s equivalence principle. According to this principle, a gen­ eralized theory of relativity became a theory for gravity at the same time. Now Einstein had to tackle the problem of how to construct a new theory that included his new insight and how to devise the proper mathematical form to capture it. That turned out to be exceedingly difficult. After plugging away for many years, he developed the insight that he needed the mathematics of curved spaces to formulate his theory successfully. Curved spaces are spaces in which the shortest route between two points is no longer a straight line, but a curved one, in the same way that the shortest route between two points on earth is the curved line across the surface of the globe. Anyone who has ever looked at the navigation screen in the cabin during an intercontinental flight is able to conjure up this image. It would lead the reader too far afield to go into much more detail here about Einstein’s theoretical considerations. Suffice it to say that in 1915, after much trial and error, Einstein eventually managed to present a theory for gravity. His theory had a mathematical form that was totally new for most physicists. If one wanted to try and put the General Theory of Relativity into words instead of using a mathematical equation, this would, of course, not do complete justice to the theory’s form. Should one want to try anyway, then it would be best to take the solar system as a starting point. The theory’s essence could then be expressed in the following statement: The sun tells space how to curve and space tells the planets how to move. In other words, the geometry of space-­time determines how bodies move, and this geometry is the result of the presence of matter—in this case, the sun.


Quantum theory and the General Theory of Relativity

Lorentz and general relativity From the beginning, Lorentz was fascinated by Einstein’s general relativity theory. As early as 1914, he went to work on an early version of the theory, a version that was actually dropped by Einstein a year later because it turned out to be funda­ mentally flawed. Lorentz’s thorough study of the theory resulted in a series of extraordinarily technical articles—published in 1915 and 1916—in which he ana­ lyzes the relativity theory extensively and provides it with a new theoretical foun­ dation.25 Very soon the complicated mathematical formalism held no secrets for Lorentz, as is evidenced not only by his first article on the theory, published in 1915, but also by the great quantity of calculations and notes that have been ­preserved among his papers. When Einstein’s definitive theory eventually appeared, at the end of 1915, Lorentz and Ehrenfest immediately became intensely interested. Soon, Lorentz, Ehrenfest, and Einstein embarked on a lively discussion, in an extensive cor­res­ pond­ence that has, for the most part, been preserved.26 The final result was that Lorentz and Ehrenfest accepted Einstein’s theory completely—but not before Einstein had cleared up a number of objections they had raised. Lorentz was so enthused about the theory and he had so quickly gained such complete command of the formalism, that he decided to devote his Monday morning lectures to the theory. As a rule, Lorentz’s lectures were attended mainly by advanced students, for whom the classes were intended, but in this case he also found colleagues among his audience. Because Lorentz was able to explain com­ plicated material so clearly, this was a better way for many of them to familiarize themselves with the theory than by reading Einstein’s often somewhat arcane publications. As was said already, Lorentz published extensively about the General Theory of Relativity. He also inspired other Dutch physicists and mathematicians, including Adriaan Fokker, Hendrik Kramers, and Jan Arnoldus Schouten, to work in this field.27 Another one was Lorentz’s doctoral student Johannes Droste, a talented mathematician who later became professor of mathematics in Leiden. In his dis­ sertation research he had managed to find a solution for Einstein’s equations for a special case: the gravitational field of one single material body, for example of the sun.28 The German astronomer Karl Schwarzschild had been working on the same problem and he published the same solution as Droste, unfortunately just slightly earlier. As a result, Droste has receded somewhat into the background and

25  See Lorentz  1915a, Lorentz  1916b, Lorentz  1916c, Lorentz  1916d, and Lorentz  1917d. See also Janssen 1992 for a discussion. 26  See Kox  2008 and Kox  2018 for the correspondence of Lorentz with Einstein and Ehrenfest respectively; the correspondence between Einstein and Ehrenfest from this period is published in CPAE-­8. Its content is also discussed in Chapter 7. 27 28  See Kox 1992c for an overview.  See Droste 1915 and his dissertation Droste 1916.

Lorentz and general relativity


the result is known nowadays as the Schwarzschild solution.29 In a follow-­up to their earlier work, Lorentz and Droste together tackled a problem that was even more complicated: the field of a number of material bodies. Unfortunately, their important results did not become as widely known as they deserved to be. They were only published in Dutch, in the Zittingsverslagen of the Academy, which is why they went unnoticed for quite some time.30 Because of the war, it had not been possible to have the articles translated into English to include them, as usual, in the English-­language Proceedings of the Academy. This is why others physicists have—unbeknownst to them—duplicated the work by Lorentz and Droste. The war was also the reason why the English no longer had access to German scientific literature and had completely missed out on the new developments in physics in Germany. Fortunately, the Leiden astronomer Willem de Sitter had fol­ lowed Lorentz’s lectures on Einstein’s theory with keen interest. Using the know­ ledge he had acquired there, he summarized the General Theory of Relativity—and even extended it to a certain extent—in a number of articles for an English scien­ tific journal. Through De Sitter’s publications the physics community in England was able to familiarize itself with Einstein’s work, albeit with considerable delay. In his own research, apart from his role in the dissemination of Einstein’s work, De Sitter also made a number of important contributions to general relativity, in par­ ticular in its application to cosmology. Apart from his technical discussions and his teaching, Lorentz was also instru­ mental in exposing a broader audience to Einstein’s theory by writing a number of popular articles on the subject.31 There was also another way in which he was an important mediator for the propagation of Einstein’s theory. To explain this, it is necessary to return briefly to the theory’s content. Einstein’s theory made two extremely curious predictions that captured the imagination of many.32 The first prediction was that the path of a ray of light in a gravitational field is not a straight orbit, but a curved one. The second prediction was that the wavelength of light emitted by a source in a gravitational field shifts toward red, and more so as the field becomes stronger. This prediction of red shift was extremely difficult to test and could not be experimentally confirmed until many years later.33 For an experimental test of the curved path of a light ray under the influence of gravity, a solar eclipse offered an excellent opportunity. A solar eclipse made it  See Eisenstaedt 1982 for historical background.  See Lorentz and Droste 1917a and Lorentz and Droste 1917b. See also Havas 1989 for the later work. 31   See, for example, Lorentz 1917e. 32   In addition, he had an immediate success. Soon after the publication of his theory, Einstein man­ aged to provide an explanation for the, until then, mysterious deviation in the orbit of the planet Mercury: “the anomaly in the perihelium motion” of this planet. It is interesting to note that Lorentz had already used this deviation in 1900 to test his own theory of gravity—with a negative result. 33   A possibility was to look, for example, at the spectral lines in the solar atmosphere. The final confirmation was not made until 1959. See also Will  1993 for the experimental confirmations of ­relativity. 29 30


Quantum theory and the General Theory of Relativity

possible to see stars in the sky that are very close to the sun. As the light of such stars passed so closely by the sun, in its very strong gravitational field, this light would by necessity have to be deflected, according to Einstein’s theory. If, during a solar eclipse, the exact position of the stars in question could be recorded by means of photographs, and if these photographs were then compared to photo­ graphic records of the same part of the sky when the sun was not close by, Einstein’s prediction could be tested. If the positions of the stars were different, Einstein would be right—at least if the size of the differences was in accordance with his prediction. Testing Einstein’s prediction of the deflection of light was precisely the ob­ject­ ive of two English expeditions, in May of 1919, to places where a solar eclipse could be observed.34 They were directed by the astronomer Arthur Eddington, one of the English scientists who, thanks to De Sitter’s articles—and so, indirectly, thanks to Lorentz—had become interested in the theory of general relativity. Later, after the expeditions had returned home and the measurements had been processed, Lorentz’s involvement was more direct. When preliminary results became known among a small circle of professionals and when it had become clear that they confirmed Einstein’s theory, Lorentz was one of the first people to hear about it. He immediately sent Einstein a telegram with the momentous news.35 The Great War had ended so recently that contacts between the Anglo-­ ­ Saxon and the German scientific world were still minimal. Without Lorentz’s intervention it would have taken much longer before Einstein would have heard about these experimental results that were so important to him. The confirmation by the solar eclipse results of Einstein’s theory captivated people’s imagination worldwide, and this certainly did not apply only to scientific circles. There was something magical about a theory that was rumored to be understood only by very few people in the world and whose predictions somehow appeared to have upended the world of science.36 After the official scientific ­publication of the results,37 in the beginning of November, leading newspapers all over the world reported the big news. Papers like the Times of London and the New York Times provided massive coverage. Even the typography of the headlines in the New York Times on November 10, 1919 showed how great the excitement was that was generated by the expedition results, also for the general public. “Lights all askew in the heavens” shouted the front page in all caps, followed by

  The expeditions went to Sobral in Brazil and the island of Principe before the West-­African coast.   The telegram is dated September 22, 1919. See Kox 2008, 339, for the text. 36  An anecdote has it that during the presentation of the solar eclipse results a journalist asked Eddington: “Am I right that there are only three people in the world who understand the theory of relativity?” Eddington, who was hardly the epitome of modesty, is supposed to have answered: “Who is the third one, then?” 37   The final results were: Sobral 1″.98 ± 0″.30; Principe 1″.61 ± 0″.30. Einstein’s prediction was 1″.74. See also Kennefick 2019 for an historical account of the expeditions and their aftermath and a refutation of the persistent rumor that Eddington had manipulated the results in Einstein’s favor. 34 35

Lorentz and general relativity


a further series of screaming headlines about the important events that had just happened.38 In the Netherlands, Lorentz himself wrote an article, on November 19, for the nationally circulated daily Nieuwe Rotterdamsche Courant.39 Its tone was somewhat more subdued than the loud American headlines quoted earlier, and later, in December of the same year, the New York Times also published this more calmly formulated analysis. The somewhat overheated publicity of 1919 was the start of the enormous, worldwide fascination with Einstein among the public at large, which would only become greater over the years. In the words of Abraham Pais, one of Einstein’s biographers, this was the beginning of the “canonization” of Einstein and the “birth of the legend.”40 To conclude, there is one more intriguing question to discuss. In Chapter  4, Lorentz’s objections to the special theory of relativity were already mentioned, and in particular his problems with the disappearance of the ether. Now the ­question is how Lorentz could, nevertheless, be so enthused about the General Theory of Relativity. This theory was, after all, a generalization of Einstein’s special theory that had done away with the ether. The answer is that to accept the General Theory of Relativity, there was no need at all for Lorentz to leave the concept of the ether behind. The ether could remain part of the theory. In a paper presented in 1917, he argued how exactly it could play a role again in Einstein’s theory: There is, for example, no doubt that the electromagnetic waves, which we use in wireless telegraphy, do not share in the rotation of the earth, so that, if we could only observe their propagation more accurately than we can do now, we could find proof of this rotation in the details of their propagation. But all motion is relative. Nowadays more than ever. If we say that the earth rotates, we must, therefore, add with respect to what it does so, and then the electromagnetic waves must be connected with this “what,” in one way or another. [. . .] Would it not be so much simpler to imagine a stationary ether, with respect to which the earth rotates and in which the electromag­ netic waves propagate without being disturbed by the motions of the earth?41

38   They read: “Men of science more or less agog over results of eclipse observations.” “Einstein theory tri­ umphs.” “Stars not where they seemed or were calculated to be, but nobody need worry.” “A book for 12 wise men. No more in all the world could comprehend it, said Einstein, when his daring publishers accepted it.” 39   Nieuwe Rotterdamsche Courant, December 19, 1919. A translation appeared in the New York Times of December 21, 1919. 40  See Pais 1982, 303, 306. 41   “Het lijdt, om een voorbeeld te noemen, geen twijfel, dat de elektromagnetische golven waarmede wij bij de draadlooze telegraphie werken, niet in de aswenteling der aarde delen, zoodat wij, als wij hun voortplanting maar veel nauwkeuriger dan nu konden waarnemen, in de bijzonderheden daarvan bewij­ zen voor die wenteling zouden kunnen vinden. Maar alle beweging is relatief. Tegenwoordig meer dan ooit. Als wij zeggen dat de aarde draait, moeten wij er dus bijvoegen ten opzichte van wat zij dat doet en met dat ‘wat’ moeten dan de electromagnetische golven op een of andere wijze verbonden zijn. [. . .] Zou het niet wel zoo eenvoudig zijn, om een stilstaanden aether voor te stellen, ten opzichte waarvan de aarde draait en in welken de electromagnetische golven zich voortplanten zonder zich aan de bewegingen der aarde te storen?” Lorentz  1917c, 41–42. The argument of the rotating electromagnetic waves also appears in Lorentz to Einstein, June 6, 1916 (Kox 2008, 313) and in Lorentz 1923c. In that publication Lorentz also discusses a number of other possibilities to determine the rotation of the earth.


Quantum theory and the General Theory of Relativity

Einstein publicly reacted to this line of reasoning in his inaugural address, in October 1920, when he became special professor in Leiden.42 He met Lorentz halfway by suggesting that the mathematical structure that the General Theory of Relativity imposes on space could be interpreted as a new kind of ether. It has been suggested that Einstein only made this concession out of politeness toward his host and role model, but this seems unlikely, because Einstein was hardly the kind of man who tended to choose politeness over scientific rigor.

42   The title of the lecture is telling: “Ether and the general theory of relativity” (Äther und allgemeine Relativitätstheorie). See CPAE-­7, 38, for the text; see also Kox 1988 for a discussion.

Chapter 9 World W   ar One

On June 18, 1914, Archduke Franz Ferdinand, heir to the throne of the ­Austro-­Hungarian Empire, was assassinated in Sarajevo. This put a series of events into motion that rapidly escalated into what is now known as World War One.1 Austria declared war on Serbia on July 23, Germany followed, declaring war against Russia on August 1 and against France on August 3. The following day the German army invaded neutral Belgium on its way to France, thus avoiding the  fortifications on France’s eastern border. That same day Great Britain joined the war in support of Belgium and France. The Germans proceeded to occupy a large part of Belgium, as well as parts of northern France: The Great War, as it was called at the time, had begun. During the next four years, Germany, ­ Austria-­ Hungary, and Bulgaria—the “central powers”—fought the “allied powers,” of which England and France were the most important ones. The main fighting went on in Europe, in the trenches of Belgium and northern France, but battles were also fought in Africa and in the Middle East. Nine million men and women died, including many physicists. In the Netherlands, the German violation of Belgium’s neutrality caused a great uproar. The Netherlands were neutral as well, so what had happened to Belgium might also happen there. By the end of August, the Dutch had already mobilized their army to guard the German and Belgian borders. It would remain on the borders until the end of the war.2 Germany left the Netherlands entirely untouched during the war, but that had little to do with the army presence at the border. The Dutch forces were weak and poorly equipped and they were no match for the Germans. For Germany, it was simply of strategic interest to have a neutral country in the vicinity—though the Germans did not foresee at the time that this very same neutral country would harbor the Kaiser after their defeat.

  An interesting recent book on the events leading to the outbreak of the war is Clark 2012.  See Moeyes 2001 for a history of the Netherlands during the war.

1 2

“A Living Work of Art”: The Life and Science of Hendrik Antoon Lorentz. A. J. Kox and H. F. Schatz, Oxford University Press (2021). © A. J. Kox and H. F. Schatz. DOI: 10.1093/oso/9780198870500.003.0010


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The peace movement Not surprisingly, the outbreak of the war caused great consternation among the Dutch organizations that promoted world peace. In the Netherlands, there had been an active peace movement for quite some time. It was particularly vocal in  the aftermath of the first great international peace conference, which was held  in The Hague in 1899. In the first decade of the twentieth century there was sufficient justification for the existence of peace movements, as a number of crises occurred in succession, all with the attendant danger of war. The Boer War, 1899–1902, was followed by the Morocco crisis, a dangerous international crisis in North Africa, and in the years preceding 1914 many were convinced that Europe was heading for another war.3 Already before the war, Lorentz had become involved in one of the peace organizations: the Algemeene Nederlandsche Bond “Vrede door Recht” (General Dutch Union “Peace through Justice”). This left-­leaning union, established in 1901 as a result of a merger between two other peace organizations,4 mainly advocated the use of arbitration for the prevention of international conflicts. One became a member by donating 1 guilder per year, and whenever a town had acquired a large enough membership a local chapter was established. This is how, in April 1904, a Leiden chapter came into existence. The founding meeting of the Leiden chapter, held at the Lorentz home, elected Lorentz as vice-­chairman. Just like the other chapters, it organized meetings that featured a speaker at least once a year. The first meeting took place in November 1904 and was chaired by Lorentz. Four years later, the usually detailed newspaper accounts of the organization’s events mention Lorentz again, this time as the chairman of the chapter’s annual meeting, but from then on, it appears that he was no longer on the board. In 1913, an article by Lorentz in a special issue of the union’s official publication showed that he was still a member.5 In the piece he lauded international co­oper­ation in scientific circles, citing his own experience as a physicist and a number of large international projects that transcended strictly national interests, as well as the great number of scientific conferences in many different fields. That did not mean, he asserted, that there was no more room for individual scientific work. Quite the opposite. There should be room for individual nations with their own separate characteristics. His conclusion—appearing out of the blue—was

3   See also Riemens 1995 and Riemens 2005 for the Dutch peace movements before and during World War One. 4  The two organizations were the languishing Het Algemeene Nederlandsche Vredesbond (General Dutch Peace Union) (the neuter Dutch article “het” is grammatically incorrect here but was used on purpose to indicate that the organization was impartial) and the very active Nederlandsche Vrouwenbond ter Internationale Ontwapening (Dutch Women’s Union for International Disarmament). See also Schakenraad  1984 for more about Vrede door Recht. 5   Lorentz 1913a.

The peace movement


that ­scientific cooperation would eventually “engender feelings of appreciation, solidarity and good friendship”6 and that this in turn would promote peace. It may not have been his best piece of work, but it did show that the peace question was dear to Lorentz’s heart. The outbreak of the war caused great concern in Vrede door Recht, but the members were as indecisive as they were concerned. Even after a month the members still had not managed an adequate response. This shocking incompetence prompted some to take a truly national peace initiative, but even under pressure of war the various Liberal, Christian, and Socialist peace organizations were in­cap­able of joint action. Finally, in October 1914, a group of private citizens took the bull by the horns and founded the Nederlandsche Anti-­Oorlog Raad (NAOR) (Netherlands Anti War Council). A later offshoot of the NAOR was the Centrale Organisatie voor een Duurzamen Vrede (Central Organization for a Lasting Peace), a very successful international pacifist organization which, at its peak, had a membership of some 40,000. Among other things, it worked to formulate the conditions necessary to create a peaceful society after the war. Lorentz was never really active in the NAOR. He attended a general assembly in January of 1915, but a month later he sent a note that he was unable to attend the next meeting and, after that, his name no longer appeared in the NAOR archive.7 Like dozens of other prominent Dutch citizens, Lorentz signed its “Appeal to the Dutch Nation,” which was carried by a number of large dailies,8 but after his support for this appeal Lorentz disappeared from view, both in the NAOR and in Vrede door Recht. That does not mean that Lorentz was no longer involved in war issues. In an  article in the weekly De Amsterdammer, Lorentz showed a somewhat more ­personal involvement, though his approach is decidedly cautious. The issue of September 13, 1914 carried five articles by representatives of science and the arts who had been asked to write about their reactions to the recent destruction of the Belgian city of Louvain by the German army. This act of war had reduced hundreds of houses to ashes, had destroyed precious works of art in the cathedral, and had completely annihilated the university library filled with countless ­irreplaceable manuscripts and books. Lorentz was one of the authors invited by the editors.9 Almost everyone outside Germany placed the responsibility squarely with the German army, but Lorentz’s overly careful judgment was that the degree of responsibility of the parties involved was still difficult to establish. Apparently not wanting to be seen to take sides, he

  “gevoelens van waardering, solidariteit en goede kameraadschap.”   See Archive NAOR, International Institute of Social History, Amsterdam.   See, for example, Algemeen Handelsblad, October 13, 1914. 9  The others were the architect Pierre Cuypers, who made short shrift of the request by only ­writing  one paragraph, the composer Alphons Diepenbrock, with a thorough discussion about “La grande Allemagne” (The Great Germany), the author Frederik van Eeden, and the historian Samuel Muller Fzn. 6 7 8


World W   ar One

wrote that he did not want to discuss Louvain, but only “that which I can judge from up close.”10 Instead of condemning the death and destruction brought about in Belgium, Lorentz sang the praises of Ernest Solvay. Without a doubt, Solvay deserved praise, but it seemed somewhat out of place in this context. Going into quite some detail about the importance of the Solvay conferences and the ac­tiv­ ities of the Solvay Institute, he emphasized that the subsidies disbursed by this Institute had gone, in large part, to German scientists. At the end of the piece he lauded Solvay and somewhat perfunctorily expressed his “compassion [. . .] with the tribulations of the nation of which he is one of the high-­minded and best representatives.”11

Aufruf an die Kulturwelt Like so many others in the Netherlands, Lorentz was very indignant about the German invasion in Belgium and about the unscrupulous warfare by the Germans, who had caused innumerable civilian casualties and enormous collateral damage. Lorentz was initially cautious not to be overly partisan, apparently hoping to sound a note of moderation. He became really agitated, though, after reading the Aufruf an die Kulturwelt (Appeal to the World of Culture), a manifesto published in October 1914 in which ninety-­three prominent German intellectuals and artists unreservedly took the side of the German army. They defended the invasion of Belgium and resolutely rejected all accusations of wrongdoing and intentional destruction. Lorentz was struck especially painfully when he counted many of his colleagues and acquaintances among the manifesto’s signatories, such as Felix Klein, Walther Nernst, Max Planck, Wilhelm Röntgen, and Wilhelm Wien. The core of the Aufruf consisted of six statements, all preceded by the phrase “Es ist nicht wahr, dass” (It is not true that). Germany, according to the manifesto, bore no guilt for the war, had not violated Belgium’s neutrality, had only in exceptional cases injured the lives and goods of Belgian civilians, and had not violated the rules of engagement. The fight against German militarism was a fight, according to the Aufruf, against German culture. In brief, the manifesto’s drift was that the people and the army of Germany were one and the same, that the heritage of Goethe, Beethoven, and Kant was sacred for the German nation, and that Germany, as a Kulturvolk (nation of culture), would continue its struggle to the very end. Not surprisingly, the Aufruf caused great indignation on the part of the allies and led to flaming counter-­manifestos.

  “over wat ik van nabij kan beoordeelen.”   “deernis [. . .] met het zwaar beproefde volk, waarvan hij een der hoogstaande en beste vertegenwoordigers is.” At the behest of Lorentz, the article also appeared in translation in Belgium (l’Indépendance belge, October 4, 1914), France (Revue du Mois, 18 (1914): 456–458), and Germany (Naturwissenschaften 2 (1914): 997–998; without the last sentence), and in abridged form in Vossische Zeitung (November 22, 1914). See also the correspondence with Marcel Brillouin (LA 11) and Emil Warburg (LA 86), who mediated in France and Germany respectively. 10 11

Correspondence with V   oigt


After Lorentz’s rather gratuitous participation in Vrede door Recht and the NAOR, he could no longer stand by passively when it became inescapably clear— partly as a result of the Aufruf—how radically the relations between the scientists of the warring countries had broken down. In the allied countries scientists also took sides, albeit not in such a radical way as they did in Germany. It appears that Lorentz had decided to direct his efforts for a future peaceful society toward the world where he felt most at home and where he had considerable authority: the world of science.

Correspondence with Voigt Things began to heat up when Lorentz’s German colleague and regular cor­res­ pond­ent Woldemar Voigt wrote a letter, in November, to create more understanding for the German standpoint.12 He downplayed the role of Germany, arguing that it had been forced into the war, especially by England, and that the country was now struggling for its very survival. Voigt praised the unity of the German nation, about which, he complained, the lying English press was systematically keeping silent. He compared the German struggle to the seventeenth-­century Dutch uprising against the Spanish—known in the Netherlands as the Eighty Years’ War. Two weeks later, he sent Lorentz a booklet about his experiences as a soldier in the Franco-­German war of 1870–1871, in which the Germans had been victorious.13 In the accompanying letter he took a swing at the Belgians, claiming that they had treated the Germans like animals, and he maintained that the dis­cip­ line of the German troops was much greater than that of the British army during the Boer War. At the end of December, Lorentz sent his reply.14 He wrote back that the invasion of Belgium and the ravages of the war that was fought there had “un­speak­ ably saddened” him. He then asked whether Voigt agreed with him that nations, like individuals, should never commit an injustice, even if not doing so would be to their own detriment. The booklet he would read later, he said, because “now I am not in the mood for it.”15 Voigt replied in a long letter in which he almost caricaturized the war.16 He had been in the occupied part of Belgium, he wrote, to visit his sons who worked there as nurses, and everything was completely normal there: not a trace of devastation and everybody happy and cheerful. All reports of damage in Belgium were pure British propaganda. Everything the Germans did was justified, since Germany was struggling for survival. Lorentz did not reply to the letter.

  Voigt to Lorentz, November 17, 1914 (Kox 2008, 271).   See Voigt to Lorentz, November 30, 1914 (Kox 2008, 273). The booklet is Voigt 1913. 14   Lorentz to Voigt, December 27, 1914 (Kox 2008, 276). 15   “unsäglich betrübt haben.” “jetzt bin ich dazu nicht in der Stimmung.” 16   Voigt to Lorentz, January 13, 1915 (Kox 2008, 277). 12 13


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Committee for the Union of European States The correspondence between Lorentz and Voigt was resumed in February 1915, when Voigt reacted to a petition summoning European governments, heads of state, and members of parliament to forgive any pain that had been inflicted during the war and to strive for a just peace.17 Voigt had received the petition in the mail. It was initiated by the committee De Europeesche Statenbond (Committee for the Union of European States). Lorentz had been involved in drawing up the manifesto and was also one of its signatories. Others were the writer Nico van Suchtelen, who was the secretary of the committee, the psychologist Gerard Heymans, the novelist Frederik van Eeden, and Lorentz’s friend and colleague, the physicist Pieter Zeeman.18 The idea behind the petition was to immediately collect as many signatures as possible in the warring countries, but to only publicize the final result after peace negotiations were opened officially.19 As it turned out, there was little international enthusiasm to sign the petition. One of the very few foreigners who signed was Albert Einstein.20 Not surprisingly, Voigt’s reaction to the petition was skeptical. He had little hope for a just peace and, as a supporting argument for his skepticism, he cited the warmongering of British statesmen, among them Winston Churchill. Their public statements boiled down to the idea that Germany should be destroyed completely. In his reply, Lorentz expressed doubt about the warmongering cited by Voigt. In the Dutch newspapers he had never seen anything of the sort, and a Leiden historian whom he had consulted was also totally unaware of any inflammatory statements by British politicians. Nevertheless, Voigt stuck to his guns.21 In France the reaction to the petition was also quite negative. Marcel Brillouin, for example, a professor at the Collège de France and a member of the Comité ­scientifique of the Solvay Institute, was very indignant.22 In particular, the appeal for forgiveness had provoked his anger.

  Voigt to Lorentz, February 19, 1915 (Kox 2008, 281).   The petition had been preceded by a telegram to the German emperor, sent by Van Suchtelen, Van Eeden, and a few others, making an urgent appeal to prevent disastrous events like the ones that had taken place in Louvain from happening in the future. Despite an insistent plea, Lorentz had not signed. See Van Suchtelen to Lorentz, August 31, 1914 (LA 77) for the text, and Lorentz to L. Simons, undated draft (LA 70), for his reaction. 19   See Lorentz to Nico van Suchtelen, undated draft, Van Suchtelen’s reply of January 18, 1925, and Van Suchtelen to Lorentz, April 15, 1915 (all in LA 77). 20   Einstein himself had also initiated an appeal in Germany, a kind of counter-­manifesto against the Manifesto of the 93. It was only signed by two others, and it did not exactly enhance Einstein’s reputation in Germany (see CPAE-­6, 8). 21   See Lorentz to Voigt, March 11, 1915 and Voigt to Lorentz, March 16, 1915 (Kox 2008, 282 and 283). 22  Others were the Swiss physicist Charles-­Édouard Guillaume and the French mathematicians Jacques Hadamard and Émile Picard. 17 18

Committee for the Union of European States


I feel compelled to reply in a negative way to your circular, which, I must admit, has first caused me to burst out in great rage and then made me somewhat sad. [. . .] We neither want to forgive, nor forget; we have nothing to be forgiven for.23

A signature at this time, under this petition, so Brillouin continued, meant to prostrate oneself before the Kaiser, throw up one’s hands and plead for forgiveness. In his answer to Brillouin, Lorentz explained the origins of the petition.24 He called Van Suchtelen an idealist with ideas to which he himself and the other ­signatories subscribed. Zeeman and Lorentz had suggested names of other ­colleagues who might want to support the petition, he wrote, but he had been surprised that the petition had already been publicized so soon, given the original plan to do so only after the war. He did admit that in supporting the initiative he had ignored the strong feelings in the warring nations and he offered his apologies for the pain he might have caused Brillouin. Brillouin was stung by the word idealist in Lorentz’s letter, and he sent a cutting reply: One must be extraordinarily idealistic not to hear from such a close distance the fusillades of the old men and children in Belgium, and the cries of the women and young girls, being raped without any intervention by the officers to protect them, or by these officers themselves.25

According to Brillouin, everyone with relatives or acquaintances in Belgium knew about these kinds of atrocities. Among those of his students who had not been able to bring their families to safety before being called to arms, there was one whose wife and daughter had disappeared without a trace, while another’s wife and daughter had both been raped and were pregnant. Assessing the whole episode of the petition, it is fair to say that on the part of Van Suchtelen and his fellow petitioners it was all a question of well-­meaning and naive—though misguided—idealism, mixed with a healthy dose of complacency. Given the neutral status of their country, the Dutch obviously felt justified to tell the warring parties how they should resolve their conflict.26 It is regrettable that Lorentz, perhaps carried away by his desire for peace, allowed himself to get involved in this somewhat self-­congratulatory enterprise.

23   “Je m’empresse de répondre négativement à votre circulaire, qui, je vous l’avoue, m’a d’abord mis fort en colère, et ensuite fait quelque peine. [. . .] Nous ne voulons ni pardonner, ni oublier; nous n’avons rien à nous faire pardonner.” Brillouin to Lorentz, February 25, 1915 (LA 11). 24   Lorentz to Brillouin, undated draft (LA 11). 25   “Il faut être extraordinairement idéaliste pour ne pas entendre à si petite distance les fusillades de vieillards et d’enfants en Belgique, et les cris des femmes et jeunes filles violées sans intervention des officiers pour les protéger, ou par eux-­mêmes.” Brillouin to Lorentz, March 28, 1915 (LA 11). 26   Some Dutch readers may be reminded of the term “Nederland gidsland” (Netherlands guiding-­ country), in use in the 1960s and ’70s.


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Wilhelm Wien and Max Planck The correspondence with Voigt illustrates to what extent the relations between the warring nations had been poisoned by propaganda and hate and how even a representative of a neutral country was powerless in the face of such deep-­seated feelings. A similarly skeptical attitude to Voigt’s was also on display in an exchange of letters with two other German colleagues. In March of 1915 Lorentz addressed two identical letters to Wilhelm Wien and Max Planck.27 His message was unequivocal. The saddening bitterness that reigned in France, as witnessed by colleagues, family, and friends, was to be ascribed primarily to the method of warfare the Germans had employed in the occupied territories. Lorentz referred to all kinds of printed sources and offered to send these to Wien, although he did not expect them to have much effect: “What is the use of writing this to you? I hardly know.”28 Nevertheless, Lorentz said, it was impossible for him to continue his silence in light of all the information he had at his disposal. In the final paragraph of his letter, he explained his feelings in eloquent terms: You know how sincerely I admire your science, your poetry and philosophy, your diligence and the energy of your nation, how I venerate your great men and how much value I have always attached to the relations with German colleagues. But you will understand that, just because of that, I believed I needed to write this to you. I have done so with a heavy heart.29

Both Planck and Wien replied with the usual German arguments, roughly repeating those in the Aufruf, though Planck’s letter was somewhat subtler than Wien’s.30 Germany was struggling for survival, he wrote, and that overshadowed everything. He deplored the increasing obduracy of the respective standpoints and the impossibility of making sense of all the contradictory information. On the German side, too, there was no end of suffering. A cousin of his, for example, as well as two sons of his Berlin colleague Walther Nernst, had been killed on the battlefield, while his own son Erwin had been made a prisoner of war. Yet, he did see a positive side to all this suffering: And nevertheless, more than one thousand years of German history have never, at any time, seen the German nation so united. Would it truly be a bad thing, something

27   Lorentz to Wien, March 22, 1915 (Kox 2008, 284). In the letter Lorentz mentioned that he had sent an identical letter to Max Planck. That letter has been lost, however. 28   “Welchen Nutzen es hat, dass ich Ihnen dieses schreibe? Ich weiss es kaum.” 29   “Sie wissen, wie aufrichtig ich Ihre Wissenschaft, Ihre Dichtkunst und Philosophie, den Fleiss und die Arbeitskraft Ihres Volkes bewundere, wie ich Ihre grossen Männer verehre und auf den Verkehr mit den deutschen Fachgenossen immer hohen Wert gelegt habe. Aber Sie werden verstehen, dass ich eben deshalb gemeint habe, Ihnen dieses schreiben zu müssen. Ich tat es schweren Herzens.” 30   Planck to Lorentz, March 28, 1915 (Kox 2008, 285).

Wilhelm Wien and Max Planck


that engenders such willingness to sacrifice, such pure, sacred inspiration? I cannot believe it. For I also know no purer and more beautiful sacrifice than that made for the good of the fatherland, to which we owe everything that we are and that we possess, not in the least also the possibility to live (even now!) our science.31

It was no longer important now who was at fault for the war, according to Planck. His motive to sign the infamous Aufruf had been to make it clear that German scientists did not want to separate their activities from those of the German ­government and the German army. Wien reacted much more sharply than Planck. In two letters in reply he accused the allies in no uncertain terms of waging a systematic campaign of lies that was mostly responsible for the embittered atmosphere in France.32 He denied any misconduct by the German troops and forwarded newspaper clippings as con­firm­ ation. At the end of one of the letters, he emphasized that especially the neutral nations needed to play their part in an objective judgment of all the accusations that had been put forward on both sides. In his answer to Wien, Lorentz defended himself against the implicit accusation that he had allowed himself to be influenced by the British and French press.33 His judgment, he emphasized, was based on letters and personal accounts and on trustworthy information in brochures and articles. He added that the newspapers in the Netherlands also reported calmly and objectively about the events. Lorentz also raised the Aufruf once more. If only it had said: “We cannot believe that…” instead of “It is not true that…” Wien answered in a long letter in which he did not bring up any more arguments than he had done in his previous letters.34 It threatened to become a repeat performance, so Lorentz, prudently, left it at that. In his letter to Wien, Lorentz also mentioned that he had spoken with Planck and other colleagues during a recent visit to Berlin.35 On that occasion he had told Planck how happy it would make him if the signatories to the Aufruf could somehow find a way to soften the impression it had left. In a letter to Marcel Brillouin, he gave a lengthier account of this Berlin visit.36 He had noticed, he said, that the Germans were very poorly informed about what happened abroad. A colleague had even assured him that it was absolutely impossible that illegal acts and

31   “Und dennoch hat die übertausendjährige deutsche Geschichte noch zu keiner Zeit das deutsche Volk so einig gesehen. Sollte das wirklich eine schlechte Sache sein, die eine solche Opferwilligkeit, eine solche reine, heilige Begeisterung zeitigt? Ich kann es nicht glauben. Denn auch ich kenne kein reineres und schöneres Opfer, als das für das Wohl des Vaterlandes, dem wir alles verdanken, was wir sind und was wir besitzen, nicht zum mindesten auch die Möglichkeit, unserer Wissenschaft (auch jetzt!) zu leben.” 32   Wien to Lorentz, April 4 and 23, 1915 (Kox 2008, 286 and 287). 33   Lorentz to Wien, May 3, 1915 (Kox 2008, 289). In this letter he quoted the passage from the ­letter by Brillouin of March 28, 1915, cited earlier. 34   Wien to Lorentz, undated (after May 3, 1915) (Kox 2008, 290). 35   Lorentz was in Berlin to visit his daughter Berta and to accompany her and her children back to the Netherlands (see also later in this chapter). 36   Lorentz to Brillouin, May 26, 1915, draft (LA 11).


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­ isconduct had been committed by the German army, because of the army’s m strict discipline.37 It had also become clear to Lorentz that there were German colleagues who disapproved of the Aufruf and who regretted their signature. They had signed the manifesto out of misguided patriotism and—to his ­surprise—they had not realized sufficiently what kind of impression it would make outside Germany. In speaking to Planck, Lorentz had made the somewhat moralizing remark that Germany did not have the right to view itself as superior. Planck, as it turned out, agreed with this assessment and Lorentz found his modesty ­credible— without really wondering how many of Planck’s colleagues actually shared this opinion. Lorentz left Berlin still hoping that he might be able to contribute in some way to the restoration of relations with Germany, even though the reactions by his colleagues must have dampened his mood considerably. In view of their reactions, one may wonder why Lorentz still continued his ideal­is­tic attempts to inspire some reason in his German colleagues, perhaps even against his better judgment. His most important motivation was most likely his great regret about the breakdown of international scientific relations that were once so fruitful, and his concern over how these relations could ever be restored in the future. Restoration of international scientific contacts would, by the way, remain an important motivation for him until the end of his life. After his activities in the first year of the war, and especially after the fiasco of the petition put forward by the Committee for the Union of European States, Lorentz had exhausted his possibilities to actively pursue the reconciliation and restoration of the scientific relations, at least while the war was still ongoing. Nevertheless, he became involved in another conciliatory initiative, this time on the German side. At the end of February 1916, after his conversation with Lorentz, Max Planck wrote an open letter, qualifying his support for the Aufruf. He submitted the letter to Lorentz, asking for his assistance in having it published in a Dutch news­paper.38 Lorentz took his time to mull over Planck’s request and did not answer until three weeks later.39 He had read the piece and was pleasantly surprised, he wrote. He did suggest some changes, partly about the content, but mostly with respect to the harsh tone the letter displayed in places. He suggested, among other things, to add a passage stating that the judgment about the responsibility for the human suffering in the war should be left to future, objective investigation. Planck agreed,40 and Lorentz used his authority in the national and international scientific community to ensure publication in Dutch and foreign media.41 37  The colleague was Emil Warburg, director of the Berlin Physikalisch-­Technische Reichanstalt (Physical-­Technical State Institute). See later in this chapter for more about this institute. 38   See Planck to Lorentz, February 27, 1916 (Kox 2008, 308). 39   Lorentz to Planck, March 20, 1916 (Kox 2008, 309). 40   See Planck to Lorentz, March 28, 1916 (Kox 2008, 310). 41   The piece appeared first in Algemeen Handelsblad of April 11, 1916 and afterwards, among other places, in Vossische Zeitung and Observatory (39 [1916]: 284–285). The French media also paid attention to Planck’s letter.

The “Scientific Commission”


In his letter, Planck called the Aufruf an act of defense, first and foremost, inspired in part by patriotic fervor. It was a defense against the accusations that had been leveled against the German army. German scientists and men of arts and letters were, after all, unable to separate their own interests from those of the German army. “For the German army is nothing other than the German nation in arms, and, like those in other walks of life, scientists and artists are also inseparably connected to it.”42 At the same time, wrote Planck, one cannot be held accountable for any individual act by an individual. Planck ended his letter with an attempt to rise above earthly, human affairs: What I do wish to emphasize, though, especially to you, is the firm conviction that is not to be shaken, not even by the events of this present war, that there are areas of the mental and the moral world that lie beyond the struggle of nations, and that sincere cooperation in the care for these international cultural goods, and, no less, the personal esteem for subjects of a hostile state are quite compatible with glowing love and energetic labor for one’s own fatherland.43

The international reactions were mixed. In the Netherlands, the declaration was met with general approval, but in England and France it was judged very critically. One point of criticism was that Planck had not spoken on behalf of the other signatories of the Aufruf—in his earlier letter he told Lorentz that a joint declaration had turned out to be impossible to achieve. Also, the way in which the German scientists identified with the German army did not sit well with many. In Germany, there was criticism as well. According to a German newspaper, the declaration might be abused to create the impression that some of those who had signed the Aufruf had withdrawn their support.44 Still, there were also reactions that were considerably more reasonable.45

The “Scientific Commission” At the end of 1917, Lorentz began to direct his attention toward the war effort in his own country. As the chairman of the section of science of the Royal Academy of Science, he played an important role in the establishment of a scientific

42  “Denn das deutsche Heer ist nichts anderes als das deutsche Volk in Waffen, und wie alle Berufsstände, so sind auch die Gelehrten und Künstler unzertrennlich mit ihm verbunden.” 43   “Was ich aber weiter mit besonderem Nachdruck gerade Ihnen gegenüber zu betonen wünsche, ist die feste, auch durch die Ereignisse des gegenwärtigen Krieges nie zu erschütternde Ueberzeugung, dass es Gebiete der geistigen und der sittlichen Welt gibt, welche jenseits der Völkerkämpfe liegen, und dass eine ehrliche Mitwirkung bei der Pflege dieser internationalen Kulturgüter, wie auch nicht minder die persönliche Achtung vor Angehörigen eines feindlichen Staates, wohl vereinbar ist mit glühender Liebe und tatkräftiger Arbeit für das eigene Vaterland.” 44 45   Newspaper clipping from Vossische Zeitung in LA 61.   See also Heilbron 1986, 70–79.


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c­ommission charged with delivering assistance to the Dutch government to resolve problems resulting from the war. In those days, the situation in the Netherlands was far from rosy, in spite of the country’s neutrality. There were shortages of fuel and food, and many primary needs were rationed. The sea blockade of the allies also affected the neutral Netherlands and made the import of foodstuffs almost impossible. To make things worse, the Netherlands were forced to export agricultural products to Germany on a massive scale, in order to be able to import direly needed German coal. These coal imports were indispensable for households as well as industry, but the population suffered immensely because of the food exports that had to be delivered in exchange. So, there were problems galore in the Netherlands. The idea for the scientific commission had germinated in October 1917, during a meeting of the Academy’s board. A request by some younger Academy members, among whom was the important mathematician Luitzen E. J. Brouwer, was the subject of some discussion. The young members wanted the Academy’s support to obtain deferment of their military service in the Landstorm, the reserve forces of the regular Dutch army. After some back and forth, it was decided to support the request. The argument was that the government could make better use of their knowledge by enlisting their assistance to resolve problems generated by the circumstances of the war. Lorentz then proceeded to broaden the issue by asking more generally “if it would not be good to point out once again, especially to the Ministry of War, that within the Academy there are forces that could be employed in times of war.” The meeting decided that Lorentz should request a meeting with Prime Minister and Minister of the Interior Cort van der Linden to advocate for deferment of military service for any member of the Academy. On that occasion, he would also offer the Academy’s assistance for the resolution of problems generated by the war.46 In a summary that was sent to the minister prior to the meeting, Lorentz focused on “the question if, more than was the case until now, men of science, in particular members of the Academy, could not offer certain services to the Government, in the interest of both our defense and the well-­being of the nation.”47 The minister was quite positive about this suggestion,48 and, as a result, a commission came about with the cumbersome name Wetenschappelijke Commissie van Advies en Onderzoek in het Belang van Volkswelvaart en Weerbaarheid (Scientific Commission of Advice and Investigation in the Interest of National Welfare and Defense), commonly known as the Scientific Commission. Lorentz was appointed chairman and Zeeman became the commission’s secretary.

  Minutes of the board meeting of October 27, 1917 (Archive Royal Academy of Sciences, NHA).   “de vraag of niet, meer dan tot dusverre het geval was, wetenschappelijke personen, in het bijzonder leden der Akademie, zekere diensten aan de Regering zouden kunnen bewijzen, in het belang zoowel van onze weerbaarheid als van de volkswelvaart.” Lorentz to Minister of the Interior, November 10, 1917 (draft in LA 166 and copy in ZA 794). 48   Minister of the Interior to Pieter Zeeman, November 17, 1919 (ZA 794). 46 47

Private concerns about the war


The commission gave a broad interpretation to its official mandate. Not only did it occupy itself with specific assignments by the government, it also took initiatives on its own and supported those by others. In a number of sub-­commissions, the members worked on a great variety of problems, ranging from developing food surrogates and pressing oil from beech nuts to manufacturing explosives and interpreting aerial photographs. Judging by the quantity of available archival material, both Lorenz and Zeeman spent a great deal of time on their work for the commission.49 For the most part, they occupied themselves with organizational issues like the management of the commission and its sub-­commissions. Sometimes they also had to deal with more personal issues. The commission member Luitzen Brouwer, for instance, who was known as a querulous fellow, caused considerable problems in connection with the method he had developed to take and interpret aerial photographs without needing to fly directly over the objects to be photographed. Brouwer believed that he was not receiving adequate credit and funding for his contributions and was constantly harassing the board members about this pet peeve. Eventually the problem was resolved by forming a separate sub-­commission for his project, which Brouwer was, of course, allowed to chair. A definitive assessment of the effect and the usefulness of the commission has never been made, but it appears that the great efforts of the board members bore no relationship to either the quality or the quantity of the results achieved. Lorentz’s verdict, at least, in a letter to the minister in 1921, was that the commission had had “limited success.” After Lorentz’s judgment that the commission was already “dead” and his request to be discharged of his duties, it was dissolved in 1922.50 A new commission was established, nonetheless, this time chaired by Friedrich A. F. C. (Frits) Went, chairman of the Royal Academy and former board member of the earlier Scientific Commission. This new commission laid the foundation for the Organisatie voor Toegepast Natuurwetenschappelijk Onderzoek (TNO) (Organization for Applied Research in Natural Science), which was very successful and is still in existence.

Private concerns about the war Beyond the breakdown of international scientific relations, Lorentz also had concerns in his private life on account of the war. He was worried, first of all, about his daughter Berta and her husband Wander de Haas who were living in Berlin at

49   The Zeeman Archive (NHA) in particular contains a great deal of material about the commission’s work. 50   See Lorentz to Minister for Education, November 11, 1921 (LA 166); Lorentz to commission board, November 28, 1921 (LA 166); Section of Sciences, Royal Academy of Sciences, to Minister of Education, February 3, 1922, and Minister to Section of Sciences, July 17, 1922 (Archive Royal Academy of Sciences, 350, NHA).


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the time. De Haas, an experimental physicist who had completed his doctorate under Kamerlingh Onnes in 1912, had started work in 1911 as an assistant at the University of Berlin. When that job came to an end in 1913, he had found a pos­ ition at the Physikalisch-­Technische Reichsanstalt (PTR) (Physical-­Technical State Institute). The PTR was a very innovative public organization. It worked mainly on precision measurements, for instance of weights and measures, not only for public organizations but also for private enterprise. Because of this unusual ­public–private partnership, the PTR was a focal point where scientific, technical, and industrial interests converged. The work by De Haas at the PTR became known primarily because of an experiment he conducted there in 1915 together with Albert Einstein.51 Not without reason, Lorentz was worried about the situation for foreigners in Germany. On July 31, 1914, just before the outbreak of the war, at a time when everyone was already fearing the worst, he wrote to Berta and Wander, this time still in Dutch: We want to hope with all our hearts that everything will work out for the best and that you will not experience any difficulties. I believe, as long as one stays calm, the risk is not great. It is true that I do not have any sense at all of such circumstances, but it will surely be unthinkable that foreigners would have to leave the country just like that. And subjects of a friendly and neutral state, at that.52

Starting in early August, Lorentz began to write to Berta in German to ease the task of the German censors. Rudolf Lorentz, who had been staying with his sister that summer, had meanwhile returned to Haarlem and was drafted into the army in August. By August 29, Lorentz had become so worried, that he suggested that the family return to the Netherlands and that Wander find a teaching job there—if a suitable position would become available.53 A month later, Berta and the two children returned to the Netherlands while Wander stayed behind. After some time, the situation appears to have stabilized somewhat, as Berta and the children were back in Berlin in January 1915. Around April 1 that year, Wander had to come back to the Netherlands suddenly, as his mother had died. He decided not to return to Germany, and at the

51   The experiment provided an explanation—incorrectly, as it turned out later—of the well-­known phenomenon that a ferromagnetic substance like iron is magnetized under the influence of a per­man­ ent magnet. Their experiment appeared to confirm Ampère’s hypothesis that this magnetization resulted from microscopic currents caused by orbiting electrons. It is, incidentally, the only published experiment performed by Einstein. 52   “Wij willen van harte hopen dat zich alles gunstig zal schikken en dat jelui geen moeilijkheden zult hebben. Mij dunkt, als men zich maar kalm houdt, is het gevaar daarvoor ook niet groot. Ik heb wel is waar van zulke omstandigheden niets geen verstand, maar het zal toch wel ondenkbaar zijn, dat vreemdelingen zoo maar het land zouden moeten verlaten. En nog wel onderdanen van een bevrienden en onzijdigen staat.” Lorentz to Berta and Wander de Haas, July 31, 1914 (FC). 53   See Lorentz to Wander de Haas, August 29, 1914 (FC).

Paris and the war’s aftermath


end of the month Lorentz traveled to Berlin to fetch Berta and the children. Wander tried hard to find work as soon as possible, and by the end of May he had the chance to teach for a few hours a week at an HBS in Amsterdam. Later, Einstein took it upon himself to move the family’s effects to the Netherlands. That turned out to take quite some doing, and it elicited Einstein’s half-­ joking, ­half-­serious comment: “If ever I were to find myself in the situation that I would need to make a declaration of love to someone, I would tell him: ‘For you I would arrange a move abroad in wartime.’ ”54 In the subsequent war years Lorentz and Aletta had no further concerns about the safety of Berta or their other children, but—like all other Dutch families—they could not escape the daily hardships, great and small, that were caused by the war. The increasing food shortages, because of the exports to Germany, forced the government to introduce a rationing system. Not only were many food items rationed, but the government also regulated the composition of many primary foodstuffs. White bread, for instance, the bread eaten daily by almost everyone, was replaced by the dark “government bread,” so hated by many. The stores also began to sell eenheidsworst (uniform sausage), whose composition was determined by the government. In the Dutch language, the term denoting this sausage survives to this day as a metaphor for any dull and unattractive product. Despite the coal imports from Germany, fuel became scarce during the war years and many people in the Netherlands had a difficult time heating their houses in winter. Fortunately, there was no question of serious deprivation in the Netherlands, and there were no great shortages or famines like there were in some parts of Germany toward the end of the war.55

Paris and the war’s aftermath In spite of all their contacts with colleagues, all the newspaper reports, and all the first-­hand accounts by eye-­witnesses, it was not until six months after the ar­mis­ tice of November 11, 1918 that Lorentz and Aletta experienced, for the first time, the shocking reality of the unspeakable horrors that had taken place in France and Belgium. Only then did it truly dawn on them how enormous the destruction had been, how immense the hardships, and how unfathomable the human suffering. All this happened when they decided to venture, in the spring of 1919, on a trip to Paris. They wanted to see Aletta’s sister Betsy, who had been living in Paris for quite some time and from whom they had not had much news during the war years. Besides, Lorentz wanted to speak to his colleagues about the future of the scientific contacts that had been broken off so dramatically. Since connections

54   “Wenn ich je in den Fall kommen sollte, jemand eine Liebeserklärung zu machen, würde ich ihm sagen: ‘Für Sie würde ich zu Kriegszeiten einen Umzug ins Ausland besorgen.’ ” CPAE-8, 107. 55   See also Moeyes 2001 for more about the situation in the Netherlands.


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over land were still unreliable, and possibly even unsafe, Lorentz and Aletta decided to make the first leg of the journey by sea. There was a boat connection between Rotterdam and Dunkirk, and from there they could continue by train. The boat would leave early in the morning and was to arrive mid-­afternoon in Dunkirk. Lorentz wrote to Berta about their travel plans.56 The letter caused a slight panic in the De Haas household. Berta did not agree at all with the plans, because the situation at sea was much more dangerous than her parents believed.57 The North Sea was full of mines, she wrote, and recently two ships had sunk after hitting a mine.58 In the two letters he sent in reply, Lorentz stuck to his guns.59 Yes, there were mines floating around, but the journey would take place by day and, so he had heard, men with binoculars were positioned on the forward deck of all ships to scan the horizon for mines that were then detonated by gunshots. Aletta and Lorentz had thought about it long and hard, he wrote, and they had taken a decision that they did not want to reverse: “So often in the world one has to surrender in good faith and we will just do so now. And if, after long deliberation, one has taken a decision, it is best to remain cheerful about it.”60 For some unknown reason, the travel plan was changed after a few days. They ended up going by train after all, traveling by way of Brussels. Surprisingly enough—certainly in light of Berta’s insistent safety concerns—the third wave of the Spanish flu, which raged in Paris in the spring, appears not to have been an issue for the Lorentz couple. It certainly must have made the news, as even President Woodrow Wilson suffered from the Spanish flu in April 1919 while negotiating the crucial treaty ofVersailles to end theWorldWar.61 In the Netherlands, with its population of around 6.5 million, the devastating pandemic had affected around fifty percent of the population in 1918 and had claimed the lives of four out of every one thousand people.62 Yet Lorentz mentioned it only once in passing, when he observed that sadly several colleagues in the office of his son-­in-­law Henri Leemhorst had died from the disease. Berta’s husband Wander and their children also contracted the flu and, according to Lorentz, their recovery was rather slow.63

  Lorentz to Berta, April 4, 1919 (FC).   See Lorentz to Berta, April 30, 1919 (FC), in which he reacted to her objections. 58   In early April, the Dutch steamer De Amstel was lost with all hands on its way from Gothenburg to Rotterdam. The well-­known actress Enny Vreede, on her way back from her honeymoon, had been one of the ship’s passengers, so the newspapers devoted much attention to this disaster. From Lorentz’s letter to Berta of April 30 it can be gathered that she had mentioned this drama, as well as the disaster that had befallen an English ship, probably the Hughli, which had sunk after running into a mine near Ostend on April 26. 59   Lorentz to Berta, April 30, 1919 and Lorentz to Wander de Haas, May 6, 1919 (FC). 60   “Men moet zich zoo dikwijls in de wereld met goed vertrouwen overgeven en dat zullen wij nu ook maar doen. En als men na wikken en wegen een besluit heeft genomen is het het best verder maar blijmoedig te zijn.” 61 62  See Barry 2018, chap. 32.  See Moeyes 2001, chap. 9. 63   Lorentz to Berta, October 13, 1918 and January 21, 1919 (FC). 56 57

Paris and the war’s aftermath


Aletta wrote a travel diary about the trip, with numerous interesting observations and details.64 On May 10, the two departed. In the border town of Roosendaal their passports and luggage were checked, but they had an easy time of it. “To the question whether we had to open [the suitcases] the answer was: ‘No, for professor Lorentz that is not necessary.’ The young man blushed with joy and said: ‘At the HBS I already learned about you.’ ”65 After endless formalities in Antwerp, they arrived in Brussels where they spent the night. The following morning, they went on to Paris. The train journey through the devastated landscape in Belgium and Northern France they found extremely depressing: From Bergen until a good hour before Paris, all bridges and overpasses were destroyed; probably because of bombs from airplanes the entire road and its proximity were full of holes, sometimes filled with water; completely bent rails were still lying by the roadside and along the railroad tracks the roofs were all destroyed. [. . .] Near Feignies the devastation really set in, the station and the houses, too, were all rubble. How can one ever overcome this misery, I thought, when I saw one little old man chipping away at bricks, in the midst of all these vast fields full of rubble-­heaps and fragments of walls. [. . .] There are many places where nothing else can be found that even reminds one of living quarters for human beings, everything has been laid waste and is deserted. [. . .] We were just sitting down in the dining car when we were riding through the regions that suffered the greatest devastation; the contrast was so stark and the misery and desolation outside were so great that we sometimes had ­trouble swallowing. Not only did we see destroyed what had been built up with difficulty by human hands, but also, of vast forests nothing was left but fields from which only splintered points were still sticking up, or large orchards where the stumps of the trees looked like small round tables. [. . .] German army camps and hiding places with all kinds of placards were still discernible, one was called Fliederheim [Lilac House]. The abundant grain could not be tamed, everything grew and flourished as if it wanted to cover the misdeeds and misery, but the contrast became all the starker for it.66

  LA 732 and 739.   “Op de vraag of we [de koffers] niet behoefden open te maken was het antwoord: ‘Neen, voor prof. Lorentz is dat niet noodig.’ De jonge man kreeg een kleur van pleizier en zeide: ‘Op de H.B.S. heb ik al van U geleerd.’ ” 66   “Van Bergen tot een goed uur voor Parijs waren alle bruggen en viaducten vernield; waarschijnlijk door bommen uit vliegtuigen waren de hele weg en zijn naaste omgeving vol gaten waarin soms water stond; de geheel gekromde rails lagen nog langs den weg en van alle huizen langs de spoorbaan waren de daken vernield. [. . .] Bij Feignies begon de verwoesting eerst recht, het station en ook heel veel huizen waren enkel puinhopen. Hoe kan men deze ellende ooit te boven komen dacht ik toen ik te midden van deze onafzienbare velden vol hopen puin en stukken muur één oud mannetje zag steenen bikken. [. . .] Veel plaatsen zijn er waar niets meer te vinden is wat aan een woning voor menschen doet denken, daar is dan ook alles woest en verlaten. [. . .] Wij zaten juist in de restauratiewagen toen wij door de meest verwoeste streeken reden; het kontrast was zoo sterk en de ellende en de eenzaamheid buiten waren zoo groot dat het slikken ons soms moeite kostte. Niet alleen zagen wij verwoest wat met moeite door menschenhanden was opgebouwd maar ook van uitgestrekte bosschen was niets meer over dan velden waaruit de splinterige punten nog omhoog staken, of groote boomgaarden waar de stukken stam op kleine ronde tafeltjes leken. [. . .] Duitsche leger- en schuilplaatsen met allerlei opschriften waren nog te onderscheiden, een heette ‘Fliederheim.’ Het welige graan liet zich niet onderdrukken, alles groeide en bloeide alsof het de misdaad en ellende wilde bedekken maar het kontrast werd er slechts des te sterker door.” 64 65


World W   ar One

Once they had arrived in Paris, Lorentz wrote to Berta about what he had noticed there: What has struck me very much is that on the boulevards and in the streets one sees nothing that reminds one of “victory,” I mean no expressions at all of “reveling in victory.” One sees many soldiers of all sorts; many Americans are on leave here, to see Paris before they return to their country, something they do sometimes all heaped together on a “sight-­seeing car.” Of the war one is further reminded by the many women in mourning; sometimes, in a short while, one sees one after the other. But the mood among the people appears to be calm; as [Betsy’s brother-­in-­law] Jean says, there was a deep sense of fatigue and it is a relief to everyone that it is over.67

Aletta provided her own observations about the Americans: They said that at this moment there were more than a million foreigners in Paris. The most noticeable of these were the American soldiers, on the whole good-­looking fellows, well-­dressed and well-­cared for, everything khaki, even down to the handkerchiefs. [. . .] In general, they are not much loved, but that does not stop them from walking arm in arm with a girl, just like the other military men.68

The reunion with Betsy and with various acquaintances was happy, and there was much to catch up on—the first evening late into the night. Lorentz also spoke with French colleagues, in particular with his “best acquaintances” Marie Curie, Paul Langevin, and Jean Perrin. They also went to the opera and the theater and saw the Mona Lisa in the Louvre for the first time. They enjoyed the fine weather on their walks through the Bois de Boulogne and during an outing of several days to Fontainebleau. On the return journey, Aletta expressed her great concern about the country’s future when she saw the wrecked and devastated factories in Belgium. When one travels through this devastated industrial land, one is filled with the ­fearsome thought: what must become of this once so industrious nation? Later we realized that this was the prevalent thinking in Belgium, what must become of this nation? The factories are empty, it will take years before they can again be back to full

67   “Wat mij zeer heeft getroffen is dat men op de boulevards en de straten niets ziet dat aan de ‘overwinning’ doet denken, ik bedoel in het geheel geen uitingen van den ‘overwinningsroes.’ Men ziet veel soldaten van allerlei soort; vele Amerikanen zijn met verlof hier om voor zij naar hun land teruggaan, Parijs te zien, wat zij soms, opeengehoopt op een sight-­seeing car doen. Aan den oorlog wordt men verder herinnerd door de vele vrouwen in rouw; soms ziet men er kort na elkaar een stuk of wat. Maar de stemming onder de menschen schijnt kalm te zijn; zooals [Betsy’s zwager] Jean zegt, was er een diepen vermoeidheid en geeft het iedereen een opluchting dat het uit is.” Lorentz to Berta, May 22, 1919 (FC). 68   “Men zeide dat er op dit oogenblik meer dan een millioen vreemdelingen in Parijs waren. Hiervan vielen het meest in ’t oog de amerikaansche soldaten, over het algemeen flinke kerels, goed gekleed en verzorgd, alles kaki tot zelfs de zakdoeken. [. . .] Over het algemeen zijn zij niet erg bemind maar dat verhindert niet dat zij evenals de andere militairen meestal met een meisje aan de arm wandelen.”

Paris and the war’s aftermath


strength and in the meantime this nation, already so demoralized by the occupation, will run wild, even more so than it does now. The worst is, though, that the market will shift, Germany is already beginning to work and deliver now. All over the place, people say that the country’s occupation and its systematic looting will have far more serious consequences for the country than the invasion with all its killing and devastation. On the surface it looks like life is taking its normal course, but in the hearts of the people there is worry and fear for the future.69

On the return journey, Lorentz and Aletta traveled again by way of Brussels, where they were hoping to meet Ernest Solvay, whose news they had long gone without. About this visit to Brussels no further particulars are known, other than that they also made a side trip to Louvain.70 On May 30 they were back home, safe and secure in the Netherlands. Once they were home, Lorentz’s life began to return to normal, bringing him more national and international recognition. Later in 1919 he was honored with the Copley medal of the London Royal Society, its oldest and most prestigious award, conferred for the first time in 1731.71 He was granted the medal “On the ground of his distinguished researches in mathematical physics.” Because Lorentz did not attend the award ceremony at the Society’s meeting on November 30, a medal was sent to him a few months later. To his disappointment, it was not the official medal in gold, but a copper replica. In those post-­war days it was impossible to buy gold at a reasonable price, so the Society explained.72

69   “Wanneer men dit verwoeste industrieland doortrekt wordt men vervuld van de bange gedachte: wat moet er van dit eens zo nijvere volk worden? Later bleek ons dat dit de heerschende gedachte in België is, wat moet er van dit volk worden? De fabrieken zijn leeg, het zal jaren duren eer zij weer op sterkte kunnen zijn en in die tijd verwildert dit volk nog meer dat nu reeds zo gedemoraliseerd is door de overheersching. Het ergste is echter dat de markt zich zal verplaatsen, Duitschland begint nu reeds te werken en af te leveren. Algemeen zegt men dan ook dat de bezetting van het land en de stelselmatige uitplundering ervan veel erger gevolgen voor het land zullen hebben dan de inval met al zijn moorden en verwoestingen. Oppervlakkig gezien gaat het leven zijn gewonen gang maar in de harten der menschen heerscht zorg en angst voor de toekomst.” 70   See Lorentz to Zeeman, May 28, 1919 (ZA 106). 71   See Arthur Schuster to Lorentz, November 7, 1918 (LA 197). 72   See R. Harrison to Lorentz, February 17, 1919 (LA 31).

Chapter 10 The Zuiderzee Commission

On the 15th of July 1918, three days before his sixty-­fifth birthday, an important appointment turned into a life-­changing event for Lorentz. On that day he was installed as Chairman of the Zuiderzee Commission. This commission was charged with the task of establishing how closing off the Zuiderzee would affect the water levels of the Wadden Sea (a branch of the North Sea), especially during storm surges, along the northern coast of the Dutch provinces of Noord-­Holland, Friesland, and Groningen. The Zuiderzee was an inland sea in the northern part of the Netherlands, a large body of water that had existed since the late Middle Ages. With a surface area of roughly 6,000 square kilometers, or roughly 3,700 square miles, it was an important resource for the Dutch economy. It connected directly to the open water of the Wadden Sea. As it provided the harbors along its coast with direct access to open sea, the Zuiderzee was not only an important shipping route, but also a rich fishing ground. Several Dutch provinces, as well as a number of the coastal Wadden Islands, partially enclosed the Zuiderzee. Storm surges caused regular and often devastating floods in many of these surrounding coastal areas. After two shipping canals across the province of Noord-­Holland had been dug— in 1824 and 1875—quick and direct access for shipping to the North Sea had improved, and once the technological possibilities had advanced sufficiently, concrete suggestions were made to reclaim a large part of the Zuiderzee. Such a project would increase the roughly 16,200 square miles (roughly 42,000 square kilometers) of Dutch land mass by about one quarter and would create new fertile agricultural land. Besides, it would considerably reduce the length of the coastline, thereby also reducing the risk of flooding during storm surges. When Lorentz accepted the appointment, he could never have guessed that for eight long years the work for the commission would take up most of his time. Used to the position of independent scientist, Lorentz now became the director of a large-­scale research project—a completely new role for him. Besides, it would become clear very soon that he was the only member of the commission with the  scientific wherewithal to successfully tackle the problems involved.1 From


  This chapter is based for the most part on Kox 2007.

“A Living Work of Art”: The Life and Science of Hendrik Antoon Lorentz. A. J. Kox and H. F. Schatz, Oxford University Press (2021). © A. J. Kox and H. F. Schatz. DOI: 10.1093/oso/9780198870500.003.0011

Reclamation or closure


a scientific point of view, closer observation of Lorentz’s work in the Zuiderzee Commission is very interesting, as the methodology he developed has greatly influenced the science of hydraulics.2

Reclamation or closure For a proper insight into Lorentz’s work on the Zuiderzee Commission, it is ne­ces­sary to first discuss in some more depth the background to the plans for closing it off and the events leading up to the actual work of the commission. Closing off the Zuiderzee was the first phase of a plan that envisioned draining this large body of water, or at least part of it. A large area of the sea’s bottom consisted of fertile clay and reclaiming this soil was an attractive proposition because, as was said earlier, it would increase the surface area of precious arable land by about one quarter. Once the large body of water in the Zuiderzee would no longer be subject to tidal influences and storm surges, the eventual reclamation process would become far easier. The first plan to close off and reclaim the Zuiderzee dates from the seventeenth century and was drawn up by Hendric Stevin, son of the famous Dutch scientist Simon Stevin.3 He proposed to first connect the Wadden Islands and the mainland by means of dams and then reclaim the resulting lake.4 This plan did not come out of the blue. Especially because of the pioneering work of Jan Adriaanszoon Leeghwater, the struggle against the water in the western part of the Netherlands had taken a serious turn. In the first decades of the seventeenth century many lakes, like the Beemster, Purmer, and Schermer lakes, had become polders, or reclaimed land. In the end, Stevin’s plan came to nothing, as it was far too ambitious for its time.5 In the nineteenth century, renewed interest in closing off and reclaiming the Zuiderzee resulted in a series of plans that ranged from creating a modest polder on the southern end of the Zuiderzee to a complete reclamation of both the Zuiderzee and the Wadden Sea. This new interest led to the establishment, in 1886, of the Zuiderzee-­Vereeniging (Zuiderzee Association). This association was the culmination of activities in the preceding years by a number of influential 2  See Mazure 1963, in which Lorentz’s work is characterized as a “turning point in the treatment of hydraulic problems involved in maritime engineering” (123). 3   Simon Stevin carried out important scientific work, among much else, in the field of mathematics and hydrostatics. His endeavors to use the Dutch language as the language of science have left a lasting imprint on the Dutch language because of the terms he introduced (like wiskunde for mathematics, omtrek for circumference, and evenwijdig for parallel). See, for example, Kox  1990 for more about Stevin. 4   See, for example, Fuchs en Simons 1972 for more about Stevin’s plan and the background of closing off the Zuiderzee. 5   Stevin’s pioneering work was honored by attaching his name to one of the two complexes of locks in the Afsluitdijk, the dike that eventually closed off the Zuiderzee. The other complex was named after Lorentz. (The names were only given in 1952.)


The Zuiderzee Commission

parliamentarians and private individuals to generate support—financial and other­wise—for the idea of closing off and reclaiming the Zuiderzee. Impatient with the lack of initiative on the part of the authorities, the objective of the association’s founders was to exert pressure on the government by drawing up a feas­ ible plan. The Zuiderzee Association contracted Cornelis Lely, a young and brilliant engineer.6 In the period between 1886 and 1892, Lely produced eight memoranda, supported by careful scientific and technical analysis, laying out the plans that were later adopted almost to the letter. Now it was a question of waiting for the government to adopt these plans.7 The first opportunity for this to happen arose in 1891, when Lely became Minister of Water Management, Trade and Economic Affairs. He instituted a State Commission that he chaired himself—actually without participating in its activities. It was to draw up recommendations with regard to the feasibility of the plans. The commission’s report appeared in 1894 and was favorable, but after the general election in that same year Lely was no longer a member of the government and there was no follow-­up to the report. When he became a cabinet minister for the second time, from 1897 to 1901, Lely went a step further. In 1901 he proposed legislation to close off and reclaim the Zuiderzee, based on his own plans and the recommendations of the State Commission. Again, an election threw a spanner in the works; this time it was the 1901 general election. Lely was not asked to become part of the new government, and the proposed legislation was withdrawn by his successor. It was not until his third term as minister, this time as Minister of Water Management from 1913 to 1918, that Lely was successful. In 1916 he proposed a new law to close off and partly reclaim the Zuiderzee, and two years later he successfully managed to ­shepherd the law through parliament. Two factors played an important part in the law’s acceptance. In January of 1916, a devastating storm surge had claimed several dozen casualties and had inundated large parts of the province of Noord-­Holland—even parts of Amsterdam had been under threat for some time. It was clear that closing off the Zuiderzee could have prevented this disaster. In addition, food shortages resulting from the still raging World War had made it clear to what extent the Netherlands depended on neighboring countries for its food supply. Availability of more arable land was expected to make the country less dependent on other nations.8 The debate in the Dutch Lower House took place over the course of five evening sessions in March of 1918. The Upper House needed less time: It decided in 6  See Ham 2007 for a biography of Lely with special attention to his work on closing off and reclaiming the Zuiderzee. The earlier nineteenth-­century plans are also discussed extensively there. 7   The plans involved closing off the Zuiderzee with a dike and the subsequent creation of four polders to be reclaimed. These are now known as Wieringermeer, Noordoostpolder, Flevoland, and Markerwaard (a polder that was never actually realized). Wieringermeer had already become dry land in 1930, even before the Zuiderzee was closed off in 1932. 8   The total amount to be reclaimed was 200,000 hectares (ca. 500,000 acres) of agricultural land: ten times the surface area of the Haar­lem­mer­meer pol­der.

Reclamation or closure


only one session, in June.9 In both houses of parliament the law was passed without a roll-­call vote. An important point of discussion during the debates, especially in the Lower House, was the question of the safety of those parts of the provinces of Friesland and Noord-­Holland that were outside the projected closing dike. Of particular importance was the question of whether closing off the Zuiderzee would cause an increase of the tidal water levels along the coastlines of the Wadden Sea and the province of Noord-­Holland, especially during storm surges. If this did turn out to be the case, it was important to determine whether or not such an increase would aggravate the risk of flooding, considering the height of the existing dikes. Meanwhile, a number of brochures were circulated, discussing this issue at length.10 The authors of these pamphlets assumed an increase of the water ­levels for a fact, but they had serious differences of opinion as to how great an increase had to be reckoned with. At the location where the closing dike would meet the coastline of Friesland, for example, the results of their calculations of water level increases varied between six inches (fifteen centimeters) and over thirteen feet (four meters). Anyhow, everyone was certain that the tides in the Wadden Sea would change once the Zuiderzee was closed off during storm surges. During heavy storms the open Zuiderzee functioned as a reservoir for the masses of water that were propelled inward, so that in most cases the storm would already be past its climax by the time the Zuiderzee was “filled to capacity.” It was also presumed that the normal tidal differences would be influenced by a dike that closed off part of the body of water, but tidal activity in the Wadden Sea was very complex, so the critics could shed little light on this question. In the Explanatory Memorandum, attached to his proposed law, Lely had downplayed the safety problem by stating that only on the Friesian coastline, very close to the dike, would a minor tidal increase occur.11 He remained convinced that his view was correct, but in order to pacify the Lower House he agreed to appoint a State Commission that was to investigate the question scientifically, so that the dikes along the Wadden Sea could be raised sufficiently to avoid flooding during storm surges, if that turned out to be necessary.

9   Apart from the publication in the general proceedings of the Dutch Lower and Upper House, the discussions were published in Zuiderzee 1920. 10  See Mansholt, L.  H.  1916, Mansholt, D.  R.  1917, Gallé  1917, Kooper  1918, and Lely  1918. D. R. Mansholt and L. H. Mansholt, father and son, were prominent gentleman farmers in the province of Groningen, P. H. Gallé was vice director of the KNMI, the Dutch Meteorological Institute, J. Kooper was chief engineer of the Provincial Department of Water Management in Groningen, and C. W. (Wim) Lely, son of Cornelis, was engineer at the Ministry of Water Management. 11   In this respect he followed the increased level predicted in one of his memoranda (and confirmed by the State Commission in 1891) of six inches (fifteen centimeters) at the Friesian end of the dike and a fast decrease of this higher level in a northerly direction.


The Zuiderzee Commission

The State Commission Lorentz Once the law had been passed in the Upper House, Lely proceeded energetically. The promised State Commission was instituted by Royal Decree of July 4, 1918. The commission was charged with the convoluted assignment “to investigate to what extent it is to be expected, as a result of closing off the Zuiderzee, pursuant to the law of 14 June 1918, that higher water levels and a greater wave surge than is currently the case will occur during storms on the coasts of mainland ­Noord-­Holland, Friesland and Groningen, as well as on the North Sea Islands situated in front of these coasts.”12 Soon after the law’s passing Lely had already decided whom he wanted to include in the State Commission. In a letter dated June 17 he asked Lorentz to become its chairman, and at the same time submitted the commission members’ names for Lorentz’s approval.13 Lorentz and Lely knew each other already, because Lely had been a member of the Royal Academy of Sciences since 1895. After some deliberation, Lorentz accepted the chairmanship.14 He wrote to Lely: “I will consider it a duty to lend assistance, to the best of my ability, in resolving the important question the Commission will have to address.”15 During the installation of the commission, on July 15, he expressed himself in the same vein: It is an honorable duty for every Dutchman, if he is offered such an opportunity, to promote the success, to however modest a degree, of the great national enterprise which is to be undertaken at this moment.16

Lorentz expressed a sentiment here that was felt by many Dutchmen. A project was being undertaken of which the Netherlands could be proud and which would inspire admiration and respect abroad. Such feelings of national pride were

12   The assignment is to be found in Staatscommissie 1926. In it, an additional assignment is formulated as well: To investigate the influence of an envisioned partial reclamation of the Wadden Sea. This latter influence was found to be minor. Wave surge is the phenomenon of waves running up against a dike as a result of high winds. Obviously, it was important to prevent waves from running up or across the Afsluitdijk (the dike that was to close off the Zuiderzee). Because Lorentz had little involvement in the study of wave surge, this topic is not discussed any further in this chapter. 13   Lely to Lorentz, June 17, 1918 (CT). 14   Ironically enough, a few months earlier, in his speech accepting an honorary doctorate at the Delft Technical University, Lorentz had said that he hoped that this award would not mean “that you will, now or in the future, expect technical knowledge on my part.” (dat gij nu of in de toekomst technische kennis van mij zoudt verlangen.) Nieuwe Rotterdamsche Courant, March 7, 1918. 15   “Ik zal het tot een plicht rekenen, aan de oplossing van het belangrijke vraagstuk waarmede de commissie zich zal moeten bezighouden, zoo goed ik kan mede te werken.” Lorentz to Lely, undated draft (written on Lely to Lorentz, June 17, 1918, CT). 16  “Het is voor iederen Nederlander een eervolle plicht, zoo hem de gelegenheid daartoe wordt geboden, het welslagen te bevorderen in hoe bescheiden mate het ook moge zijn, van het groote natio­ nale werk, dat thans staat ondernomen te worden.” Report on the commission’s installation in Nieuwe Rotterdamsche Courant, July 15, 1918.

The State Commission Lorentz


­ arbored by many. A group of prominent Dutchmen, for instance, called for the h “National Tricolor” to be flown on the day that the law officially went into effect.17 In the Lower House one of the representatives even pointed out the “powerful educational momentum” that would emanate from this “great enterprise.”18 Because of his status as a celebrated physicist, nationally and internationally, Lorentz was the obvious choice to lead the commission. He was undoubtedly aware of this, and must have been convinced that he would be able to carry through this momentous task to the end, certainly with the support of the other members of the commission. Besides, the chairmanship would no doubt contribute further to his national fame. The State Commission Zuiderzee, which would later become known as the State Commission Lorentz, counted seventeen members. Among them were four authors of critical brochures that had been discussed in the Lower House.19 By virtue of their membership, as Lely had apparently reasoned, they would share the responsibility for the final report and would no longer be able to level criticisms from the sidelines. Under Lorentz, there were two deputy chairmen: H. Wortman, inspector of the Ministry of Water Management, and E. van Everdingen, director of the KNMI, the national meteorological institute, and extraordinary professor at the Utrecht University. One of the members, W.  F.  Stoel, chief engineer at the Ministry of Water Management, held the position of first secretary.20 The commission further consisted of four sections: a central section for coordination, headed by Lorentz; section A for hydraulic data and observations, headed by Wortman; section B for meteorological data and observations, headed by Van Everdingen; and section C for theoretical problems, also headed by Lorentz.21 Besides the first secretary, there was also a second secretary who was not a member of the commission but who was responsible for daily operations. That position was filled in June 1919 by a young engineer from the Ministry of Water Management who had originally been recruited to carry out observations. This young man, Johannes Theodoor Thijsse, quickly turned into an invaluable

 See Nieuwe Rotterdamsche Courant, June 29, 1918.   “groote opvoedende kracht.” “grootsche werk.” Lower House member R. R. L. de Muralt, in the session of March 7, 1918 (Zuiderzee 1920, 230). 19   P. H. Gallé, J. Kooper, C. W. Lely, and L. H. Mansholt. 20  The other members were: W.  K.  Behrens, professor at the Technical University Delft, R. H. Gockinga, chief engineer of the Provincial Department of Water Management in Friesland and Drenthe, A.  R.  van Loon, chief engineer of the Provincial Department of Water Management in Noord-­Holland, R. R. L. de Muralt, Member of the Lower House, J. M. Phaff, Head of the Department of Hydrography of the Ministry of the Navy, C.  J.  A. Reigersman, chief engineer of the Provincial Department of Water Management in Noord-­Holland, J.  P.  van der Stok, director of the National Meteorological Institute KNMI, D. F. Wouda, chief engineer of the Provincial Department of Water Management in Friesland, and J. P. Wijtenhorst, chief engineer of the Provincial Department of Water Management in Groningen. 21   Additionally, there were a few sub-­commissions, for current measurements and for the study of the origin of current channels, among others. 17 18


The Zuiderzee Commission

discussion partner for Lorentz and became the coordinator of the substantial ­calculation work that needed to be carried out.22

The commission’s work After the commission’s first official meeting on July 15, it set to work on the collection of large quantities of data on water levels and currents in the Wadden Sea and the Zuiderzee. Surprisingly enough, it turned out that nothing was known about the North Sea tidal activities outside the sea inlets between the Wadden Islands, so the work had to start with measurements at those locations. It was a very time-­consuming job, for which special instruments had to be developed. At the same time, Lorentz began to familiarize himself with the subject matter, which was largely new to him. Characteristic for his systematic approach was that he began by thoroughly studying the critical brochures by Gallé, Kooper, and Lely Junior.23 He then set out to acquire sufficient knowledge about tides and storm surges. In doing so, he encountered two completely different approaches. Meteorologists knew much about the influence of wind on water motion and about phenomena that changed over time. Since they generally worked on oceanic circumstances, with water masses of great depth, they did not know anything, however, about the way in which water motion was influenced by the sea bottom’s resistance. Among the water management experts, the situation was exactly the reverse. They knew much about the motion of liquids in channels with resistance at the bottom, but they generally limited themselves to currents that were constant over time and in their work the influence of wind was not taken into consideration at all. In situations where other factors had to be considered, they resorted to analogies and estimates. Lorentz’s great contribution—and this contribution goes far beyond the specific work on the Zuiderzee issues—was that, for the first time, he was able to combine these two methodologies in such a way that he provided a scientific basis for fluid dynamics for non-­permanent phenomena. His approach was of crucial importance for the resolution of later hydrological problems. Examples are the problems that needed to be solved during the preparation of the Delta Works, the major flood protection works in the 1950s and ’60s, after the great flood of February 1953 that claimed as many as 1,836 casualties.24 After an initial period, when he stayed somewhat in the background, Lorentz realized that he had to be the one to take the initiative, as the other members

22   Thijsse’s predecessor as second secretary was L. M. de Nerée tot Babberich. Thijsse was the son of the well-­known biologist Jac. P. Thijsse. He became one of the most prominent hydrologists in the Netherlands. 23   His notes have been preserved in a notebook in CT. 24   See also the quote from Mazure 1963 in note 2.

The normal tides


turned out to be incapable of dealing with the problem at hand.25 This was the beginning of a period of eight years of hard work: longer than anybody could have predicted.26 As the final report presented not only the conclusions of the commission, but also a more or less chronological account of all the work that was carried out, it offered a good overview of the enormous amount of work that was required and the difficulties that had to be overcome. Interestingly, the exceptionally w ­ ell-­written report—for the most part written by Lorentz himself—not only showed the successes, but also the methods that ended up not leading anywhere.

The normal tides It was clear from the beginning that the problem was best approached by dividing it into two parts: First study how the closing dike would influence normal tidal activity, and subsequently take stock of the storm surges. Because normal tides are periodical in nature, the first problem was simpler than the second one, since it involved storm surges that are unique and have a high degree of non-­periodicity. The normal tides could be studied by modeling the Wadden Sea and the Zuiderzee as a system of current channels and describing the currents in these channels by means of hydrodynamic equations.27 These equations described the currents under the influence of external forces, such as water level differences between the beginning and the end in rectangular channels. The resistance at the bottom was also accounted for in these calculations. The equations were then transformed in such a way that they could be solved numerically. The calculation method that was being used essentially started with the channels that began in the sea inlets of the Wadden Islands. On the basis of known data, like water levels and current velocities in these inlets, the values of these entities at the end of the channels were then calculated. Subsequently, the results were taken as the initial data for the following channels. At an intersection of three or more channels it had to be taken into account that the water level calculated for each

25   Lorentz never publicly expressed himself about the degree of his involvement and the competence of his fellow commission members, but it was generally known that he was almost solely responsible for the theoretical work, and this impression is confirmed by the available archival material. See also the memories of Thijsse in Thijsse 1957 and in “Ten years with Lorentz on the Zuiderzee problem” (manuscript CT). The full commission only met four times during the entire ten-­year project— the last time in 1922—and this also speaks volumes about its degree of involvement in the work. 26   In the minutes of the second general meeting of the commission on February 27, 1920 (FAK and ASZ), the expectation is expressed that the final report will appear in the course of 1921. ASZ also contains a printed draft of the introduction, dated 1921 (1522). 27   Calculations were carried out on various systems with an increasing number of channels. The most extensive system consisted of forty-­five “main channels,” but because a number of these actually consisted of two or three parallel channels, the total number came to 121. The parallel channels all ran in the same direction and were of equal length, but they had different widths and depths. Because they were connected horizontally, the water level in all parallel channels was the same.


The Zuiderzee Commission

channel had to have the same value and also that water should not accumulate there. In this way, the water motions for each of the periodical tides could be calculated in the model.28 By doing so, it turned out to be possible to satisfactorily reproduce the water levels that had, in effect, been measured on the Zuiderzee coast. Just to be sure, the method was also tested in other comparable situations. In the Gulf of Suez, the tides at Suez—where no current exists—were successfully used to calculate the water levels and currents in the Gulf of Aqaba. The Bristol Channel between Wales and Cornwall29 was also chosen as a testing ground. It was expected that, after closing off the Zuiderzee, the tidal differences in the Wadden Sea would become greater than those in the sea inlets. A similar situation existed in the Bristol Channel. The funnel effect that occurred there caused the tide in the town of Chepstow, at the mouth of the river, to be almost twice as high as on the side of the open ocean. Here, too, the results were satisfactory, so the way was clear to apply the method to a closed off Zuiderzee. As was said before, Thijsse was in charge of the enormous quantity of calculation work which required the utmost accuracy on the part of the—mainly female—calculators. According to Lorentz himself, he tried his hand at this numerical work a few times, but he had to give it up because he made too many mistakes.30 The calculations, for that matter, were very elaborate and ­time-­consuming. Unfortunately, little is known about the precise proceedings, other than that two mechanical calculation machines were used and two calculators spent several months on the calculation of the most extensive system of channels.31 To be able to model the closed off Zuiderzee as well, an adjusted system of channels was used, ending at the envisioned location of the closing dike. In other aspects, the calculations went exactly the same as in the case of the open Zuiderzee. The results showed that the highest tides, especially near the Friesian end point of the closing dike, would increase slightly and that the velocity of the currents in the sea inlets would increase substantially. This latter result appeared to be strange at first sight. A smaller reservoir can contain less water, so it was expected that less water would be propelled into the Wadden Sea and that this would coincide with lower current velocities. The 28   In the nineteenth century, Lord Kelvin had shown that tidal movements could be well described as a combination of periodic motions with different periods. The most important of these is the “lunar tide” M2, which has a period of roughly twelve hours (see Cartwright  1999 for more details). In the  Zuiderzee Commission’s calculations, the most important of these components were taken into account. 29   The information about the test in the Gulf of Suez is derived from Thijsse 195; in Staatscommissie 1926 this test is not mentioned. 30  See Thijsse 1927, 723. 31  The mechanical calculators were Millionaire and Burroughs machines (Staatscommissie  1926, 311). Thijsse 1935, B259, speaks of one month’s work and Mazure 1963, 126, about three months’ work, in both cases by two calculators. It is unclear whether they are referring here to the same system of channels.

Storm surges


solution to this paradox was found in the dimensions of the open Zuiderzee. Each time when the outgoing tidal current from the Zuiderzee and the Wadden Sea reached the sea inlets, it almost coincided with a new incoming current, so that the net current actually became smaller. In a closed-­off Zuiderzee this would no longer be the case.32

Storm surges Storm surges were less easy to master than normal tides. Not until 1925 was an approach finally found that led to an even slightly satisfactory result.33 Two methods were tried and then abandoned, because they turned out to be unreliable. For the sake of completeness, they were still included in the final report. The third method, which ended up being successful, consisted of two phases. In the first phase, the commission made an approximate calculation of the water currents in a system of channels, just like for the normal tides, albeit that in this case a simpler system with fewer channels was used.34 The most important aspect of this “third” approach was that the state of the current was assumed to be permanent, or stationary; that is to say, it was constant over time. This—actually unrealistic—assumption boiled down to the idea that, on the coast, water that was driven into the Wadden Sea by the wind was somehow shunted off into a kind of drain. In this way, the water levels were the same at every moment in time. The calculation then went as follows: The starting point was a given water level in the North Sea, derived from data about past storm surges, in particular one in 1894. Then an assumption was made for the current in the first channel and, based on that assumption, what the height and velocity of the current would be at the end, at the point where the channel forks into two, was subsequently calculated. At each fork, new assumptions were made about the spread of the current over the new channels. This procedure continued until the current had eventually returned to the North Sea. The level calculated there had to match what was known from the data. If it did not match, the calculation had to be redone with a different value for the velocity of the initial current. It stands to reason that this system of trial and error was very time-­consuming. Hence the choice of a simplified system of channels.

32   In somewhat more technical terms: The distance between the Afsluitdijk (the dike closing off the Zuiderzee) and the sea inlets was approximately a quarter of the wavelength of the tidal wave, and this caused a resonance phenomenon. 33   See “Ten years with Lorentz on the Zuiderzee problem” (manuscript, CT). In the ASZ there are two manuscripts by Thijsse with calculations based on the new approach (“Memorie E,” from August 1925 and “Memorie F,” from January 1926; both in 1535). 34   Instead of working with 121 channels, thirty-­five channels were now used.


The Zuiderzee Commission

Now, in order to be able to eliminate the assumption of permanence, Lorentz devised the following procedure.35 He succeeded in developing an exact method for an even more simplified system. It actually consisted of only one channel with five sections. Exact, in this case, meant that the hydrodynamical equations could be solved without the requirement of permanence. Dropping this requirement made the calculations substantially more time-­consuming and laborious, however, because now the course of the currents had to be calculated for a period of at least six hours, instead of just for one point in time.36 Of course, a permanent state could also be calculated for the simplified system, so it was possible now to determine the influence of permanence, at least for this system. Based on the correction factor found in this way, an estimate was made of the correction that needed to be applied to the earlier results to compensate for the “non-­permanence” of the current. This is how the increases were calculated eventually. In the end, an additional correction of twenty percent was applied to the final result to allow for uncertainties and approximations.37

Recommendations and predictions On the basis of all these calculation results, the commission recommended raising the dikes all along the Wadden Sea. The results also formed the basis for the recommendation to move the Friesian end of the closing dike to the north, from the village of Piaam to the village of Zurich.38 The background for this decision was the concern, among the engineers preparing the dike’s construction, that the quality of the soil near Piaam was not sufficiently solid. It might be too weak to bear the weight of the heavy dike. They suggested Zurich as an alternative for Piaam, and this led to the request to the commission for additional calculations for a dike to Zurich. The commission eventually preferred this more northerly route because the calculations showed that this option would lead to a lesser increase of the current velocities in the sea inlets.39

35   On June 17, 1925, during a meeting in Lorentz’s home, it was concluded that an earlier approach that assumed the correction for “non-­permanence” to be only slight, had to be rejected; see “Ten years with Lorentz on the Zuiderzee problem” (manuscript, CT). The new, exact method was formulated for the first time in a letter to Thijsse, dated one day later (CT). 36   The calculations were done, this time with a slide rule, for eight points in time, each separated from the other by 5,000 seconds. 37   More precisely: The error analysis led, in the first instance, to a decrease in the results by thirteen percent; to this decreased result the correction of twenty percent was applied, so that an increase of five percent of the original values resulted eventually. 38   An undesirable consequence of moving the dike was that it would now cross a deep channel with strong currents at an angle that would create difficulties for the builders. To avoid this, a dogleg bend was introduced in the design so that the dike would cross the channel perpendicular to the current (see Thijsse 1972, 96). 39   In anticipation of possibly moving the Friesian end of the closing dike, the Zuiderzee law had been changed in such a way that Piaam was no longer mentioned as its end point.

The time spent by Lorentz


The predictions by the Lorentz Commission, made on the basis of the calculation results, can be summarized as follows. Near Zurich, the water level at high tide and with no wind would increase by about twelve inches (thirty centimeters). This meant an increase of the tidal difference from around forty-­two inches (105 centimeters) to roughly sixty-­three inches (160 centimeters), or around sixty percent. Near Wieringen the normal tidal difference would double from around twenty-­eight inches (seventy centimeters) to approximately fifty-­five inches (140 centimeters). During storm surges the highest level at Zurich would increase by some thirty-­eight inches (ninety-­five centimeters) and near Wieringen by a quantity between about thirty-­two and forty-­four inches (eighty and 110 centi­ meters). In all other places the increase would be lower, during normal tides as well as ­during storm surges. How accurate were these predictions? Those regarding the normal tides were fine, even in places where the tidal differences were predicted to double. For the storm surges the situation was different. For these, the predictions were too optimistic; in other words, systematically too low. The difference for the closing dike itself turned out to be about twelve inches (thirty centimeters). This difference was actually not all that dramatic, since the dike was made extra high to withstand wave surges up to twenty-­three feet (seven meters) above sea level.40 It should also be noted that storm surges are rare, so that definitive conclusions could not be drawn until many decades later, on the basis of a thorough statistical analysis. Nonetheless, the dikes have held perfectly adequately for all these years. The final report of the commission was published in November of 1926. In the Royal Decree of January 22, 1926 dissolving the Lorentz Commission, the chairman is thanked especially for “the excellent direction of the investigations of the Commission and for his preponderant part in the work, which has ensured that the results of the investigations have become of very high importance.”41 Thijsse’s work was also rewarded. He received a royal decoration in the high rank of Officer in the Order of Orange-­Nassau.42

The time spent by Lorentz How much time did Lorentz spend on the Zuiderzee problem during the eight years of the commission’s existence?43 This is a question that is not easy to answer, as it is difficult to make an accurate estimate of the time consumed by the Zuiderzee project. However, it is known that the theoretical work regularly required Lorentz’s complete and undivided attention for week-­long periods.

41  See Thijsse 1972, 214–221.   Copy in ASZ, 1522.  See Nieuwe Rotterdamsche Courant, January 28, 1926. 43   The information in this paragraph is derived from material in CT, in particular the correspondence between Thijsse and Lorentz and the manuscript “Ten years with Lorentz on the Zuiderzee problem” (CT). 40 42


The Zuiderzee Commission

That Lorentz had time for little else during that period can be concluded from the correspondence with Thijsse and from the frequency of the visits Thijsse made to Haarlem to discuss progress or obstacles they found on their way. In a letter from 1926, Lorentz referred to their ups and downs, recounting how “quite a few times” they had been “down in the dumps in friendly companionship” and had then “worked their way back out” and had “shared success and misery like good friends.”44 According to calculations by Thijsse, he had some 120 such meetings with Lorentz over the course of those years, meetings which often lasted all day. Especially in 1925, when the problem of the storm surges turned out to be tougher than they had thought, Lorentz concentrated almost completely on his work on the Zuiderzee for quite some time. During the period of writing the final report, Lorentz also had little time for other business. Of the 300-­page final report, he wrote at least half himself and he commented extensively on the remaining text. Well documented in Thijsse’s archive is also the enormous amount of additional time Lorentz spent on the final proofreading. In these proofs he made a great number of corrections, large and small, and this work must have taken him many weeks. Of one important correction it is known that Lorentz worked on it over an entire weekend. Just to be sure, he had performed a simple consistency test while reading the proofs. The test boiled down to reassuring himself that no water surges occurred in the current channels: At the channels’ intersections the incoming and outgoing quantities of water should be equal. To his great dismay, Lorentz found that in a certain situation this was not the case. His heart sank when, at first, he could not think of any solution other than doing the calculations all over again. Eventually, after a brilliant idea and a Sherlock Holmes-­like reconstruction, he was able to conclude that the mistake was most probably a copying error. Adding a minus sign to a number in one of the tables solved the problem completely and made everything consistent again. Lorentz’s letter to Thijsse, in which he recounts all this, is fascinating to read.45 In all, it is difficult to estimate what percentage of his time Lorentz spent on the work for the commission, but it is clear that the amount of time it consumed was considerable. It also seems clear that, when he started, he was not aware how much of a workload he was taking on; all the more so because at the start he trusted his fellow commission members to be able to shoulder a substantial part of the burden. That Lorentz was working on a great and time-­consuming problem certainly did not escape his physics colleagues. Thijsse recounts, for example, how he was taken aside after Lorentz’s funeral by Albert Einstein. He reproached Thijsse,

44   “heel wat keren kameraadschappelijk in de put hebben gezeten en er weer zijn uitgekomen.” “als goede vrienden lief en leed hebben gedeeld.” Lorentz to Thijsse, July 21, 1925 (CT). 45   Lorentz to Thijsse, August 23, 1925 (CT).

The time spent by Lorentz


saying that the Zuiderzee work had done a great disservice to physics by keeping Lorentz away too much from his physics work.46 Nevertheless, Lorentz managed to have time left over for other activities. During the same period, he chaired two Solvay conferences, requiring much preparation, and he traveled to the US twice to lecture for a few months. Not only did these American lectures require much time to prepare, but his lectures in Pasadena, in 1922, were also published as a book.47 Lorentz’s correspondence from that period shows that he still succeeded reasonably well in keeping up with the most important developments in physics. His correspondence, for example, with Paul Ehrenfest, his successor in Leiden, showed a thorough command of the new developments in quantum theory, such as matrix mechanics and wave mechanics.48 Besides, he managed to produce a continuous flow of publications. In looking back, Lorentz later said about his work on the commission that “he had shied away from it somewhat” and that he “had been quite reluctant, as a physicist is not used to problems with such a degree of complication and with so few definite facts.” Nonetheless, his final judgment was that it had turned out better than he had expected. He was especially satisfied that “we were able to make more of it than we had imagined.”49 Though the investigations may have snowballed, in the sense that they took much longer than was originally anticipated, Lorentz himself apparently thought that the result justified the efforts.

  See Thijsse’s manuscript “Ten years with Lorentz on the Zuiderzee problem” (CT).   This actually did not happen until 1927, without a doubt partly because of his preoccupation with the work on the Zuiderzee. 48  See Kox 2018. 49   “daarvoor wel wat [was] teruggeschrikt.” “erg tegen opzag, want een physicus is niet gewoon aan problemen met zulke mate van ingewikkeldheid en met zoo weinig vaststaande gegevens.” “wij er meer van [hebben] kunnen maken dan wij ons hadden voorgesteld.” Thijsse 1927, 723. 46 47

Chapter 11 International cooperation in crisis

The Great War had finally come to an end with the armistice of November 11, 1918. After the victory of the allied powers, the time had now come to make a new beginning by clearing away the debris. Literally, by clearing the devastated areas in France and Belgium, where whole villages had been wiped off the face of the earth, but also figuratively, by restoring international relations. In 1919, after the Versailles peace treaty, the victorious nations took the initiative to establish the League of Nations. This organization—which counted ­fifty-­eight member-­countries at its peak in 1935—was founded in an effort to prevent another gruesome war and to bring reconciliation to the warring parties. Yet, there was little feeling of reconciliation in the conditions imposed by the Treaty of Versailles, as they were extremely harsh for Germany. In the restoration of international relations in the scientific community, a similarly vindictive atmosphere prevailed.1

The Conseil International de Recherches Before the war, the coordination of international scientific contacts was the remit of the Association Internationale des Académies. This umbrella organization represented the science academies of all countries that amounted to anything in the world of science. The association was founded in 1899 to create some order in the  plethora of international scientific societies and organization and in the steadily expanding number of international conferences on many different ­scientific topics. Once the war was over, the association was replaced by a completely new organization for the exact sciences: the Conseil International de Recherches (International Research Council; abbreviated hereafter as CIR). During the war, preparations had already been made to establish this new institution for scientific

1   See for an overview of the development of international scientific cooperation after World War One Schröder-­Gudehus 1966, Schröder-­Gudehus 1978, and, especially for the role of the Netherlands, Otterspeer and Schuller 1997.

“A Living Work of Art”: The Life and Science of Hendrik Antoon Lorentz. A. J. Kox and H. F. Schatz, Oxford University Press (2021). © A. J. Kox and H. F. Schatz. DOI: 10.1093/oso/9780198870500.003.0012

The Conseil International de Recherches


cooperation, and at the time, it had already been determined that the central powers, consisting of Germany, the Austro-­ ­ Hungarian Empire, the Ottoman Empire, and Bulgaria, would be excluded from participation. For Belgium and France in particular, this was a matter of principle. A less implacable stance was taken by Great Britain and the United States, which had not become actively involved in the war until 1917. A preparatory first conference was held in London from October 9 to 11, 1918, even before the official armistice of November 11. The following month, in a meeting in Paris from November 26 to 29, statutes were drafted and a committee was formed to prepare the formal foundation of the CIR. Member countries would be represented by academies of science, national research councils, or directly by their governments. It was further decided that the CIR would function as an umbrella organization for separate unions for individual disciplines, which were to be established later. For admission to one of the unions, membership of the CIR was required. Day-­to-­day management of the CIR was in the hands of a Comité Exécutif (Executive Committee), consisting of five members representing France, Belgium, Great Britain, the United States, and Italy. In practice, it turned out that the Comité Exécutif was dominated by two men: the French mathematician Émile Picard, the Secrétaire Perpétuel (Permanent Secretary) of the French Académie des Sciences and the British physicist Arthur Schuster, who represented the Royal Society in London.2 During the London preparatory conference, in a preamble to the draft statutes, the participants motivated explicitly why the central powers had to be excluded from membership. They were motivated to do so not so much by the abom­in­ ations of the war, but rather by the fact that these atrocities had been committed as a result of conscious decisions. “The central powers have broken the laws of civilization,” so they proclaimed, and they were held responsible for “the horrors, organized and thought out, from the beginning, with the sole purpose of terrorizing innocent populations.”3 This strong condemnation would later cause serious problems with regard to the accession of Germany to the CIR. The text of the draft statutes was a source of great agitation and irritation among the neutral countries as well. It was stipulated that neutral countries could only be admitted with a majority of three quarters of the vote, while other countries only required a two thirds majority. The Groningen chemist and Academy member Frans Maurits Jaeger was quite incensed about this inequality, according to a letter he wrote to Lorentz. He believed it was not right that “Serbian, Portuguese, Montenegran, Japanese, Siamese and Brazilian seventh-­class-­forces”

2   The other members were the Belgian astronomer Georges Lecointe, the US astronomer George Ellery Hale, and the Italian mathematician Vito Volterra. 3  “[...] les Puissances centrales ont enfreint les lois de la civilisation,” “les horreurs organisées, encouragées et imaginées, dès l’origine, dans le seul but de terroriser les populations inoffensives.” Schröder-­Gudehus 1966, 91.


International cooperation in crisis

were to decide whether great scientists like Lorentz, Kamerlingh Onnes, Zeeman, and Hugo de Vries “would be deemed worthy of cooperating in scientific matters.”4 He added: The scientists have not exactly shown themselves as mentally equilibrated. But such insanity as this latest academic product has not been produced until now; and should the matter not have more than one serious consequence, then it would be ridiculous enough to turn it into an operetta.5

The CIR statutes also emphatically stipulated that any change required a ­two-­thirds majority. This stipulation was meant to prevent the neutral countries, or a block of smaller, less vengeful nations from forcing the admission of Germany against the will of Belgium and France by means of a statutory amendment. In addition, the number of votes per country was assigned in proportion to the number of inhabitants, colonies included. By this calculation, Belgium, which had only about 7.5 million inhabitants at the time, managed to acquire the maximum number of five votes, equal to that of France or Great Britain, thanks to the size of its possessions in the Belgian Congo. In the Netherlands and in other neutral countries protests were raised against these stringent conditions for admission, while in the United States and England some parties advocated for admission of the neutral nations as soon as possible. To accommodate the objections, the Comité Exécutif decided, in the spring of 1919, to propose in the planned founding meeting of July 1919 that neutral countries could be invited without a roll-­call vote in the Conseil. As was stated earlier, the CIR was meant for the exact sciences. A similar organization was founded as a counterpart for the humanities, the Union Académique de Recherches et de Publications (Academic Union of Research and Publications), later renamed the Union Académique Internationale (UAI) (International Academic Union). An important difference between the UAI and the CIR was that the UAI’s statutes did not specifically exclude the central powers. In practice, the CIR and the UAI operated quite independently, and for this reason, the UAI will be given little further attention in this chapter.

The Netherlands and the Conseil In all these developments Lorentz played an important role, initially for the most part in the Netherlands. He was the chairman of the Section of Sciences of the

  “waardig zouden worden bevonden om mee te werken op wetenschappelijk gebied.”   “De geleerden hebben zich tijdens deze vier oorlogsjaren nu niet juist als geestelijk-­geëquilibreerden doen kennen. Maar zulk een krankzinnigheid als dit nieuwste academisch product is er tot dusverre niet voortgebracht; en kon de zaak niet meer dan één ernstig gevolg hebben, dan zou ze ridicuul genoeg zijn om er eene operette van te maken.” Jaeger to Lorentz, January 8, 1919 (LA 146). 4 5

The Netherlands and the Conseil


Academy of Science and, therefore, once every two years he was also the chairman of the entire Academy. Since the Academy was the organization designated to represent the Netherlands in the CIR and the UAI, Lorentz was a central figure in the discussion about the role of the Netherlands in the post-­war scientific world order. Very soon after the Paris meeting, the discussions heated up. A group of members from Groningen was especially vocal, among them the astronomer Johannes Kapteyn, the psychologist Gerard Heymans, and Jaeger—already mentioned earl­ ier. They insisted that the Academy should take a strong stand and should only join if there was a clear prospect of a speedy inclusion of the central powers. Besides, they wanted the Academy to state clearly that it would continue to maintain its contacts with German and Austrian academies. Although the initiative soon fizzled, the group from Groningen even considered establishing a competing organization, together with other neutral countries, publishing an open letter to the neutral countries to that effect. The Groningen group also envisioned a new role for the old Association Académique. Formally, it had never ceased its activities, although, in effect, it was no longer in existence. Lorentz, in his usual cautious way, initially took a wait-­and-­see approach, to the great displeasure of the members from Groningen. Jaeger reproached him that the Academy took the “despicable, compromising standpoint of our diplomacy” and, without mincing words, he called Lorentz’s attitude “all too weak.”6 During a meeting of the Academy’s Section of Sciences on January 25, 1919, the matter was discussed. Lorentz, who chaired the discussion, saw three possibilities: creating a separate organization for the neutral countries, joining the central countries, or aspiring to membership of the CIR. In the latter case, admission of the central powers as quickly as possible should be an additional goal. Not unexpectedly, Lorentz himself favored joining the CIR, and he would consistently adhere to that idea in the years to come. In the debate that followed, the meeting was unable to reach a decision. A commission, instituted in the course of the meeting, had to find a way out by formulating a proposal that the boards of the Academy’s two sections could present to their entire membership. This commission turned out to be equally incapable of resolving the issue. Eventually, it became altogether superfluous, because during the founding meeting of the CIR in Brussels, in July 1919, it was decided that the neutral countries would also be invited to become members. The decision was unanimous, so the provision requiring three quarters of the vote did not need to be adapted or struck down. The CIR communicated the decision to Lorentz in a letter by Arthur Schuster, the British member of the Comité Exécutif that was clearly meant to

6   “verwerpelijk schipperig standpunt onzer diplomatie.” “al te slap.” Jaeger to Lorentz, January 20, 1919 (LA 146).


International cooperation in crisis

gauge the opinion of the Dutch Academy.7 In his response, Lorentz wrote that it was “very tempting” to accept the invitation.8 He did stress that the final goal should be an organization in which all countries were represented, even though he understood—as Schuster had emphasized—that cooperation with the central powers would, as yet, be impossible. On September 19, the official invitation to join the CIR arrived in Amsterdam.9 Lorentz called a special meeting of the entire Academy for a decision about the Section of Sciences and the Section of Humanities respectively joining the CIR and UAI. Unsurprisingly, the Academy’s board, dominated by chairman Lorentz, was in favor of joining. Lorentz had prepared for the meeting very carefully. Contrary to his normal habit, he wrote down his argumentation word for word in a statement of no fewer than fifteen pages. He was in favor of membership because it would increase the effect of any Dutch efforts to undo the exclusion of the central powers, more so than if the Netherlands were to remain on the sidelines. After an extensive and sometimes emotional debate, the proposal was accepted, albeit not with an overwhelming majority. Humanities passed it with twenty-­four votes for and six against, while Sciences voted twenty-­one for and fifteen against. In total, the result was forty-­five against and twenty-­one for: a scant two-­thirds majority.10 The meeting also decided that the Netherlands had to insist that the CIR needed to bring about true international cooperation. Additionally, the Academy reserved the right to continue its contacts with the academies of the central powers, particularly those of Germany and Austria.11 A decision had finally been reached, but that did not mean that all Academy members took the decision sitting down. The members Heymans and Kapteyn from Groningen made their displeasure crystal clear to everyone. Heymans ­formally gave up his membership of the Academy, while Kapteyn just never appeared at any further Academy meetings.

Attempts toward normalization The Academy’s accession to the CIR was the parting shot for a long series of attempts to normalize international cooperation and undo the isolation of the central powers. These efforts by the Academy, as well as by Lorentz personally, stretched out over a period of many years. The second CIR Assembly in Brussels, from July 25 through 29, 1922, was the first opportunity to test any willingness to

  Schuster to Lorentz, August 13, 1919 (LA 146).   Undated draft (LA 146).   Schuster to Secretary of the Academy (Pieter Zeeman), September 19, 1919 (LA 146). 10   Prior to the meeting there had been an internal discussion about whether or not a two-­thirds majority would be required for a valid decision, but eventually this idea had been abandoned. 11   Berkel 2011, 50. In letters to the academies in Berlin and Vienna it was emphasized that the Dutch decision to join the Conseil did not imply that the relations with these two academies would be severed. 7 8 9

Attempts toward normalization


admit the central powers. The biologist Frits Went, who had succeeded Lorentz in 1921 as chairman of the Academy’s Section of Sciences, and the chemist Arnold Holleman, vice-­chairman of the section, represented the Netherlands. In the Assembly, the Swedish delegation put forward a proposal to admit Germany. The Academy was aware of the proposal, but, given the various sensitivities, it was left to Went and Holleman to decide whether or not they would support the proposal during the meeting. Afterwards, Went gave an account of the meeting in a letter to Lorentz.12 The Swedish representative had defended the proposal without much conviction “among great agitation of the Belgians and French who were present,”13 after which the Englishman Glazebrook had proposed to let the matter rest. The Swiss representative Gautier had then expressed his expectation that France and Belgium would take the initiative to admit all countries. The response of the Belgian and French delegates was furious, according to Went. Then there followed a cry of hate, in the form of a “jamais” [“never”] that I will never forget. Gautier repeated that he could fully understand the state of mind of the Belgians and the French, but that it saddened him on behalf of science, that the ­gentlemen did not want anything else. Again, came the: “jamais.”14

Went was so indignant and so frustrated that he threatened in conversations with participants from other countries to completely withdraw the Academy from the CIR. In his letter to Lorentz, he also suggested the possibility that Germany might encourage admission by showing a slightly more humble attitude. Lorentz was pessimistic about this possibility. Still, in the fall of 1923, he tried to persuade Planck, one of the prominent signatories of the infamous Aufruf an die Kulturwelt, to initiate a declaration to recall the inflammatory Aufruf. In vain. Planck wanted to do nothing of the sort and expressed his opinion in a letter to Lorentz in no uncertain terms: that signing this declaration would undoubtedly create the impression in the public sphere that the German scientists, driven by dire necessity, declare themselves prepared to let go of the convictions they have expressed before, that the war was not caused by the Germans alone and that, once it had broken out, it was Germany’s duty to take the side of the German army. This conviction, in fact, still exists today, although nobody among us with any insight can deny the great failings and injustices included in the manifesto of the 93. [. . .] We are the vanquished, the defeated, and, as one might also say, the tortured. Expecting from us, in this condition, a further concession, without the slightest guarantee of success, means to impose a penance

  Went to Lorentz, June 30, 1922 (LA 147).   “onder grote opwinding van de aanwezige Belgen en Franschen.” 14   “Daarop volgde een kreet van haat, in de vorm van een ‘jamais’ die ik nooit zal vergeten. Gautier herhaalde, dat hij de geestesgesteldheid van Belgen en Franschen volkomen kon begrijpen, maar dat het hem toch voor de wetenschap leed deed, dat de heeren niet anders wilden. Weer kwam het: ‘jamais.’ ” 12 13


International cooperation in crisis

on the defeated enemy in retrospect, a penance especially, that is even harsher than that of political and economic servitude.15

The Union de Physique During the CIR meeting in 1922, a union for physics was founded—among others. The question arose as to which organization should represent the ­ Netherlands in this Union de Physique Pure et Appliquée (Union of Pure and Applied Physics). The Nederlandsche Natuurkundige Vereniging (NNV) (Dutch Physics Association) was the obvious choice. Even though this organization was still quite young—it had been founded only a year before, in 1921—it was the only professional organization for all Dutch physicists. On February 24, 1923, the NNV had called a meeting to discuss a letter from Lorentz to chairman Gilles Holst proposing the association as the representative of the Netherlands in the Union de Physique.16 The meeting’s participants were divided. They condemned the exclusion of the central powers, but had a difference of opinion about the preferred course of action—just like the Academy in its meeting at the end of 1922. Should the NNV remain on the sidelines or should it join the CIR to be able to effect reforms from within? Of course, Lorentz, who was present at the meeting, advocated for the second alternative, but a decision could not be reached. On September 29, the NNV met again. In the seven months since February, the board had not managed to formulate a unified standpoint or advance a proposal. This time the meeting finally reached a decision. By twenty-­two votes against three it was decided not to join the Union, in spite of Lorentz’s appeal.17 This was a bitter disappointment for Lorentz. As he wrote indignantly to his friend Zeeman, a couple of days later: “[the meeting] lasted until about six o’clock and I arrived

15  “dass eine Unterzeichnung dieser Erklärung in der Oeffentlichkeit zweifellos vielfach den Eindruck erwecken würde, dass die deutschen Gelehrten, von ihrer bitteren Notlage getrieben, sich bereit erklären, ihre früher ausgesprochene Ueberzeugung preiszugeben, dass der Krieg nicht von Deutschland allein verschuldet worden ist, und dass es, nachdem er einmal ausgebrochen war, in ihrer Pflicht lag, sich auf die Seite des deutschen Heeres zu stellen. Diese Ueberzeugung besteht aber tatsächlich auch heute noch, wenn auch kein Einsichtiger unter uns die in jenem Aufruf der 93 enthaltenen groben Mängel und Unrichtigkeiten abläugnen kann. [. . .] Wir sind die Besiegten, Geschlagenen und, wie man wohl auch sagen darf, Gemarterten. In diesem Zustand uns noch ein weiteres Zugeständnis abverlangen, ohne die mindeste Gewähr für einen Erfolg, heisst dem überwundenen Feind noch nachträglich eine Busse auferlegen, und zwar eine Busse, die noch härter ist als die der politischen und wirtschaftlichen Knechtung.” Max Planck to Lorentz, December 5, 1923 (Kox 2008, 382). He replied to a letter by Lorentz of November 13, which is lost. Planck had been visited by the Amsterdam chemist Charles Boissevain, who had presented to him a draft declaration of the retraction of the manifesto. See also Boissevain’s account of his visit in his letter to Lorentz of December 3, 1923 (LA 147). 16   Lorentz to Holst, December 17, 1922 (Archive NNV, Utrecht). 17   Minutes general assembly NNV, February 24 and September 29, 1923 (Archive NNV, Utrecht).

December events in Paris


home late and somewhat in a huff.”18 Emphasizing how strategically important membership would have been, he described his feelings: “I defended accession still more warmly than I thought I would, a few days earlier, for thinking about it some more, I realized more and more that, if the neutral countries stay on the sidelines now, the chance to achieve general cooperation becomes much smaller.”19 Lorentz was all the more disappointed because his expected appointment as vice-­chairman of the Union de Physique would not be feasible now, and even his participation in the first general assembly of the Union, planned for December in Paris, was now in doubt. Fortunately, a slightly artificial solution was created to send a Dutch representation to the Union’s assembly anyway. In a personal cap­ acity, a group of physicists, among whom were Lorentz, Ehrenfest, and Zeeman, formed a “National Committee” that was also open to non-­members of the Academy. The Committee was supposed to promote international cooperation between Dutch and foreign physicists from all countries, not just from member countries of the Union. On December 10, 1923, Lorentz and Zeeman went to the Union assembly in Paris to represent the Netherlands on behalf of this National Committee. Other disciplines made different choices than the physicists. The geographers, for example, as well as the chemists, the astronomers, and the biologists joined their respective Unions without much further ado. Following the lead of these other disciplines, the Academy decided in early 1923 to institute a special ­commission to promote international cooperation in a broader context than strictly among physicists. It went by the name of Wetenschappelijke Internationale Samenwerkingscommissie (Scientific International Cooperation Commission), or WIS-­commission. Apart from the Academy’s board, many of the commission’s members were representatives from the disciplines represented in the various CIR Unions. It goes without saying that Lorentz represented physics. Amendment of the CIR statutes to facilitate Germany’s admission, so ardently advocated by the Netherlands but rejected in 1922, continued to be the focus of attention for the Academy. Plans were made for the commission to raise the issue again during the upcoming CIR meeting of July 1925, with broader support than in 1922 this time, or so it was hoped. Lorentz was very active in attempts to achieve this goal.

December events in Paris The meeting of the Union de Physique was a reason for Lorentz and Aletta to travel to Paris, but not the only reason. From December 5 to 8, Lorentz participated in

18   “Het duurde tot ongeveer 6 uur en ik kwam laat en wel wat in mijn wiek geschoten thuis.” Lorentz to Zeeman, October 1, 1923 (Kox 2018, 260). 19   “Ik heb de aansluiting toch nog warmer verdedigd dan ik eenige dagen te voren dacht te zullen doen, want verder nadenkende, zag ik meer en meer in dat, als de neutralen zich nu afzijdig houden, de kans om tot een algemeene samenwerking te geraken veel kleiner wordt.”


International cooperation in crisis

the Third Session of the Commission Internationale de Coopération Intellectuelle (CICI) (International Committee on Intellectual Cooperation), of which he was a member. His involvement with this committee is discussed in a later section in this chapter. The CICI meeting was followed immediately by the celebration of the fiftieth anniversary of the Société Française de Physique (French Society of Physics) from December 8 to 15.20 These meetings incorporated big social events: 200 guests were welcomed at the Bienvenue Française (French Welcome) of the CICI, and Prime Minister Raymond Poincaré—brother of the mathematician—hosted a formal dinner for the CICI members and one hundred other guests in the Foreign Ministry at the Quai d’Orsay. Lorentz’s toast on that occasion was very well received and Aletta was proud: When Pa got up, a great applause went through the hall [. . .] Pa never says anything he does not mean and there is always feeling in it while some others let themselves get carried away and run on in beautiful sentences, like a Belgian socialist Destrée who is almost moved to tears by his own humility.21

At the solemn celebratory session of the Société Française de Physique Lorentz gave a scientific lecture22 and a formal speech on behalf of the foreign visitors. Of course, this celebration was also accompanied by several social events, including a festive dinner where Lorentz gave a table speech and raised another toast. Aletta was not in luck. One of her table-­companions was quite uncommunicative and the other one was seated on the side of Aletta’s deaf ear. She took it all in her stride: “I am so dyed in the wool that things like that do not bother me, I enjoy myself anyway by just looking around.”23 At the end of the month Lorentz made another quick trip to Paris, this time for the celebration of the twenty-­fifth anniversary of the discovery of radium by Pierre and Marie Curie. On the evening of Christmas he boarded the night train to Paris, where he arrived at around eight the next morning. After a quick nap in his hotel, he attended the official session, complete with speeches and music, held at the Sorbonne. The French President, Alexandre Millerand, addressed the meeting, taking the opportunity to announce that Marie Curie had been awarded a lifelong pension of 40,000 francs annually. Lorentz also spoke briefly on behalf of

20  See Le livre du cinquantenaire de la Société française de Physique (Paris: Éditions de la Revue d’Optique Théorique et Instrumentale, 1925) for an account of the celebration. 21   “Toen Pa opstond ging er een groot applaus door de zaal. [. . .] Pa zegt nooit iets wat hij niet meent en er is altijd gevoel in terwijl sommige anderen zich laten gaan en verloopen in prachtige zinnen, zooals een Belgische socialist Destrée die haast tot tranen bewogen wordt door zijn eigen nederigheid.” Aletta to Marie van Vollenhoven, December 8, 1923 (LA 748). Jules Destrée was a member of the CICI. 22   Lorentz 1925h. 23   “Ik ben zoo door de wol geverfd dat die dingen mij niets kunnen schelen, ik vermaak mij toch wel met rondkijken.” Aletta to Berta and Wander de Haas, December 16, 1923 (FC).

The failure of 1925


the foreign visitors, and the event was closed in the evening with a formal dinner served in the sumptuous residence of the wealthy banker Edmond de Rothschild. The following day, Lorentz took the train back to Haarlem, where he arrived in the evening, “glad to be done with all that traveling and to once again calmly go to work.”24

The failure of 1925 For the 1925 Assembly of the Conseil the Academy prepared a proposal to remove the exclusion article from the statutes and to reverse the 1922 decision that membership of the Conseil was required to become a member of a Union. As early as November 4, the Academy’s board informed the CIR about this plan in a letter to the Comité Exécutif.25 The Danish and Swiss academies were asked to act as co-­sponsors of the proposal.26 The Danish agreed, but the Swiss refused. In December 1924, Lorentz was in Paris and spoke with Émile Picard, the dominant French member of the Comité Exécutif. In a letter to Went dated December 7, he summarized the conversation.27 Lorentz had advocated a more lenient attitude on the part of the Comité Exécutif, and he warned Picard of the risk that the Conseil could fall apart. He had also suggested that Picard might take an initiative on behalf of the Comité to allow Germany as a member and, by doing so, show the generosity of the French. Picard had not given an inch. He persisted in his opinion that admission of Germany was beyond discussion and said that Germany would do well to become a member of the League of Nations first. This happened soon afterwards, since Germany joined the League as early as the following year. Lorentz continued to hope that Picard, on second thoughts, would be more indulgent, he told Went. Still, he did not harbor resentment against Picard: “Let me add that Picard, even though he is a little stubborn, is still a noble and civilized man,” he wrote.28 To add some force to the Dutch-­Danish proposal, Lorentz drafted a letter to the Conseil and asked a number of well-­known Nobel laureates from neutral countries to sign it.29 It was his intention, in case the Dutch-­Danish proposal was

24   “blij al dat reizen achter de rug te hebben en eens weer wat rustig aan het werk te gaan.” Lorentz to Berta and Wander de Haas, December 30, 1923 (FC). The details about the trip are also from this letter. 25   LA 147. The letter is addressed to Émile Picard, President of the Comité, and signed by chairman Went and secretary Bolk. 26   See Went to Lorentz, December 11, 1923 (LA 147). 27   Lorentz to Went, December 7, 1924 (RB). 28  “Laat ik er bijvoegen dat Picard, al is hij wat hardnekkig, toch wel een nobel en beschaafd man is.” 29  See Fernandez Santarén et al. 2015 for more about the plan and the text of the letter. The letter is also reproduced in Report 1925, the Proceedings of the Third Assembly of the CIR. Curiously enough, Cajal’s name was missing there, so that only Dutch signatories remained and the letter obviously lost authority. An omission, or subtle sabotage?


International cooperation in crisis

accepted, to submit this letter to a number of German Nobel laureates as well, and to create publicity for the initiative in the international press. Zeeman, Kamerlingh Onnes, and Einthoven were, of course, willing to sign. Lorentz also hoped that the Spanish histologist Santiago Ramón y Cajal, to whom he had recently been introduced in Madrid, as well as the Danish physicist and 1922 Nobel laureate Niels Bohr, were prepared to sign. Cajal agreed, but Bohr refused to get involved. In a letter to Lorentz he explained that he did not want to get in the way of the Danish delegation by signing his name.30 The letter to the Conseil stated that now, after six years, the time was right for a change of course as the beginnings of cooperation were already visible in organizations outside the Conseil and in commissions of the League of Nations. The letter proclaimed “that the moment has now come to lend to scientific endeavors the air of universality that they should have, as much as possible, by virtue of the nature of science itself, and we think that one can do so without hesitation.”31 Somewhat speculatively, the signatories further professed: But we do not forget that, even in the war years, there have been German scholars who have shown good will towards their adversaries and we are convinced that, when Germany will have a place in the Conseil de Recherches, those scholars who will participate in the meetings and who will take part in the joint efforts, will show proof, by doing so, of this spirit of reconciliation that we would like to see reigning in the world and which is the only thing that can protect it against war and violence. They will feel, together with you, that the participation in the international efforts implies that one has the desire to achieve true peace, which is our ideal for all and which should, in fact, be the ideal of any man of science. The decision which, we dare hope, you are about to take, will show a confidence that will not fail to provoke a reciprocal feeling and we will thus, fortunately, see the shadows recede somewhat that have darkened the lives of the nations.32

The third CIR Assembly in Brussels took three days, from July 7 through 9. In the morning of the second day, the Dutch-­Danish proposal was tabled.33 As the leader

  Bohr to Lorentz, July 2, 1925 (LA 146).  “que le moment est venu de rendre aux efforts scientifiques le caractère d’universalité qu’ils doivent avoir, autant que possible, en vertu de la nature de la science même, et nous pensons qu’on peut faire ce pas sans hésiter.” 32   “Mais nous n’oublions pas que, même dans les années de guerre, il y a eu des savants allemands qui ont montré de la bonne volonté envers leurs adversaires et nous sommes convaincus que, lorsque l’Allemagne aura une place dans le Conseil de Recherches, ceux de ses savants qui assisteront aux réunions et qui prendront part au travail commun, feront preuve par cela même, de cet esprit de ­réco­n­cili­ation que nous aimerions voir régner dans le monde et qui seul peut le protéger contre la guerre et la violence. Ils sentiront, avec vous, que la participation aux travail international implique qu’on désire arriver à la véritable paix, qui est notre idéal à tous, qui doit, en effet, être celui de tout homme scientifique. La résolution que, nous osons l’espérer, vous allez prendre, montrera une confiance qui ne manquera pas d’éveiller un sentiment réciproque, et nous serons heureux de voir ainsi se dissiper un peu les ombres qui obscurcissent la vie des peoples.” 33   See also Report 1925 and Otterspeer and Schuller 1997 for accounts of the meeting. 30 31

The failure of 1925


of the nine-­man Dutch delegation, Lorentz read the letter to the assembled delegates of the Conseil, but in the debate that followed the Belgian representative Paul Pelseneer maintained that it was still too early to admit Germany. As the Netherlands and Denmark had not participated in the war, they were not able to choose the right moment to remove the exclusion article from the statutes, and for that reason he requested that the Dutch and Danish delegations withdrew their proposal. When they refused to go along with Pelseneer’s request, he put forward a Belgian counter-­proposal: Admission of Germany could only be taken into consideration once the country had become a member of the League of Nations. In further discussions it became clear that the British, US, and Italian delegations supported the Dutch-­Danish proposal, while the French joined Pelseneer. In a last-­ditch effort Lorentz made an urgent appeal to the Assembly. He feared for the continuity of the Conseil and declared that he did not think much of the Belgian proposal, as it would only raise new barriers for German membership. During the afternoon session, both proposals were put to a vote. The Belgian counter-­proposal was roundly rejected. Six countries voted in favor, casting nineteen votes, and ten voted against, with their twenty-­eight votes. Spain did not want to get involved in the fray and abstained (five votes). The Dutch-­Danish plan was accepted, unsurprisingly with the same proportion of votes, but now in reverse. All the countries that had first voted down Pelseneer’s proposal now voted in favor of the Dutch-­Danish plan, and the other way around. Again, Spain abstained. Then came the moment of truth. Chairman Picard announced that, naturally, a two-­thirds majority was sufficient for a statutory amendment. The hitch turned out to be that this two-thirds majority had to include all member states instead of only those countries that happened to be assembled at the meeting. Several participating countries were absent, and passage of the proposal required at least fifty-­three votes: one more vote than the total number cast in the assembly. This meant, according to Picard, that the plan had not passed, after all, because of a lack of valid votes.34 A further proposal, to uncouple the membership of the Unions from that of the Conseil, was also rejected. The vote caused quite a stir in the Assembly. Especially Picard’s attitude gave rise to great indignation. He had known well before the start of the meeting that  there would not be a sufficient number of votes for any statutory change whatsoever, yet he had kept quiet about it until after all the votes were cast. Did he want to humiliate the delegates who had put forward the proposal? Was it a ­power-­play to show that, in the end, the Belgians and the French would still be calling the shots? Be this as it may, besides idealism, the supporters of the proposal had shown a hefty dose of naivete. By not making sure ahead of time that they would be able to 34   Picard was actually mistaken: The total number of votes of all the members was seventy-­six, so that fifty-­one votes were required for a change of the statutes. Fifty-­three was the number mentioned in all accounts of the meeting (see, for example, Nature 116 [1925]: 138), but in the proceedings the correct numbers are given.


International cooperation in crisis

garner the required number of votes, they had more or less invited their own defeat. It was remarkable, to say the least, that none of them had taken the trouble to make a realistic estimate of the chance that their proposal would actually pass. Still, in the months to follow it would become clear that Picard had overplayed his hand this time. Lorentz was outraged, and his letter to Zeeman, a few days later, clearly betrayed his indignation. In contrast to his usual sedate style, the letter was peppered with exclamation marks: It is actually very strange that a proposal about which England, the United States, Italy, Japan, Switzerland, The Netherlands, and the Scandinavian countries agree, does not make it through because a number of countries that are of little significance stay out of it. Among the opponents were Poland, and other such countries that are closely tied to France. Morocco (!) as well and Egypt (!) voted in the same spirit.35

At the Assembly Lorentz still managed to remain his polite self. By the end of the meeting he had overcome his annoyance and had thanked Picard for his leadership and for his work on behalf of the CIR—“not in very warm words,” as he recounted,36 and also adding a word of caution. I add that we are going through a rather serious crisis, but I have faith in the Comité Exécutif, and I hope, given the spirit that has animated our debates, that it will succeed in finding approaches for détente in a way that will get us through this crisis while maintaining the Conseil International de Recherches safe and sound.37

As he later wrote to Went, he almost regretted these polite words after hearing Picard’s speech at the close of the meeting: It was tragi-­comical that finally, on closing the meeting, Picard read a speech that had been prepared beforehand. In it, as if nothing had happened, he reiterated once more what he had said, totally inappropriately, in his opening speech. It was strange, after all the trust I had expressed and it made me almost regret having done so.38

35   “Het is eigenlijk wel heel gek dat een voorstel, waarmee Engeland, de Vereenigde Staten, Italië, Japan, Zwitserland, Nederland en de Skandinavische landen het eens zijn, er niet door gaat omdat een aantal weinig beteekenende landen niets van zich laten hooren. Onder de tegenstanders waren Polen, en dergelijke nauw met Frankrijk verbonden landen. Ook Marokko (!) en Egypte (!) brachten in denzelfden geest hun stem uit.” Lorentz to Zeeman, July 10, 1925 (Kox 2018, 268). Of course, there was no question now of submitting the letter to German Nobel laureates. 36   “niet in erg warme woorden.” Lorentz to Went, July 20, 1925 (RB). 37   “J’ajoute que nous traversons une crise assez grave mais j’ai confiance dans le Comité exécutif, et j’espère, étant donné l’esprit qui a animé nos débats, qu’il réussira a trouver des formules d’entente de manière à nous traverser cette crise, en maintenant sain et sauf le Conseil international de recherches.” Report 1925, 18. 38  “Het was wel tragi-­comisch dat ten slotte bij het sluiten der vergadering Picard een vooraf opgestelde toespraak voorlas. Waarin hij, alsof er niets gebeurd was, nog eens herhaalde wat hij, geheel te onpas in zijne openingsspeech had gezegd. Het was zonderling na het vertrouwen dat ik had ­uitgesproken en deed mij daar bijna spijt van hebben.” Lorentz to Went, July 20, 1925 (RB).

The failure of 1925


Lorentz was referring here in particular to a section in Picard’s opening address in which he emphasized that mutual trust was a requirement for fruitful co­oper­ ation. In order not to lose mutual trust, he had stated, it might be better to wait and encourage rapprochement toward the excluded countries only after real feelings of goodwill toward one another would have taken hold in the various countries. In his closing address, Picard repeated this sentiment. He pointed out that the six years that had gone by since the establishment of the CIR might be sufficient for some to leave the past behind, but that this certainly did not apply to those who had suffered first-­hand from the war. In the scientific community, the reactions to the Assembly’s outcome were generally negative. An anonymous author said in an account of the meeting in Nature: “This result is in every way most unfortunate for international science.”39 In reports in the Dutch newspapers similar conclusions were drawn.40 On July 12 the daily Het Vaderland, in The Hague, even carried the headline: “The war continues in the world of science.”41 Meanwhile, Lorentz had informed Academy chairman Went in a letter that Great Britain and the United States were also very unhappy about the course of events during the Assembly and that these countries were considering steps to remedy the situation. As the organizer of the founding meeting of 1918 in London, the Royal Society now felt the responsibility to break the impasse that had arisen. Lorentz had received a letter from Arthur Schuster, telling him confidentially that the Royal Society would urge Picard to call a meeting of the Comité Exécutif as soon as possible.42 As Schuster wrote, “The Royal Society is deeply impressed by the danger which threatens the International Research Council unless some speedy solution is found.” Lorentz then suggested to Went that the Netherlands had better leave the initiative to Great Britain and the United States for now. Lorentz argued for patience, but if no perspective emerged for Germany after a year or two, the Netherlands would have to leave the CIR, and if the Conseil fell apart, he told Went, the differences would only become sharper. Caution was his motto, small steps, one at a time, and no great gestures or drastic decisions, and it typifies Lorentz that he ended his letter by citing three reasons why patience would be the best course of action. The Belgian amendment to tie Germany’s accession to the CIR to its membership of the League of Nations could already be considered quite a big step forward, he argued. Secondly, the Dutch might also be able to achieve cooperation by working from within the CIR.That would become impossible if the Netherlands

  Nature 116 (1925): 138.   To ensure objective reporting of the meeting, the Dutch dailies received an account by the Delft mathematician A. J. Kluyver, who had been a member of the Dutch delegation (see Went to Lorentz, July 1925, LA 147). The newspapers reproduced the account verbatim. 41   “De oorlog duurt voort in de wetenschappelijke wereld.” 42   Schuster to Lorentz, July 27, 1925 (LA 147). The letter to Picard, dated August 7, 1925 (copy in LA 147), urged the admission of the central powers. 39 40


International cooperation in crisis

left the Conseil. Finally, if Germany was admitted successfully before too long, “we do something (though it may perhaps not be all that meaningful) in favor of the rapprochement of the nations.”43 Went, by no means pleased with the result of the Brussels Assembly, wanted to call a meeting of the WIS-­commission in September to discuss the failure in Brussels and determine an Academy standpoint. He was expecting the ­WIS-­commission to recommend leaving the CIR. On September 26, the commission had its meeting. Leaving the CIR was indeed viewed as a viable option, but a decision was postponed, since the Comité Exécutif would be convening a special meeting on October 13. Especially the Royal Society had insisted on this special meeting, and the pressure on Picard and his Belgian colleague had become so great that they could not do anything but concede. Admitting Germany was no longer completely out of bounds, and one way or another the Comité was expected to reverse the outcome of the Brussels Assembly. The Comité decided to call a special General Assembly of the CIR, devoted exclusively to the German question. During the meeting, planned for June 29, 1926, the British would table a proposal for an amendment of the statutes that allowed membership for the central powers. The WIS-­commission supported the proposal wholeheartedly.

The “small conference” Meanwhile, in the run-­up to the General Assembly, Lorentz had come up with a new plan. In Amsterdam he wanted to bring together a small number of scientists from Germany, Great Britain, France, and Belgium to discuss in a friendly atmosphere how the goal of admitting Germany to the CIR could best be achieved. Lorentz suggested that he could visit the French mathematician and Minister of War Paul Painlevé during his upcoming trip to Paris to discuss the matter.44 This conversation with Painlevé took place in the presence of Went, who had rushed to Paris at very short notice after having received Lorentz’s invitation. The two scholars found Painlevé very willing to listen, and the conversation marked the start of a series of feverish consultations between the various parties. Besides, of course, Lorentz and Academy chairman Went, these consultations mainly involved the German chemist Fritz Haber and the Dutch chemists Ernst Cohen and Hugo Kruyt.45 Kruyt and Went followed up with a lightning visit to Berlin to consult with eminent scholars like Max Planck. Soon they returned with a document signed by Planck and his two fellow secretaries of the Prussian

43   “doen wij iets (het moge dan misschien niet veel beteekenen) ten gunste van de toenadering der volken.” 44   See Lorentz to Went, December 17, 1925. Lorentz left for Paris the next day to conduct business for the Institute for International Cooperation. 45  See Otterspeer and Schuller 1997, chap. 8, for more details.

The “small conference”


Academy, Heinrich Lüders and Max Rubner. The Germans laid out their wishes about accession to the CIR in stern language. The German document was to be discussed in what had meanwhile been dubbed the “small preparatory conference.”46 Because of the meeting’s con­cili­ atory nature and the participation of Germany, a comparison with the Locarno peace conference in October 1925 was obvious. Germany had been represented at this first peace conference and it had brought France and Germany decidedly closer together. No wonder that the “small conference” was viewed as the “Locarno of science,” and that the participants hoped it would have the same effect as its political predecessor.47 The document drawn up by the Germans was full of demands. “The other side” needed to be forthcoming in its desire to cooperate and had to extend a clear invitation for the small conference. During the consultations a show of goodwill toward the Germans was expected, as well as a sense that the other participants truly harbored the desire to depoliticize science and cooperate with the Germans. As for Germany’s CIR membership, their demands were equally forbidding. Not only did the CIR statutes need to be amended, Germany should also be invited in such a manner that it would be clear to anyone that the earlier grounds for exclusion were now considered null and void. Additionally, Germany and Austria claimed seats on the Comité Exécutif and demanded that in discussions and official documents the German language be given a position equivalent to that of English and French. The demand of voiding the exclusion grounds was especially important, since the central powers had felt deeply humiliated by the reasons why they had been excluded. The other demands were inspired by the strong feelings of resentment about how international cooperation had been organized during the past few years: The central powers had been completely ignored. At the end of March 1926 Lorentz went to Paris again, this time to chair a session of Commission Internationale de Coopération Intellectuelle (CICI). He took the opportunity to speak to Painlevé again about the progress of his conference plan. On Palm Sunday, March 28, Painlevé organized a luncheon at his home to discuss the German demands. Besides Lorentz, Einstein, Marie Curie (both CICI members), and the mathematicians Émile Borel and Jacques Hadamard were also invited. In a letter to Went, Lorentz gave an account of the meeting: When I arrived, I found Einstein already busy, toning down various sentences in the document, in which our German friends were making rather too many demands. I put a stop to that by pointing out that we should view this declaration of the three secretaries of the Berlin Academy as the fruit of thorough consideration, in which

46   A copy of the statement, dated March 19, 1926, is in LA 147. It includes the modifications made by Einstein that are mentioned here. 47  See Otterspeer en Schuller 1997, p. 153.


International cooperation in crisis

they had already come a long way in reversing their opinions and there could be no question of forcing them to make changes in their statement.48

After the luncheon, Lorentz thoroughly explained everything once more, in an attempt to “make clear that the Berlin gentlemen really meant no harm.”49 He even ventured a somewhat dubious linguistic analysis by declaring that the word Bedingungen (conditions), which Einstein had changed into Umständen (circumstances), could also be taken to mean circumstances in German, just like the word conditions in French. In this explanation, Lorentz completely ignored the context that made the meaning of Bedingungen crystal clear. It obviously denoted demands there. The translation of the passage in question actually read: “that the accession to the Conseil could be allowed to become possible under the following Bedingungen.”50 The content of the German document, for example the demand that the earlier grounds for exclusion should be nullified, also met with objections. Nonetheless, it was decided to continue planning the small conference. Two days after the luncheon, Lorentz and Painlevé also spoke to C. G. Simpson, board member of the Royal Society.51 When Lorentz showed him the German statement, Simpson was surprised: The British were under the impression that only the French would create problems. Now that the Germans, too, appeared to be creating barriers, it seemed sensible to Simpson that the British also participated in the small conference. The Germans would have to be made to understand that their document could not serve as the starting point, especially if they demanded to nullify the exclusion grounds. It was decided to invite four Frenchmen, four Germans, two Englishmen, two Italians, one Belgian, and a Dutchman and to choose Paris as the location for the small conference because of its symbolic value. Lorentz proceeded to ask Einstein to suggest names of possible German participants.52 Then things started to unravel. Instead of coming up with his own choice of names, Einstein had consulted Planck, to the great dismay of Lorentz.53 From Planck, Einstein had heard that within the Berlin Academy there was great resistance against accession to the CIR, much greater than Planck and his sympathizers had anticipated when they had drafted their document. In his letter to Went,

48  “Toen ik kwam vond ik Einstein al bezig, verschillende volzinnen in het stuk, waarin onze Duitsche vrienden wat veel eischen stelden met potlood te verzachten. Daar heb ik een stokje voor gestoken door erop te wijzen dat wij deze verklaring van de drie secretarissen der Berlijnse Akademie als de vrucht van grondige overweging moesten beschouwen, waarbij zij reeds een heel eind van hunne oorspronkelijken opvatting waren teruggekomen en dat er geen sprake van kon zijn, hen tot wijzigingen in hun verklaring te nopen.” Lorentz to Went, April 14, 1926 (RB). The copy of the document in LA 147 contains Einstein’s modifications. 49   “doen uitkomen dat de Berlijnsche heeren het zoo kwaad niet meenden.” 50   “[...] dass sich der Anschluss an der Conseil unter folgenden Bedingungen ermöglichen lassen dürfte.” 51   Of this conversation Lorentz also gave an account in the letter to Went quoted earlier. 52   See Lorentz to Einstein, April 6, 1926 (Kox 2008, 406). 53   See Einstein to Lorentz, April 12, 1926 (Kox 2008, 407).

The “small conference”


Lorentz adamantly expressed his indignation about the contradiction between the resistance in Berlin and the document by Planck and his friends. “And it was no less than [a document written by] the secretaries of the Academy.”54 He also saw with some concern how the plans for the small conference now appeared to be totally up in the air again; “You see that it does not look very promising yet and that all sorts of strange things can still happen.” Still, he remained optimistic: “My hope is only that, once some Germans are received cordially in Paris, many objections will disappear.”55 Given the attitude in Germany, this turned out to be idle hope and not very realistic to boot. The small conference came to nothing. As a result of its cancellation, the special General Assembly of the CIR, on June 29, 1926 in Brussels, had to be held without prior informal consultations with the Germans. The Netherlands were represented by Lorentz, Went, and Kruyt. The only item on the agenda was the British proposal to amend the statutes and make admission of the central powers pos­ sible. It was unanimously accepted and the following day Germany, Austria, Hungary, and Bulgaria were already invited for membership. The newly invited countries did not accept immediately, in part because the exclusion clause had not been discussed at all in Brussels. German acceptance would signify an implicit admission of prior offences and an equally implicit declaration that they had now changed their ways. Besides, the matter had become less urgent for German scientists, as they were increasingly being admitted to various international conferences. As a consequence, nothing happened beyond the CIR decision, and that was a very disappointing result. To get things moving again, Lorentz tried to convene another meeting, this time a more formal one, under the auspices of the CICI. A preliminary plan was drawn up after consulting CICI member Robert  A.  Millikan and the German Albert Dufour-­ Féronce, one of the permanent undersecretaries of the League of Nations.56 As Lorentz formulated it in his letter to Millikan, the meeting’s ob­ject­ ive was “To consider in a friendly spirit the way in which we may best reach the really universal collaboration which we all desire.” The meeting was to take place in  February 1928, either in Geneva—preferred by Lorentz—or in Haarlem. Representatives of Germany, the CICI, the CIR, the Royal Society, and the Dutch Academy would join, and the CICI would send out the formal invitations. Some issues still needed to be resolved, though. The Germans were not at all in favor of a formal role for the CICI, and they wanted Lorentz to make the invitation in a personal capacity. After his earlier experiences with the small conference,

  “En het waren nog wel de secretarissen der Akademie.”   “Gij ziet dat het er nog niet veelbelovend uitziet en dat wij nog rare dingen beleven kunnen.” “Mijn hoop is alleen nog dat, wanneer eenmaal eenige Duitsers te Parijs vriendelijk ontvangen worden, vele bezwaren zullen verdwijnen.” 56  More details about the choice of participants, among other things, in Lorentz to Frits Went, January 10, 1928 (RB) and Lorentz to Millikan, January 20, 1928, draft (LA 187, Lorentz’s last, unfinished letter). 54 55


International cooperation in crisis

Lorentz did not see any point in an informal meeting. In any event, it all came to nothing in the end. Because of Lorentz’s unexpected death, on February 4, all initiatives came to a grinding halt and nobody took up Lorentz’s role as inter­ nation­al mediator after he had passed away. The Germans joined the CIR eventually, but that did not come to pass until 1931.

Commission Internationale de Coopération Intellectuelle How did Lorentz become a member and later chairman of the CICI? His involvement started in 1923 with a letter from Eric Drummond, Secretary General of the League of Nations. It contained a very flattering request: Lorentz was invited to become a member of the CICI. In his reply, Lorentz declared that he was greatly honored by the invitation and would gladly accept it.57 Privately, he did have some misgivings, especially about the usefulness of the commission and the time the work for the commission would cost him. To his daughter Berta he wrote: “That I have great expectations of its work (in the interest of good relations between the “intellectuals”), I cannot say, and it will also take quite some doing, but I feel that I cannot refuse.”58 The League of Nations had been founded in 1920.59 In September 1921, during the second General Assembly, the CICI was formally instituted to study inter­ nation­al questions concerning intellectual cooperation and education and promote peace and good relations between the nations. After final approval in the sixteenth session of the League of Nations Council, the CICI was convened for the first time in August of 1922. It consisted of no fewer than twelve members with a broad range of backgrounds, from eleven different countries. Its chairman was the French philosopher Henri Bergson.60 The members were explicitly expected to join as individuals and not as representatives of their respective countries. In this 57   Drummond to Lorentz, April 26, 1923 and Lorentz to Drummond, undated draft (both in LA 176). The invitation was based on a decision by the League’s Council on April 23, 1923. The Council was the executive body of the League. 58   “Dat ik veel van het werk daarvan (in het belang van de goede verstandhouding der ‘intellectueelen’) verwacht, kan ik niet zeggen, en het zal nog al wat te doen geven, maar ik vind dat ik het niet kan laten.” Lorentz to Berta, May 1923 (FC). 59   For more about the history of the CICI, see Renoliet 1999 and, especially for the Dutch involvement, Riemens 2005. 60  The other members were: Devendra Nath Bannerjea, professor of political economics at the University of Calcutta, Kristine Bonnevie, professor of zoology at the University of Kristiania, Aloisio de Castro, professor of medicine at the University of Rio de Janeiro, Marie Curie, professor of physics at the University of Paris, Jules Destrée, Belgian author and politician, Albert Einstein, George Ellery Hale, director of the Mount Wilson Observatory in Pasadena, California, Gilbert A. Murray, Regius professor of Greek at the University of Oxford and vice-­chairman, Gonzague de Reynold, professor of French literature at the University of Bern, Francesco Ruffini, professor of canon law at the University of Turin and member of the Italian Senate, and Leonardo de Torres-­Quevedo, engineer and member of the Spanish Royal Academy of Sciences. Hale was replaced in 1922 by Robert Millikan.

Commission Internationale de Coopération Intellectuelle


way, it was possible to appoint people from countries that had not joined the League of Nations. This is how Albert Einstein could be a member of the commission from the very beginning—actually, Germany did not take part in the League of Nations until 1926. Einstein’s participation was made easier because he was a Swiss as well as a German citizen. Already in the first session, concrete steps were undertaken to further the goals of the commission. An international survey of the circumstances of post-­war intellectual life in the various member countries was carried out to make an inventory of problems the commission might deal with. The focus was on providing financial support to countries where intellectual life was considered to be in danger. Three sub-­commissions were instituted, whose participants did not necessarily need to be members of the CICI. One sub-­commission occupied itself with inter­ nation­al scientific documentation and cooperation in the field of bibliography, another one took care of inter-­university cooperation, and the third one dealt with the protection of intellectual property. All these three issues were considered of prime academic importance. Einstein had been quite hesitant to join the CICI.61 The silence of the League of Nations about the occupation of the Ruhr area by French and Belgian troops in January of 1923 was reason enough for him to leave the commission again, as little as eight months after the first session. He had actually not even managed to attend the session, because he was in the Far East at the time. Lorentz took his place, but a little more than a year later Einstein changed his mind and notified the commission that he wanted to rejoin, if the commission still wanted him. In June the commission did indeed reappoint him—this time to represent German ­science—and in July he attended the third session.62 During the second session of the CICI, from July 26 to August 2, 1923, Lorentz was officially installed as a member. In a letter to Zeeman he described the meeting and expressed his skepticism about the commission’s possibilities to achieve results in practice. We had 6 days of two long meetings each day, morning and afternoon, with a few evening meetings en petit comité and on the 7th day a public event. We discussed a great many topics: the assistance (books, journals, etc.) to needy countries, the exchange of professors and students, international information agency for uni­ver­ sities, ensuring the right to “intellectual property,” bibliography, esperanto, etc. Time will tell whether we can really achieve any practical results now. My general impression is very satisfactory; I notice that in L. of N. circles there are aspirations towards

61   Einstein’s hesitations were motivated at first by his skepticism about what the commission would be able to achieve and were subsequently exacerbated by the murder, in June of 1922, of German Chancellor Walther Rathenau, a good friend of his. He did not want to be a representative of a nation with a mentality that had given rise to such an atrocity. See CPAE-­13, Introduction, liv–lvi, for more details. 62  See CPAE-­14, Introduction, lxii–lxiii.


International cooperation in crisis

true universality, even though now this cannot be realized completely under the ­current political circumstances.63

A few years later, Lorentz was scarcely more optimistic about the possibilities to achieve practical results. In a speech in 1926 he reviewed the commission’s work in the past few years.64 He expressed his regret that he could not give this speech fifteen years later, “because then, one may hope, much that has been started now and is in the process of growing, will, to a certain extent, have come to fruition, because then, we hope, there is less need to be speaking of plans and more possibility of demonstrating results. If no new disasters befall us.”65 Historical judgment supports Lorentz’s modest expectations of the CICI.66 In spite of all its goodwill and sincere efforts, the CICI did not manage to achieve much in the way of concrete results. In spite of the hard work of intellectuals from many countries, precious little became of the ambitions for true reconciliation and understanding between the nations—less and less so as the years progressed. The deteriorating political situation in Europe was to blame for this. An additional factor that made the commission less effective than it should have been was that national governments were leery of turning over even a small part of their independent decision-­making power to a supra-­national organization. As a result, the CICI was chronically underfinanced, which seriously diminished its effectiveness.

Lorentz as chairman of the CICI Even at the first meeting of the CICI he attended, Lorentz immediately stood out because of his linguistic proficiency and his diplomacy. In a note to the Leiden Professor of International Law, Willem van Eysinga, undersecretary general of the League of Nations, Inazō Nitobe, described his excellent impression of Lorentz’s first appearance in the CICI:

63   “Wij hebben 6 dagen telkens twee lange vergaderingen, ’s morgens en ’s middags gehad, met nog enkele avondvergaderingen en petit comité, en den 7den dag een openbare bijeenkomst. Heel wat onderwerpen hebben wij behandeld: de hulp (boeken, tijdschriften, enz) aan de noodlijdende landen, de uitwisseling van professoren en studenten, internationaal informatiebureau voor universiteiten, verzekering van het recht op den ‘intellectueelen eigendom,’ bibliographie, esperanto, enz. De tijd zal leeren of wij nu werkelijk eenige practische resultaten kunnen bereiken. Mijn algemeene indruk is zeer bevredigend; ik merk op dat in de kringen van den V.B. inderdaad het streven naar werkelijke universaliteit bestaat, al kan dat nu onder de tegenwoordige politieke omstandigheden niet ten volle tot zijn recht komen.” Lorentz to Zeeman, September 2, 1923 (Kox 2018, 259). 64   The speech was given in Haarlem on September 4, 1926 at the meeting of the Association of teachers in Dutch Gymnasia (secondary schools). See LA 189 for a draft and Algemeen Handelsblad, September 5, 1926, for a newspaper account. 65   “omdat men mag hopen dat dan veel dat nu begonnen is en bezig is te groeien tot zekeren wasdom zal zijn gekomen omdat dan, naar wij hopen minder van plannen behoefde gesproken te worden en meer op uitkomsten zal kunnen worden gewezen. Als er geen nieuwe rampen komen.” 66  See Renoliet 1999 and Riemens 2005, chap. 5.

Lorentz as chairman of the CICI


Dr. Lorentz rendered no small service in expressing his opinions in the Committee— opinions founded on large and varied experience. His spirit is such that he always played a conciliatory part whenever there arose a divergence of opinion. This he could do with wonderful tact, but above all by the atmosphere he created for goodwill and cooperation.67

Because of his impressive performance, Lorentz was—not surprisingly—the ­obvious successor when chairman Bergson was forced to leave the commission because of illness, in September 1925. On January 14, 1926, the opening day of the seventh session, he was elected chairman by acclamation. In all, Lorentz attended eight CICI sessions, as well as countless numbers of sub-­commission meetings. Two days after his election, the Institut International de Coopération Intellectuelle was opened officially. This institute, which was to serve as the executive body of the CICI, was established because of the commission’s dissatisfaction with the scant means at its disposal. A continuous scarcity of funding severely hampered its possibilities. The Institut was a gift from the French government, which footed the bill for practical needs like office space and logistical support. The French government’s offer already dated from two years earlier, in July 1924,68 but apparently the establishment of the Institut proved more difficult than an­tici­pated. Because of France’s financial stake in the Institut, it pretty much became a French affair. The director was French, as well as the chairman of the Comité de Direction, the management board of the Institut.69 It is true that Lorentz was also a member of this management board, but he gave up his membership after a year. As chairman of the CICI, Lorentz was one of the speakers at the formal opening ceremony. In his speech—according to newspaper reports “a fine and witty and enthusiastically acclaimed address”70—Lorentz compared the Institut with a living being. It is a truly harmonious organism. For an organ is an eye or a hand, but the Institut is the combination of those two and of quite some more. It sees and understands for us; nothing of what happens in the world, in its domain, will escape its attention. It

67   Nitobe to Van Eysinga, August 4, 1923 (copy in LA 178). It can also be concluded from this letter that Van Eysinga had played a role in Lorentz’s acceptance of the commission’s membership. Nitobe thanked Van Eysinga for “his good offices regarding the appointment of Dr. Lorentz.” 68  See Riemens 2005, chap. 5.B for more details. 69   According to Riemens 2005, 242, the substantial French role in the Institut had also been a factor in the choice for Lorentz as CICI chairman. Appointment as chairman of Paul Painlevé, successor of Bergson as member of the commission, would have been less desirable, especially since Painlevé also chaired the Comité de Direction. 70   “een fijn geestige en geestdriftig toegejuichte rede.”


International cooperation in crisis

thinks for us, while proposing problems to us and questions for us to study. It speaks for us in all languages.71

Lorentz especially caused some hilarity when he remarked that inhabitants of other planets—according to the seventeenth-­century scholar Christiaan Huygens, not all that different from earthlings—would have to contact the Institut, in case they would ever want to find out about life on earth.72

The CICI, a time-­consuming task Based on the amount of available archival material regarding the CICI, especially correspondence and the published minutes of the sessions, Lorentz must have spent a considerable portion of his time on the commission, certainly while he was chairman. In his speech of September 1926, he said: “We meet regularly in Geneva for five days at a time in July and I can assure you that those are busy days because of the variety of issues.”73 The League of Nations was a bureaucratic institution that produced mountains of paper, so even on his trips abroad the correspondence followed Lorentz around, especially during his stay in the United States in the fall of 1926 and the first months of 1927. Lorentz was also confronted by the sometimes mysterious ways of international and national politics. It is not surprising that fellow commission member Gilbert Murray remarked in a letter to Lorentz, after the ninth session in July 1927: “You seemed very tired when you went away [...] I think the meetings were too long & the work too heavy.”74 The Dutch Ministry of Foreign Affairs followed the CICI with suspicion, since there was a great deal of distrust at the ministry against the kind of internationalism propagated by the League of Nations. As a result, Lorentz became involved in a “cause célèbre” that was almost grotesque. The short version of the story is that, as part of the League’s survey of the academic circumstances in a great number of countries, CICI member De Reynold had produced—and published—a report about the Dutch situation. It was full of errors and sloppy mistakes. De Reynold had given the report short shrift, in part due to a serious lack of cooperation on

71   “Il est un vrai organisme harmonieux. Car un organe, c’est un œil ou une main, mais l’Institut est la combinaison de ces deux et de bien d’autres encore. Il voit et il entend pour nous; rien de ce qui se passe au monde, dans son domaine, ne lui échappera. Il pense pour nous, nous suggérant des pro­ blèmes et des questions à étudier. Il parle pour nous dans toutes les langues.” Quoted in Riemens 2005, 242. Riemens also remarks that the speech greatly impressed the Dutch Ambassador because Lorentz gave it by heart. The ambassador did not know that Lorentz learned all his speeches by heart ahead of time. 72  See Nieuwe Rotterdamsche Courant, January 17, 1926. 73   “Wij vergaderen geregeld te Genève gedurende een dag of vijf in de maand juli en ik kan u verzekeren dat dit wegens de veelheid der onderwerpen drukke dagen zijn.” 74   Murray to Lorentz, July 30, 1927 (LA 177).

The CICI, a time-­consuming task


the part of the Ministry of Education and a number of Dutch universities.75 The Ministry could obviously have limited the affair to a factual correction of the report, but instead it chose to blow the question totally out of proportion and turned it into a major incident, including letters to the editors in all the major national newspapers. In the end, Lorentz had to spend precious time to calm the waters. In an extensive correspondence, and later also in a personal discussion in Brussels, he managed to persuade De Reynold—who at first objected fiercely—to write an official apology and promise a new, corrected version of the report— which never arrived, of course.

75  See Riemens  2005, 247–259, for a detailed account of this “high-­level affair.” It is perhaps no coincidence that Albert Einstein called De Reynold an “ass” (Esel) (see Einstein to Tatiana Ehrenfest-­ Afanassjewa, November 10, 1924 [CPAE-­14, 368]).

Chapter 12 The final years: Travel and tributes

In the first years after World War One, Lorentz was busier than ever, mostly with business that had little direct bearing on his role as a professor of physics. He spent an inordinate amount of time on his work for the Scientific Commission of the Academy. His activities to promote international scientific cooperation, especially as member, and later chairman, of the CICI, also took up a great deal of his time. Lorentz also spent much time and energy on his chairmanship of the Department of Higher Education of the Education Council. This independent advisory body, instituted by law in 1919, delivered solicited and unsolicited legislative and policy advice for all levels of education in the Netherlands—and still does so today. Its recommendations support policy decisions of the government, the lower and upper houses of parliament, and local municipalities. Lorentz played a key role in drafting a new Academic Statute to regulate, among other things, the form and content of all academic examinations in Dutch universities. He did not resign from the Council until just before his second journey to the United States, in the fall of 1926.1 Even though it had little to do with his work at the university, the extensive work on the Zuiderzee Commission, which took up a major part of Lorentz’s time, was more scientifically oriented. Just after the war, he also made a start on the ­time-­consuming task of publishing a number of his Leiden lectures, assisted to some extent by Berta and a few former students, who took care of the editing work.2 Besides, he continued his Monday morning lectures and produced a steady stream of scientific articles. In view of all this activity, it is astounding that Lorentz had any free time left at all during those busy years. Still, he managed to

1   Earlier, in 1908, because of his interest in education reform, Lorentz had become involved in the creation of the Lyceum, a new type of secondary school offering both HBS and Gymnasium diplomas. A general preparatory period of one or two years after finishing primary school enabled students to postpone the selection of a specific school type. 2  In the period 1919–1925, eight volumes of Lessen over theoretische natuurkunde aan de Rijks-­ Universiteit Leiden gegeven (Lessons about Theoretical Physics Given at the State University Leiden) were published. They were also translated into English, German, and Russian.

“A Living Work of Art”: The Life and Science of Hendrik Antoon Lorentz. A. J. Kox and H. F. Schatz, Oxford University Press (2021). © A. J. Kox and H. F. Schatz. DOI: 10.1093/oso/9780198870500.003.0013

Second visit to the United States


travel to familiar and unfamiliar places to receive tributes from various sources, give ­lectures, make speeches, and visit conferences.3

Second visit to the United States After his unforgettable stay in New York in 1906, Lorentz had always wanted to return to the United States, but this time in the company of Aletta. He had already received two invitations, one from Columbia University—a second one—and one from the University of Chicago. In both cases he had been invited to lecture during the summer semester, but he did not feel much like accepting these invitations, as he dreaded the summer heat in these two cities.4 Yet, he could not let go of the prospect of another stay in the United States, and at the 1921 Solvay Conference he spoke about his wish with Robert A. Millikan. At the time, Millikan was still a professor at the University of Chicago, but he would be leaving for the California Institute of Technology (Caltech) in Pasadena later that year. He had been hired to become director of the Norman Bridge Laboratory and, at the same time, chairman of the Executive Committee and de facto president of the university. Apart from being an outstanding physicist—he received the Nobel Prize in 1923—he was also highly ambitious and an extremely competent organizer and manager. It is in large part due to Millikan’s talents and drive that Caltech developed into one of the most prominent universities in the United States during the years of his leadership. Millikan’s transfer from the University of Chicago, at the time a rather insignificant institution, was engineered by George Ellery Hale, director of the Mount Wilson observatory in the mountains above Pasadena. Hale was not only a brilliant astronomer, but also a gifted organizer who had great ambitions for Caltech and had succeeded in raising large sums of research money for this institute.5 Hale had invited Lorentz to Mount Wilson several times. In 1910 he had offered him a visiting position as a Research Associate of the Carnegie Institution, the institution that administered the observatory. Lorentz declined.6 In 1920, at the recommendation of Hale, the Carnegie Institution offered him the position of Research Associate, with a remuneration of 1,000 dollars. Again,

3   Unless stated otherwise, the information about the journeys described in this chapter is taken from travel journals by Aletta (LA 733: USA 1922–1923; LA 734: England and Scotland 1923; and LA 737: USA 1926–1927) and letters by Lorentz and Aletta to the children (LA 742, LA 743, and FC). 4  The invitations were for the summer semesters of 1920 and 1921 respectively. See the cor­re­ spond­ence with George B. Pegram in LA 59; Albert A. Michelson to Lorentz, December 11, 1920, telegram (LA 54); and Lorentz to Millikan, February 10, 1921 (Caltech Archives). 5  See Goodstein 1991 for a history of Caltech. Millikan’s propensity for self-­promotion was ridiculed at Caltech. The general measurement unit for publicity was designated the “kan,” so the story went, but because it was so unwieldy, the “milli-­kan” was a more practical unit for everyday use. 6   See Hale to Lorentz, January 12 and February 5, 1910 (LA 30).


The final years: Travel and tributes

Lorentz declined.7 As he wrote to his son-­in-­law, he hoped that he would be able to accept the offer and use the money during a visit to the USA sometime in the future.8 Shortly after the 1921 Brussels conference, Millikan announced in a letter to Lorentz that he would hold Lorentz to his promise to visit America once again and that he would soon come up with some suggestions.9 In June his invitation arrived: lectures in Pasadena from January to mid-­March 1922, for a fee of 3,000 dollars. Lorentz would be able to give the same lectures in Chicago first, in the months of October and December 1921, in which case the fee would be raised to 5,000 dollars.10 This was certainly no mean sum, as one dollar was worth about three guilders in those days, and Lorentz’s yearly salary as a professor was 7,000 guilders. The fee included 1,000 dollars from the Carnegie Institution, which had renewed his appointment as a Research Associate for the year 1922.11 Lorentz responded to the invitation about two weeks later and was enthusiastic about the plans.12 He liked the idea of visiting both Chicago and Pasadena very much, but suggested an alternative plan, because he could not stay away from Leiden for more than four months at a time. From January 1 to March 1 he would lecture in Pasadena and then, after a week’s travel, he would go on to lecture in Chicago. Then he would travel to the East Coast for a short stay, to be able to be back in Leiden around May 1. In spite of the shorter lecturing period, he would still like to receive the full remuneration of 5,000 dollars. Millikan agreed.13 Lorentz had to advance a sizeable sum to pay for his travel expenses. His own funds were apparently insufficient, so he borrowed 3,000 guilders from a fund that Ernest Solvay had made available to him for scientific purposes—of course after having first requested Solvay’s permission to do so.14 On December 16, 1921, Lorentz and Aletta set sail on the Nieuw Amsterdam, the Dutch steamer that would be carrying them from Rotterdam to New York. From there, their journey by train would take them clear across the United States to Pasadena. They would be away for a total of around five months and it would be the trip of a lifetime. Many details about the journey are known, because Aletta kept a detailed travel journal and both she and Lorentz corresponded extensively with their children. Lorentz and Aletta were treated like honored guests. For the crossing they were offered a luxury cabin by the Holland America Line, and their special treatment

  See Robert S. Woodward (Carnegie Institution) to Lorentz, May 11, 1920 (LA 89).   See Lorentz to Robert  S.  Woodward (Carnegie Institution) October 3, 1920 (draft, misdated 1919; LA 89) and Lorentz to Wander de Haas, December 14, 1920 (FC). 9   Millikan to Lorentz, April 15, 1921 (LA 54). 10   Millikan to Lorentz, June 14, 1921 (LA 54). 11   See John C. Merriam (Carnegie Institution) to Lorentz, December 21, 1921 (LA 13). 12   Millikan to Lorentz, August 4, 1921 (LA 54). 13   Lorentz to Millikan, July 3, 1921 (Caltech Archives). 14   See Lorentz to Solvay, draft, November 8, 1921 and Solvay to Lorentz, November 10, 1910 (LA 73). 7 8

Second visit to the United States


included a tour of the entire ship, including the engine room, as well as a place at the Captain’s table. Their table companions were obviously a diverse group. One of them was, in Aletta’s words, “a communist who thoroughly enjoyed the benefits of our capitalist society.”15 On board they had a pretty regular existence. They got up at eight every morning and were among the first passengers to have a simple breakfast consisting of tea and bread and cheese, even though all kinds of luxury foods were on offer. Breakfast was followed by a brisk walk, eight times around the ship’s deck, followed by work and reading in the tea lounge, another stroll on deck before getting dressed for dinner with the ship’s Captain, and then back to the cabin at seven to retire for the night. The passage was slower than usual because of the rough seas, and many passengers were seasick, but Lorentz and Aletta were just fine: they did not suffer seasickness at all. Christmas on board was celebrated with a special eight-­course “private dinner” for seven guests offered by the captain16 and a “religious exercise during which the violin and the organ were played, the Nativity story was read, and hymns were sung,” under the direction of a Romanian opera singer. “It was funny,” wrote Aletta, “that all the Jews were happily singing Hallelujah Christ is born.”17 On December 28, the Nieuw Amsterdam docked in Hoboken, New Jersey, on the Hudson River. After time-­consuming formalities at customs and passport control, the two travelers took the ferry to Manhattan, across the river, where they spent the night in a hotel. The following morning, they boarded the train to Chicago. They had their own compartment, a “drawing room” with benches that were turned into beds at night, as well as a chair and a table, and even their own little “toilet room.” The landscape near Chicago was somewhat of a deception. “Everything looked run-­down, fields with fallen trees, full of pits and holes, in between much water, large rivers and lakes, but also muddy pools.”18 After the long journey they arrived in Chicago, of course somewhat disheveled. They had booked a hotel that was so grand that they did not feel they could go and dine there in their travel clothes. Instead, they went out to eat in a different, simpler, hotel. The following morning in their hotel, they met Millikan and Max Mason, professor of physics at the University of Wisconsin at Madison. Both had arrived by train from Toronto. Mason came to arrange the lectures Lorentz would be giving in Madison, while Millikan had planned to travel with them to Pasadena. The arrival of the two gentlemen had apparently been announced by telephone, and  using the phone was clearly difficult for Aletta. In one of her letters she

  “communist die volop genoot van de voordelen van onze kapitalistische maatschappij.”   The menu, signed by the participants, is in LA 704. 17   “godsdienstoefening waar viool en orgel gespeeld werd, het kerstverhaal voorgelezen en hymnes gezongen.” “Aardig was dat al de joden vroolijk zongen Halleluja Christus is geboren.” 18   “Alles zag er even verwaarloosd uit, velden met omgevallen boomen, vol kuilen en gaten, er tussen veel water groote rivieren meeren maar ook modderpoelen.” 15 16


The final years: Travel and tributes

c­ omplained: “If I do not understand immediately, they yell so loudly that nothing is to be understood at all.” For Lorentz using the telephone was a little easier, but “for a long time afterwards we shuddered if a telephone was calling out to us.”19 Chicago was the starting point of a long train journey to Pasadena, through endless plains, but also through the Rocky Mountains and along the Grand Canyon, where they thoroughly enjoyed the impressive views. “Everything was color, red and blue and yellow and glowing hues.”20 They spent New Year’s Eve on the train and were served a special New Year’s Day dinner the next day. Aletta was surprised—and also quite critical—about the looks of the ladies she met on the train. Thanks to the hairdressing salon on the train, they were all “beautifully coiffed, with permanent waves” and “so heavily powdered, besides, that everything in the toilet room is covered with a dusting of white.”21 At dinner, she remarked, they all appeared with their necks and arms powdered white. On the final leg of their journey they delighted in views of a beautifully colored sunrise over the California desert, grassy fields, great mansions, and endless orange groves. On January 4, the couple arrived in Pasadena. They spent the first  night in a hotel and a few nights with the Millikan family, but soon they moved into a guest house. Lorentz did not dawdle: The day after his arrival, he immediately started his lectures and right away he was also given an office in the laboratory.

Life in Pasadena In the 1920s, Pasadena was still rural, but it was changing quickly into a ­fast-­growing city: In the period between 1920 and 1930 the number of inhabitants grew from 45,000 to 76,000. The mild climate made Pasadena a popular place for well-­to-­do East Coast residents to spend the winter months. Lorentz and Aletta’s first impressions were not undividedly positive: “Pasadena, that city praised so highly by all Americans, did not do us the honor of exceeding our expectations, even though the roads are nicely planted, the houses are surrounded by gardens with lawns in front, but the streets are so long.” At first, they found the city just too large, especially for walking. “We did walk around for a few blocks, but all the streets are alike, so it is very monotonous.”22

19   “Als ik niet dadelijk goed versta schreeuwen zij zoo hard dat er helemaal niets te begrijpen is.” “Nog langen tijd daarna beefden wij als een telefoon ons riep.” 20   “Alles was kleur, rood, blauw en geel en gloeiende tinten.” 21  “prachtig gekapt met ingebrande golven.” “zoo geblanket dat dat alles in de toiletkamer wit ­bestoven is.” 22   “Pasadena, de door alle Amerikanen hoog geprezen stad had niet de eer ons mee te vallen, wel zijn de wegen mooi begroeid, liggen de huizen midden in tuinen met grasvelden er voor, maar de straten zijn zóó lang.” “Wij wandelden wel een paar blokjes om maar alle straten zijn aan elkaar gelijk zoodat het erg eentoonig is.”

Life in Pasadena


Later, their appreciation grew when they saw the lush vegetation along the streets: palms, pepper trees, eucalyptus, and green oaks (“live oaks”). They also admired the colors of the mountains in the North, among them Mount Wilson, and the beautifully clear and starry night sky. They loved the weather, too. Almost continuously they had “clear sunshine and sometimes almost summer weather,” but when it did rain it was “abundant and relentless.”23 Aletta marveled at how different daily life was in Pasadena. “All and sundry drive cars here, people are surprised that we are not buying a car here, and when they hear that we cannot “drive,” they look at us as if we are prehistoric beings.”24 Even though they did not drive themselves, that did not stop them from seeing much of the surrounding area, as they were taken on many beautiful driving tours during the months of their stay. An example was their two-­day, 250-­kilometer (155-­mile) trip—with an average speed of fifty-­six kilometers (thirty-­five miles) per hour!—to Santa Barbara, a coastal town North of Pasadena. They were less enamored with the mostly open cars. Lorentz was wearing his scarf all the time and Aletta wore her “fur.” The modern comforts in the homes were a constant source of wonder: hot water, gas heating, “one opens a closet door and a large ironing board pops out, firm and solid right away, complete with an electrical outlet.”25 The visit by Lorentz was, of course, reported in the local news outlets. Going by the news coverage, the Pasadena population must have had the impression that Lorentz was some kind of superhuman being. In the press release by Caltech, Lorentz was announced not only as the author of many physics publications, but also as the leader of two expeditions to Dutch New Guinea. Not for the first time, his namesake, the well-­known explorer Hendrikus Albertus Lorentz, had created confusion. Lorentz was also confused regularly with another near-­namesake, the Austrian orthopedic surgeon Adolf Lorenz. He had become quite a celebrity in the United States and was the father of the—later well-­known—biologist Konrad Lorenz. This confusion apparently bothered Lorentz a bit, as is clear from a remark he made during his after-­dinner speech in Washington on the occasion of the yearly meeting of the National Academy of Sciences on April 25, 1922. Reportedly, he expressed his appreciation for the friendliness he had encountered in various parts of the United States, calling it greater than he deserved and, so he had noticed, at times due to a case of mistaken identity when he had been confused with Adolf Lorenz.26

23   “bijna altijd helderen zonneschijn en soms bijna zomerweer.” “overvloedig en onverbiddelijk.” Lorentz to Frits Went, February 9, 1922 (RB). 24   “Alles rijdt hier auto, men is verbaasd dat wij hier geen auto koopen en als men hoort dat wij niet kunnen ‘drive’ zien zij ons aan voor voorwereldlijke wezens.” 25   “een kastdeur doet men open en er springt een groote strijkplank uit die meteen stevig staat, een stopcontact er bij.” 26  See Science 55 (1922): 467.


The final years: Travel and tributes

Very soon the Lorentz couple showed their usual enterprising spirit. They took a train to Mount Lowe, an hour-­and-­a-­half journey with “scary trams and a funicular railway.”27 Once they were up on the mountain, they took a delightful but strenuous hike, sometimes along narrow paths that made Aletta a little anxious. Other “wonderful hikes” took them to Big Dalton Canyon in Glendora and to Mount Rubidoux in Riverside County, South of Pasadena. Meanwhile, Aletta was amazed at the attire of other hikers. “Ladies in their hiking costume, yellow trousers, colored sweater, and beautifully styled hair over a painted face, the ensemble was far from beautiful, especially as most ladies were fat and bulging.”28 A memorable climb was their hike to the top of “Old Baldy,” a snow-­covered peak towering high over the countryside. It was not an easy hike. First “we bought 6 raisin ‘rolls’ (sandwiches) for a ‘dime’ [twenty-­five Dutch cents] and when we hiked through an orange grove, we picked up two oranges off the ground, and that is how the uphill hike began, first a wide roadway, but after some waterworks a small footpath remained, sometimes full of loose rocks, I was sometimes very childish and crawled along the rock face, leaning against it, but Pa walks along sheer drops as if he is a practiced mountaineer, he turns around and comes back as if it is nothing.”29 After three hours of climbing they reached the mountaintop from where they could see almost all the way down to the foothills of “Old Baldy.” No wonder that Lorentz began to look very healthy and tanned as a result of all this outdoor activity: “Pa is chestnut-­colored up to his crown,” Aletta wrote to the children.30

Everyday life in Pasadena Aletta observed sharply what she found striking during their stay. Not only did she make notes on what she admired or what surprised her, she also noticed the lesser sides of daily life in Pasadena. After a dinner party at the house of one of their new acquaintances in Pasadena, for example, Aletta was quite critical about American table manners: “The food was good, but was served in an awful way, an American is not happy if he does not have five little plates surrounding his dinner plate and then he takes turns taking a

  “griezelige trams en kabelspoor.”   “Dames in hun hiking costume gele broek, gekleurde trui en prachtig gekapte haren boven een geverfd gezicht, het geheel was ver van mooi vooral daar de meeste dames dik en slobberig waren.” 29   “kochten wij voor een dime (een kwartje) 6 rolls (broodjes) met rozijnen en toen wij door een orange grove liepen pikten wij twee sinaasappels van den grond en zoo begon de klim, eerst een breede rijweg maar na eenige waterwerken bleef er een smal voetpad soms vol losse stenen, ik was soms erg kinderachtig en kroop langs de rotswand, daartegen leunende maar Pa wandelt langs afgronden alsof hij een geoefend bergbeklimmer is, hij keert zich om en komt terug alsof het niets is.” 30   “Pa is roodbruin tot in zijn kruintjes.” 27 28

The lectures at Caltech


small bite from each, on a small dish there are fresh olives, a long celery stalk, a sour date, and sometimes some other things.”31 Aletta was also struck by other details and differences from life in the Netherlands. When she wrote that she needed to buy a dress for “everyday dinner” she remarked: “the eternal dressing-­up here is very oppressive, I do not need to keep up with it, but I cannot stand out too much.”32 Lorentz, in turn, writes with self-­deprecating humor about an encounter in the guest house that he found a little curious: “After dinner, we went into the parlor to do a bit of socializing, but I ended up finding myself sitting in a circle of seven old ladies (Ma had ended up in another corner) and had to tell them about radium, Mrs. Curie etc. In the end, when the scene struck me as somewhat comical, I made my way upstairs.”33 Another relaxing pastime was a visit to the cinema, something that must not have been all that common in Haarlem in those days. “Pa likes that when he is tired and it only lasts till nine,” according to Aletta.34

The lectures at Caltech In the meantime, Lorentz was busy lecturing. Four times a week, for a period of eight weeks, he lectured to some forty listeners. For the audience the lectures were a resounding success: “One student said it is just like you yourself find everything so beautiful as well and Pa said, yes, it is so beautiful.”35 Lorentz’s lectures were organized so that he was “singing in refrain,”36 as he called it, with Paul Epstein. Lorentz taught at five in the afternoon, and Epstein an hour earlier. Epstein, a physicist originally from Russia, had been working at Caltech since 1921, but Lorentz already knew him personally because he had spent some time in Leiden as an assistant. Although both lecture series touched on one another in terms of subject matter, they did not interfere. Epstein discussed quantum theory, while Lorentz’s classes were announced under the title “Light and matter,” but he ended up discussing all the major topics of physics. In Lorentz’s own words: “various optical questions, special theory of relativity, structure of atoms, and now some things from the 31   “Het eten was lekker maar weer akelig voorgediend, een Amerikaan is niet gelukkig als hij niet rondom zijn bord vijf kleine bordjes heeft en dan neemt hij van alles beurt om beurt een hapje, op een schaaltje liggen versche olijven een lang stuk selderij, een zure dadel en soms nog wat anders.” 32   “Dat eeuwige gekleed is hier erg benauwend, ik behoef wel niet mee te doen, maar kan toch ook niet al te zeer afsteken.” 33   “Na het eten gingen we nog een poosje in de hall om wat aan gezelligheid te doen, maar het liep zoo dat ik in een kring van zeven oude dames zat (Moe was in een anderen hoek terecht gekomen) en hun van radium, Mevr. Curie enz moest vertellen. Toen het ietwat komische daarvan mij trof ben ik maar naar boven gestapt.” 34   “Dat vindt Pa wel prettig als hij moe is en het duurt maar tot negen uur.” 35   “Een student zeide het is net alsof U zelf alles ook zoo mooi vindt en Pa zei, ja, het is zoo mooi.” 36   See Lorentz to Paul Ehrenfest, March 6, 1921 (Kox 2018, 251).


The final years: Travel and tributes

quanta theory; the gravitational theory will be saved for last.”37 Lorentz’s lectures were published—with considerable delay—in 1927.38 The astronomers of the great Mount Wilson observatory, situated high on the mountain that was easy to spot from all over the city of Pasadena, showed their interest and attended the lectures as well. Outside the lecture hall, Lorentz was also in regular contact with the astronomers, as they had their headquarters down in Pasadena.39 In particular, Lorentz had regular discussions with George Ellery Hale, director and driving force behind the observatory. A tour of the observatory was something Lorentz must really have looked forward to. There he would be able to see the latest equipment for observation of the starry skies in full action. Unfortunately, a planned visit had to be postponed several times because of rain—and in January even snow—or because of the bad state of the mountain road. On February 28, a few days before their departure, the trip finally happened. They went up the mountain in a group led by Hale. With the 100-­inch telescope, then the largest in the world, they viewed the Orion Nebula and other constellations. In a “stage,” a kind of open tour bus, they negotiated the difficult track up the mountain. In this day and age the Mount Wilson observatory is an easy car ride away from Caltech, along a newly built asphalt road, but in those days getting up there was a very adventurous trip along a rutted, unpaved road. Aletta made the trip with apprehension: “Often I held on for dear life, out of fear when the car went on the side of the deep tracks and listed to one side where a deep drop was laughing at us.”40 In the evening the group went down again, shrouded in deep darkness. Without a doubt, it must have been a frightening experience to descend along the twisting and turning, pitch-­dark mountain track. An important official event, in which Lorentz also had a part, was the opening ceremony of the new Norman Bridge Laboratory: Millikan’s domain. As the most important guest of Caltech, Lorentz was, of course, expected to speak a few words. Apart from the obligatory congratulations, he took the opportunity to emphasize the need to intensify international scientific contacts, especially in a time when “they have to a certain extent been severed in the disastrous period through which the world has passed.”41

37   “Verschillende optische quaesties, specieele relativiteitstheorie, structuur van atomen en nu een en ander uit de quanta theorie; de gravitatietheorie wordt voor het laatst bewaard.” Lorentz to Zeeman, February 9, 1922 (Kox 2018, 249). 38   Lorentz 1927f. 39   Although the observatory was financed by the Carnegie Foundation and was institutionally sep­ ar­ate from Caltech, there was much contact between the Caltech staff and that of Mount Wilson because of the observatory’s location. 40   “Ik klemde mij dikwijls vast van angst als de wagen eens op de kanten der diepe wagensporen kwam en ter zijde overhelde waar een diepe afgrond ons tegenlachte.” 41   Lorentz 1922a.

The return journey


The return journey On March 5, Lorentz and Aletta left Pasadena on their way to Yosemite Park, where they spent one day and one night to be able to admire the spectacular landscape. It was an enormous change after the mild weather in Pasadena. They took a tour in the snow in a sleigh drawn by two horses, wearing rented galoshes. They also slithered around—Aletta’s words—over slippery footpaths. Everybody was exceptionally nice and polite and, according to Aletta, they were not “fathered over, but grandmothered and mothered over” in their hotel.42 The weeks after their departure from Pasadena were very busy. Before sailing back at the end of April, they visited no fewer than eleven different places, where Lorentz gave as many as twenty-­three lectures:43 an impressive program for a sixty-­eight-­year-­old man, then as well as now. Relaxation during those busy weeks after Pasadena did not arrive until they had reached the East Coast. While they were in Toronto, they took a side-­trip to Niagara Falls, where they spent two ­leisurely days. The people they met were often quite impressed by the spry elderly couple. In a letter to his daughter, Lorentz remarks, not without pride, that they were praised because they were “so strong.” In an earlier letter he had written to her—equally proudly—about how at some point they were complimented on the fact that they looked younger then than they did when they had first arrived in Pasadena. After Yosemite the next stop was Berkeley, where they thoroughly enjoyed themselves. They were staying in a good hotel where they had their usual American breakfast: “half a grapefruit, or a ‘baked apple’, a dish of ‘corn flakes’ with milk and sugar, toasted bread and tea.”44 Lorentz took the opportunity to have his beard trimmed as well, as it had meanwhile undergone “all sorts of metamorphoses.”45 The comparison between Northern and Southern California was clearly un­favor­able for Pasadena: “if we were to live in California, we would rather be in the North, in Berkeley or Stanford, than in Pasadena. The latter, however beautiful, is too much ‘fashionable movement’ and makes too much of an impression of having been wrested from the desert by an overabundance of irrigation.”46

  “niet bevaderd maar begrootmoederd en bemoederd.”  The places he visited (with the number of lectures) were, in order: University of California, Berkeley (1); Stanford University, Palo Alto (1); Iowa State University, Iowa City (1); University of Chicago (2); University of Wisconsin, Madison (4); again University of Chicago (2); Case School of Applied Science and Western Reserve University, Cleveland (1); Harvard University, Cambridge (3); Columbia University, New York (1); Cornell University, Ithaca (2); University of Toronto (4); National Academy of Sciences and Carnegie Institution, Washington (1). 44   “Een halve grapefruit of wel een ‘baked apple,’ een schoteltje ‘corn flakes’ met melk en suiker, geroosterd brood en thee.” 45   “allerlei gedaantewisselingen.” 46   “Als wij in Californië moesten wonen zouden wij liever in het Noorden ervan, in Berkeley of Stanford zijn, dan in Pasadena. Dit laatste, hoe mooi ook, is te veel ‘mondain beweeg’ en maakt te veel den indruk aan de woestijn door overvloed van irrigatie ontwoekerd te zijn.” 42 43


The final years: Travel and tributes

From Berkeley, they began their long journey eastward. The train trip from Iowa City to Chicago provided an opportunity for Aletta to grumble some more about the appearance of the American women. Because they had their own “drawing room” and toilet facilities on the train, this time, there was no need for them to wash “amid all these painted and glued madams. The lips on the outside, the mouths on the inside, the eyes, the eyebrows, everything is painted, not to even mention the cheeks and foreheads.” Aletta was more laconic when it came to her own safety. While they were in Chicago, her hostess walked her home with a dog, for “it is not safe here in the streets at night, there is shooting, bombs are thrown etc. etc.”47 She appeared not overly impressed by these serious-­sounding dangers. There were irritations as well. Lorentz’s passport turned out to have expired and to be issued a new one he needed proof that they had paid their taxes, or that they were tax-­exempt in the United States. “Pa is indignant that he has to make an effort to be allowed back into his own country and also that he needs to pay up the taxes on his hard-­earned cash.”48 On April 29 Lorentz and Aletta departed from New York, again sailing on the Nieuw Amsterdam, to arrive in Rotterdam on May 9. Millikan joined them onboard. He was on his way to Brussels to give a lecture series which he was planning to deliver in French. Since his command of French was “very bad,” he was grateful for Lorentz’s coaching in writing down the first few lectures.49 A few weeks after his return from the United States, Lorentz traveled to Brussels in great haste for the funeral, on May 29, 1922, of Ernest Solvay, who had died three days earlier at the age of eighty-­five. The Solvay family was very appreciative that Lorentz had taken the trouble to come to the funeral.50 They expressed their appreciation in more than words alone: to thank him for all he had done for the Solvay Institute, Solvay’s heirs donated 100,000 Belgian francs (some 18,000 Dutch guilders at the time), as a contribution to “your grand and precious work.”51 Kamerlingh Onnes also attended the funeral and likewise received 100,000 francs.52 The report of the third Solvay Conference contained a striking obituary written by Lorentz.53

Foreign commendations and honors For Lorentz, the arrival of a new century in 1900 had marked the beginning of a long stream of foreign commendations and honors, starting with an honorary   “het is hier niet veilig ’s avonds op straat, er wordt geschoten, bommen gegooid enz. enz.”   “Pa is verontwaardigd dat hij nog moeite moet doen om in zijn eigen land te worden toegelaten en ook om van zijn zuur verdiende geldje belasting te betalen.” 49   Millikan to Lorentz, June 26, 1922 (LA 54). 50   See Charles Lefébure to Lorentz, June 2, 1922 (LA 48). Lefébure was private secretary of Solvay. 51   “vos grands et précieux travaux.” See Charles Lefébure to Lorentz, August 25, 1922 (LA 48). 52 53  See Delft 2007, 483. Onnes invested the money in his laboratory.   Lorentz 1923h. 47 48

Foreign commendations and honors


doctorate from the University of Göttingen in 1899 and his decoration in 1900 in the Netherlands as Ridder in de Orde van de Nederlandse Leeuw, on the occasion of the twenty-­fifth anniversary of his doctorate. In the decades to follow, the initial trickle of commendations and honorary appointments had swollen to a steady stream. By 1920 he had received no fewer than six honorary doctorates, while he had meanwhile also become a member, an honorary member, or a foreign or corresponding member of as many as thirty-­five academies and other learned so­ci­ eties abroad. In the last eight years of his life, Lorentz received additional honorary doctorates from the universities of Cambridge and Paris, and also from Leiden, his alma mater. In addition, he was awarded a number of high decorations in the Netherlands and abroad. In 1922 he became Commandeur de l’Ordre la Couronne in Belgium, Commandeur de la Légion d’Honneur in France, and Commandeur in de Orde van Oranje-­Nassau in the Netherlands. To top it all off, he enjoyed the rare honor, in 1925, of receiving the seldom awarded high distinction of Grootkruis in de Orde van Oranje-­Nassau.54 An exceptional list of tributes for a physicist. Attending the ceremony for the honorary doctorate in Cambridge was part of an extended trip through England and Scotland in May and June of 1923. Very early in the morning of May 13, Lorentz and Aletta departed, on a freezing cold train to the harbor town of Vlissingen, where they took a steamer to England. They found the white cliffs of Dover very impressive, but the crowded city of London turned out to be less to their liking. In England and Scotland, the program of the Lorentz couple was again very busy. In two months, they traveled all over the country and Lorentz lectured at no fewer than six universities. In Cambridge, they were accommodated in grand style in Trinity Lodge. In these luxurious living quarters of the Master of Trinity College, they were waited on hand and foot, sometimes even without asking. “All the time there is new hot water waiting for me, but I cannot wash myself all the time just to please them,” wrote Aletta, slightly irritated.55 About the fancy reception she remarked coolly: “My best dress was just very simple compared to all those ladies.”56 On May 15, Lorentz was afforded the high honor of addressing a large audience in Cambridge during the prestigious yearly Rede Lecture. Lorentz’s speech, on Maxwell’s electromagnetic theory, was an enormous success: “There was no end to the applause,”57 according to Aletta. Two days later he spoke in London, at University College, about the rotation of the earth.58

54   An overview of the honors Lorentz received can be found in LA 192. According to this list, at the time of his death in 1928 Lorentz was a member of at least forty-­one foreign societies, including those in Paris, Berlin, Vienna, and the Royal Society of London. It is not certain, though, that this list is complete. 55   “Steeds staat er maar weer warm water op me te wachten maar ik kan me niet altijd wasschen voor hun plezier.” 56   “Mijn beste japon was maar heel simpel bij al die prachtige dames.” 57   “Er kwam geen eind aan het applaus.” The lecture was published as Lorentz 1923f. 58   Published as Lorentz 1923c.


The final years: Travel and tributes

After Cambridge they traveled to Scotland. They had an entire week without obligations and made time for one of their favorite pastimes, hiking. Highlights of the trip were a boat ride on the lake of Loch Lomond and climbing Ben Lomond, a mountain in the Scottish Highlands and a popular tourist destination, even today. During the cruise on the lake “it rained steadily, but we found dry stools and sat on them like two steadfast soldiers, so we would still see something.”59 Climbing the mountain was not so pleasant, especially in the beginning. First, they got lost and sank into the morass up to the tops of their boots, but they were not bothered by it all: “Fearlessly we continued to climb.” Once they were higher up on the mountain, the weather did not exactly help either. According to Aletta, it rained and snowed and it was “fiercely cold, Pa had nine layers and a scarf.” They still enjoyed the hike and took the cold and the rain in their stride. Back in the hotel, they dried their wet feet in front of the fire and “drank tea, tea, tea,” because they had been given a packed lunch of very dry sandwiches.60 After their hiking trip, they continued on to Edinburgh, where Lorentz gave a lecture at the university. He followed up with lectures in Manchester, Leeds, and Liverpool. In Aletta’s terms: “Pa is holding a triumphal march through the country.”61 In Liverpool, after Lorentz’s lecture, there was a large dinner with many toasts and speeches, sometimes “full of jokes, but fortunately Pa’s toast was more serious.”62 A great deal of wine was consumed and there was much smoking, while the dinner guests became more and more rowdy and undisciplined. “The heads turned redder all the time, also because they were applauding with hands and feet and were stomping on the floor when the speaker finished, also they called ay, ay, no, no; or good, good, which altogether caused a terrible ruckus and I was glad when it was all over.”63 The trip through the United Kingdom ended with another visit to Cambridge and London. In Cambridge Lorentz received his honorary doctorate, at the same time as Niels Bohr and Viscount Edward Gray, a British former Foreign Secretary and ambassador.64 During the ceremony, Lorentz and the other honorary doctors were addressed by the Public Orator, the “voice” of the university at public events. He gave an  overview of Lorentz’s merits and lauded him in particular for his didactic

59   “regende het flink maar wij vonden drooge krukjes en zaten daarop als twee standvastige soldaten om toch nog iets te zien.” 60   “Onverschrokken klommen wij door.” “vinnig koud, Pa had negen lagen en een das.” “dronken ze thee, thee, thee.” 61   “Pa houdt als het ware een zegentocht door het land.” 62   “vol moppen maar gelukkig was de toast op Pa ernstiger.” 63  “De hoofden werden steeds rooder ook doordat zij steeds met handen en voeten klapten en stampten als de spreker ermee ophield, ook riepen ze ay, ay, no, no; of good, good, wat alles te zamen een afschuwelijk leven maakte en ik was blij toen het afgeloopen was.” 64  See Nature 111 (1923): 818, 900.

Lorentz’s seventieth birthday


qualities. The speech, full of puffery and in Latin, ascribed a plethora of virtues to Lorentz. I introduce to you Henricus Antonius Lorentz [. . .] through his knowledge of many languages, his insight in many questions, the miraculous sharpness of his mind, he is so much the driving force for his students, that he is a support to those who investigate the secrets of nature, even if they do not penetrate them thoroughly, especially by his words, his example, and his goodness.65

A festive dinner followed, attended by ninety-­six men only, and the Master of Trinity College first proposed the traditional toast to the King, followed by a toast to the newly minted doctores honoris causa. Both Lorentz and Viscount Gray were obliged to answer the Master’s toast in a table speech. In London, Lorentz made an address to the venerable Royal Institution. He also gave a series of lectures on June 4, 5, and 6 at University College and on June 6 he addressed the ­Anglo-­Batavian Society. His lecture there was part of an exchange between the Netherlands and England, organized by this society, whose objective it was to further relations between the two countries by means of lectures by Dutch ­scholars. The whole affair ended with a banquet.

Lorentz’s seventieth birthday On June 14, 1923, Lorentz and Aletta were back home in Haarlem. A month later a festive event awaited them there: Lorentz’s seventieth birthday. Less festive was the fact that Lorentz would now be pensioned off as professor of physics, which would put an end to his teaching activities. Lorentz had more or less reconciled himself to this idea—in May he had already given his farewell lecture—but apparently others had different ideas. At the initiative of a number of friends he was invited, on behalf of the Leidsch Universiteits Fonds (Leiden University Fund), to continue his Monday morning lectures in the following ­academic year.66 The actual birthday celebration, on July 18, was quite informal. Zeeman, Ehrenfest, Haga, De Sitter,67 the children, and the grandchildren all came to dinner. The festivities, in which all the grandchildren participated, ended outside on

65   “Duco ad vos Henricum Antonium Lorentz.” [. . .] “Multa linguarum Scientia, multa rerum cognition, miro ingenii acumine, talis est studiorum hortatory, ut illis qui Naturae secreta explorant etiam si non adhuc penitus inveniunt verbo exemplo benignitate praecipue subveniat.” 66  See Leidsch Universiteits Fonds to Lorentz, July 17, 1923 (LA 114). The letter states that a request had been made to the Crown to give permission for the invitation on the basis of article 183 of the Law on Higher Education. A remuneration of 2,500 guilders was raised by his friends. It supplemented his pension almost to the amount of his professor’s salary of 7,000 guilders (see the letter about his pension rights in LA 115). The invitation was subsequently repeated every year. 67   Kamerlingh Onnes could not join them because he was taking a cure in Baden near Zürich.


The final years: Travel and tributes

the square in front of the Lorentz home, with an atmospheric walk with paper lanterns around the central lawn.68 It goes without saying that Lorentz received many letters congratulating him in flowery terms, but when it came to flowery formulations, the congratulations by Einstein took the cake. In daily life shame forbids us to make declarations of love to those whom we venerate the most. But your seventieth birthday may well break this ban. How often have I found deep consolation in your noble and outstanding personality, when human affairs around me looked hopelessly sad. For a human being like you offers consolation and lifts up, by virtue of his existence and his example alone. Beyond that, I am fortunate to be particularly deeply connected to you, as I may honor you in matters concerning science as my teacher, in whose footsteps I follow, for this represents the greatest meaning of my life. But not only in our science, but also in your attitude towards the individual human being and towards human affairs of greater importance you are and remain my shining, albeit unattainable, example.69

Lorentz replied in an appropriate manner: I still vividly remember how, more than thirteen years ago, you explained in my room in Leiden that my clock would go slightly faster if it would be in the proximity of the ceiling. That was the beginning of many a lesson and so the revolution in our science, which we owe to you, has contributed much to keep me young. Very valuable for me was that we are, also in other questions than scientific ones, fundamentally always of the same opinion, just like we also feel the torment of these times in the same way.70

  See Lorentz to Ehrenfest, July 20, 1923 (Kox 2018, 258).   “Im täglichen Leben verbietet es die Scham, denen, die wir am meisten verehren, Liebeserklärungen zu machen. Aber Ihr siebzigster Geburtstag darf wohl diesen Bann brechen. Wie oft habe ich in Ihrer edlen und überragenden Persönlichkeit tiefen Trost gefunden, wenn die menschlichen Dinge um mich her hoffnungslos traurig aussahen. Denn ein Mensch wie Sie tröstet und erhebt durch sein blosses Dasein und Beispiel. Darüber hinaus bin ich glücklich, besonders tief mit Ihnen verbunden zu sein, indem ich Sie in Dingen der Wissenschaft als meinen Lehrer verehren darf, auf dessen Wegen zu gehen, meinen grössten Lebensinhalt ausmachte. Aber nicht nur in unserer Wissenschaft sondern auch in Ihrer Stellung zu den einzelnen Menschen und menschlichen Angelegenheiten grösseren Stiles sind und bleiben Sie mir leuchtendes, wenn auch unerreichbares Vorbild.” Einstein to Lorentz, July 15, 1923 (Kox 2008, 377). 70  “Ich erinnere mich noch lebhaft, wie Sie mir vor mehr als 13 Jahren in meinem Zimmer in Leiden auseinandersetzten, meine Uhr würde etwas rascher gehen, wenn sie sich nahe der Decke befände. Das war der Anfang mancher Belehrung und so hat die Umwälzung in unserer Wissenschaft, die wir Ihnen verdanken, viel dazu beigetragen, mich jung zu erhalten. Sehr wertvoll war es mir dabei dass wir auch in anderen als wissenschaftlichen Fragen im Grunde immer derselben Meinung sind, wie wir auch die Drangsal dieser Zeiten in derselben Weise fühlen.” Lorentz to Einstein, September 15, 1923 (Kox 2008, 380). The first encounter between Lorentz and Einstein took place in February 1911. The remark about the clock being faster refers to Einstein’s “equivalence principle” from 1906 (see Chapter 8). 68 69

New travel plans


No wonder that his seventieth birthday was reason for Lorentz to look back on his scientific career. Somewhat nostalgic and at the same time modest about his own achievements, he wrote to Ehrenfest: It would be very unkind of me if I did not sometimes say something about the “old” physics; certainly I shall be very glad to do so. It will then be seen how easy things were for us in those days, compared with now; there were no clouds (of quanta and the like) and we worked unhindered and in a rapture of delight. Besides, there were still so many not-­too-­difficult questions that one could easily make a start and find a suitable way. I have been very fortunate to have lived in such a fine, I would almost say festive, period. But I was even more fortunate in having won the affection and friendship of many good people, as has once again been made clear.71

Three days after the birthday celebration, Lorentz and Aletta left on another trip. By way of Paris they traveled to Geneva, where it was back to the grindstone for Lorentz. He attended a CICI meeting for the first time. Immediately afterwards, though, they took a well-­deserved vacation in the Vosges mountains in Northeast France, perfect for plenty of hiking and relaxation. As Aletta wrote: “For Pa it is really good that he will have three weeks of lazing about.”72

New travel plans In the fall of 1923, Lorentz began to make travel plans again, this time for a second stay in Pasadena. In a letter to Millikan of November 30, Lorentz wrote that he was somewhat envious of Ehrenfest, who was about to leave to spend a few months in Pasadena. Ehrenfest had come to say goodbye, and they had had a long conversation about Caltech. Lorentz mentioned in his letter that Millikan had made him an offer for a permanent position earlier, but, he wrote, “I must consider this as one of those illusions that cannot be fulfilled, though I am grateful for the idea. In my language we have an adage to the effect that old trees must not be transplanted and I am becoming an old tree indeed. It may even well be, though I  should like (another illusion!) to come once more, that I shall never see your

71   “Ik zou nu wel erg onvriendelijk zijn als ik niet eens wat van de ‘oude’ physica ging vertellen; ik zal het trouwens graag doen. Men zal dan zien hoe gemakkelijk wij het vergeleken met nu, in die dagen hadden; er waren geen wolken (van quanta en dergelijke) en wij werkten onbekommerd en in blijde opgetogenheid. Bovendien waren er nog zooveel niet al te moeilijke vraagstukken dat men allicht een begin kon maken en een geschikten weg kon vinden. Ik ben wel heel gelukkig geweest in zo’n mooie, ik zou haast zeggen feestige periode geleefd te hebben. Maar grooter geluk was het nog dat ik de genegenheid en de vriendschap van vele goede menschen verworven heb, wat ook nu weer bleek.” Lorentz to Ehrenfest, July 20, 1923 (Kox 2018, 258). Lorentz also thanked Ehrenfest for his piece “written with so much love” (met zooveel liefde geschreven) about his work in the Nieuwe Rotterdamsche Courant of July 17, 1923. 72   “Voor Pa is het erg goed dat hij eens drie weken luieren zal.” Aletta to Berta and Wander, July 5, 1923 (FC).


The final years: Travel and tributes

country again. In that case I shall be content with my good and beautiful recollections.”73 A few months later, Millikan responded with an invitation to Lorentz to come to Pasadena in the first months of 1926.74 To his colleague, Ernest Merritt, the dean of the physics department at Cornell University in Ithaca, New York, Millikan had apparently made it look as if Lorentz had already made the decision to come to the United States. At any rate, Merritt invited Lorentz to come and lecture at Cornell for ten weeks in early January, prior to his stay in Pasadena. He offered a salary of no less than 3,000 dollars.75 Lorentz accepted, and everything seemed done and dusted. Early in 1925, Lorentz’s colleagues and friends got wind of his travel plans, which would prevent him from being in the Netherlands on December 11 of that year, the day of the fiftieth anniversary of his doctorate. Under pressure from, among others, Ehrenfest and Einstein, Lorentz realized that it would be out of the question for him to cele­ brate this anniversary while in the United States. As a result, the trip was canceled and postponed to the following year.76

Honors in Spain Although the trip to the United States had been postponed, this did not mean that Lorentz and Aletta remained in the Netherlands throughout the year 1925. In Spain Lorentz was honored with yet another prestigious decoration. He was awarded the Spanish Echegaray Medal, a gold medal, named after the Spanish mathematician, minister, and playwright José Echegaray, who had been awarded the Nobel Prize for literature in 1904. The medal was the highest scientific distinction of the Real Academia de Ciencias Exactas, Físicas y Naturales, the Spanish Academy of Sciences. In an official letter dated January 29, 1925, Lorentz was informed that the Academia had not only unanimously awarded him the medal the day before, but had also appointed him as a corresponding member.77 Receiving this award offered the Lorentz couple the opportunity to visit a country where neither Lorentz nor Aletta had ever been.   Lorentz to Millikan, November 30, 1923 (Caltech Archives).   Millikan to Lorentz, February 25, 1924 (LA 54). 75   Merritt to Lorentz, January 10, 1924 (LA 81). 76   See Lorentz to Ehrenfest, February 14, 1925 (Kox 2018, 265). The decision to cancel the trip, it appears from the letter, had been taken a day earlier in a conversation with Einstein, and that very same day Lorentz had sent a telegram to Ernest Merritt. On February 28, he wrote to Millikan (Caltech Archives) and proposed a year’s postponement. Typical for Lorentz is that he speaks in his letter about “reasons, which do not matter now” that necessitated the postponement. Millikan’s invitation to come and lecture for a fee of 3,500 dollars during the first months of 1926 only reached Lorentz after his cancellation (see Millikan to Lorentz, February 25, 1925, LA 54). 77   Academia de Ciencias to Lorentz, January 29, 1925 (LA 52). The letter was signed by Secretary General José De Madariaga. 73 74

Golden anniversary of Lorentz’s doctorate


After their arrival in Madrid, on May 14, 1925, Lorentz received the medal on May 18 from the hands of King Alfonso XIII. The day after the ceremony he was also received in a private audience with the King. It was obviously not completely clear to Lorentz how solemn the ceremony of the royal audience would be, for he was in doubt whether or not to wear the top hat he had borrowed for the occasion. In the end, he decided to do without the hat and, as Aletta later wrote to Rudolf, “act according to Dutch fashion. The King was not cross about it.”78 Evidently, Lorentz was also expected to lecture in Madrid. He gave three lectures this time, two at the university about magnetism and one for engineers. All three lectures were received with enthusiasm. He was also introduced to some Spanish colleagues in physics and met the histologist and Nobel laureate Santiago Ramón y Cajal, to whom he would turn later that year for support in his efforts to restore international cooperation. After the more ceremonious part of the visit, Lorentz and Aletta also took the chance to do some sightseeing in Spain. They made a day trip by train to Toledo. As honored guests, they were extended every possible courtesy by the Spanish railways. Since a member of the railway directorate had joined them on the trip, they even traveled in a separate carriage. After Toledo they first visited Granada— where they admired the Alhambra—then Seville and Cordoba, and eventually they returned to Madrid for a day of museum visits. On their journey through Spain they encountered a well-­known linguistic phenomenon that they found quite humorous: Two ladies in their train compartment “wanted to shout their Spanish ever louder to make us understand.” And later, in Granada: “the custodian in one of the towers bellowed the words and, in the end, she had someone fetch a school textbook with large letters from below and when we did not even understand that, she had a good laugh.”79 On the way back, by way of San Sebastian, they visited Aletta’s sister Betsy in Paris. On Tuesday, June 2 they were back home in Haarlem.

Golden anniversary of Lorentz’s doctorate As early as October 1924, planning had started for the fiftieth anniversary of Lorentz’s doctorate, at least according to a letter Einstein wrote to Kamerlingh Onnes on November 4, 1924.80 It was not so much a question of organizing ­festivities, but rather of establishing a fund that was supposed to promote the practice of theoretical physics: the Lorentz Doctorate Golden Anniversary Fund (later called the Lorentz Fund for short). The promotion of physics would also

  Aletta to Rudolf, n.d. (LA 742).   “wilden het spaansch steeds harder schreeuwen om het ons te doen begrijpen.” “De wachteres op een van de torens brulde de woorden uit en eindelijk liet zij van beneden een spaansch schoolboekje halen met groote letters er in en toen we ook dat niet begrepen moest ze erg lachen.” 80  See CPAE-­14, 361. 78 79


The final years: Travel and tributes

include a continuation of the funding that enabled Lorentz to continue his teaching activities, something that was financed initially by the Leiden University Fund. The official founding charter of the Lorentz Fund stipulated explicitly that it could also disburse money to Lorentz’s surviving family after his death.81 On September 1, 1925, the plan was officially unveiled in a confidential announcement to physicists at home and abroad, on behalf of a National Committee made up of sixty colleagues and prominent Dutchmen. An Executive Committee was in charge of making the plan a reality. It consisted of Kamerlingh Onnes as chairman, Lorentz’s student and professor in Delft Adriaan Fokker as  secretary, and his fellow Nobel laureate and friend, Pieter Zeeman. In the announcement, a financial contribution was solicited. The fund’s objectives were described and the idea was launched that, during his lifetime, Lorentz himself would be able to dispose of the money in the fund. An announcement went out to important foreign physicists requesting them to join an international committee tasked with recruiting further donors. The international committee was joined by no fewer than seventy-­two members, among whom were prominent scientists, but also representatives of big industrial firms such as General Electric and Siemens & Halske. The whole enterprise was a great success. Hundreds of donors signed up. It goes without saying that most of the donations—almost 1,000—came from the Netherlands, including the substantial gift of 25,000 guilders from a certain P. Reineke. He had met Lorentz on the boat to New York in 1906 while on his way to Mexico to “find out about the culture of fibers” and he had cherished that encounter.82 Among the foreign contributors France and the United States were well represented, with 186 and 185 participants respectively.83 Germany, with 69 donors, was somewhat underrepresented, certainly given the scale of physics in Germany. Because of his attitude during the war, Lorentz was no longer so loved by many of his German colleagues. The celebrations consisted of several parts. First, Lorentz was awarded an honorary doctorate in medicine during a private ceremony in the Senate Chamber of Leiden University. This was in recognition of his long-­term devotion to teaching the first-­year students of medicine. This private award ceremony was followed by the public celebration, which turned out to be less public than it appeared, as seating space was extremely limit­ed and many people who wanted to attend had to be disappointed. The cele­ bra­tion took place in the great auditorium of Leiden University, which was filled to capacity with countless dignitaries and a select group of Dutch and foreign scientists. Among those who attended were such icons of physics as Niels Bohr,

  See LA 118 for a draft of the charter.   “de vezelcultuur na te gaan.” See P. Reineke to Lorentz, December 12, 1925 (copy in PC). The man in question is probably Pieter Reineke, director of the Handelsvereniging Amsterdam, at the time one of the largest agricultural enterprises in the world. 83   The numbers are taken from an undated manuscript written in Berta’s hand in PC. 81 82

Golden anniversary of Lorentz’s doctorate


Marie Curie, and Arthur Eddington. On behalf of the Royal Family, Prince Consort Hendrik was also present, while the government was represented by none other than Prime Minister Hendrik Colijn, and Minister of Education, Arts, and Sciences, Victor Rutgers.84 The celebration was livened up at the beginning and at the end with music played by a string quartet made up of science students. A direct radio transmission was also planned, but that turned into an abject failure. Because of a poor telephone connection and the wrong placement of the microphones, the ceremony was impossible to follow, and after forty-­five minutes the transmission was suspended. The celebration in the auditorium consisted mainly of a long series of laudatory speeches. The parting shot was for the university rector, Willem de Sitter, who had also chaired the honorary doctoral ceremony. He addressed Lorentz in language that was unusually pompous, even for those days.85 He called him unreservedly “the greatest physicist” of his time and praised him as “such a good human being and for all of us such a good friend.” Interestingly enough, he also pointed out that Lorentz did not suffer from false modesty, observing “that you, knowing that you are a great man, are not embarrassed about that.”86 This set the tone for the rest of the speeches.87 Kamerlingh Onnes, the second speaker, also praised Lorentz’s scientific merits. His speech was couched in equally pompous terms as the one by De Sitter. Kamerlingh Onnes went into great detail about his warm ties of friendship with Lorentz and about Lorentz’s willingness to be helpful. With very good reason, as he had profited like no other when Lorentz took over his lectures for medical students, first temporarily and later permanently. According to Kamerlingh Onnes, Lorentz had “saved his health” by doing so, but one may wonder whether he had not in fact abused Lorentz’s helpfulness. Kamerlingh Onnes also framed the tension between Lorentz’s desire to experiment and Onnes’s drive to expand the Leiden laboratory in a rather positive light.

84  Details in the description that follows are derived from the speeches published in Physica 6 (1926): 1–29 and from extensive newspaper reports (which even record at which moments the ­speakers were interrupted by applause). See also Bataviaans Nieuwsblad, December 14, 1925, for an account of a celebration in Bandung in the Dutch East Indies (now Indonesia). 85  He used unusually archaic turns of phrase comparable to English usage like “thou wished to know”, “thou wast,” “thou didst,” and the like. 86   “zoo’n goed mensch en voor ons alleen zoo’n goed vriend.” “dat gij, wetend dat gij een groot man zijt, U daarvoor niet schaamt.” 87   Lorentz was well aware of his status, as is illustrated by the following anecdote. Returning from one of their foreign trips, Lorentz and Aletta arrived at the Haarlem train station, where a large crowd and even a brass band had assembled on the platform. Lorentz was quite pleased with this reception, but his mood changed after he went up to shake someone’s hand in thanks and this person asked him “Who are you?” It turned out that the crowd was waiting for the local football team to arrive, to cele­ brate their victory in an important game. Disappointed, Lorentz went home. (Personal communication from Robert Boekelaar. His grandmother, who served as a housekeeper in the Lorentz household, had heard the story from Aletta.)


The final years: Travel and tributes

Working with extremely poor equipment, the problems with the ever-­increasing lack of space, you have endured them by involving yourself in the difficulties of the la­bora­tory, and by putting your sympathy for what was endeavored there into the balance, offsetting the burdens that the realization of these endeavors meant for you.88

As regards the work in the laboratory, Kamerlingh Onnes also explicitly made note of the “Lorentz Series,” the series of experiments in the field of ­magneto-­optics that was inspired by Lorentz’s electromagnetic theory and culminated in the discovery of the Zeeman effect. Also, Lorentz’s important role in the Solvay Conferences was memorialized eloquently by Onnes. For his compatriots it was a source of pride to see Lorentz in action at the Solvay meetings, he noted. Leading the meetings that went on for whole days at a stretch, and thinking in three languages at once to catch everyone’s opinions and then present them to the others, sometimes even in a clearer form, finding the right tone and creating the desired atmosphere of confidentiality.89

Kamerlingh Onnes ended his speech by announcing the happy news that for the Lorentz Fund the impressive sum of “one ton’s worth of gold” (100,000 guilders in gold) had been collected. The final sum, as specified in the founding charter of the fund, was around 145,000 guilders, of which 31,700 guilders were contributed by foreign donors.90 As noted earlier, financial support for Lorentz’s surviving family was an explicit goal of the fund. As it turns out, in the period to 1940 the Lorentz family received a total sum of 40,000 guilders, which was no less than half the proceeds of the capital.91 After Kamerlingh Onnes, Prime Minister Colijn gave a speech, on behalf of the entire Dutch nation, as he stated. At the end of his somewhat perfunctory speech, he announced that it had pleased Queen Wilhelmina to present Lorentz with the Grootkruis (Great Cross) in the Order of Orange-­Nassau, the highest rank in this order. The Prime Minister’s address was followed by a speech by the chairman of the Royal Academy, Frits Went. He lauded Lorentz’s achievements in the Academy during the more than ten years of his chairmanship—from 1910 to 1921—of the 88  “Het werken met uiterst gebrekkige hulpmiddelen, de bezwaren van het steeds nijpender ruimtegebrek, gij hebt ze U getroost, door U in de moeilijkheden van het laboratorium te verplaatsen, en Uwe sympathie voor wat daar beoogd werd in de weegschaal te werpen tegenover de lasten, die de verwezenlijking ervan voor U medebracht.” 89   “bij het leiden der zich over den ganschen werkdag uitstrekkende vergaderingen, en het in drie talen tegelijk denken om de meeningen van ieder op te vangen en deze dan aan de anderen, soms zelfs in helderder vorm weer te geven, bij het treffen van den juisten toon en bij het wekken van den gewenschten geest van vertrouwelijkheid.” 90  See Nieuwe Rotterdamsche Courant, January 15, 1926. The exact amount as noted in the foundation charter was 145,615.06 guilders. 91   Personal communication to one of the authors (AJK) from P. Plantenga, author of an unpublished financial history of the Leiden University Fund.

Golden anniversary of Lorentz’s doctorate


Section of Sciences. He also praised Lorentz’s importance after his chairmanship had ended, especially for the restoration of international scientific cooperation. To honor him, Went announced, the Academy had instituted a gold Lorentz medal for great achievements in the field of physics. Lorentz was able to present the first medal to Max Planck in the spring of 1927, and to this day the medal is still awarded every four years. Marie Curie congratulated Lorentz on behalf of the Solvay Institute, and as a member of the CICI she commended him for his valuable contributions to that commission’s work. Einstein gave the final speech. In his address there was an interesting twist. On behalf of the Prussian Academy of Sciences, he presented Lorentz with a cassette containing a printed congratulatory text.92 This text had been drafted by Einstein himself, but during his speech he studiously avoided presenting himself as a representative of the Academy. In one paragraph his spoken words differed in essential ways from the printed version. He read a passage out loud that sounded very much like a paragraph that had been excised from his original text after a number of Prussian Academy members had raised serious objections. In the offending passage he praised Lorentz’s personality and said about his work in the interest of international cooperation: You have suffered as few others under the misfortune of Europe and under the strife that still splits in two the scientific world of our day.You have done everything in your power to heal the wounds and once again bring about united cooperation.93

That had clearly touched a raw nerve for a number of fanatical German-­nationalist members of the Prussian Academy.94 It became all the more clear that Lorentz was not loved by everyone in Germany. Einstein ended his address with a metaphor that he had used before: “To few it is given to create works of art, and to even fewer is given the power to turn their own lives into a work of art.You have this gift and we consider you fortunate because of it.”95 After all the illustrious speakers, it was Lorentz’s turn to speak. He expressed his appreciation for the presence of all those who had taken the trouble to come, in particular dignitaries like the Prince Consort, and he thanked the speakers for

 See CPAE-­15, 121, for the text.   “Sie litten wie wenige unter dem Unglück Europas und unter dem Hader, der noch immer die wissenschaftliche Welt unserer Tage entzweit. Sie haben sich mit aller Kraft bemüht, die Wunden zu heilen und wieder einträchtige Zusammenarbeit herbeizuführen.” See CPAE-­15, 116. 94   The text of the speech he pronounced in Leiden was in the same vein. “You suffered as few ­others have under the misfortune of Europe and you have used all your power to bring about once again a united cooperation. That is why we venerate you and love you, also as a human being and not just as a scientific investigator.” (Sie litten wie wenige unter das Unglück Europas und Sie haben sich mit aller Kraft bemüht, wieder einträchtige Zusammenarbeit herbeizufuhren. Darum lieben und verehren wir in Ihnen auch den Menschen und nicht nur den Forscher.) 95   “Wenigen ist es gegeben, Kunstwerke zu schaffen, noch viel weniger vermögen ihr eigenes Leben zu einem Kunstwerk zu gestalten. Ihnen ist es gegeben und wir preisen Sie darum glücklich.” 92 93


The final years: Travel and tributes

their kind praise. He was impressed by the diverse membership of the national committee, recognizing physicists from many universities, but also representatives from countless educational institutions and several learned societies. He further reflected on his life, his school years in Arnhem, his student years, and his professorship, bringing up the reminiscences that have already been quoted elsewhere in this book. Lorentz also took the opportunity to emphasize, once again, his great confidence in international cooperation. He spoke about a “true brotherhood of the practitioners of science in all countries” which “will persist and will increase in power and intensity.”96 He ended his speech by expressing the desire that Leiden University would “continue to grow and flourish, a breeding ground for the knowledge and the disposition by which the well-­being and the happiness of the people is increased.”97 Besides speeches, there were gifts as well. The certificate of the Prussian Academy was impressive, but other organizations had also designed their congratulatory messages beautifully. On behalf of the Solvay Institute, Lorentz received a silver platter, for example, with the engraved signatures of all the members of the scientific and administrative committees. A particularly special gift was the little wooden “chest of drawers” made out of Macassar Ebony with initials and years inlaid in Ceylon Satinwood, with the dedi­ ca­tion: “To doctor Hendrik Antoon Lorentz, for fifty years of serving and leading, thanks.”98 The drawer contained two rows of small cards on which all the donors to the Lorentz Fund had written their names.99 After the ceremony, there was first a reception in Leiden that was attended by a great many guests. Lorentz then traveled with a smaller group to Haarlem for a festive dinner in the building of the Hollandsche Maatschappij der Wetenschappen. Those who were present were immortalized in a photograph, a group portrait that has become very well known. Apart from Lorentz and his wife and children, a colorful collection of foreign and Dutch physicists and dignitaries—like the mayor of Haarlem and the Minister of Education—can be recognized in the photo.100 Afterwards, Lorentz was happy and touched by the way in which so many ­people had paid such attention to his golden doctorate. The celebration had been

96   “een ware broederschap van de beoefenaars der wetenschap in alle landen die zal blijven bestaan en in kracht en innigheid zal toenemen.” 97   “blijven groeien en bloeien, een kweekplaats van de kennis en de gezindheid waardoor het welzijn en het geluk der menschen verhoogd worden.” 98   “Aan doctor Hendrik Antoon Lorentz, voor vijftig jaren van dienend leiden dank.” 99   The silver platter is in RB. The “chest of drawers,” a small file drawer with a wooden chest around it, is in the possession of the Royal Netherlands Academy of Sciences. In fact, it is the second chest that was made, as the first one turned out to be too small to accommodate all the cards. The second chest of drawers was not completed until several months after the celebration, because the woodworker was busy with other, more pressing assignments (see J. P. Fokker to Lorentz, March 3, 1926, LA 25). 100   Of the fifty-­four guests at the dinner, fifty-­two are in the photo. They have been identified in the booklet Verhalen rond een foto. Lorentz’ gouden doctoraat: een huldiging zonder weerga by F. Berends, published in 2014 by the Koninklijke Hollandsche Maatschappij der Wetenschappen.

Back to Pasadena


just the way he liked it: festive, but not too formal. In a letter to Berta, a week later, he looked back upon the day with great satisfaction. He did have to admit that, ahead of time, he had had mixed feelings about the whole affair. Yes, it was a very beautiful day, more beautiful than I could ever have imagined. I had, of course, been nervous about it, although in the last few days less than before, but the sense of dread disappeared immediately after the first words by De Sitter. [. . .] In this way it was a large ceremony, but still not “ceremonious” and that made it possible for me to keep a check on my emotions, even though sometimes, also on rereading and remembering, the tears have welled up in my eyes.101

Immediately after the celebration, Lorentz had another few busy days. There was a luncheon with the foreign guests, while others, Einstein among them, visited Lorentz in connection with the CICI. Thijsse came by as well, for some work on the Zuiderzee project, which was now beginning to near its completion. Time for relaxation was still scarce even after all this activity, for on December 19 Lorentz left for Paris on business for the Institute for International Cooperation, which was to open in January of the following year. The new year 1926 began with yet another festivity. On January 7 Rudolf married Maria Elisabeth van Vollenhoven, a music teacher. The following month, though, Lorentz was confronted with a sad event. On February 21 his old friend Kamerlingh Onnes passed away after a short illness, at the age of 72. Recently, his already weak health had been declining slowly but steadily; so much so, that he had not even been able to attend the dinner for Lorentz’s golden doctorate. At the funeral, on February 25, Lorentz was the only speaker.

Back to Pasadena After the postponement in connection with the celebrations for his golden doctorate, Lorentz was now ready for his next trip to the United States. On September 21, 1926, Lorentz and Aletta departed from Rotterdam for the crossing to New York: the second time in five years. Before his departure, Lorentz had done “a cleanup” of his activities, as he wrote to Zeeman in August.102 On September 1, the Zuiderzee Commission had met for the last time to approve the final report. Lorentz had also resigned from the Education Council and from the Comité de Direction of the Institut International de

101  “Ja, het was een heel mooie dag, mooier dan ik mij ooit had kunnen voorstellen. Ik had er ­ at­uurlijk wel tegen opgezien, schoon in de laatste dagen minder dan daarvoor, maar het gevoel van n beklemming week onmiddellijk na de eerste woorden van de Sitter.” [. . .] “Zoo was het wel een groote plechtigheid en toch niet ‘plechtig’ en dat maakte het mij mogelijk, mijne aandoening meester te blij­ ven, al zijn mij wel eens, ook bij het herlezen en herdenken, de tranen in de oogen gekomen.” Lorentz to Berta, December 18, 1925 (FC). 102   Lorentz to Zeeman, August 20, 1926 (ZA 107).


The final years: Travel and tributes

Coopération Intellectuelle. “When I am back [from the United States] in the spring, I will not know how easy a time I am going to have,” he wrote to Zeeman.103 Just like in 1921, they crossed on the Nieuw Amsterdam and they received special treatment. Again, they were seated at the Captain’s table and again they were given an extensive tour of the vessel. The first day, Lorentz familiarized himself with “the use of the stepladder to get up into the upper ‘berth.’ ” That did not give him much trouble, and neither did coming down, but “turning around up there in the small space at one’s disposal” remained a challenge.104 The voyage was easy. According to Aletta’s travel journal, there were no great waves and they did not participate in the “amusements” onboard. “We had no inclination at all, according to what we heard about it, to see the Charleston and similar negro-­dances [sic!].”105 Instead they read books. On their arrival in New York on October 1, they noticed that the formalities had become considerably more time-­consuming, since a new law had come into effect that made immigration into the United States much more difficult.106 Many forms had to be filled out, some in Rotterdam at the start of the journey. “The important matter in all of this turned out to be that one does not have the intention to remain in the United States,” Aletta noted.107 The immigration officer who received Lorentz noticed that he had also been in the United States five years earlier. He cordially welcomed him back and expressed the wish that he would be doing “good work.” Lorentz wrote: “I already felt flattered and encouraged and realized only later that I had once again been confused with my Austrian namesake.”108 That namesake was, again, the orthopedic surgeon Adolf Lorenz, with whom he had also been confused on his previous trip. In New York most of the luggage was sent on to Ithaca, but Lorentz and Aletta first traveled to Cambridge, Massachusetts, where Lorentz was to present a paper. On the 6th of October they arrived in Ithaca for a ten-­week stay. As Schiff Lecturer, Lorentz delivered a series of thirty lectures entitled “The Quantum Theory. Present Day Problems and Outstanding Questions on the Quantum Theory.” As usual, the two went on quite a few excursions to the surrounding countryside. For the first time they experienced Halloween and Thanksgiving. Other novel experiences for the couple were the large quantities of snow and the ­seriously

  “Als ik in het voorjaar [uit de VS] terug ben, zal ik niet weten hoe gemakkelijk ik het heb.” Ibid.   “het gebruik van het trapje om in de bovenste ‘berth’ te komen.” “het ronddraaien boven in de kleine ruimte die men tot zijn beschikking heeft.” 105  “Wij verlangden naar wat wij er van hoorden in het geheel niet de Charleston en dergelijke ne­gerdansen [sic!] te zien.” 106   The Johnson-­Reed Act of 1924. In order to realize the purpose of the act, “to preserve the ideal of U.S.  homogeneity,” immigration quota were introduced that chiefly affected immigrants from Southern and Eastern Europe and Asia. 107   “De hoofdzaak van dit alles bleek te zijn dat men niet voornemens is in de Vereenigde Staten te blijven.” 108   “goed werk.” “Ik voelde me al gevleid en bemoedigd en bedacht eerst later dat ik weer met mijn Oosterrijksche naamgenoot verwisseld was.” 103 104

Back to Pasadena


low temperatures, which caused a waterpipe to freeze in the house where they were staying. In early December they left the bitter cold behind to go to the West Coast. They traveled by way of Rochester, New York, where Lorentz gave two lectures at the Eastman Kodak factory, and then went on to Chicago and Madison, Wisconsin. After Madison they traveled to Portland, Oregon, for a visit to Lorentz’s cousin David Henny,109 and to San Francisco, where they took a short break on the coast to see Monterey, Carmel, and Santa Barbara. On January 5, Lorentz and Aletta arrived in Pasadena for the second time. Lorentz wasted little time and began his lecture series on January 11. It was a repeat performance of what he had done at Cornell. He gave three hours of classes each week, followed by an hour of question time, for an audience of forty. It was hard work for Lorentz, Aletta wrote, for “people know so much about the quanta here and have all, like Epstein, kept up to date on all the literature, so that Pa has to work hard to always keep ‘ahead of them.’ ”110 During Lorentz’s stay, the Austrian physicist Erwin Schrödinger also visited Pasadena as part of a three-­month tour of the United States. He arrived in ­mid-­February to give lectures over a two-­week period. A year earlier, Lorentz had carried on an extensive correspondence with Schrödinger about his then ­brand-­new wave mechanics.111 That topic was also on the agenda this time. In January, Lorentz and Schrödinger had already agreed on a division of labor. Lorentz was to discuss the foundations of the new mechanics and Schrödinger the more advanced topics.112 The lectures were scheduled one after the other: Schrödinger’s ran from a quarter to four until a quarter to five, and after a ­fifteen-­minute break for tea Lorentz took over for the next hour of class. That Schrödinger’s class had two Nobel laureates in the front row—Lorentz and Millikan—was somewhat intimidating for him. Yet he thoroughly enjoyed Lorentz. In his diary he called the lectures “of dear old Lorentz a special delight every time. [...] It is so beautiful to listen to him, one certainly feels guided along and not for a moment bored by unavoidable lengths—as almost always in other cases during a lecture on a topic discussed in a mathematical treatment.”113

109   Henny’s mother was a sister of Lorentz’s father. After emigrating to the United States Henny became a respected engineer, who, among other things, was involved in the building of the Hoover Dam. 110  “men weet hier zooveel van de kwanten en heeft zooals Epstein al de literatuur bijgehouden zoodat Pa hard moet werken om altijd ahead of them te blijven.” 111   Four letters in the months of May and June 1926 (Kox 2008, 412, 413, 416, and 418). See also Kox 2013a and Kox 2013b for discussions of the correspondence. 112   See Lorentz to Schrödinger, January 21, 1927 and Schrödinger to Lorentz, January 26, 1927 (Kox 2008, 420 and 421). 113   “des lieben alten Lorentz jedesmal ein besonderer Genuß. [. . .] Es ist so schön ihm zuzuhören, man fühlt sich sicher geführt und keinen Augenblick durch unvermeidliche Längen gelangweilt—wie sonst fast stets beim Vortrag eines Gegenstandes in mathematischer Behandlung.” Quoted in Meyenn 1982.


The final years: Travel and tributes

Schrödinger was generally quite happy during his time in the United States, but there was one detail that he was very unhappy about: prohibition, which had banned the sale and consumption of alcohol in the whole country since 1920. Interestingly enough, this does not seem to have bothered the Lorentz couple at all. None of the letters or travel journals by Aletta or Lorentz—who enjoyed an occasional glass of wine—devotes even one word to this total prohibition of alcohol. An important event during Lorentz’s stay was his participation in a two-­day small-­scale conference on the new results of Dayton Miller. This physicist had replicated the famous experiment by Michelson and Morley in Cleveland and had found results that appeared to indicate the existence of an “ether wind.” If that were true, the whole of physics would be upended, as one of the basic principles of the theory of relativity, the impossibility of demonstrating the motion of the earth through the ether, would then have been refuted. In physics circles Miller’s results were causing quite a stir, although there was considerable skepticism as well. At any rate, Einstein was not convinced, and Lorentz had serious objections.114 The Caltech meeting did not lead to a definitive judgment, but when others were unable to duplicate Miller’s results, the conviction grew that in his experiments he had not sufficiently considered possible disruptive influences like temperature fluctuations.115 After a final, more popular public lecture, on Friday, March 10, Lorentz and Aletta prepared to leave Pasadena. Just like five years earlier, they took the opportunity to further explore the United States. After a repeat visit of a few days to the Grand Canyon—where it snowed for an entire day—they traveled to Albuquerque, in New Mexico. From there they took a bus into the desert and on to Santa Fe and a few Indian reservations. By train they traveled onwards, first to Las Vegas, then to Chicago and on to Columbus, Ohio, for a lecture. From there they went to Washington, DC, where they stayed from March 24 to 29, after which they arrived in Philadelphia for a final lecture at the Franklin Institute on March 31. On April 7, Lorentz and Aletta departed again for the ocean crossing from New York, this time on the steamer Veendam, which arrived in Rotterdam on April 18.

CICI, Como, and Solvay Although Lorentz was looking forward to having somewhat of an easier time during the remainder of the year 1927, there were three important events ahead for him: one of a diplomatic nature and two scientific meetings. The first one was the ninth session of the CICI, which he chaired. The meeting, which marked the fifth anniversary of the Committee, took place from July 20 to

114  See CPAE-­15, Introduction, sec. V, and Lorentz to Zeeman, September 10, 1925 (Kox 2018, 270). 115  See Lalli 2012 for more details on Miller’s experiments and their ultimate refutation.

CICI, Como, and Solvay


26. In his opening speech116 Lorentz showed himself to be positive about the state of international collaboration: “More and more have the clouds begun to roll away and a new day to shine. Expe­rience has already clearly shown that the nations which were engaged in the war can begin again to work in harmony as they did formerly.” He then made a case for the existence of the Committee and the Institute. He listed tasks they could take on, such as making sure that important books were not lost but preserved for posterity. Of the scientific meetings, the first one was a large conference taking place in Como, Pavia, and Rome between September 11 and 20. There the most prom­in­ ent physicists would commemorate the one hundredth anniversary of the death of the famous Italian physicist Alessandro Volta. The organizers shamelessly used the meeting to extoll the greatness of Italy and boost the image of the new— fascist—regime. The jarring press image of several world-­ famous physicists, including Lorentz, Zeeman, and Millikan, sharing a table at a garden party with official host Mussolini, caused particular consternation in the Dutch newspapers, and Lorentz’s attendance provoked sharp criticism.117 Shortly thereafter, from October 24 to 29, the fifth Solvay Conference was held, attended mostly by the same participants as earlier Solvay Conferences. It dealt with the topic of “Électrons et photons” and was mainly characterized by heated discussions about all sorts of interpretation problems in the new quantum theory, which had made great advances in the past few years with the development of matrix mechanics and wave mechanics. Nowadays, the fifth Solvay Conference is well known, chiefly because of the discussion between Einstein and Niels Bohr about the interpretation of the “uncertainty relations,” then recently formulated by Werner Heisenberg. About this, by the way, there is nothing to be found in the conference proceedings.118 It is outside the scope of this book to go into this topic any further; all the more so because during the meeting Lorentz limited himself to making contributions to the discussion. Nevertheless, he was once again praised abundantly for the way in which he had chaired the meeting and for his thorough knowledge of the new physics, according to the letters of condolence to his widow that were written a few months later. The conference differed from the two previous ones, in 1921 and 1924, because now, for the first time after World War One, some Germans had also been invited. Of the twenty-­two participants, as many as six were from Germany this time. Although there had never been any official boycott of Germans by the Solvay Conferences, it had simply not been considered proper to invite German scholars for a meeting in Brussels under the auspices of a foundation that had such strong historic ties to Belgium.

  Minutes 1927, 9­–11.   The picture appeared in Algemeen Handelsblad, November 3, 1927; the socialist daily Het Volk wrote in its edition of November 9, 1927 that “it hurts” (pijn doet) to see Lorentz as a guest of Mussolini. 118   See also Bacciagaluppi and Valentini 2009 for a detailed description of the conference. 116 117


The final years: Travel and tributes

For one man an exception had been made, both in 1921 and 1924. It was Albert Einstein. Not only was he both a Swiss and a German citizen, he had also been quite outspoken during and after the war about how much he despised German militarism and nationalism.119 Actually, Einstein accepted neither invitation. In 1921 a trip to the United States prevented him from attending the conference, and in 1924 he felt that accepting the invitation would be disloyal toward his German colleagues. In a letter to Lorentz he wrote how he believed that one should separate politics and science and that individuals should not be held responsible for the acts committed by their country.120 An interesting detail is that the conference was interrupted halfway through by a short excursion to attend part of the elaborate commemorative ceremonies for the one hundredth anniversary of the death of Augustin Fresnel.121 On October 27, a group of eighteen participants left for Paris where the commemoration took place, among them a number of Germans. Lorentz expressed his great admiration for Fresnel in an evening lecture122 that was quoted in Chapter 1. The following day the whole group returned to Brussels, where the conference sessions were resumed in the afternoon. In the years since its establishment, the Solvay Conference had become an event of importance in Belgium—this much is clear. At the end of the conference, some participants, including Lorentz, Aletta, Einstein, and Bohr, were even received for a luncheon by King Leopold and Queen Elizabeth.

Como, Lorentz’s swan song The Solvay Conference had been dominated by the latest developments in quantum theory, but at the meeting in Como a month later, only the last day was devoted to the new physics. The main topic of the four preceding days had been classical physics, and in particular the heritage of Volta. On that last day Lorentz presented a paper that was to be his scientific swan song.123 For this reason, but also because of his choice of topic, it is worthwhile to briefly discuss its content. In his paper, Lorentz discussed the motion of a negative particle that not only revolved around a positively charged center, but also around its own axis. He marshalled the heaviest artillery of classical mechanics and electrodynamics, and not one single effect went unmentioned. He suggested, for example, the possibility that the particle changed shape under the influence of the electromagnetic forces that were present. Lorentz was able, once more, to show his complete proficiency

119  The first invitation, for that matter, had led to Edmond van Aubel, a physicist from Ghent, ­leaving the Commission Scientifique in protest (see the correspondence with Van Aubel in LA 2). 120   Einstein to Lorentz, August 16, 1923 (Kox 2008, 379). 121 122   See Lorentz to Went, November 1, 1927 (RB).   Lorentz 1927b. 123   Lorentz 1928c. Lorentz also gave a speech to conclude the meeting, once again emphasizing the importance of international collaboration (Lorentz 1928d).

Como, Lorentz’s swan song


in classical physics and, even today, he commands respect and admiration for the ease with which he was able to bend the complicated mathematical formalisms to his will. Yet, there was something curious about the paper. The title was “Sur la rotation d'un électron qui circule autour d'un noyau” (About the Rotation of an Electron that Revolves around a Nucleus). Lorentz was emphatically referring here to the atomic model of Niels Bohr, in which an atomic nucleus was surrounded by revolving electrons. He also followed Bohr in the implicit assumption that the electrons revolved in fixed, stable orbits. In Bohr’s model, the quantum theory already played an important role and in the years preceding the Como conference the role of quantum theory had only become greater in more advanced atomic models. In the meantime, a consensus was growing that it was no longer sensible to speak about particles, since matter could also manifest itself as waves. In the brand-­new theory of wave mechanics particles were viewed as some kind of spread-­out waves, so it was now possible only to speak about the probability of finding such an object in any given place. To begin with, even the title of Lorentz’s speech ran contrary to these new ideas. The younger physicists in the audience, who were meanwhile turning traditional physics completely on its head, must have heard the title with some surprise and possibly with some shaking of heads. Lorentz was well aware of this. About a special hypothesis he had worked out he first remarked that, because of it, his considerations might be a little less important. Then he continued: And one will consider this of even lesser importance, if one takes the point of view of the modern mechanics of quanta. All the more so, as the “classical” formulas that will be developed here will be able to serve as an introduction to the application of wave mechanics or matrix mechanics to our problem.124

Lorentz claimed here that his considerations could serve as the basis for the new theories, but it did not look very likely that his exposé would be of any use for the further development of quantum theory. It was couched too much in language that had little in common with the formalisms that were now being used to formulate the new theories of physics. In a certain sense, Lorentz had come full circle in his paper. After all the scientific developments he had lived to see and to which he had made essential contributions, he now reverted to “classical” physics, the physics of which he had been the undisputed master. And all of this took place at a conference session where attention was focused on quantum theory, the theory that had definitively done away with the presuppositions of classical physics.125

124   “On trouvera cet intérêt moindre encore si on se place au point de vue de la mécanique des quanta moderne. Tout au plus les formules “classiques” qui seront développées ici, pourront servir d’introduction à l’application à notre problème de la mécanique ondulatoire ou de la mécanique des matrices.” 125   The contribution to the conference by Niels Bohr, for example, was very influential.


The final years: Travel and tributes

One wonders whether it was a conscious decision by Lorentz to put himself so much on display as a classical physicist at the end of his long career. It cannot be claimed that he rejected the modern developments. He continued to follow the new theories with great care and interest, lectured about them, and discussed them in correspondence with his fellow physicists. In 1925 he had even pointed out the need to develop a new mechanics, “the mechanics of quanta, the mechanics of discontinuities,” which was to replace classical mechanics.126 Yet classical physics was still close to his heart. In this context it is interesting to note the reminiscences of the Russian physicist Abram Joffe, who quoted a conversation with Lorentz in 1924. In it, Lorentz presumably commented somewhat dejectedly on the uncertainties that quantum theory had brought to physics and questioned the use of carrying on in the face of these fundamental changes. Today I claim in a discussion of electromagnetic theories that the electron moving in a curved orbit emits energy, and tomorrow I say to the same audience that the electron revolving around the nucleus does not lose energy. Where lies the truth, if one can make contrary, mutually exclusive statements about this? Are we at all capable of recognizing the truth and does it make sense to occupy oneself with science?127

According to Joffe, Lorentz then proceeded to make a statement that is curious, at the very least: I have lost the conviction, that my work has led to an objective truth, and I do not know what I have lived for; I regret only that I did not already die five years ago, when everything still seemed clear to me.128

It is difficult to determine the veracity of these quotes, as Joffe’s book is not always reliable. Although it is undeniable that Lorentz’s life’s work had been undermined by the new theories of the late 1920s, the spirit expressed in Joffe’s quote contrasts starkly with Lorentz’s somewhat nostalgic, but certainly positive outlook on life on his seventieth birthday, only one year earlier.

  “la mécanique des quanta, la mécanique des discontinuités.” Lorentz 1925h, 110.   “Heute behaupte ich bei der Besprechung der elektromagnetischen Theorien daß das auf einer krummlinigen Bahn sich bewegende Elektron Energie aussendet, und morgen sage ich demselben Auditorium, daß das um den Kern kreisende Elektron keine Energie verliert. Wo liegt die Wahrheit, wenn man darüber sich gegenseitig ausschließende Behauptungen machen kann? Sind wir überhaupt fähig, die Wahrheit zu erkennen, und hat es Sinn, sich mit der Wissenschaft zu befassen?” Quoted in Joffe 1967, 60. 128   “Ich habe die Überzeugung verloren, daß meine Arbeit zu einer objektiven Wahrheit geführt hat, und ich weiß nicht, wofür ich gelebt habe; ich bedaure nur, daß ich nicht vor fünf Jahren gestorben bin, als mir noch alles klar erschien.” Ibid. 126 127

Illness and death


Illness and death Lorentz started the new year in 1928 in good spirits and in good health.129 However, he was not looking forward to a new round of anniversary festivities to celebrate the fifty years of his professorship on January 25. After the still so recent 1925 celebrations, he hardly relished the thought of again being the center of attention, so he did everything in his power to prevent another official celebration. When he got wind of plans by the daily De Telegraaf to publish a piece about him for the occasion, he insistently asked the paper’s editors to abandon this idea.130 De Haas, professor in Leiden since 1924, was commissioned to notify the Leiden faculty informally that Lorentz did not want any more fuss.131 That worked, but obviously there was still a stream of congratulatory letters, from Zeeman and Ehrenfest among others. Ehrenfest did not limit himself to congratulations alone. He could not help himself and immediately launched into one of his customary emotional expressions of inferiority: Unfortunately, it is now so that I must disappoint you in many essential expectations, especially regarding diligence, uniformity in scientific work and in the fulfillment precisely of the most normal duties.132

Whatever celebratory program may still have been in the works, though, all plans were thwarted when Lorentz fell ill. On Monday, January 16, Lorentz went home with a fever after finishing his lecture. He turned out to have facial erysipelas, an acute and painful bacterial skin infection usually accompanied by high fever. A small lesion is often enough to cause an infection, and weak health can facilitate the condition. Erysipelas in the face is considered one of the more serious forms and is often accompanied by very high fever, intense pain, and sometimes edema. Nowadays, erysipelas can be easily cured with antibiotics. In a letter to Einstein at the end of January, Ehrenfest provided some more detail about Lorentz’s condition.133 The start of the infection was probably a small lesion on Lorentz’s lip or in the corner of his mouth. Ehrenfest wrote, at any rate, that the infection had begun on the mouth and had subsequently spread via the nose to the face. He called the situation very serious and even entertained the ­possibility of a fatal outcome.

  According to his daughter Berta in Haas-­Lorentz 1957.  Already at the end of November a number of dailies, among them De Telegraaf, the Nieuwe Rotterdamsche Courant, De Tijd, Het Vaderland, and Algemeen Handelsblad, had published announcements of the upcoming anniversary. 131   See Lorentz to Zeeman, December 28, 1927 (Kox 2018, 292). 132   “Leider ist es ja so, dass ich Sie in vielen wesentlichen Erwartungen enttäuschen musste vor allem was Fleiss und Gleichmässigkeit in wissenschaftlicher Arbeit und in der Erfüllung gerade der meist normalen Pichten betrifft.” Ehrenfest to Lorentz, January 24, 1928 (Kox 2018, 293). 133   Ehrenfest to Albert Einstein, January 25, 1928 (to be published in CPAE, Vol. 16). 129 130


The final years: Travel and tributes

Apparently, Lorentz’s illness did not develop very fast initially, but on January 20 he had to take to his bed and was no longer able to finish a letter he was writing to Millikan. The unfinished letter would be his last. Not much else is known about the cause of Lorentz’s illness. Alfred Zimmern, one of Millikan’s collaborators, wrote later that he had heard from friends of Lorentz’s while attending the funeral that Lorentz had become visibly weaker during the last months of 1927 and that he had suffered from pneumonia earlier that year.134 Berta, however, does not mention any of this in her biographical notes. Illness was actually not something that Lorentz had had many dealings with during his lifetime. In his speech for Lorentz’s golden doctorate, De Sitter recalled a story that Lorentz had only been ill once, but that even then he had just continued his lectures. On January 23, the newspapers first reported about “Professor Lorentz’s indisposition,” and from then on the readership was being kept informed about his condition on a daily basis. Apparently, the family issued daily medical bulletins. On January 24 and 25, Lorentz’s condition was reportedly deteriorating. He had a high fever of more than 39°C (more than 102°F). On the evening of January 26, his condition improved and his fever went down to slightly above 38°C (100°F). Although his temperature continued to fluctuate over the next few days, Lorentz seemed to be improving, and briefly there was some hope for a complete recovery. On Thursday, February 2, however, the fever increased sharply and he took a turn for the worse. The following evening Lorentz’s condition was even called “alarming.” On the afternoon of Saturday, February 4, around one-­thirty, Lorentz passed away “peacefully.”135 The sad tiding was announced in a grandiose manner in the national dailies. Immediately a stream of commemorative articles began to appear in all the news media. On behalf of the Royal Family, Prince Consort Hendrik came to the Lorentz home to pay his respects to the widow.136 Lorentz’s funeral took place on February 9, amid great public interest.137 Preceded by a carriage completely filled with flowers—in spite of the family’s request to refrain from sending flowers—and followed by a procession of fifteen carriages, Lorentz’s body made its final journey through the center of Haarlem.138 134   Alfred Zimmern to Robert Millikan, February 10, 1928 (Caltech Archives). Zimmern was deputy director of the Institut International de Coopération Intellectuelle. 135   It is ironic that, later in that same year of 1928, the Englishman Alexander Fleming discovered penicillin. It should be remembered, though, that this antibiotic could not be isolated successfully in useful quantities until 1940. 136   The Prince Consort was unable to attend the funeral because he had to attend the opening of the Winter Olympics in St. Moritz. 137   A few fragments of newsreels have also been preserved, which show how the coffin is brought out through a ground floor front window in Lorentz’s home and how Einstein gets into one of the carriages. There is also footage of the procession of the carriages. 138   According to a report in the Nieuwe Rotterdamsche Courant of February 9, apart from the family, twenty-­ eight people were seated in the carriages. Among them were representatives of Queen Wilhelmina and Prince Hendrik, the mayor of Haarlem, representatives of Leiden University, and Dutch and foreign colleagues like Pieter Zeeman, Paul Ehrenfest, Ernest Rutherford, Paul Langevin, Marie Curie, and Albert Einstein.

Illness and death


Bells were ringing from the steeples of all the churches and from the town hall, including the Roeland Bell of the Great Church of Haarlem, which dated back to 1503 and was rarely heard. Flags were flown at half-­mast on many houses and stores and on all public buildings. Through the dense crowds the funeral procession progressed slowly, from the Julianastraat, the street where Lorentz had lived, through the old center of Haarlem, to the Grote Markt. On the square an enormous crowd had gathered, and people reverently tipped their hats when the carriage with Lorentz’s coffin approached. In front of the town hall the procession came to a halt, and a wreath was placed on the coffin on behalf of the City Council. On it went, through the city to the Kleverlaan Cemetery. As a national show of respect, the National Telegraph Service was suspended for three minutes at noon, the hour of the funeral. As the newspapers wrote, this event was all the more impressive because “the staff spontaneously got up from their seats in empathic reverence.”139 At the cemetery, a large crowd awaited the eulogies and the interment.140 The first, and most important, eulogy was delivered by Ehrenfest. In a touching note on February 5, Aletta had asked him to speak: We all think that my husband would be very glad to know that a single friend was to speak an affectionate word at his graveside, he did not like many speeches but no speech at all, he always said, is certainly very short and cold. Would you as his friend and successor oblige us by making a short speech at my husband’s graveside; be assured that I and all the children will be very grateful to you.141

Ehrenfest came up with more than a cordial word. It was an exceedingly emotional outpouring of admiration and deference, reflecting the extreme emotions to which he had fallen prey after Lorentz’s death. For some in the audience, his eulogy may have been surprising, and even nowadays, readers might shake their heads at the many superlatives. The curious form and the emotional wording make it worthwhile to reproduce a major part of the text here: Hendrik Antoon Lorentz dead. Death closing the eyes of Lorentz—the old doubt, the age-­old question rises up in our hearts: What can still be the meaning of life then?

  “het personeel zich in begrijpende vereering spontaan van de zetels verhief.”  The Nieuwe Rotterdamsche Courant of February 9 lists more than sixty names, among them various dignitaries and representatives of the Dutch academic community. 141   “Wij allen meenen dat mijn man het ’t meest op prijs zou stellen wanneer hij wist dat een enkele vriend een hartelijk woord aan zijn graf zou willen spreken, hij vond veel toespraken niet sympathiek en heelemaal zonder toespraak zeide hij altijd, dat is toch wel erg kort en koud. Zoudt gij nu als zijn vriend en opvolger ons den dienst willen bewijzen om aan het graf van mijn man een korte toespraak te houden; wees verzekerd dat ik en de kinderen allen, U daarvoor zeer dankbaar zullen zijn.” Aletta to Paul Ehrenfest, February 5, 1928 (Kox  2018, 294). Aletta’s first choice as speaker had been Pieter Zeeman, but she had ceded to strong pressure from the Leiden colleagues to invite Ehrenfest instead (see Aletta to Zeeman, February 5, 1928, ZA 81). 139 140


The final years: Travel and tributes

And we ponder on the life of Lorentz. And our hearts are moved in their depths. Through our doubt grows a warming feeling of certainty, to our surprise. At first, we do not dare to admit it. Not to ourselves, not to our closest friends. Whence, at this grave, this uplifting feeling of certainty? It is an honorable place, that the fatherland, that the world, accorded Lorentz and great are the tributes that the fatherland and the world have bestowed on him. Sincere appreciation accompanies him and, what is more, sincere love. But we must search deeper for the source of our certainty. The investigator Lorentz, the teacher! Truly beautiful is what he was privileged to find in his science, to achieve; in physics, which he loved so dearly. [...] And we lose ourselves more deeply in viewing the image of this man. In viewing this so completely mature harmony. In each of his expressions of life his entire personality reveals itself to us, as if by miracle, time and again with full intensity. And we can no longer distinguish line from line and we would not even want to! The irony in his finely expressed smile, the irony, through which he relaxes our souls, where in his smile does the irony separate itself from the silence of the emotion with which he helps us to ennoble our joy? [...] Truly, Lorentz is faithful! He enjoys fully the great beauty of California or of the Alps. But at home he feels in the woods and on the heath of the Veluwe and in the dunes. And so it is for him with the canals, with our Leiden Academy Building and with the small lecture room in the physics laboratory. And so it is for him with the sweet, familiar sounds and words of his mother tongue. So it is for Lorentz, the world citizen! Lorentz, whose mind is so receptive, he is true to himself. Hence his deep respect for the personality of his fellow humans and for the freedom of everyone’s truly lived conviction. Confronted with imperiousness, coercion, and cunning, one sees Lorentz suddenly lose his calm. And regain with difficulty. Regain for the tough struggle against these for him most serious enemies of human value. And always he carries on this struggle from within, he distrusts the struggle from without, from the depth of his being. So he leads us here at home towards tolerance and he does so abroad, among the nations. And Lorentz remains in his devout faithfulness to the very last a helper in times of need, the comforting consoler in bitter sorrow. He, whose heart, until the last, is so youthfully involved in the work of the youngest, he whose warm love, until the last, accompanies precisely that growing life—he faces death with the calm of wisdom, when death comes to claim a well-­spent life. [. . .]142

142   “Hendrik Antoon Lorentz dood. Dat de dood de oogen van Lorentz vermocht te sluiten—de oude twijfel, de oer-­oude vraag rijst in ons hart op: Wat kan dan nog de zin van het leven zijn? En wij peinzen over het leven van Lorentz. En ons hart wordt in zijn diepte bewogen. Door onzen twijfel heen groeit een verwarmend gevoel van zekerheid, tot onze verrassing. Wij durven het eerst niet bekennen. Niet aan ons zelven, niet aan onze naaste vrienden. Van waar, aan dit graf, dit verheffend gevoel van zekerheid? Het is een

Illness and death


After the funeral, Ehrenfest suffered for days with severe headaches that made it impossible for him to work.143 Despite Aletta’s wish that only “one good friend” would speak at the graveside, there were three more speakers who represented the international scientific community. First Ernest Rutherford, on behalf of the London Royal Society, and then Paul Langevin, in the name of French science and the Solvay Institute. Finally, Albert Einstein stood up to speak, not only to represent the Prussian Academy of Science, but especially as a “student and loving admirer.”144 In his short speech he said about Lorentz: “His life he has shaped like a costly work of art, up to the smallest things,”145 an echo of the statement during the ­celebration of the golden doctorate quoted earlier in this chapter. Lorentz’s son Rudolf ended the ceremony with a word of thanks and the request to scatter ­flowers on the grave. In the days after the funeral, the newspapers were full of commemorative ­articles, often phrased in stilted superlatives.146 Few, however, have been able to

eervolle plaats, die het vaderland, die de wereld aan Lorentz toewees en groot zijn de eerbewijzen, die het vaderland en de wereld hem schonken. Oprechte waardeering begeleidt hem en wat meer is, oprechte liefde. Maar wij moeten dieper zoeken naar de bron van onze zekerheid. De onderzoeker Lorentz, de leermeester! Waarlijk schoon is wat hem vergund was in zijn wetenschap te vinden, te volbrengen; in de natuurkunde, die hij zoo van harte liefhad. [. . .] En wij verliezen ons dieper in de beschouwing van het beeld van dezen man. In de beschouwing van deze zoo volkomen gerijpte harmonie. In ieder van zijn levensuitingen openbaart zich ons, als door een wonder, telkens weer met volle spanning, zijn heele persoonlijkheid. En wij kunnen niet meer lijn van lijn scheiden en wij zouden het ook niet willen! De ironie in zijn fijn sprekenden glimlach, de ironie, waardoor hij onze ziel ontspant, waar scheidt zich in zijn glimlach deze ironie van de stilte der ontroering, waarmee hij ons helpt onze vreugde te veredelen? [. . .] Waarlijk Lorentz is trouw! Hij geniet volop van de grootsche schoonheid van Californië of van de Alpen. Maar thuis gevoelt hij zich in de bosschen en op de heide der Veluwe en in de duinen. En zoo gaat het hem ook met de grachten, met ons Leidsch Academiegebouw en met de kleine college-­kamer in het natuurkundig laboratorium. En zoo gaat het hem met de lieve, vertrouwelijke klanken en woorden van zijn moedertaal. Zoo gaat het Lorentz den wereldburger! Lorentz, wiens geest zoo ontvankelijk is, hij is trouw aan zich zelf. Vandaar zijn diep ontzag voor de persoonlijkheid van zijn medemensch en voor de vrijheid van ieders oprecht-­doorleefde overtui­ ging. Tegenover heerschzucht, dwang, sluwheid ziet men Lorentz ineens zijn kalmte verliezen. En moeilijk herwinnen. Herwinnen voor den taaien strijd tegen deze voor hem ergste vijanden van menschenwaarde. En steeds voert hij dezen strijd van binnen uit, hij wantrouwt den strijd van buiten af uit de diepte van zijn wezen. Zoo leidt hij ons hier thuis tot verdraagzaamheid en hij doet het daarginds tusschen de volken. En Lorentz blijft in zijn vrome trouw tot het laatst de helper in nood, de verkwikkende trooster in bitter verdriet. Hij, wiens hart tot het laatst zoo jeugdig bij het werk der jongsten is, hij, wiens warme liefde tot het laatst juist het groeiende leven begeleidt—hij staat met de kalmte der wijsheid tegenover den dood, wanneer de dood een welbesteed leven komt opeischen. [. . .]” See Ehrenfest 1928 for the full text. 143   See Ehrenfest to Albert Einstein, February 13, 1928 (to be published in CPAE, Vol. 16). The Leidsche Courant announced on February 15: “Prof. P. Ehrenfest [...] has been indisposed since a few days ago and will not be lecturing this week.” (Prof. P. Ehrenfest [. . .] is sedert enige dagen ongesteld en zal deze week geen college geven.) 144   “Schüler und liebender Verehrer.” 145   “Sein Leben hat er gestaltet wie ein köstliches Kunstwerk bis ins Kleinste.” (Einstein 1934, 25). Of the eulogy there are, apart from the version published in 1934, a number of short newspaper reports, for example in the Nieuwe Rotterdamsche Courant of February 9. 146   There were some critical notes as well. Apart from the sour remark in the Tribune, quoted in the prologue, an anonymous author complained about the secular nature of the funeral ceremony: “Whoever does not place himself in the light of eternity when pondering the deceased, can speak


The final years: Travel and tributes

formulate Lorentz’s role on the crossroads of classical and modern physics as strikingly as Gilbert Murray, his successor as chairman of the CICI: The solid world in which we were educated in the eighties has dissolved beneath our feet. The gelatinous, incompressible ether, the indivisible atoms, the indestructible solid matter, the law of gravitation itself, seem no longer to offer a firm basis for our faith or speculation. The comprehensible, or apparently comprehensible world of the nineteenth-­century physicist has disappeared and in its stead we are confronted with a world which, as it seems to a layman, can at present neither be comprehended nor explained, nor even described save in mathematical formulae. [...] All through the process from the first stage to the last, originating, assimilating, combining, ­co-­ordinating, a spring of new suggestion in himself and a constant encouragement to the ideas of others, presided the luminous intelligence and serene equanimity of Hendrik Antoon Lorentz.147

On the day after the funeral, during his commemorative speech in the great auditorium of the university, even Einstein, who was generally not prone to hyperbole, could not completely suppress a tendency toward superlatives. He characterized Lorentz as “both the noblest and the most powerful personality, the greatest and strongest human being that I have known.”148 The better part of his speech was devoted to Lorentz’s work, but at the end Einstein emphasized Lorentz’s efforts to promote international cooperation, while making the interesting observation that “Lorentz cannot be praised highly enough, because he has surmounted within himself the doubt and the skepticism, these characteristically European afflictions.”149 Twenty-­five years later, in an article on the occasion of the one hundredth anniversary of Lorentz’s birth, he echoed his earlier characterization of Lorentz with the words: “He was, to me personally, more than all others that I have encountered on my life’s journey.”150 Already on the day before the funeral, the Haarlem City Council had decided to rename the Van Eedenplein, the little park that could be seen from Lorentz’s house on Julianastraat 49. From then on, it would be known as Lorentzplein, and in the park a bust of Lorentz would be placed. The bust, made by the sculptor Arend Odé, was unveiled by Prince Consort Hendrik in 1929, and is still on display there. It was the first of many opportunities to honor Lorentz posthumously by naming streets or institutions after him.

­ eautifully, but [. . .] leaves the heart cold and the mind dissatisfied. Human nature cries then for light b from heaven [. . .] even though one does not dare admit it!” Leidsche Courant, February 11, 1928.   Minutes 1928, 9.  zoowel de edelste en krachtigste persoonlijkheid, als de grootste en sterkste mensch dien ik gekend heb.” The quotes are from a Dutch text in the Nieuwe Rotterdamsche Courant of February 11, 1928. The text is most likely based on notes by Ehrenfest (Ehrenfest Archive, RB). 149  “Lorentz niet hoog genoeg geprezen kan worden, omdat hij de twijfel en sceptisme, deze karakteristiek-­Europeesche ziekten, in zichzelf overwonnen heeft.” 150  “Er war mir persönlich mehr als alle andern, die mir auf dem Lebenswege begegnet sind.” Einstein 1953, 7. 147 148

In conclusion


The most important and largest monument to Lorentz is the one in Park Sonsbeek in his native Arnhem, the initiative of a national committee. It was sculpted by the Dutch sculptor Ludwig Oswald Wenckebach and was unveiled on September 10, 1931 by Princess Juliana. A larger than life-­size statue of Lorentz is flanked on either side by stone tablets showing images of six famous predecessors and contemporaries of Lorentz whom he admired. On the left are Christiaan Huygens, Augustin Fresnel, and James Clerk Maxwell, on the right, Max Planck, Albert Einstein, and Niels Bohr. Unfortunately, in a misguided attempt to “update” the monument in 2008, it has been disfigured by chiseling the names of no fewer than 142 physicists from the years after Lorentz in the stone tablets flanking Lorentz’s statue.

In conclusion Aletta moved to Amsterdam in the fall of 1928 to be closer to Hannie’s family, “the Hannen.” She died there, three years later, at the age of 73. During her final years she received generous financial support from the Lorentz Fund.151 Wander de Haas, together with Willem Keesom, succeeded Kamerlingh Onnes as professor and director of the Leiden laboratory and retained that position until his retirement in 1948. He came out of World War Two with a somewhat tarnished reputation.152 Berta remained active in physics, for example by publishing a textbook on thermodynamics, but she never had a paid position. She died in 1973, thirteen years after her husband Wander. They had five children, some of whom changed their surname to “Lorentz de Haas.” Henri Leemhorst became the mayor of the town of Hoorn in 1933. During World War Two, he refused to cooperate with the Germans. He had to go into hiding and was replaced in 1943 by a mayor who belonged to the Dutch National Socialist Party. Henri died in 1973 and Hannie in 1980. They had four children. Rudolf Lorentz studied Classics in Leiden and obtained a doctorate by writing a dissertation on a topic from church history. In 1925 he became the librarian of Teylers Foundation. He died in 1977.

151   She received a yearly allowance of 3,000 guilders, although, as she admitted herself, she really did not need the money (see Aletta to Ehrenfest, April 6, 1928 [Ehrenfest Archive, RB]). According to an official document in LA 686, Lorentz’s capital amounted to 91,000 guilders at the time of his death, of which Aletta inherited 66,000 guilders. 152  See Delft and Hoeneveld 2011.


This book began with the question of who Hendrik Antoon Lorentz was, and why he has become such a towering figure in physics. His life, as described in this book, shows a more nuanced view of Lorentz’s personality than is apparent from the superlative terms in which he is frequently praised as a figure of almost superhuman dimensions. Lorentz is often presented as an ideal human being in every sense, a brilliant scholar, a universally loved, serene personality. Despite the almost complete lack of really personal biographical material, this book shows clearly that Lorentz was, above all, a supportive husband and father, a faithful friend, a gifted teacher, and a good colleague. He was helpful, interested in others, and prepared to invest energy in matters he believed in. In his personal life, Lorentz was somewhat reserved and averse to affectations. For him it counted as a compliment to call someone “modest.”1 He regularly demonstrated a refined sense of humor, especially in his speeches. Lorentz was a warm and involved father to his children, who always signed his letters to them with “Your loving Pa.”2 He showed much personal involvement in his students and doctoral candidates. Apart from all this, or perhaps in the first place, he was a brilliant and extremely influential physicist with a great passion for his work and impressive energy and staying power. He received a great deal of recognition, nationally and internationally, and this recognition was absolutely deserved. In his scientific work, Lorentz showed a praiseworthy quality that not all scientists have. Work by others that was contrary to his own ideas he was willing to evaluate in an honest and balanced manner, while at the same time he continued to prefer his own ideas. An example is his attitude toward Einstein’s theory of relativity. He admired the theory and he even lectured about it, without renouncing his own totally different approach. A second example is his correspondence, in 1920, with Willem Julius, about his theory on the solar atmosphere.3 Although it is clear from Lorentz’s letters that he held no brief for this theory, time and again he commented extensively on Julius’s arguments, trying patiently to convince him of their incorrectness, even when Julius decided, on dubious grounds, that the general theory of relativity had to be rejected. 2  “eenvoudig.”   “Je liefhebbende Pa.”   Published in Kox 2018. Soon after Julius’s death in 1925, his theory was completely forgotten.

1 3

“A Living Work of Art”: The Life and Science of Hendrik Antoon Lorentz. A. J. Kox and H. F. Schatz, Oxford University Press (2021). © A. J. Kox and H. F. Schatz. DOI: 10.1093/oso/9780198870500.003.0014



Lorentz’s life went by in great harmony, without far-­ reaching conflicts or dramatic events, at least as far as can be gathered from the material that has been preserved. There were very few days that passed without him being involved in scientific work in one way or another. In the development of his illustrious career he was supported by his wife Aletta, who busied herself—as was the norm in those days—with household duties and child care. She also found time to be active in the women’s movement and women’s suffrage, something that was quite unusual at the time. Until World War One, Lorentz’s social activities were limited and mainly related to his work as a professor of physics. His daily existence was rather clearly laid out and was mainly focused on science. The war caused great upheaval, and for Lorentz it was a traumatic experience. The human suffering, the great destruction, and the hate between the warring nations, also in the world of science, motivated him to use his by then considerable international authority to promote peace among the nations. His energetic involvement in the Conseil International de Recherches and his membership of the Commission Internationale de Coopération Intellectuelle must be viewed mainly in this light. Another expression of his social conscience was Lorentz’s chairmanship of the Zuiderzee Commission, a role he took on a few months before the end of the war. This work, time-­consuming though it was, captured the imagination in the Netherlands and was of great consequence for generations to come. Lorentz’s great scientific reputation and his social activities contributed enormously to his fame in the Netherlands. Yet there was more to his enormous fame. In the eyes of the general public in the Netherlands, his friendship with Albert Einstein made him one of the few who could fathom the mysterious theory of relativity and who was able to discuss it with him on an equal footing. When Einstein came to visit Lorentz in Haarlem, the neighborhood was abuzz: “Einstein is visiting professor Lorentz!”4 Especially after the confirmation of the theory of relativity by the solar eclipse expeditions of 1919 and the crystal-­clear explanation by Lorentz in the daily Nieuwe Rotterdamsche Courant, Einstein’s great reputation also reflected on Lorentz. Another important reason for Lorentz’s fame in the Netherlands was that he lived in an age in which science and its practitioners were held in high esteem, and also in an age in which society had great expectations of the results of new scientific discoveries. The daily and weekly press regularly published articles by scientists about the latest developments in their area of expertise and reports on the discussions during meetings of the Academy also appeared in the newspapers. Lorentz was an ambitious man who, as he grew more famous, was well aware of his status as an international scientific luminary. This was why he found it difficult sometimes to accept diverging points of view. In his own mind he was convinced that his ideas—rational, after all, and well thought-­out—were evidently the most sensible. The troubles in the Academy in connection with the CIR and   Communicated to one of the authors (AJK) by J. C. Leemhorst, grandson of Lorentz.




the accusations by the Academy members from Groningen are clear illustrations of what could be called Lorentz’s blind spot. Also typical was his reaction to the decision by the Nederlandse Natuurkundige Vereniging to bow out of the Union de Physique: he was “in a huff.”5 Albert Einstein made a striking observation in an article about Lorentz’s involvement in the efforts for international cooperation. He compared Lorentz to a doctor who administers a medication to a recalcitrant patient because it is good for him, even though the patient is unwilling to see it that way.6 Until a ripe old age, Lorentz was a curious man, greatly interested in the world around him. His curiosity did not only express itself in how he stayed abreast of the latest professional developments until the very end of his life, but also in his fondness for travel, something that was quite unusual in those days. Without a doubt he was also encouraged by Aletta, who was equally curious and interested to find out what new experiences their often lengthy trips would bring them. In personal matters, Lorentz sometimes showed a certain degree of naivete and a lack of empathy with other people’s feelings. His role in promoting the petition by Van Suchtelen illustrates how Lorentz, because of his idealism—and in this case probably also a dose of naivete—was not at all prepared for the predictably very negative reactions the petition would provoke. His inability to react adequately to other people’s feelings was also apparent in his rather tepid reaction to the tormented and emotional outpourings by his successor Paul Ehrenfest. He often showed a certain degree of discomfort with such extremes of emotion. Lorentz himself was definitely not a man of great emotional outbursts, of extreme peaks and troughs—or certainly not in the public sphere. For the most part, his daily existence reflected his personality. Fully in accordance with the standards of decency, it was uneventful, frugal, and perhaps a trifle dull, at times. His frugality and concern for money were evidenced in his bargaining about remuneration, despite the generous salary he enjoyed throughout his career. That Lorentz had little use for extremes explains his tendency to avoid conflict. He took it for granted that he could—of course—rely on others to behave reasonably. As Adriaan Fokker put it in a biographical article: “When it was possible to avoid a conflict, he would rather circumnavigate the obstacle. That fit in completely with his sensible, thoughtful sense of reason.”7 In his many efforts to mediate and play a conciliatory role in the restoration of international relations, this characteristic trait appears to have been more of a hindrance than a help to him.8 Lorentz was often hesitant in the face of demands or requests, as evidenced by his reluctance to say no to Kamerlingh Onnes when he was asked to take over 6   “in zijn wiek geschoten.”  See Einstein 1928.   “Wanneer het mogelijk was een conflict te vermijden, omzeilde hij de klip liever. Dat paste geheel in zijn verstandige, bedachtzame redelijkheid.” Fokker 1946, 63. 8   In a 1963 interview, Fokker commented on the failed effort to admit Germany to the CIR: “Bohr sometimes told me that he thought that if Lorentz had used his authority a little more frankly, then the thing would have come out all right.” Interview of Adriaan Fokker by John L. Heilbron on April 1, 1963, Niels Bohr Library & Archives, American Institute of Physics, College Park, MD USA. 5 7



the classes for medical students. Fokker suggested that Aletta had protected Lorentz many times from people “who wanted to abuse his all too trusting nature,”9 but Lorentz’s tendency to avoid conflict may also have played a part. It is certainly transparent in Lorentz’s somewhat surprising refusal to speak out in the case of Sacco and Vanzetti, the two anarchists who were sentenced to death and executed in the US among worldwide protests.10 From a letter by Aletta to Ehrenfest, after Lorentz’s death, it is clear that at home, Lorentz was not always quite so controlled and so consistently good humored as his serene and balanced public persona suggested. If he felt that he was not being listened to, she wrote, “then he could be so upset and worked up that I became very worried.”11 Further qualification of Lorentz’s idealized image is to be found in a letter of condolence to Aletta from one of his close colleagues: “We all know how much you meant to him and how much support you have given to him, for to him life was not so easy as it appeared because of his equanimity.”12 As for the stirrings of Lorentz’s soul, the diary of author Frederik van Eeden allows a slightly more intimate glimpse into his reflections on death and the hereafter. In one entry Van Eeden noted: “He said that he could not believe in a continued personal existence. He did believe in the spirit being primary, and viewed himself as part of a great whole. But he believed that he would then want to be finished as a person. He had no need for a personal continued existence.” In a later entry Van Eeden wrote: “We had long conversations. About Freud and his work, about existence after Death. Lorentz thought about it as he thought about a faint spell he had had. That is how dying must be.”13 To summarize this analysis of Lorentz as a person, the conclusion is justified that Lorentz was a giant among the physicists of his day. It is also clear— fortunately—that he was a man of flesh and blood, with his flaws and foibles. His human weaknesses, however, pale in comparison with his great achievements for the development of natural science, both in the Netherlands and abroad. In the final analysis, what remains is the image of Lorentz in Einstein’s florid language: “A living work of art.”

  “die van zijn goed vertrouwen misbruik hadden kunnen maken.” Fokker 1946, 63.   See Lorentz to Pieter Zeeman, August 28, 1927 (ZA 107). The controversial conviction in 1921 of Nicola Sacco and Bartolomeo Vanzetti gave rise to worldwide protests. After a series of unsuccessful appeals they were executed in 1927. Fifty years later they were exonerated. 11   “dan kon hij zoo opgewonden en druk worden dat ik mij erg bezorgd maakte.” Aletta to Ehrenfest, February 11, 1929 (Ehrenfest Archief, RB). 12   “Wij allen weten hoeveel u voor hem waart en hoeveel steun u hem hebt verleend, want zóó ge­makkelijk als het door zijn evenwichtigheid scheen, was hem het leven toch niet.” D. J. Korteweg to Aletta, February 4, 1928 (LA 672). 13  “Hij zei dat hij niet aan een persoonlijk voortbestaan kon gelooven. Hij geloofde wel in het primair-­zijn van den geest, en achtte zichzelf een deel van een groot geheel. Maar hij meende dat hij als persoon dan wel wenschte afgedaan te hebben. Hij had geen behoefte aan persoonlijk voortbestaan.” “We hadden lange gesprekken. Oover Freud en zijn werk, oover het bestaan na den Dood. Lorentz dacht er aan zooals hij dacht aan een flaauwte die hij gehad had. Zoo moest doodgaan zijn.” Eeden 1971, entries of July 9, 1913 and March 20, 1914. Van Eeden was also a psychiatrist. 9



The short titles of the publications by Lorentz are in conformity with those in the complete Lorentz bibliography in Kox 2008. The abbreviation C.P. refers to Lorentz’s ninevolume Collected Papers. Arnhem 1933 Arnhem. Zeven eeuwen stad. Arnhem: Hijman, Stenfert Kroese en Van der Zande, 1933. Atti 1909 Atti del IV Congresso Internazionale dei Matematici (Roma, 6–11 Aprile 1908). Roma: R. Accademia dei Lincei, 1909. Bacciagaluppi and Valentini 2009 Bacciagaluppi, Guido and Antony Valentini. Quantum Theory at the Crossroads: Reconsidering the 1927 Solvay Conference. Cambridge: Cambridge University Press, 2009. Badash 1969 Badash, Lawrence, ed. Rutherford and Boltwood. Letters on Radioactivity. New Haven: Yale University Press, 1969. Barkan 1999 Kormos Barkan, Diana. Walther Nernst and the Transition to Modern Physical Science. Cambridge: Cambridge University Press, 1999. Barry 2018 Barry, John M. The Great Influenza. New York: Penguin Random House, 2018. Berends 2015 Berends, Frits. “Lorentz, the Solvay Councils and the Physics Institute.” European Physical Journal Special Topics 224 (2015): 2091–2111. Berkel 1985 Berkel, Klaas van. In het voetspoor van Stevin. Meppel, Amsterdam: Boom, 1985. Berkel 2008 Berkel, Klaas van. De stem van de wetenschap. Geschiedenis van de Koninklijke Akademis van Wetenschappen. Vol. 1, 1808–1914. Amsterdam: Bert Bakker, 2008. Berkel 2011 Berkel, Klaas van. De stem van de wetenschap. Geschiedenis van de Koninklijke Akademis van Wetenschappen. Vol. 2, 1914–2008. Amsterdam: Bert Bakker, 2011. Born 1969 Born, Max, ed. Albert Einstein, Hedwig und Max Born. Briefwechsel 1916–1955. Munich: Nymphenburger Verlagshandlung, 1969. Bremer 1955 Bremer, Gerardus  J.  B. Wijsgerige aspecten van het natuurkundig tijdbegrip. Doctoral dissertation University of Utrecht, 1955. Brillouin 1926 Brillouin, Marcel. “M. H. A. Lorentz en France et en Belgique. Quelsques souvenirs.” Physica 6 (1926): 30–35. Candolle 1885 Candolle, A. de. Histoire des sciences et des savants depuis deux siècles. 1885. Cartwright 1999 Cartwright, David E. Tides: A Scientific History. Cambridge: Cambridge University Press, 1999. Casimir 1983 Casimir, Hendrik  B.  G. Het toeval van de werkelijkheid. Amsterdam: Meulenhoff, 1983. Clark 2012 Clark, Christopher. The Sleepwalkers: How Europe Went to War in 1914. London: Allan Lane, 2012. Coupain 2015 Coupain, Nicolas. “Ernest Solvay’s scientific networks. From personal research to academic patronage.” European Physical Journal Special Topics 224 (2015): 2075–2089. CPAE-3 Klein, Martin J., A. J. Kox, Jürgen Renn, and Robert Schulmann, ed. The Collected Papers of Albert Einstein.Vol. 3, The SwissYears: Writings, 1909–1911. Princeton: Princeton University Press, 1993.



CPAE-5 Klein, Martin  J., A.  J.  Kox, and Robert Schulmann, ed. The Collected Papers of Albert Einstein. Vol. 5, The Swiss Years: Correspondence, 1902–1914. Princeton: Princeton University Press, 1993. CPAE-6 Kox, A.  J., Martin  J.  Klein, and Robert Schulmann, ed. The Collected Papers of Albert Einstein. Vol. 6, The Berlin Years: Writings, 1914–1917. Princeton: Princeton University Press, 1996. CPAE-7 Janssen, Michel, József Illy, Christoph Lehner, and Diana K. Buchwald, ed. The Collected Papers of Albert Einstein. Vol. 7, The BerlinYears:Writings, 1918–1921. Princeton: Princeton University Press, 2002. CPAE-8 Schulmann, Robert, A.  J.  Kox, and Michel Janssen, ed. The Collected Papers of Albert Einstein. Vol. 8, The Berlin Years: Correspondence, 1914–1917. Princeton: Princeton University Press, 1998. CPAE-13 Kormos Buchwald, Diana, József Illy, Ze’ev Rosenkranz, and Tilman Sauer, ed. The Collected Papers of Albert Einstein.Vol. 13, The BerlinYears:Writings & Correspondence, January 1922–March 1923. Princeton: Princeton University Press, 2012. CPAE-14 Kormos Buchwald, Diana, József Illy, Ze’ev Rosenkranz, Tilman Sauer, and Osik Moses, ed. The Collected Papers of Albert Einstein. Vol. 14, The Berlin Years: Writings & Correspondence, April 1923–May 1925. Princeton: Princeton University Press, 2015. CPAE-15 Kormos Buchwald, Diana, József Illy, A.  J.  Kox, Dennis Lehmkuhl, Ze’ev Rosenkranz, and Jennifer Nollar James, ed. The Collected Papers of Albert Einstein. Vol. 15, The Berlin Years:Writings & Correspondence, June 1925–May 1927. Princeton: Princeton University Press, 2018. Crawford 1984 Crawford, Elisabeth. The Beginnings of the Nobel Institution. Cambridge: Cambridge University Press, 1984. Crawford et al. 1987 Crawford, Elisabeth, J. L. Heilbron, and Rebecca Ullrich. The Nobel Population 1901–1937: A Census of the Nominators and Nominees for the Prizes in Physics and Chemistry. Berkeley: Office for History of Science and Technology; Uppsala: Office for History of Science, 1987. Darrigol 1995 Darrigol, Olivier. “Henri Poincaré’s criticism of Fin de Siècle electrodynamics.” Studies in the History and Philosophy of Modern Physics 26 (1995): 1–44. Darrigol 2000 Darrigol, Olivier. Electrodynamics from Ampère to Einstein. Oxford: Oxford University Press, 2000. Darrigol 2014 Darrigol, Olivier. “The quantum enigma.” In The Cambridge Companion to Einstein, pp. 117–142. Michel Janssen and Christoph Lehner, ed. Cambridge: Cambridge University Press, 2014. Delft 2005 Delft, Dirk van. Heike Kamerlingh Onnes. Een biografie. Amsterdam: Bert Bakker, 2005. Delft 2007 Delft, Dirk van. Freezing Physics: Heike Kamerlingh Onnes and the quest for Cold. Amsterdam: Koninklijke Akademie van Wetenschappen, 2007. Transl. of Van Delft 2005. Delft 2008 Delft, Dirk van. “De afscheidsbrief van Paul Ehrenfest.” Nederlands Tijdschrift voor Natuurkunde 74 (2008): 18–20. Delft and Hoeneveld 2011 Delft, Dirk van, and Friso Hoeneveld. “Wander Johannes de Haas en de Tweede Wereldoorlog.” Nederlands Tijdschrift voor Natuurkunde 77 (2011): 254–257, 298–301. Dongen 2012 Dongen, Jeroen van. “Mistaken identity and mirror images: Albert and Carl Einstein, Leiden, Berlin, relativity and revolution.” Physics in Perspective 14 (2012): 126–177. Droste 1915 Droste, Johannes. “Over het veld van een enkel centrum in Einstein’s theorie der zwaartekracht.” Verslagen van de Gewone Vergaderingen der Wis- and Natuurkundige



Afdeeling, Koninklijke Akademie vanWetenschappen te Amsterdam 23 (1914–1915): 968–981. English transl. “On the field of a single centre in Einstein’s theory of gravitation.” Proceedings of the Section of Sciences, Koninklijke Akademie van Wetenschappen te Amsterdam 17 (1914–1915): 998–1011. Droste 1916 Droste, Johannes. Het zwaartekrachtsveld van een of meer lichamen volgens de theorie van Einstein. Leiden: Brill, 1916. Doctoral dissertation, Leiden University. Duncan and Janssen 2019 Duncan, Anthony and Michel Janssen. Constructing Quantum Mechanics.Vol. 1, The Scaffold, 1900–1923. Oxford: Oxford University Press, 2019. Eckert 2013 Eckert, Michael. Arnold Sommerfeld. Atomphysiker und Kulturbote 1868–1951. Göttingen: Wallstein Verlag, 2013. Eeden 1971 Eeden, Frederik van. Dagboek 1878–1923. Deel III, 1911–1918. Culemborg: Tjeenk Willink-Noorduijn, 1971. Ehrenfest 1906 Ehrenfest, Paul. “Zur Planckschen Strahlungstheorie.” Physikalische Zeitschrift 7 (1906): 528–532. Ehrenfest 1912 Ehrenfest, Paul. Zur Krise der Lichtäther-Hypothese. IJdo, 1912. Ehrenfest 1928 Ehrenfest, Paul. “Grafrede.” Physica 8 (1928): 101–104. Ehrenfest, P. and T. 1911 Ehrenfest, Paul, and Tatiana Ehrenfest. “Begriffliche Grundlagen der statistischen Auffassung in der Mechanik.” In Encyklopädie der mathematischen Wissenschaften, mit Einschluss ihrer Anwendungen, Vol. IV 32, pp. 1–90. Leipzig: Teubner, 1911. Einstein 1905 Einstein, Albert. “Zur Elektrodynamik bewegter Körper.” Annalen der Physik 17 (1905): 549–560. Einstein 1907 Einstein, Albert. “Über das Relativitätsprinzip und die aus demselben ­gezogen Folgerungen.” Jahrbuch der Radioaktivität und Elektronik 4 (1907): 411–462; 5 (1908): 98–99 (“Berichtigung”). Einstein 1909 Einstein, Albert. “Zum gegenwärtigen Stand des Strahlungsproblems.” Physikalische Zeitschrift 10 (1909): 185–193. Einstein 1915 Einstein, Albert. “Die Relativitätstheorie.” In Die Kultur der Gegenwart, Ihre Entwicklung und ihre Ziele, Teil 3, Abteilung 3, Band 1, Physik, pp. 703–713. Emil Warburg, ed. Leipzig: Teubner, 1915. Einstein 1926 Einstein, Albert. “W. H. Julius 1860–1925.” Astrophysical Journal 63 (1926): 196–198. Einstein 1928 Einstein, Albert. “H. A. Lorentz et la coopération internationale.” In Grotius. Annuaire international pour l’année 1928, pp. 1–4. The Hague: Nijhoff, 1928. Einstein 1934 Einstein, Albert. “Nachruf Paul Ehrenfest 1880–1933.” Almanak van het Leidsche Studentencorps 120 (1934): 94–97. Einstein 1934 Einstein, Albert. Mein Weltbild. Amsterdam: Querido, 1934. Einstein 1953 Einstein, Albert. H. A. Lorentz als Schöpfer und Persönlichkeit. Mitteilung No. 91 aus dem Rijksmuseum voor de Geschiedenis der Natuurwetenschappen. Leiden, 1953. Eisenstaedt 1982 Eisenstaedt, Jean. “Histoire et singularités de la solution de Schwarzschild (1915–1923).” Archive for History of Exact Sciences 27 (1982): 157–198. Fernandez Santarén et al. 2015 Fernandez Santarén, J., A. J. Kox and Jose Sánchez-Ron. “Beyond disciplinary borders: H. A.  Lorentz and S.  Ramón y Cajal.” Archives Internationales d’Histoire des Sciences 64 (2015): 497–521. Fokker 1946 Fokker, Adriaan D. “Hendrik Antoon Lorentz.” In Nederlandsche helden der wetenschap, pp. 53–94. T. P. Sevensma, ed. Amsterdam: Kosmos, 1946. Forman et al. 1975 Forman, Paul, John  L.  Heilbron, and Spencer Weart. “Physics circa 1900.” Historical Studies in the Physical Sciences 5 (1975): 1–185.



Frisch 2005 Frisch, Mathias. “Mechanisms, principles, and Lorentz’s cautious realism.” Studies in History and Philosophy of Modern Physics 36 (2005): 659–679. Fuchs and Simons 1972 Fuchs, J. M. and W. J. Simons. De Afsluitdijk. Recht door zee. Haren: Knoop en Niemeijer, 1972. Gallé 1917 Gallé, Pieter H. Stormvloeden langs de Noordzee- en de Zuiderzeekusten. Uitgegeven door de Zuiderzee-Vereeniging. Leiden: Brill, 1917. Gedenkboek 1966 R. Feis, W. Sjoerdsma, and J. B. Stomph, ed. Transformaties. Van Franse school tot Athenaeum. Gedenkboek uitgegeven ter gelegenheid van het honderd jarig bestaan van de gemeentelijke H.B.S., thans Lorentz-H.B.S. te Arnhem. Arnhem, 1966. Goodstein 1991 Goodstein, Judith R. Millikan’s School. New York: Norton, 1991. Haas-Lorentz 1957 Haas-Lorentz, G. L. de. “Reminiscences.” In H. A. Lorentz: Impressions of his Life and Work, pp. 5–47, 82–120, 145–153, 160–164. Amsterdam: North-Holland, 1957. Haga 1900 Haga, Herman. “De ontwikkeling der natuurkunde in de 19de eeuw.” In Jaarboek der Rijks-Universiteit te Groningen 1899–1900, pp. 1–16. Groningen: Wolters, 1900. Ham 2007 Ham, W. van der. Verover mij dat land. Lely en de Zuiderzeewerken. Amsterdam: Boom, 2007. Harman 1982 Harman, Peter M. Energy, Force and Matter. The Conceptual Development of Nineteenth-century Physics. Cambridge: Cambridge University Press, 1982. Harman 2002 Harman, Peter  M., ed. The Scientific Papers and Letters of James Clerk Maxwell. Vol. 3. Cambridge: Cambridge University Press, 2002. Havas 1989 Havas, Peter. “The early history of the ‘Problem of motion’ in general relativity.” In Einstein and the History of General Relativity, pp. 234–276. D.  Howard and J. Stachel, ed. Boston: Birkhäuser, 1989. Heilbron 1986 Heilbron, John L. The Dilemmas of an Upright Man. Max Planck as Spokesman for German Science. Berkeley: Universty of California Press, 1986. Hermann 1969 Hermann, Armin. Frühgeschichte der Quantentheorie (1899–1913). Mosbach in Baden: Physik Verlag, 1969. Hollestelle 2011 Hollestelle, Marijn. Paul Ehrenfest. Worstelingen met de moderne wetenschap, 1912–1933. Leiden: Leiden University Press, 2011. Hoorn 1998 Hoorn, Marijn van. “The Physics Laboratory of the Teyler Foundation (Haarlem) under Professor H. A. Lorentz.” Bulletin of the Scientific Instrument Society No. 59 (1998): 14–21. Huijnen and Kox 2007 Huijnen, Pim and A. J. Kox. “Paul Ehrenfest’s rough road to Leiden: A physicist’s search for a position, 1904–1912.” Physics in Perspective 9 (2007): 186–211. Janssen 1992 Janssen, Michel. “H. A. Lorentz’s attempt to give a coordinate-free formulation of the general theory of relativity.” In Studies in the History of General Relativity, pp. 344–363. J. Eisenstaedt and A. J. Kox, ed. Boston: Birkhäuser, 1992. Janssen 2002 Janssen, Michel. “Reconsidering a scientific revolution: The case of Einstein versus Lorentz.” Physics in Perspective 2 (2002): 421–446. Janssen 2009 Janssen, Michel. “Drawing the line between kinematics and dynamics in special relativity.” Studies in History and Philosophy of Modern Physics 20 (2009): 26–52. Janssen 2014 Janssen, Michel. “ ‘No success like failure. . .’ Einstein’s quest for General Relativity.” In The Cambridge Companion to Einstein, pp. 167–227. Michel Janssen and Christoph Lehner, ed. Cambridge: Cambridge University Press, 2014. Janssen 2019 Janssen, Michel. “How did Lorentz find his theorem of corresponding states?” Studies in History and Philosophy of Modern Physics 67 (2019): 167–175.



Janssen and Kox 2011 Janssen, Michel and A.  J.  Kox. “Lorentz als wegbereider voor de speciale relativiteitstheorie.” Nederlands Tijdschrift voor Natuurkunde 77 (2011): 344–347. Joffe 1967 Joffe, A. F. Begegnungen mit Physikern. Basel: Pfalz-Verlag, 1967. Jungnickel and McCormmach 1986 Jungnickel, Christa and Russell McCormmach. Intellectual Mastery of Nature:Theoretical Physics from Ohm to Einstein. Chicago; London: University of Chicago Press, 1986. Kelvin 1901 Kelvin, Lord. “Nineteenth century clouds over the dynamical theory of heat and light.” Philosophical Magazine (6) 2 (1901): 1–40. Kennefick 2019 Kennefick, Daniel. No Shadow of a Doubt. The 1919 Eclipse that Confirmed Einstein’s Theory of Relativity. Princeton; Oxford: Princeton University Press, 2019. Kevles 1978 Kevles, Daniel J. The Physicists:The History of a Scientific Community in Modern America. New York: Knopf, 1978. Klein 1970 Klein, Martin  J. Paul Ehrenfest.Vol. 1, The Making of a Theoretical Physicist. Amsterdam: North-Holland, 1970. Kooper 1918 Kooper, J. Nota betreffende den te verwachten invloed van de afsluiting van de Zuiderzee op de waterhoogten langs de Friesche en Groningse kusten. Uitgegeven door de Zuiderzee-Vereeniging. Leiden: Brill, 1918. Kox 1988 Kox, A. J. “Hendrik Antoon Lorentz, the ether, and the general theory of relativity.” Archive for History of Exact Sciences 38 (1988): 67–78. Kox 1990 Kox, A. J. “Simon Stevin (1548–1620).” In Van Stevin tot Lorentz, pp. 10–19. A. J. Kox, ed. Amsterdam: Bert Bakker, 1990. Kox 1990b Kox, A. J. “H. A. Lorentz’s contributions to kinetic gas theory.” Annals of Science 47 (1990): 591–606. Kox 1992c Kox, A. J. “General relativity in the Netherlands, 1915–1920.” In Studies in the History of General Relativity, pp. 39–56. Jean Eisenstaedt and A.  J.  Kox, ed. Boston: Birkhäuser, 1992. Kox 1993a Kox, A. J. “Einstein and Lorentz: More than just good colleagues.” Science in Context 6 (1993): 43–56. Kox 1993b Kox, A.  J. “Boltzmann’s influence on H. A.  Lorentz.” In Proceedings of the International Symposium on Ludwig Boltzmann (Rome, February 9–11,1989), pp. 95–111. Giovanni Battimelli, Maria Grazia Ianniello, and Otto Kresten, ed. Wien: Verlag der Österreichischen Akademie der Wissenschaften, 1993. Kox 1997 Kox, A.  J. “The discovery of the electron: II. The Zeeman effect.” European Journal of Physics 18 (1997): 139–144. Kox 2005 Kox, A. J. “H. A. Lorentz, Lectures on electron theory, first edition (1909).” In Landmark Writings in Western Mathematics 1640–1940, pp. 778–783. Ivor Grattan Guinness, ed. Amsterdam [etc.]: Elsevier, 2005. Kox 2007 Kox, A. J. “Uit de hand gelopen onderzoek in opdracht: H. A. Lorentz’ werk in de Zuiderzeecommissie.” In Onderzoek in opdracht. De publieke functie van het universitaire onderzoek in Nederland sedert 1876, pp. 39–52. L. J. Dorsman and P. J. Knegtmans, ed. Hilversum: Verloren, 2007. Kox 2008 The Scientific Correspondence of H. A. Lorentz. Vol. 1. A. J. Kox, ed. New York: Springer, 2008. Kox 2010 Kox, A.  J. “Hendrik Antoon Lorentz en Paul Ehrenfest: Twee tegenpolen in gesprek.” In Wetenschapsarchieven in het Noord-Hollands Archief, pp. 41–54. Frans W. Lantink and Jaap Temminck, ed. Hilversum: Verloren, 2010.



Kox 2012 Kox, A. J. “De Nederlandse natuurkunde rond 1900.” In Zekerheden in waarnemingen. Natuurwetenschappelijke ontwikkelingen in Nederland rond 1900, pp. 11–24. George B. Huitema, H. de Jong, and J. Guichelaar, ed. Hilversum: Verloren, 2012. Kox 2013a Kox, A. J. “Hendrik Antoon Lorentz’s struggle with quantum theory.” Archive for History of Exact Sciences 67 (2013): 149–170. Kox 2013b Kox, A. J. “The debate between Hendrik A. Lorentz and Schrödinger on wave mechanics.” In Erwin Schrödinger—50 Years After, pp. 149–158. Wolfgang L. Reiter and Jakob Yngvason, ed. 2013. Kox 2018 The Scientific Correspondence of H. A. Lorentz. Vol. 2, The Dutch Correspondents. A. J. Kox, ed. New York: Springer, 2018. Kox 2019 Kox, A.  J. “Een levend kunstwerk.” Hendrik Antoon Lorentz (1853–1928), ­natuurkundige. Amsterdam: Balans, 2019. Kox and Troelstra 1998 Kox, A. J. and Willemien P. Troelstra. “Uit het Zeeman-archief: De ontdekking van het Zeeman-effect.” Gewina 19 (1998): 153–166. Kragh 1999 Kragh, Helge. Quantum Generations. A History of Physics in the Twentieth Century. Princeton: Princeton University Press, 1999. Kuhn 1978 Kuhn, Thomas S. Black-body Theory and the Quantum Discontinuity, 1894–1912. Oxford: Clarendon Press; New York: Oxford University Press, 1978. Lalli 2012 Lalli, Roberto. “The reception of Miller’s ether-drift experiments in the USA: The history of a controversy in relativity revolution.” Annals of Science 69 (2012): 153–214. Lambert 2015 Lambert, Franklin J. “Einstein’s witches’ sabbath in Brussels: The legend and the facts.” European Physical Journal Special Topics 224 (2015): 2023–2040. Lely 1918 Lely, C. W. Verhooging van de stormvloedstanden op de Friesche kust, tengevolge van de afsluiting der Zuiderzee. Uitgegeven door de Zuiderzee-Vereeniging. Leiden: Brill, 1918. Lorentz 1875b Lorentz, Hendrik Antoon. Over de theorie der terugkaatsing en breking van het  licht. Arnhem: Van der Zande, 1875. Doctoral dissertation, Leiden University, 11 Dec. 1875. C.P. 1, pp. 1–192. French transl. C.P. 1, pp. 193–383. Lorentz 1877a Lorentz, Hendrik Antoon. “Ueber die Theorie der Reflexion und Refraction des Lichtes. Erste Mittheilung.” Zeitschrift für Mathematik und Physik 22 (1877): 1–30. Lorentz 1877b Lorentz, Hendrik Antoon. “Ueber die Theorie der Reflexion und Refraction des Lichtes. Zweite Mittheilung.” Zeitschrift für Mathematik und Physik 22 (1877): 205–219. Lorentz 1878a Lorentz, Hendrik Antoon. De moleculaire theoriën in de natuurkunde. Redevoering, bij de aanvaarding van het hoogleeraarsambt aan de Rijksuniversiteit te Leiden den 25sten Januari 1878 Arnhem: Van der Zande, 1878. C.P. 9, pp. 1–25. English transl. C.P. 9, pp. 26–49. Lorentz 1878b Lorentz, Hendrik Antoon. “Ueber die Theorie der Reflexion und Refraction des Lichtes. Dritte Mittheilung.” Zeitschrift für Mathematik und Physik 23 (1878): 197–210. Lorentz 1879a Lorentz, Hendrik Antoon. “Over het verband tusschen de voortplantings­ snelheid van het licht en de dichtheid en samenstelling der middenstoffen.” Verhandelingen der Koninklijke Akademie van Wetenschappen te Amsterdam 18 (1879): 1–112 (separately paginated). English transl. C.P. 2, pp. 1–119. Lorentz 1881a Lorentz, Hendrik Antoon. “Ueber die Anwendung des Satzes vom Virial in der kinetischen Theorie der Gase.” Annalen der Physik 12 (1881): 127–136, 660–661 (“Nachtrag”). C.P. 6, pp. 40–50.



Lorentz 1886a Lorentz, Hendrik Antoon. “Over den invloed, dien de beweging der aarde op de lichtverschijnselen uitoefent.” Verslagen en Mededeelingen der Koninklijke Akademie van Wetenschappen (Amsterdam), Afdeeling Natuurkunde (3) 2 (1885–1886): 297–372. French transl. “De l’influence du mouvement de la terre sur les phénomènes lumineux.” Archives Néerlandaises 21 (1887): 103–176. C.P. 4, pp. 153–214. Lorentz 1891b Lorentz, Hendrik Antoon. “Over electriciteit en ether.” Handelingen van het Nederlandsch Natuur- en Geneeskundig Congres 3 (1891): 40–50. C.P. 9, pp. 89–101. Lorentz 1892c Lorentz, Hendrik Antoon. “De relatieve beweging van de aarde en den aether.” Verslagen der Zittingen van de Wis- en Natuurkundige Afdeeling der Koninklijke Akademie van Wetenschappen (Amsterdam) 1 (1892–1893): 74–79. English transl. C.P. 4, pp. 219–223. Lorentz 1892f Lorentz, Hendrik Antoon. “La théorie électromagnétique de Maxwell et son application aux corps mouvants.” Archives néerlandaises des sciences exactes et naturelles 25 (1892): 363–552. C.P. 2, pp. 164–343. Lorentz 1895b Lorentz, Hendrik Antoon. Versuch einer Theorie der electrischen und optischen Erscheinungen in bewegten Körpern. Leiden: Brill, 1895. C.P. 5, pp. 1–138. Lorentz 1899d Lorentz, Hendrik Antoon. “Vereenvoudigde theorie der electrische en optische verschijnselen in lichamen die zich bewegen.” Verslagen van de Gewone Vergaderingen der Wis- en Natuurkundige Afdeeling, Koninklijke Akademie van Wetenschappen te Amsterdam 7 (1898–1899): 507–522. English transl. “Simplified theory of electrical and optical phenomena in moving systems.” Proceedings of the Section of Sciences, Koninklijke Akademie van Wetenschappen te Amsterdam 1 (1898–1899): 427–442. French transl. C.P. 5, pp. 139–155. Lorentz 1900a Lorentz, Hendrik Antoon. “Electromagnetische theorieën van natuurkundige verschijnselen. Rede, uitgesproken door den Rector Magnificus op den 325sten verjaardag der Rijksuniversiteit te Leiden, 8 Februari 1900.” In Jaarboek der RijksUniversiteit te Leiden 1899–1900, pp. 3–28 (in third part). Leiden: Brill, 1900. German transl. C.P. 8, pp. 333–352. Lorentz 1900b Lorentz, Hendrik Antoon. “Beschouwingen over de zwaartekracht.” Verslagen van de Gewone Vergaderingen der Wis- en Natuurkundige Afdeeling, Koninklijke Akademie van Wetenschappen te Amsterdam 8 (1899–1900): 603–620. English transl. “Considerations on gravitation.” Proceedings of the Section of Sciences, Koninklijke Akademie van Wetenschappen te Amsterdam 2 (1899–1900): 559–574. French transl. C.P.  5, pp. 198–215. Lorentz 1900e Lorentz, Hendrik Antoon. “Théorie des phénomènes magnéto-optiques récemment découverts.” In Rapports présentés au Congrès International de Physique réuni à Paris en 1900 sous les auspices de la Société française de Physique, Vol. 3, pp. 1–33. Ch.-Éd. Guillaume and L. Poincaré, ed. Paris: Gauthier–Villars, 1900. C.P. 7, pp. 35–65. Lorentz 1901d Lorentz, Hendrik Antoon. Zichtbare en onzichtbare bewegingen.Voordrachten, op uitnoodiging van het bestuur van het Departement Leiden der Maatschappij tot Nut van ’ t Algemeen (Cursus van Hooger Onderwijs buiten de Universiteit), in Februari en Maart 1901. Leiden: Brill, 1901. Lorentz 1903c Lorentz, Hendrik Antoon. “Het emissie- en absorptievermogen der metalen in het geval van groote golflengten.” Verslagen van de Gewone Vergaderingen der Wis- en Natuurkundige Afdeeling, Koninklijke Akademie van Wetenschappen te Amsterdam 11 (1902–1903): 787–807. English transl. “On the emission and absorption by metals of rays of heat of great wave-lengths.” Proceedings of the Section of Sciences, Koninklijke



Akademie van Wetenschappen te Amsterdam 5 (1902–1903): 666–685. C.P.  3, pp. 155–176. Lorentz 1904c Lorentz, Hendrik Antoon. “Electromagnetische verschijnselen in een stelsel dat zich met willekeurige snelheid, kleiner dan die van het licht, beweegt.” Verslagen van de Gewone Vergaderingen der Wis- en Natuurkundige Afdeeling, Koninklijke Akademie van Wetenschappen te Amsterdam 12 (1903–1904): 986–1009. English transl. “Electromagnetic phenomena in a system moving with any velocity smaller than that of light.” Proceedings of the Section of Sciences, Koninklijke Akademie van Wetenschappen te Amsterdam 6 (1903–1904): 809–831. C.P. 5, pp. 172–197. Lorentz 1904e Lorentz, Hendrik Antoon. “Maxwells elektromagnetische Theorie.” In Encyklopädie der mathematischen Wissenschaften mit Einschluss ihrer Anwendungen. Vol. 5, Physik, Vol. 2, pp. 63–144. Arnold Sommerfeld, ed. Leipzig: Teubner, 1904–1922. Lorentz 1904f Lorentz, Hendrik Antoon. “Weiterbildung der Maxwellschen Theorie. Elektronentheorie.” In Encyklopädie der mathematischen Wissenschaften mit Einschluss ihrer Anwendungen. Vol. 5, Physik, Vol. 2, pp. 145–280. Arnold Sommerfeld, ed. Leipzig: Teubner, 1904–1922. Lorentz 1905c Lorentz, Hendrik Antoon. “De wegen der theoretische natuurkunde.” Nederlandsch Tijdschrift voor Geneeskunde 49 (1905): 204–223. C.P. 9, pp. 53–76. Lorentz 1905h Lorentz, Hendrik Antoon. “La thermodynamique et les théories cinétiques.” Bulletin des Séances de la Société Française de Physique (1905): 35–63. Lorentz 1905v Lorentz, Hendrik Antoon. “Nobel-Vorlesung.” In Les prix Nobel en 1902, pp. 1–20. Stockholm: Norstedt, 1905. C.P. 7, pp. 66–86. Lorentz 1906c Lorentz, Hendrik Antoon. “On positive and negative electrons.” Proceedings of the American Philosophical Society 46 (1906): 103–109. C.P. 8, pp. 152–158. Lorentz 1906g Lorentz, Hendrik Antoon. Abhandlungen über theoretische Physik. Vol. 1, part 1. Leipzig; Berlin: Teubner, 1906. Lorentz 1907c Lorentz, Hendrik Antoon. “Ludwig Boltzmann.” Verhandlungen der Deutschen Physikalischen Gesellschaft 9 (1907): 206–238. C.P. 9, pp. 359–390. Lorentz 1907i Lorentz, Hendrik Antoon. Abhandlungen über theoretische Physik. Vol. 1, part 2. Leipzig; Berlin: Teubner, 1907. Lorentz 1908b Lorentz, Hendrik Antoon. Le partage de l’énergie entre la matière pondérable et l’éther. Roma: R. Accademia dei Lincei, 1908. C.P. 7, pp. 317–343. Lorentz 1908e Lorentz, Hendrik Antoon. “Le partage de l’énergie entre la matière pondérable et l’éther.” Nuovo Cimento 16 (1908): 5–34. C.P. 7, pp. 317–343. Lorentz 1909g Lorentz, Hendrik Antoon. “Le partage de l’énergie entre la matière pondérable et l’éther.” Revue générale des sciences 20 (1909): 14–26. C.P. 7, pp. 317–343. Lorentz 1909h Lorentz, Hendrik Antoon. “Le partage de l’énergie entre la matière pondérable et l’éther.” In Atti del IV Congresso Internazionale dei Matematici (Roma, 6–11 Aprile 1908), Vol. 1, pp. 145–165. Roma: R. Accademia dei Lincei, 1909. C.P. 7, pp. 317–343. Lorentz 1909i Lorentz, Hendrik Antoon. The Theory of Electrons and its Applications to the Phenomena of Light and Radiant Heat. A Course of Lectures Delivered in Columbia University, New York, in March and April 1906. Leipzig: Teubner, 1909. Lorentz 1910g Lorentz, Hendrik Antoon. “Alte und neue Fragen der Physik.” Physikalische Zeitschrift 11 (1910): 1234–1257. C.P. 7, pp. 205–257. Lorentz 1913a Lorentz, Hendrik Antoon. “De internationale wetenschap bevordert de vrede.” Orgaan van den Algemeenen Nederlandschen Bond “Vrede Door Recht” 14 (1913): 5–7.



Lorentz 1915a Lorentz, Hendrik Antoon. “Het beginsel van Hamilton in Einstein’s theorie der zwaartekracht.” Verslagen van de Gewone Vergaderingen der Wis- en Natuurkundige Afdeeling, Koninklijke Akademie van Wetenschappen te Amsterdam 23 (1914–1915): 1073–1089. English transl. “On Hamilton’s principle in Einstein’s theory of gravitation.” Proceedings of the Section of Sciences, Koninklijke Akademie van Wetenschappen te Amsterdam 19 (1916–1917): 751–765. C.P. 5, pp. 229–245. Lorentz 1916b Lorentz, Hendrik Antoon. “Over Einstein’s theorie der zwaartekracht. I.” Verslagen van de Gewone Vergaderingen der Wis- en Natuurkundige Afdeeling, Koninklijke Akademie van Wetenschappen te Amsterdam 24 (1915–1916): 1389–1402. English transl. “On Einstein’s theory of gravitation. I.” Proceedings of the Section of Sciences, Koninklijke Akademie van Wetenschappen te Amsterdam 19 (1916–1917): 1341–1354. C.P.  5, pp. 246–313. Lorentz 1916c Lorentz, Hendrik Antoon. “Over Einstein’s theorie der zwaartekracht. II.” Verslagen van de Gewone Vergaderingen der Wis- en Natuurkundige Afdeeling, Koninklijke Akademie van Wetenschappen te Amsterdam 24 (1915–1916): 1759–1774. English transl. “On Einstein’s theory of gravitation. II.” Proceedings of the Section of Sciences, Koninklijke Akademie van Wetenschappen te Amsterdam 19 (1916–1917): 1354–1369. C.P.  5, pp. 246–313. Lorentz 1916d Lorentz, Hendrik Antoon. “Over Einstein’s theorie der zwaartekracht. III.” Verslagen van de Gewone Vergaderingen der Wis- en Natuurkundige Afdeeling, Koninklijke Akademie van Wetenschappen te Amsterdam 25 (1916–1917): 468–486. English transl. “On Einstein’s theory of gravitation. III.” Proceedings of the Section of Sciences, Koninklijke Akademie van Wetenschappen te Amsterdam 20 (1917–1918): 2–19. C.P. 5, pp. 246–313. Lorentz 1916e Lorentz, Hendrik Antoon. Les théories statistiques en thermodynamique. Conférences faites au Collège de France en novembre 1912. L. Dunoyer, ed. Leipzig; Berlin: Teubner, 1916. Lorentz 1917c Lorentz, Hendrik Antoon. “De gravitatietheorie van Einstein en de grondbegrippen der natuurkunde.” Handelingen van het Nederlandsch Natuur- en Geneeskundig Congres 16 (1917): 23–43. C.P. 9, pp. 244–263. Lorentz 1917d Lorentz, Hendrik Antoon. “Over Einstein’s theorie der zwaartekracht. IV.” Verslagen van de Gewone Vergaderingen der Wis- en Natuurkundige Afdeeling, Koninklijke Akademie van Wetenschappen te Amsterdam 25 (1916–1917): 1380–1396. English transl. “On Einstein’s theory of gravitation. IV.” Proceedings of the Section of Sciences, Koninklijke Akademie van Wetenschappen te Amsterdam 20 (1917–1918): 20–34. C.P. 5, pp. 246–313. Lorentz 1917e Lorentz, Hendrik Antoon. “De gravitatietheorie van Einstein en de grondbegrippen der natuurkunde.” De Ingenieur 32 (1917): 649–655. C.P. 9, pp. 244–263. Lorentz 1921c Lorentz, Hendrik Antoon. “La prévision scientifique.” Revue de l’Université de Bruxelles 26 (1921): 445–460. C.P. 8, pp. 48–75. Lorentz 1922a Lorentz, Hendrik Antoon. “Research in the Norman Bridge Laboratory.” Science 55 (1922): 334–335. Lorentz 1922j Lorentz, Hendrik Antoon. Lessen over theoretische natuurkunde aan de RijksUniversiteit te Leiden gegeven. Vol. 6, Het relativiteitsbeginsel voor eenparige translaties (1910–1912). Adriaan D. Fokker, ed. Leiden: Brill, 1922. Lorentz 1923c Lorentz, Hendrik Antoon. “The rotation of the earth and its influence on optical phenomena.” Nature 112 (1923): 103–104. C.P. 7, pp. 173–178. Lorentz 1923f Lorentz, Hendrik Antoon. Clerk Maxwell’s Electromagnetic Theory. The Rede Lecture for 1923. Cambridge: Cambridge University Press, 1923. C.P. 8, pp. 353–366.



Lorentz 1923h Lorentz, Hendrik Antoon. “À la mémoire d’Ernest Solvay.” Solvay 1921, p. VII. Lorentz 1924a Lorentz, Hendrik Antoon. “The radiation of light.” Nature 113 (1924): 608–611. C.P. 8, pp. 17–27. Lorentz 1925h Lorentz, Hendrik Antoon. “l’Ancienne et la nouvelle mécanique.” In Le livre du cinquantenaire de la Société française de Physique, pp. 99–114. Paris: Éditions de la Revue d’Optique Théorique et Instrumentale, 1925. C.P. 7, pp. 274–289. Lorentz 1927b Lorentz, Hendrik Antoon. “Allocution de M. H.-A. Lorentz, au nom des délégations étrangères.” Revue d’optique 6 (1927): 515–516. C.P. 9, pp. 340–342. Lorentz 1927f Lorentz, Hendrik Antoon. Problems of Modern Physics. A Course of Lectures Delivered in the California Institute of Technology. H. Bateman, ed. Boston [etc.]: Ginn, 1927. Lorentz 1928c Lorentz, Hendrik Antoon. “Sur la rotation d’un électron qui circule autour d’un noyau.” In Atti del Congresso Internazionale dei Fisici, 11–20 Settembre 1927-V, Como—Pavia—Roma. Onoranze ad Alessandro Volta nel primo centenario della morte, Vol. 2, pp. 3–32. Bologna: Zanichelli, 1929, C.P. 7, pp. 179–204. Lorentz 1928d Lorentz, Hendrik Antoon. “Discorso del Prof. H.A.  Lorentz.” In Atti del Congresso Internazionale dei Fisici, 11–20 Settembre 1927-V, Como—Pavia—Roma. Onoranze ad Alessandro Volta nel primo centenario della morte, Vol. 2, pp. 621–630. Bologna: Zanichelli, 1928. Lorentz and Droste 1917a Lorentz, Hendrik Antoon and Johannes Droste. “De beweging van een stelsel lichamen onder den invloed van hunne onderlinge aantrekking, behandeld volgens de theorie van Einstein. I.” Verslagen van de Gewone Vergaderingen der Wisen Natuurkundige Afdeeling, Koninklijke Akademie van Wetenschappen te Amsterdam (1917–1918): 392–403. C.P. 5, pp. 330–355. Lorentz and Droste 1917b Lorentz, Hendrik Antoon and Johannes Droste. “De beweging van een stelsel lichamen onder den invloed van hunne onderlinge aantrekking, behandeld volgens de theorie van Einstein. II.” Verslagen van de Gewone Vergaderingen der Wisen Natuurkundige Afdeeling, Koninklijke Akademie van Wetenschappen te Amsterdam 26 (1917–1918): 649–660. C.P. 5, pp. 330–355. Lunteren and Hollestelle 2013 Lunteren, Frans van and Marijn Hollestelle. “Paul Ehrenfest and the dilemmas of modernity.” Isis 104 (1013): 504–536. Maas 2001 Maas, Ad. “Tachtigers in de wetenschap. Een nieuwe kijk op het ontstaan van de ‘Tweede Gouden Eeuw’ in de Nederlandse natuurwetenschap.” Tijdschrift voor Geschiedenis 114 (2001): 354–376. Mansholt, D. R. 1917 Mansholt, Derk R. Het wetsontwerp op de afsluiting en indijking der Zuiderzee, eenige kritische beschouwingen. Groningen: Werkman, 1917. Mansholt, L. H. 1916 Mansholt, Lambertus Helprig. De afsluiting der Zuiderzee, een ernstig gevaar voor Friesland en Groningen. Groningen: Noordhoff, 1916. Maxwell 1874 Maxwell, James Clerk. “Van der Waals on the continuity of the gaseous and liquid states.” Nature 10 (1874): 477–480. Maxwell 1875 Maxwell, James Clerk. “On the dynamical evidence of the molecular constitution of bodies.” Nature 11 (1875): 357–374. McCormmach 1970 McCormmach, Russell. “H. A. Lorentz and the electromagnetic view of nature.” Isis 61 (1970): 459–487. Mehra 1975 Mehra, Jagdish. The Solvay Conferences on Physics. Dordrecht: Reidel, 1975. Mehra and Rechenberg 1982–2001 Mehra, Jagdish and Helmut Rechenberg. The Historical Development of Quantum Theory. 6 vols. in 9 parts. New York, Heidelberg, Berlin: Springer, 1982–2001.



Meyenn 1982 Meyenn, Karl von. “Schrödinger in Amerika.” Der Physikunterricht 16 (4) (1982): 27–41. Minutes 1927 International Committee on Intellectual Co-operation. Minutes of the Ninth Session, July 1927. Geneva: Publications of the League of Nations, 1927. Minutes 1928 International Committee on Intellectual Co-operation. Minutes of the Tenth Session, July 1928. Geneva: Publications of the League of Nations, 1928. Moeyes 2001 Moeyes, Paul. Buiten schot. Nederland tijdens de Eerste Wereldoorlog, 1914–1918. Amsterdam, Antwerp: Arbeiderspers, 2001. Nobel 1902 Les Prix Nobel en 1902. Stockholm: Norstedt, 1905. Norton 1991 Norton, John  D. “Thought Experiments in Einstein’s Work.” In Thought Experiments in Science and Philosophy, pp. 129–148. Tamara Horowitz and Gerald  J. Massey, ed. Savage, MD: Rowman and Littlefield, 1991. Nye 1976 Nye, Mary Jo. Molecular Reality: A Perspective on the Scientific Work of Jean Perrin. London: MacDonald, 1976. Otterspeer and Schuller 1997 Otterspeer, Willem and Joke Schuller tot Peursum-Meijer. Wetenschap en wereldvrede. De Koninklijke Akademie van Wetenschappen en het herstel van de internationale wetenschap tijdens het Interbellum. Amsterdam: KNAW, 1997. Pais 1982 Pais, Abraham.“Subtle is the Lord…”: The Science and the Life of Albert Einstein. Oxford: Clarendon Press; New York: Oxford University Press, 1982. Pais 1991 Pais, Abraham. Niels Bohr’s Times in Physics, Philosophy, and Polity. Clarendon Press, 1991. Peeperkorn 1985 Peeperkorn-Van Donselaar, Loes  A. Twee eeuwen technisch onderwijs. Leiden, 1985. Poincaré 1902 Poincaré, Henri. La science et l’hypothèse. Paris: Flammarion, 1902. Poincaré 1905 Poincaré, Henri. La valeur de la science. Paris: Flammarion, 1905. Poincaré 1999 Poincaré, Henri. La correspondance entre Heni Poincaré et Gösta Mittag-Leffler. Basel [etc.]: Birkhäuser, 1999. Poincaré 2007 Poincaré, Henri. La correspondance entre Henri Poincaré et les physiciens, chimistes et ingénieurs. Basel [etc.]: Birkhäuser, 2007. Pol 1925 Pol, Balthasar van der. “Doctor H. A. Lorentz.” Physica 5 (1925): 321–324. Purrington 1997 Purrington, Robert D. Physics in the Nineteenth Century. New Brunswick: Rutgers University Press, 1997. Renoliet 1999 Renoliet, Jean-Jacques. l’Unesco oubliée: La Société des Nations et la coopération intellectuelle (1919–1946). Paris: Publications de la Sorbonne, 1999. Report 1925 Schuster, Arthur, ed. International Research Council. Third Assembly held at Brussels, July 7th to July 9th, 1925. Report of Proceedings. London, 1925. Riemens 1995 Riemens, Michael. Een vergeten hoofdstuk. De Nederlandsche Anti-Oorlog Raad en het Nederlands pacifisme tijdens de Eerste Wereldoorlog. Groningen: Wolters-Noordhoff, 1995. Riemens 2005 Riemens, Michael. De passie voor vrede. Doctoral dissertation University of , 2005. Roozen 1983 Roozen, J. M. “Leidse bibliotheken rond de eeuwwisseling.” Jaarboekje voor de geschiedenis en oudheidkunde van Leiden en omstreken 75 (1983): 149–166. Rosenfeld 1956 Rosenfeld, Léon. “The velocity of light and the evolution of electrodynamics.” Nuovo Cimento Supplement 4 (1956): 1630–1669. Schakenraad 1984 Schakenraad, Ad. De Algemeene Nederlansche Bond “Vrede door Recht” 1901–1914. Utrechtse Historische Cahiers 5 (1985) nr 4, 1984. Schirrmacher 2015 Schirrmacher, Arne. “Who made quantum theory popular with physicists and beyond? The Solvay Model, a new center for quantum physics, and science communication.” European Physical Journal Special Topics 224 (1015): 2113–2125.



Schröder-Gudehus 1966 Schröder-Gudehus, Brigitte. Deutsche Wissenschaft und internationale Zusammenarbeit 1914–1918. Genève: Dumaret & Golay, 1966. Schröder-Gudehus 1978 Schröder-Gudehus, Brigitte. Les scientifiques et la paix. Montréal: Presses de l’Université de Montréal, 1978. Snelders 1987 Snelders, H.  A.  M. “De bemoeienissen van Lorentz en Einstein met de Utrechtse leerstoel voor theoretische fysica (1911–1914).” Tijdschrift voor de Geschiedenis der Geneeskunde, Natuurwetenschappen,Wiskunde en Techniek 10 (1987): 57–71. Solvay 1911 La théorie du rayonnement et les quanta. Rapports et discussions de la réunion tenue à Bruxelles du 30 octobre au 3 novembre 1911, sous les auspices de M. E. Solvay. Paris: Gauthier-Villars, 1912. Solvay 1913 La structure de la matière. Rapports et discussions du Conseil de Physique tenu à Bruxelles du 27 au 31 octobre 1913 sous les auspices de l’Institut International de Physique Solvay. Paris: Gauthier-Villars, 1921. Solvay 1914 Die Theorie der Strahlung und der Quanten.Verhandlungen auf einer von E. Solvay einberufenen Zusammenkunft (30. Oktober bis 3. November 1911), mit einem Anhange über die Entwicklung der Quantentheorie vom Herbst 1911 bis Sommer 1913. Halla a.S.: Knapp, 1914. Solvay 1921 Atomes et électrons. Rapports et discussions du Conseil de Physique tenu à Bruxelles du 1er au 6 avril 1921 sous les auspices de l’Institut International de Physique Solvay. Paris: Gauthier-Villars, 1923. Solvay 1924 Conductibilité électrique des métaux et problèmes connexes. Rapports et discussions du quatrième Conseil de Physique tenu à Bruxelles du 24 au 29 avril 1924 sous les auspices de l’Institut International de Physique Solvay. Paris: Gauthier-Villars, 1927. Solvay 1927 Électrons et photons. Rapports et discussions du cinquième Conseil de Physique tenu à Bruxelles du 24 au 29 octobre 1927 sous les auspices de l’Institut International de Physique Solvay. Paris: Gauthier-Villars, 1928. Staatscommissie 1926 Verslag van de Staatscommissie benoemd bij Koninklijk Besluit van 4 Juli 1918 No. 30 met opdracht te onderzoeken in hoeverre, als gevolg van de afsluiting van de Zuiderzee, ingevolge de wet van 14 Juni 1918 (Staatsblad No. 354), te verwachten is, dat tijdens storm hoogere waterstanden en een grootere golfoploop, dan thans het geval is, zullen voorkomen vóór de kust van het vaste land van Noord-Holland, Friesland en Groningen, alsmede vóór de daarvoor gelegen Noordzee-eilanden. ‘s-Gravenhage: Algemeene Landsdrukkerij, 1926. Steen 2008 Steen, Agnes van. “Vrouwenkiesrechtstrijd in Leiden. De knuppel in het hoenderhok.” In Negentiende en Twintigste Jaarboek der sociale en economische geschiedenis van Leiden en omstreken 2007/2008, pp. 177–220. Steen 2011 Steen, Agnes van. “ ‘Vol moed en blakende van ijver’. Aletta Lorentz-Kaiser en de vrouwenbeweging in Leiden (1881–1912)” In Jaarboek Dirk van Eck 2011, pp. 124–166. Stuewer 2014 Stuewer, Roger  H. “The experimental challenge of light quanta.” In The Cambridge Companion to Einstein, pp. 143–166. Michel Janssen and Christoph Lehner, ed. Cambridge: Cambridge University Press, 2014. Theunissen 2000 Theunissen, Bert. “Nut en nog eens nut.”Wetenschapsbeelden van Nederlandse natuuronderzoekers, 1800–1900. Hilversum: Verloren, 2000. Thijsse 1927 Thijsse, J. Th. “De verwachtingen omtrent de waterbeweging bij de afsluiting van de Zuiderzee op grond van het verslag der staatscommissie-Lorentz en van de ­verrichte modelmetingen.” De Ingenieur 42 (1927): 713–724. Thijsse 1935 Thijsse, J. Th. “De toepassings-mogelijkheden van verschillende getijberekeningen.” De Ingenieur 50 (1935): B259–B262.



Thijsse 1957 Thijsse, J. Th. “Enclosure of the Zuiderzee.” In H. A. Lorentz. Impressions of his Life and Work, pp. 129–144. G. L. de Haas-Lorentz, ed. Amsterdam: North-Holland, 1957. Thijsse 1972 Thijsse, J. Th. Een halve eeuw Zuiderzeewerken, 1920–1970. Groningen: Tjeenk Willink, 1972. Ven 1998 Ven, Anne-Marie van de. Het Natuurkundig Genootschap Wessel Knoops. 175 jaar Arnhemse historie. Zutphen: Walburg Druk, 1998. Voigt 1908 Voigt, Woldemar. Magneto- und Elektrooptik. Leipzig: Teubner, 1908. Voigt 1913 Voigt, Woldemar. Erinnerungsblätter aus dem deutsch-französischen Kriege 1870/71. Göttingen: Dieterich, 1913. Weiss 2013 Weiss, Martin. The Masses and the Muses. A History of Teylers Museum in the Nineteenth Century. Doctoral dissertation, Leiden University, 2013. Whittaker 1951–1953 Whittaker, Edmund T. A History of the Theories of Aether and Electricity. London, New York: T. Nelson, 1951–1953. Wien 1900 Wien, Wilhelm. “Über die Möglichkeit einer elektromagnetischen Begründung der Mechanik.” Archives néerlandaises des sciences exactes et naturelles 5 (1900): 96–107. Reprinted as Wien 1901. Wien 1901 Wien, Wilhelm. “Über die Möglichkeit einer elektromagnetischen Begründung der Mechanik.” Annalen der Physik 5 (1901): 501–513. Reprint of Wien 1900. Will 1993 Will, Clifford. Was Einstein Right? Putting General Relativity to the Test. New York: Basic Books, 1993. Willink 1988 Willink, Bastiaan. Burgerlijk sciëntisme en wetenschappelijk toponderzoek. Doctoral dissertation University of Amsterdam, 1988. Willink 1991 Willink, Bastiaan. “Origins of the Second Golden Age of Dutch Science after 1860: Intended and unintended consequences of educational reform” Social Studies of Science 21 (1991): 503–526. Willink 1998 Willink, Bastiaan. De Tweede Gouden Eeuw. Nederland en de Nobelprijzen voor natuurwetenschappen, 1870–1940. Amsterdam: Bert Bakker, 1998. Wylick 1966 Wylick,W. A. H. van. Röntgen en Nederland. Röntgen’s betrekkingen tot Nederland en de opkomst der Röntgenologie hier te lande. Utrecht: Hoeijenbos, 1966 Doctoral dissertation Free University Amsterdam. Zeeman 1896 Zeeman, Pieter. “Over den invloed eener magnetisatie op den aard van het door een stof uitgezonden licht.” Verslagen van de Gewone Vergaderingen der Wis- en Natuurkundige Afdeeling, Koninklijke Akademie van Wetenschappen te Amsterdam 5 (1896–1897): 181–184. Zeeman 1900 Zeeman, Pieter. Experimenteele onderzoekingen over deelen kleiner dan atomen. Amsterdam,1900. Zeeman 1909 Zeeman, Pieter. “Rede uitgesproken bij de overhandiging van den gouden eerepenning van het Genootschap ter Bevordering van Natuur- Genees- en Heelkunde, aan Prof. Van der Waals, op Woensdag 18 November 1908.” Nederlandsch Tijdschrift voor Geneeskunde 53 (1909): 522–525. Zeeman 1928 Zeeman, Pieter. “Hendrik Antoon Lorentz, in memoriam.” De Gids 22 II (1928): 105–115. Zuiderzee 1920 Handelingen en bijlagen van de beide Kamers der Staten-Generaal betreffende het ontwerp van wet tot afsluiting en droogmaking van de Zuiderzee. Uitgegeven door de Zuiderzee-Vereeniging. Leiden: Brill, 1920.


A Abraham, Henri (1858–1943) 98 Abraham, Max (1875–1922)  73, 74 Afanassjewa, Tatiana (1876–1964)  133, 136 Aletta Catharina (1858–1931)  passim Ampère, André-Marie (1775–1836)  55, 176 Arrhenius, Svante (1859–1927) 94 Aubel, Edmond van (1864–1941) 248 Aurivillius, Per Olov (1843–1928) 91

B Baehr, George (1812–1898) 20 Bemmelen, Jacob Maarten van (1830–1911)  9, 11, 12, 13, 14, 30, 40 Bergson, Henri (1859–1941)  214, 217 Besso, Michele (1873–1955) 125 Bjerknes,Vilhelm (1862–1951) 103 Bockwinkel, Hermann (1881–1960) 102 Bohr, Niels (1885–1962)  109, 143, 144, 154–155, 206, 232, 238, 247, 248, 249, 257, 260 Boissevain, Charles (1893–1946) 202 Boltzmann, Ludwig (1844–1906)  31, 32–33, 75, 88, 100, 102, 103, 133, 134, 149 Borel, Émile (1871–1956)  211 Born, Max (1882–1970)  109 Bosscha, Johannes (1831–911)  30, 114, 115

Brillouin, Marcel (1854–1948)  98, 123, 166, 168–169, 171 Brouwer, Luitzen (1881–1966)  106, 174, 175 Butler, Nicholas (1862–1947) 103

C Candolle, Alphonse de (1806–1893) 77 Cohen, Ernst (1869–1944) 210 Colijn, Hendrik (1869–1944)  239, 240 Cort van der Linden, Pieter (1846–1935) 174 Curie, Marie (1867–1934)  1, 97, 98, 124, 125, 180, 204, 211, 214, 227, 239, 241, 252 Curie, Pierre (1859–1906)  97 Cuypers, Pierre (1827–1921)  165

D D’Alembert, Jean-Baptiste le Rond (1717–1783)  57 D’Aumerie, Martinus (1831–1919) 45 Debye, Peter (1884–1966)  129, 135 Destrée, Jules (1853–1936)  204 Diepenbrock, Alphons (1982–1921) 165 Dorp, Lizzy van (1872–1945)  45, 46, 49, 50 Droste, Johannes (1886–1963)  28, 158–159 Drucker, Hendrik (1857–1917) 49 Drucker, Wilhelmina (1847–1925) 49 Drummond, Eric (1876–1951) 214 Dufour-Féronce, Albert (1868–1945) 213

E Echegaray y Eizaguirre, José (1832–1916) 236 Eddington, Arthur (1882–1944)  160, 239 Eeden, Frederik van (1860–1932)  34, 165, 168, 261 Ehrenfest, Paul (1880–1933)  1, 57, 87, 118, 133–145, 158, 195, 203, 233, 235, 236, 251, 255, 260–261 Ehrenfest, Wassily (Wassik) (1918–1933) 144–145 Eijkman, Christiaan (1858–1930) 85 Einstein, Albert (1879–1955)  1, 2, 5, 22, 24, 54, 66, 67, 70–74, 79, 109, 116–138, 140, 143, 152–154, 156–162, 168, 176–177, 194, 211–212, 215, 219, 234, 236, 241, 246, 247, 248, 255, 256, 259–261 Einthoven, Willem (1860–1927)  85, 206 Elias, Gerhard Joan (1879–1951) 112, 115, 116 Epstein, Paul (1871–1939)  227, 245 Everdingen, Ewoud van (1873–1955) 187 Exner, Franz (1849–1926)  102 Eysinga, Willem van (1878–1961)  216, 217

F Fabius, Maria Cornelia (1848–1924) 44 Fock, Cornelis (1828–1910)  101, 113 Fokker, Adriaan (1887–1972)  5, 28, 36, 42, 46, 116, 142, 158, 238, 260–261



Franklin, Benjamin (1706–1790) 105 Fresnel, Augustin (1788–1827)  16, 17, 39, 248, 257

G Gallé, Pieter Hendrik (1873–1934) 185, 187, 188 Gautier, Raymond (1885–1957) 201 Ginkel, Geertruida van see Lorentz-Van Ginkel, Geertruida Glazebrook, Richard (1854–1935) 201 Gray, Viscount Edward (1862–1933)  232, 233 Grinwis, Cornelius (1831–1899) 20 Groenewegen, Hermanus (1862–1930) 26 Grossmann, Marcel (1878–1936) 129 Guillaume, Charles-Édouard (1861–1938) 168

H Haas, Wander Johannes de (1878–1960)  45, 109, 116, 135, 136, 138, 175–176, 178, 251, 257 Haas-Lorentz, Geertruida Luberta de see Lorentz, Geertruida Luberta Haber, Fritz (1868–1934)  210 Hadamard, Jacques (1865–1963)  168, 211 Haga, Herman (1852–1936)  8, 10–12, 15, 76, 79, 81, 84, 88, 113, 233 Hale, George Ellery (1868–1938)  197, 214, 221, 228 Hallock,William (1857–1913) 103 Hasenöhrl, Fritz (1874–1915), 134 Heisenberg, Werner (1901–1976) 247 Helmholtz, Hermann von (1821–1894)  16, 58, 59 Hertz, Heinrich (1857–1894)  39, 57, 80, 81

Heymans, Gerard (1857–1930) 168, 199, 200 Hilbert, David (1862–1943)  108, 109–110 Hoff, Jacobus Henricus van ’t (1852–1911)  85, 87 Holleman, Arnold (1859–1953) 201 Holst, Gilles (1886–1968)  202 Hugenholtz, Petrus (1834–1911) 38 Hupkes, Lubberta see Lorentz-Hupkes, Lubberta Huygens, Christiaan (1629–1695)  28, 99, 218, 257

J Jacobs, Aletta (1854–1929)  45 Jaeger, Frans (1877–1945)  197, 199 Janssen, Hendrik Jan Jacob (1850–?) 4 Janssen, Jan Jacob (ca. 1824–1850)  4 Jeans, James Hopwood (1877–) 151 Joffe, Abram (1880–1960)  250 Jong, W.H. de  10, 12 Julius,Victor August (1851–1902) 79 Julius, Willem Henri (1860–1925)  79, 127–129, 130, 138, 258

K Kaiser, Aletta Catharina see Aletta Catharina Kaiser, Aletta Rebecca  44 Kaiser, Elisabeth (Betsy) (1854–1934)  36, 37 Kaiser, Frederik (1808–1872)  9, 13, 31, 36 Kaiser, Johan Wilhelm (1813–1900)  36, 37, 39, 88 Kaiser, Pieter Jan (1838–1916)  36, 44 Kaiser, Rudolf (1861–?)  36, 37–38

Kamerlingh Onnes, Heike (1853–1926)  30, 35, 63, 77, 78, 85, 88, 115, 116, 117, 122, 123, 124, 127, 136, 143, 176, 198, 206, 230, 233, 238, 239–240, 243, 257, 260 Kapteyn, Johannes (1870–1949)  199, 200 Kaufmann,Walter (1871–1947) 69 Keesom,Willem (1876–1956)  117, 136, 257 Kelvin, Lord (William Thomson) (1824–1907)  84, 190 Kirchhoff, Gustav (1824–1887) 148 Klein, Felix (1849–1925)  31, 166 Kluyver, Albert (1888–1956)  52, 209 Kluyver, Clasina (1884–1974) 52 Knoops,Wessel (1800–1879) 7 Kooper, Johan (1876–1939)  185, 187, 188 Krantz, Cornelis Hendrik (?–1924) 42 Krantz-Van Dijk, E. M. H. 42 Kruyt, Hugo (1882–1959)  210, 213 Kuenen, Johannes (1866–1922)  102, 114, 115, 136

L Langevin, Paul (1872–1946)  98, 180, 252, 255 Laplace, Pierre Simon (1749–1827) 55 Lecointe, Georges (1869–1929) 197 Leeghwater, Jan Adriaanszoon (1575–1650) 183 Leemhorst, Hendrik Carel (Henri) (1884–1973)  138, 178, 257 Leemhorst-Lorentz, Johanna Wilhelmina see Lorentz, Johanna Wilhelmina Legendre, Adrien-Marie (1752–1833) 57

Index Lely, Cornelis (1854–1929)  184–187 Lely, Cornelis Willem (Wim) (1885–1932) 185, 187, 188 Lincker, Otto  16 Lindemann, Frederick (1886–1957) 125 Lorentz, Alexander Cornelis (1858–1861)  5, 6 Lorentz, Friedrich Gottlob (ca.1750–1820) 4 Lorentz, Geertruida Luberta (Berta, Ber) (1885–1973)  5, 6, 10, 13, 26, 27, 28, 33, 38, 39, 45, 49, 103–106, 108, 109, 138, 175–177, 178, 220, 252, 257 Lorentz, Gerrit (1893–1894)  39, 43 Lorentz, Gerrit Fredrik (1822–1893)  4, 6, 16 Lorentz, Hendrikus Albertus (1871–1944) 225 Lorentz, Johanna Wilhelmina (Hannie, Han) (1889–1980)  39, 49, 52, 108, 138, 257 Lorentz, Rudolf (1895–1977)  39, 43, 52, 138, 176, 243, 255, 257 Lorentz, Tobias Hendrik (1791–1857) 4 Lorentz-Hupkes, Lubberta (1820–1897) 6 Lorentz-Van Vollenhoven, Maria Elisabeth (1900–?)  243 Lorentz-Van Ginkel, Geertruida (1826–1861)  4, 6 Lorenz, Adolf (1854–1946)  225, 244 Lorenz, Konrad (1903–1989)  225 Lorenz, Ludvig (1829–1891)  62 Lüders, Heinrich (1869–1943)  211 Lummer, Otto (1860–1925)  148

Mansholt, Lambertus Helprig (1875–1945)  185, 187 Marić, Mileva (1875–1948)  116 Mason, Charles Max (1877–1961) 22 Maupertuis, Pierre Louis (1698–1759) 57 Maxwell, James Clerk (1831–1879)  16, 18, 19, 39, 56–61, 80, 81, 88, 92, 257 Merritt, Ernest (1865–1948)  236 Michaelis, Gerrit Jan 9,  15 Michelson, Albert Abraham (1852–1931)  65–68, 70, 246 Miller, Dayton (1866–1941)  246 Millerand, Alexandre (1859–1943) 204 Millikan, Robert (1868–1953)  153, 213, 214, 221–224, 228, 230, 235–236, 245, 247, 252 Minkowski, Hermann (1864–1909)  73, 128 Mittag-Leffler, Gösta (1846–1927)  60, 70, 92–94 Morley, Edward (1838–1923)  65–68, 246 Muller, Samuel (1848–1922)  165 Murray, Gilbert (1866–1957)  214, 218, 256

N Nernst, Walther (1864–1941)  31, 121–122, 166, 170 Newton, Isaac (1643–1727)  57, 131, 147 Nitobe, Inazō (1862–1933)  216, 217

O Odé, Arend (1865–1955)  256 Ornstein, Leonard (1880–1941)  28, 136 Oersted, Hans Christian (1777–1851) 55



Mansholt, Derk Roelfs (1842–1921) 185

Painlevé, Paul (1863–1933)  210, 211–212, 217


Pelseneer, Jean (1863–1945)  207 Perrin, Jean (1870–1942)  22, 98, 125, 180 Picard, Émile (1856–1941)  168, 197, 205, 207–210 Planck, Max (1858–1947)  33, 73, 88, 92, 103, 108, 117, 122, 147–150, 153, 166, 170–173, 201, 210, 212–213, 241, 257 Poggendorff, Johan Christian (1796–1877) 85 Poincaré, Henri (1854–1912)  66, 70, 88, 92–93, 98, 108, 125, 126 Poincaré, Raymond (1860–1934) 204 Pringsheim, Alfred (1850–1941) 148 Pupin, Michael (1858–1935)  103

R Ramón y Cajal, Santiago (1852–1934) 205, 206, 237 Rayleigh, Lord (John William Strutt) (1842–1919)  88, 98, 122, 148 Reedt Dortland, Pieter van (1871–1941) 52 Reineke, Pieter (1870–1948)  238 Reynold, Gonzague de (1880–1970) 214, 218–219 Rijke, Petrus Leonardus (Pieter) (1812–1899)  13, 17, 18, 19–20, 30 Roemers Visscher, Maria Tesselschade (1594–1649)  43 Röntgen, Wilhelm (1845–1923)  79, 92, 100, 102, 166 Rubner, Max (1854–1932)  211 Rutgers, Arend (1903–1998)  144, 145 Rutgers, Victor (1877–1945)  239 Rutherford, Ernest (1871–1937)  121, 124, 154, 252, 255



S Sande Bakhuyzen, Hendricus Gerardus van de (1838–1923) 30 Savornin Lohman, Witius Hendrik de (1864–1932)  130 Schrödinger, Erwin (1887–1961)  155, 245–246 Schuster, Arthur (1851–1934)  197, 199, 200, 209 Schwarzschild, Karl (1873–1916) 158 Siertsema, Lodewijk (1863–1945)  36, 101 Sillevis, Annie (1880–1960)  52 Simpson, George Clarke (1878–1965) 212 Sissingh, Remmelt (1858–1927) 78 Sitter,Willem de (1872–1934)  159, 160, 233, 239, 243, 252 Solvay, Ernest (1838–1922)  120–122, 125, 126, 166, 181, 222, 230 Sommerfeld, Arnold (1886–1951)  90, 108, 121, 133, 134, 136 Stadt, Hendrik van de (1842–1915)  9, 10, 11, 12, 13, 14, 16, 30 Stevin, Hendric (1614–1668)  183 Stevin, Simon (1548–1620)  183 Stoel, Willem Frederik (1831–1903) 187

Suchtelen, Nicolaas van (1878–1949)  168, 169, 260 Swaters, Daniel Johannes (1822–1888)  6, 7

T Teyler van der Hulst, Pieter (1702–1778) 111 Thijsse, Johannes Theodoor (1893–1984)  187, 188, 189, 190, 193, 194, 243 Thomson, Joseph John (1856–1940)  64, 88, 98 Thorbecke, Johan Rudolph (1798–1872) 8 Timmer, Geurt Kornelis  7–8, 9

V Ven, Elisa van der (1883–1909) 112 Voigt, Woldemar (1850–1919)  31, 77, 88, 109, 117, 167–168 Vollenhoven, Maria Elisabeth van see Lorentz-Van Vollenhoven, Maria Elisabeth Volta, Alessandro (1745–1827) 247 Volterra, Vito (1869–1940)  197 Vreede, Enny (1883–1919)  178 Vries, Hugo de (1848–1935)  85, 105, 198

W Waals, Johannes Diderik van der (1837–1923)  18–19,

20, 22–23, 30, 75, 76, 78, 83, 85, 86, 87, 88, 113, 114, 122, 130–131, 135 Waals Jr., Johannes Diderik van der (1873–1971)  114 Warburg, Emil (1846–1931)  172 Waterink, Jan (1890–1966)  144 Wenckebach, Ludwig Oswald (1895–1962) 257 Went, Frits (1863–1935)  175, 201, 210, 213, 240–241 Wien, Wilhelm (1864–1928)  33, 69, 83, 103, 124, 148–149, 166, 170–171 Wiener, Otto (1862–1927)  100 Wind, Cornelis Harm (1867–1911)  79, 127 Wolfskehl, Paul (1856–1906)  108 Wortman, Hendrik (1859–1939) 187

Z Zangger, Heinrich (1874–1957) 127 Zeeman, Pieter (1865–1943)  1, 29, 47, 62–64, 70, 76, 77, 78, 82–83, 84, 87, 88, 91–97, 113–114, 144, 168, 169, 174–175, 198, 203, 206, 233, 238, 247, 251, 253 Zimmern, Alfred (1879–1957) 252

(1) Jan Jacob Janssen ±1824–1850

5 children

Wander Johannes 1910 de Haas 1878–1960

Geertruida Luberta (Berta) Lorentz 1885–1973

4 children

Hendrik Carel (Henri) 1912 Leemhorst 1884–1973

Gerrit Lorentz 1893–1894

4 other children

Maria Elisabeth 1926 van Vollenhoven 1900–?

Rudolf Lorentz 1895–1977

Rudolf Kaiser 1861–?

Johanna Buisman 1821–1907

5 brothers and sisters Frederik Kaiser 1808–1872

Elisabeth (Betsy) Kaiser 1854–1934


Johan Wilhelm Kaiser 1813–1900

Johanna Wilhelmina (Hannie) Lorentz 1889–1980

Hendrik Antoon Lorentz Aletta Catharina Kaiser 1853–1928 1858–1931 1881

Alexander Cornelis Hendrik Jan Jacob Lorentz Janssen 1858–1861 1850–?

(2) (3) Lubberta Gerrit Frederik Geertruida Hupkes 1862 Lorentz 1852 van Ginkel 1820–1897 1822–1893 1828–1861

1. The first portrait photograph of Hendrik Antoon Lorentz at the age of six, made at the country fair he visited with his mother in 1860.

2. Lorentz as a secondary-school student.

3. The final class of the Arnhem HBS in 1860. From left to right: Herman Haga,W. H. de Jongh, and Lorentz. The rifle against the windowsill was owned by the school’s janitor.

4. Lorentz in 1875, at the time of his dissertation defense.

5. Lorentz in 1902, on the occasion of the Nobel Prize.

6. Young Aletta Kaiser.

7. The Lorentz family in their vacation home in Noordwijkerhout, around 1900. From left to right: Lorentz, Berta, Rudolf, Aletta, presumably a maid, and Hannie.

8. Lorentz around 1900.

9. Pieter Zeeman in 1902, on the occasion of the Nobel Prize.

10. Max Planck in 1919. This German physicist laid the foundation for quantum theory.

11. Albert Einstein with cigar, around 1920.

12. The Austrian Paul Ehrenfest, Lorentz’s successor in Leiden.

13. Einstein, Ehrenfest, and Paul Ehrenfest Jr. at the Ehrenfest home.

14. Heike Kamerlingh Onnes with the helium liquefactor in the Leiden laboratory in 1913.

15. Ehrenfest, Lorentz, Niels Bohr, and Kamerlingh Onnes in the Leiden laboratory in 1925.

16. The German physicistWoldemarVoigt, who carried on a regular correspondence with Lorentz after 1883.

17. Nobel laureate Marie Curie in her laboratory.

18. The 1911 Solvay Conference participants. Lorentz and Ernest Solvay seated next to each other at the head of the table. Marie Curie in conversation with Henri Poincaré.To the right are Einstein and Kamerlingh Onnes. In the middle is Planck, with his radiation law written out on the blackboard behind him.

19. Lorentz and Einstein at the Ehrenfest home around 1922.

20. Lorentz at his farewell lecture in May 1923.

21. Dinner guests on December 11, 1925 in the building of the Hollandsche Maatschappij der Wetenschappen. Seated beside Lorentz are Marie Curie, Berta, Rudolf’s fiancée Maria Elisabeth van Vollenhoven, and Rudolf himself; beside Aletta are the Minister of Education, Arts and Science,Victor H. Rutgers, and Hannie.

22. Lorentz and Aletta on the fiftieth anniversary of his doctorate, on December 11, 1925.

23. Informal portrait of Lorentz in middle age.

24. The funeral procession on Grote Markt in Haarlem, February 9, 1928.

25. Lorentz in 1925, on the occasion of the fiftieth anniversary of his doctorate.