THE SOLVAY CONFERENCES ON PHYSICS Aspects of the Development of Physics since 1911 The Solvay Conferences on Physics

ASPECTS OF THE DEVELOPMENT OF PHYSICS SINCE 1911

by Jagdish Mehra

With a Foreword by Werner Heisenberg

D. Reidel Publishing Company Dordrecht-Holland / Boston-U.S.A. 1975 LibflU)' of Congress Cataloging in Publication Data

Mehra, Jagdish. The Solvay Cooferc:ooes on Physics.

locludes bibliographical references and index. I. Solvay Cooferenceon Physics. 2. Solvay, Emest,1838-1922. 3. Physics-History. QCI.5792M43 530'.09 75-28332 ISBN-13: 978-94-010-1869-2 e-ISBN-13: 978-94-010-1867-8 DOl: 10.10071978-94-010-1867-8

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All Rights Reserved Copyright C 1975 by D. Reidel Publishing Company, Dordrecht, Holland Sottcover reprint orthe hardcover 1st edition 1975 No pari of the material protected by this copyriJhl notice may be reproduced or utili:oed in any form or by any means, electronic or mechanical, including photocopyi.og, recording or by any infonnational storage and retrieval system, without written permission from the copyright owner Foreword

Jagdish Mehra's historical account of the Solvay Conferences from 1911 to 1973 demonstrates not only the great influence which these conferences have had on the development of modern physics, but it also shows clearly how far-sighted and well• planned were the intentions of Ernest Solvay when he took the initiative for organizing a new type of international conferences. In contrast to the conventional meetings in which reports are given on the successful solution of scientific problems, the Solvay Conferences were conceived to help directly in solving specific problems of unusual difficulty. The importance of the quantum structure of Nature had become well under• stood already by 1911, but at that time there was no hope for an answer to the ex• tremely difficult new questions posed by the atomic phenomena. The new conferences should therefore be devoted primarily to thorough discussions of such problems be• tween a small number of the most competent physicists, and Ernest Solvay was guided by the hope that the discussions would eventually lead to a real and substantial progress. The earliest Solvay Conferences which I attended were those of 1927, 1930 and 1933, and they served this purpose extremely well. In 1926 the mathematical formalism of quantum- and wave-mechanics approached its final shape, but the interpretation was still controversial. Schrodinger hoped that his matter waves could be considered as waves in three-dimensional space and time, and that the discontinuous feature of quantum 'jumps' could be avoided thereby. Born, in his theory of collisions, had given the statistical interpretation of the waves in a many-dimensional configuration space. The members of the Copenhagen group, primarily Bohr, Kramers, Pauli and I, after a thorough analysis of the uncertainty principle and the concept of com• plementarity, had come to the conviction that the paradoxa of quantum theory could be finally resolved within their philosophy, and that the new interpretation would answer all the hard questions for a well-defined experimental situation. However, there were many problems in which this final answer had not yet been given ex• plicitly. Therefore the discussions at the 1927 Solvay Conference, from the very beginning, centred around the paradoxa of quantum theory. The Compton effect emphasized the apparent wave-particle duality; the flexibility of the mathematical formalism demonstrated that the two pictures, waves and particles, may be compatible, if the limited range of their applicability is taken properly into account. Einstein criticized this very limitation because it seemed to undermine the ideal of an objective descrip• tion of Nature, which had been considered to lie at the basis of physics. Besides it in• troduced a statistical element into the foundations of physics, which Einstein would VI FOREWORD not admit. Einstein therefore suggested special experimental arrangements for which, in his opinion, the uncertainty relations could be evaded. But the analysis carried out by Bohr and others during the Conference revealed errors in Einstein's arguments. In this situation, by means of intensive discussions, the Conference contributed direct• ly to the clarification of the quantum-theoretical paradoxa. The next meeting (1930) dealt largely with the applications of quantum mechanics to problems of general interest in magnetism, such as the magnetic behaviour of solid bodies or the fine structure of spectral lines. The discussions made visible the wide field that had been opened up by the final understanding of quantum theory and they spread the knowledge of the new methods and their use in many parts of physics. The discussions on the paradoxa of quantum theory were taken up again between Bohr and Einstein. When Einstein discussed an experiment in which the energy of a photon was measured by its gravity, Bohr was able to demonstrate that the influence of the gravitational field on the frequency of light, as described in Einstein's theory of general relativity, was indeed just sufficient to guarantee the uncertainty relations; the inner consistency of quantum mechanics could not be better demonstrated. The whole weight of the Conference lay on the discussions, not on the reports, and the results justified the hopes of Ernest Solvay that this style would immediately foster progress in physics. Three years later, in the meeting of 1933, the interest had changed from quantum theory to the structure of the atomic nucleus. The discovery of the neutron by Chad• wick, and of the positron by Dirac, Anderson and Blackett, had raised entirely new theoretical problems. If the nucleus consists of protons and neutrons, does it also contain electrons? Or are the electrons created in p-decay out of energy, as the posi• tron-electron pairs are created by jI-quanta? Pauli enunciated his hypothesis of the neutrino. Again, theoretical research was actually carried forward at the Con• ference by means of discussions between those who had the best insight into the dif• ficult new problems. There can be no doubt that in those years the Solvay Conferen• ces played an essential role in the history of physics. I have taken up these reminiscences in this foreword in order to emphasize that the historical influence of the Solvay Conferences on the development of physics was connected with the special style introduced by their founder: a small group of the most competent specialists from various countries discussing the unsolved problems of their field and thereby finding a basis for their solution. During the period following the Second World War the situation in physics had changed. Progress was mainly due to the new experimental results, e.g. the observa• tions concerning the spectrum and the interaction of particles. Mter the radical changes brought about by the discovery (in 1932) of anti-particles and anti-matter had been interpreted and understood, no fundamental difficulty had appeared which would foreshadow new radical changes in the foundation of physics. The main obstacle to further progress seemed to be the high degree of complexity in the spectrum of par• ticles, and with this obstacle the methods of the Solvay Conferences were perhaps less efficient than with the fundamental problems of the early 1920s. Nevertheless the FOREWORD vn

Solvay Meetings have stood as an example of how much well-planned and well• organized conferences can contribute to the progress of science, and this book provides a testimony of that progress.

Munich. 19 November 1974 'Acknowledgements

On several occasions during the 1970 Solvay Conference (on the symmetry proper• ties of nuclei) I found myself answering various questions about the Solvay Confer• ences on Physics: their origin, the fundamental problems which had come up for dis• cussion and highlights of the encounters between famous physicists at the earlier Conferences, etc. It was possible to answer some of these questions because in previous visits to Brussels I had received the opportunity of studying many documents * relating to the Conferences. Moreover, I had had the benefit of conversations with Professors Niels Bohr, P. A. M. Dirac, Werner Heisenberg, Wolfgang Pauli, and Leon Rosen• feld t, all of whom attended the 1927 and 1930 Conferences (on quantum mechanics and magnetism, respectively) and had witnessed the Einstein-Bohr discussions. Pro• fessors Bohr, Heisenberg, Pauli and Rosenfeld continued to take an active interest in the later Conferences, and I had been able to gather much interesting and useful in• formation from them. Hitherto the only published accounts of the early Solvay Conferences, other than references to them in letters and memoirs, were: (1) Maurice de Broglie, Les Premiers Congres de Physique Solvay (Editions Albin Michel, Paris 1951), which gave short biographical sketches of the participants in the first Conseil Solvay and a very brief report on the first three Conferences, and (2) N. Bohr, 'The Solvay Meetings and the Development of Quantum Physics' (presented at the 1961 Solvay Conference), in which Bohr discussed the fundamental problems which came into focus at different times. During the 1970 Conference Professor I1ya Prigogine, Director of the I nstituts Internationaux de Physique et de Chimie (Solvay), expressed to me the wish that the Solvay Institutes would welcome a book giving a survey of the scientific content of the reports and discussions at the Solvay Conferences, and he thought that it might also be of interest to physicists and historians of science. Mr Jacques Solvay, President, the Administrative Council of the Solvay Institutes and great-grandson of Ernest Solvay, concurred in the project. I take great pleasure in thanking Professor Ilya Prigogine for inviting me to write this work as a scientific member of the Solvay Institutes during the academic year

• Extracts from a number of these documents are quoted in Jean Pelseneer's (unpublished) Historique des Instituts Internationaux de Physique et de Chimie (Solvay). t Leon Rosenfeld was not an invited participant in the 1927 and 1930 Conferences. He went to the former in order to take up contact with Max Born about the possibility of working with him in Gottingen. He worked with Born during 1927-29, then went to Pauli in Zurich. At the 1930 Confer• ence, Rosenfeld recalled, 'I did not attend it, but being in Brussels at the time, I hovered around, especially at the Club of the Fondation Universitaire, where the participants gathered after the ses• sions.' x ACKNOWLEDGEMENTS

1973-74, and for his warm hospitality during my stay in Brussels. I also wish to thank Professor Andre Jaumotte, President, Universite libre de BruxeIIes, for election to an Invited Professorship and hospitality of the University. Mr and Madame Jacques Solvay took a personal interest in my work and encouraged me throughout, for which I am grateful.

JAGDISH MEHRA Contents

Foreword, by Werner Heisenberg v

Acknowledgements IX

List of Plates XII

Introduction XIII

1 Ernest Solvay and the Origin of Solvay Conferences on Physics 2 Radiation Theory and the Quanta 13 3 The Structure of Matter 75 4 Atoms and Electrons 95 5 The Electrical Conductivity of Metals 115 6 Electrons and Photons 133 7 Magnetism 183 8 The Structure and Properties of Atomic Nuclei 207 9 Towards the Spectrum of Elementary Particles and the Hierarchy of Interactions 227 10 The Elementary Particles 239 11 Quantum Field Theory 269 12 Fundamental Problems in Elementary Particle Physics 299 13 Symmetry Properties of Nuclei 323 14 Solid State Physics 339 15 Astrophysics, Gravitation, and the Structure of the Universe 381

Index of Names 405 List of Plates

Ernest Solvay XXXIV

First Solvay Conference (1911) 12 Second Solvay Conference (1913) 74 Third Solvay Conference (1921) 94 Fourth Solvay Conference (1924) 114 Fifth Solvay Conference (1927) 132 Sixth Solvay Conference (1930) 182 Seventh Solvay Conference (1933) 208 Eighth Solvay Conference (1948) 238 Ninth Solvay Conference (1951) 338 Tenth Solvay Conference (1954) 356 Eleventh Solvay Conference (1958) 382 Twelfth Solvay Conference (1961) 268 Thirteenth Solvay Conference (1964) 388 Fourteenth Solvay Conference (1967) 298 Fifteenth Solvay Conference (1970) 322 Sixteenth Solvay Conference (1973) 396

Signatures of Some Famous Physicists, from the Fifth Solvay Conference (1927) 180

Some Drawings and Doodles, by H. J. Bhabha 266 Introduction

In a meeting with the distinguished German physico-chemist Walther Nernst in spring 1910, Ernest Solvay learnt about the difficulties of reconciling the consequences of the MaxwelI-Boltzmann molecular-kinetic theory with the quantum conceptions of Planck and Einstein in the domains of specific heats and the theory of radiation. To discuss and hopefulIy to solve these difficulties, Ernest Solvay convened an interna• tional conseil de physique in October 1911, to which he invited some of the most prominent physicists of the day - including Lorentz, Planck, Einstein, Sommerfeld, Wien, Kameriingh Onnes, Rutherford, Marie Curie, Jean Perrin, Langevin, Marcel Brillouin and Poincare. The formal reports at the conseil covered a large range of problems and opinions, from James Jeans's attempt to explain all the apparent failures of the classical theory without invoking the quantum ideas to Einstein's arguments in favour ofthe absolute inevitability of the quantum structure of radiation. H. A. Lorentz served as chairman at alI the sessions of the conseil. The discussions after each report were free, intense and often very pointed. For instance, Poincare dismissed Jeans's attempts with the remark: 'That is not the role of physical theories. One must not introduce as many arbitrary constants as there are phenomena to be explained. The goal of physical theory is to establish a connection between diverse experimental facts, and above all to predict.' Einstein complained that Planck's way of using the probability was 'somewhat shocking', because it deprived the Boltzmann relation of any physical content. Lorentz needed alI his vast scientific knowledge, mastery of languages, and incomparable tactfulness, to keep the discussions focused on the problems at hand and yet allowing each participant's views to come through. The conseil succeeded in sharpening the issues. Paul Langevin acknowledged the power of quantum theory to discover totalIy unexpected relationships among phe• nomena as apparently distinct as optical absorption frequencies and specific heats. Marcel Brillouin concluded that, 'From now on we will have to introduce into our physical and chemical ideas a discontinuity, something that changes in jumps, of which we had no notion at alI a few years ago.' Planck's fear, expressed to Nernst before the conseil, that hardly anyone would feel the 'urgent necessity' of a reform lead• ing to a solution of the 'unresolved problems', could now be dismissed. Ernest Solvay's foresight and initiative, Walther Nernst's organizing ability, and Hendrik Lorentz's scientific authority had played the decisive role in the success of the conseil. Einstein's attitude towards the conseil seems to have been ambiguous. To Nernst he had written: 'I shall gladly prepare the report assigned to me. The whole enterprise appeals to me, and I hardly doubt that you are the spirit behind it.' But in letters to XIV INTRODUcnON his friend Michele Besso, he complained that he was 'plagued with my tittle-tattle' (his report on specific heats) for the 'witches' sabbath in Brussels'.1 A few weeks after the conseil, he wrote to Besso again: 'I have not made any further progress in electron theory. In Brussels, too, one lamented at the failure of the theory without finding a remedy. This Congress had an aspect similar to the wailing at the ruins of Jerusalem. Nothing positive came out of it. My treatment of fluctuations aroused great interest, but elicited no serious objection. I did not benefit much, as I did not hear anything which was not known to me a1ready.'2* However, in his letter to Ernest Solvay (from Prague, 22 November 1911), Einstein wrote: 'I thank you sincerely for the extremely beautiful week which you provided us in Brussels, and not the least for your hospitali• ty. The Solvay Congress shall always remain one of the most beautiful memories of my life.' The sentiment which Einstein expressed in his letter to Solvay reflected the feelings of the others as well, all of whom sent enthusiastic messages of appreciation for the opportunity they had had of participating in the 'Conseil Solvay'.

• Immediately on his return to Prague from the first Solvay Conference, Einstein wrote to his friend Heinrich Zangger, Director of the Institute for Forensic Medicine of Zurich University: 'I returned home yesterday from Brussels, where I spent much time together with Jean Perrin, Paul Langevin and Madame Curie, and I am just delighted with these people. The latter even promised me to visit us with her daughters. The thiiller t spread around in the newspapers is nonsense. That Langevin wants to get a divorce has been known for some time. If he loves Madame Curie and she him, then they don't need to elope as they have enough opportunity of meeting each other in Paris. I did not at all get the impression that there was anything special between them; rather [they] enjoyed being together in a harmless way. I also don't believe that Madame Curie is either domineering or has some other such amiction. She is a straightforward, honest person whose duties and burdens are just too much for her. She has a sparkling intelligence. but in spite of her passionateness she is not attractive enough to become dangerous for anyone ... H. A. Lorentz presided [at the Conference] with incomparable tact and incredible virtuosity. He speaks all three languages [French. German. English] equally well and possesses unique scientific acumen. I was able to convince Planck to a large extent about my views. now that he has resisted them for years. He is a totally honest man who does not worry about himself.' Einstein continued his comments on the conseil in another letter to Zangger, dated Prague. 16 November 1911: 'It was most interesting in Brussels. Other than the French - Curie. Langevin. Perrin and Poincare -. and the Germans - Nernst. Rubens. Warburg and Sommerfeld -, Rutherford and Jeans were present. Of course. H. A. Lorentz and Kamerlingh Onnes as well. Lorentz is a miracle of intelligence and subtle tact - a living work of art. In my opinion he was the most intelligent of all the theoreticians present. Poincare was altogether simply negative about the relativity theory and. in spite of all [his] acumen. showed little understanding for the situation. Planck is untractable about certain preconceived ideas which are. without any doubt. wrong ...• but nobody really knows. The whole thing [the consell] would have been a delight for the diabolical Jesuit fathers.'

Note: Einstein's letters to Heinrich Zangger. quoted in Carl Seelig (Ed.). Helle Zeit - Dunkele Zeit. In Memorium Albert Einstein, Europa Verlag. Zurich 1956. have been translated into English and used by permission of the publisher. t The reference is to the 'breath of scandal' concerniDs the Curio-Lan8cvin 'dalr' which fint made its way into Le Journal of Paris on 4 November 1911 while Madame Curie and Paul Lansevln were still in Brussels. The newspaper implied that the two had eloped from Paris. L'lndlpendanu beIge took up the story the next day in Brussels, and printed a vehement declara• tion of Madame Curie and Jean Perrin'. clarification of the circumstances and his expression of complete IUpport for Curie and Lanaevin. For details of the Curio-Lansevin relatioDihip, see A tro.er6 MWC !Iicle.: Sou.enlrs Itt rencontrlt6 by Camille Marbo (dauihter of the French mathematician Emile Borel), Editions Bernard Grasset, Paris 1968, and Robert Reid, Mark Curie, Collins, London 1974. INTRODUCTION xv

Encouraged by the success of this conference Ernest Solvay, with the counsel and help of H. A. Lorentz, established a foundation on 1 May 1912, initially for a period of thirty years, to be called the Institut International de Physique, with the goal 'to encourage the researches which would extend and deepen the~knowledge of natural phenomena'. The new foundation was intended to concentrate on 'the progress of physics and physical chemistry', including the problems pertaining to them in other branches of the natural sciences.3 The inclusion of physical chemistry, as one of the fields to be encouraged by the Institute, was intended as a tribute to the role which Walther Nernst had played in the organization and success of the filst Solvay Con• ference. The activities of the Institute were to be directed by two committees: (a) An Ad• ministrative Council, consisting of three Belgian members: (1) Ernest Solvay or a person designated by him, (2) a member designated by the King of the Belgians, and (3) a member designated by the Universite libre de Bruxelles. In the absence of Mr Solvay, the other two members would invite one of his descendants to become a member; (b) An International Scientific Committee, consisting of nine regular mem• bers, to which an 'extraordinary' member could be added. This Committee was given the responsibility for directing the scientific activities, and at the time of the establish• ment of the Institute it consisted of: H. A. Lorentz, President, Madame Marie Curie, Marcel Brillouin, R. B. Goldschmidt, H. Kamerlingh Onnes, M. Knudsen, W. Nernst, E. Rutherford, and E. Warburg. Article 10 of the Statutes3 required that, 'At times determined by the Scientific Committee a Conseil de Physique, analogous to the one convened by Mr Solvay in October 1911, will gather in Brussels, having for its goal the examination of significant problems of physics or physical chemistry.' In 1913, after a series of exchanges with Wilhelm Ostwald and William Ramsay, Ernest Solvay established another foundation, the Institut International de CMmie, and the responsibility for activities related to chemistry was passed on to it. The two foundations were ultimately united into Les Instituts Internationaux de Physique et de Chimie, each of them having its own Scientific Committee. The first Solvay Conference on Physics had set the style for a new type of scientific meetings, in which a select group of the most well informed experts in a given field would meet to discuss the problems at its frontiers, and would seek to define the steps for their solution. But for the interruptions caused by the two World Wars, these international conferences on physics have taken place almost regularly since 1911 in Brussels. They have been unique occasions for physicists to discuss the fundamental problems which were at the centre of interest at different periods, and have stim• ulated the development of physical science in many ways . • • • H. A. Lorentz, in his capacity as President of the Scientific Committee, directed the work of the first five conferences. These conferences dealt with radiation theory and the quanta (1911), the structure of matter (1913), the atoms and electrons (1921), XVI INTRODUcnON the electrical conductivity of metals (1924), and the electrons and photons (1927). At the second Solvay Conference (1913), the centre of interest was Laue's discovery in 1912 of the diffraction of X-rays in crystals. The structure of matter was discussed on the basis of J. J. Thomson's atomic model, to which Madame Curie took serious objection in view of the laws of radioactive transformations. The reference to Ruther• ford's nuclear model of the atom was made by Rutherford himself. At the third Solvay Conference Rutherford gave a detailed account of his atomic model and of the many phenomena which, in the meantime, had been successfully interpreted on its basis. During the discussions Rutherford mentioned his conception of the neutron. 'It has occurred to me', he declared, 'that the hydrogen of the nebulae might consist of particles which one might call "neutrons", which would be formed by a positive nucleus with an electron at a very short distance. These neutrons would hardly exercise any force in penetrating into matter .... The electron is much closer to the nucleus in the neutron than in hydrogen.' Also during the discussions at the third Solvay Conference, Madame Curie con• tended that while the electrostatic forces could explain the speeds of p-particles, they were not compatible with the stability of the nucleus. The latter would require non• Coulomb forces acting within a very short distance of the nucleus. The new quantum mechanics, complete with the Copenhagen interpretation, was presented at the fifth Solvay Conference (1927). After Compton's lecture, Madame Curie made the prophetic remark that the Compton effect might have important ap• plications in biology, and that the high voltage technique employed in the production of high frequency X-rays would find important uses for 'therapeutic' purposes. Louis de Broglie gave an exposition of his theory of the 'double solution' in the form of the 'pilot wave', but 'It received hardly any attention. Physicists such as Planck, Lorentz and Langevin, accustomed to the old methods, were hoping for an interpretation of wave mechanics in accordance with classical concepts but they made no pronouncement upon its nature. Schrodinger remained faithful to a pure wave interpretation. Only Einstein encouraged me somewhat on the path I wished to tread. But I was faced with redoubtable adversaries: Niels Bohr and Max Born, scientists of world renown. And there was also the group of young researchers who formed the Copenhagen School, amongst them, in particular, Pauli, Heisenberg and Dirac, who were already authors of remarkable works. They interpreted the duality of corpuscles and waves by the theory of complementarity recently proposed by Bohr and, no longer attributing to the arbitrarily normalised wave of Schrodinger any more than the role of representing the probability of certain observations being obtained, they concluded by abandoning any clear picture of a wave or a particle. I was very distressed. I found Bohr's complementarity quite obscure and I did not like abandon• ing physical images which had guided me for many years. However, the probabilistic interpretation of "quantum mechanics", developed by numerous young keen re• searchers possessing great facility in mathematical calculations, rapidly took the form of elegant and rigorous mathematical formalism.'4 The new orthodoxy had already taken shape. For instance, in the general discussion INTRODUC'I10N xvu following Bohr's report on 'The quantum postulate', Dirac commented at length on the essential differences between the classical and quantum descriptions of physical processes. Quantum theory, he said, describes a state by a time-dependent wave func• tion !/I, which can be expanded at a given time 11 in a series containing wave functions !/In with coefficients en. The wave functions !/In are such that they do not interfere at an instant 1>11• Now Nature makes a choice sometime later and decides in favour of the state !/In with the probability /cn/ 2• This choice cannot be renounced and deter• mines the future evolution of the state. Heisenberg opposed this point of view by as• serting that there was no sense in talking about Nature making a choice, and that it is our observation that gives us the reduction to the eigenfunction. What Dirac called a 'choice of Nature', Heisenberg preferred to call 'observation', showing his predilection for the language he and Bohr had developed together. During the general discussion, Lorentz did his best to give the floor to one speaker at a time. However, many of the participants felt strongly about the interpretation of the new theory and wished to express their views. The discussion became very animated, with several participants speaking at the same time, each in his own language. Ehren• fest went to the blackboard, which successive speakers had used, and wrote on it: 'The Lord did there confound the language of all the Earth.' The Einstein-Bohr dialogue on deterministic description versus statistical causality also started at the fifth Solvay Conference. (See Chapter 6, Appendix.) During one of the lectures, Paul Ehrenfest passed on a note to Einstein, saying 'Don't laughl There is a special section in purgatory for professors of quantum theory, where they will be obliged to listen to lectures on classical physics ten hours every day.' To which Einstein replied, 'I laugh only at their naivete. Who knows who would have the laugh in a few years l' Upon Lorentz's death in February 1928, Paul Langevin became President of the Scientific Committee and the two following conferences, on magnetism (1930) and on the structure and properties of atomic nuclei (1933), were held under his leadership. During the sixth Solvay Conference (1930) Pauli remarked on the impossibility of measuring the magnetic moment of a free electron, and Bohr emphasized that 'the direct inobservability of the intrinsic magnetic moment of the electron does not imply that the concept of spin has lost its significance as a means of explaining the fine struc• ture of spectral lines and the polarization of electron waves. Only the manner in which the concept of spin appears in the formalism of quantum mechanics is such that it does not lend itself to an independent interpretation based on classical notions.' The Einstein-Bohr discussions, on the question whether the quantum mechanical description of physical reality is 'complete', were continued at the sixth Solvay Con• ference. These discussions took place at the Club of the Fondation Universitaire. Leon Rosenfeld has recalled: 'It was the occasion when Einstein thought to have found a counter-example of the uncertainty principle with his well-known box from which a photon is emitted at a certain time, and a weighing of the box before and after the emission determines the energy of the emitted photon. It was quite a shock for Bohr to be faced with this problem; he did not see the solution at once. During the xvm INTRODUcnON whole evening he was extremely unhappy, going from one to the other and trying to persuade them that it couldn't be true, that it would be the end of physics if Einstein were right; but he couldn't produce any refutation. 1 shall never forget the vision of the two antagonists leaving the Club: Einstein, a tall, majestic figure, walking quietly, with a somewhat ironical smile, and Bohr trotting near him, very excited, ineffectually pleading that if Einstein's device would work, it would mean the end of physics. The next morning came Bohr's triumph and the salvation of physics; Bohr had found the answer: the displacement of the box in the gravitational field used for the weighing would disturb the frequency of the clock governing the photon emission just to the amount needed to satisfy the uncertainty relation between energy and time .... Who knows, this may change. It is one of the great open questions whether gravitation does or does not play any part in the interactions at the subnuclear level.'s Rosenfeld also remembered the occasion during the sixth Solvay Conference when, 'I met Louis de Broglie- in the street near the Club, and we got into conversation. 1 told him that Heisenberg and Pauli had started building up quantum electrodynamics and were encountering great difficulties. With an expression of great surprise he asked: "What difficulties 1" '5 Ernest Solvay had died on 26 May 1922. His monument, erected near the Univer• site libre de Bruxelles on what is now called Avenue Franklin Roosevelt, was unveiled on 16 October 1932. On this occasion a national homage, under the patronage of King Albert of the Belgians, was rendered to Solvay, and Paul Langevin gave one of the main eulogies.6 Armand Solvay, Ernest's eldest son, carried on the tradition of his father; he re• presented the family on the Administrative Council of the Institute at the 1924 and 1927 Conferences. Armand died in February 1930 and his son, Ernest-John Solvay, took over his functions at the 1930 Conference. Eduard Herzen, professor at the Ecole des Hautes Etudes de Bruxelles, represented the Solvay family at the 1933 Conference. At the seventh Solvay Conference (1933), Rutherford expressed his pleasure at the development of the 'modem alchemy', the science of nuclear and particle physics. Rutherford reported the results of his recent experiments, with Oliphant, on the bombardment of lithium with protons and deuterons, yielding evidence about the existence of hitherto unknown isotopes of hydrogen and helium with atomic mass 3. Chadwick reported on the discovery of the neutron. Heisenberg had immediately grasped this discovery as the new foundation of nuclear structure, with neutrons and protons as the proper nuclear constituents. He had developed a theory of nuclear structure on its basis, and gave a report on it at the seventh Solvay Conference. The Joliot-Curies discussed the assumption concerning the complex structure of the proton, it being formed of a neutron and a positron. They would soon (1934) go on to discover artificial radioactivity. Blackett told the story of the discovery of the positron by C. D. Anderson and

• L. de Broglie was not an invited participant at the sixth Solvay Conference (1930), but he was visiting Brussels at the time. INTRODUctION XIX himself in cosmic ray researches, and its interpretation in terms of Dirac's relativistic quantum theory. Dirac, in his report on the theory of the positron, announced a logarithmically divergent charge renormalization. But he also pointed out a fascinating finite correc• tion to electrodynamics, to the polarization of the vacuum. This was the beginning of a new development which was to bear its most valuable fruits fifteen years later in renormalization theory. Niels Bohr had suggested earlier that the laws of energy and momentum conserva• tion might possibly be violated in p-decay. At the 1933 Conference, Pauli expressed his belief that the conservation principles of energy, momentum, angular momentum, and statistics were valid in al/ elementary processes, and that his conception of the neutrino, which he had proposed in June 1931 at Pasadena, would help in upholding them in p-decay. Assuming that energy and momentum conservation could be applied, and that only one neutrino was emitted in p-decay, Francis Perrin made a relativistic calculation and reported that the intrinsic mass of the neutrino had to be zero. Fermi (1934) would use the conception of the neutrino in his theory of p-decay, but twenty-two years later an important conservation law (that of parity) would be found to break down in weak decays. • • • The Solvay Conferences had been held every three years since 1921, and plans were made to hold the eighth Solvay Conference in 1936. The Scientific Committee, con• sisting of Langevin, Bohr, Cabrera, Debye, De Donder, Richardson, and Verschaffelt met on 29-31 October 1935 at the Club of the Fondation Universitaire in Brussels to decide upon a programme for the meeting. They selected 'Cosmic Rays and Nuclear Physics' as its theme, to be discussed from 26 to 31 October 1936, and left the rest in the hands of Paul Langevin. A long period of illness during 1935-36 prevented Langevin from taking the initiative for holding the eighth Solvay Conference in 1936, and it was postponed to 1937. Rapid scientific developments in nuclear physics and Langevin's continuing bad health made a further delay necessary. The Scientific Committee met again on 22-23 October 1938 to devise a programme and to prepare a list of speakers and invited participants. It was decided that the con• ference would deal with the problems of elementary particles and their mutual inter• actions, and would be held from 22 to 29 October 1939. The invited speakers and their subjects were: W. Pauli and W. Heisenberg (general properties of elementary particles); G. Wentzel (interactions between protons, neu• trons and electrons); W. Heider (the heavy electron, i.e. meson, from a theoretical point of view); P. Blackett (the meson from an experimental point of view); J. Solo• mon (the beta-spectrum and the theory of the neutrino); P. Auger (the showers from the experimental viewpoint); L. de Broglie (the theory of the photon); C. F. von Weizsacker (astronomical indications concerning the properties of particles); and F. Bloch (the magnetic moments of protons and neutrons). xx INTRODUcnON

The other invited participants included: C. D. Anderson, W. Bothe, A. H. Comp• ton, P. A. M. Dirac, E. Fermi, G. Gamow, H. Geiger, Irene Joliot-Curie, F. Joliot, H. A. Kramers, Lise Meitner, F. Perrin, A. Proca, F. Rasetti, B. Rossi, M. A. Tuve, E. J. Williams, and H. Yukawa. The Scientific Secretaries were to be: E. Stahel, H. J. Bhabha, M. Cosyns, J. Geheniau, C. Meller, L. Rosenfeld, and J. Solomon. World War II started on 3 September 1939. In a letter addressed on the same day to Charles Lefebure, Secretary of the Administrative Council, Paul Langevin won• dered whether the Conference could still be held; he thought that a final decision should be taken by IS September, and hoped that it would be favourable. On S Sep• tember 1939 the Administrative Council declared that the circumstances then prevail• ing had made it necessary to postpone the eighth Solvay Conference indefinitely. Early in August 1939 Pauli and Heisenberg were still collaborating by mail on their joint report for the Solvay Conference. But on 29 August 1939 Pauli informed Heisenberg that the meeting which he had organized in Zurich (to be held in Septem• ber) had been cancelled on account of the difficulties of transportation and communi• cation. Pauli said: 'It's a great pity; I would have so much liked once again to talk about physics in detail with you and the others.' For the next six years at least, normal contacts between scientific colleagues were suspended and, in many cases, personal relationships were permanently altered. • • • The first Solvay Conference (1911), the 'witches' sabbath in Brussels' as he had de• scribed it to Besso, was not Albert Einstein's first contact with Belgium. During his years at the Patent Office in Bern, he visited his 'favourite uncle' Cisar Koch in Ant• werp several times in his vacations. Nor was it the first occasion of some importance which he had attended - Geneva and Salzburg had preceded Brussels. It was certainly the first to which he had gone as a young celebrity on whom many eyes were focused. In anticipation of his first academic appointment at Zurich University, starting October 1909, Einstein had resigned on 6 July from the Federal Department of Justice and Police where he was formally employed at the Patent Office. From 7 to 9 July 1909 he attended the celebrations commemorating the 3S0th anniversary of the founding of the University of Geneva· by Calvin. Over two hundred delegates from universities and learned societies had been invited as guests of honour. It rained so much during the celebrations that the journalist, covering the official procession as it wended its way on 8 July to St Peter's Cathedral, reported: 'Le defile etait un peu trop silencieux et meme funebre. '7 The convocation to confer honorary degrees took place in Victoria Hall under the aegis of the Federal President Adolphe Deucher. In a veritable orgy of academic dissipation, and a most un-Calvinistic display of ostentation, the five Geneva faculties bestowed over one hundred and ten honorary doctorates. Among those who received them were Ernest Solvay, Marie Curie, Wilhelm Ostwald (who a few months later was awarded the Nobel Prize in chemistry for his work on catalysis),

• The Academia Genevensis, founded by lohn Calvin in 1559, bad two sections: (1) Scho/a Privata, i.e. 'college' or secondary school, and (2) Scho/a Publica, also called 'l'acadCmie', for higher education. INTRODUcnON XXI

Albert Einstein, and Ernst Zahn, the restaurant keeper of Goschnen (in Canton Uri), whose coarse prose was considered in those days as the authentic representation of the voice of the Swiss mountain folk. Einstein had received his doctorate honoris causa thanks to the enthusiasm of Charles Eugene Guye, Professor of Experimental Physics (1900-1930) at the Universi• ty of Geneva. Guye had taught (1894-1900) electrical engineering and done research on alternating currents, polyphase generators and hysteresis phenomena at the E.T.H. in Zurich, where Einstein had attended his course.8 With the advent of Lorentz's theory of the electron and Max Abraham's rival theory, Guye became interested in using his accurate instrumental techniques to test for evidences of the Fitzgerald• Lorentz contraction hypothesis and transformation equations. He thought that a crucial experiment ought to be possible to decide between Lorentz's conception of a deformable electron, with a shape dependent on its velocity, and Abraham's notion of permanently spherical electrons. The opportunity seemed all the more inviting when Einstein's theory of special relativity, which had a close connection with Lorentz's work, began to stir controversy after 1905 - W. Kaufmann's experiments appeared to support Abraham and contradict the predictions of Lorentz and Einstein. Guye became an admirer of Einstein's work. For over a decade, beginning in 1907, he carried on a series of experiments with charged particles moving through electro• magnetic fields. In collaboration with M. Ratnowsky and Charles Lavanchy, Guye developed very precise techniques for measuring particle deflections in carefully con• trolled electric and magnetic fields. In 1916 and 1921 Guye published the accounts of his experimental techniques and pronounced results in favour of the Lorentz trans• formations and Einstein's theory. He had, however, demonstrated his confidence in the latter by persuading the University of Geneva to honour Einstein already in 1909. Charles Guye served as a member of the Scientific Committee of the Solvay Institute for Physics from 1925 to 1934. Two months after his visit to Geneva, Einstein attended the 81 st Congress of Ger• man Natural Scientists and Physicians in Salzburg (19-25 September 1909). Already in summer 1908 Rudolf Ladenburg had gone from Berlin to Bern to invite Einstein to address the Congress. Einstein delivered his lecture, 'On the development of our views concerning the nature and structure of radiation', on 21 September 1909. In Salzburg he met Planck, Wien, Rubens, Sommerfeld ('a splendid chap'), Max Born, and Lud• wig Hopf who became his assistant in Zurich and Prague. As he told a colleague on this occasion, he had 'never met a real physicist' before he was thirty. He would take up his own first academic appointment in the following month. In January 1911 H. A. Lorentz invited Einstein and his wife, Mileva, to visit Leyden. They stayed with the Lorentz family in February for a few days, and also stopped by in Antwerp to see Uncle casar. Shortly afterwards Einstein was formally invited to attend the first Solvay Conference in Brussels. In Brussels, Einstein met Lorentz, Planck and Nernst on equal terms for the first time, and made an enormous impression on his colleagues. Frederick Lindemann, who had accompanied Walther Nernst as one of the scientific secretaries to the Con- xxn INTRODUcnON ference, wrote home to his father the day after the meeting. Lindemann, the future Viscount Cherwell and scientific adviser to Winston Churchill, described Ernest Sol• vay as 'a very nice man, unfortunately though with rather liberal views.' He continued, 'I got on very well with all the people here, even with Madame Curie who is quite a good sort when one knows her. I got on very well with Einstein who made the most impression on me except perhaps Lorentz .... He says he knows very little mathemat• ics, but he seems to have had a great success with them.'9 And, almost half a century later, Lindemann wrote: 'I well remember my co-secretary, M. de Broglie, saying that of all those present Einstein and Poincare moved in a class by themselves.'lO Others present at the Conference also had the opportunity of sizing up Einstein. * Among those in Brussels who were most deeply impressed with his ability were Max Planck and Walther Nernst, twin pillars of the Prussian scientific establishment. A year and a half after the first Solvay Conference, Planck and Nernst, together with Rubens and Warburg who had also attended the Conference, recommended to the Prussian Academy of Sciences to elect Einstein (he was thirty-four) to full member• ship and award him a research professorship. What had motivated them, in addition to the theory of relativity, was Einstein's work on the quantum theory of matter, especially on specific heats, about which he had spoken at the Conference. In the minds of these people, in particular Planck's, who drafted the recommendation, Einstein's ideas on the structure of radiation were in doubt, for they remarked, 'That

• After the first Solvay Conference Einstein's friends, Heinrich Zangger and Marcel Grossmann, initiated the moves to bring Einstein from the German University of Prague to the E.T.H. (the Swiss Federal Institute of Technology) in Zurich. In order to further these plans Pierre Weiss, Professor of Physics at the E.T.H., had the brilliant idea of asking the opinions of Marie Curie and Henri Poin• care, both of whom had just recently become personally acquainted with Einstein in Brussels. In her reply, dated Paris, 17 November 1911, Madame Curie wrote: 'I have greatly admired the papers published by Mr Einstein on questions dealing with modern theoretical physics. Moreover, I believe that the mathematical physicists all agree that these works are of the highest order. In Brus• sels, where I attended a scientific conference in which Mr Einstein took part, I was able to appreciate the clarity of his mind, the vastness of his documentation and the profundity of his knowledge. If one considers that Mr Einstein is still very young, one has every right to justify the greatest expectations from him, and to see in him one of the leading theoreticians of the future ....' And Henri Poincare wrote: 'Mr Einstein is one of the most original thinkers I have ever met. In spite of his youth, he has already achieved a very honourable place among the leading savants of his age. What one has to admire in him above all is the facility with which he adapts himself to new con• cepts and knows how to draw from them every possible conclusion. He has not remained attached to classical principles, and when faced with a problem of physics he is prompt in envisaging all its possibilities. A problem which enters his mind unfolds itself into the anticipation of new phenomena which may one day be verified by experiment. I do not mean to say that all these anticipations will withstand the test of experiment on the day such a test would become possible. Since he seeks in all directions one must, on the contrary, expect most of the trails which he pursues to be blind alleys. But one must hope at the same time that one of the directions he has indicated may be the right one, and that is enough. This is indeed how one should proceed. The role of mathematical physics is to ask the right questions, and experiment alone can resolve them. The future will show more and more the worth of Mr Einstein, and the university intelligent enough to attract this young master is certain to reap great honour.' Note: These letters of Marie Curie and Henri Poincare are quoted in original French in Carl Seelig, Albert Einstein, Eine Dokumentarische Biographie, Europa Verlag, Zurich 1954, pp. 162-163, and have been re-translated into English and reproduced here by permission of the publisher. INTRODUcnON xxm he may sometimes have missed the target in his speculations, as for example, in his hypothesis of light quanta, cannot really be held too much against him, for it is not possible to introduce really new ideas, even in the most exact sciences, without some• times taking a risk. '11 In spring 1913 Einstein was invited to attend the second Solvay Conference, to be held in October. On 29 April 1913 he informed the Scientific Committee that he would be glad to participate in the Conference, but requested that he should not be asked to give a report as he was 'already very overloaded beyond the call of professional duties' .12 Einstein's talk on 'the status of the problem of specific heats' at the first meeting would remain as the only report which he presented at the Solvay Conferences. Plans were made during summer 1920 for the third Solvay Conference to be held in April 1921. Following World War I the feeling against the participation of German scientists in international meetings was still strong. 'An exception was made,' Tassel wrote, 'in the case of Einstein, of ill-defined nationality, Swiss I believe, who was roundly abused in Berlin during the war because of his pacifist sentiments which have never varied for a moment.'13 Rutherford put it differently: 'The only German invited is Einstein who is considered for this purpose to be international.'14 Einstein accepted 'with great pleasure'.15 Lorentz informed Rutherford that Einstein would speak on 'The electron and magnetism; gyroscopic effects'.lS However, on 22 February 1921 Einstein wrote to Lorentz, sending his regrets that he would not be present at the Con• ference. 'The Zionists have planned to establish a university in Jerusalem,' he wrote, and they had requested Einstein to speak for them in the United States in March and April so that through his personal cooperation 'the rich American Jews will be persuaded to pay Up.'17 Nevertheless, Einstein wished the Conference every success. At the Conference, W. J. de Haas gave a report on 'the angular momentum in a magnetic body', based on the notes supplied by Einstein. Niels Bohr, who had been invited to attend the 1921 Conference, was not able to do so on account of illness. His report on the 'application of the theory of quanta to atomic problems' was presented by Paul Ehrenfest. Two years later the 1924 Conference was being planned, and again only Einstein was to be invited from Germany. On 16 August 1923 he wrote to Lorentz from Lautrach in southern Germany: 'This letter is hard for me to write but I have to write it. I am here together with Sommerfeld. He is of the opinion that it is not right for me to take part in the Solvay Congress because my German colleagues are excluded. In my opinion it is not right to bring politics into scientific matters, nor should individu• als be held responsible for the government of the country to which they happen to belong. If I took part in the Congress I would by implication become an accomplice to an action which I consider most strongly to be distressingly unjust.' But he still looked forward to a future international cooperation, and continued: 'I should be grateful if you would see to it that I do not even receive an invitation to the Congress. I want to be spared the necessity of declining - an act which might hinder the gradual reestablishment of friendly collaboration between physicists of various countries.' International scientific relations began to return to normal after Germany joined XXIV INTRODUcnON

the League of Nations in 1926. 'Now,' Lorentz made a private note, 'I am able to write to Einstein.'ls The fifth Solvay Conference was being planned and Einstein's name had been proposed as a member of the Scientific Committee to replace Kamer• lingh Onnes who had died on 21 February 1926. However, it was thought proper that the approval of King Albert of the Belgians should be sought. On 2 April 1926, Lorentz was given an audience at which the King specifically approved Einstein's nomination to the Scientific Committee. The proposal to invite Planck and the other German physicists was also approved. Lorentz subsequently reported that, 'His Majesty expressed the opinion that, seven years after the war, the feelings which they aroused should be gradually damped down, that a better understanding between peoples was absolutely necessary for the future, and that science could help to bring this about. He also felt it necessary to stress that in view of all that the Germans had done for physics, it would be very difficult to pass them over.'IS- In any case, the 1927 Conference was planned to discuss quantum mechanics, and it could hardly have proceeded without Heisenberg, Born, Planck, and Einstein. Surprisingly, Sommerfeld was not invited. The fifth Solvay Conference witnessed the start of the discussions between Einstein and Bohr, which became the nucleus of the ever-growing literature on the interpreta• tion of the quantum principle. Einstein, to the end, did not like Heisenberg's un• certainty and Bohr's complementarity principles. In spring 1929 Einstein made one of his regular trips from Berlin to Leyden and, as usual, he called on his Uncle Casar in Antwerp. Here he received an invitation to visit Queen Elizabeth of the Belgians at the Chateau de Laeken on Monday, 20 May 1929. King Albert, who had a genuine interest in science, was absent only because he was visiting Switzerland. The Queen, formerly Princess Elizabeth of Bavaria, was unconventional and artistic. On 20 May Einstein, with his violin, 'spent the first of many musical afternoons' at the Palace, Her Majesty 'playing second fiddle'.19 'There followed tea under the chestnuts and a walk in the grounds, followed by dinner at 7: 30.' 20 A few days later Einstein received prints of the photographs which the Queen had taken. In a covering note she expressed the King's regrets that he had been away, and added: 'It was unforgettable for me when you came down from your peak of knowledge and gave me a tiny glimpse into your ingenious theory.' 20 An unusual friendship thus began between Queen Elizabeth and Albert Einstein and, during the next four years until he left Europe for ever for the U.S.A., he was al• ways invited to Laeken when he visited Belgium. A royal chauffeur would be sent to meet Einstein at the train, who at times would miss the man with the violin case. Einstein would then go to a small cafe nearby, request the use of the telephone, call the Palace and ask to speak to the Queen to announce his arrival. At the Palace there would be music, trios and quartets, with Einstein, the Queen, her lady-in-waiting and another Palace guest keeping it going for several hours. On one occasion he wrote to his (second) wife Elsa: 'Then they all went away, and I stayed behind alone for dinner with the King, vegetarian style, no servants. Spinach with hard-boiled eggs and potatoes, period.' 21 INTRODUcnON xxv

Einstein would write a post-card or send a note to the Queen from wherever he was, and she would always reply. Before going to Pasadena in the winter of 1932-33, Einstein called on the King and Queen at Laeken. In a letter of thanks to Her Majesty, he wrote: 'It was a great happiness for me to explain to you something of the mysteries in front of which the physicists stand silent.' 22 From the Rayben Farm Hope Ranch in California, which Queen Elizabeth had visited during a state visit to the United States in 1919, Einstein sent a post-card to her with the lines: 'A tree stands in the cloister garden / Which was planted by your hand. / It sends a little twig as greeting / Because there it must forever stand. / It sends a friendly greeting with Yours, A. Einstein.' 23 In December 1932 Kurt von Schleicher had become Chancellor of Germany, and for some weeks he desperately tried to form a stable government. He failed, the third man to do so in as many years. Then on 30 January 1933 President Hindenburg turned over the government to the leader of the National Socialists. Einstein's reac• tion in far-away Pasadena was immediate and unqualified: he could not return to Germany under the circumstances. On 10 March, the eve of his departure from Pasa• dena, Einstein made his decision public in an interview with Evelyn Seeley of the New York World-Telegram: 'As long as I have any choice in the matter,' he said, 'I shall live only in a country where civil liberty, tolerance, and equality of all citizens before the law prevail. Civil liberty implies freedom to express one's political convictions, in speech and in writing; tolerance implies respect for the convictions of others what• ever they may be. These convictions do not exist in Germany at the present time.' 24 Einstein and his wife crossed the Atlantic on the Belgenland, which docked in Ant• werp on 28 March 1933. Einstein was immediately driven to Brussels where, in the German embassy, he formally surrendered the rights of full German citizenship that he had taken after the First World War. He retained his Swiss nationality, and handed in his German passport. As he descended the steps of the embassy, Albert Einstein, 'the Swabian from Ulm, left German territory for the last time.' 25 On the same day, 28 March, he wrote to Berlin formally announcing his resigna• tion from the Prussian Academy of Sciences, on the grounds that he was no longer able to serve the Prussian state. An important personal reason for his immediate resignation from the Academy was his concern for the difficult position into which his old friends Planck and Nernst would be thrust; if he were expelled they would find it dangerous to protest, yet disloyal not to. These were the men who had persuaded the Academy in the first place to elect him to full membership in 1913, thereby conferring on him 'the greatest boon', and freeing him 'from the distractions and cares of a pro• fessional life and so making it possible for me to devote myself entirely to scientific studies.' 26 This connection was now severed finally. Albert and Elsa Einstein took temporary refuge at 'Canteroy', a historic manor house outside Antwerp, which belonged to the family of Professor A. de Groodt of Ghent University. Then they made a base, for their last six months in Europe, at Le Coq-sur-mer, a small resort near Ostend on the Belgian coast. King Albert wrote to Einstein: XXVI INIRODUcnON

My dear Professor,

We are delighted that you have set foot on our soil. There are men who by their work and intellec• tual stature belong to mankind rather than to anyone country, yet the country they choose as their asylum takes keen pride in that fact. The Queen joins me in sending you best wishes for a pleasant stay in Belgium. Please accept my expression of high esteem. A1bertl7

Europe had begun to prepare itself for a tumult, and the few months which Einstein spent at Le Coq were hectic. He had great concern about all that was happening and anxiety about what might happen. From Le Coq, he wrote to his friend Maurice Solovine: 'My great fear is that this hate and power epidemic will spread throughout the world. It comes from below to the surface like a flood, until the upper regions are isolated, terrified and demoralized, and then also submerged.'28 Einstein had much to do, numerous visitors called on him, and he did much travelling. His science was still his main concern and on three separate evenings he gave lectures on general relativity and the unified theory to invited audiences at the Club of the Fondation Universitaire in Brussels. There were, now and then, some moments of relaxation as well. He was invited to Laeken, where he and the Queen played the violin together. In meetings with Abraham Flexner at Oxford and Caputh (Einstein's summer home) during summer 1932, Einstein had agreed to visit, on a regular basis, starting in the fall of 1933, the Institute for Advanced Study which Flexner was planning to establish in Princeton. After the events leading to his refuge in Le Coq-sur-mer, the arrangement in Princeton was going to be permanent. Einstein left Le Coq on 9 September 1933 and took a Channel boat from Ostend to England. There he spent a month full of meetings with scientists and politicians. He boarded the Wester/and, bound for New York, on the evening of 7 October 1933; his wife Elsa, Miss Helen Dukas, his secretary, and Dr. Walther Mayer, his assistant, had already embarked in Antwerp. As the Wester/and sailed up the approaches to New York Harbour after an uneventful voyage, a tugboat, with two trustees of the Institute for Advanced Study in it, came aside, and Einstein and his party were helped aboard. The Wester/and continued on its way and, long before it docked, Einstein and his 'family' had been transferred to a car and were being driven to Princeton. The seventh Solvay Conference (22-29 October 1933) took place in Brussels two weeks after Einstein had left Europe for good. In a letter (Le Coq-sur-mer, 12 August 1933) to Charles Lefebure he had sent his regrets concerning his inability to attend the Conference in view of his impending departure for Princeton at the begin• ning of October. Einstein's absence was felt keenly at the Conference and Paul Langevin, on behalf of all the participants, sent Einstein a message of greetings. On 26 November, Einstein wrote to Lefebure to thank him for the group photograph of the Conference. On Friday, 27 October 1933, King Albert of the Belgians received the participants INTRODUcnON xxvn in the Conference for a dinner· at Laeken. The royal couple missed Einstein's presence. A few days after the dinner, King Albert wrote to Lefebure: 'The Queen and I were very happy to receive "ces Messieurs les Physiciens", and to see that the great Ernest Solvay's noble efforts continue to perpetuate themselves in the most effective service of science and the relations between the savants of all countries.'29 Einstein settled down in Princeton. 'Princeton is a wonderful little spot', he wrote, giving his first impressions to Queen Elizabeth of Belgium, 'a quaint and ceremonious village of puny demigods on stilts. Yet, by ignoring certain special conventions, I have been able to create for myself an atmosphere conducive to study and free from distrac• tion. Here, the people who compose what is called "society" enjoy even less freedom than their counterparts in Europe. Yet they seem unaware of this restriction since their way of life tends to inhibit personality development from childhood.' 30 And, more than a year later, he wrote to her: ' ... as an elderly man, I have remained es• tranged from the society here ... .'31 As always, he would remain an outsider. King Albert died in a mountaineering accident in 1934. Queen Elizabeth, now the Queen Mother, and Einstein continued to exhange letters for another twenty years. Early in 1939 he wrote to her: 'The work has proved fruitful this past year. I have hit upon a hopeful trail, which I follow painfully but steadfastly in company with a few youthful fellow workers. Whether it will lead to truth or fallacy - this I may be unable to establish with any certainty in the brief time left to me. But I am grateful to destiny for having made my life into an exciting experience so that life has appeared meaning• ful ... .'32 The eighth Solvay Conference was scheduled for 22-29 October 1939. Einstein was, of course, invited but he had informed the Solvay Institute already on 20 April that, 'Unfortunately, it would not be possible for me to come to Europe this year.' To the Belgian Queen Mother he wrote: 'The moral decline we are compelled to witness, and the suffering it engenders, are so oppressive that one cannot ignore them even for a moment. No matter how deeply one immerses oneself in work, a haunting feeling of inescapable tragedy persists.' 32 The tragedy had begun, and the 1939 Conference was cancelled . • • • After World War II Sir Lawrence Bragg became President of the Scientific Committee of the Solvay Institute for Physics. He directed the Conferences on the elementary particles (1948), the solid state (1951), the electrons in metals (1954), the structure and evolution of the universe (1958), and quantum field theory (1961). Upon Bragg's re• signation, J. Robert Oppenheimer became President of the Committee, and directed the Conference on the structure and evolution of galaxies (1964). After Oppenheimer died on 18 February 1967, C. Maller acted as President of the Scientific Committee of the fourteenth Solvay Conference on fundamental problems in elementary par-

• Diner du 27 Octobre 1933, LaekeD: Creme a la SCvign6; SaumOD sauce GeDcvoise, Pommes a l'Anglaise; Filet de boeuf a la Fran~se, Chicor6es de Bruxelles g1ac6es; Poulardes rOties cresson, Salade coeurs de laitues, Compote de reineUes; Bombe F6dora; Corbeilles de fruits; Desserts. XXVIII INTRODUCTION ticle physics (1967). Edoardo Amaldi then took over the Presidency of the Scientific Committee, and directed the fifteenth and sixteenth Conferences on symmetry prop• erties of nuclei (1970) and astrophysics and gravitation (1973). Ernest-John Solvay continued to represent the Solvay family at all the Conferences until 1967. At the 1967 Conference his son, Jacques Solvay, also became a member and, later on President (1970), of the Administrative Council of the International (Solvay) Institutes of Physics and Chemistry. Until 1958 the administrative functions of the Solvay Institutes were executed by the successive Secretaries of the Administrative Council (E. Tassel, J. E. Verschaffelt, C. Lefebure, F. H. van den Dungen). Ilya Prigogine became a member of the Council in 1958 and, in 1959, was appointed its Secretary. Under the new Statutes, Prigogine was appointed Director of the International (Solvay) Institutes of Physics and Chemistry in 1970. • • • The eighth Solvay Conference, and the first after World War II, took place in 1948 and discussed the problems of elementary particles. This field had grown immensely since the 1933 Conference, especially on account of the appearance of mesons as car• riers of the nuclear interactions. At the 1948 Conference, the latest sensation 'was the production of artificial mesons in Berkeley, about which Serber was supposed to give exciting details. However the technical aids were not then as refined as they are now: there were no microphones, so that Serber could hardly be heard, and the projection apparatus was so bad that it was hardly possible to see the meson tracks that he showed on the screen. This somewhat subdued the expected excitement, and at the banquet concluding the Conference, poor Serber was lampooned in a "meson song" ,'33 an adaptation by Teller of another composition, sung to the music composed by O. R. Frisch for the occasion. Here is the relevant verse (See Chapter 10, Appendix). Some beautiful pictures are thrown on the screen; Though the tracks of the mesons can hardly be seen, Our desire for knowledge is most deeply stirred, When the statements of Serber can never be heard. What, not heard at all? No, not heard at all! Very dimly seen And not heard at all! The conclusion of the song gave an adequate description of the degree of under• standing that had been reached at the time: From mesons all manners of forces you get, The infinite part you simply forget, The divergence is large, the divergence is small, In the meson field quanta there is no sense at all. What, no sense at all? No, no sense at all! Or, if there is some sense, It's exceedingly small. I!IrI'RODUCTlON XXIX

In 1929 Heisenberg and Pauli had laid a systematic foundation of quantum field theory. When Leon Rosenfeld wrote to Pauli (in 1929) whether he could come to Zurich to work under his guidance, Pauli sent a friendly reply, pointing out that the time was quite favourable since Heisenberg and he had just started work on the quan• tization of electrodynamics - 'ein Gebiet,' he said, 'das noch nicht ganz abgedroschen ist' (a domain that is not yet threshed out). When Rosenfeld arrived in Zurich, he immediately got the proof-sheets of their first paper to read. 'The first thing I did,' said Rosenfeld, 'was to correct a minor mistake in it. I was very proud of it, and wrote a little note which started with the words: "In ihrer grundlegenden Arbeit, haben Heisenberg und Pauli ... " (In their fundamental paper ..• ) When he saw this, Pauli laughed and remarked: "Es ist ein ziemlich morscher Grund, den wir da gelegt haben!" (It is a rather swampy foundation we have laid.)'34 The renormalization technique in quantum electrodynamics had just come into view at the time of the 1948 Conference. It developed very rapidly, and quantum field theory continued to follow its adventurous course until the 1961 Conference. The twelfth Solvay Conference in 1961 was also the occasion at which the fiftieth an• niversary of the first Conseil Solvay was celebrated. Feynman took note of the distance one had travelled since then when he remarked: 'Fifty years ago at this Conference one of the problems most energetically discussed was the apparent quantum nature of the interaction of light and matter. It is a privilege to be able, after half a century, to give a report on the progress that has been made in its solution. No problem can be solved without it dragging in its wake new problems to be solved. But the incom• pleteness of our present view of quantum electrodynamics, although presenting us with the most interesting challenges, should not blind us to the enormous progress that has been made. With the exception of gravitation and radioactivity, all of the phenomena known to physicists and chemists in 1911 have their ultimate explana• tion in the laws of quantum electrodynamics. '35 At the 1961 Conference, Rosenfeld recalled, 'The heroes of the day were our friends Chew and Gell-Mann. We then witnessed the birth of the bootstrap idea and the canonization of Regge. There was some alarm when Regge poles were first mentioned, and I remember Professor Wigner's polite and ineffectual efforts to induce somebody to define these Regge poles. However, the discussion went on happily without such a definition. At one point, Chew and Gell-Mann seemed preoccupied with understand• ing what the old people could have expected from such queer concepts as Lagrangians. Gell-Mann was suggesting: "They may have thought this and this ... ," and Chew replied: "No, probably they thought that and that .... " It did not occur to them that Heisenberg was sitting there and that it might have been simpler to ask him. What I found most remarkable, however, was that Heisenberg himself did not volunteer any statement about this point.' In Rosenfeld's view, the 'swampy foundation' which Heisenberg and Pauli had laid over thirty years previously had not yet become terra firma, and 'we are still plodding in this mud, some of us trying to dig a little deeper in the hope of finding some firmer ground, others trying to extricate themselves by pull• ing their own bootstraps.'36 xxx INTRODUcnON

All the Solvay Conferences until 1948 had been devoted to the discussion of the development of quantum physics. After the 1961 Conference on quantum field theory, the fundamental problems in elementary particle physics again came up for discussion at the fourteenth Solvay Conference (1967). The symmetry properties of nuclei were discussed in the following Conference (1970). Mter World War II the field of solid state physics had emerged into prominence and many fundamental problems occupied physicists working in it in different countries. Thus the 1951 Conference was devoted to the solid state, and that of 1954 to the electrons in metals. Technological progress during the Second World War had also opened new horizons in the study of astronomy, especially since micrometer wavelength radiation became observable. The realm of radio stars and very distant radio galaxies was now ready for investigation. The eleventh Solvay Conference (1958) was devoted to the discussion of the structure and evolution of the universe. The thirteenth Conference (1964) dealt with the structure and evolution of galaxies, the recent discovery of quasars engender• ing appropriate enthusiasm. The discussion of black holes and pulsars at the sixteenth Solvay Conference (1973) showed how some of the most fundamental concepts of physics had become important for the understanding of problems in astrophysics and gravitation. • • • Numerous participants in the various Solvay Conferences have given accounts of the direct beneficial effects which they experienced on these occasions. Reports given by the most well-informed scholars and discussions among the authoritative experts of a field could not but stimulate all who came together. Take Henri Poincare. As the direct result of his participation in the first Solvay Conference, Poincare wrote two memoirs on quantum theory a few months before his death on 17 July 1912, taking the reports of Planck and Sommerfeld as his points of departure.37 Extensive refer• ences to Solvay Conferences in the correspondence or recollections of various par• ticipants - such as Lorentz, Einstein, Rutherford, Madame Curie, Langevin, Joffe, Oebye, Bohr, Compton, Gamow, Pauli, Heisenberg, de Broglie, Rosenfeld, and others - bear witness to the importance of these meetings as occasions for strong scientific interaction in an agreeable setting. For some they became the happiest memory of their lives. The indirect effects of the Solvay Conferences are incalculable. Niels Bohr, for in• stance, heard a first-hand account of the proceedings of the first Solvay Conference from Rutherford when Bohr visited him in Manchester a few weeks after Rutherford's return from Brussels.3s The nineteen-year-old Louis de Broglie, whose older brother, Maurice, was one of the scientific secretaries of the first Solvay Conference, read the discussions as the manuscript was being prepared for publication. 'With all the ardour of my youth,' he has written, 'I was swept away by my enthusiasm for the problems discussed and 1 resolved to devote all my efforts to understanding the true nature of the mysterious quanta that Max Planck had introduced ten years earlier.'39 INTRODUCTION XXXI

It is quite possible that others may also have been inspired, or might be in the future, by the knowledge of what some of the greatest physicists of the twentieth century sought to understand at these Conferences. As Niels Bohr once said: 'The careful recording of the reports and of the subsequent discussions at each of these meetings will in the future be a most valuable source of information for students of the history of science wishing to gain an impression of the grappling with the new problems raised in the beginning of our century. Indeed, the gradual clarification of these problems through the combined effort of a whole generation of physicists was in the following decades not only so largely to augment our insight in the atomic con• stitution of matter, but even to lead to a new outlook as regards the comprehension of physical experience. '40 'A new outlook as regards the comprehension of physical experience,' - this is precisely what Ernest Solvay had been thinking when he convened his first conseil and then took the initiative to provide the forum and the means for the dialogue to go on.

REFERENCES

1. A. Einstein to M. Besso, 21 October 1911. See Albert Einstein - Michele Besso, Correspontiance 1903-1955 (Ed. P. Speziali), Hermann, Paris 1972. 2. A. Einstein to M. Besso, 26 December 1911, Ref. 1. 3. Institut International de Physique (Solvay), Status, 1 May 1912. Revised 1 July 1919; the earlier Article 10 now became Article 13. 4. L. de Broglie, 'Beginnings of Wave Mechanics', in Wave Mechanics, the First Fifty Years (Eds. W. C. Price, S. S. Chissick, and T. Ravensdale), Butterworths, London 1974, pp. 16-17. 5. L. Rosenfeld, 'Some Concluding Remarks and Reminiscences', in Fundamental Problems in Elementary Partides Physics, Proceedings of the fourteenth Solvay Conference on Physics, John Wiley-Interscience, New York 1968, p. 232. 6. Hommage National aErnest Solvay, Inauguration du Monument, 16 October 1932, Brussels 1933, pp.31-44. 7. C. Seelig, Albert Einstein, Staples Press, London 1956, p. 93. 8. Biographical Notice of Charles Eugene Guyc in Dictionary of Scientific Biography, Volume V, Charles Scribner's Sons, New York 1972, pp. 597-598. 9. F. W. F. Smith, Earl of Birkenhead, The Professor and the Prime Minister, Houghton Miffiin, Boston 1962, p. 43. 10. Quoted in R. W. Clark, Einstein, The Life and Times, World Publishing Company, New York 1971; Avon Books, New York, 1972. See the latter edition, p. 185. 11. Ref. 7, p. 14S. 12. A. Einstein to E. Tassel, 29 Apri11913. 13. E. Tassel to M. Huisman, 23 March 1921. 14. E. Rutherford to B. B. Boltwood, 28 February 1921. 15. A. Einstein to H. A. Lorentz, 15 June 1920. 16. H. A. Lorentz to E. Rutherford, 16 December 1920. 17. A. Einstein to H. A. Lorentz, 22 February 1921. 18. H. A. Lorentz's private note on the back of a telegram (documents of the Solvay Institutes). 18a. H. A. Lorentz, Report to the Administrative Council, Solvay Institute, 3 April 1926. 19. Queen Elizabeth's personal notes in her agenda book: Royal Archives, Brussels. Quoted in Ref. 10, p. 511. 20. Ref. 19. Quoted in Ref. 10, p. 512. 21. A. Einstein's letter to his wife Elsa, quoted in O. Nathan and H. Norden (Eds.), Einstein on Peace, London 1963, p. 662. 22. A. Einstein to Queen Elizabeth, 19 September 1932, Royal Archives, Brussels; Ref. 10, p. 546. xxxn INTRODUCTION

23. A. Einstein to Queen Elizabeth, 19 February 1933, Royal Library, Brussels; Ref. 10, p. 555. 24. Interview with Evelyn Seeley, New York World-Telegram, 10 March 1933; Ref. 10, p. 557. 25. R. W. Clark, Ref. 10, p. 565. 26. A. Einstein, inaugural address at the Prussian Academy of Sciences, 2 July 1914, Sitz. Ber. KgI. Akad. Wiss. 28, 739-742 (1914). 27. King Albert to A. Einstein, 24 July 1933, Royal Archives, Brussels; quoted in Ref. 10, p. 595. 28. A. Einstein to M. Solovine, 28 April 1933, quoted in Ref. 7, p. 192. 29. King Albert to Charles Lefebure, 5 November 1933 (documents of the Solvay Institutes). 30. A. Einstein to Queen Elizabeth, 20 November 1933, Royal Archives, Brussels; Ref. 10, p. 643. 31. A. Einstein to Queen Elizabeth, Ref. 10, p. 643. 32. A. Einstein to Queen Elizabeth, 9 January 1939, Royal Library, Brussels; Ref. 10, p. 656. 33. L. Rosenfeld, Ref. 5, pp. 232-233. 34. L. Rosenfeld, Ref. 5, p. 231. 35. R. P. Feynman, 'The Present Status of Quantum Electrodynamics', in La TMorie Quantique des Chomps, Douzieme Conseil de Physique, 1961, Interscience, New York, and R. Stoops, Brussels, 1962, p. 61. 36. L. Rosenfeld, Ref. 5, p. 231. 37. H. Poincare, 'Sur la tMorie des quanta', J. Phys. Theor. Appl. (Paris) 2, 5 (1912); 'The Quantum Theory', Chapter VI in Mathematics and Science: Last Essays, Dover Publications, New York, 1963 (originally published in Derni~res Pensees, Ernest F1ammarion, 1913). 38. N. Bohr, Essays 1958-1962 on Atomic Physics and Human Knowledge, Interscience, New York 1963, pp. 31, 83. 39. Lauis de Broglie, Physicien et Penseur, Editions Albin Michel, Paris 1953, p. 458; see p. 425 for the comments of Maurice de Broglie. Also M. de Broglie, Les Premiers Congr~s de Physique Solvay, Editions Albin Michel, Paris 1951. 40. N. Bohr, 'The Solvay Meetings and the Development of Quantum Physics', in La Theorie Quantique des Champs, Douzieme Conseil de Physique, 1961, Interscience, New York, and R. Stoops, Brussels, 1962, p. 13.

Note: I would also like to cite M. J. Klein, 'Einstein, Specific Heats, and the Early Quantum Theory', Science 148, 173-180 (1965). I take special pleasure in thanking my friend Ronald W. Clark, the author of Einstein: The Life and Times, for allowing me to use some information which he had pains• takingly gathered. Ernest Solvay 16 April 1838 - 26 May 1922