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THE PHYSICIST's CONCEPTION of NA TURE the Physicist's Conception of Nature

THE 'S CONCEPTION OF NA TURE The Physicist's Conception of

Edited by Jagdish Mehra

D. Reidel Publishing Company

Dordrecht-Holland / Boston-U. S.A.

1973 Library of Congress Catalog Card Number 73-75765

ISBN-13: 978-94-010-2604-8 e-ISBN-13: 978-94-010-2602-4 DOl: 10.1007/978-94-010-2602-4

Published by D. Reidel Publishing Company, P.O. Box 17, Dordrecht, Holland

Sold and distributed in the U.S.A., Canada, and Mexico by D. Reidel Publishing Company, Inc. 306 Dartmouth Street, Boston, Mass. 02116, U.S.A.

All Rights Reserved Copyright © 1973 by D. Reidel Publishing Company, Dordrecht, Holland Softcover reprint of the hardcover I st edition 1973 No part of this book may be reproduced in any form, by print, photoprint, microfilm, or any other means, without written permission from the publisher Dedicated to

PAUL ADRIEN MAURICE DIRAC

on the occasion of his seventieth birthday SYMPOSIUM ON THE DEVELOPMENT OF THE PHYSICIST'S CONCEPTION OF NATURE IN THE TWENTIETH CENTURY

Held at the International Centre for Theoretical , Miramare, Trieste, Italy, 18-25 September 1972

Sponsoring Committee:

Co-Chairmen: H. B. G. CASIMIR ( Research Laboratories, Eindhoven),

EUGENE P. WIGNER (Princeton University)

EDOARDO AMALDI (University of ),

PIERRE AUGER (Universite de Paris),

BRIAN FLOWERS (U.K. Science Research Council, London),

RUDOLF E. PEIERLS (University of ), FRANCIS PERRIN (College de France),

ISIDOR I. RABI (Columbia University),

GUNNAR RANDERS (NATO, Brussels), LEON ROSENFELD (NORDITA, ),

ABDUS SALAM (Imperial College, London, and International Centre for Theoretical Physics, Trieste)

Symposium Director:

JAGDISH MEHRA (The University of Texas at Austin)

Financial Sponsorship:

The Science Committee, North Atlantic Treaty Organization (Brussels, Belgium); Minna-James-Heineman Stiftung (Hanover, Germany); Consortia of the City and the Region of Trieste (Trieste, Italy); The Lorentz Foundation (Haarlem, The ) Contents

page Preface XI Contributors xv

Opening remarks at the symposium XIX

1 Development of the physicist's conception of nature, by P. A. M. DIRAC 1

Part I Space, Time, and Geometry

2 The universe as a whole, by DENNIS W. SCIAMA 17 3 A chapter in the astrophysicist's view of the universe, by s. CHANDRASEKHAR 34 4 Fundamental constants and their development in time, by P. A. M. DIRAC 45 5 The expanding earth, by PASCUAL JORDAN 60 6 The nature and structure of spacetime, by JVRGEN EHLERS 71 7 Einstein, Hilbert, and the theory of gravitation, by JAGDISH MEHRA 92 8 Theory of gravitation, by ANDRZEJ TRAUTMAN 179 9 From relativity to mutability, by JOHN ARCHIBALD WHEELER 202

Part II Quantum Theory

10 The wave-particle dilemma, by LEON ROSENFELD 251 11 Development of concepts in the history of quantum theory, by 264 12 From matrix mechanics and wave mechanics to unified , by B. L. V AN DER W AERDEN 276 13 Early years of quantum mechanics: some reminiscences, by PASCUAL JORDAN 294 14 The mathematical structure of elementary quantum mechanics, by JOSEF M. JAUCH 300 15 Relativistic equations in quantum mechanics, by EUGENE P. WIGNER 320 16 The : development of the first elementary particle theory, by FRITZ ROHRLICH 331 17 The development of , by RUDOLF E. PEIERLS 370 18 Quantum theory of fields (until 1947), by GREGOR WENTZEL 380 19 Development of quantum electrodynamics, by SIN-ITIRO TOMONAGA 404 20 A report on quantum electrodynamics, by 413 x CONTENTS

21 Progress in renormalization theory since 1949, by 430 22 Some concepts in current elementary particle physics, by CHEN NING YANG 447 23 Crucial experiments on discrete symmetries, by v. L. TELEGDI 454 24 and , by H. B. G. CASIMIR 481

Part III Statistical Description of Nature

25 Problems of statistical physics, by GEORGE E. UHLENBECK 501 26 Phase transitions, by MARK KAC 514 27 Approach to thermodynamic equilibrium (and other stationary states), by WILLIS E. LAMB, JR. 527 28 Kinetic approach to non-equilibrium phenomena, by E. G. D. COHEN 548 29 Time, irreversibility and structure, by IL Y A PRIGOG INE 561 30 The origin of biological information, by 594

Part IV Physical Description, Epistemology, and Philosophy

31 Classical and quantum descriptions, by c. F. VON WEIZSACKER 635 32 Wavefunction and observer in the quantum theory, by LEON N. COOPER 668 33 The problem of measurement in quantum mechanics, by JOSEF M. JAUCH 684 34 Subject and object, by J. S. BELL 687 35 Subject, object, and measurement, by R. HAAG 691 36 Measurement process and the macroscopic level of quantum mechanics, by IL Y A PRIGOGINE 697 37 Why a new approach to found quantum theory?, by G. LUDWIG 702 38 A process conception of nature, by DA VID FINKELSTEIN 709 39 Quantum logic and non-separability, by BERNARD D'ESPAGNAT 714 40 Physics and philosophy, by c. F. VON WEIZSACKER 736

Part V Memorial Lectures

41 Recollections of Lord Rutherford, by P. L. KAPITZA 749 42 W. Pauli's scientific work, by CHARLES P. ENZ 766 43 Remarks on , by s. CHANDRASEKHAR 800

Part VI Celebration of P. A. M. Dirac's 70th Birthday 44 The banquet of the symposium - in honour of , including an address on: The classical mind, by c. P. SNOW 805

Appendix 1 Programme of the symposium 820 Appendix 2 Participants 823 Index of names 830 Preface

The fundamental conceptions of twentieth-century physics have profoundly influenced almost every field of modern thought and activity. Quantum Theory, Relativity, and the modern ideas on the Structure of Matter have contributed to a deeper understand• ing of Nature, and they will probably rank in history among the greatest intellectual achievements of all time. The purpose of our symposium was to review, in historical perspective, the current horizons of the major conceptual structures of the physics of this century. Professors Abdus Salam and Hendrik Casimir, in their remarks at the opening of the symposium, have referred to its origin and planning. Our original plan was to hold a two-week symposium on the different aspects of five principal themes: 1. Space, Time and Geometry (including the structure of the universe and the theory of gravita• tion),2. Quantum Theory (including the development of quantum mechanics and quantum field theory), 3. Statistical Description of Nature (including the discussion of equilibrium and non-equilibrium phenomena, and the application of these ideas to the evolution of biological structure), 4. The Structure of Matter (including the discus• sion, in a unified perspective, of atoms, molecules, nuclei, elementary particles, and the physics of condensed matter), and finally, 5. Physical Description and Epistemo• logy (including the distinction between classical and quantum descriptions, and the epistemological and philosophical problems raised by them). These themes, taken together, might be regarded as constituting almost the whole of modern physics, and as a programme for one symposium it would have seemed to be too ambitious. It was, however, not all that insane. Our intention was not to cover the details of all the problems that come under these themes. Rather we had hoped to discuss the historical development of the principal conceptions, and the current horizons which have arisen from them, which would emphasize the structural and conceptual unity of the body of physical knowledge about Nature. These would be presented by some of their most authentic exponents. The Sponsoring Committee came to the conclusion that the programme, even for a two-week symposium, would be too heavy. They thought that it would be advisable to hold two symposia to cover the intended programme, and that it could be done without damaging the unity of the general theme. They decided that we should deal with four themes in the first symposium, leaving the Structure of Matter to the second. Our financial sponsors also thought that this was a good idea, that based upon the success of the first symposium the second one could be held with confidence a couple of years later. We thus finally adopted the programme which is reproduced in Appendix 1. xu PREFACE

With a programme so rich in scientific and historical fare, and with many stars among the lecturers, every occasion at the symposium represented a highlight. Yet, there were several exceptional occasions which many of us will specially cherish. Dirac's evening lecture on the 'Development of the Physicist's Conception of Nature' was such an occasion. In pin-drop silence, all ears attuned to his words, Dirac developed his theme. He talked about classical mechanics and relativity, about quantum mechanics and the quantum theory of fields, and about how the development of physics could be pictured as a rather steady development with many small steps, superposed on which were a number of big jumps, the latter consisting usually of over• coming prejudices. Dirac wondered about the present state of physical theory and invoked the twin principles of 'beauty and logic' as the guidelines for the developing architecture of physics. He declared his belief that the physicist's conception of Nature has not stopped in growing: it is at an interim state at present and fundamental developments will occur in the future that will change it. As he spoke, his voice feeble but firm, the sentences perfectly formed as only Dirac's sentences can be, his spirit grew and filled the hall. The audience shrank in size if not in numbers. As he finished his talk, one felt the presence of only this frail man who had the strength of character to match his wisdom, a man who, with a few kindred spirits, had created the language of modem theoretical physics, and who now enjoined his audience of distinguished to cast away prejudice and seek a deeper understanding of Nature. In his beautiful historical talk, dealing with a period of a few weeks in March and April 1926 when the passage took place from matrix mechanics and wave mechanics to a unified quantum mechanics, B. 1. van der Waerden discussed at length a letter from Pauli to Jordan, written in April 1926, in which Pauli had referred to Lanczos' work on the integral equations. Van der Waerden pointed out in delectable detail how Pauli and SchrOdinger had imperfectly understood the work of Lanczos, and how the latter's integral equation was equivalent to Schrodinger's. At the end of this talk, which had shown how deep and original had been Lanczos' understanding of the mathematical structure of quantum mechanics, it was pointed out that Professor Cornelius Lanczos was in the lecture hall and had listened to this forceful vindication of his early work against Pauli's criticism of it. As Lanczos got up to acknowledge the announcement of his presence Van der Waerden, who did not know him personally, was shocked with pleasant surprise, and the audience broke into sustained applause. It was a thrilling moment for all. Another occasion with enormous impact came on Wednesday morning, 20 Septem• ber 1972. This was just the fact that the speakers that morning, one after the other, were Paul Dirac, Werner Heisenberg, and . An occasion with such a succession, in one session of a conference, of speakers who had participated in the creation of quantum theory had probably not occurred since the first and the fifth Solvay conferences, and the audience was conscious of a sense of history. This was the veritable celebration of the seventieth birthday of physics in the twentieth century. An evening we will all remember and cherish was the banquet of the symposium in PREFACE XJII honour of Dirac. The gracefulness of the occasion was highlighted by both the com• pany present and the remarks that were made. Lord Snow's address on 'The Classical Mind' underscored the joy and pride of the occasion which we were allowed to share. The impromptu telling of the 'Dirac stories' by several people, and Dirac's own story about , delighted all who were present. These remarks have been gathered in a chapter in this volume, and hopefully many a reader will enjoy them. Many of those who attended the symposium have written to me how 'successful, beautiful, historic, unique, etc.' this symposium was. It gives me great pleasure to thank those who contributed to the uniqueness, beauty, and success of this event. First of all, the members of the Sponsoring Committee, all of whom helped at one time or another during the organization. Professors Edoardo Amaldi, Pierre Auger, Brian Flowers, Rudolf Peierls, Francis Perrin, I. I. Rabi and Leon Rosenfeld gave much wise counsel and help. Professor Gunnar Randers' encouragement, cooperation, and assistance were invaluable throughout. Professors Hendrik Casimir, Abdus Salam, and Eugene Wigner encouraged, helped, and sustained me at every step of the way in the effort to organize the symposium, and it was their continuous support that made this task both possible and a joy. The association of all these distinguished scientists with the symposium had assured its success, and every expression of gratitude will be inadequate to acknowledge the significance of their sponsorship. The distinguished lecturers were most cooperative. The subject of the lecture was not of their choice at times, yet they accepted their assignments graciously. They made wonderful contributions to the programme and it is what they delivered that gave content and meaning to the symposium. Their papers, often with the discussions that followed the talks, are reproduced here. Also reproduced in this volume are the papers, prepared for other occasions, of those who were invited to give talks but preferred not to do so (Wentzel), and of those whom circumstances prevented from attending (Kapitza, Tomonaga). We can be truly grateful for the treat which the contributors to this volume have offered us. The chairmen of sessions preserved a relaxed atmosphere throughout. The par• ticipants in the symposium were just marvellous. They braved a heavy programme, participating fully in all sessions and contributing to the discussions. The lecturers, chairmen, and participants stayed throughout the symposium, which was also a unique experience, deserving of the gratitude of the organizers. The holding of the symposium was made possible by the financial sponsorship of NATO's Scientific Committee. The Minna-James-Heineman Foundation of Hanover, Germany, also contributed funds. At the request of Professor Casimir, the Lorentz Foundation made a timely contribution. Several friends of the Symposium Director, who wish to remain anonymous, rendered financial help, and the Consortia of the City and the Region of Trieste gave invaluable support for the social events, as did the Prince of Torre e Tasso. The publishers of this volume have supported the symposium enormously, and have cooperated in meeting our wishes regarding the publication. We are grateful for all this wonderful support. The symposium was held at the beautiful premises of the International Centre for XIV PREFACE

Theoretical Physics in Miramare, by courtesy of the Director, Professor Abdus Salam. Professor Paolo Budini organized crucial assistance when the holding of the sympo• sium was threatened with disruption. The Administrative Officer, Dr Andre Hamende, and the staff of the Centre, gave every help and assistance most readily and cheerfully. My gratitude to them is everlasting. This volume is dedicated to Paul Dirac. I hope it will give him, and others who will read it, some pleasure.

JAGDISH MEHRA Contributors

J. S. BELL, Theory Division, C.E.R.N., Geneva, Switzerland. H. B. G. CASIMIR, Philips Research Laboratories, Eindhoven, The Netherlands. s. CHANDRASEKHAR, Laboratory for Astrophysics and Space Research, University of , Chicago, Illinois, U.S.A. E. G. D. COHEN, The Rockefeller University, New York, New York, U.S.A. LEON N. COOPER, Physics Department, Brown University, Providence, Rhode Island, U.S.A. P. A. M. DIRAC, Department of Physics, Florida State University, Tallahassee, Florida, U.S.A. JURGEN EHLERS, Max-Planck-Institut flir Physik und Astrophysik, Munich, Fed. Rep. Germany. MANFRED EIGEN, Max-Planck-Institut flir Biophysikalische Chemie, Gottingen• Nikolausberg, Fed. Rep. Germany. CHARLES P. ENZ, Institut de Physique Theorique, Universit6 de Geneve, Geneva, Switzerland. BERNARD D'ESPAGNAT, Laboratoire de Physique Theorique et Particules El6men• taires, Universit6 Paris-Sud, Orsay, France. DA VID FINKELSTEIN, Department of Physics, Belfer Graduate School of Science, Yeshiva University, New York, New York, U.S.A. R. HAAG, II. Institut flir Theoretische Physik, Universitat Hamburg, Hamburg, Fed. Rep. Germany. WERNER HEISENBERG, Max-Planck-Institut flir Physik und Astrophysik, Munich, Fed. Rep. Germany. JOSEF M. JAUCH, Institut de Physique Theorique, Universite de Geneve, Geneva, Switzerland. PASCUAL JORDAN, Universitat Hamburg, Hamburg, Germany. MARK KAC, The Rockefeller University, New York, New York, U.S.A. P. L. KAPITZA, U.S.S.R. Academy of Sciences, Moscow, U.S.S.R. WILLIS E. LAMB JR., Physics Department, Yale University, New Haven, Connecticut, U.S.A. G. LUDWIG, Institut flir Theoretische Physik, Universitat Marburg, Marburg, Fed. Rep. Germany. JAGDISH MEHRA, The University of Texas at Austin, Austin, Texas, U.S.A. RUDOLF E. PEIERLS, Department of Theoretical Physics, , Oxford, England. XVI CONTRIBUTORS

ILY A PRIGOGINE, Faculte des sciences, Universite Libre de Bruxelles, Brussels, Belgium, and Center for , The University of Texas at Austin, Austin, Texas, U.S.A. FRIT Z RO HRLI CH, Department of Physics, Syracuse University, Syracuse, New York, U.S.A. LEON ROSENFELD, NORDITA, Copenhagen, Denmark. ABDUS SALAM, Imperial College of Science and Technology, London, and Interna• tional Centre for Theoretical Physics, Miramare, Trieste, Italy. JULIAN SCHWINGER, Physics Department, University of California, Los Angeles, California, U.S.A. DENNIS W. SCIAMA, Department of Astrophysics, University of Oxford, Oxford, England. c. P. SNOW, 85 Eaton Terrace, London S.W.I, England. v. L. TELEGDI, Enrico Fermi Institute, University of Chicago, Chicago, Illinois, U.S.A. SIN-ITIRO TOMONAGA, Tokyo University of Education, Tokyo, Japan. ANDRZEJ TRAUTMAN, Institute of Theoretical Physics, University of , Warsaw, Poland. GEORGE E. UHLENBECK, The Rockefeller University, New York, New York, U.S.A. B. L. VAN DER WAERDEN, Universitat Zurich, Zurich, Switzerland. c. F. VON WEIZSACKER, Max-Planck-Institut zur Erforschung der Lebensbeding• ungen der wissenschaftlich-technischen Welt, Starnberg, Fed. Rep. Germany. GREGOR WENTZEL, 77 Via Collina, Ascona, Switzerland. JOHN ARCHIBALD WHEELER, Joseph Henry Laboratories, Princeton University, Princeton, New Jersey U.S.A. EUGENE P. WIGNER, Joseph Henry Laboratories, Princeton University, Princeton, New Jersey, U.S.A. CHEN NING YANG, Department of Physics, State University of New York, Stony Brook, Long Island, New York, U.S.A. Symposium Lecturers, Chairmen, and Members of the Sponsoring Committee Sitting: C. F. von Weizsiicker, F. Perrin, G. E. Uhlenbeck, R. Haag, E. P. Wigner, W. Heisenberg, P. A. M. Dirac, C. N. Yang, C. P. Snow, S. Chandrasekhar, H. B. G. Casimir, M. Eigen, D. W. Sciama, C. M0lIer, J. Schwinger. Standing: J. M. Jauch, W. E. Lamb, Jr., F. Rohrlich, J. S. Bell, B. L. van der Waerden, R. E. Peierls, C. P. Enz, J. Mehra, [V. L. Telegdi obscured], A. Salam, J. A. Wheeler, M. Kac, E. G. D. Cohen, G. Randers, A. Trautman, J. Ehlers. Opening Remarks at the Symposium

Abdus Salam: Mr Mayor, Mr Rector, Professor Casimir, ladies and gentlemen. It is a very great privilege for me to welcome you on behalf of the International Centre for Theoretical Physics. This gathering of physicists is the most distinguished we have had in the eight years of the Centre's history, and might well perhaps be the most distin• guished ever assembled. We have been privileged to welcome some of you here before and we are looking forward to making more friendships among those who have come here for the first time. I would like to introduce the Centre to those who have not been here before. The Centre was set up in 1964 by the International Atomic Energy Agency at Vienna, with the cooperation of the government of Italy, the University of Trieste and the City of Trieste. As you know, the International Atomic Energy Agency is a part of the United Nations family of agencies. A few years later, the International Atomic Energy Agency was joined by Unesco, and the Centre is presently a joint enterprise of these two United Nations agencies. Besides furthering international cooperation and the pursuit of theoretical physics, one of the many aims of the Centre, and this is what makes it a unique organization within the United Nations family, is to help senior physicists from developing countries to remain active in research while still teaching and working in their own home countries. We recognize that a good theoretical physicist, in order to remain at the peak of his powers, must come in contact with his fellows and peers for at least a fraction of the working year. The Centre aims to provide financial support and other facilities for this fraction of the physicist's working year in a stimulating environment of research, and annually some five hundred physicists from developing countries, out of a total of about eight hundred, make use of the facilities of the Centre. In order to keep the Centre's research environment as invigorating and stimulating as possible it is imperative that the great physicists of the world remain associated with it and identify themselves with the Centre's aims and ideals. This is one of the reasons why it gives us the greatest happiness to welcome this very distinguished gathering here today. We believe that the Centre has the first faculty of a future United Nations University, that will serve as a prototype for further academic enterprises of a similar kind for furthering international co-operation and devoted to the task of building up science in the so-called 'third world'. The symposium we are holding today had its origin in April 1970. There was a dis• cussion between Jagdish Mehra and myself about the idea of holding a series of sym• posia on the physicist's concept of nature throughout centuries. The Centre, at that time, was planning for an occasion to celebrate the seventieth birthday of one of the greatest physicists of all time, P. A. M. Dirac, who is with us today, and to whom all xx OPENING REMARKS AT TIm SYMPOSIUM of us wish many happy returns of his birthday. Jagdish Mehra thought it would be a wonderful idea to combine the two occasions, of holding the symposium envisaged by him and saluting Dirac at the same time. He took it from there, and we are very happy that his idea has become a reality. A most exciting programme has been drawn up by the most distinguished Sponsoring Committee for the week ahead. As part of the United Nations Organization, the Centre cannot officially identify itself with any organizations devoted exclusively to the West or the East. This political circumstance, however, in no way diminishes the warmth of the welcome the Centre extends to the symposium by placing its facilities at your fullest disposal, although the University of Trieste is the official host of the symposium in Trieste. It now gives me the greatest pleasure to request one of the two very distinguished co-chairmen of the Sponsoring Committee, a great physicist and a very old friend of the Centre, Professor Casimir, to take over from here.

Hendrik Casimir: Ladies and gentlemen. I have been looking forward to this sym• posium all along as you also have probably. When Jagdish Mehra first approached me in this connection in September 1970 and explained the idea, I at once felt that this symposium could become an extremely interesting and an extremely important occa• sion. I think that my enthusiasm was shared by practically everyone we approached about this project. Why is that? First of all, it is stereotyped to say that there has been an enormous growth of physics, but it just happens to be true that we are living in an age with an enormous expansion of empirical and theoretical knowledge; an age of enormous specialization in various branches of our subject. Therefore, a meeting where one tries not so much to go into the fine details of all the techniques of cal• culation and measurement, but where one can review the fundamental concepts and their background would appear to be extremely useful. Second, there may also be for some of us, who are now of an older generation, a certain nostalgic element in a gathering of this kind, because it makes us think back to a period when there were fewer physicists around and they knew each other, when it was still possible to read most of the really important papers on basic theoretical physics, and even to under• stand them, something which is becoming increasingly difficult today, in any case, for older people. I don't know how the youngsters manage. For these and other reasons, our Sponsoring Committee believes, and all of us expect, that a programme of this kind will be extremely interesting, instructive, and rewarding. I believe I should here say a few words about certain criticisms that have been, and are being directed against a symposium of this kind. They are of different natures at various levels. On the one hand, there is a sort of philosophical level that believes that it is meaningless to discuss physics in the abstract, and not within the compass of some sort of philosophy of mankind, of society, of political structure. Such ideas evi• dently are not new. They have occurred even long ago in the disputes between the Church and Galileo. We may remember the textbook of physics called Die Deutsche Physik, German Physics, written during the Nazi period in Germany, which happened to be quite a good textbook, if you took only those chapters which were not partic- OPENING REMARKS AT THE SYMPOSIUM XXI ularly German. We may recall discussions on genetics in Soviet Russia. I am sure we still believe, as physicists, that we can discuss the basic issues and the abstract ideas of our subject without going into political questions. So I would like to discard that criticism. That does not mean that all of us are not in some way or the other, formed and shaped in our ideas and our beliefs and our ways of expressing ourselves by the background we come from. Of course we are. A historian might find it interesting later on, from the proceedings of a symposium like this one, to try to distinguish the various backgrounds of the participants, but it does not have to do with the subject matter which we are going to treat. A second criticism is more down to earth. It points out that in our world physics has had an enormous impact on the whole society. Whereas, perhaps, older technolo• gies were empirical and craftsmanlike, the new technologies have increasingly been based on the basic science that preceded them, often preceded them many years before. This development has been so enormous and has changed the pattern of our lives to a great extent. It holds such great promise, on the one hand, and such great dangers on the other, that the questions of this impact are more urgent and more important to some people than the abstract problems of the structure of physics. One can have sympathy with this point of view, but in my opinion it does not mean that any and every gathering of physicists should deal with that particular matter. If people find that it is more interesting to discuss these social implications - well, by all means, one can organize meetings and discussions on these questions. But I feel that it is a part of liberty and democracy that physicists who want to get together to discuss certain aspects of their own subject should be free to do so, and it should not be necessary at all always to include these other questions which, needless to say, are very im• portant questions of our age. I should like to say that, after discussions of our Sponsoring Committee, we arrived at the conclusion that although we don't feel that we should modify in any way the programme of the meeting as it stands, unless a majority of those present here would desire such a change of programme. I would like to stress the point that although we believe that we have every right to go on with the programme as it stands, it doesn't mean that many of us would not be willing to discuss other questions, if it is requested and if it can be done in an orderly manner. I should like, at this moment, to ask Jagdish Mehra to step forward and to say a few words about the programme, and get our work started.

Jagdish Mehra: Mr Mayor, Mr Rector, Professor Casimir, Professor Salam, honoured guests, fellow participants, ladies and gentlemen: 'Nature is written in mathematical language.' This phrase first occurred in Galileo's Saggiatore in 1623 and expressed a singularly revolutionary idea. 'By a stroke of the pen, Galileo had abolished the Natura of the ancients with its substances, forms and qUalities. Nature had become the sum total of quantitative phenomena and the very purpose of scientific research was henceforth completely changed.'* * History of Science, Vol. II, Edited by R. Taton, Basic Books, New York, 1963. XXII OPENING REMARKS AT THE SYMPOSIUM

Galileo himself was not able to put his maxim entirely into practice, but from Galileo and Newton to James Clerk Maxwell, to Planck, Einstein, Rutherford, and Bohr, to Werner Heisenberg, Paul Dirac, Eugene Wigner and their successors, physical science has created a conception of Nature which would belong to the permanent intellectual heritage of man. And what a heritage it already is. This symposium has been organized to discuss the current horizons of physical theory in the context of its historical development in the twentieth century. It is a unique development. It has affected our intellectual premises and modes of thought. If the theory of relativity completed the logical development of classical physics, the quantum theory made a break with the past. Paul Dirac initiated the possibility of their unification. But the current frustration about the lack of theoretical structures that would unite them in lasting harmony might well bear out 's pro• phecy that 'no man shall join what God hath put asunder.' The approach to truth is at best asymptotic. John Maynard Keynes, in an address delivered to the Royal Society Club in 1942 [and read again by Geoffrey Keynes at the Newton Tercentenary Celebrations in July 1946] said that 'Newton was not the first of the age of reason. He was the last of the magicians, the last of the Babylonians and Sumerians, the last great mind which looked out on the visible and intellectual world with the same eyes as those who began to build our intellectual inheritance rather less than 10000 years ago.' It is quite possible that a future age will look back on Einstein, Heisenberg, and Dirac as magicians. But in a certain sense we recognize the quality of magic in their work even today. For after all, it is in the apprehension of a sense of mystery in the search for the understanding of Nature, in faulting attempts at its math• ematical and empirical description, that science grows and is never final. As the alchem• ist said in Anatole France's novel La ROtisserie de la Reine Pedoque, 'If the repast I am about to offer you is not well prepared, it is not the fault of my cook, but of chemistry which is in its state of infancy.' The characteristic of science, if not infancy, is perpetual adolescence. The approach to truth is asymptotic. Still, I believe that we have cause for celebration. On the occasion of the seventieth birthdays of men like Dirac, Heisenberg and Wigner, we can look back at what has been served before us. They are among the principal authors of the conceptions to which our symposium is devoted. In seeking to understand the achievements of scientists of their calibre, one might well wonder about the play of chance or the choice of destiny that brought fulfilment to their undertakings. A historian does think about the conditions in which exceptional growth of science took place in the past, and the choices and encounters that led great minds to their pursuits and achievements. It is quite possible that Heisenberg, who was already enamoured of Plato while in school, could have pursued philosophy and the classics. He may have pursued pure mathematics, if the mathematician Lindemann at Munich, whom Heisenberg first visited for advice about his studies at the university, had been less austere and just a bit more friendly. But good fortune, his no less than of physics, led him to visit Sommerfeld. Sommerfeld's kindly guidance and careful advice - such as his remark OPENING REMARKS AT THE SYMPOSWM XXllI that 'when kings go abuilding, wagoners have to do the work' - put a harness on Heisenberg and liberated his genius. One finds it quite remarkable that , who had been invited by Hilbert and the Wolfskehl committee to lecture on atomic theory in Gottingen in June 1921, could not do so until the following year - just when Heisenberg's preparation was ripe enough to encounter him. Sommerfeld may not have encouraged him to go along with him to Gottingen the previous year, and one can only wonder who else would have accompanied Bohr for those Socratic dialogues on the Hainberg and questioned him with such tenacity as did Heisenberg a year later. The pilgrimage which began in 1922 at the Hainberg in Gottingen, has continued for fifty years, and several generations have followed Heisenberg in it and shall continue to do so. Dirac could very well have pursued a career in electrical engineering, a field which he pursued at Bristol University much to the disappointment of the mathematicians there. On graduation in 1921 he looked for a job in engineering. He couldn't find one because there was a depression on at that time. He went back to Bristol to study mathematics. Dirac and a Miss Dent were the only two students in the honours course in mathematics. Miss Dent was quite determined to pursue applied mathematics, and in order that the mathematical faculty should not have to give two sets of courses, Dirac also decided on applied mathematics. This was his way back to science, and he never looked for a job in engineering again. Thanks to Miss Dent and the depression, the choice had been made for him. And just eleven years later he succeeded Sir Joseph Larmor as Lucasian Professor of Mathematics in Cambridge - a chair whose first occupant was Isaac Barrow, Newton's teacher, and the second, Newton himself. Wigner's boyhood ambition was to do pure physics, but it did not show promise as a career, because there were only two chairs of physics in all of Hungary in those days, and his father was not sure whether he should pin his future to occupying one of them. The family business, a prosperous one, was a tanning factory, and Wigner appropriately studied chemical engineering in Budapest in order to be useful to the enterprise. For• tunately, as a young man he was not an expert on employer-employee relations in the factory, and the loss to leather-tanning and management was surely minimal com• pared to the gain to science when he left Budapest and business for and physics. Since 1925, when Wigner wrote his thesis on 'Bildung und Zerfall von Molekiilen' under Polanyi in Berlin, he has known two ways of doing physics: either by pronounc• ing the first word on a subject or the last, and at times both. We can only be happy that these great masters elected to do physics in their careers, a science which owes many of its riches to their conceptions. We are delighted that they are attending this symposium. It has been my good fortune to have been associated with the organization of this sym• posium. I have enjoyed the utmost encouragement, confidence, and help from distin• guished friends and supporters, especially Professors Casimir, Wigner, Salam, Randers, and the other eminent members of the Sponsoring Committee. We have a programme that might well test your stamina. Welcome to our symposium, and I wish you a pleasant and memorable stay at the symposium and in the beautiful city of Trieste. P. A. M. Dirac "The physicist has to replace prejudices by something more precise, leading to some entirely new conception of nature."

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