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  • Q U A N T U M T H E O R Y A T T H E C R O S S R O A D S

    The 1927 Solvay conference was perhaps the most important meeting in the historyof quantum theory. Contrary to popular belief, the interpretation of quantum theorywas not settled at this conference, and no consensus was reached. Instead, a rangeof sharply conflicting views were presented and extensively discussed, includingde Broglies pilot-wave theory, Born and Heisenbergs quantum mechanics, andSchrdingers wave mechanics. Today, there is no longer an established ordominant interpretation of quantum theory, so it is important to re-evaluate thehistorical sources and keep the interpretation debate open.

    This book contains a complete translation of the original proceedings, withbackground essays on the three main interpretations of quantum theory presentedat the conference, and an extensive analysis of the lectures and discussions in thelight of current research in the foundations of quantum theory. The proceedingscontain much unexpected material, including extensive discussions of de Brogliespilot-wave theory (which de Broglie presented for a many-body system), and atheory of quantum mechanics apparently lacking in wave function collapse orfundamental time evolution. This book will be of interest to graduate students andresearchers in physics and in the history and philosophy of quantum theory.

    GUIDO BACCIAGALUPPI is a Senior Lecturer in the Department of Philosophy,University of Aberdeen. His research interests lie mainly in the philosophy ofphysics. He has contributed significantly to the development and critique of modalinterpretations of quantum mechanics. He has since worked widely in variousapproaches to the foundations of quantum theory, as well as in the philosophyof probability and of time and in the history of quantum mechanics.

    ANTONY VALENTINI is a Research Associate in the Theoretical Physics Group,Imperial College London. He has proposed that the universe began with a non-quantum distribution of hidden variables, which later relaxed to the quantum equi-librium state we see today. He has pioneered the development of the new physicsof quantum non-equilibrium, in de BroglieBohm theory and in hidden-variablestheories generally, and has explored its possible role in cosmology, black holesand information theory. He also works in the history and philosophy of modernphysics.

    www.cambridge.org in this web service Cambridge University Press

    Cambridge University Press978-0-521-81421-8 - Quantum Theory at the Crossroads: Reconsidering the 1927 SolvayConferenceGuido Bacciagaluppi and Antony ValentiniFrontmatterMore information

    http://www.cambridge.org/9780521814218http://www.cambridge.orghttp://www.cambridge.org

  • QUANTUM THEORY AT THECROSSROADS

    Reconsidering the 1927 Solvay Conference

    G U I D O B A C C I A G A L U P P IA N T O N Y V A L E N T I N I

    www.cambridge.org in this web service Cambridge University Press

    Cambridge University Press978-0-521-81421-8 - Quantum Theory at the Crossroads: Reconsidering the 1927 SolvayConferenceGuido Bacciagaluppi and Antony ValentiniFrontmatterMore information

    http://www.cambridge.org/9780521814218http://www.cambridge.orghttp://www.cambridge.org

  • C A M B R I D G E U N I V E R S I T Y P R E S SCambridge, New York, Melbourne, Madrid, Cape Town, Singapore, So Paulo, Delhi

    Cambridge University PressThe Edinburgh Building, Cambridge CB2 8RU, UK

    Published in the United States of America by Cambridge University Press, New York

    www.cambridge.orgInformation on this title: www.cambridge.org/9780521814218

    c G. Bacciagaluppi and A. Valentini 2009This publication is in copyright. Subject to statutory exception

    and to the provisions of relevant collective licensing agreements,no reproduction of any part may take place without

    the written permission of Cambridge University Press.

    First published 2009

    Printed in the United Kingdom at the University Press, Cambridge

    A catalogue record for this publication is available from the British Library

    Library of Congress Cataloguing in Publication dataBacciagaluppi, Guido.

    Quantum theory at the crossroads : reconsidering the 1927 Solvay conference / Guido Bacciagaluppi,Antony Valentini.

    p. cm.Includes bibliographical references and index.

    1. Quantum theoryCongresses. I. Valentini, Antony. II. Title.QC173.96.B33 2008

    530.12dc222008019585

    ISBN 978-0-521-81421-8 hardback

    Cambridge University Press has no responsibility forthe persistence or accuracy of URLs for external or

    third-party internet websites referred to in this publication,and does not guarantee that any content on such

    websites is, or will remain, accurate or appropriate.

    www.cambridge.org in this web service Cambridge University Press

    Cambridge University Press978-0-521-81421-8 - Quantum Theory at the Crossroads: Reconsidering the 1927 SolvayConferenceGuido Bacciagaluppi and Antony ValentiniFrontmatterMore information

    http://www.cambridge.org/9780521814218http://www.cambridge.orghttp://www.cambridge.org

  • To the memory of James T. Cushing

    www.cambridge.org in this web service Cambridge University Press

    Cambridge University Press978-0-521-81421-8 - Quantum Theory at the Crossroads: Reconsidering the 1927 SolvayConferenceGuido Bacciagaluppi and Antony ValentiniFrontmatterMore information

    http://www.cambridge.org/9780521814218http://www.cambridge.orghttp://www.cambridge.org

  • Contents

    List of illustrations page xiiPreface xv

    Abbreviations xxi

    Typographic conventions xxiii

    Note on the bibliography and the index xxiii

    Permissions and copyright notices xxiii

    Part I Perspectives on the 1927 Solvay conference 11 Historical introduction 3

    1.1 Ernest Solvay and the Institute of Physics 3

    1.2 War and international relations 6

    1.3 Scientific planning and background 8

    1.4 Further details of planning 15

    1.5 The Solvay meeting 18

    1.6 The editing of the proceedings 20

    1.7 Conclusion 21

    Archival notes 232 De Broglies pilot-wave theory 27

    2.1 Background 27

    2.2 A new approach to particle dynamics: 19231924 33

    2.2.1 First papers on pilot-wave theory (1923) 34

    2.2.2 Thesis (1924) 39

    2.2.3 Optical interference fringes: November 1924 49

    2.3 Towards a complete pilot-wave dynamics: 19251927 51

    2.3.1 Structure: Journal de Physique, May 1927 55

    2.3.2 Significance of de Broglies Structure paper 65

    vii

    www.cambridge.org in this web service Cambridge University Press

    Cambridge University Press978-0-521-81421-8 - Quantum Theory at the Crossroads: Reconsidering the 1927 SolvayConferenceGuido Bacciagaluppi and Antony ValentiniFrontmatterMore information

    http://www.cambridge.org/9780521814218http://www.cambridge.orghttp://www.cambridge.org

  • viii Contents

    2.4 1927 Solvay report: the new dynamics of quanta 672.5 Significance of de Broglies work from 1923 to 1927 76Archival notes 79

    3 From matrix mechanics to quantum mechanics 803.1 Summary of Born and Heisenbergs report 813.2 Writing of the report 843.3 Formalism 85

    3.3.1 Before matrix mechanics 853.3.2 Matrix mechanics 863.3.3 Formal extensions of matrix mechanics 90

    3.4 Interpretation 923.4.1 Matrix mechanics, Born and Wiener 933.4.2 Born and Jordan on guiding fields, Bohr on collisions 943.4.3 Borns collision papers 963.4.4 Heisenberg on energy fluctuations 983.4.5 Transformation theory 1003.4.6 Development of the statistical view in the report 1033.4.7 Justification and overall conclusions 107

    Archival notes 1104 Schrdingers wave mechanics 111

    4.1 Planning of Schrdingers report 1124.2 Summary of the report 1144.3 Particles as wave packets 1164.4 The problem of radiation 1204.5 Schrdinger and de Broglie 1244.6 The conflict with matrix mechanics 125

    4.6.1 Early days 1264.6.2 From Munich to Copenhagen 1284.6.3 Continuity and discontinuity 132

    Archival notes 135

    Part II Quantum foundations and the 1927 Solvay conference 1375 Quantum theory and the measurement problem 139

    5.1 What is quantum theory? 1395.2 The measurement problem today 141

    5.2.1 A fundamental ambiguity 1415.2.2 Measurement as a physical process: quantum theory

    without observers 143

    www.cambridge.org in this web service Cambridge University Press

    Cambridge University Press978-0-521-81421-8 - Quantum Theory at the Crossroads: Reconsidering the 1927 SolvayConferenceGuido Bacciagaluppi and Antony ValentiniFrontmatterMore information

    http://www.cambridge.org/9780521814218http://www.cambridge.orghttp://www.cambridge.org

  • Contents ix

    5.2.3 Quantum cosmology 1475.2.4 The measurement problem in statistical interpretations

    of 1496 Interference, superposition and wave packet collapse 152

    6.1 Probability and interference 1526.1.1 Interference in de Broglies pilot-wave theory 1536.1.2 Interference in the quantum mechanics of Born

    and Heisenberg 1556.2 Macroscopic superposition: Borns discussion of the cloud

    chamber 1606.2.1 Quantum mechanics without wave packet collapse? 161

    6.3 Dirac and Heisenberg: interference, state reductionand delayed choice 164

    6.4 Further remarks on Born and Heisenbergs quantum mechanics 1717 Locality and incompleteness 175

    7.1 Einsteins 1927 argument for incompleteness 1757.2 A precursor: Einstein at Salzburg in 1909 1787.3 More on non-locality and relativity 181

    8 Time, determinism and the spacetime framework 1848.1 Time in quantum theory 1848.2 Determinism and probability 1898.3 Visualisability and the spacetime framework 192

    9 Guiding fields in 3-space 1979.1 Einsteins early attempts to formulate a dynamical theory

    of light quan