MANY WORLDS? This page intentionally left blank Many Worlds? Everett, Quantum Theory, and Reality Edited by SIMON SAUNDERS, JONATHAN BARRETT, ADRIAN KENT, AND DAVID WALLACE 1 1 Great Clarendon Street, Oxford ox2 6dp Oxford University Press is a department of the University of Oxford. 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Enquiries concerning reproduction outside the scope of the above should be sent to the Rights Department, Oxford University Press, at the address above You must not circulate this book in any other binding or cover and you must impose the same condition on any acquirer British Library Cataloguing in Publication Data Data available Library of Congress Cataloging in Publication Data Library of Congress Control Number: 2009944012 Typeset by Laserwords Private Limited, Chennai, India Printed in Great Britain on acid-free paper by CPI Antony Rowe, Chippenham, Wiltshire ISBN 978–0–19–956056–1 13579108642 To my parents (JB) To Adriana Kent and Patrick Rabbitt, with gratitude and love (AK) To Kalypso, Ari, and Daphne, with love and appreciation; to Itamar Pitowsky, in remembrance (SS) To Lynne and Phil Wallace (DW) This page intentionally left blank Preface This paper seeks to clarify the foundations of quantum mechanics ...The aim is not to deny or contradict the conventional formulation of quantum theory, which has demonstrated its usefulness in an overwhelming variety of problems, but rather to supply a new, more general and complete formulation, from which the conventional interpretation can be deduced. So wrote Hugh Everett III in 1957, ambitiously and yet understatedly. His ‘relative state formulation of quantum mechanics’ was and is still more radical: it claims that the formalism of quantum mechanics, taken completely literally, describes a reality where every macroscopic superposition of quantum states is really a splitting of the universe into parallel copies. Hence the more standard, and more nakedly radical, name for Everett’s proposal: many-worlds quantum theory. It sounds like science fiction: suggest to a non-physicist that this is what our best theory of the world tells us and the response is often incredulity. Yet over the fifty years since Everett’s proposal, the many-worlds concept has become enormously influential in theoretical physics. It remains highly controversial. Many physicists accept some version of many worlds as the correct reading of quantum theory; many others take the idea very seriously; many, too, firmly reject it. Meanwhile, the last two decades have seen a renaissance in the conceptual and technical development of Everett’s ideas: work in areas as disparate as decoherence theory and decision theory has helped to clarify exactly what a commitment to many worlds might—or perhaps must—involve. Fifty years on, the time seems ripe for an assessment of many-worlds quan- tum theory. The four of us do not agree on its merits. Two of us (Saunders and Wallace) argue that Everett’s proposal is the only coherent way to under- stand unitary quantum theory, and so—in the absence of viable alternatives to unitary quantum theory—current physics makes many-worlds quantum theory compulsory. One of us (Kent) argues that the conventional formu- lation of quantum theory and the experimental predictions that support it cannot in fact be deduced from Everett’s proposal: in other words, that Everett failed in his stated aim. One of us (Barrett) remains agnostic. But we do agree about the profound challenge posed by Everett’s ideas and the work that has come from them, and the concomitant importance of a critical assessment. In the summer of 2007—to mark the 50th anniversary of Everett’s original paper—we organized two conferences, to bring together the best advocacy of the viii Preface Everett interpretation and engage it with the sharpest criticism. The venues and attendees of these conferences illustrate the interdisciplinary nature of research on many-worlds quantum theory and its wide-ranging importance. The first was held in Oxford, organized by Saunders (a philosopher of physics) and under the aegis of the Philosophy Faculty of Oxford University; the second was held in the Perimeter Institute for Theoretical Physics in Waterloo, Canada, and was organized by Barrett and Kent (theoretical physicists) and by Wallace (a physicist-turned-philosopher). This book grew out of the contributions to those conferences, but it is more than a ‘simple’ conference proceedings. Instead, we have drawn on the expertise present at the Oxford and Perimeter meetings to put together a collection of papers which illustrates, as clearly as possible, the state of play of many-worlds quantum theory fifty years on. The four of us agree that there are powerful arguments in its favour, and serious counter-arguments that need to be overcome for it to succeed. We believe that the best of each were on display at the Oxford and PI conferences, and we hope that the best of each can be found within this volume. The book begins with an introduction to many-worlds quantum theory, and a summary of the main arguments that follow. They are arranged in six parts. The first and second centre on whether and how the formalism of quantum theory, when applied to macroscopic systems, already contains within it a description of many worlds. Part 1 gives arguments in favour of that claim; Part 2 gives arguments against. Part 3 is about probability, and the case for recovering or enforcing the probability interpretation of standard quantum theory given the branching structure of reality as argued for in Part 1. Part 4 is also largely on probability and the relationship between many-worlds and standard quantum theory, but its authors aim to refute those claims of Part 3, and particularly those resting on decision theory and Bayesian methods, or on any likely variant of them. Part 5 is given over to some of the main contemporary rivals to many worlds—to proposed alternative realist versions or variants of quantum theory. In contrast Part 6 considers, from the perspective of many worlds, old and new questions: about Everett’s original ideas and their early reception; about time-symmetric quantum mechanics; and about the multiverse concept as it independently arises in contemporary cosmology. Neither this book nor the conferences from which it emerged would have been possible without support from the Fundamental Questions Institute Foundation (FQXi), and from the Perimeter Institute (PI). In connection with the PI meeting, we would particularly like to thank Mike Lazarides, Perimeter Institute’s Founder and Board Chair, for making everything possible, Howard Burton, PI’s then Executive Director, and all the faculty, researchers and staff of Perimeter Institute for their support of and many contributions to the meeting. Special thanks go to Kate Gillespie and Dawn Bombay, for their cheerfully efficient administration. Preface ix The Oxford meeting is indebted to the support of the Philosophy Faculty and its administrators, headed by Thomas Moore, Secretary to the Faculty, to Andrew Davies, IT Manager, for his help with filming the conference and building the conference website, and to all the staff at 10 Merton Street for their cooperation and hospitality. Special thanks go to Peter Taylor, who gave freely and unstintingly of time and energy in managing the conference, and to Paul Tappenden, for preparing transcripts of discussions. Jonathan Barrett Adrian Kent Simon Saunders David Wallace November 2009 This page intentionally left blank Contents Many Worlds? An Introduction 1 Simon Saunders 1. WHY MANY WORLDS? Chapter 1: Decoherence and Ontology 53 David Wallace Chapter 2: Quasiclassical Realms 73 Jim Hartle Chapter 3: Macroscopic Superpositions, Decoherent Histories, and the Emergence of Hydrodynamic Behaviour 99 Jonathan Halliwell 2. PROBLEMS WITH ONTOLOGY Chapter 4: Can the World be Only Wavefunction? 121 Tim Maudlin Chapter 5: A Metaphysician Looks at the Everett Interpretation 144 John Hawthorne Commentary: Reply to Hawthorne: Physics before Metaphysics 154 James Ladyman Transcript: Ontology 161 3. PROBABILITY IN THE EVERETT INTERPRETATION Chapter 6: Chance in the Everett Interpretation 181 Simon Saunders Chapter 7: A Fair Deal for Everettians 206 David Papineau xii Contents Chapter 8: How to Prove the Born Rule 227 David Wallace Chapter 9: Everett and Evidence 264 Hilary Greaves and Wayne Myrvold 4. CRITICAL REPLIES Chapter 10: One World Versus Many: The Inadequacy of Everettian Accounts of Evolution,