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Physics Meets Philosophy at the Planck Scale: Contemporary This page intentionally left blank [18:13 2000/10/16 g:/tex/key-tex/callendr/3663-PRE.tex] Ref: 3663 CALLENDER: Physics Meets Philosophy Prelims Page: ii i–x Physics meets philosophy at the Planck scale The greatest challenge in fundamental physics is how quantum mechanics and general relativity can be reconciled in a theory of ‘quantum gravity’. The project suggests a profound revision of our notions of space, time, and matter, and so has become a key topic of debate and collaboration between physicists and philosophers. This timely volume collects classic and original contributions from leading experts in both fields for a provocative discussion of all the issues. This volume contains accessible introductions to the main and less well known approaches to quantum gravity. It includes exciting topics such as the fate of spacetime in various theories, the so-called ‘problem of time’ in canonical quantum gravity, black hole thermodynamics, and the relationship between the interpretation of quantum theory and quantum gravity. This book will be essential reading for anyone interested in the profound implications of trying to marry the two most important theories in physics. Craig Callender is an Assistant Professor of Philosophy at the University of California at San Diego. Formerly a Senior Lecturer in Logic, Philosophy and Scientific Method at the London School of Economics, he has worked as Deputy Editor of the British Journal for the Philosophy of Science and as Associate Editor of Mind. His research interests include the philosophical foundations of modern physics, the problem of the direction of time, and the metaphysics of science. He has published in physics, law and philosophy journals. Nick Huggett is an Assistant Professer of Philosophy at the University of Illinois at Chicago, who earned his Ph.D. in Philosophy from Rutgers University. He has published and lectured extensively on topics concerning the philosophical foundations of quantum mechanics, quantum field theory, and spacetime theories. He is the author of the book Space from Zeno to Einstein: Classic Readings with a Contemporary Commentary. [18:13 2000/10/16 g:/tex/key-tex/callendr/3663-PRE.tex] Ref: 3663 CALLENDER: Physics Meets Philosophy Prelims Page: i i–x [18:13 2000/10/16 g:/tex/key-tex/callendr/3663-PRE.tex] Ref: 3663 CALLENDER: Physics Meets Philosophy Prelims Page: ii i–x Physics meets philosophy at the Planck scale Contemporary theories in quantum gravity edited by Craig Callender University of California at San Diego Nick Huggett University of Illinois at Chicago [18:13 2000/10/16 g:/tex/key-tex/callendr/3663-PRE.tex] Ref: 3663 CALLENDER: Physics Meets Philosophy Prelims Page: iii i–x The Pitt Building, Trumpington Street, Cambridge, United Kingdom The Edinburgh Building, Cambridge CB2 2RU, UK 40 West 20th Street, New York, NY 10011-4211, USA 477 Williamstown Road, Port Melbourne, VIC 3207, Australia Ruiz de Alarcón 13, 28014 Madrid, Spain Dock House, The Waterfront, Cape Town 8001, South Africa http://www.cambridge.org © Cambridge University Press 2004 First published in printed format 2001 ISBN 0-511-03382-6 eBook (Adobe Reader) ISBN 0-521-66280-X hardback ISBN 0-521-66445-4 paperback For Joanna and Lisa [18:13 2000/10/16 g:/tex/key-tex/callendr/3663-PRE.tex] Ref: 3663 CALLENDER: Physics Meets Philosophy Prelims Page: v i–x [18:13 2000/10/16 g:/tex/key-tex/callendr/3663-PRE.tex] Ref: 3663 CALLENDER: Physics Meets Philosophy Prelims Page: ii i–x Contents Prefaceix 1Introduction1 Craig Callender and Nick Huggett Part I Theories of Quantum Gravity and their Philosophical Dimensions 2Spacetimeandthephilosophicalchallengeofquantumgravity33 Jeremy Butterfield and Christopher Isham 3Naivequantumgravity90 Steven Weinstein 4Quantumspacetime:Whatdoweknow?101 Carlo Rovelli Part II Strings 5Reflectionsonthefateofspacetime125 Edward Witten 6Aphilosopherlooksatstringtheory138 Robert Weingard 7Blackholes,dumbholes,andentropy152 William G. Unruh Part III Topological Quantum Field Theory 8Higher-dimensionalalgebraandPlanckscalephysics177 John C. Baez vii [18:13 2000/10/16 g:/tex/key-tex/callendr/3663-PRE.tex] Ref: 3663 CALLENDER: Physics Meets Philosophy Prelims Page: vii i–x Contents Part IV Quantum Gravity and the Interpretation of General Relativity 9Ongeneralcovarianceandbestmatching199 Julian B. Barbour 10Pre-Socraticquantumgravity213 Gordon Belot and John Earman 11 The origin of the spacetime metric: Bell’s ‘Lorentzian pedagogy’ and its significanceingeneralrelativity256 Harvey R. Brown and Oliver Pooley Part V Quantum Gravity and the Interpretation of Quantum Mechanics 12 Quantum spacetime without observers: Ontological clarity and theconceptualfoundationsofquantumgravity275 Sheldon Goldstein and Stefan Teufel 13Ongravity’sroleinquantumstatereduction290 Roger Penrose 14Whythequantummustyieldtogravity305 Joy Christian References339 Notesoncontributors357 Index361 viii [18:13 2000/10/16 g:/tex/key-tex/callendr/3663-PRE.tex] Ref: 3663 CALLENDER: Physics Meets Philosophy Prelims Page: viii i–x Preface Both philosophy and quantum gravity are concerned with fundamental questions regarding the nature of space, time, and matter, so it is not surprising that they have a great deal to say to each other. Yet the methods and skills that philosophers and physicists bring to bear on these problems are often very different. However, and especially in recent years, there is an increasing recognition that these two groups are indeed tackling the same issues, and moreover, that these different methods and skills may all be of use in answering these fundamental questions. Our aim in this volume is thus to introduce and explore the philosophical foundations of quantum gravity and the philosophical issues within this field. We believe that some insight will be gained into the many deep questions raised by quantum gravity by examining these issues from a variety of perspectives. Toward this end, we collected together ten original and three previously published con- tributions on this topic from eminent philosophers of science, mathematicians, and physicists. Though the papers assume that the reader is scientifically literate, the majority are written with the non-specialist in philosophy or quantum gravity in mind. The brief that we gave the contributors was to write pieces that would introduce the key elements of the physics and philosophy, whilst making original contributions to the debates. The book should therefore be of interest to (and appro- priate for) anyone challenged and fascinated by the deep questions facing the cutting edge of fundamental theoretical physics. We also feel that these essays will lay the foundation for a wider consideration of quantum gravity in the philosophical com- munity, and hopefully a fruitful dialogue between physicists and philosophers. After all, many of the greatest advances in physics were inseparable from philosophical reflection on foundational questions. We could not have completed this volume without a great deal of support and assistance, and we owe thanks to many people: to all our contributors for their efforts; to Carlo Rovelli and John Baez especially, for their encouragement and advice in the ix [18:13 2000/10/16 g:/tex/key-tex/callendr/3663-PRE.tex] Ref: 3663 CALLENDER: Physics Meets Philosophy Prelims Page: ix i–x Preface early stages; to Jeeva Anandan, Jossi Berkovitz, Jeremy Butterfield, Joy Christian, Carl Hoefer, Jeffrey Ketland, Tom Imbo, and Steve Savitt for help and comments on a number of issues; to Adam Black and Ellen Carlin, our editors, and Alan Hunt of Keyword Publishing Services Ltd. for their enthusiasm, help, and (most of all) patience. We are also grateful to Andrew Hanson for letting us use his wonderful 4-D projection of Calabi-Yau space on the cover. We also thank Damian Steer for his efforts with the bibliography and Jason Wellner for his help preparing the reprinted articles. Nick thanks the Centre for Philosophy of Natural and Social Sciences at the London School of Economics for support during a residence, and the Center for the Humanities and the Office of the Vice Chancellor for Research at the University of Illinois at Chicago for a faculty fellowship and Grants-in-Aid support. Finally, we thank Robert Weingard who taught us that these ideas existed, and of course our families, Joanna and Lisa, Ewan and Lily, without whom none of this would be worthwhile. Craig Callender Nick Huggett x [18:13 2000/10/16 g:/tex/key-tex/callendr/3663-PRE.tex] Ref: 3663 CALLENDER: Physics Meets Philosophy Prelims Page: x i–x 1 Introduction Craig Callender and Nick Huggett In recent years it has sometimes been difficult to distinguish between articles in quantum gravity journals and articles in philosophy journals. It is not uncommon for physics journals such as Physical Review D, General Relativity and Gravitation and others to contain discussion of philosophers such as Parmenides, Aristotle, Leibniz, and Reichenbach; meanwhile, Philosophy of Science, British Journal for the Philosophy of Science and others now contain papers on the emergence of spacetime, the problem of time in quantum gravity, the meaning of general covariance, etc. At various academic conferences on quantum gravity one often finds philosophers at physicists’ gatherings and physicists at philosophers’ gatherings. While we exaggerate a little, there is in recent years a definite trend of increased communication (even collaboration) between physicists working in quantum gravity and philosophers of science. What explains this trend? Part of the reason
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