“It would be hard to imagine a better guide to this difficult subject.” PHYSICS —SCIENTIFIC AMERICAN The Holy Grail of modern physics is the theory of “quantum gravity.” It is a search for a view of the Universe that unites two seemingly opposed pillars of modern science: Einstein’s theory of general relativity, which deals with large scale phenomena like planets, solar systems and galaxies, and quantum theory, which deals with the world of the very small—molecules, atoms and electrons. In the last few years physicists have made steps toward their goal of a completely new theory of space, time and the universe, a “theory of everything.” In Three Roads to Quantum Gravity, Lee Smolin, who has spent his career at the forefront of these new discoveries, presents for the first time the main ideas behind the new developments that have brought a quantum theory of gravity in sight. Written with exceptional style and clarity, Three Roads to Quantum Gravity confronts the deepest questions of the nature of the universe and provides a preview of some of the remarkable scientific developments we can look forward to in the twenty-first century. Praise for THREE ROADS TO QUANTUM GRAVITY “[Smolin] argues lucidly and effectively….This is a deeply philosophical work OHEN that asks us to rethink the epistemological roots of the mental pictures we C EW ORK IMES OOK EVIEW make about nature. —N Y T B R ARC M “Absolutely compelling…Lee Smolin is one of a small handful of physicists around the world who have been able to make genuine progress on an extraordinarily dif- ficult problem that has stumped some of the world’s finest minds for 80 years. [R]equired reading for anyone interested in the problem of quantum gravity and HE MERICAN CIENTIST the future of theoretical physics.” —T A S OVER DESIGN BY C LEE SMOLIN is a leading contributor to the search for a quantum theory of gravity. He is currently a researcher at Perimeter Institute for Theoretical Physics and WITH A NEW POSTSCRIPT BY THE AUTHOR Professor of Physics at Waterloo University. He is also the author of Life of the Cosmos. US $14.00 / $20.95 CAN A Member of the Perseus Books Group www.basicbooks.com THREE ROADS TO QUANTUM GRAVITY LEE SMOLIN A Member of the Perseus Books Group SmolinNRM.qxd 5/16/02 2:37 PM Page iv To my parents Pauline and Michael Copyright © 2001 by Lee Smolin Published by Basic Books, A Member of the Perseus Books Group First Published in Great Britian in 2000 by Weidenfeld and Nicolson All rights reserved. Printed in the United States of America. No part of this book may be reproduced in any manner whatsoever without written permission except in the case of brief quotations embodied in critical articles and reviews. For information, address Basic Books, 387 Park Avenue South, New York, NY 10016-8810. Typeset at the Spartan Press Ltd Lymington, Hants A CIP record for this book is available from the Library of Congress. ISBN 0-465-07836-2 020304/10987654321 CONTENTS ............................................................................................ Acknowledgments vii Prologue: The quest for quantum gravity 1 Part I Points of departure 15 1 There is nothing outside the universe 17 2 In the futurewe shall know more 26 3 Many observers,not many worlds 33 4 The universe is made of processes, not things 49 Part II What we have learned 67 5 Black holes and hidden regions 69 6 Acceleration and heat 77 7 Black holes are hot 88 8 Area and information 95 9 How to count space 106 10 Knots,links and kinks 125 11 The sound of space is a string 146 vi THREE ROADS TO QUANTUM GRAVITY PartIII Thepresentfrontiers 167 12Theholographicprinciple169 13Howtoweaveastring179 14Whatchoosesthelawsofnature?194 Epilogue:Apossiblefuture 207 Postscript 212 Glossary 226 Suggestions for further reading 235 Index 241 ACKNOWLEDGMENTS I must thank first of all the friends and collaborators who formed the community within which I learned most of what I know about quantum gravity. If I had not had the luck to know Julian Barbour, Louis Crane, John Dell, Ted Jacobson and Carlo Rovelli, I doubt that I would have got very far with this subject at all. They will certainly see their ideas and views represented here. Fotini Markopoulou-Kalamara is re- sponsible for changing my views on several important as- pects of quantum gravity over the last several years, which got me out of space and back into spacetime. Large parts of Chapters 2, 3 and 4 are suffused by her thinking, as will be clear to anyone who has read her papers. I am also indebted to Chris Isham for the several ideas of his that have seeped into my work, in some cases without my fully realizing it, and for his friendship and for the example of his life and thought. Stuart Kauffman has taught me most of what I know about how to think about self-organized systems, and I thank him for both the gift of his friendship and his willingness to walk with me in the space between our two subjects. I am also very indebted for discussions, collaborations and encouragement to Giovanni Amelino-Camelia, Abhay Ashte- kar, Per Bak, Herbert Bernstein, Roumen Borissov, Alain Connes, Michael Douglas, Laurant Friedel, Rodolfo Gambini, Murray Gell-Mann, Sameer Gupta, Eli Hawkins, Gary Horowitz, Viqar Husain, Jaron Lanier, Richard Livine, Yi Ling, Renate Loll, Seth Major, Juan Maldecena, Maya Paczuski, Roger Penrose, Jorge Pullin, Martin Rees, Mike Reisenberger, viii THREE ROADS TO QUANTUM GRAVITY Jurgen Renn, Kelle Stelle, Andrew Strominger, Thomas Thiemann and Edward Witten. I add a special word of appreciation for the founders of the ®eld of quantum gravity, including Peter Bergmann, Bryce DeWitt, David Finkelstein, Charles Misner, Roger Penrose and John Archibald Wheeler. If they see many of their ideas here, it is because these are the ideas that continue to shape how we see our problem. Our work has been supported generously by the National Science Foundation, for which I especially have to thank Richard Issacson. In the last several years generous and unexpected gifts from the Jesse Phillips Foundation have made it possible to concentrate on doing science at a time when nothing was more important than the time and freedom to think and work. I am grateful also to Pennsylvania State University, and especially to my chair, Jayanth Banavar, for the supportive and stimulating home it has given me over the last six years, as well as for showing some understanding of the con¯icting demands placed upon me when I found myself with three full-time jobs, as a scientist, teacher and author. The theory group at Imperial College provided a most stimulating and friendly home from home during the year's sabbatical during which this book was written. This book would not exist were it not for the kind encouragement of John Brockman and Katinka Matson, and I am also very grateful to Peter Tallack of Weidenfeld & Nicolson, both for his encouragement and ideas and for being such a good editor, in the old-fashioned sense. Much of the clarity of the text is also due to the artistry and intelligence of John Woodruff's copy editing. Brian Eno and Michael Smolin read a draft and made invaluable suggestions for the organiza- tion of the book, which have greatly improved it. The support of friends has continued to be essential to keeping my own spirit alive, especially Saint Clair Cemin, Jaron Lanier and Donna Moylan. Finally, as always, my greatest debt is to my parents and family, not only for the gift of life but for imparting in me the desire to look beyond what is taught in school, to try to see the world as it may actually be. Lee Smolin London, July 2000 PROLOGUE ............................................................................................ THE QUEST FOR QUANTUM GRAVITY This book is about the simplest of all questions to ask: `What are time and space?' This is also one of the hardest questions to answer, yet the progress of science can be measured by revolutions that produce new answers to it. We are now in the midst of such a revolution, and not one but several new ideas about space and time are being considered. This book is meant to be a report from the front. My aim is to communicate these new ideas in a language that will enable any interested reader to follow these very exciting developments. Space and time are hard to think about because they are the backdrop to all human experience. Everything that exists exists somewhere, and nothing happens that does not happen at some time. So, just as one can live without questioning the assumptions in one's native culture, it is possible to live without asking about the nature of space and time. But there is at least a moment in every child's life when they wonder about time. Does it go on for ever? Was there a ®rst moment? Will there be a last moment? If there was a ®rst moment, then how was the universe created? And what happened just a moment before that? If there was no ®rst moment, does that mean that everything has happened before? And the same for space: does it go on and on for ever? If there is an end to space, what is just on the other side of it? If there isn't an end, can one count the things in the universe? I'm sure people have been asking these questions for as long as there have been people to ask them. I would be surprised if 2 THREE ROADS TO QUANTUM GRAVITY the people who painted the walls of their caves tens of thousands of years ago did not ask them of one another as they sat around their ®res after their evening meals.