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PHYSICS
——SSCCIIEENNTTIIFFIICCAAMMEERRIICCAANN
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.
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LLEEEE SSMMOOLLIINNis 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
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T H R E E R O A D S TO
Q U A N T U M G R AV I T Y
LEE SMOLIN
AMember 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
02 03 04 / 10 9 8 7 6 5 4 3 2 1
CONTENTS
............................................................................................
Acknowledgments vii
Prologue:Thequestforquantumgravity 1
Part I Points of departure 15
1 Thereisnothingoutsidetheuniverse 17
2 Inthefutureweshallknowmore 26
3 Manyobservers,notmanyworlds 33
4 Theuniverseismadeofprocesses,notthings 49
Part II What we have learned 67
5 Blackholes and hidden regions 69
6 Acceleration and heat 77
7 Blackholes arehot 88
8 Areaand information 95
9 Howtocount space 106
10 Knots,linksandkinks 125
11 Thesound of space isastring 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 field of quantum gravity,
including Peter Bergmann, Bryce DeWitt, David Finkelstein,
Charles Misner, Roger Penrose and John Archibald Wheeler.
Iftheyseemanyoftheirideashere,itisbecausethesearethe
ideas that continue to shape how we see our problem. Our
workhasbeensupportedgenerouslybytheNationalScience
Foundation, for which I especially have to thank Richard
Issacson. In the last several years generous and unexpected
giftsfromtheJessePhillipsFoundationhavemadeitpossible
to concentrate on doing science at a time when nothing was
moreimportantthanthetimeandfreedomtothinkandwork.
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 conflicting
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 homeduring the year’s sabbaticalduring
whichthisbookwaswritten.
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,bothforhisencouragementandideasandforbeing
such a good editor, in the old-fashioned sense. Much of the
clarityofthetextisalsoduetotheartistryandintelligenceof
JohnWoodruff’scopyediting.BrianEnoandMichaelSmolin
readadraftandmadeinvaluablesuggestionsfortheorganiza-
tionofthebook,whichhavegreatlyimprovedit.Thesupport
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
parentsandfamily,notonlyforthegiftoflifebutforimparting
inmethedesiretolookbeyondwhatistaughtinschool,totry
toseetheworldasitmayactuallybe.
LeeSmolin
London,July2000
PROLOGUE
............................................................................................
THE QUEST FOR QUANTUM GRAVITY
Thisbook is about the simplest ofall questionsto ask:‘What
aretimeandspace?’Thisisalsooneofthehardestquestions
to answer, yet the progress of science can be measured by
revolutionsthatproducenewanswerstoit.Wearenowinthe
midstofsucharevolution,andnotonebutseveralnewideas
about space and time are being considered. This book is
meanttobeareportfromthefront.Myaimistocommunicate
these new ideas in a language that will enable any interested
readertofollowtheseveryexcitingdevelopments.
Spaceandtimearehardtothinkaboutbecausetheyarethe
backdrop to all human experience. Everything that exists
existssomewhere,andnothinghappensthatdoesnothappen
atsometime.So,justasonecanlivewithoutquestioningthe
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 first moment?
Willtherebealastmoment?Iftherewasafirstmoment,then
how was the universe created? And what happened just a
moment before that? If there was no first moment, does that
meanthateverythinghashappenedbefore?Andthesamefor
space:doesitgoonandonforever?Ifthereisanendtospace,
what is just on the other side of it? If there isn’t an end, can
onecountthethingsintheuniverse?
I’msurepeoplehavebeenaskingthesequestionsforaslong
astherehavebeenpeopletoaskthem.Iwouldbesurprisedif
