John D. Barrow, Paul C. W. Davies, Charles L. Harper, Jr-Science and Ultimate Reality_ Quantum Theory, Cosmology, And Complexity-Cambridge University Press (2004)

8/18/2019 John D. Barrow, Paul C. W. Davies, Charles L. Harper, Jr-Science and Ultimate Reality_ Quantum Theory, Cosmolo…

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SCI EN CE A N D U LTI M A TE REA LI TY

Quantum Theory, Cosmology, and Complexity

This volume provides a fascinating preview of the future of physics, covering

fundamental physics at the frontiers of research. It comprises a wide variety of

contributions from leading thinkers in the field, inspired by the pioneering work

of John A. Wheeler. Quantum theory represents a unifying theme within the book,

along with topics such as the nature of physical reality, the arrow of time, models

of the universe, superstrings, gravitational radiation, quantum gravity, and cosmic

inflation. Attempts to formulate a final unified theory of physics are discussed,

along with the existence of hidden dimensions of space, spacetime singularities,hidden cosmic matter, and the strange world of quantum technology.

J o h n A r c h i b a l d W h e e l e r is one of the most influential scientists of the

twentieth century. His extraordinary career has spanned momentous advances in

physics, from the birth of the nuclear age to the conception of the quantum computer.

Famous for coining the term “black hole,” Professor Wheeler helped lay the foun-

dations for the rebirth of gravitation as a mainstream branch of science, triggering

the explosive growth in astrophysics and cosmology that followed. His early contri-

butions to physics include the S matrix, the theory of nuclear rotation (with Edward

Teller), the theory of nuclear fission (with Niels Bohr), action-at-a-distance electro-

dynamics (with Richard Feynman), positrons as backward-in-time electrons, the

universal Fermi interaction (with Jayme Tiomno), muonic atoms, and the collective

model of the nucleus. His inimitable style of thinking, quirky wit, and love of the

bizarre have inspired generations of physicists.


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John Archibald Wheeler, 1987. (Photograph by Robert Matthews, courtesy of Princeton University.)


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S C I E N C E A N D U L T I M A T E R E A L I T Y

Quantum Theory, Cosmology, and Complexity

Edited by

J O H N D . B A R R O WUniversity of Cambridge

PAUL C. W. DAVIES

Macquarie University

and

C H A R L E S L . H A R P E R , J r . John Templeton Foundation


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p u b l i s h e d b y t h e p r e s s s y n d i c a t e o f t h e u n i v e r s i t y o f c a m b r i d g e

The Pitt Building, Trumpington Street, Cambridge, United Kingdom

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C Cambridge University Press 2004

This book is in copyright. Subject to statutory exceptionand to the provisions of relevant collective licensing agreements,

no reproduction of any part may take place withoutthe written permission of Cambridge University Press.

First published 2004

Printed in the United Kingdom at the University Press, Cambridge

Typeface Times 11/14 pt. System LATEX 2ε [tb]

A catalog record for this book is available from the British Library

Library of Congress Cataloging in Publication dataScience and ultimate reality : quantum theory, cosmology, and complexity / edited by

John D. Barrow, Paul C. W. Davies, and Charles L. Harper, Jr.p. cm.

Includes bibliographical references and index.ISBN 0 521 83113 X

1. Quantum theory. 2. Cosmology. 3. Wheeler, John Archibald, 1911–

I. Barrow, John D., 1952– II. Davies, P. C. W. III. Harper, Charles L., 1958– QC174.12.S4 2004

530.12 – dc22 2003055903

ISBN 0 521 83113 X hardback

The publisher has used its best endeavors to ensure that the URLs for external websites referred to in this bookare correct and active at the time of going to press. However, the publisher has no responsibility for the websites

and can make no guarantee that a site will remain live or that the content is or will remain appropriate.

Reprinted 2005 (twice)


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Contents

List of contributors page viii

Foreword xi Editors’ preface xiii

Preface xvii

Acknowledgments xx

Part I An overview of the contributions of John Archibald

Wheeler

1 John Archibald Wheeler and the clash of ideas 3

Paul C. W. Davies

Part II An historian’s tribute to John Archibald Wheeler and

scientific speculation through the ages2 The heritage of Heraclitus: John Archibald Wheeler and the itch

to speculate 27

Jaroslav Pelikan

Part III Quantum reality: theory

3 Why is nature described by quantum theory? 45

Lucien Hardy

4 Thought-experiments in honor of John Archibald Wheeler 72

Freeman J. Dyson

5 It from qubit 90

David Deutsch

6 The wave function: it or bit? 103

H. Dieter Zeh

7 Quantum Darwinism and envariance 121

Wojciech H. Zurek

8 Using qubits to learn about “it” 138

Juan Pablo Paz

v


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vi Contents

9 Quantum gravity as an ordinary gauge theory 153

Juan M. Maldacena

10 The Everett interpretation of quantum mechanics 167

Bryce S. DeWitt

Part IV Quantum reality: experiment11 Why the quantum? “It” from “bit”? A participatory universe?

Three far-reaching challenges from John Archibald Wheeler and

their relation to experiment 201

Anton Zeilinger

12 Speakable and unspeakable, past and future 221

Aephraim M. Steinberg

13 Conceptual tensions between quantum mechanics and general

relativity: are there experimental consequences? 254

Raymond Y. Chiao14 Breeding nonlocal Schrödinger cats: a thought-experiment to

explore the quantum–classical boundary 280

Serge Haroche

15 Quantum erasing the nature of reality: or, perhaps, the reality

of nature? 306

Paul G. Kwiat and Berthold-Georg Englert

16 Quantum feedback and the quantum–classical transition 329

Hideo Mabuchi

17 What quantum computers may tell us about quantum mechanics 345

Christopher R. Monroe

Part V Big questions in cosmology

18 Cosmic inflation and the arrow of time 363

Andreas Albrecht

19 Cosmology and immutability 402

John D. Barrow

20 Inflation, quantum cosmology, and the anthropic principle 426

Andrei Linde

21 Parallel universes 459

Max Tegmark

22 Quantum theories of gravity: results and prospects 492

Lee Smolin

23 A genuinely evolving universe 528

Jo˜ ao Magueijo

24 Planck-scale models of the universe 550

Fotini Markopoulou


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Contents vii

25 Implications of additional spatial dimensions for questions in

cosmology 564

Lisa Randall

Part VI Emergence, life, and related topics

26 Emergence: us from it 577Philip D. Clayton

27 True complexity and its associated ontology 607

George F. R. Ellis

28 The three origins: cosmos, life, and mind 637

Marcelo Gleiser

29 Autonomous agents 654

Stuart Kauffman

30 To see a world in a grain of sand 667

Shou-Cheng ZhangAppendix A Science and Ultimate Reality Program Committees 691

Appendix B Young Researchers Competition in honor of John

Archibald Wheeler for physics graduate students,

postdoctoral fellows, and young faculty 694

Index 697


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Contributors

Andreas Albrecht, University of California, Davis, USA

John D. Barrow, University of Cambridge, UKRaymond Y. Chiao, University of California, Berkeley, USA

Philip D. Clayton, Claremont School of Theology and Claremont Graduate

University, Claremont, California, USA

Paul C. W. Davies, Macquarie University, Sydney, Australia

David Deutsch, University of Oxford, UK

Bryce S. DeWitt, University of Texas, Austin, USA

Freeman J. Dyson, Institute for Advanced Study, Princeton, New Jersey, USA

George F. R. Ellis, University of Cape Town, South Africa

Berthold-Georg Englert, National University of Singapore, MalaysiaMarcelo Gleiser, Dartmouth College, Hanover, New Hampshire, USA

Lucien Hardy, Perimeter Institute for Theoretical Physics and the University

of Waterloo, Ontario, Canada

Serge Haroche, College of France, Paris

Stuart Kauffman, Santa Fe Institute, Santa Fe, New Mexico, USA

Paul G. Kwiat, University of Illinois, Champaign–Urbana, USA

Andrei Linde, Stanford University, Stanford, California, USA

Hideo Mabuchi, California Institute of Technology, Pasadena, USA

João Magueijo, Imperial College of Science, Technology and Medicine,

London, UK

Juan M. Maldacena, Institute for Advanced Study, Princeton, New Jersey,

USA

Fotini Markopoulou, Perimeter Institute for Theoretical Physics and the

University of Waterloo, Ontario, Canada

Christopher R. Monroe, University of Michigan, Ann Arbor, USA

Juan Pablo Paz, University of Buenos Aires, Argentina

Jaroslav Pelikan, Yale University, New Haven, Connecticut, USA

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List of contributors ix

Lisa Randall, Harvard University, Cambridge, Massachusetts, USA

Lee Smolin, Perimeter Institute for Theoretical Physics and the University of

Waterloo, Ontario, Canada

Aephraim M. Steinberg, University of Toronto, Ontario, Canada

Max Tegmark, University of Pennsylvania, Philadelphia, Pennsylvania, USAH. Dieter Zeh, University of Heidelberg, Germany

Anton Zeilinger, University of Vienna, Austria

Shou-Cheng Zhang, Stanford University, Stanford, California, USA

Wojciech H. Zurek, Los Alamos National Laboratory, Los Alamos, New

Mexico, USA


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Foreword

I am immensely pleased with this wonderful volume, and humbled by it. It demon-

strates the incredible vibrancy of fundamental physics, both theoretical and exper-imental, as a new century gets under way. Just as unimagined vistas of the physical

world were revealed in the early years of the twentieth century, so too we are encoun-

tering unimagined wonders a hundred years later. If there is an end to physics, an

end to understanding the reasons for existence, it lies far in the future.

Who would have guessed in 1925, or even in 1950, that quantum mechanics

would remain for so many decades such a fertile field of research? Who would

have guessed then that its reason for being would remain mysterious for so long?

Like many of the authors in this book, I remain convinced that some deeper reason

for quantum mechanics will one day emerge, that eventually we will have an answer to the question, “How come the quantum?” And to the companion question, “How

come existence?”

And who could have guessed in 1975 – when the black hole was coming to be

accepted, when an explanation of pulsars was at hand, when primordial black-body

radiation had been identified – who could have guessed then that an incredible

confluence of deep thinking and stunning experimental techniques would push our

understanding of cosmology – of the beginnings, the history, and the fate of the

universe – to its present astonishing state?

Niels Bohr liked to speak of “daring conservatism” in pursuing physics. That is

what I see in this volume. Nearly every chapter reveals a scientist who is hanging

on to what is known and what is valid while, with consummate daring (or should

I say derring-do?), pushing beyond the limit of what current observation confirms

onward to the outer limit of what current theory allows. Here are scientists daring

to share their visions of where future knowledge may lie.

The organizers of the symposium on which this book is based are to be congrat-

ulated for pulling together and so beautifully integrating the threads of quantum

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xii Foreword

physics, cosmology, and the emergence of complexity. In the 1920s and 1930s, as

my own career got started, I was inspired by Niels Bohr, Werner Heisenberg, Albert

Einstein, and others. I hope that the young people who read this volume will find

similar inspiration in it.

Princeton University John A. Wheeler

Princeton, New Jersey


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Editors’ preface

This book project began as part of a special program, Science and Ultimate Reality,

developed in honor of the ninetieth birthday of renowned theoretical physicistJohn Archibald Wheeler.1 Having long yearned for a comprehensive, integrated

understanding of the nature of the universe, Wheeler has blended scientific rigor

with an unusually adventurous approach to research in physics and cosmology

over a career spanning almost 70 years. Known for investigating many of the

most fundamental and challenging issues in physics, Wheeler has often worked

at the frontiers of knowledge where science and philosophy meet, probing the deep

nature of physical reality. His vision, shaped in part by his influential mentor Niels

Bohr, still flourishes today amid ongoing research activities pursued by several

generations of those he has influenced over the course of much of the twentiethcentury.

With Wheeler as its inspiration, the Science and Ultimate Reality program was

developed with a focus on the future. It brought together a carefully selected group of

outstanding contemporary research leaders in the physics community to explore the

frontiers of knowledge in areas of interest to Wheeler and to map out major domains

and possibilities for far-reaching future exploration. Its two principal components –

(1) this book and (2) a previously held symposium2 – were developed to take

Wheeler’s vision forward into a new century of expanding discovery.

In addition to his role as a research leader in physics, Wheeler has been an

inspirational teacher of many of the twentieth century’s most innovative physicists.

In this context, the program developers, many of whom contributed chapters to this

volume, were asked to offer recommendations for their best candidates – not only

1 Born July 9, 1911 in Jacksonville, Florida.2 The symposium, Science and Ultimate Reality: Celebrating the Vision of John Archibald Wheeler , was held

March 15–18, 2002 in Princeton, New Jersey, United States. See www.metanexus.net/ultimate reality/ for more information and to order the symposium proceedings on DVD. We wish to acknowledge the support of Dr. William Grassie, Executive Director of theMetanexus Institute,and hisexpert stafffor helping to organize thesymposium and for hosting this website. Also see Appendix A for a listing of the program committee members.

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xiv Editors’ preface

distinguished, well-established research leaders, but also highly promising, up-and-

coming young innovators – to address the great questions of physical science in the

twenty-first century. It is well known that, in physics and mathematics at least, the

most powerful insights often come from surprisingly young people. By including

young researchers in the Science and Ultimate Reality program, its developershoped to identify future research leaders for the coming decades.3

In formulating the program, the developers purposely solicited research topics in

areas close to some of Wheeler’s most passionate interests. Some of the questions

the developers kept in mind were: What can Wheeler’s vision imply for the century

ahead? What surprises lie in store for physics? What are the best ways to obtain

deep insights into the heart of reality? What are the great unsolved problems of

cosmology? How might the next generation of researchers tackle some of Wheeler’s

“Really Big Questions,” such as: “Why the quantum?” “How come existence?” “It

from bit?” “A participatory universe?” “What makes ‘meaning’?”This book is intended to stimulate thinking and research among students, profes-

sional physicists, cosmologists, and philosophers, as well as all scholars and others

concerned with the deep issues of existence. Authors were invited to be bold and

creative by developing themes that are perhaps more speculative than is usual in a

volume of this sort. Specifically, they were asked to reflect on the major problems

and challenges that confront fundamental science at this time and to animate their

discussions by addressing the “Really Big Questions” for which Wheeler is so

famous. This book is therefore more than a retrospective celebration of Wheeler’s

ideas and inspirations, or a simple survey of contemporary research. Rather, theeditors sought to develop a collection of chapters that also point to novel approaches

in fundamental research.

The book’s first two chapters provide an overview of John Wheeler’s contribu-

tions (Part I) and an historian’s look at scientific speculation through the ages (Part

II). The remaining twenty-eight chapters are grouped according to four themes:

Part III: Quantum reality: theory

Part IV: Quantum reality: experiment

Part V: “Big questions” in cosmologyPart VI: Emergence, life, and related topics.

The Science and Ultimate Reality program has provided a high-level forum in which

some of the most visionary and innovative research leaders in science today could

present their ideas. It continues to be an important mechanism for funding serious

3 In conjunction with the program,a special Young Researchers Competition was held in which 15 youngscientistschosen competitively from among applicants under age 32 presented short talks at the symposium. One of the researchers who tied for first place – Fotini Markopoulou – contributed a chapter to this volume. Seewww.metanexus.net/ultimate reality/competition.htm/ for more information. Also see Appendix B for a listingof the competition participants and overseers.


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Editors’ preface xv

scientific research and to engage with issues of “ultimate reality” in fascinating

ways. Its overarching goal is to provide a risk-taking stimulus for research leading

to (at least a few) major advances in knowledge of the nature of physical reality. We

hope that John A. Wheeler’s example will continue to stimulate the imaginations

of new generations of the world’s best and brightest students and researchers inscience and that this book will serve to carry that vision forward.

University of Cambridge John D. Barrow

Macquarie University Paul C.W. Davies

John Templeton Foundation Charles L. Harper, Jr.


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Preface

My first encounter with John Archibald Wheeler was in the fall of 1945 in the

reading room of the Science Library in London, a warm and comfortable placewhere anyone could walk in off the street to escape from rain and fog or to browse

at leisure in scientific books and journals. I had just been released from war service

and was eager to get back into science. I found the classic paper of Bohr and Wheeler,

“The mechanism of nuclear fission,” in volume 56 of the Physical Review, pages

426–450. It was published on September 1, 1939, the day on which Hitler’s armies

marched into Poland and the Second World War began. Bohr and Wheeler wrote

the paper in Princeton, where Bohr was visiting in the spring of 1939, a few months

after the discovery of fission. The paper is a masterpiece of clear thinking and

lucid writing. It reveals, at the center of the mystery of fission, a tiny world whereeverything can be calculated and everything understood. The tiny world is a nucleus

of uranium 236, formed when a neutron is freshly captured by a nucleus of uranium

235.

The uranium 236 nucleus sits precisely on the border between classical and

quantum physics. Seen from the classical point of view, it is a liquid drop composed

of a positively charged fluid. The electrostatic force that is trying to split it apart

is balanced by the nuclear surface tension that is holding it together. The energy

supplied by the captured neutron causes the drop to oscillate in various normal

modes that can be calculated classically. Seen from the quantum point of view,

the nucleus is a superposition of a variety of quantum states leading to different

final outcomes. The final outcome may be a uranium 235 nucleus with a re-emitted

neutron, a uranium 236 nucleus with an emitted gamma-ray, or a pair of fission-

fragment nuclei with one or more free neutrons. Bohr and Wheeler calculate the

cross-section for fission of uranium 235 by a slow neutron and get the right answer

within a factor of two. Their calculation is a marvelous demonstration of the power

of classical mechanics and quantum mechanics working together. By studying this

process in detail, they show how the complementary views provided by classical

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xviii Preface

and quantum pictures are both essential to the understanding of nature. Without the

combined power of classical and quantum concepts, the intricacies of the fission

process could never have been understood. Bohr’s notion of complementarity is

triumphantly vindicated.

The Wheeler whose dreams inspired this book is another Wheeler, different fromthe one I encountered in London. Throughout his life, he has oscillated between two

styles of writing and thinking that I like to call prosaic and poetic. In the fission paper,

I met the prosaic Wheeler, a master craftsman using the tools of orthodox physical

theory to calculate quantities that can be compared with experiment. The prosaic

Wheeler always has his feet on the ground. He is temperamentally conservative,

taking the existing theories for granted and using them with skill and imagination

to solve practical problems. But from time to time, we see a different Wheeler,

the poetic Wheeler, who asks outrageous questions and takes nothing for granted.

The poetic Wheeler writes papers and books with titles such as “Beyond the blackhole,” “Beyond the end of time,” and “Law without law.” His message is a call

for radical revolution. He asks, “Should we be prepared to see someday a new

structure for the foundations of physics that does away with time?” He proclaims,

“Proud unbending immutability is a mistaken ideal for physics; this science now

shares, and must forever share, the more modest mutability of its sister sciences,

biology and geology.” He dreams of a future when “as surely as we now know how

tangible water forms out of invisible vapor, so surely we shall someday know how

the universe comes into being.”

The poetic Wheeler is a prophet, standing like Moses on the top of Mount Pisgah,looking out over the Promised Land that his people will one day inherit. Moses

did not live long enough to lead them into the Promised Land. We may hope

that Wheeler will live like Moses to the age of 120. But it is the young people

now starting their careers who will make his dreams come true. This book is a

collection of writings by people who take Wheeler’s dreams seriously and dare to

think revolutionary thoughts. But in science, as in politics and economics, it is not

enough to think revolutionary thoughts. If revolutionary thoughts are to be fruitful,

they must be solidly grounded in practical experience and professional competence.

What we need, as Wheeler says here in his Foreword, is “daring conservatism.”

Revolutionary daring must be balanced by conservative respect for the past, so that

as few as possible of our past achievements are destroyed by the revolution when

it comes.

In the world of politics as in the world of science, revolutionary leaders are

of two kinds, conservative and destructive. Conservative revolutionaries are like

George Washington, destroying as little as possible and building a structure that

has endured for 200 years. Destructive revolutionaries are like Lenin, destroying as

much as possible and building a structure that withered and collapsed after his death.


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Preface xix

The people who will lead us into the new world of physics must be conservative

revolutionaries like Wheeler, at home in the prosaic world of practical calculation

as well as in the poetic world of speculative dreams. The prosaic Wheeler and the

poetic Wheeler are equally essential. They are the two complementary characters

that together make up the John Wheeler that we know and love.

Institute for Advanced Study Freeman J. Dyson

Princeton, New Jersey


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Acknowledgments

The editors wish to acknowledge the John Templeton Foundation (see

www.templeton.org), and Sir John Templeton personally, for making this projectpossible. Sir John was enthusiastic about recognizing John A. Wheeler as one of the

greatest living scientific leaders exemplifying a bold and far-sighted vision com-

bined with tough-minded scientific rigor. We would also like to thank The Peter

Gruber Foundation (see www.petergruberfoundation.org) for additional financial

support.

Freeman Dyson and Max Tegmark, contributors to this volume, played key roles

in organizing the program. Artur Ekert, Robert Laughlin, Charles Misner, William

Phillips, and Charles Townes also provided valuable program advisory assistance,

as did many of the contributors to this volume. (See Appendix A for a listing of theprogram committee members.)

Pamela Bond, working with the John Templeton Foundation, deserves special

thanks for her dedicated work. Pam helped to organize the symposium and oversaw

and coordinated the communications, manuscripts, artwork, and other practical

aspects of producing this book.

We wish to express our special gratitude to Kenneth Ford, a long-time Wheeler

colleague, who served as Senior Program Consultant for both the book and the sym-

posium. Ken worked tirelessly to bring a bold idea into fruition. His commitment,

skillful diplomacy, and hard work are deeply appreciated.

Finally, we wish to thank Cambridge University Press, and particularly Tamsin

van Essen, for their support of this book project.

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Part I

An overview of the contributions of

John Archibald Wheeler


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1

John Archibald Wheeler and the clash of ideas

Paul C. W. Davies

Macquarie University

History will judge John Archibald Wheeler as one of the towering intellects of the

twentieth century. His career spanned the transition from the celebrated GoldenAge of physics to the New Physics associated with the Space Age, the information

revolution and the technological triumphs of quantum and particle physics. His

contributions, ranging from trailblazing work in nuclear physics to general relativity

and astrophysics, are too numerous to list here.1 His influence on three generations

of physicists is immense.

But Wheeler has been more than a brilliant and influential theoretical physicist.

The decision to hold a symposium Science and Ultimate Reality in his honor reflects

the fact that he is also an inspiring visionary who brought to physics and cosmology

a unique style of thought and mode of reasoning, compared by Jaroslav Pelikan inthis volume to that of the Greek philosopher Heraclitus.

“Progress in science,” Wheeler once remarked to me, “owes more to the clash

of ideas than the steady accumulation of facts.” Wheeler has always loved con-

tradiction. After all, the Golden Age of physics was founded on them. The theory

of relativity sprang from the inconsistency between the principle of relativity of

uniform motion, dating back to Galileo, and Maxwell’s equations of electromag-

netism, which predicted a fixed speed of light. Quantum mechanics emerged from

the incompatibility of thermodynamics with the continuous nature of radiation

energy.

Wheeler is perhaps best known for his work in the theory of gravitation, which

receives its standard formulation in Einstein’s general theory of relativity. Although

hailed as a triumph of the human intellect and the most elegant scientific theory

1 See Wheeler’s autobiography Geons, Black Holes, and Quantum Foam: A Life in Physics (W. W. Norton, NewYork, 1998) for more background information.

Science and Ultimate Reality, eds. J. D. Barrow, P. C. W. Davies and C. L. Harper, Jr. Published by Cambridge University Press.C Cambridge University Press 2004.

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4 Paul C. W. Davies

known, the general theory of relativity was for decades a scientific backwater.

Its renaissance in the 1950s and 1960s was due in large measure to the work

and influence of John Wheeler and his many talented students. It was Wheeler

who coined the terms black hole, wormhole, spacetime foam, no-hair theorems,

and many other ubiquitous expressions of gravitational physics. And it was in thetheory of gravitation that Wheeler confronted the starkest contradiction, the most

iconoclastic clash of ideas in science, and one that underscored so much of his

imaginative later work. In the 1960s, astronomical evidence began to accumulate

that compact massive bodies such as the cores of burnt-out heavy stars could not

avoid imploding, suddenly and catastrophically, under their own immense weight, a

phenomenon dubbed gravitational collapse. Wheeler was fascinated by the paradox

implied by the final stages of collapse – the formation of a so-called spacetime

singularity, in which matter is squeezed into a single point of infinite density, and

the gravitational field rises without limit. The fate of a star in this respect resembleson a small scale and in time reverse the origin of the entire universe in a big bang,

where spacetime is hypothesized to have exploded into existence from an initial

singularity.

Gravitational collapse evidently signals the end of . . . what? The general theory

of relativity? The concept of spacetime? Physical law itself? Here was a state of

affairs in which a physical theory contained within itself a prediction of its own

demise, or at least its own inherent limitation. “Wheeler’s style,” a colleague once

told me, “is to take a perfectly acceptable physical theory, and extrapolate it to

the ultimate extreme, to see where it must fail.” With gravitational collapse, thatultimate extreme encompassed not just the obliteration of stars but the birth and

perhaps death of the entire universe.

Two words that recur frequently in the Wheeler lexicon are transcendence and

mutability. At what point in the extrapolation of a theory would the physical situation

become so extreme that the very concepts on which the theory is built are over-

taken by circumstances? No topic better epitomizes this philosophy than the black

hole. When a massive star collapses the gravitational field rises higher and higher

until even light itself is trapped. The material of the core retreats inside a so-called

event horizon and effectively disappears as far as the outside universe is concerned.

What, one may ask, happens to the imploding matter? What trace does it leave in

the outside universe of its erstwhile existence? Theory suggests that only a handful

of parameters survive the collapse – mass, electric charge, and angular momentum

being the three principal conserved quantities. Otherwise, cherished conservation

laws are not so much violated as transcended; they cease to be relevant. For example,

a black hole made of matter cannot be distinguished from one made of antimat-

ter, or neutrinos, or even green cheese, if the few conserved parameters are the

same.


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John Archibald Wheeler and the clash of ideas 5

Although the concept of the black hole had been implicit in the general theory

of relativity for some decades, it was only in the late 1960s, in large part through

the work of John Wheeler, together with Roger Penrose, Stephen Hawking, Robert

Geroch, Brandon Carter, and others, that the extraordinary physical properties of

such objects became understood. Already in those early days it was clear that onevery basic law of physics – the second law of thermodynamics – was threatened

by the existence of black holes, since they could apparently swallow heat and thus

reduce the entropy of the universe. Conversely, if a black hole is perfectly black, its

own entropy (normally measured as energy divided by temperature) would seem

to be infinite. Again, physical theory extrapolated to the limit led to a nonsen-

sical result. Wheeler brilliantly spotted that quantum mechanics might provide a

way around this and come to the rescue of the second law of thermodynamics,

in some ways the most cherished of all physical laws. Together with his student

Jacob Bekenstein, Wheeler surmised that the event horizon area of the black holeconstitutes a completely new form of entropy, so that when a black hole swallows

heat, it swells in size, and its entropy will rise by at least the loss of heat entropy.

These early ideas were placed on a sound theoretical basis in 1975 when Hawking

showed by applying quantum mechanics to black holes that they are not black at

all, but glow with heat radiation at a temperature directly related to their mass. So

quantum mechanics rescues the second law of thermodynamics from an apparent

absurdity when it is combined with general relativity. This episode provides a

wonderful example of the internal consistency of theoretical physics, the fact that

disparate parts of the discipline cunningly conspire to maintain the deepest laws.But what of the fate of the imploding matter? The general theory of relativity

makes a definite prediction. If the core of the star were a homogeneous spherical ball,

continued shrinkage can result in one and only one end state: all matter concentrated

in a single point of infinite density – the famed singularity. Since the general theory

of relativity treats gravitation as a warping or curvature of spacetime, the singularity

represents infinite curvature, which can be thought of as an edge or boundary to

spacetime. So here is a physical process that runs away to infinity and rips open

space and/or time itself. After that, who can say what happens?

As a general rule, infinity is a danger signal in theoretical physics. Few physicists

believe that any genuinely infinite state of affairs should ever come about. Early

attempts to solve the problem of spacetime singularities by appealing to departures

from symmetry failed – a wonky star may not implode to a single point, but, as

Penrose proved, a spacetime singularity of some sort is unavoidable once the star

has shrunk inside an event horizon or something similar.

To Wheeler, the singularity prediction was invested with far-reaching significance

that conveyed his core message of mutability. He likened the history of physics to a

staircase of transcendence, at each step of which some assumed physical property


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6 Paul C. W. Davies

dissolved away to be replaced by a new conceptual scheme. Thus Archimedes

established the density of matter as an important quantity, but later high-pressure

technology showed it was not conserved. Nuclear transmutations transcended the

law of conservation of the elements. Black holes transcended the law of conservation

of lepton number. And so on. Wheeler went on to conclude:

At the head of the stairs there is a last footstep of law and a final riser of law transcended.

There is no law of physics that does not require “space” and “time” for its statement.

Obliterated in gravitational collapse, however, is not only matter, but the space and time

that envelop that matter. With that collapse the very framework falls down for anything one

ever called a law of physics.

The lesson to be learned from this? “Law cannot stand engraved on a tablet of

stone for all eternity . . . All is mutable.” Indeed, mutability was Wheeler’s own

choice when I asked him, in the mid 1980s, what he regarded as his most importantcontribution to physics. He summed up his position with a typical Wheelerism:

“There is no law other than the law that there is no law.”

It has always been Wheeler’s belief that gravitational collapse is a pointer to a

deeper level of reality and a more fundamental physical theory, and that this deeper

level will turn out to be both conceptually simple and mathematically elegant: “So

simple, we will wonder why we didn’t think of it before,” he once told me. But

delving beyond the reach of known physical law is a daunting prospect, for what can

be used as a guide in such unknown territory? Wheeler drew inspiration from his

“law without law” emblem. Perhaps there are no ultimate laws of physics, he mused,only chaos. Maybe everything “comes out of higgledy-piggledy.” In other words,

the very concept of physical law might be an emergent property, in two senses.

First, lawlike behavior might emerge stepwise from the ferment of the Big Bang at

the cosmic origin, instead of being mysteriously and immutably imprinted on the

universe at the instant of its birth: “from everlasting to everlasting,” as he liked to

put it. In this respect, Wheeler was breaking a 400-year-old scientific tradition of

regarding nature as subject to eternal laws. Second, the very appearance of lawlike

behavior in nature might be linked in some way to our observations of nature –

subject and object, observer and observed, interwoven. These were radical ideas

indeed.

One beacon that Wheeler has employed throughout his search for a deeper level

of reality is quantum mechanics. The quantum is at once both an obstacle and an

opportunity. Twentieth-century physics was built largely on the twin pillars of the

general theory of relativity and quantum mechanics. The former is a description of

space, time, and gravitation, the latter a theory of matter. The trouble is, these two

very different sorts of theories seem to be incompatible. Early attempts at combining

them by treating the gravitational field perturbatively like the electromagnetic field,


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with hypothetical gravitons playing a role analogous to photons, ran into severe

mathematical problems. Although a few physical processes could be satisfactorily

described by this procedure, most answers were infinite in a way that could not be

circumvented by simple mathematical tricks. The predictive power of this approach

to quantum gravity was fatally compromised.In spite of these severe conceptual and mathematical problems, Wheeler realized

that at some level quantum effects must have a meaningful impact on gravitational

physics, and since the gravitational field receives a geometrical interpretation in

Einstein’s general theory of relativity, the net result must be some sort of quantum

spacetime dynamics, which Wheeler called quantum geometrodynamics. Here,

spacetime geometry becomes not only dynamical, but subject to quantum rules

such as Heisenberg’s uncertainty principle.

One expression of the uncertainty principle is that physical quantities are subject

to spontaneous, unpredictable fluctuations. Thus energy may surge out of nowherefor a brief moment; the shorter the interval the bigger the energy excursion. Simple

dimensional analysis reveals that for durations as short as 10−43 s, known after

Max Planck as the Planck time, these energy fluctuations should be so intense that

their own gravity seriously distorts the microscopic structure of spacetime. The size

scale for these distortions is the Planck length, 10−33 cm, about 20 powers of ten

smaller than an atomic nucleus, and hence in a realm far beyond the reach of current

experimental techniques. On this minuscule scale of size and duration, quantum

fluctuations might warp space so much that they change the topology, creating a

labyrinth of wormholes, tunnels, and bridges within space itself. As ever, Wheeler was ready with an ingenious metaphor. He compared our view of space to that of an

aviator flying above the ocean. From a great height the sea looks flat and featureless,

just as spacetime seems flat and featureless on an everyday scale of size. But with

a closer look the aviator can see the waves on the surface of the sea, analogous to

ripples in spacetime caused by quantum fluctuations in the gravitational field. If the

aviator comes down low enough, he can discern the foam of the breaking waves, a

sign that the topology of the water is highly complex and shifting on a small scale.

Using this reasoning, Wheeler predicted in 1957 the existence of spacetime foam

at the Planck scale, an idea that persists to this day.

In his later years Wheeler drew more and more inspiration from the quantum

concept. In 1977 he invited me to spend some time with him in Austin at the

University of Texas, where he was deeply involved with the nature of quantum

observation. He was fond of quoting the words of his mentor Niels Bohr concerning

the thorny question of when a measurement of a quantum system is deemed to have

taken place. “No measurement is a measurement until it is brought to a close

by an irreversible act of amplification,” was the dictum. Accordingly, I delivered

a lecture on quantum mechanics and irreversibility that I hoped might have some


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8 Paul C. W. Davies

bearing on this project, but it was clear that Wheeler saw something missing in mere

irreversibility per se. A true observation of the physical world, he maintained, even

something as simple as the decay of an atom, must not only produce an indelible

record, it must somehow impart meaningful information. Measurement implies a

transition from the realm of mindless material stuff to the realm of knowledge. So itwas not enough for Wheeler that a measurement should record a bit of information;

that lowly bit had to mean something. Applying his usual practice of extrapolating

to the extreme, he envisaged a community of physicists for whom the click of the

Geiger counter amounted to more than just a sound; it was connected via a long

chain of reasoning to a body of physical theory that enabled them to declare “The

atom has decayed!” Only then might the decay event be accorded objective status

as having happened “out there” in the physical world.

Thus emerged Wheeler’s idea of the participatory universe, one that makes full

sense only when observers are implicated; one that is less than fully real untilobserved. He envisaged a meaning circuit , in which atomic events are amplified

and recorded and delivered to the minds of humans – events transformed into

meaningful knowledge – and then conjectured a return portion of that meaning

circuit, in which the community of observers somehow loops back into the atomic

realm.

To bolster the significance of the “return portion” of the circuit, Wheeler con-

ceived of a concrete experiment. It was based on an adaptation of the famous

Young’s two-slit experiment of standard quantum mechanics (see Fig. 1). Here a

pinhole light source illuminates a pair of parallel slits in an opaque screen, closelyspaced. The image is observed on a second screen, and consists of a series of bright

and dark bands known as interference fringes. They are created because light has

a wave nature, and the waves passing through the two slits overlap and combine.

Where they arrive at the image screen in phase, a bright band appears; where they

are out of phase a dark band results.

According to quantum mechanics light is also made up of particles calledphotons.

A photon may pass through either one slit or the other, not both. But since the

interference pattern requires the overlap of light from both slits there seems to be

a paradox. How can particles that pass through either one slit or the other make a

wave interference pattern? One answer is that each photon arrives at a specific spot

on the image screen, and many photons together build up a pattern in a speckled

sort of way. As we don’t know through which slit any given photon has passed,

and as the photons are subject to Heisenberg’s uncertainty principle, somehow each

photon “knows” about both slits. However, suppose a mischievous experimenter

stations a detector near the slit system and sneaks a look at which slit each photon

traverses? There would then be a contradiction if the interference pattern persists,

because each photon would be seen to encounter just a single slit. It turns out that


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if this intervention is done, the pattern gets washed out. One way to express this is

to say that the experimenter can choose whether the wavelike or particlelike nature

of light shall be manifested. If the detector is positioned near the slits, light takes

on the properties of particles and there is no wave interference pattern. But if the

experimenter relinquishes information about which slit each photon traverses, lightbehaves like a wave and the pattern is observed. In this manner the experimenter

helps determine the nature of light, indeed, the nature of reality. The experimenter

participates in deciding whether light is made up of waves or particles.

Wheeler’s distinctive adaptation of this experiment came from spotting that the

experimenter can delay the choice – wave or particle – until after the light has

passed through the slit screen. It is possible to “look back” from the image screen

and deduce through which slit any given photon came. The conclusion is that the

experimenter not only can participate in the nature of physical reality that is, but

also in the nature of physical reality that was. Before the experimenter decided,the light was neither wave not particle, its status in limbo. When the decision was

made, the light achieved concrete status. But . . . this status is bestowed upon the

light at a time before the decision is made! Although it seems like retrocausation,

it isn’t. It is not possible for the experimenter to send information back in time, or

to cause a physical effect to occur in the past in a controlled way using this set-up.

In typical Wheeler fashion, John took this weird but secure result and extrapolated

it in the most extreme fashion imaginable. I first learned about this when I ran into

Wheeler at a conference in Baltimore in 1986. “How do you hold up half the ghost

of a photon?” he asked me quizzically. What he had in mind was this. Imaginethat the light source is not a pinpoint but a distant quasar, billions of light years

from Earth. Suppose too that the two slits are replaced by a massive galaxy that

can bend the light around it by gravitational lensing. A given photon now has a

choice of routes for reaching us on Earth, by skirting the galaxy either this way

or that. In principle, this system can act as a gigantic cosmic interferometer, with

dimensions measured in billions of light years. It then follows that the experimenter

on Earth today can perform a delayed choice experiment and thereby determine

the nature of reality that was billions of years ago, when the light was sweeping

by the distant galaxy. So an observer here and now participates in concretizing the

physical universe at a time that life on Earth, let alone observers, did not yet exist!

John’s query about holding up half the ghost of a photon referred to the technical

problem that the light transit times around the galaxy might differ by a month or so,

and light coming around one way might have to be coherently stored until the light

coming round the other way arrives in order to produce an interference pattern.

So Wheeler’s participatory universe ballooned out from the physics lab to encom-

pass the entire cosmos, a concept elegantly captured by his most famous emblem,

shown in Fig. 26.1 on p. 578. The U stands for universe, which originates in a Big


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granted. Students often ask why there is indeterminism in the atomic domain, or

where Schrödinger’s equation comes from. The standard answer is simply, “The

world just happens to be that way.” But some scientists, following Wheeler’s lead,

are not content to simply accept quantum mechanics as a God-given package of

rules: they want to know why the world is quantum mechanical. Wheeler’s insis-tent question, “How come the quantum?” received a good deal of attention at the

symposium.

One way to approach this problem is to ask if there is anything special about the

logical and mathematical structure of quantum mechanics that singles it out as a

“natural” way for the universe to be put together. Could we imagine making small

changes to quantum mechanics without wrecking the form of the world as we know

it? Is there a deeper principle at work that translates into quantum mechanics at the

level of familiar physics?

Lucien Hardy of the Centre for Quantum Computation at the University of Oxfordhas attempted to construct quantum mechanics from a set of formal axioms. Boiled

down to its essentials, quantum mechanics is one possible set of probabilistic rules

with some added properties. The question then arises as to whether it has spe-

cially significant consequences. Could it be that quantum mechanics is the simplest

mechanics consistent with the existence of conscious beings? Or might quantum

mechanics be the structure that optimizes the information processing power of

the universe? Or is it just an arbitrary set of properties that the world possesses

reasonlessly?

The flip side of the challenge “How come the quantum?” is to understand howthe familiar classical world of daily experience emerges from its quantum under-

pinnings, in other words, “How come the classical?” Few disputes in theoretical

physics have been as long-running or as vexed as the problem of how the quantum

and classical worlds join together. Since the everyday world of big things is made

up of little things, quantum mechanics ought to apply consistently to the world

as a whole. So why don’t we see tables and chairs in superpositions of states, or

footballs doing weird things like tunnelling through walls?

Melding the madhouse world of quantum uncertainty with the orderly operation

of cause and effect characteristic of the classical domain is a challenge that has

engaged many theorists. Among them is Wojciech Zurek from the Los Alamos

National Laboratory. In a nutshell, Zurek’s thesis is that quantum systems do not

exist in isolation: they couple to a noisy environment. Since it is central to the quan-

tum description of nature that matter has a wave aspect, then quantum weirdness

requires the various parts of the waves to retain their relative phases. If the environ-

ment scrambles these phases up, then specifically quantum qualities of the system

are suppressed. This so-called “decoherence” seems to be crucial in generating a

quasi-classical world from its quantum components, and Zurek has played a leading


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12 Paul C. W. Davies

role in establishing the theoretical credibility of this explanation. There is, however,

a snag. A decohering environment well explains how, for example, Schrödinger’s

cat will always be seen either alive or dead, never in a ghostly amalgam of the two

conditions. But what if there is no environment? Suppose the system of interest is

the universe as a whole?The application of quantum mechanics to the entire universe is perhaps the most

audacious extrapolation of physical theory in the history of science, and once again

John Wheeler played a key role in its inception. When I was a student in the

1960s the ultimate origin of the universe was widely regarded as lying beyond the

scope of physical science. To be sure, cosmological theory could be applied to

the early moments of the universe following the Big Bang, but the initiating event

itself seemed to be decisively off-limits – an event without a cause. The singularity

theorems suggested that the Big Bang was a boundary to spacetime at which the

gravitational field and the density of matter were infinite, and physical theory brokedown. This meant that there was very little that could be said about how the universe

came to exist from nothing, or why it emerged with the properties it has.

Wheeler drew the analogy between the instability of the classical atom, result-

ing in the emission of an infinite quantity of radiation, and the infinite spacetime

curvature of gravitational collapse. He conjectured that just as quantum mechanics

had saved classical mechanics from diverging quantities, and predicted a stable,

finite atom, so might quantum mechanics ameliorate the Big Bang singularity –

smearing it with Heisenberg uncertainty. But could one take seriously the appli-

cation of quantum mechanics, a theory of the subatomic realm, to cosmology, thelargest system that exists? Wheeler, with typical boldness, believed so, and with

Bryce DeWitt produced a sort of Schrödinger equation for the cosmos. Thus was

the subject of quantum cosmology born.

Today, most cosmologists agree that the universe originated in a quantum pro-

cess, although the application of quantum mechanics to the universe as a whole

remains fraught with both conceptual and technical difficulties. In spite of this,

the subject of quantum cosmology received a fillip in the early 1980s from Alan

Guth’s inflationary universe scenario, which was predicted by applying the quan-

tum theory of fields to the state of matter in the very early universe, while largely

neglecting the quantum aspects of the expanding spacetime itself. Some cosmol-

ogists went on to suggest that quantum fluctuations in the inflationary era created

the small primordial irregularities in the early universe that served as the seeds for

its large-scale structure. If so, then the slight temperature variations detected in the

cosmic background heat radiation – the fading afterglow of the Big Bang – are none

other than quantum fluctuations from the dawn of creation inflated and writ large in

the sky.


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Andrei Linde, now at Stanford University, has been involved in the quantum

cosmology program since the early days of inflation. Indeed, he developed his own

variant of the inflationary theory, termed chaotic inflation, which has been lucidly

explained in his expository articles and books. One fascinating prediction of chaotic

inflation is that what we call “the universe” might be merely a “Hubble bubble”within a multiverse of vast proportions. There could be other “bubbles” out there,

way beyond the scope of even our most powerful instruments, distributed in their

infinite profusion.

If “our universe” is indeed but a minute component of a far more extensive and

complex system, the philosophical consequences are profound. Since Copernicus,

scientists have clung to the notion that there is nothing special about our location

in the universe. We inhabit a typical planet around a typical star in a typical galaxy.

But the multiverse theory suggests that, on a super-Hubble scale, our “bubble”

might be far from typical. It might represent a rare oasis of habitability in a desertthat is generally hostile to life. If so, perhaps many of the felicitous features we

observe, including the fact that the physics of “our universe” seems so bio-friendly,

might actually be the product of a cosmic selection effect. We live in such a special

bubble only because most of the multiverse is unfriendly to life. This idea gener-

ally goes under the name anthropic principle, a misnomer since no special status

is accorded to Homo sapiens as such. Alternatively, it may be that bio-friendly

regions of the universe are also those that grow very large and so occupy the lion’s

share of space. Again, it would be no surprise to find ourselves living in one such

region.Why might our region of the universe be exceptionally bio-friendly? One pos-

sibility is that certain properties of our world are not truly fundamental, but the

result of frozen accidents. For example, the relative strengths of the forces of nature

might not be the result of basic laws of physics, but merely reflect the manner in

which the universe in our particular spatial region cooled from a hot Big Bang. In

another region, these numbers may be different. Mathematical studies suggest that

some key features of our universe are rather sensitive to the precise form of the

laws of physics, so that if we could play God and tinker with, say, the masses of the

elementary particles, or the relative strengths of the fundamental forces, those key

features would be compromised. Probably life would not arise in a universe with

even slightly different values.

A more radical idea is Wheeler’s mutability, according to which there are actually

no truly fixed fundamental laws of physics at all. Many years ago Wheeler suggested

that if the universe were to eventually stop expanding and collapse to a Big Crunch,

the extreme conditions associated with the approach of the spacetime singularity

might have the effect of “reprocessing” the laws of physics, perhaps randomly.


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Imagine, then, that the universe bounces from this Big Crunch into a new cycle of

expansion; then the laws of physics in the new cycle might be somewhat different

from those with which we are familiar.

Is it credible to imagine “different” laws of physics in this way? Einstein once

remarked that the thing that most interested him was whether God had any choice inthe nature of his creation. What he was referring to is whether the physical universe

necessarily has the properties it does, or whether it might have been otherwise.

Obviously some of the fine details could be different, such as the location of this

or that atom. But what about the underlying laws? For example, could there be a

universe in which gravity is a bit stronger, or the electron a bit heavier?

Physicists remain divided on the issue. Some have flirted with the idea that if we

knew enough about the laws of physics, we would find that they are logically and

mathematically unique. Then to use Einstein’s terminology, God would have had

no choice: there is only one possible universe. However, most scientists expect thatthere are many – probably an infinite number – of possible alternative universes in

which the laws and conditions are self-consistent. For example, the universe could

have been Newtonian, consisting of flat space populated by hard spheres that fly

about and sometimes collide. It would probably be a boring world, but it’s hard to

say why it is impossible.

Perhaps the most extreme version of a multiverse has been suggested by

Max Tegmark, a theoretical physicist at the University of Pennsylvania. Tegmark

proposes that all mathematically self-consistent world descriptions enjoy real exis-

tence. There is thus a sliding scale of cosmic extravagance, ranging from mul-tiplying worlds with the mathematical laws fixed, to multiplying laws within an

overall mathematical scheme, to multiplying the mathematical possibilities too.

Some people conclude that invoking an infinity of unseen worlds merely to explain

some oddities about the one we do see is the antithesis of Occam’s razor. Others

believe the multiverse is a natural extension of modern theoretical cosmology.

One of the basic properties of the world that seems to be crucial to the emergence

of life is the dimensionality of space. Must space have three dimensions? A hundred

years ago, Edwin Abbott delighted readers with his book Flatland , an account of

a two-dimensional world in which beings of various geometries lived weird lives

confined to a surface. Strange it may be, but logically there is no reason why the

universe could not be like this. However, the physics of a two-dimensional world

would be odd. For example, waves wouldn’t propagate cleanly as they do in three

dimensions, raising all sorts of problems about signaling and information trans-

fer. Whether these differences would preclude life and observers – which depend

on accurate information processing – is unclear. Other problems afflict worlds

with more than three dimensions; for example, stable orbits would be impossible,


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precluding planetary systems. So a three-dimensional world might be the only one

in which physicists could exist to write about the subject.

Is it just a lucky fluke that space has three dimensions, or is there a deeper

explanation? Some theorists speculate that space emerged from the hot Big Bang

with three dimensions just by chance, and that maybe there are other regions of theuniverse with different dimensionality, going unseen because they cannot support

life. Others suggest that space is actually not three-dimensional anyway, but only

appears that way to us – an idea that dates back to the 1920s.

There are two ways to conceal extra space dimensions. One is to “roll them up”

to a tiny size. Imagine viewing a hosepipe from afar: it looks like a wiggly line. On

closer inspection, the line is revealed as a tube, and what was taken to be a point on

the line is really a little circle going around the circumference of the tube. Similarly,

what we take to be a point in three-dimensional space could be a tiny circle going

around a fourth dimension, too small to detect. It is possible to conceal any number of extra dimensions by such a “compactification” process.

The other way to conceal a higher dimension is to suppose that light and matter

are restricted by physical forces to a three-dimensional “sheet” or “membrane,”

while allowing some physical effects to penetrate into the fourth dimension. In

this way we see just three dimensions in daily life. Only special experiments could

reveal the fourth dimension. Our “three-brane” space need not be alone in four

dimensions. There could be other branes out there too. Recently it was suggested

that the collision of two such branes might explain the Big Bang. Lisa Randall

explains her take on these brane worlds in Chapter 25.No starker clash of ideas, no more enduring a cosmological conundrum, can

be given than that of the arrow of time, a topic with which Wheeler has a long

association. The problem is easily stated. Almost all physical processes display a

unidirectionality in time, yet the underlying laws of physics (with a minor exception

in particle physics) are symmetric in time. At the molecular level, all processes

are perfectly reversible; only when a large collection of molecules is considered

together does directionality emerge. To express it Wheeler’s way: “Ask a molecule

about the flow of time and it will laugh in your face.”

How does asymmetry arise from symmetry? This puzzle has exercised the atten-

tion of many of the world’s greatest physicists since the time of James Clerk

Maxwell and Ludwig Boltzmann. It was discussed at the symposium by Andreas

Albrecht. Most investigators conclude that the explanation for time’s arrow can

be traced to the initial conditions of the system concerned. Seeking the ultimate

origin of time asymmetry in nature inevitably takes one back to the origin of the

universe itself, and the question of the cosmic initial conditions. It seems reasonable

to assume that the universe was born in a low-entropy state, like a wound clock,


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from which it is sliding irreversibly toward a high-entropy state, or heat death, like

a clock running down.

Therein lies a puzzle. From what we know about the state of the universe at,

say, 1 second after the Big Bang, it was very close to thermodynamic equilibrium.

Evidence for this comes from the cosmic background heat radiation, which hasa so-called Planck spectrum, resembling the state of a furnace that has settled

down to a uniform temperature. So at first sight the universe seems to have been

born in a high-, rather than a low-, entropy state. However, first impressions are

misleading, because it is necessary to take into account the thermodynamic effects

of gravitation. Self-gravitating systems tend to go from smooth to clumpy, so the

fact that matter in the early universe was spread evenly through space (disturbed

only by the tiny ripples that triggered the growth of large-scale structure) suggests

that, gravitationally, the universe actually did begin in a low-entropy state. The

arrow of time derives ultimately from the initial cosmological smoothness.Naturally that conclusion begs the question: “Why did the universe start out

smooth?” One explanation is given by the inflation theory: when the universe

jumped in size by a huge factor, any primordial irregularities would have been

stretched away, just as an inflating balloon decrinkles. However, the inflation pro-

cess is a product of laws of physics that are symmetric in time, so, plausible though

this explanation may be, it appears to have smuggled in asymmetry from symme-

try. Clearly there is still much room for disagreement on this historic and vexatious

topic.

John Wheeler has contributed to the discussion of the arrow of time in a number of ways. Perhaps best known is the work he did in the 1940s with Richard Feynman,

formulating a theory of electrodynamics that is symmetric in time. The arrow of

time manifests itself in electromagnetic theory through the prevalence of retarded

over advanced waves: a radio transmitter, for example, sends waves outwards (into

the future) from the antenna. In the Wheeler–Feynman theory, a radio transmitter

sends half its signal into the future and half into the past. Causal chaos is avoided

by appealing to cosmology. It turns out that given certain cosmological boundary

conditions – namely the ability of cosmic matter to absorb all the outgoing radiation

and turn it into heat – a frolic of interference between the primary and secondary

sources of electromagnetic radiation occurs. This serves to wipe out the advanced

waves and build up the retarded waves to full strength. Thus the electromagnetic

arrow of time derives directly from the thermodynamic arrow in this theory.

Many of these big questions of cosmology run into trouble because they appeal to

physics that occurs at extremely high energies, often approaching the Planck values

at which quantum gravitational effects become important. Producing a consistent

and workable theory of quantum gravity has become something of a Holy Grail

in theoretical physics. About 20 years ago it became fashionable to assume that a


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successful theory of quantum gravity would have to form part of a bigger amalga-

mation, one that incorporated the other forces of nature too. The misleading term

theories of everything was adopted. The idea is that there exists a mathematical

scheme, maybe even a formula simple enough to wear on a T-shirt, from which

space, time, gravitation, electromagnetism, the weak and strong nuclear forces plusdescriptions of all the varieties of subatomic particles would emerge, in more or

less their familiar form, at low energies.

An early candidate for such a unified theory went by the name of superstrings.

In simple terms, the idea is that the world isn’t made up of tiny particles, in the

tradition of Greek atomism. Instead, the ultimate building-blocks of the physical

universe are little loops of string that wiggle about, perhaps in a space with 10 or

26 dimensions, in such a way as to mimic particles in three space dimensions when

viewed on a larger scale. Since then, string theory has been incorporated into a

more ambitious scheme known cryptically as M theory. It is too soon to proclaimthat M theory has produced a final unification of physics, though some proponents

are extremely upbeat about this prospect.

Part of the problem is that M theory is characterized by processes that occur at

ultra-high energies or ultra-small scales of size (the so-called Planck scale). Testing

the consequences of the theory in the relatively low-energy, large-scale world of

conventional particle physics isn’t easy. Another problem is the plethora of abstract

mathematical descriptions swirling around the program. Not only does this make the

subject impenetrable for all but a select few workers, it also threatens to smother

the entire enterprise itself. There seem to be so many ways of formulating unifiedtheories, that, without any hope of experimental constraint, there is a danger that

the subject will degenerate into a battle of obscure mathematical fashions, in which

progress is judged more on grounds of philosophical and mathematical appeal than

conformity with reality.

But perhaps that is too cynical a view. There are hints of a meta-unification, in

which the welter of unified theories are themselves unified, and shown to be merely

alternative languages for the same underlying structure. The ultimate hope is that

this meta-unification will stem from a deep physical principle akin to Einstein’s

principle of general covariance, and we will find that all the fundamental properties

of the physical universe flow from a single, simple reality statement.

Although M theory is the currently popular approach to quantum gravity, it is by

no means the only show in town. An alternative scheme, known as loop quantum

gravity, has been painstakingly developed by Lee Smolin and others over many

years, and is described in Chapter 22 of this volume.

Let me turn now to another major theme of the symposium, Wheeler’s “It from

bit,” and the nature of information. Many of the contributors have been working

on quantum information processing and the quest for a quantum computer. These


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researchers often take a leaf out of Wheeler’s book and describe nature in infor-

mational terms. Some even go so far as to claim that the universe is a gigantic

computer. The sociology underlying this line of thinking is interesting. There is a

popular conception that science drives technology rather than the other way about,

but a study of history suggests a more subtle interplay. There has always been atemptation to use the pinnacle of contemporary technology as a metaphor for nature.

In ancient Greece, the ruler and compass, and musical instruments, were the latest

technological marvels. The Greeks built an entire world view based on geometry

and harmony, from the music of the spheres to the mystical properties associated

with certain geometrical shapes. Centuries later, in Renaissance Europe, the clock

represented the finest in craftsmanship, and Newton constructed his theory of the

clockwork universe by analogy. Then in the nineteenth century the steam engine

impressed everybody, and lo and behold, physicists began talking about the universe

as a thermodynamic system sliding toward a final heat death.In recent times the digital computer has served as a seductive metaphor for

nature. Rather than thinking of the universe as matter in motion, one could regard

it as information being processed. After all, when the planets orbit the Sun they

transform input date (the initial positions and motions) into output data (the final

positions and motions) – just like a computer. Newton’s laws play the role of the

great cosmic program.

As technology moves on, we can already glimpse the next step in this concep-

tual development. The quantum computer, if it could be built, would represent a

technological leap comparable to the introduction of the original electronic com-puter. If quantum computers achieve their promise, they will harness exponentially

greater information processing power than the traditional computer. Unfortunately

at this stage the technical obstacles to building a functional quantum computer

remain daunting. The key difficulty concerns the savage decohering effect of the

environment, which I explained above.

In spite of, or perhaps because of, these technical problems, the race to build a

quantum computer has stimulated a new program of research aimed at clarifying

the nature of information and its representation in states of matter, as elucidated in

Chapter 8 by Juan Pablo Paz of the Ciudad Universitaria in Buenos Aires. This work

has led to the introduction of the so-called qubit (or qbit) as the “atom” of quantum

information, the counterpart of the bit in classical information theory. Inevitably the

question arises of whether the universe is “really” a vast assemblage of frolicking

qubits, i.e., a cosmic quantum information processing system. It could be a case of

“it from qubit” as Wheeler might put it.

If quantum information processing is really so powerful, it makes one wonder

whether nature has exploited it already. The obvious place to look is within liv-

ing organisms. A hundred years ago the living cell was regarded as some sort of


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magic matter. Today we see it as a complex information processing and replicat-

ing system using a mathematical organization very similar to a digital computer.

Could quantum mechanics play a nontrivial role in this, at least at certain crucial

stages such as the origin of life from nonlife? Does life manipulate qubits as well

as bits?No topic better illustrates the interplay of information, quantum, and gravitation

than the quantum black hole. Hawking showed that the temperature of a black hole

depends inversely on its mass, so as the hole radiates energy it gets hotter. The

process is therefore unstable, and the hole will evaporate away at an accelerating

rate, eventually exploding out of existence.

Theorists remain sharply divided about the ultimate fate of the matter that fell

into the hole in the first place. In the spirit of Wheeler’s “it from bit,” one would

like to keep track of the information content of the collapsing body as the black

hole forms. If the matter that imploded is forever beyond our ken, then it seems asif all information is irretrievably lost down the hole. That is indeed Hawking’s own

favored interpretation. In which case, the entropy of the black hole can be associated

with the lost information. However, Hawking’s position has been challenged by

Gerard ’t Hooft and others, who think that ultimately the evaporating black hole

must give back to the universe all the information it swallowed. To be sure, it comes

out again in a different form, as subtle correlations in the Hawking radiation. But

according to this position information is never lost to the universe. By contrast,

Hawking claims it is.

Theorists have attacked this problem by pointing out that when matter falls intoa black hole, from the point of view of a distant observer it gets frozen near the

event horizon. (Measured in the reference frame of the distant observer it seems

to take matter an infinite amount of time to reach the event horizon – the surface

of the hole.) So from the informational point of view, the black hole is effectively

two-dimensional: everything just piles up on the horizon. In principle one ought

to be able to retrieve all the black hole’s information content by examining all the

degrees of freedom on the two-dimensional surface. This has led to the analogy

with a hologram, in which a three-dimensional image is created by shining a laser

on a two-dimensional plate.

Despite heroic attempts to recast the theory of quantum black holes in holo-

graphic language, the issue of the fate of the swallowed, or stalled, information is

far from resolved. There is a general feeling, however, that this apparently esoteric

technical matter conceals deeper principles that relate to string theory, unification,

and other aspects of quantum gravity. Juan Maldacena, a theoretical physicist from

Princeton’s Institute for Advanced Study, discusses a specific model that exam-

ines a black hole in a surrounding model universe. Whilst artificial, this so-called

anti-de Sitter universe admits of certain transparent mathematical properties that


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help elucidate the status of the holographic analogy, and the nature of black hole

information and entropy.

At the symposium, there was no better example of Wheeler’s clash of ideas than

the conflict between locality and nonlocality. This is an aspect of the tension that

exists between gravitation and relativity on the one hand, and quantum mechanicson the other. The theory of relativity requires one to specify the state of matter, and

describe the gravitational field, or the geometry, at each spacetime point. In other

words, it is a “local” theory. By contrast, quantum mechanics is “nonlocal” because

the state of a quantum system is spread throughout a region of space. To take a

simple illustration, in general relativity one might specify the gravitational field of

the Sun, and compute the orbit of a planet moving in that field. But in an atom,

there is no well-defined orbit for an electron going around a nucleus. Quantum

uncertainty “smears out” the motion, so that it is simply not possible to say from

one moment to the next where the electron is located. So what happens when wewant to develop the theory of the gravitational effects of a quantum particle? Even

harder is to describe the gravitational effects of a collection of quantum particles

that may exist in a so-called entangled state – a state in which the whole is not

merely the sum of its parts, but an indivisible, inseparable unity containing subtle

correlations that may extend over a wide region of space.

The most dramatic examples of nonlocal quantum states are the quantum fluids,

such as superconductors or Bose–Einstein condensates, in which the weird quantum

effects extend over macroscopic distances. Such systems have been successfully

created in the laboratory in recent years, and Raymond Chiao, a physicist at theUniversity of California, Berkeley, used them to illustrate novel gravitational exper-

iments. In particular, Chiao claims that quantum systems will make superefficient

gravitational wave antennas. The search for gravitational waves is a major research

theme in general relativity and astrophysics, using high-cost laser interferometers.

Chiao’s ideas could revolutionize that search as well as provide new insights into

the apparent clash between gravitation theory and quantum physics.

The final theme of the symposium focused on the philosophical concept of emer-

gence. In the foregoing I have referred to the que

Documents

John D. Barrow, Paul C. W. Davies, Charles L. Harper, Jr-Science and Ultimate Reality_ Quantum Theory, Cosmology, And Complexity-Cambridge University Press (2004)