Table Of ContentColin Bruce
Joseph Henry Press
Washington, DC
Joseph Henry Press • 500 Fifth Street, N.W. • Washington, D.C. 20001
The Joseph Henry Press, an imprint of the National Academies Press, was created
with the goal of making books on science, technology, and health more widely
available to professionals and the public. Joseph Henry was one of the founders of
the National Academy of Sciences and a leader in early American science.
Library of Congress Cataloging-in-Publication Data
Bruce, Colin.
Schrödinger’s rabbits : the many worlds of quantum / Colin Bruce.
p. cm.
ISBN 0-309-09051-2 (case)
1. Quantum theory—Popular works. I. Title.
QC174.12.B78 2004
530.12—dc22
2004021021
Any opinions, findings, conclusions, or recommendations expressed in this volume
are those of the author and do not necessarily reflect the views of the National
Academy of Sciences or its affiliated institutions.
Cover design by Michele de la Menardiere.
Copyright 2004 by Colin Bruce. All rights reserved.
Hand-drawn illustrations by Laura Dawes from sketches by Colin Bruce.
Printed in the United States of America
Dedicated to
Paul Dirac
physicist extraordinary
who believed we must seek visualizable processes
and
Jim Cushing
philosopher of science
who believed we must find local stories
PREFACE
D
oes the weirdness of quantum indicate that there is a deep
problem with the theory? Some of the greatest minds in phys-
ics, including Einstein, have felt that it does. Others prefer to
believe that any conceptual difficulties can be ignored or finessed away.
I would put the choice differently. The flip side of a problem is an
opportunity, and the problems with the old interpretations of quan-
tum present us with valuable opportunities.
First, there is the hope of finding ways to think more clearly about
the subject. I have several times seen highly respected scientists—
physicists whose ability to work with the math of quantum mechanics
is certainly better than my own—make appalling freshman howlers in
describing what the result of an experiment would be, because their
qualitative thinking about such matters as quantum collapse was as
fuzzy as everyone else’s. Better conceptual tools are badly needed—
and now they are becoming available.
Second, there is the possibility that a clearer view of quantum will
cause us to see the universe in a fundamentally different way, with
implications both practical and philosophical. Then, as has happened
so many times in physics, the resolution of a seemingly arcane prob-
lem will open our eyes to great new wonders. To ignore such an op-
portunity would be sheer cowardice.
The past few years have seen a sudden explosion of light in the
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viii / Preface
murkier corners of quantum. The old stories, involving such quaint
characters as dead-alive cats and conscious observers with the power
to “collapse” the whole universe, or even split it in two, are passé. There
are new stories to choose from, one of them particularly promising. It
restores us to a classical universe where things behave predictably
rather than randomly and where interactions between things are local
rather than long range. But it comes at a price. We must accept that
the universe we inhabit is much vaster than we thought, in an unex-
pected way.
Although the many-worlds view was invented in the United States,
it is in Europe, and especially in Oxford, that it has developed to matu-
rity. That is my good luck, for I have had the privilege of seeing the
process at first hand. Here I describe the remarkable new picture that
has recently emerged, which I dub the Oxford Interpretation.
My warmest thanks go to my editor Jeff Robbins at Joseph Henry
Press for his vision and determination in ensuring that this book came
to be. Also to many physicists and philosophers at Oxford and else-
where for valuable advice and discussion, including in particular
Harvey Brown, David Deutsch, Roger Penrose, Simon Saunders, David
Wallace and Anton Zeilinger. Special thanks to Lev Vaidman, Jacob
Foster, and Heather Bradshaw, who read the manuscript at an ad-
vanced stage and made many useful comments. Responsibility for any
mistakes that remain, and any controversial opinions expressed herein,
is of course entirely my own.
Colin Bruce
Oxford, 2004
CONTENTS
1 A Magical Universe 1
2 Clinging to the Classical 13
3 Collapse by Inference 27
4 A Horror Story Writ Large 40
5 The Old Testament 57
6 Let’s All Move into Hilbert Space 74
7 Pick Your Own Universe 92
8 A Desirable Locality 106
9 Introducing Many-Worlds 126
10 Harnessing Many-Worlds 1: Impossible Measurements 140
11 Harnessing Many-Worlds 2: Impossible Computers 155
12 Many-Worlds Heroes and Dragons 169
13 The Terror of Many-Worlds 185
14 The Classical Warrior: Roger Penrose 198
15 The New Age Warrior: Anton Zeilinger 211
16 Proving and Improving Many-Worlds 228
Appendix 251
Notes 253
Index 261
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