Conceptual Foundations of Quantum Physics: An Overview from Modern Perspectives

399 Pages·1997·10.56 MB·English

by  Dipankar Home (auth.)

Conceptual Foundations
of Quantum Physics
An Overview from
Modern Perspectives
Conceptual Foundations
of Quantum Physics
An Overview from
Modern Perspectives
DIPANKAR HOME
Bose Institute
Calcutta, India
Springer Science+Business Media, LLC
Library of Congress Catalog1ng-1n-PublIcatIon Data
Home, Dlpankar.
Conceptual foundations of quantum physics : an overview from
modern perspectives / Dlpankar Home,
p. cm.
Includes bibliographical references and Index.
ISBN 978-1-4757-9810-4
1. Quantum theory. I. Title.
QC174.12.H65 1997
530.12—dc21 97-31703
CIP
ISBN 978-1-4757-9810-4 ISBN 978-1-4757-9808-1 (eBook)
DOI 10.1007/978-1-4757-9808-1
© 1997 Springer Science+Business Media New York
Originally published by Plenum Press, New York in 1997
Softcover reprint of the hardcover 1st edition 1997
All rights reserved
1098 765432 1
No part of this book may be reproduced, stored in a retrieval system, or transmitted in any form
or by any means, electronic, mechanical, photocopying, microfilming, recording, or otherwise,
without written permission from the Publisher
For Anindita
FOREWORD
It may tum out that, like certain other phenomena studied by sociologists, bouts of
interest in the foundations of quantum mechanics tend to come in 60-year cycles.
It is hardly surprising that in the first decade or so of the subject the conceptual
puzzles generated by this strange new way of looking at the world should have
generated profound interest, not just among professional physicists themselves but
also among philosophers and informed laymen; but this intense interest was
followed by a fallow period in the forties and fifties when the physics establishment
by and large took the view that the only puzzles left were the product either of
incompetent application of the formalism or of bad philosophy, and only a few
brave individualists like the late David Bohm dared to suggest that maybe there
really was something there after all to worry about. As Bell and Nauenberg,
surveying the scene in 1966, put it: "The typical physicist feels that [these questions 1
have long ago been answered, and that he will fully understand how if ever he can
spare twenty minutes to think about it." But gradually, through the sixties and
seventies, curiosity did revive, and the last ten years or so have seen a level of
interest in foundational questions, and an involvement in them by some of the
leading figures of contemporary physics, which is probably unparalleled since the
earliest days.
What are the origins of this remarkable revival? A cynic might perhaps suggest
(no doubt as with other 60-year cycles!) that one major component is that the
generation who had "solved" the problems to their own satisfaction in the twenties
have by now by and large died off, leaving the field to those who, as these founding
fathers might have seen it, have never received the correct intellectual vaccinations
in their youth. A rosier view may be that over the last thirty years or so it has
gradually become clear that at least some of the questions which had been dismissed
by the community of hard-headed practicing physicists as "merely philosophical"
could actually generate interesting experiments par excellence on the "quantum
nonlocality" associated with Bell's theorem, but also on the quantum behavior of
collective (macroscopic) degrees of freedom, on single-atom shelving and much
else. Although in all these cases the standard formalism of quantum mechanics,
when competently applied, makes unambiguous predictions, and to date these
predictions have been confirmed, not everyone in the field would necessarily have
staked his or her life in advance of the event that Nature would continue to respect
vii
viii Foreword
the quantum prescriptions under these extreme conditions. The last five years or so
have seen an even more exciting development: the realization that some of the
"weirdest" aspects of quantum theory may actually have useful practical applica
tions, in quantum cryptography and quantum computing. (A development, inciden
tally, which has its negative aspects; not all of us welcome the thought that
nowadays among the audience at conferences on Bell's theorem and such "inno
cent" subjects there may be people whose prime affiliation is with some military
intelligence establishment!)
While the last few years have seen a plethora of books on the conceptual
problems of quantum mechanics for a lay audience, and at least a handful intended
for the more technically fluent reader, I believe the present book will fill a special
niche. On the one hand, Dipankar Home writes on the assumption that his reader
is a professional physicist, or at least will be able to follow complex technical
discussions, and thus keeps the argument on a rigorous level, without the oversim
plifications that inevitably have to be made in books for a lay readership. On the
other hand, he does not, unlike some other books in this category, assume a priori
that quantum mechanics is the ultimate truth about the world and/or that worries
about the conceptual foundations merely reflect an inadequate appreciation of the
subtleties involved in applying it. Indeed a major theme of the book is the profound
and fundamental difficulties which any version of the "orthodox" interpretation has
in explaining the existence, in our everyday experience, of definite outcomes to
experiments where the final state predicted by quantum mechanics is a superposition
the classic "quantum measurement paradox," which in his opinion (and mine!) has
got no nearer a solution for all the words expended on it over the last sixty years.
At the same time, he does not advocate a particular line of solution to this problem to
the exclusion of all others; rather, his emphasis is on the different kinds of experimental
test that someday may (or may not!) set limits to the validity of the quantum description
and/or prove one or other of these "non-standard" approaches correct.
A second special point is the extensive treatment of topics such as non
inequality-based demonstrations of quantum nonlocality, quantum teleportation,
and experiments on the "quantum Zeno effect," which have emerged only very
recently. (The subject of quantum computation was, however, unfortunately too
"late-breaking" to be included). I believe that this book is essential reading for any
physicist who is seriously concerned about the foundations of the theory which, for
all its curious and counterintuitive aspects, is still (to adapt a famous charac
terization by a former British Cabinet member of the prime minister of the day)
"the best theory of the world we've got."
Anthony Leggett
Department of Physics
University of Illinois at Urbana-Champaign
PREFACE
It is the customary fate of new truths to begin as heresies and to end as
superstitions.
THOMAS HENRY HUXLEY
In June 1925, while recuperating after an attack of hay fever on the island of
Heligoland, Werner Heisenberg conceived the crucial ideas which became the
genesis of modern quantum mechanics. On July II, Heisenberg gave Max Born
the final manuscript of his seminal paper entitled "Quantum Theoretical Reinter
pretation of Kinematic and Mechanical Relations." In a letter to Einstein dated July
IS, Born wrote: "Heisenberg's latest paper appears rather mystifying but is cer
tainly true and profound." That signalled the advent of quantum mechanics wrapped
in mystery. Curiously, this sense of elusiveness continues to persist across a span
of more than seventy years.
Rarely in the history of science has there been a theory like quantum mechanics
which strikes at the root of traditional scientific concepts. The spectacular success
of quantum mechanics in accounting for an enormous variety of empirical facts
concerning all known microsystems is so overwhelming that the theory has almost
come to seem "final." To date, quantum mechanics is not known to be contradicted
by any observed phenomenon. Yet the interpretational aspects of the quantum
mechanical formalism have remained an issue of vigorous dissension. The founda
tions of quantum mechanics continue to present an array of intriguing problems
that have challenged the ingenuity of some of the most insightful minds in physics.
Buoyed by the empirical success of their theory, the founders of quantum
mechanics now seem to us, with hindsight, to have made light of the unfirm ground
they trod. It is, of course, true that many of the great achievements in science are
not the result of a brick-by-brick construction, gradually carried out from the
foundations to the top. Nevertheless, foundational problems can become acute
when the discipline is fully developed. This is just the case with present-day
quantum mechanics.
On the face of it, the conceptual problems plaguing the basic principles of
quantum mechanics do not appear to have any immediate bearing on the predictions
of experimentally meaningful results. It is therefore not surprising that practicing
physicists tend to relegate such issues outside the purview of hard-core physics.
ix
x Preface
However, the refinement of the conceptual structures does play an important role
in molding our understanding of the physical world. Often these analyses require
considerable time to generate any significant impact. The increasing interest in the
foundational problems of quantum mechanics which we are witnessing today is
perhaps on account of the fact that quantum mechanics has begun to react on its
own foundations and we are now able to sharpen the delicate questions of funda
mental principles. It is this development that motivates the present book.
This book seeks to provide an overview of the present status of the foundational
issues of nonrelativistic quantum mechanics. It is intended to supplement the usual
graduate courses on quantum mechanics and should also be useful for active
researchers on the foundational problems of quantum mechanics as well as all those
interested in the philosophy of science. The emphasis is on the conceptual issues
and careful explanations of how a number of these problems have become amenable
to precise quantitative formulations as well as experimental studies. The reader will
be given an adequate exposition of the standard interpretation and a critique of its
problematic features. The need to go beyond the standard interpretation is a focal
point of the book. A highlight of the book is its detailed expositions of various
alternative schemes such as the Bohmian approach, the decoherence models, and
the dynamical models of wave function collapse.
In particular, the book seeks to provide an adequate background for anyone
with a graduate-level knowledge of quantum mechanics to follow the exciting new
developments in this area. I also hope that it will contribute to ongoing research
efforts by indicating some new avenues for further studies.
Here I should like to emphasize that the present book does not claim to be a
comprehensive treatise on this subject. Though I have attempted to give a fair and
balanced presentation of different viewpoints, notwithstanding my own preference
for the so-called "realist" approaches, some biases are bound to creep in. The
complexity and enormity of the pertinent literature also impose serious constraints
on the range of topics that can be covered in such an endeavor. Besides, my own
limitations in grasping certain features of some of the issues and viewpoints may
have resulted in a number of omissions or deficiencies in the treatments. With the
focus firmly on the conceptual aspects, details of an experimental or mathematical
nature have been minimized wherever possible. Nevertheless, I have tried to
provide the relevant references adequately so that an interested reader may study
them in depth.
Writing such a book on the foundations of quantum mechanics is highly
demanding, extremely stimulating, and utterly draining! I shall feel amply rewarded
if this can provoke at least a few students and skeptical physicists to take a hard
look at the foundational issues of quantum mechanics.
Preface XI
ACKNOWLEDGMENTS. I've been thinking about the foundational aspects of quantum
mechanics since my graduate student days. My interests concerning such issues
were spurred on by lively interactions with my teacher, Shyamal Sengupta. It was
he who motivated me to take up the challenge of this rather unfashionable research
topic as a subject of my dissertation work. Apart from an inspiring guidance, he
tried to inculcate in me an appreciation of the true spirit underlying scientific
pursuits. Formal words of acknowledgment appear too insignificant to express the
debt of gratitude lowe to him.
From my postdoctoral research days I was indeed fortunate to have the
opportunity of closely interacting with a number of the best minds in physics and
distinguished researchers on this topic. To them I'm indebted for not only learning
enormously but also for the exposure to a wide range of diverse viewpoints. I've
tried to make the best use of these incisive interactions in writing this book. I must
specially mention the late David Bohm, Harvey Brown, James Cushing, Augusto
Garuccio, Daniel Greenberger, Peter Holland, Basil Hiley, Anthony Leggett, Roger
Penrose, Helmut Rauch, the late Euan Squires, Franco Selleri, Emilio Santos, Akira
Tonomura, Andrew Whitaker, and Anton Zeilinger.
Franco Selleri suggested to me the idea of writing a book on this topic. Ken
Derham of Plenum Press was very kind and patient with me though I had repeatedly
failed to meet the scheduled deadline. When taking up this project I had hardly
realized what a formidable job it was. At the end, I feel not only relieved but also
must confess enjoying this opportunity which enabled me to make a concerted
comprehensive study of this subject. As a fitting finale, I'm honored that Anthony
Leggett agreed to write the Foreword. He also painstakingly prepared a list of
several important corrections which were incorporated while checking the proof
pages.
It is a pleasure to place on record my heartiest thanks to all the collaborators
of my various research works used in writing this book. They include Girish
Agarwal, Sougato Bose, Ron Brown, Rajagopal Chattopadhayaya, Amitava Datta,
Partha Ghose, Lucien Hardy, Pan Kaloyerou, Gino Lepore, Archan Majumdar,
Trevor Marshall, Yutaka Mizobuchi, Ranjit Nair, Amlan Ray, Amitava Raychaud
huri, Franco Selleri, Shyamal Sengupta, the late Euan Squires, M.D. Srinivas, and
Andrew Whitaker. With a deep sense of gratitude I recall that I had the privilege
of having perceptive and stimulating discussions with G.S. Agarwal, N. Mukunda,
S.M. Roy, and Y. Singh. Here I also thankA.N. Mitra, G. Rajasekaran, R. Ramanna,
and B.Y. Sreekantan for their warm support of my research work on this "hetero
dox" topic.
None of the above acknowledgments, of course, is meant to imply that any of
these physicists necessarily agrees with the positions I've taken in this book, nor
are they to be implicated in any errors in the present work. Moreover, I suspect and