This page intentionally left blank www.com The concept of quantum physics led Einstein to state that ‘God does not play dice’. The difficulty he, and others, had with quantum physics was the great conceptual leap it requires us to make from our conventional ways of thinking about the physical world. Rae’s introductory ex- ploration into this area has been hailed as a ‘masterpiece of clarity’ and is an engaging guide to the theories on offer. This new edition has been revised throughout to take account of developments in this field over the past fifteen years, including the idea of ‘consistent histories’ to which a completely new chapter is devoted.com Quantum physics Illusion or reality? www.com Quantum Physics Illusion or Reality? Second Edition Alastair I.
Rae School of Physics and Astronomy University of Birmingham www.com Cambridge, New York, Melbourne, Madrid, Cape Town, Singapore, São Paulo Cambridge University Press The Edinburgh Building, Cambridge , UK Published in the United States of America by Cambridge University Press, New York www.org Information on this title: www.org/9780521542661 © Cambridge University Press 1986, 2004 This publication is in copyright. Subject to statutory exception and to the provision of relevant collective licensing agreements, no reproduction of any part may take place without the written permission of Cambridge University Press. First published in print format 2004 - ---- eBook (EBL) - --- eBook (EBL) - ---- paperback - --- paperback Cambridge University Press has no responsibility for the persistence or accuracy of s for external or third-party internet websites referred to in this publication, and does not guarantee that any content on such websites is, or will remain, accurate or appropriate.com To Ann www.com I like relativity and quantum theories Because I don’t understand them And they make me feel as if space shifted About like a swan that can’t settle Refusing to sit still and be measured And as if the atom were an impulsive thing Always changing its mind. Lawrence Time present and time past Are both perhaps present in time future And time future contained in time past.
Eliot Do you think the things people make fools of themselves about are any less real and true than the things they behave sensibly about? Bernard Shaw www.com Contents Preface to the first edition page xi Preface to the second edition xiii 1 · Quantum physics 1 2 · Which way are the photons pointing? 19 3 · What can be hidden in a pair of photons? 34 4 · Wonderful Copenhagen? 52 5 · Is it all in the mind? 67 6 · Many worlds 81 7 · Is it a matter of size? 95 8 · Backwards and forwards 109 9 · Only one way forward? 120 10 · Can we be consistent? 128 11 · Illusion or reality? 139 Further reading 149 Index 153 ix www.com Preface to the first edition Quantum physics is the theory that underlies nearly all our current understanding of the physical universe. Since its invention some sixty years ago the scope of quantum theory has expanded to the point where the behaviour of subatomic particles, the properties of the atomic nucleus and the structure and properties of molecules and solids are all successfully described in quantum terms. Yet, ever since its beginning, quantum theory has been haunted by conceptual and philosophical problems which have made it hard to understand and difficult to accept. As a student of physics some twenty-five years ago, one of the prime fascinations of the subject to me was the great conceptual leap quantum physics required us to make from our conventional ways of thinking about the physical world.
As students we puzzled over this, encouraged to some extent by our teachers who were nevertheless more concerned to train us how to apply quantum ideas to the understanding of physical phenomena. At that time it was difficult to find books on the conceptual aspects of the subject – or at least any that discussed the problems in a reasonably accessible way. Some twenty years later when I had the opportunity of teaching quantum mechanics to undergraduate students, I tried to include some references to the conceptual aspects of the subject and, although there was by then a quite extensive literature, much of this was still rather technical and difficult for the non- specialist. With experience I have become convinced that it is possible to explain the conceptual problems of quantum physics without requiring either a thorough understanding of the wide areas of physics to which quantum theory has been applied or a great competence in the mathematical techniques that professionals find so useful.
This book is my attempt to achieve this aim. The first four chapters of the book set out the fundamental ideas of quantum physics and describe the two main conceptual problems: non- locality, which means that different parts of a quantum system appear to influence each other even when they are a long way apart and even although there is no known interaction between them, and the ‘measurement problem’, which arises from the idea that quantum systems possess properties only when these are measured, although there is apparently nothing outside quantum physics to make the xi www.com xii Preface to the first edition measurement. The later chapters describe the various solutions that have been proposed for these problems. Each of these in some way challenges our conventional view of the physical world and many of their implications are far-reaching and almost incredible.
There is still no generally accepted consensus in this area and the final chapter summarises the various points of view and sets out my personal position. I should like to thank everyone who has helped me in the writing of this book. In particular Simon Capelin, Colin Gough and Chris Isham all read an early draft and offered many useful constructive criticisms. I was greatly stimulated by discussions with the audience of a class I gave under the auspices of the extra-mural department of the University of Birmingham, and I am particularly grateful for their suggestions on how to clarify the discussion of Bell’s theorem in Chapter 3.
I should also like to offer particular thanks to Judy Astle who typed the manuscript and was patient and helpful with many changes and revisions.com Preface to the second edition My aims in preparing this second edition have been to simplify and clarify the discussion, wherever this could be done without diluting the content, and to update the text in the light of developments during the last 17 years. The discussion of non-locality and particularly the Bell inequalities in Chapter 3 is an example of both of these. The proof of Bell’s theorem has been considerably simplified, without, I believe, damaging its validity, and reference is made to a number of important experiments performed during the last decade of the twentieth century. I am grateful to Lev Vaidman for drawing my attention to the unfairness of some of my criticisms of the ‘many worlds’ interpretation, and to him and Simon Saunders for their attempts to lead me to an understanding of how the problem of probabilities is addressed in this context.
Chapter 6 has been largely rewritten in the light of these, but I am sure that neither of the above will wholeheartedly agree with my conclusions. Chapter 7 has been revised to include an account of the influential spontaneous-collapse model developed by G. Significant recent experimental work in this area is also reviewed. There has been considerable progress on the understanding of irreversibility, which is discussed in Chapters 8, 9 and 10.
Chapter 9, which emphasised ideas current in the 1980s, has been left largely alone, but the new Chapter 10 deals with developments since then. This edition has been greatly improved by the input of Chris Timpson, who has read and criticised the manuscript with the eye of a professional philosopher: he should recognise many of his suggested redrafts in the text. I gratefully acknowledge useful discussions with the speakers and other participants at the annual UK conferences on the foundations of physics – in particular Euan Squires whose death in 1996 deprived the foundations-of-physics community of an incisive critical mind and many of us of a good friend. At the editing stage, incisive constructive criticism from Susan Parkinson greatly improved the text.
Of course, any remaining errors and mistakes are entirely my responsibility.com 1 · Quantum physics ‘God’, said Albert Einstein, ‘does not play dice’. This famous remark by the author of the theory of relativity was not intended as an analysis of the recreational habits of a supreme being but expressed his reaction to the new scientific ideas, developed in the first quarter of the twentieth century, which are now known as quantum physics. Before we can fully appreciate why one of the greatest scientists of modern times should have been led to make such a comment, we must first try to understand the context of scientific and philosophical thought that had become established by the end of the nineteenth century and what it was about the ‘new physics’ that presented such a radical challenge to this consensus. What is often thought of as the modern scientific age began in the sixteenth century, when Nicholas Copernicus proposed that the motion of the stars and planets should be described on the assumption that it is the sun, rather than the earth, which is the centre of the solar system.
The opposition, not to say persecution, that this idea encountered from the establishment of that time is well known, but this was unable to prevent a revolution in thinking whose influence has continued to the present day. From that time on, the accepted test of scientific truth has increasingly been observation and experiment rather than religious or philosophical dogma. The ideas of Copernicus were developed by Kepler and Galileo and notably, in the late seventeenth century, by Isaac Newton. Newton showed that the motion of the planets resulted directly from two sets of laws: first, the laws of motion, which amount to the statement that the acceleration of a moving body is equal to the force acting on it divided by the body’s mass; and, second, the law of gravitation, which asserts that each member of a pair of physical bodies attracts the other by a gravitational force proportional to the product of their masses and inversely proportional to the square of their separation.
Moreover, he realised that the same laws applied to the motion of ordinary objects on earth: the apple falling from the tree accelerates because of the force of gravity acting between it and the earth. Newton’s work also consolidated the importance of mathematics in understanding physics.com 2 Quantum physics: illusion or reality? The ‘laws of nature’ were expressed in quantitative form and mathematics was used to deduce the details of the motion of physical systems from these laws. In this way Newton was able not only to show that the motions of the moon and the planets were consequences of his laws but also to explain the pattern of tides and the behaviour of comets. This objective mathematical approach to natural phenomena was continued in a number of scientific fields.
In particular, James Clerk Maxwell in the nineteenth century showed that all that was then known about electricity and magnetism could be deduced from a small number of equations (soon to be known as Maxwell’s equations) and that these equations also had solutions in which waves of coupled electric and magnetic fields could propagate through space at the speed of light. This led to the realisation that light itself is just an electromagnetic wave, which differs from other such waves (e. radio waves, infrared heat waves, x-rays etc.) only in the magnitudes of its wavelength and frequency. It now seemed that the basic fundamental principles governing the behaviour of the physical universe were known: everything appeared to be subject to Newton’s mechanics and Maxwell’s electromagnetism.