The Meaning of Quantum Theory: A Guide for Students of Chemistry and Physics (Oxford Science Publications)

243 Pages·1992·8.93 MB·English

SCIENCE
JIM BAGGOTT
The Meaning of Quantum
Theory
A Guide for Students of Chemistry and Physics
Jim Baggott
OXFORD NEW YORK TOKYO
OXFORD UNIVERSITY PRESS
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Boggofl, J. E.
The meaning oj Quantum theory .- IJ guide for s(udf'nl5 of chemistry
and physics I Jim Baggoft.
Includes bibliographical refe-rences and index.
1. Quantum theory. 2. QuanfUm chemisfry. f. Title.
QCJ74,12.834 1992 503.1'2-dclO 91-)4937
ISBN O-19-855575-X (Pbk)
Pdnfed in Harrisonburg, Va. by
R.R.Donnelley &: Sons Company
To Timothy
KERNER: Now we come to the exciting par!. We will watch the bullets
of light to see which way they go. This is not difficult, the apparatus
is simple. So we look carefully and we see the bullets one at a lime,
and some hit the armour plate and bounce back, and some go through
one slit, and some go through the other slit, and, of course, none go
through both slits.
B LA i R: I knew that.
KERNER: You knew that. Now we come to my favourite bit. The wave
pattern has disappeared! It has become particle pattern, jusllike with
real machine-gun bullets.
BLAIR: Why?
KERNER: Because we looked. So, we do it again, exactly the same
except now without looking to see which way the bullets go; and
the wave pattern comes back. So we try again while looking, and
we get particle pattern. Every time we don't look we get wave pattern.
Every time we look to see how we get wave pattern, we get particle
pattern. The act of observing determines the reality.
Tom Stoppard, Hapgood
Preface
Why have I written this book? Perhaps a more burning question for YOll
is: Why should you read it? .
. i' I wrote this book because in August 1987 I made a discovery that
~ shocked me. If, before this date, you had asked me at what stage in the
process of emission and subsequent detection of a photon its state of
polarization is established, I would have answered: At thc moment of
emission, of coursel Imagine then that two photons emitted in rapid suc-
cession from an excited calcium atom are obliged, by the laws of atomic
physics, to be emitted in opposite states of circular polarization: one left
circularly polarized and one right circularly polarized. Surely, they set
off from the atom towards their respective detectors already in those
states of circular polarization. Yes?
Well, ... no. I have since learned that this view-the assumption that
the physical states of quantum particles like photons are 'real' before they
are measured - is called (rather disparagingly, I sometimes think) naive
realism. Now in the J92O$ and 19305, some of the most famous figures
in twentieth century physics were involved in a big debate about the
meaning of the new quantum theory and its implications for physical
reality. In August 1987 I knew a lillIe bit about this debate. But I had
assumed that it had the status of a philosophical debate, with little or no
, relevance to practical matters that could be settled in the laboratory. I
had been trained as a scientist, and although I enjoyed reading about
philosophy (like I enjoyed listening to musk), I was too busy with more
important matters to dig deeply into the subject.
In July and August 1987, I made a short study visit to the University
of Wisconsin at Madison, where I bought a book (always dangerous)
from the University bookstore. This was a book published in celebration
of the centenary of the birth of Niels Bohr. In it were lots' of articles
about his contribution to physics and his great debate with Einstein
on the meaning of quantum theory. One of these articles, written by
N. David Mermin, gave me a tremendous shock. Mermin described the
results of experiments that had been carried out as recently as 198210 test
something called Bell's theorem using two-photon 'cascade' emission
viii Preface
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A pictorial history of the Einstein-Podolsky-Rosen paradox. Montage by p, J.
Kennedy. Reprinted by permission ot the publishers from French, A. P. and
Kennedy, P. J. {eds) (1985). Niels Bohr: 8 centenary volume. Harvard Univer-
sjty Press. MA. Copyright © 1985 by The International Commission on Physics
Education.
Preface ix
from excited calcium atoms. Put simply, Bell's theorem says that my idea.
of naive realism is in conflict with the predictions of quantum theory
in a way that can be tested in the laboratory in spedal experiments on
pairs of quantum particles. These experiments had been done: quantum
theory had been proved right and naIve realism wrong! There in a
montage was a pictorial history of the debate about reality and the
experiments that had been done to test it (reproduced opposite).
This work struck me as desperately important to my understanding
of physical reality, something that as a scientist I felt I ought to know
about. This discovery also made me feel rather embarrassed. Here I was,
proud of my scientific qualifications and with almost 10 years' experi-
ence in chemical physics research at various prestiflious institutions
around the world, and I had been going around with a conception of
physical reality that was completely wrong! Why hadn't somebody told
me about this before?
I could not rest until I had sorted all this ouL How can it be that quan-
tum particles are not '':91]' until they are detected? Are alternative inter-
pretation' of quantum theory ·possible? If so, what arc they like? I
bought lots more books (some very expensive) and spent hours and hours
trying to understand what was going on. There are many excellently
wrilleu popular books on Ihis subject that are easy to understand,
but these left me dissatisfied. These books just told me that there is a
problem, whereas I needed to know why there is a problem: to know
what it is aboutlhe mathematics of quantum theory that leads to all these
difficultie., s .
The trouble is that many of the most important works pUblished on
the imerpretatiofl of quantum theory are heavy going and I (with a
mathematical background I will flatteringly describe as 'poor') made
heavy wdther of them. Nevertheless, I persevered and managed to arrive
at something approaching comprehension. I decided to write it all out in
a book, in such a way that undergraduate students of chemistry and
physics should be able to comprehend the material without needing to
spend hours poring over more advanced texts. And this, of course,
answers the second question: this is why you should read this book.
Students of physical science are usually introduced to the subject
matter of quantum theory at an early, sensitive period in their under-
graduate sludies. Sensitive, because their earlier instruction will not have
taught them the whoie truth about the nature of scientific activity and
the way in which scientific progress is made. Sensitive also because they
will not yet have been trained to question what they are told or what they
read in textbooks.
Quantum theory is unnerving. Not only is the theory mathematically
x Preface
complex, it is also conceptually challenging. For the first time, students
arc taught about a theory which they have to accept and which they have
to learn how to apply, but for which Ihey cannot expect to be told its
meaning. Many will not realize that their inability to understand the
theory is due not to a failing on their part, but to the fact that quantum
theory in its present form is inherently non-understandable. As Richard
Feynman has said: ' ... I think I can safely say that nobody understands
quantum mechanics" (my italics).
Most undergraduate courses on quantum theory never touch on the
theory's profound conceptual problems. This is because the theory
brings us right back to some of the cenlral questions of philosophy and,
as we know, there is no room for philosophy in a modern science degree.
I find this an absurd situation. It is my opinion, expressed in this book,
that quantum theory is philosophy. 011, we can dress it up in grand
phrases littered with jargon-state vector, hermitian operator, Hilbert
space, projection amplitude, and SO on - we can make it all very
mechanistic and mathematical and scientific, b\Jt this does not com-
pletely hide the truth. Beneath the formalism must be an interpretation,
and the interpretation is pure philosophy.
This is the reason why quantum theory is such a difficult subject. Until
they reach undergraduate level, students of chemistry and physics are
brought up on classical science in which there appears 10 be no need for I
philosophy. They arc consequently ill prepanod 10 come 10 terms with
quantum theory. And be warned: students are rarely told the whole truth
about this theory. Instead they are fed the orthodox interpretation either
by design or default. It is perhaps surprising that for a theory so funda-
mental to our understanding of much of chemistry and physics our
teachers do not find it necessary to explain that it has many alternative
interpretations. This is a great pity, Students have a right 10 know the
truth, even if it is bizarre.
In this book I have posed five questions which I believe students
might have expected to be provided with answers. These five questions
are:
Why is quantum theory necessary?
How does it work?
What does it mean?
How can it be tested?
What are the alternatives?
I have tried to answer these questions, one in each of the book's five
chapters. While I am no! in a position to tell you what quantum theory
means, I can tell you why its meaning is so elusive,
I feynman,. R, p, (1%1). The characfer of physical law. M,LT Press, Cambridge, MA.
Preface Xl
Thanks go to Mike Pilling, Ian Smith and Brian Elms for their con-
structive comments on early drafts of this book. Spedal thanks go to
Peter Atkins, not only for the very helpful comments he made on the
draft manuscript in his role as reviewer for Oxford University Press,
but also for his excellent textbook Molecular quantum mechanics from
which much of my knowledge of quantum theory and its applications
has been derived. This book would oot have been possible without the
encouragement of my editor at OUP. r will remain eternally grateful for
having an opportunity to get this lot off my chest.'
JOHN S. BELL
It was with great sadness that I learned of the death of the physicist
John S. Bell during my writing of this book. I had never met Bell, nor
heard him lecture, but in my reading of his scientific papers I have
developed a great admiration for him and his work. I have especially
admired hrs attempts to dismantle the orthodox Copenhagen interpreta-
tion of quantum theory, written with such tremendous style and obvious
enjoyment. Although in this book I have tried to present a balanced
account -arguing one way and then another-l hope that I have done
justice to Bell's superbly constructed criticisms. The debate Over the
meaning of quantum theory will certainly be poorer without him.
Reading
April 1991 J. E. B.