Monday, July 2, 2007

Reality in the Quantum World

Quantum mechanics, created early this century in response to
certain experimental facts which were inexplicable according to
previously held ideas (conveniently summarised by the title
‘classical physics’), caused three great revolutions. In the first place
it opened up a completely new range of phenomena to which the
methods of physics could be applied: the properties of atoms and
molecules, the complex world of chemical interactions, previously
regarded as things given from outside science, became calculable in
terms of a few fixed parameters. The effect of this revolution has
continued successfully through the physics of atomic nuclei, of
radioactivity and nuclear reactions, of solid-state properties, to
recent spectacular progress in the study of elementary particles. In
consequence all sciences, from cosmology to biology, are, at their
most fundamental level, branches of physics. Through physics they
can, at least in principle, be understood. Indeed, on contemplating
the success of physics, it is easy to be seduced into the belief that
‘everything’ is physics-a belief that, if it is intended to imply that
everything is understood, is certainly false, since, as we shall see,
the very foundation of contemporary theoretical physics is
mysterious and incomprehensible.

The second revolution was the apparent breakdown of determinism,
which had always been an unquestioned ingredient and an
inescapable prediction of classical physics. Note that we are using the word ‘determinism’ solely with regard to physical systems,
without at this stage worrying about which systems can be so
described; that is, we are not here concerned with such concepts as
free will. In a deterministic theory the future behaviour of an
isolated physical system is uniquely determined by its present state.
If, however, the world is correctly described by quantum theory,
then, even for simple systems, this deterministic property is not
valid. The outcome of any particular experiment is not, even in
principle, predictable, but is chosen at random from a set of
possibilities; all that can be predicted is the probability of particular
results when the experiment is repeated many times. It is important
to realise that the probability aspects that enter here do so for a different
reason than, for example, in the tossing of a coin, or throw
of a dice, or a horse race; in these cases they enter because of our
lack of precise knowledge of the orginal state of the system,
whereas in quantum theory, even if we had complete knowledge of
the initial state, the outcome would still only be given as a
probability.
Naturally, physicists were reluctant to accept this breakdown of
a cherished dogma-Einstein’s objection to the idea of God playing
dice with the universe is the most familiar expression of this
reluctance-and it was suggested that the apparent failure of determinism
in the theory was due to an incompleteness in the description
of the system. Many attempts to remedy this incompleteness,
by introducing what are referred to as ‘hidden variables’, have been
made. These attempts will form an important part of our later
discussion.

We are accustomed to regarding the behaviour, at least of simple
mechanical systems, as being completely deterministic, so if the
breakdown of determinism implied by quantum mechanics is
genuine, it is an important discovery which must affect our view of
the physical world. Nevertheless, our belief in determinism arises
from experience rather than logic, and it is quite possible to conceive
of a certain degree of randomness entering into mechanics; no
obvious violation of ‘common sense’ is involved. Such is not the
case with the third revolution brought about by quantum
mechanics. This challenged the basic belief, implicit in all science
and indeed in almost the whole of human thinking, that there exists
an objective reality, a reality that does not depend for its existence
on its being observed. It is because of this challenge that all who endeavour to study, or even take an interest in, reality, the nature
of ‘what is’, be they philosophers or theologians or scientists,
unless they are content to study a phantom world of their own
creation, should know about this third revolution.

To provide such knowledge, in a form accessible to nonscientists,
is the aim of this book. It is not intended for those who
wish to learn the practical aspects of quantum mechanics. Many
excellent books exist to cover such topics; they convincingly
demonstrate the power and success of the theory to make correct
predictions of a wide range of observed phenomena. Normally
these books make little reference to this third revolution; they omit
to mention that, at its very heart, quantum mechanics is totally
inexplicable. For their purpose this omission is reasonable because
such considerations are not relevant to the success of quantum
mechanics and do not necessarily cast doubt on its validity. In
1912, Einstein wrote to a friend, ‘The more success the quantum
theory has, the sillier it looks.’ [Letter to H Zangger, quoted on
p 399 of the book Subtle is the Lord by A Pais (Oxford: Clarendon
1982).] If it is true that quantum mechanics is ‘silly’, then it is so
because, in the terms with which we are capable of thinking, the
world appears to be silly. Indeed the recent upsurge of interest in
the topic of this book has arisen from the results of recent
experiments; results which, though they beautifully confirm the
predictions of quantum mechanics, are themselves, quite
independent of any specific theory, at variance with what an
apparently convincing, common-sense, argument would predict

We can emphasise the essentially observational nature of the
problem we are discussing by returning to the experimental facts we
mentioned at the start of this section, and which gave birth to quantum
mechanics. Although, by abandoning some of the principles of
classical physics, quantum theory predicted these facts, it did not
explain them. The search for an explanation has continued and we
shall endeavour in this book to outline the various possibilities. All
involve radical departures from our normal ways of thinking about
reality.

On almost all the topics which we shall discuss below there is a
large literature. However, since this book is intended to be a
popular introduction rather than a technical treatise, I have given very few references in the text but have, instead, added a detailed
bibliography. For the same reason various ifs and buts and
qualifying clauses, that experts might have wished to see inserted
at various stages, have been omitted. I hope that these omissions
do not significantly distort the argument.
I have tried to keep the discussion simple and non-technical,
partly because only in this way can the ideas be communicated to
non-experts, but also because of a belief that the basic issues are
simple and that highly elaborate and symbolic treatments only
serve to confuse them, or, even worse, give the impression that
problems have been solved when, in fact, they have merely been
hidden. The appendices, most of which require a little more
knowledge of mathematics and physics than the main text, give
further details of certain interesting topics.

Finally, I conclude this section with a confession. For over thirty
years I have used quantum mechanics in the belief that the problems
discussed in this book were of no great interest and could, in
any case, be sorted out with a few hours careful thought. I think
this attitude is shared by most who learned the subject when I did,
or later. Maybe we were influenced by remarks like that with which
Max Born concluded his marvellous book on modern physics
[Atomic Physics (London: Blackie 1935)] : ‘For what lies within
the limits is knowable, and will become known; it is the world of
experience, wide, rich enough in changing hues and patterns to
allure us to explore it in all directions. What lies beyond, the dry
tracts of metaphysics, we willingly leave to speculative philosophy.’
It was only when, in the course of writing a book on elementary
particles, I found it necessary to do this sorting out, that I
discovered how far from the truth such an attitude really is. The
present book has arisen from my attempts to understand things
that I mistakenly thought I already understood, to venture, if you
like, into ‘speculative philosophy’, and to discover what progress
has been made in the task of incorporating the strange phenomena
of the quantum world into a rational and convincing picture of
reality.

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