Early history and the Copenhagen interpretation

We have not, in this book, been greatly concerned with the
historical development of quantum theory. When an idea is new
many mistakes are made, blind alleys followed, and the really
significant features can sometimes be missed. Thus history is unlikely to be a good teacher. Nevertheless, it is of interest to look
back briefly on how the people who introduced quantum theory
into physics interpreted what they were doing.
Already we have noted that Einstein, surely the premier scientist
of this century, was always unhappy with quantum theory, which
he considered to be, in some way, incomplete. Initially his objections
seemed to be to the lack of causality implied by the theory,
and to the restrictions imposed by the uncertainty principle. He had
a long running controversy with Bohr on these issues, a controversy
which it is fair to say he lost. In addition, however, Einstein was
one of the first to realise the deeper conceptual problems. These he
was not able to resolve. Many years after the time when he was the
first to teach the world about photons, the particles of light, he admitted
that he still did not understand what they were.
Even more remarkable, perhaps, was the attitude of Schrodinger .
We recall that it was he who introduced the equation that bears his
name, and which is the practical expression of quantum theory,
with solutions that contain a large proportion of all science. In
1926, while on a visit to Copenhagen for discussions with Bohr and
Heisenberg, he remarked: ‘If all this damned quantum jumping
were really to stay, I should be sorry I ever got involved with quantum
theory.’ (This quote, which is of course a translation from the
original German, is taken from the book by Jammer, The
Philosophy of Quantum Mechanics, p 57). The ‘jumping’
presumably refers to wavefunction reduction, a phenomenon
Schrodinger realised was unexplained within the theory, which he,
like Einstein, therefore regarded as incomplete. To illustrate the
problem in a picturesque way he invented, in 1935, the
‘Schrodinger cat’ story, which we have already discussed in §4.4.
He considered it naive to believe that the cat was in an uncertain,
dead or alive, state until observed by a conscious observer, and
therefore concluded that the quantum theory could not be a proper
description of reality.
Next we mention de Broglie, who, it will be recalled, was the first
to suggest a wave nature for electrons. He was also unhappy with
the way quantum theory developed, and took the attitude that it
was wrong to abandon the classical idea that particles followed
trajectories. He believed that the role of the wavefunction was to
act as a pilot wave to guide these trajectories, an idea which paved
the way for hidden-variable theories. Thus, of the four people (Planck, Einstein, Schrodinger, de
Broglie) who probably played the leading roles in starting quantum
theory, three became, and remained, dissatisfied with the way it
developed and with its accepted ‘orthodoxy’. This orthodoxy is
primarily due to the other three major figures in the early development
of the theory, Bohr and, to a lesser extent, Heisenberg and
Born. It has become known as the ‘Copenhagen’ interpretation.
A precise account of what the Copenhagen interpretation actually
is does not exist. Quotations from Bohr’s articles do not always
seem to be consistent (which is not surprising in view of the fact
that the ideas were being developed as the articles were being
written). Almost certainly, two present-day physicists, who both
believe that they subscribe to the orthodox (Copenhagen) interpretation,
would give different accounts of what it actually means.
Nevertheless there are several key features which, with varying
degrees of emphasis, would be likely to be present. We shall
endeavour to describe these.
(i) Bohr made much use of the notion of ‘complementarity’:
particle and wave descriptions complement each other; one is
suitable for one set of experiments, the other for different
experiments. Thus, since the two descriptions are relevant to
different experiments, it does not make sense to ask whether they
are consistent with each other. Neither should be used outside its
own domain of applicability.
(ii) The interpretation problems of quantum theory rest on
classical ways of thinking which are wrong and should be abandoned.
If we abandon them then we will have no problems. Thus
questions which can only be asked using classical concepts are not
permitted. Classical physics enters only through the so-called ‘correspondence’
principle, which says that the results of quantum
theory must agree with those of classical mechanics in the region
of the parameters where classical mechanics is expected to work.
This idea, originally used by Planck, played an important role in
the discovery of the correct form of quantum theory.
(iii) The underlying philosophy was strongly ‘anti-realist’ in tone.
To Bohr: ‘There is no quantum world. There is only an abstract
quantum physical description. It is wrong to think that the task of
physics is to find out how nature is. Physics concerns what we can say about nature.’ Thus the Copenhagen interpretation and the
prevailing fashion in philosophy, which inclined to logical
positivism, were mutually supportive. The only things that we are
allowed to discuss are the results of experiments. We are not
allowed to ask, for example, which way a particle goes in the interference
experiment of 61.4, The only way to make this a sensible
question would be to consider measuring the route taken by the
particle. This would give us a different experiment for which there
would not be any interference. Similarly, Bohr’s reply to the alleged
demonstration of the incompleteness of quantum theory, based on
the EPR experiment, was that it was meaningless to speak of the
state of the two particles prior to their being measured. (It should
be noted that Einstein himself had made remarks which were in this
spirit. Indeed Heisenberg, a convinced advocate of the Copenhagen
interpretation, was apparently helped along this line by one such
remark: ‘It is the theory which decides what we can observe.’)
(iv) All this leaves aside the question of what constitutes a
‘measurement’ or an ‘observation’. It is possible that somewhere in
the back of everyone’s mind there lurked the idea of apparatuses
that were ‘classical’, i.e. that did not obey the rules of quantum
theory. In the early days the universality of quantum theory was
not appreciated, so it was more reasonable to divide the world into,
on the one hand, observed systems which obeyed the rules of
quantum mechanics, and, on the other, measuring devices, which
were classical.
These, then, are the ingredients of the Copenhagen interpretation.
It is very vague and answers few of the questions; anybody
who thinks about the subject today would be unlikely to find it
satisfactory: yet it became the accepted orthodoxy. We have
already, in $5.2, suggested reasons why this should be so. The
theory was a glorious success, nobody had any better answers to the
questions, so all relaxed in the comfortable glow of the fact that
Bohr had either answered them or told us that they should not be
asked.
1 was a research student in Manchester in the 1950s. Rosenfeld
was the head of the department and the Copenhagen interpretation
reigned unquestioned. One particular Christmas, the department
visited the theoretical physics department in Birmingham to sing
carols (that, at least, was the excuse). Some of the carols were parodied. In particular, I remember the words we used for the carol
that normally begins ‘The boar’s head in hand bear 1’. They were:
At Bohr’s feet I lay me down,
For I have no theories of my own
His principles perplex my mind,
But he is oh so very kind.
Correspondence is my cry, I don’t know why,
I don’t know why.
But we were all afraid to ask!