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420 THE QUANTUM THEORY

equations do hold in the average and the experiment on AX refers only to the
average change of the wavelength. In fact, at the time of the BKS proposal, there
did not exist any direct experimental proof of energy-momentum conservation or
of causality in any individual process. This is one of the reasons why the objections
to BKS (held by many, 'perhaps the majority' of physicists [PI]* were initially
expressed in a somewhat muted fashion. Thus, Pauli wrote to Bohr that he did
not believe in the latter's theory but that 'one cannot prove anything logically and
also the available data are not sufficient to decide for or against your view' [PI].
All this was to change soon.
There was a second reason, I believe, for the subdued character of comments
by others. The physics community was witness to a rare occurrence. Einstein, of
course, did not care at all for BKS. Earlier he had given thought to energy non-
conservation and rejected it. To give up strict causality went deeply against his
grain. Thus Einstein and Bohr, the two leading authorities of the day, were locked
in conflict (the word conflict was used by Einstein himself**). To take sides meant
choosing between the two most revered physicists. Ideally, personal considerations
of this kind ought to play no role in matters scientific, but this ideal is not always
fully realized. Pauli reflected on this in a letter concerning the BKS issue: 'Even
if it were psychologically possible for me to form a scientific opinion on the
grounds of some sort of belief in authority (which is not the case, however, as you
know), this would be logically impossible (at least in this case) since here the
opinions of two authorities are so very contradictory' [PI].
Even the interaction between the two protagonists was circumspect during that
period. They did not correspond on the BKS issue [El2]. Nor (as best I know)
were there personal meetings between them in those days, even though Bohr had
told Pauli repeatedly how much he would like to know Einstein's opinion [PI].
Heisenberg wrote to Pauli that he had met Einstein in Goettingen and that the
latter had 'a hundred objections' [H2]. Sometime later, Pauli also met Einstein,
whereupon he sent Bohr a detailed list of Einstein's criticisms [PI].
Einstein had given a colloquium on this paper, at which he had raised objec-
tions. The idea (he wrote Ehrenfest) 'is an old acquaintance of mine, which I do
not hold to be an honest fellow, however' ( ... den ich aber fur keinen reellen
Kerl halte) [E13]. At about that time, he drew up a list of nine objections, which
I shall not reproduce here in detail. Samples: 'What should condition the virtual
field which corresponds to the return of a previously free electron to a Bohr orbit?
(very questionable). ... Abandonment of causality as a matter of principle should
be permitted only in the most extreme emergency' [El4]. The causality issue


*Born, Schroedinger, and R. Ladenburg were among the physicists who initially believed that BKS
might be a step in the right direction.
**On October 25, 1924, the Danish newspaper Politiken carried an item on the Bohr-Einstein
controversy. This led the editor of a German newspaper to send a query to Einstein [Jl]. Einstein
sent a brief reply [E12], acknowledging that a conflict existed and adding that no written exchanges
between himself and Bohr had resulted.

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