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INTERLUDE: THE BKS PROPOSAL 419

vation was taken up by Nernst [Nl].* Not later than January 1922, Sommerfeld
remarked that the 'mildest cure' for reconciling the wave theory of light with
quantum phenomena would be to relinquish energy conservation [S4]. Similar
speculations were made by other physicists as well [Kl]. Thus the BKS proposal
must be regarded as an attempt to face the consequences of an idea that had been
debated for quite some time.
In order to understand Bohr's position in 1924, it is above all important to
realize that to him the correspondence principle was the principal reliable bridge
between classical and quantum physics. However, the correspondence principle
is, of course, no help in understanding light-quanta: the issue of photons versus
waves lies beyond that principle. The photon-wave duality was the earliest
known instance of what was later to be called a complementary situation. The
BKS theory, with its rejection of photons and its insistence on the continuous pic-
ture of light at the price of nonconservation, historically represents the last stand
of the old quantum theory. For very good reasons, this proposal was characterized
some years later by one of the principal architects of quantum mechanics as rep-
resenting the height of the crisis in the old quantum theory [HI]. Nor was non-
conservation of energy and momentum in individual processes the only radical
proposal made by BKS.


The Second Paradox. Another question that had troubled Einstein since 1917
(as we have seen) was, How does an electron know when to emit radiation in
making a spontaneous transition?
In its general form, the BKS answer to this question was that there is no truly
spontaneous emission. They associated with an atom in a given state a 'virtual
radiation field' that contains all the possible transition frequencies to other sta-
tionary states and assumed that 'the transitions which in [the Einstein theory of
1917] are designated as spontaneous are, in our view, induced [my italics] by the
virtual field.' According to BKS, the spontaneous transition to a specific final state
is connected with the virtual field mechanism 'by probability laws analogous to
those which in Einstein's theory hold for induced transitions.' In this way, 'the
atom is under no necessity of knowing what transitions it is going to make ahead
of time' [S2]. Thus, spontaneous emission is ascribed to the action of the virtual
field, but this action is noncausal. I shall not discuss details of the BKS picture of
induced emission and absorption and other radiative processes. Suffice it to say
that all of these are supposed to be due to virtual fields and that in all of these
causality is abandoned. In a paper completed later in 1924, Slater [S2] noted that
the theory 'has unattractive features. .. [but] it is difficult at the present stage to
see how [these are] to be avoided.'
But what about the Compton effect? The successfully verified Eq. 21.21 rests
on the conservation laws Eqs. 21.19 and 21.20. However (BKS argued), these


"The title of Nernst's paper is (in translation) 'On an attempt to revert from quantum-mechanical
considerations to the assumption of continuous energy changes.'
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