418 THE QUANTUM THEORYof the properties of interaction between radiation and matter. The BKS proposal
represents the extreme example of this position. Its authors suggested that radia-
tive processes have highly unconventional properties 'the cause of [which] we shall
not seek in any departure from the electrodynamic theory of light as regards the
laws of propagation in free space, but in the peculiarities of the interaction
between the virtual field of radiation and the illuminated atoms' [Bl]. Before
describing these properties, I should point out that the BKS paper represents a
program rather than a detailed research report. It contains no formalism what-
soever.* This program was not to be the right way out of the difficulties of the old
quantum theory, yet the paper had a lasting impact in that (as we shall see) it
stimulated important experimental developments. Let us discuss next the two
main paradoxes addressed in BKS.
The first paradox. Consider an atom that emits radiation in a transition from
a higher to a lower state. BKS assume that in this process 'energy [is] of two kinds,
the continuously changing energy of the field and the discontinuously changing
atomic energy' [S2]. But how can there be conservation of an energy that consists
of two parts, one changing discontinuously, the other continuously? The BKS
answer [Bl]: 'As regards the occurrence of transitions, which is the essential fea-
ture of the quantum theory, we abandon ... a direct application of the principles
of conservation of energy and momentum.' Energy and momentum conservation,
they suggested, does not hold true for individual elementary processes but should
hold only statistically, as an average over many such processes.
The idea of energy nonconservation had already been on Bohr's mind a few
years prior to the time of the BKS proposal [B5].** However, it was not Bohr
but Einstein who had first raised—and rejected—this possibility. In 1910 Einstein
wrote to a friend, 'At present, I have high hopes for solving the radiation problem,
and that without light quanta. I am enormously curious as to how it will work
out. One must renounce the energy principle in its present form' [E9]. A few days
later he was disenchanted. 'Once again the solution of the radiation problem is
getting nowhere. The devil has played a rotten trick on me' [E10]. He raised the
issue one more time at the 1911 Solvay meeting, noting that his formula for the
energy fluctuations of blackbody radiation could be interpreted in two ways. 'One
can choose between the [quantum] structure of radiation and the negation of an
absolute validity of the energy conservation law.' He rejected the second alterna-
tive. 'Who would have the courage to make a decision of this kind?. .. We shall
agree that the energy principle should be retained' [Ell]. Others, however, were
apparently not as convinced. In 1916 the suggestion of statistical energy conser-
*The same is true for a sequel to this paper that Bohr wrote in 1925 [B4]. Schroedinger [SI] and
especially Slater [S2] did make attempts to put the BKS ideas on a more formal footing. See also
Slater's own recollections of that period [S3].
**A letter from Ehrenfest to Einstein shows that Bohr's thoughts had gone in that direction at least
as early as 1922 [E8[.