404 THE QUANTUM THEORYEquation 21.5 suggests (loosely speaking) that the particle and wave aspects of
radiation occur side by side. This is one of the arguments which led Einstein in
1909 to summarize his view on the status of the radiation theory in the following
way!*I already attempted earlier to show that our current foundations of the radiation
theory have to be abandoned. ... It is my opinion that the next phase in the
development of theoretical physics will bring us a theory of light that can be
interpreted as a kind of fusion of the wave and the emission theory.... [The]
wave structure and [the] quantum structure ... are not to be considered as
mutually incompatible. ... It seems to follow from the Jeans law [Eq. 19.17]
that we will have to modify our current theories, not to abandon them
completely.This fusion now goes by the name of complementarity. The reference to the
Jeans law we would now call an application of the correspondence principle.
The extraordinary significance for twentieth century physics of Einstein's sum-
ming up hardly needs to be stressed. I also see it as highly meaningful in relation
to the destiny of Einstein the scientist if not of Einstein the man. In 1909, at age
thirty, he was prepared for a fusion theory. He was alone in this. Planck certainly
did not support this vision. Bohr had yet to arrive on the scene. Yet when the
fusion theory arrived in 1925, in the form of quantum mechanics, Einstein could
not accept the duality of particles and waves inherent in that theory as being fun-
damental and irrevocable. It may have distressed him that one statement he made
in 1909 needed revision: moving light-quanta with energy hv are not pointlike.
Later on, I shall have to make a number of comments on the scientific reasons
that changed Einstein's apartness from that of a figure far ahead of his time to
that of a figure on the sidelines. As I already indicated earlier, I doubt whether
this change can be fully explained on the grounds of his scientific philosophy
alone.
(As a postscript to the present section, I add a brief remark on Einstein's energy
fluctuation formula. Equations 21.3-21.5 were obtained by a statistical reasoning.
One should also be able to derive them in a directly dynamic way. Einstein himself
had given qualitative arguments for the case of Eq. 21.3. He noted that the fluc-
tuations come about by interference between waves with frequencies within and
without the dv interval. A few years later, Lorentz gave the detailed calculation,
obtaining Eq. 21.3 from classical electromagnetic theory [LI]. However, difficul-
ties arose with attempts to derive the Planck case (Eq. 21.5) dynamically. These
were noted in 1919 by Leonard Ornstein and Frits Zernike, two Dutch experts
on statistical physics [Ol]. The problem was further elaborated by Ehrenfest [E5].
*In the following quotation, I combine statements made in the January and in the October paper.