THE LIGHT-QUANTUM 383he had used this law as an experimental fact without explaining it. Above all, it
was obvious to him from the start that grave tensions existed between his principle
and the wave picture of electromagnetic radiation—tensions which, in his own
mind, were resolved neither then nor later. A man as perfectly honest as Einstein
had no choice but to emphasize the provisional nature of his hypothesis. He did
this very clearly in 1911, at the first Solvay congress, where he said, 'I insist on
the provisional character of this concept [light-quanta] which does not seem recon-
cilable with the experimentally verified consequences of the wave theory' [El2].
It is curious how often physicists believed that Einstein was ready to retract.
The first of these was his admirer von Laue, who wrote Einstein in 1906, 'To me
at least, any paper in which probability considerations are applied to the vacuum
seems very dubious'[L7], and who wrote him again at the end of 1907, 'I would
like to tell you how pleased I am that you have given up your light-quantum
theory' [L8]. In 1912 Sommerfeld wrote, 'Einstein drew the most far-reaching
consequences from Planck's discovery [of the quantum of action] and transferred
the quantum properties of emission and absorption phenomena to the structure of
light energy in space without, as I believe, maintaining today his original point of
view [of 1905] in all its audacity' [S3]. Referring to the light-quanta, Millikan
stated in 1913 that Einstein 'gave... up, I believe, some two years ago' [M3],
and in 1916 he wrote, 'Despite... the apparently complete success of the Einstein
equation [for the photoeffect], the physical theory of which it was designed to be
the symbolic expression is found so untenable that Einstein himself, I believe, no
longer holds to it' [M4].
It is my impression that the resistance to the light-quantum idea was so strong
that Einstein's caution was almost hopefully mistaken for vacillation. However,
judging from his papers and letters, I find no evidence that he at any time with-
drew any of his statements made in 1905.
- Electromagnetism: Free Fields and Interactions. Einstein's March paper
is the second of the revolutionary papers on the old quantum theory. The first one
was, of course, Planck's of December 1900 [P4]. Both papers contained proposals
that flouted classical concepts. Yet the resistance to Planck's ideas—while cer-
tainly not absent—was much less pronounced and vehement than in the case of
Einstein. Why?
First, a general remark on the old quantum theory. Its main discoveries con-
cerned quantum rules for stationary states of matter and of pure radiation. By and
large, no comparable breakthroughs occurred in regard to the most difficult of all
questions concerning electromagnetic phenomena: the interaction between matter
and radiation. There, advances became possible only after the advent of quantum
field theory, when the concepts of particle creation and annihilation were formu-
lated. Since then, progress on the interaction problems has been enormous. Yet
even today this is not by any means a problem area on which the books are closed.
As we saw in Section 19a, when Planck introduced the quantum in order to
describe the spectral properties of pure radiation he did so by a procedure of quan-