Why were these elementary equations not published five or even ten years earlier,
as well they could have been? Even those opposed to quantized radiation might
have found these relations to their liking since (independent of any quantum
dynamics) they yield at once significant differences from the classical theories of
the scattering of light by matter** and therefore provide simple tests of the photon
idea.
I have no entirely satisfactory answer to this question. In particular, it is not
clear to me why Einstein himself did not consider these relations. However, there
are two obvious contributing factors. First, because photons were rejected out of
hand by the vast majority of physicists, few may have felt compelled to ask for
tests of an idea they did not believe to begin with. Second, it was only in about
1922 that strong evidence became available for deviations from the classical pic-
ture. This last circumstance impelled both Compton and Debye to pursue the
quantum alternative.f Debye, incidentally, mentioned his indebtedness to Ein-
stein's work on needle radiation [D2]. Compton in his paper does not mention
Einstein at all.
The same paper in which Compton discussed Eqs. 21.19 and 21.20 also con-
tains the result of a crucial experiment. These equations imply that the wave-
length difference AX between the final and the initial photon is given by
where 6 is the photon scattering angle. Compton found this relation to be satisfied
* Einstein attached great importance to an advance in another direction that took place in the inter-
vening years: the effect discovered by Otto Stern and Walther Gerlach [E21]. Together with Ehren-
fest, he made a premature attempt at its interpretation [E22].
**For details on these classical theories, see Stuewer's fine monograph on the Compton effect [S2].| Nor is it an accident that these two men came forth with the photon kinematics at about the same
time. In his paper, Debye acknowledges a 1922 report by Compton in which the evidence against
the classical theory was reviewed. A complete chronology of these developments in 1922 and 1923 is
found in [S2], p. 235. For a detailed account of the evolution of Compton's thinking, see [S2J, Chap-
ter 6.
THE PHOTON 413
it is not surprising that he would look for new ways in which the existence of
photons might lead to observable deviations from the classical picture. In this he
did not succeed. At one point, in 1921, he thought he had found a new quantum
criterion [E19], but it soon turned out to be a false lead [E20, Kl]. In fact, after
1917 nothing particularly memorable happened in regard to light-quanta until
capital progress was achieved* when Arthur Compton [Cl] and Debye [D2]
independently derived the relativistic kinematics for the scattering of a photon off
an electron at rest:
(21.19)
(21.20)