414 THE QUANTUM THEORYwithin the error.* The quality of the experiment is well demonstrated by the value
he obtained for the Compton wavelength: h/mc « 0.024-2 A, which is within less
than one per cent of the modern value (for the current state of the subject, see
[W2]). Compton concluded, 'The experimental support of the theory indicates
very convincingly that a radiation quantum carries with it directed momentum as
well as energy.'**
This discovery 'created a sensation among the physicists of that time' [Al].
There were the inevitable controversies surrounding a discovery of such major
proportions. Nevertheless, the photon idea was rapidly accepted. Sommerfeld
incorporated the Compton effect in his new edition of Atombau und Spektrallinien
with the comment, 'It is probably the most important discovery which could have
been made in the current state of physics' [S3].
What about Einstein's response? A year after Compton's experiments, Einstein
wrote a popular article for Berliner Tageblatt, which ends as follows: 'The posi-
tive result of the Compton experiment proves that radiation behaves as if it con-
sisted of discrete energy projectiles, not only in regard to energy transfer but also
in regard to Stosswirkung (momentum transfer)' [E24]. Here then, in projectile
(that is, particle) language, is the 'momentum postulate,' phrased in close analogy
to the energy quantum postulate in 1905. In both cases, we encounter the phra-
seology, 'Radiation... behaves ... as if it consists of... .'
Still, Einstein was not (and would never be) satisfied. There was as yet no real
theory. In the same article he also wrote, 'There are therefore now two theories
of light, both indispensable, and—as one must admit today despite twenty years
of tremendous effort on the part of theoretical physicists—without any logical
connection.'
The years 1923-24 mark the end of the first phase of Einstein's apartness in
relation to the quantum theory. Yet there remained one important bastion of
resistance to the photon, centering around Niels Bohr.
References
Al. S. K. Allison, Biogr. Mem. Nat. Acad. Sci. 38, 81 (1965).
Bl. M. Born, W. Heisenberg, and P. Jordan, Z. Phys. 35, 557 (1925).
B2. M. V. Berry, Ann. N.Y. Ac. Set. 357, 183 (1980).
Cl. A. H. Compton, Phys. Rev. 21, 483 (1923).
Dl. L. Davis, A. S. Goldhaber, and M. M. Nieto, Phys. Rev. Lett. 35, 1402 (1975).
D2. P. Debye, Phys. Zeitschr. 24, 161 (1923).*K-line X-rays from a molybdenum anticathode were scattered off graphite. Compton stressed that
one should use only light elements as scatterers so that the electrons will indeed be quasi-free. Scat-
tered X-rays at 45°, 90°, and 135° were analyzed.
**The work of Compton and Debye led Pauli to extend Einstein's work of 1917 to the case of
radiation in equilibrium with free electrons [P3]. Einstein and Ehrenfest subsequently discussed the
connection between Pauli's and Einstein's Stosszahlansatz [E23].