THE PHOTON 409The parallels between Eqs. 21.5 and 21.17 are striking. The respective first terms
dominate if hv/kT S> 1, the regime in which p is approximated by Wien's expo-
nential law. Recall that Einstein had said of the first term in Eq. 21.5 that it
corresponds to 'independently moving pointlike quanta with energy hv.' One
might therefore expect that the first term in Eq. 21.17 would lead Einstein to
state, in 1909, the 'momentum quantum postulate': monochromatic radiation of
low density behaves in regard to pressure fluctuations as if it consists of mutually
independent momentum quanta of magnitude hv/' c. It is unthinkable to me that
Einstein did not think so. But he did not quite say so.
What he did say was, 'If the radiation were to consist of very few extended
complexes with energy hv which move independently through space and which
are independently reflected—a picture that represents the roughest visualization
of the light-quantum hypothesis—then as a consequence of fluctuations in the
radiation pressure there would act on our plate only such momenta as are repre-
sented by the first term of our formula [Eq. 21.17].' He did not refer explicitly to
momentum quanta or to the relativistic connection between E = hv and p =
hv/c. Yet a particle concept (the photon) was clearly on his mind, since he went
on to conjecture that 'the electromagnetic fields of light are linked to singular
points similar to the occurrence of electrostatic fields in the theory of electrons'
[E3]. It seems fair to paraphrase this statement as follows: light-quanta may well
be particles in the same sense that electrons are particles. The association between
the particle concept and a high degree of spatial localization is typical for that
period. It is of course not correct in general.
The photon momentum made its explicit appearance in that same year, 1909.
Johannes Stark had attended the Salzburg meeting at which Einstein discussed
the radiative fluctuations. A few months later, Stark stated that according to the
light-quantum hypothesis, 'the total electromagnetic momentum emitted by an
accelerated electron is different from zero and ... in absolute magnitude is given
by hv/c [SI]. As an example, he mentioned Bremsstrahlung, for which he wrote
down the equation
the first occasion on record in which the photon enters explicitly into the law of
momentum conservation for an elementary process.
- Momentum Fluctuations: 1916. Einstein himself did not explicitly intro-
duce photon momentum until 1916, in the course of his studies on thermal equi-
librium between electromagnetic radiation and a molecular gas [E10, Ell]. In
addition to his new discussion of Planck's law, Einstein raised the following prob-
lem. In equilibrium, the molecules have a Maxwell distribution for the transla-
tional velocities. How is this distribution maintained in time considering the fact
that the molecules are subject to the influence of radiation pressure? In other
words, what is the Brownian motion of molecules in the presence of radiation?