4 1 6 D O C . 4 2 5 R E M A R K O N J O R D A N S P A P E R
425. “Remark on P. Jordan’s Paper ‘On the Theory of
Quantum Radiation’”[1]
[Einstein 1925o]
Received 22 January 1925
Published February-April 1925
In: Zeitschrift für Physik 31 (1925): 784–785.
It is shown that the hypotheses on which the author bases his statistical theory of elementary pro-
cesses of radiation are irreconcilable with the existence of an absorption coefficient.
In the cited incisive investigation, P. Jordan has attempted to refute the thesis
postulated by me, according to which it should be theoretically necessary that the
transfer of an impulse in the amount of occur in every elementary process of
emission and absorption onto the emitting, or the absorbing, molecule. The logical
correctness of P. Jordan’s arguments certainly does seem to be evident. How is it
possible for the author to arrive at results that, according to my earlier consider-
ations, are excluded? The attempt made by the author in §6 of his paper to indicate
the reason for this incongruence does not seem to me to be entirely successful.[2]
That is why I want to try to show below what the grounds are for the author to arrive
at Planck’s formula without the hypothesis of “needle radiation,” i.e., the men-
tioned transfer of momentum onto the molecule in each elementary process.
I contend that the author arrives at this result on the basis of a hypothesis about
the elementary process that I did not consider at all because it is contradictory to
the experiments on light absorption. This hypothesis is expressed in §5 in equations
(12), (13), (18), and (18′).[3]
In order to be able to present clearly what matters to me, I shall confine myself
to the special case of an immobile molecule. The elementary process consists in the
absorption, or, as the case may be, emission of the energy hν. It does not, however,
come from or go into a definite direction, but rather is distributed over the direc-
tions satisfying a definite angular function σ that is independent of the radiation’s
directed distribution. This hypothesis can safely be postulated for emission without
[p. 784]

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