THE PHOTON 411Besso about the needle rays, he wrote, 'Damit sind die Lichtquanten so gut wie
gesichert' [E13].* And, in a phrase contained in another letter about two years
later, 'I do not doubt anymore the reality of radiation quanta, although I still
stand quite alone in this conviction,' he underlined the word 'Realitat' [E14].
On the other hand, at about the same time that Einstein lost any remaining
doubts about the existence of light-quanta, we also encounter the first expressions
of his Unbehagen, his discomfort with the theoretical implications of the new
quantum concepts in regard to 'Zufall,' chance. This earliest unease stemmed
from the conclusion concerning spontaneous emission that Einstein had been
forced to draw from his consistency condition (Eq. 21.16): the needle ray picture
applies not only to induced processes (as was mentioned above) but also to spon-
taneous emission. That is, in a spontaneous radiative transition, the molecule suf-
fers a recoil hv/c. However, the recoil direction cannot be predicted! He stressed
(quite correctly, of course) that it is 'a weakness of the theory ... that it leaves
time and direction of elementary processes to chance' [Ell]. What decides when
the photon is spontaneously emitted? What decides in which direction it will go?
These questions were not new. They also apply to another class of emission
processes, the spontaneity of which had puzzled physicists since the turn of the
century: radioactive transformations. A spontaneous emission coefficient was in
fact first introduced by Rutherford in 1900 when he derived** the equation dN
= —\Ndt for the decrease of the number N of radioactive thorium emanation
atoms in the time interval dt [R2]. Einstein himself drew attention to this simi-
larity: 'It speaks in favor of the theory that the statistical law assumed for [spon-
taneous] emission is nothing but the Rutherford law of radioactive decay' [E9]. I
have written elsewhere about the ways physicists responded to this baffling life-
time problem [ P2]. I should now add that Einstein was the first to realize that the
probability for spontaneous emission is a nonclassical quantity. No one before
Einstein in 1917 saw as clearly the depth of the conceptual crisis generated by the
occurrence of spontaneous processes with a well-defined lifetime. He expressed
this in prophetic terms:
The properties of elementary processes required by [Eq. 21.16] make it seem
almost inevitable to formulate a truly quantized theory of radiation. [Ell]
Immediately following his comment on chance, Einstein continued, 'Neverthe-
less, I have full confidence in the route which has been taken' [Ell]. If he was
confident at that time about the route, he also felt strongly that it would be a long
one. The chance character of spontaneous processes meant that something was
amiss with classical causality. That would forever deeply trouble him. As early as
March 1917, he had written on this subject to Besso, 'I feel that the real joke that
the eternal inventor of enigmas has presented us with has absolutely not been
* With that, [the existence of] light-quanta is practically certain.
**Here a development began which, two years later, culminated in the transformation theory for
radioactive substances [Rl].