HERR PROFESSOR EINSTEIN 189body. I did succeed in inventing something which formally corresponds to [a quan-
tum theory], but I have conclusive grounds to consider it nonsense' [E9]. To Stark,
July 1909: 'You can hardly imagine how much trouble I have taken to invent a
satisfactory mathematical treatment of the quantum theory' [E10]. To Besso,
November 1909: 'Reflected little and unsuccessfully about light-quanta' [Ell].
Again to Besso, one month later, he writes about attempts to modify Maxwell's
equations in such a way that the new equations would have light-quantum solu-
tions: 'Here perhaps lies the solution of the light-quantum problem' [E12]. To
Laub, that same day: "I have not yet found a solution of the light-quantum ques-
tion. All the same I will try to see if I cannot work out this favorite problem of
mine' [E13].* Also to Laub, March 1910: 'I have found some interesting things
about quanta, but nothing is complete yet' [El4].
In the summer of 1910 Einstein wrote to Laub about his long review article
[E8]: '[This paper] contains only a rather broad expose of the epistemological
foundations of the relativity theory' [El5]. This would have been as good an
occasion as any to reflect on the new epistemology of the equivalence principle,
but Einstein does not do so. Rather he adds, a few lines later, 'I have not come
further with the question of the constitution of light.' In November he writes again
to Laub: 'Currently I have great expectations of solving the radiation problem
...' [E16]. A week later, once more to Laub: 'Once again I am getting nowhere
with the solution of the light-quantum problem' [El7]. In December, to Laub:
'The enigma of radiation will not yield' [E18]. Finally, by May 1911 he is ready
to give up for the time being; he writes to Besso: 'I do not ask anymore whether
these quanta really exist. Nor do I attempt any longer to construct them, since I
now know that my brain is incapable of fathoming [the problem] this way' [E19].
One month later, in June 1911, he was back to gravitation theory.
It would, of course, be absurd to suppose that Einstein did not think about
gravitation at all during those three and a half years. A letter he wrote to Som-
merfeld from Bern, just before taking up his post in Ziirich, shows that he had
indeed done so:
'The treatment of the uniformly rotating rigid body seems to me to be very
important because of an extension of the relativity principle to uniformly rotat-
ing systems by trains of thought which I attempted to pursue for uniformly
accelerated translation in the last section of ... my paper [of 1907]. [E20]**That isolated remark, important as it is, does not change my opinion that Einstein
was concentrating in other directions during this period. In later years, Einstein
himself tended to be uncommunicative about his thoughts on gravitation during
*Ich will sehen ob ich dieses Lieblingsei doch nicht ausbriiten kann.
**See also [S5], I shall return in the next chapter to the influence of the problem of rotating bodies
on Einstein's thinking.