Zurich, 17 November 1909

The photochemical process, and the hypothesis of quanta.

Einstein discusses an article on the scientific basis of photography: 'It seems that the inverse photochemical process happens on its own, especially in the reactions which correspond to a weak exposure to light'. However, he excludes any application of this to the examination of the hypothesis of quanta. Einstein declares himself very busy with his lectures, 'so that my real free time is less than in Bern. But you learn a lot by it. I have pondered very little on light quanta, and without success'. Einstein also mentions two of his close friends in his Zurich years, the Slovakian engineer Aurel Stodola ('a wonderful man') and Frank Paul Habicht, who has constructed an electrostatic machine for him, which needs improvement.

Einstein's hypothesis of the photoelectric effect in 1905 was a key step in the development of quantum mechanics (and formed the basis of his award of the Nobel Prize for 1921): here he is apparently rejecting a suggestion by his friend Besso that photochemical processes may also provide insights into the problem of quanta. Einstein had been appointed associate professor in theoretical physics at the University of Zurich on 7 May: he gave his first lecture on 15 October, and in the course of the winter term taught courses on mathematics, thermodynamics and physics.