I now continue the tale of the light-quantum, a subject on which Einstein published first in 1905, then again in 1906. Not long thereafter, there began the period I earlier called 'three and a half years of silence,' during which he was again intensely preoccupied with radiation and during which he wrote to Laub, 'I am incessantly busy with the question of radiation. . . . This quantum question is so uncommonly important and difficult that it should concern everyone' [E1]. Our next subject will be two profound papers on radiation published in 1909. The first one [E2] was completed while Einstein was still a technical expert second class at the patent office. The second one [E3] was presented to a conference at Salzburg in September, shortly after he had been appointed associate professor in Zürich. These papers are not as widely known as they should be because they address questions of principle without offering any new experimental conclusion or prediction, as had been the case for the first light-quantum paper (photoeffect) and the paper on specific heats.
In 1909 Kirchhoff's theorem was half a century old. The blackbody radiation law had meanwhile been found by Planck. A small number of physicists realized that its implications were momentous. A proof of the law did not yet exist. Nevertheless, 'one cannot think of refusing [to accept] Planck's theory,' Einstein said in his talk at Salzburg. That was his firmest declaration of faith up to that date. In the next sentence, he gave the new reason for his conviction: Geiger and Rutherford's value for the electric charge had meanwhile been published and Planck's value for e had been 'brilliantly confirmed' (Section 19a).
In Section 4c, I explained Einstein's way of deriving the energy fluctuation formula(21.1)
where 〈ε2〉 is the mean square energy fluctuation and (E) the average energy for a system in contact with a thermal bath at temperature T. As is so typical for Einstein, he derived this statistical physics equation in a paper devoted to the quantum theory, the January 1909 paper. His purpose for doing so was to apply