I N T R O D U C T I O N T O V O L U M E 1 4 x l i i i These experimental attempts also triggered theoretical investigations. The data on “secondary radiations produced by X-rays,” which Compton published in Octo- ber 1922 without as yet providing an interpretation in terms of quantum scattering,[12] caught the attention of Peter Debye in Zurich. Independently of Compton, Debye completed a note in mid-March that was published in the Physi- kalische Zeitschrift in its issue of 15 April 1923, that is, after Compton had submit- ted his interpretation, but before it had appeared in print.[13] Debye, too, interpreted Compton’s data as a scattering of a light quantum with an electron and provided the relevant formulas for computing the change of wavelength resulting from the mo- mentum transfer from light quantum to recoil electron. Wolfgang Pauli, who spent the academic year 1922–1923 at Bohr’s institute in Copenhagen, also took up Compton’s work and Debye’s interpretation. He submit- ted a paper in August 1923, when Einstein was in Lautrach, on the problem of ther- mal equilibrium between free electrons and a radiation field,[14] in which he solved a problem posed twelve years earlier by Hendrik A. Lorentz at the first Solvay conference.[15] At issue was finding a satisfactory theory for the thermal equilibri- um between radiation and free electrons that would lead to the Maxwell-Boltz- mann energy distribution for the electrons and to the Planck density distribution for the radiation. Earlier attempts by Lorentz and by Adriaan Fokker at solving the problem had failed. Pauli attacked the issue with new ammunition. First, he had Einstein’s 1916/17 papers on thermal equilibrium between atoms and radiation (Einstein 1916n [Vol. 6, Doc. 38], reprinted as Einstein 1917c) that, for the first time, treat the radiation field as an ensemble of light quanta. The sole interaction between the atoms and the radiation is, therefore, the absorption and emission of light quanta by the atoms. Second, Pauli now had Compton’s new discovery of X- ray scattering on electrons, interpreted as elementary collisions between X-ray quanta and free electrons. By applying the Compton-Debye scattering formulas, Pauli’s major result was in finding the expression for the transition probability between the initial and final state of a quantum, which leads to the Maxwell-Boltzmann distribution for the electrons, and to the Planck distribution for the radiation. Pauli’s expression seemed paradoxical, since it includes the radiation density distribution after scattering. Thus, the scattering is enhanced both by the initial radiation density and the final density. But Pauli showed that this second term was necessary for obtain- ing the Planck distribution rather than the Wien distribution. Paul Ehrenfest was interested in the topic already in early April 1923, as indicat- ed in his diaries. He discussed Pauli’s paper with Einstein in Leyden in September and October, and we find in Ehrenfest’s diary entry for 8 October 1923 the details of the calculation contained in the third paragraph of the eventual paper, carried out “together with Einstein,” as he notes. They submitted their synthesis of Pauli’s and