x l i v I N T R O D U C T I O N T O V O L U M E 1 4 Einstein’s results in a joint paper “On the Quantum Theory of Radiation Equilibri- um,” published in Zeitschrift für Physik on 8 December 1923 (Einstein and Ehren- fest 1923 [Doc. 129]). Einstein and Ehrenfest were able to resolve the seemingly paradoxical result us- ing Einstein’s ideas developed in his 1916/17 papers. As Martin J. Klein wrote: “The key point lay in recognizing that a Compton process amounted to the disap- pearance (absorption) of a quantum of frequency ν and the appearance (emission) of a quantum of frequency ν′, both appropriately specified as to direction.”[16] This is actually how modern quantum electrodynamics deals with the Compton effect. Pauli’s paradoxical result was in fact prefigured by Einstein in his paper seven years earlier, where he showed that, in order to obtain the Planck distribution, one has to assume that the quantum emission probability has a term proportional to the final radiation density (i.e., the density after the emission). It is unclear whether Einstein was convinced of the reality of the Compton effect, in spite of ongoing attempts to confirm it in Berlin. But on 26 October 1923 he list- ed Compton, together with Otto Stern, Walther Gerlach, Arnold Sommerfeld, and others, as one of his nominees for the Nobel Prize. Compton, he wrote, deserved the prize “for the discovery of the quantum scattering with X-rays” (Doc. 132). Half a year later he wrote to Michele Besso: “Among the experimental results of the last years, the only significant ones are the experiments of Stern and Gerlach as well as Compton’s experiment (scattering of Röntgen rays with change in frequen- cy), of which the first proves the independent existence of quantum states and the second proves the reality of the momentum of light quanta” (Doc. 253). Einstein and Ehrenfest’s paper, published in early November 1923 (see also Docs. 136, 137) is therefore a natural sequel to the 1916/17 papers with the limita- tion to static atoms (i.e., emitters and absorbers) removed. However, it contains an additional generalization by allowing the absorption and emission of several differ- ent light quanta at the same time. This additional generalization is the subject of Pauli’s letter to Einstein of 10 November 1923 (Doc. 142). On 20 April 1924, Einstein published a piece on the Compton effect in the Ber- liner Tageblatt, explaining its broader significance to a wider audience (Einstein 1924e [Doc. 236]). During the few weeks he spent in Kiel in May and September 1923, Einstein en- gaged in other pursuits as well. He participated in the development of the mining compass, later called a gyro-theodolite (Doc. 110), and in May 1925, on the return voyage from South America to Germany, in the design of a new, oscillated log joint support for the compass (Doc. 490). He also proposed a reversible motor, to be used in conjunction with amplifiers, for the transfer of the motion of the internal sphere of the gyrocompass to its outer sphere (the so-called follow-up, Doc. 110).