l x I N T R O D U C T I O N T O V O L U M E 1 4 strongly that the concept of a rigid body “is only approximately realized, and not even to an arbitrary degree of approximation.” In 1924, in his Lucerne lecture on the ether (Einstein 1924p [Doc. 332]), he added that the status attributed to rigid bodies also distinguishes Euclidean and non-Euclidean geometries (both of which are static) from Riemannian geometries (which are dynamic) because the rigid body is “sufficient” for static geometries but becomes more problematic in dynamic geometries (like those used in general relativity). The documents in this volume show us Einstein’s ambivalence regarding the concept of a rigid body, which is indispensable for the current state of knowledge (i.e., general relativity), but problematic as something whose existence has to be assumed rather than derived as holding approximately. VIII In an effort to counter the light quantum interpretation of the Compton effect, Niels Bohr, together with Hendrik A. Kramers and John Slater, published a paper that attempted to apply a classical wave theory by employing the models of “virtual oscillators” that absorb and emit the energy of incident radiation. The most contro- versial aspect of their theory was the abandonment of conservation of energy and momentum for individual interactions. The Bohr-Kramers-Slater (BKS) paper was submitted in January 1924 to Philosophical Magazine, and a German language version was received by Zeitschrift für Physik on 22 February 1924. Both were published in May or June.[30] While the papers were in press, rumors about Bohr’s new proposal spread. Thus, in mid-March, Fritz Haber reported to Einstein on a visit to Bohr, who had entertained him for one and a half hours with a “mixture of admiration and disap- proval of your light quantum theory […]. He strives with all his being back to the classical world” (Doc. 225). With Einstein’s light quantum hypothesis apparently vindicated by Compton’s experiment, and with the proposal of an alternative theory of quantum radiation, Bohr and Einstein became the protagonists of two mutually exclusive viewpoints on the quantum problem. Two months later, in a letter to Max and Hedwig Born, Einstein wrote: “Bohr’s opinion on radiation interests me very much. But I would not like to be driven to abandon strong causality before very different measures have been taken to protect it. I cannot bear the idea that an electron hit by radiation will, of its own accord, choose the moment and the direction at which it will jump away. If so, I would rath- er be a cobbler or an employee of a casino than a physicist” (Doc. 240). Einstein was in Kiel when the BKS paper finally appeared. He was living in a “bachelor’s apartment” that Anschütz-Kaempfe had refurbished for him, of which