x x x v i i i I N T R O D U C T I O N T O V O L U M E 1 3 best, but he nevertheless believed “absolutely nothing,” explaining again the wave groups’ behavior. He ended his letter by stressing: “Einstein, listen to your young brother, for your own well-being!!!” (Docs. 31 and 39). That same day, Einstein fi- nally admitted that Ehrenfest was “completely right.” He had found where he had been mistaken (Doc. 37), and was quick in telling Sommerfeld about it as well, concluding: “Again, [I’m] somewhat cleverer, but poorer in one’s hopes!” (Doc. 41). The manuscript on the results of the experiment presented to the Academy on 19 January was withdrawn. In its place, Einstein submitted to the Academy’s session of 2 February the paper “On the Theory of Light Propagation in Dispersive Media” (Einstein 1922f, Doc. 43). In it, he carefully laid out a first principle calculation about the propagation of light in dispersive media, explaining why his proposed ex- periment had, after all, been ill-conceived. As he had come to realize, it did, in fact, not predict a decisive, observable choice between the two theoretical alternatives. The final notes of this failed enterprise rang in February, when Ehrenfest thanked Einstein for giving in (Doc. 45), with Einstein sending Ehrenfest the cor- rections to his paper and admitting to the simplicity of its result (Doc. 47). On the same day, Einstein wrote to his sons of the failure: “The experiment on which I had placed so much importance proves nothing for and nothing against the undulatory theory, so all the labor of love [Liebesmüh] was actually in vain” (Doc. 48). By May, in a letter to Born, Einstein would call this whole affair “a monumental blun- der,” but “only death can help against such blunders” (Doc. 190). Despite these setbacks, Einstein kept thinking about ways of finding empirical evidence that could guide theoretical work on quantum theory. A lively description of Einstein’s tenacious activity in this field is found in a letter from Ehrenfest to Bohr of 8 May 1922 (Doc. 184): “He keeps coming up with new experiments to check as carefully as possible whether the wave field of a moving emitting atom would behave classically.” A number of those ideas proved ephemeral and left little documentary traces. For example, on 4 May, Einstein wrote to his wife, Elsa, from Leyden: “Zeeman will do an experiment that I came up with I am very glad about that.” Alas, nothing else is known about this proposed experiment. From a letter to Ehrenfest of 23 March 1922, we learn of one idea that Einstein had along these lines. Through a thought experiment, using a reflecting mirror, he tried to prove that light emitted by an accelerating particle cannot be monochromat- ic, an outcome that would pose a problem for the particle picture. Here Einstein mentioned the idea of a Führungsfeld (“leading field”), without, however, seeing a solution to his questions. He concludes his letter, ironically: “I am now completely ripe for the insane asylum” (Doc. 107).