I N T R O D U C T I O N T O V O L U M E 1 3 x x x v i i grips with what he welcomed as a decisive outcome. The immediate reactions by his friends and colleagues were varied and hesitant. Max Born and James Franck were “extremely shaken” but admitted they were “too dumb” to reconstruct the ex- perimental setup (Doc. 4). Hermann Weyl admitted to not understanding the exper- iment, but believed that Einstein had made a discovery: “I feel sorry for the poor ‘field,’” he proclaimed (Doc. 5). In a postcard, Einstein tried to explain the experi- ment to Born (Doc. 6), and a few days later, writing to Ehrenfest (Doc. 13), he came up with the notion of a “ghost field” (“Gespensterfeld”) that Lorentz had alluded to in the letter mentioned above. Arnold Sommerfeld reacted as well: “So, you have made a great discovery again, burying the wave theory. […] If you say that your experiment is decisive, I am willing to believe it, although I still do not understand it, despite Geiger’s ex- planations.” He ends his letter with a more general agreement: “All is working well [speaking about his spectroscopy calculations], but remains, deep down, unclear. I can only supply the techniques of the quanta. It is for you to do the philosophy. I, myself, don’t believe any longer in the spherical wave” (Doc. 14). But more substantial objections came from Ehrenfest and Max von Laue. Ehren- fest’s first reaction was: “Damn! If your light experiment, withstanding all criti- cism, has an anticlassic result, then - - - you know what, then you become scary to me.” He ends his reaction by stating: “If your result is genuine, then—it seems to me—you have found something really colossal” (Doc. 24). Ehrenfest intended to consult Niels Bohr. In letters to Born, Franck, and Sommerfeld, Einstein quoted the criticism of Laue, who believed the experimental result but who insisted that Einstein’s inter- pretation was incorrect. Laue had said that classical wave theory did not predict bending of the light ray any more than quantum theory did (Docs. 25 and 27). A day later, on 19 January, Einstein discussed the theoretical aspects of the ex- periment and its result at the plenary session of the Prussian Academy. The subject of his talk still conveys his belief in the decisive importance of the negative result: “On an Optical Experiment, the Result of Which Is Incompatible with the Undula- tory Theory” (Doc. 29). On the same day Ehrenfest, by now much more certain in his opposition to Einstein’s interpretation, drew upon an old paper by Josiah W. Gibbs for help. Ehrenfest showed in some detail that wave theory should give the same negative result. His main point was that Einstein had carried out his calcula- tion with pure waves, i.e., assuming phase velocity, while the observations relate to the waves’ group velocity, where no light deflection will result (Doc. 30). Einstein, still not convinced, reacted with what he thought was a better proof, ea- ger to hear his friend’s opinion. Ehrenfest thanked him and wished his proof all the