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 i x Another proposed experiment, which may or may not have been put to the test, was meant to observe the Stark effect on the radiation of excited atoms due to the electric field of the surrounding black-body radiation. Already in January 1921, Einstein and his colleague Peter Pringsheim had thought about the possibility of learning in this way something about the wave or quantum structure of black-body radiation. In a detailed letter to Einstein of 16 July 1922, a year and a half after their initial collaboration, Pringsheim returned to the problem of observing a spectral line broadening due to the Stark effect. But, together with James Franck, he now concluded that such an experiment would not provide a decisive distinction be- tween light quantum emission and wave emission (Doc. 300). During the period covered in this volume there was no breakthrough in Ein- stein’s understanding of the quantum problems that puzzled him. But a paper he wrote with Ehrenfest on the Stern-Gerlach experiment stands out. The article was written remarkably quickly after the results of the experiment became known and showed with uncompromising clarity that the experiment posed a problem that could not be solved in the framework of contemporary quantum theory. In mid-May 1922, Einstein wrote to Born: “The most interesting thing these days is the experiment of Stern and Gerlach. The alignment of the atoms via radi- ation and without collisions is not understandable (according to current ways of thinking). Such an alignment should, by rights, last more than a 100 years. I have made together with Ehrenfest a little calculation on it. Rubens considers the exper- imental results to be absolutely reliable” (Doc. 190). The experiment showed that a beam of silver atoms passing through an inhomo- geneous magnetic field is split into two narrow beams, one of which has the atomic magnetic moments aligned with the strong (10,000 gauss) magnetic field, while the other consists of antialigned magnetic moments. The results showed, first of all, that the silver atoms did carry magnetic moments. More importantly, it also showed the splitting into aligned and anti-aligned magnetic moments, a behavior that is contrary to the classical prediction of a continuous broadening of the beam in ac- cordance with a random continuous distribution of the direction of the magnetic moments of single atoms. Einstein and Ehrenfest’s “little calculation” dealt with the question of the align- ment of the atoms’ magnetic moments. At that time, it was held that any quantum transition between states has to take place via radiation or else mechanically through collision processes. They showed that, on the basis of this understanding, the experiment posed a real problem, given the measurement process involved. Einstein and Ehrenfest, too, assumed that the alignment of the magnetic moments can only be attained by emission or absorption of radiation. The emission