x l I N T R O D U C T I O N T O V O L U M E 1 3 results from the magnetic dipole radiation of the atoms precessing around the magnetic field. The radiation absorption comes from the surrounding heat radia- tion. The calculated alignment time they found is of the order of 109 seconds (i.e., one hundred years). Contrasting this impossible result with the actual alignment time of less than 10-4 seconds fully exposed the difficulty of identifying the real physical mechanism responsible for the alignment. The result of their questions and speculations was published in “Quantum The- oretical Comments on the Experiment of Stern and Gerlach” (Einstein and Ehren- fest 1922, Doc. 315). The paper was probably conceived during Einstein’s visit with Ehrenfest in Leyden between 29 April and 13 May 1922. After Einstein’s re- turn to Berlin, discussions and expressions of doubt continued for the next three months via correspondence. Glimpses of the debates between Einstein and Ehren- fest in preparing this paper can also be gleaned from Ehrenfest’s diary. The paper appeared in September 1922, only a couple of months after Stern and Gerlach had published their results. With dissatisfaction at the time, but with prescience when seen from today’s perspective, they conclude their paper by stating: “The difficul- ties enumerated show how unsatisfactory the two attempts at the interpretation of the results found by Stern and Gerlach are.” Ten years later, Born would write in his book Atomic Physics: “Stern and Ger- lach’s experiment is perhaps the most impressive evidence we have of the funda- mental difference between classical and quantum mechanics.”[1] For many authors and teachers, the Stern-Gerlach experiment has become the starting point for teach- ing quantum mechanics and a paradigm of quantum measurement. With its uncompromising emphasis on the problem of accounting for the split- ting of the magnetic moments by any known physical process, the paper antici- pates, in a sense, what later would become known as the quantum measurement problem. Remarkably, Einstein and Ehrenfest sensed a major conceptual problem of the emerging quantum theory right away. II Superconductivity was, for Einstein, another field of experimental work where quantum ideas could be put to the test. Superconductivity, one of the major riddles of atomic physics until the middle of the twentieth century, had been discovered in 1911 by Heike Kamerlingh Onnes in Leyden.[2] At very low temperatures, and not too strong magnetic fields, certain materials undergo a transition in which they completely lose their electrical resis- tance. The phenomenon was discovered first for mercury, and in the early 1920s only a few metals were known to have this property. Einstein’s interest in supercon-