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Z. Phys. D - Atoms, Molecules and Clusters 10, 119-120 (1988) Otto Stern and the discovery of space quantization I.I. Rabi as told to John S. Rigden As a beginning graduate student back in 1923, I was greatly influenced but not convinced by the quantum theory. I suppose that is not surprising. If one tries to think logically about atomic phenomena on the basis of an undergraduate education in classical physics, the quantum theory seems like Reaganomics: It doesn’t make too much sense. Neither did the phenomena for that matter. But I hoped that with ingenuity and inventiveness I could and ways to fit the atomic phenomena into some kind of mechanical systems. After all, J.C. Maxwell had done it with the mechanical ether: It was intricate, with all sorts of machinery, gears and pulleys. Of course, further use of Maxwell’s equations so habituated people to the ether that it became natural for them to consider the ether as a mechanical object. My hope to explain atomic phenomena mechanically died when I read about the SternGerlach experiment. In this experiment a stream of silver atoms was sent trough an inhomogeneous magnetic field. The silver atom is supposed to possess angular momentum, that is, a magnetic moment. Now there is nothing in physics to suggest that these magnetic moments and angular moments would line up in a magnetic field in any coherent fashion. Because the angular moments could point in any direction, there would be a big concentration equatorially around the zero projection. The results of that experiment were astounding, although they were hinted at by quantum theory. The separation of the beam of silver atoms into two components occurred as if these moments pointed either one way or the opposite way. There was no mechanism that would orient them in one way or another since on leaving the source they were arranged quite statistically. There was no way by which those in the negative direction could gain or lose energy. In fact, the whole thing was a mystery. Here was something that could not be explained by any mechanism that could think of. This convinced me once and for all that an ingenious classical mechanism was out and that we had to face the fact that the quantum phenomena required a completely new orientation. This new orientation did come with quantum mechanics. Of course, this resolved one mystery, but it substituted another: that is, the basic mystery of quantum mechanics. I first met Stern in the fall of 1927. I had been in Copenhagen at the Niels Bohr Institute of Theoretical Physics and Bohr made arrangements for Yoshio Nishina and me to go and work with Wolfgang Pauli at the University of Hamburg. When I got there, I was pleased to find that Stern and his associates were engaged in very exciting molecular beam experiments. While my prime interest was with Pauli in theory, I spent time in Stern’s laboratory talking with Ronald Fraser, a Scotsman, and John Taylor, an American. I came to understand the subtleties of the molecular beam experiments and recognized that the components of an atomic beam could be separated with a homogeneous magnetic field. I explained the idea to Stern and he suggested I do the experiment. I was told what an honor it was to be invited by Stern to do an experiment in his laboratory. I had no job and I had a wife to support so I was in no position to refuse an honor. My experiment was a success and when it came time to write up the results, I sow a demonstration of Stern’s generosity, his fairness, and his pride. “First, publish a letter in Nature”, said Stern. “If you publish it first in German, they’ll think it’s my thing, and it’s yours.” My paper, in Zeitschrift für Physik, became the twelth paper in Stern’s U.z.M. (“Untersuchungen zur Molekularstrahlmethode”) series.