The Stern-Gerlach experiment revisited

The Stern-Gerlach-Experiment (SGE) performed in 1922 is a seminal benchmark experiment of quantum physics providing evidence for several fundamental properties of quantum systems. Based on the knowledge of today we illustrate the different benchmark results of the SGE for the development of modern quantum physics and chemistry. The SGE provided the first direct experimental evidence for angular momentum quantization in the quantum world and therefore also for the existence of directional quantization of all angular momenta in the process of measurement. Furthermore, it measured for the first time a ground state property of an atom, it produced for the first time a fully “spin-polarized” atomic beam, and it also revealed the electron spin, even though this was not realized at the time. The SGE was the first fully successful molecular beam experiment where the kinematics of particles can be determined with high momentum-resolution by beam measurements in vacuum. This technique provided a kind of new kinematic microscope with which inner atomic or nuclear properties could be investigated. Historical facts of the original SGE are described together with early attempts by Einstein, Ehrenfest, Heisenberg, and others to reveal the physical processes creating directional quantization in the SGE. Heisenberg’s and Einstein’s proposals of an improved multi-stage SGE are presented. The first realization of these proposed experiments by Stern, Phipps, Frisch and Segrè is described. The experimental set-up suggested by Einstein can be considered as an anticipation of a Rabi-apparatus with varying fields. Recent theoretical work by Wennerstr?m and Westlund, by Devereux and others, is mentioned in which the directional quantization process and possible interference effects of the two different spin states are investigated. In full agreement with the results of the new quantum theory directional quantization appears as a general and universal feature of quantum measurements. One experimental example for such directional quantization in scattering processes is shown. Last not least, the early history of the “almost” discovery of the electron spin in the SGE is revisited.