The Stern-Gerlach experiment revisited

Abstract

The Stern-Gerlach-Experiment (SGE) performed in 1922 is a seminal benchmark experiment ofquantum physics providing evidence for several fundamental properties of quantum systems.Based on the knowledge of today we illustrate the different benchmark results of the SGEfor the development of modern quantum physics and chemistry. The SGE provided the firstdirect experimental evidence for angular momentum quantization in the quantum world andtherefore also for the existence of directional quantization of all angular momenta in theprocess of measurement. Furthermore, it measured for the first time a ground stateproperty 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. TheSGE was the first fully successful molecular beam experiment where the kinematics ofparticles 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 ornuclear properties could be investigated. Historical facts of the original SGE aredescribed together with early attempts by Einstein, Ehrenfest, Heisenberg, and others toreveal the physical processes creating directional quantization in the SGE. Heisenberg’sand Einstein’s proposals of an improved multi-stage SGE are presented. The firstrealization 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 aRabi-apparatus with varying fields. Recent theoretical work by Wennerström and Westlund,by Devereux and others, is mentioned in which the directional quantization process andpossible interference effects of the two different spin states are investigated. In fullagreement with the results of the new quantum theory directional quantization appears as ageneral and universal feature of quantum measurements. One experimental example for suchdirectional quantization in scattering processes is shown. Last not least, the earlyhistory of the “almost” discovery of the electron spin in the SGE is revisited.