The Stern-Gerlach experiment revisited

Abstract

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 Wennerström 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. * Corresponding author: hsb@atom.uni-frankfurt.de 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.

An Homage to Otto Stern

“…14). He wrote historical articles [25] with others (2011,2016) and edited all of Otto's research papers [26] into books (Fig. 15).…”

Michael Polanyi: Patriarch of Chemical Dynamics and Tacit Knowing

“…in Frankfurt in 1922 (Figure 6) and later in Hamburg. [44] Molecular beams are easily formed of the reactants Polanyi used in the flame experiments:s odium vapor,h alogens,a nd organic halides.The product sodium halide molecules as well as unreacted sodium atoms can be readily detected by surface ionization on ah ot tungsten wire.K nown since 1923, and much used in Sternsl ab,t hat very simple detector responds only to alkali metal species,w hich have ionization potentials lower than the work function of the wire.A dditionally, elastically scattered sodium atoms can be distinguished from the reactively scattered sodium halide molecules by deflection in am agnetic field, much employed by Stern, or by exciting emission from the atoms with resonant light, employed by Polanyi in many of his flame experiments.…”

“…Memorial plaque honoringOtto Stern and Walther Gerlach and depicting their celebrated molecular-beam experiment using magnetic deflection to demonstrate"space quantization" (courtesy of Horst Schmidt-Bçcking;see Ref. [44]).…”

Michael Polanyi: Vater der chemischen Dynamik und des impliziten Wissens

“…Ihm mussten die gefeierten Molekularstrahlexperimente von Otto Stern (1888-1969) und seinen Kollegen bekannt sein, die zuerst in Frankfurt im Jahr 1922 (Abbildung 6) und später in Hamburg durchgeführt wurden. [44]…”