The German-born American physicist Otto Stern (1888-1969) discovered the atomic-and molecular-beam technique and used it to provide the first direct proof of spatial quantization.
Otto Stern was born on Feb. 17, 1888, in Sorau, Upper Silesia. In 1906 he entered the University of Breslau, completing his doctoral degree in physical chemistry in 1912. He then went to the University of Prague to study under Albert Einstein and, when Einstein moved to the Swiss Federal Institute of Technology (FIT) in Zurich, Stern followed him, becoming lecturer at the FIT in 1913.
The following year Stern accepted a similar position in theoretical physics at the University of Frankfurt am Main but almost immediately found himself in military service. After the war and a brief period at the University of Berlin during 1918, Stern returned to Frankfurt. There, turning from theory to experiment, he conceived and carried out the first of the atomic-and molecular-beam experiments which brought him an international reputation and, ultimately, the Nobel Prize in 1943.
Stern realized that electrons rotating about the nucleus of an atom possess "orbital angular momentum" and produce a magnetic moment along the axis of rotation. This magnetic moment gives rise to a magnetic field identical to one which would be set up by a tiny bar magnet positioned on the axis of rotation of the electron. Therefore, if a beam of atoms, each possessing a magnetic moment, is sent through a nonuniform external magnetic field, each atom will experience a net force, the magnitude of which depends on the orientation of the magnetic moment of the atom with respect to the direction of the external magnetic field.
In the classical theory, all orientations of the atom's magnetic moment are possible, so that the external field should deflect as many atoms above as below the original beam direction, causing the beam to simply spread out. Instead, using a beam of silver atoms, Stern and Walter Gerlach found that the beam was actually split up into two separate beams, one above, the other below, the original beam direction. This observation completely contradicted classical theory; it showed that not all orientations of the atom's magnetic moment are possible; that is, it showed that there exists "spatial quantization."
In later years, whether as a lecturer at Frankfurt, as a full professor at Rostock and Hamburg, or (after fleeing Nazi persecution) as a research professor of physics at the Carnegie Institute of Technology in Pittsburgh, where he remained from 1933 until 1945, Stern devised a number of other experiments exploiting the atomic-and molecular-beam technique he developed. For example, he checked the accuracy of the Maxwell-Boltzmann velocity distribution for gas molecules; he measured nuclear magnetic moments and the magnetic moment of the proton; finally, he observed the wave nature of helium and hydrogen atoms by diffracting beams of these atoms.
In 1945, the same year in which he retired and took up residence in Berkeley, Calif., Stern was elected to the National Academy of Sciences, one of a number of honors he received during his lifetime. He died in Berkeley on Aug. 17, 1969.
Further Reading on Otto Stern
Stern gave an account of his experiments in his Nobel lecture, reprinted in Nobel Lectures in Physics, vol. 3 (1964). For the importance of his work in the context of the times see Max Jammer, The Conceptual Development of Quantum Mechanics (1966).