German–American physicist (1888–1969)

Stern, who was born at Sohrau (now in Poland), was educated at the University of Breslau where he obtained his doctorate in 1912. He joined Einstein at the University of Prague and later followed him to Zurich (1913). After teaching at a number of German universities he was appointed an associate professor of theoretical physics at Rostock in 1921. He later moved (1923) to the University of Hamburg as professor of physical chemistry, but resigned in opposition to Hitler in 1933 and emigrated to America, where he took up an appointment with the Carnegie Institute of Technology at Pittsburgh. He retired in 1945.

Stern's main research came from his work with molecular beams of atoms and molecules (beams of atoms traveling in the same direction at low pressure, with no collisions occurring within the beam). Using such beams it is possible to measure directly the speeds of molecules in a gas. In 1920 Stern used a molecular beam of silver atoms to test an important prediction of quantum theory – namely, that certain atoms have magnetic moments (behave like small magnets) and that in a magnetic field these magnets take only certain orientations to the field direction.

The phenomenon is known as space quantization, and it could be predicted theoretically that silver atoms could have only two orientations in an external field. To test this, Stern with Walter Gerlach passed a beam of silver atoms through a nonuniform magnetic field and observed that it split into two separate beams. This, the famous Stern–Gerlach experiment, was a striking piece of evidence for the validity of the quantum theory and Stern received the 1943 Nobel Prize for physics for this work.

Stern used molecular beams for other measurements. Thus he was able to measure the magnetic moment of the proton by this technique. He also succeeded in demonstrating that atoms and molecules had wavelike properties by diffracting them in experiments similar to those of Clinton J. Davisson on the electron.

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

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).

(stûrn, Ger. ô'tō shtĕrn) , 1888–1969, American physicist, b. Germany, Ph.D. Univ. of Breslau, 1912. After resigning from his post at the Univ. of Hamburg in 1933, he became professor of physics at the Carnegie Institute of Technology and later professor emeritus at the Univ. of California, Berkeley. Stern was an outstanding experimental physicist; his contributions included development of the molecular-beam method, discovery of space quantization (with Gerlach, 1922), measurement of atomic magnetic moments, demonstration of the wave nature of atoms and molecules, and discovery of the proton's magnetic moment. He was awarded the 1943 Nobel Prize in Physics.

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