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Summary Article: Stern, Otto (1888-1969)
From The Hutchinson Dictionary of Scientific Biography

Place: Germany

Subject: biography, physics

German-born US physicist who showed that beams of atoms and molecules have wave properties. He also determined the magnetic moment of the proton. For these achievements, Stern gained the 1943 Nobel Prize for Physics.

Stern was born in Sohrau, Upper Silesia (now Zory in Poland), on 17 February 1888. His family moved to Breslau (now Wrocław in Poland) in 1892. Following the custom of the time, Stern attended a number of German universities. He went to Freiburg im Breisgau, Munich, and Breslau, giving him the opportunity to attend lectures by such prominent scientists as Arnold Sommerfeld, Otto Lummer, and Ernst Pringsheim. Stern received his PhD from Breslau in 1912 for a dissertation on the kinetic theory of osmotic pressure in concentrated solutions with special reference to solutions of carbon dioxide.

Although Stern's thesis work had been in the field of physical chemistry, he was fascinated by theoretical physics. He arranged to join Albert Einstein in Prague as a postdoctoral associate. When Einstein moved to Zürich in 1913, Stern followed him and took up a post as Privatdozent (unpaid lecturer) in physical chemistry at the technical high school in Zürich.

Stern served in the army in World War I and was assigned to meteorological work on the Russian front. He was able to work on theoretical physics in his spare time, and was then transferred to more congenial laboratory work in Berlin towards the end of the war. In 1918, he took up the position of Privatdozent in the department of theoretical physics at the University of Frankfurt. Stern obtained his first professional position in 1921, when he was made associate professor of theoretical physics at the University of Rostock, but he soon moved to the University of Hamburg where, in 1923, he became professor of physical chemistry and director of the Institute of Physical Chemistry.

The next ten years marked the peak of Stern's career. He made important contributions to the understanding of quantum theory, and built up a thriving research group. Many of his associates were Jewish as indeed was Stern himself, and life became increasingly difficult during the early 1930s as the Nazis rose to power. The sacking of Stern's longtime associate Immanuel Estermann and an order to remove a portrait of Einstein from the laboratory were the last straws. Stern resigned his post in 1933 and went to the Carnegie Technical Institute in Pittsburg in the USA, where he was given the post of research professor and set up a new department for the study of molecular beams.

Stern was granted US citizenship in 1939 and worked as a consultant to the US War Department during World War II. After the war he retired and moved to Berkeley, California, where he maintained contact with the scientific community on a private basis only. He died in Berkeley of a heart attack on 17 August 1969.

The first phase of Stern's scientific career covers the period 1912-18. During these years he was absorbed in purely theoretical work and in learning how to choose the really central issues for study in an experimental context. His theoretical work was partly in the field of statistical thermodynamics, in which he proposed an elegant derivation of the entropy constant.

After World War I, Stern began to concentrate on the molecular beam method, which had been discovered by Louis Dunoyer (1880-1963) in 1911. This method consisted of opening a tiny hole in a heated container held inside a region of high vacuum. The vapour molecules inside the heated container flowed out to form a straight beam of moving particles suffering virtually no collisions in the vacuum. The beam could be narrowed still further by the use of slits, and a system of rotating slits could be used to select only those particles travelling at particular velocities. This was a powerful tool in the study of the magnetic and other properties of particles, atoms, and molecules.

Stern's first experiments with this device were completed in 1919. They dealt with the measurement of molecular velocities and confirmed the Maxwellian distribution of velocities. He then began an experiment with Walther Gerlach (1899-1979) that was intended to measure the magnetic moment of metal atoms and also to investigate the question of spatial quantization.

Developing the quantum model of the atom proposed by Niels Bohr in 1913, Sommerfeld had derived a formula for the magnetic moment of the silver atom and predicted that silver atoms in a magnetic field could orient in only two directions with respect to that field. This latter idea was not compatible with classical theory. The Stern-Gerlach experiment would determine which of these theories was correct. The experiment consisted of passing a narrow beam of silver atoms through a strong magnetic field. Classical theory predicted that this field would cause the beam to broaden, but spatial quantization predicted that the beam would split into two distinct separate beams. The result, showing a split beam, was the first clear evidence for space quantization. Stern and Gerlach also obtained a measurement of the magnetic moment of the silver atom.

Stern then went on to improve this molecular beam technique and in 1931 was able to detect the wave nature of particles in the beams. This was an important confirmation of wave-particle duality, which had been proposed in 1924 by Louis de Broglie for the electron and extended to all particles.

In 1933 Stern measured the magnetic moment of the proton and the deuteron. The magnetic moment of the proton had been predicted by Paul Dirac to be one nuclear magneton. Stern's group, despite much advice not to attempt the difficult experiment whose result was in any case already ‘known’, caused much astonishment when they demonstrated that the proton's magnetic moment was 2.5 times greater than expected. The explanation for this discrepancy lies partly in parallel proton movements.

Stern's experiments with molecular beams came at a critical time in the development of quantum theory and nuclear physics as they provided the firm experimental evidence that was needed for theories in quantum mechanics that were hitherto highly controversial. Stern's great talent lay in his experimental foresight, which was based on a solid theoretical grounding, coupled with an almost fanatical obsession with experimental and technical detail.

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