Otto Stern(1888-1969): Pioneer of molecular beam research

OTTO STERN was born on February 17, 1888 in Upper Silesia, Germany, in a prosperous Jewish family of merchants. The parents encouraged their children (two sons and three daughters) to satisfy the thirst for knowledge without setting their sights on career goals.

He attended the Johannes Gymnasium and graduated in 1906. The course emphasised classics at the expense of mathematics and the sciences. Bu he supplemented his education by private readings of the various books that his father put at his disposal.

Stern continued his studies at the Universities of Freiburg, Munich and Breslau, migrating from one university to another. So this academic tradition enabled him to attend lectures on a variety of subjects, without regard to curricula or to the time needed for completion of the course requirements. Thus Stern attended lectures on theoretical physics by the legendary Arnold Sommerfeld and on experimental physics (blackbody radiation) by Otto Lummer and Ernst Prinysheim.

The books of Boltzmann on molecular theory and statistical mechanics by Clausius and Nernst on thermodynamics seemed to have influenced his choice of his line of research.

Returning to Breslau to complete his university degree, he decided to major in physical chemistry because of the influence of two particular professors in the department. His doctoral thesis (1912) was both theoretical and experimental and set the style for his future research. He was indeed an ``experimenting theoretician''. His scientific work can be grouped under two district categories: Theoretical Work (1912-1918).

Immediately after taking his doctorate, he joined Albert Einstein in 1912 as a postdoctoral fellow in Prague, moving with him to Zurich in 1913; with Ehrenfest and Max von Lane whom he came to know in Zurich. The status of Privatdozent (lecturer without salary) - which carried prestige, that he achieved in the Federal Institute of Technology, Zurich was transferred by him to the University of Frankfurt in 1914. During 1914-18, he served as a technical officer in the German army. He considered this period as important not only for the publication of some scientific papers but a certain attitude he developed towards the selection of research problems.

Stern was stationed in a small village during 1916 as a military weather observer where the duties of a routine nature left him plenty of free time which he did not waste: he took up the tedious problem of calculating the energy of a system of coupled mass points, performing the computations in longhand.

Stern was more attracted to Einstein by his work in molecular theory than his famous theory of relativity. He applied the quantum concepts to explain the curious temperature behaviour of the specific heat of crystalline substances.

Stern acknowledged what he really learned from Einstein planted the seed for the major accomplishments of his later career (Nobel Laureate). What he really learned from Einstein was essential scientific traits: how to evaluate the importance of current physical problems which questions to ask and what experiments should be undertaken at a given time.

An ingenious solution

The second major work dealt with various problems in statistical thermodynamics, in particular the entropy of a monatomic gas. The expression for entropy contains an arbitrary constant that cannot be computed but that greatly affects such properties as the vapour press of a solid or the chemical equilibrium of reacting gases.

The method for applying quantum concepts to a perfect gas had not yet been discovered. Stem gave an ingenious solution to avoid the need to apply quantum theory to a gas. He instead considered the equilibrium of a solid crystal with its vapour at a high temperature. This satisfies the conditions to use statistical mechanics for the gas and to apply quantum concepts only to the solid. He then applied Einstein's theory for specific that and the third law of thermo dynamics developed by Nerrst and derived successfully the result.

Towards the end of World War I, many German scientists were placed at Nernst's laboratory, University of Berlin, for conducting military research. Here Stern met two excellent experimentalists James Franck and Max Volmar: it is probable that his change-over from theoretical to experimental work was the result of their influence.

Experimental work (1919-45)

On returning to Frankfurt, Stern began to work with Max Born, director of the Institute for Theoretical Physics. He began to work on some theoretical problems but felt it was necessary to provide experimental proof for the fundamental concepts used in molecular theory. So he began to develop the molecular-beam method.

Stern picked up the work done by Dunoyer in 1911 for investigating the properties of free atoms, which had been lying unnoticed. Stern applied this technique, using beams of silver atoms and confirmed the theoretical values earlier obtained (in 1850), within the limits of experimental error. Einstein's teaching, how to recognise the important problems, is evident here.

Not particularly skilled in handling experimental technique for carrying out the experiment he had designed, Stern sought the assistance of his colleague Walther Gerlach. Together they succeeded in 1920 in proving the reality of space quantization and in measuring the magnetic moment of the silver atom. The five papers dealing with the Stern-Gerlach experiment received wide attention and secured for Stern a place of honour in the history of physics.

In 1921, Stern joined as professor of theoretical physics at the University of Rostock. After two years, he received an invitation from the University of Hamburg. On 1 January 1923 he joined as professor of physical chemistry and set out to organize a laboratory for molecular-beam research and to devise a program for conducting this research. This program yielded significant results: developing new and improved techniques, demonstrating the wave nature of a particle, the assumption introduced by de Broglie (1924); measuring the magnetic moment of proton. These experiments were essential for the acceptance of new ideas that eventually laid the foundation of modern physics.

With the advent of the Nazi regime (1933), the momentum of the Hamburg laboratory came to a sudden halt. Stern's colleagues of Jewish origin were dismissed; in protest Stern resigned, foreseeing for himself a similar fate.

Stern migrated to the U.S. and took up the post as research professor in the Carnegie Institute of Technology. Though a number of significant papers issued from Carnegie, the work did not progress rapidly, as the funds during the depression years were meager. He retired in 1946 and settled in Berkley, California. Though he maintained local contacts with physicists, he shunned public appearances. He died on August 17, 1969 at the age of 81.

Stern received several honours: honorary doctorates from the University of California and ETH in Zarich; member of the National Academy of Sciences (1945) and the American Philosophical Society; Noble Prize in Physics (1943)(The dixtionary of scientific biography vol XIII)

R.Parthasarathy

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