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Stern, Otto

STERN, OTTO

(b Sohrau, Upper Silesia, Germany [now Zory, Poland], 17 February 1888; d. Berkeley, California, 17 August 1969)

Physics.

Stern was the oldest of five children (two sons and three daughters) of Oskar Stern and Eugenie Rosenthal. Before he reached school age, the family moved to Breslau (now Wroclaw, Poland), where Otto received his primary and secondary education at the Johannes Gymnasium. After graduating in 1906, he continued his studies at the universities of Freiburg im Breisgau, Munich, and Breslau, from which he received the Ph.D. in physical chemistry in 1912.

Stern’s parents belonged to a prosperous Jewish family of grain merchants and flour millers who were content to let their children satisfy their thirst for knowledge without immediate, professional goals. Even while attending the Gymnasium, which emphasized the classics at the expense of mathematics and the sciences, Stern supplemented his education by perusing various books that his father put at his disposal; and during his university studies, he explored several fields of science before deciding on a career. This approach to learning was in accordance with German academic tradition in the period before World War I, when young men of means could migrate from one university to another and attend lectures on a variety of subjects without regard to curricula or to the time needed for completion of promotion requirements. Thus, Stern attended lectures on theoretical physics by Arnold Sommerfeld, one of the most brilliant lecturers of his generation, and on experimental physics by Otto Lummer and Ernst Pringsheim, both of whom were famous for their elegant research on blackbody radiation.

Stern’s real interest, however, was aroused more by his private readings than by his formal studies. The books of Boltzmann on molecular theory and statistical mechanics and of Clausius and Nernst on thermodynamics appear to have greatly influenced his choice of career. Returning to Breslau to complete his university studies, Stern decided to major in physical chemistry because two professors in that department, R. Abegg and O. Sackur, were more closely concerned with his interest in thermodynamics and molecular theory than were the professors of physics. His doctoral dissertation on the osmotic pressure of carbon dioxide in concentrated solutions was both theoretical and experimental and set the style for his future research, which he himself later described as that of an “experimenting theoretician.”

Stern’s scientific activity can be divided into two distinct periods: the theoretical (1912 – 1919) and the experimental (1919–1945). During the first period, he was strongly influenced by his contacts with Einstein, whom he joined as a postdoctoral associate in Prague immediately after graduating from Breslau and with whom he moved to Zurich in 1913; by Ehrenfest and Laue, with whom he became acquainted in Zurich; and finally, but to a lesser extent, by Max Born, with whom he began to work after his return to Frankfurt in 1919. During these years, he took advantage of his financial independence, which allowed him to select a place of work without regard to the availability of a remunerative position. He received the venia legendi at the Eidgenössische Technische Hochschule in Zurich in 1913 and transferred it to the University of Frankfurt in 1914, thus achieving the status of Privatdsozent, which carried the right to lecture in the university without salary. From the outbreak of the war in August 1914 until the German defeat in 1918, he served in the German army, first as a private and later as a noncommissioned officer, in various technical assignments. After his demobilization he returned to Frankfurt.

This period can best be described as Stern’s Lehr- und Wanderjahre. Its most important result was not the production of scientific papers, although Stern’s papers published during these years were by no means negligible, but the development of a certain attitude toward the selection of research problems. As Stern told this writer, he was less attracted to Einstein because of his spectacular achievement in formulating the theory of relativity than by his work in molecular theory, particularly the application of the then imperfectly understood quantum concepts to the explanation of the curious temperature behavior of the specific heat of crystalline bodies.

An early paper published with Einstein contributed to one aspect of this problem, namely, the question of the existence of the so-called zeropoint energy: Are the atoms of a body at rest at the absolute zero of temperature. Or do they oscillate around their equilibrium positions with an energy of hv/2? But what Stern really learned from Einstein was the evaluation of the importance of current physical problems, which questions to ask, and what experiments should be undertaken at a given time. His association with Einstein developed into a lifelong friendship and planted the seed for the major accomplishments of Stern’s later career, culminating in his winning the Nobel Prize in physics for 1943.

Stern’s work during the years 1912–1918 was concerned with various problems in statistical thermodynamics. Two papers published during this period are worthy of mention, one because of its scientific merit and the other because of the unusual circumstances of its origin. The first dealt with the absolute entropy of a monoatomic gas. The expression for the entropy of a gas obtained by classical theories contains an arbitrary constant that cannot be computed but that greatly affects such properties as the vapor pressure of a solid or the chemical equilibrium of reacting gases. The importance of this constant had been pointed out by Nernst in the formulation of the third law of thermodynamics (also known as Nernst’s theorem).

It was fairly obvious that quantum theory held the key to the solution of this problem, but the methods for the application of quantum concepts to a perfect gas had not yet been discovered. Otto Sackur and Tetrode had already published papers giving a theoretical expression for the entropy constant; and while their result was correct, their derivation was open to justifiable criticism. Stern avoided the need of applying quantum theory to a gas by considering the equilibrium of a solid crystal with its vapor at a high temperature. Under these conditions it was perfectly correct to use classical statistical mechanics for the gas and to apply quantum concepts only to the solid, where the theory provided the necessary guidance. Using Einstein’s theory of the specific heat and Nernst’s theorem, Stern obtained the same result as Sackur and Tetrode, but this time the derivation was unobjectionable.

The second paper carries the dateline “Lomsha, Russian Poland,” probably the only scientific paper that ever originated in this small town. Stern was stationed there during 1916 as a military weather observer, and his duties of recording twice daily the readings of a few instruments left him plenty of free time. To escape boredom he tackled the relations problem of calculating the energy of a system of coupled mass points–doing all the computations in longhand.

During the last years of the war, many German physicists and physical chemists were assigned to military research at Nernst’s laboratory at the University of Berlin. There Stern met James Franck and Max Volmer, both excellent experimenters; and it is very probable that his shift from theoretical to experimental work was due to the influence of these two scientists, who also became his lifelong friends.

After returning to Frnakfurt, Stern continued to work on similar theoretical problems; and he published a paper on the surface energy of solids with Max Born, director of the Institute for Theoretical Physics, to which Stern was attached. Shortly thereafter, Stern felt compelled to provide experimental proof for the fundamental concepts used in molecular theory. For this purpose, he began to develop the molecular-beam method. In 1911 Dunoyer had shown that atoms or molecules introduced into a high-vacuum chamber travel along straight trajectories, forming beams of particles that in many respects are similar to light beams. His work had been practically forgotten until Stern realized that it was a very powerful tool for the investigation of properties of free atoms. Thus began the second period of Stern’s scientific career, which secured for him a place of honor in the history of physics.

The first application of this method was still concerned with molecular theory, namely, the measurement of molecular velocities in a gas. These quantities had been computed theoretically around 1850; and although the result had been generally accepted, no one had succeeded in providing experimental proof. In 1919 Stern performed an elegant experiment, using beams of silver atoms, and confirmed the theoretical values within the limits of experimental error. Although a nice achievement, it was not exactly world-shaking. Its real importance was that it demonstrated the usefulness of the method and encouraged its further application. (Einstein’s teaching of how to recognize the really important problems is clearly visible in this work.)

At that time, Bohr’s theory of the atom had undergone rapid development, particularly in the hands of Sommerfeld, who concluded that certain atoms-for example, those of hydrogen, the alkali metals. or silver-should possess magnetic moments of the magnitude of

where e is the electronic charge, m is the mass of an electron, c is the velocity of light, and h is Planck’s constant. Moreover, if such an atom were placed in a magnetic field, it should be able to assume only two distinct orientations, with its axis and magnetic moment either parallel or opposed to the direction of the field. (A third possible orientation–magnetic moment perpendicular to the field–was forbidden by a special selection rule.) While the first conclusion was at least compatible with the classical theory, the second was not, and very few physicists of that time were inclined to take this spatial quantization seriously. Stern recognized that the molecular-beam method was capable of giving a clear yes-or-no answer to this question: If the classical theory were correct, a narrow beam of silver atoms should be broadened when passing through a nonhomogeneous magnetic field; but if the spatial quantization theory were correct, the beam should be split into two separate beams.

In 1920 this experiment, although simple in concept, was difficult to perform. Not particularly skillful in handling experimental techniques (as opposed to designing experiments), Stern, asked Walther Gerlach, a colleague at the Institute for Experimental Physics in Frankfurt, to join in this work. Together they succeeded in proving the reality of space quantization and in measuring the magnetic moment of the silver atom. The five papers reporting this work, which soon became known as the Stern-Gerlach experiment, received wide attention and established Stern’s rank among physicists.

In 1921, when these experiments were practically completed, Stern received his first formal academic appointment as associate professor of theoretical physics at the University of Rostock. There he was joined by this writer, who had just completed his doctorate under Stern’s friend Max Volmer. At that time, an appointment in Rostock was mainly a stepping-stone to greener pastures, and it was not long before Stern received a call to the University of Hamburg as professor of physical chemistry and as director of the Institute for Physical Chemistry, still to be erected. In the meantime, Stern and this writer, who were later joined by a few additional assistants, postdoctoral guests, and graduate students, were assigned temporary quarters in the Institute for Experimental Physics.

The period 1923–1933 marks the peak of Stern’s contributions to physics. Shortly after assuming his post at Hamburg on 1 January 1923, he set out to organize a laboratory specially equipped for molecular-beam research and to devise a program for conducting this research, which was executed, to a large degree, with remarkable success. The first part of the program was concerned with completing and expanding Stern’s previous work and with developing new and improved techniques; the second, with demonstrating the wave nature of particles–a revolutionary assumption introduced in 1924 by Louis de Broglie that became the foundation of modern quantum mechanics; and the third, with measuring the magnetic moment of the proton and deuteron. The significance of the work on the wave nature of particles is similar to that of the Stern-Gerlach experiment: each provided unambiguous, direct, and thoroughly convincing proof of revolutionary concepts introduced into the foundations of physics. These experiments were essential for the acceptance of new ideas that had previously been regarded with considerable skepticism.

The last part of Stern’s program had a completely different outcome. Dirac had promulgated a theory according to which the ratio of the magnetic moment of the proton to that of the electron should have been the same as the inverse ratio of their masses. This theory was believed so generally that when Stern, O. R. Frisch, and this writer began the very difficult experiments, they were told more than once by eminent theoreticians that they were wasting their time and effort. But Stern’s perseverance paid off. Measurements showed a proton magnetic moment two or three times larger than expected. While that result has since been reproduced with greater accuracy, a really satisfactory theoretical explanation is still outstanding. It is this work that was specifically mentioned in Stern’s Nobel Prize citation.

With the advent of the Nazi regime in 1933, the work in Hamburg came to an abrupt end. Several of Stern’s closest collaborators, who happened to be of Jewish origin, were summarily dismissed, and in protest Stern submitted his resignation before his own foreseeable dismissal. Stern and this writer received invitations to come to the United States, to the Carnegie Institute of Technology, where they began to build a molecular-beam laboratory. Stern was appointed research professor of physics, but the means put at his disposal during the depression years were rather meager. The momentum of the Hamburg laboratory was never regained, although a number of significant papers originated at Carnegie. Stern retired in 1946 to Berkeley, California, where he continued to maintain some contact with local physicists but shunned public appearances. Stricken by a heart attack, he died on 17 August 1969 at the age of eighty-one.

Stern was a member of both the National Academy of Sciences and the American Philosophical Society in 1945 after having received the Nobel Prize. He was also a member of the Royal Danish Academy and received honorary doctorates from the University of California and the Eidgenössiche Technische Hochschule in Zurich.

BIBLIOGRAPHY

I. Original Works. A complete bibliography of the papers published by Stern and his associates between 1926 and 1933 is listed in Estermann (see below). His early paper on the absolute entropy of a monoatomic gas is “Zur kinetischen Theorie des Dampfdrucks einatomiger fester Stoffe und über die Entropiekonstante einatomiger Gase,” in Physikalische Zeitschrift, 14 (1913), 629–632; the memoir on the energy of a system of coupled mass points is “Die Entropie fester Lösungen,” in Annalen der Physik, 4th ser., 49 (1916), 823–841. Subsequent works cited above are “Über die Oberflächenergie der Kristalle und ihren Einfluss auf die Kristallgestalt,” in Sitzungsberichte der Preussischen Akademie der Wissenschaften zu Berlin (1919), 901–913, written with Max Born; and “Eine direkte Messung der thermischen Molekulargeschwindigkeit,” in Zeitschrift für Physik, 2 (1920), 49–56, and 3 (1920), 417–421.

The results of the Stern-Gerlach experiment were reported in “Ein Weg zur experimentellen Prüfung der Richtungsquantelung im Magnetfeld,” ibid., 7 (1921), 249–253; “Der experimentelle Nachweis des magnetischen Moments des Silberatoms,” ibid., 8 (1921), 110–111, written with W. Gerlach; “Der experimentelle Nachweis der Richtungsquantelung im Magnetfeld,” ibid., 9 (1922), 349–352, written with Gerlach; “Das magnetische Moment des Silberatoms,” ibid., 353–355, written with Gerlach; and “Über die Richtungsquantelung im Magnetfeld,” in Annalen der Physik, 4th ser., 74 (1924), 673, written with Gerlach.

Stern’s Nobel Prize lecture. “The Method of Molecular Rays,” is in Nobel Lectures 1942–1962 (Physics) (Amsterdam–London–New York, 1964), 8–16, with biography on 17–18.

II. Secondary Literature. See I. Estermann, “Molecular Beam Research in Hamburg, 1922–1933,” in his Recent Research in Molecular Beams (New York–London, 1959), 1–7, with bibliography.

I. Estermann

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Otto Stern

Otto Stern

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). □

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Stern, Otto

Otto Stern (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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