A miller’s son from Silesia
Otto Stern was born on February 17, 1888, in Sohrau in Upper Silesia, today Żory in Poland, the eldest child of Oskar Stern, a well-to-do Jewish miller and grain dealer, and his wife Eugenie, née Rosenthal. In 1892 the family moved to the regional capital, Breslau (now Wrocław), where Otto attended the humanistic Johannes Gymnasium. The family’s comfortable circumstances would matter later: they allowed him to follow his curiosity through a long, unhurried education.
After finishing school in 1906 he studied for twelve semesters, mostly physical chemistry, at Freiburg, Munich and above all Breslau. His doctoral adviser there was the young physical chemist Otto Sackur, who was working on the quantum statistics of gases and gave Stern a lifelong interest in entropy. Stern took his doctorate in April 1912 with a thesis, on a subject of his own choosing, about osmotic pressure in highly concentrated solutions.
Einstein’s first assistant
Through Sackur and Fritz Haber, Stern was introduced to Albert Einstein, then a newly appointed professor at the German University in Prague. Stern thought it presumptuous to ask to work with him but went anyway in May 1912, becoming Einstein’s first postdoctoral co-worker. He later recalled expecting a learned scholar with a large beard and finding instead a tieless young man who looked like an Italian roadmender. When Einstein moved to Zurich that autumn, he took Stern with him as his assistant; the two published a joint paper on zero-point energy, and in 1913 Stern qualified as a Privatdozent in physical chemistry at the ETH.
Stern sat through all of Einstein’s improvised lectures and absorbed his habit of unconventional thinking. In Zurich he also met Max von Laue, who became a lifelong friend. The two young men found Niels Bohr’s new quantum model of the atom so offensive to classical physics that they vowed to quit physics if “this nonsense of Bohr” proved right, an oath Wolfgang Pauli later mocked. It was Stern’s own most famous experiment that would show Bohr was on the right track.
War, and the turn to experiment
In 1914 von Laue took a chair in Frankfurt and brought Stern with him. When war broke out, Stern volunteered for the German army, trained as a meteorologist and spent much of the war at a field weather station at Łomża in Russian Poland, writing papers on entropy to keep his mind occupied. According to the historians who reconstructed his life, he also quietly helped prevent the army from stripping the Solvay Institute in Belgium of its equipment.
At the end of the war he spent a few months in Walther Nernst’s Berlin laboratory, working with James Franck and Max Volmer on the quenching of fluorescence, the origin of the Stern–Volmer relationship still used by chemists today. That stint turned the theorist into an experimenter. Back in Frankfurt, now as assistant to Max Born, he conceived the idea that would define his career: using beams of atoms or molecules travelling through a vacuum to study the properties of single, isolated particles. In 1920 he used a rotating apparatus to measure the speed of silver atoms directly, confirming the kinetic theory of gases and launching the molecular-beam method.
The Stern–Gerlach experiment
Quantum theory predicted something strange: that an atom’s magnetic moment could point only in certain fixed directions relative to a magnetic field, “space quantisation.” Most physicists, including Born and Stern himself, doubted it was real. In August 1921 Stern proposed a simple test. Send a beam of silver atoms through a non-uniform magnetic field: classically the beam should smear into a continuous band, but if space quantisation was real it should split into separate beams.
The experiment was fiendishly hard. Silver had to be heated to around 1,300 °C inside a small glass vacuum apparatus, the beam had to be aligned to within hundredths of a millimetre, and runs lasted for hours. Stern, who freely admitted he was clumsy with his hands, worked out the design; his Frankfurt colleague Walther Gerlach carried most of the laboratory burden, especially after Stern took a post at Rostock in late 1921. On the night of February 7–8, 1922, Gerlach saw it: the silver beam had split cleanly in two. He sent Niels Bohr a postcard with a photograph of the split trace, and the paper that followed stated in emphatic type that space quantisation in a magnetic field had been proven as a fact.
Stern loved to tell a story about the deposit on the plate, which at first was almost invisible. He said that, too poorly paid to afford good cigars, he smoked cheap ones full of sulphur, and that his breath turned the faint silver into black silver sulphide. In 2003 the chemists Bretislav Friedrich and Dudley Herschbach, who had heard the story from Stern himself, re-enacted it and found that cheap cigar smoke did indeed reveal the trace. The experiment became one of the founding demonstrations of quantum mechanics; after electron spin was discovered in 1925, the split beam was understood as a direct sign of it.
The Hamburg years
In 1923 Stern became professor of physical chemistry and director of a new institute at the University of Hamburg. Over the next decade he built one of the leading physics laboratories in the world, working beside the theorist Wolfgang Pauli. With his lifelong collaborator Immanuel Estermann and, later, Otto Robert Frisch, he showed that beams of helium atoms and hydrogen molecules are diffracted by crystals, confirming that whole atoms and molecules behave as waves, as Louis de Broglie had predicted. In 1933 Stern, Frisch and Estermann measured the magnetic moment of the proton and found it about two and a half times larger than theory predicted, evidence that the proton is not a simple point particle but has inner structure.
Young physicists came to Hamburg from Italy, Britain and the United States. One was Isidor Isaac Rabi, who later said that from Stern and Pauli he learned what physics should be, a matter of taste and judgement rather than mere knowledge. Rabi went on to develop the molecular-beam resonance method, which led to nuclear magnetic resonance, MRI, atomic clocks and the maser. Some twenty Nobel Prizes were eventually awarded for work built on Stern’s molecular-beam technique.
Driven out of Germany
When the Nazis came to power in 1933, the Law for the Restoration of the Professional Civil Service provided the basis for removing Jewish academics from their posts. On June 23, 1933, Stern’s Jewish co-workers, Estermann, Robert Schnurmann and Frisch, received letters of dismissal. Stern himself, as a war veteran, was temporarily exempt, but he saw clearly what was coming. On June 30 he submitted his resignation, effective October 1, and before leaving made sure his non-Jewish assistant Friedrich Knauer could complete his habilitation.
He was more fortunate than many: he and Estermann were offered research professorships at the Carnegie Institute of Technology in Pittsburgh. But Depression-era funding was meagre, and Stern never recaptured the momentum of Hamburg. He became a United States citizen on March 8, 1939, which allowed him to take part in secret wartime research. It was his Jewish descent that ended his German career. After the war he kept up warm friendships with von Laue, Lise Meitner and others and often visited Europe, but he declined offers, including from von Laue, to become a member of the Göttingen Academy of Sciences.
The long-awaited Nobel Prize
Stern was nominated for the Nobel Prize in Physics more than eighty times between 1925 and 1945, more than any other candidate in the prize’s first half-century, and more than Planck or Einstein. His nominators included Einstein, Planck, Bohr, Born, Heisenberg, Pauli, Compton and Bethe. The committee hesitated, arguing that space quantisation had already been predicted and that his proton measurement disagreed with others; in fact his 1933 value proved the closest to today’s accepted figure.
On November 9, 1944, he learned that he had been awarded the unshared 1943 prize “for his contribution to the development of the molecular ray method and his discovery of the magnetic moment of the proton.” His former Hamburg fellow Rabi received the 1944 prize, and with the war preventing the usual Stockholm ceremony, both were in New York for the celebrations that December.
A bachelor in Berkeley
Stern never married. Colleagues remembered a kind, witty and gentlemanly man who held a cigar in one hand at almost all times. Frisch recalled that when equipment began to topple, Stern never tried to catch it but raised both hands in surrender, explaining that you did less damage that way. Rabi described him as one of the “antistuffy” generation of German professors who regarded the pomposity of their predecessors with amusement. He loved the cinema and once joked in Hamburg that it would be nice to die while watching a good film.
He retired from Carnegie in 1945 and moved to Berkeley, California, where relatives lived and where he had been a frequent visiting professor since receiving an honorary doctorate there in 1930. He bought a house on Cragmont Avenue overlooking San Francisco Bay, intending to share it with his unmarried younger sister Elise, who died unexpectedly in 1945; his elder sister Berta and her family lived nearby. His niece, Lieselotte Templeton, became a noted crystallographer. He remained a regular at Berkeley’s physics colloquia, and in 1968 he attended the Nobel laureates’ meeting at Lindau.
Death and legacy
In August 1969 Stern suffered a heart attack while at a cinema in Berkeley; he died on August 17, 1969, at the age of eighty-one. His ashes were buried at Sunset View Cemetery in El Cerrito. Gerlach, who outlived him by about a decade, called him a grand seigneur of physics.
Stern’s legacy is a way of doing physics. By isolating single atoms in a beam, he made it possible to measure their quantum properties directly, and the techniques he pioneered lead straight to atomic clocks, magnetic resonance imaging, lasers and masers, and modern atomic and molecular physics. The German Physical Society’s Stern–Gerlach Medal for experimental physics bears his name, and his historians have called him the founding father of experimental atomic physics.
Sources & further reading
Otto Stern, Wikipedia · Toennies, Schmidt-Böcking, Friedrich & Lower, “Otto Stern (1888–1969): The founding father of experimental atomic physics” Annalen der Physik (2011), arXiv:1109.4864 · Friedrich & Herschbach, “Stern and Gerlach: How a bad cigar helped reorient atomic physics” Physics Today (2003) · The Nobel Prize in Physics 1943, NobelPrize.org · Researched from the sources above and checked against them. Spotted an error? Tell us.
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