Otto Stern

The chemist who turned atoms into evidence

Otto Stern was born on 17 February 1888 in Sohrau, in the Prussian province of Silesia, into a prosperous Jewish family. The family moved to Breslau when he was four, and Otto drifted between Freiburg, Munich, and Breslau as a student before taking his doctorate at Breslau in 1912 with a thesis on the kinetic theory of osmotic pressure in concentrated solutions. It was respectable, careful work in physical chemistry. Nothing in it announced the experimentalist who would, a decade later, photograph the splitting of an atomic beam and force physics to swallow a quantized spin.

What changed Stern was Einstein. Within months of finishing his thesis he wrote to Albert Einstein, then a new professor at the German University in Prague, asking to come as a postdoctoral assistant. Einstein agreed. Stern moved to Prague in 1912, followed Einstein to the ETH in Zurich in 1913, and spent the next two years in the inner orbit of the man who had just rewritten the foundations of physics. He later said he learned more from listening to Einstein think out loud during their long walks than from any course he had ever taken. Einstein, he noticed, was forever asking whether a conclusion could be tested, whether some clever experimentalist could pin it to a number. That instinct, theory must answer to measurement, became the spine of Stern’s career.

Otto Stern was born on 17 February 1888 into a Jewish family in Sohrau (now Żory) in the Province of Silesia, Kingdom of Prussia. In 1892, he moved with his parents to Breslau (now Wrocław). He studied in Freiburg im Breisgau, Munich, and Breslau. Stern completed his studies at the University of Breslau in 1912 with a doctoral dissertation in physical chemistry under supervision of Otto Sackur on the kinetic theory of osmotic pressure in concentrated solutions. He then followed Albert Einstein…

The Frankfurt experiment

By 1921 Stern was a full professor at Rostock, and he had been refining a single idea since the war. He wanted to shoot atoms, in vacuum, in narrow beams, and ask them questions one at a time. The Sommerfeld atomic theory of 1916 had insisted, on quantum grounds, that the angular momentum of an electron in an atom could only point in certain discrete directions. Most physicists treated this as a calculating convention. Stern wanted to know whether it was the literal physical truth.

In February 1922, in a cramped laboratory at the Physikalischer Verein in Frankfurt, with the younger experimentalist Walther Gerlach, he found out. They sent a beam of silver atoms through an inhomogeneous magnetic field and onto a glass plate. Classical physics required a smooth, continuous band on the plate. Sommerfeld required two clean lines. After months of fighting their vacuum pumps and oven, and a Frankfurt cigar habit that smoked enough sulfur to chemically develop the faint silver deposit, Stern and Gerlach saw the split. Two crisp spots, where classical physics demanded a smear. It was the first direct visual proof that an atomic property was quantized in space. Stern was thirty-four years old. The result would, in time, be reread as the first observation of electron spin, a concept Goudsmit and Uhlenbeck would not articulate until 1925.

Hamburg, the molecular ray, and the proton

At Hamburg, where he took over the new Institute for Physical Chemistry in 1923, Stern systematized the trick. The molecular ray method (a thin beam of neutral atoms threaded through magnets and slits and finally counted) became his signature instrument. With his lifelong collaborator Immanuel Estermann, and a rotating cast of students that included Isidor Rabi and Emilio Segre, he did one impossible measurement after another. He diffracted helium off a lithium-fluoride crystal in 1929, vindicating de Broglie’s matter wave with the same casual authority that he had vindicated Sommerfeld seven years earlier. And in 1933, in the most demanding measurement of his life, he measured the magnetic moment of the proton itself, finding it about 2.5 times larger than the simple Dirac-equation prediction. That anomaly was a flag planted on the road to the discovery, two decades later, that the proton was not elementary.

Exile and the Pittsburgh years

In January 1933 the Nazi government took power in Germany. Stern, as a Jewish professor, was a target of the new race laws from the first weeks. He did not wait to be dismissed. He resigned his Hamburg chair and accepted a research professorship at the Carnegie Institute of Technology in Pittsburgh, bringing Estermann with him and packing up what he could of his molecular-ray apparatus. He was forty-five. He would never set foot in Germany again.

In 1943 the Royal Swedish Academy gave him the Nobel Prize in Physics, the first awarded since 1939, the war having paused the ceremony. The citation read: “for his contribution to the development of the molecular ray method and his discovery of the magnetic moment of the proton.” It did not name the Stern-Gerlach experiment, an omission Stern remarked on dryly more than once. He retired from Carnegie in 1945, moved to Berkeley to be near his sisters, attended the Wednesday physics colloquia faithfully for the next twenty-four years, and quietly nominated more colleagues for the Nobel Prize than any other physicist of his generation. He died of a heart attack in a Berkeley movie theater on 17 August 1969.

What he means to the quantum story

Stern is the proof that quantum mechanics is not an interpretation problem. He took the most paradoxical predictions of the new theory (that angular momentum is quantized in space, that an atom is also a wave, that the proton has internal structure) and put each one on a glass plate that you could photograph. He is the second most-nominated person in Nobel history, with 82 nominations between 1925 and 1945. The two discrete spots on the Frankfurt plate are still the first picture in every textbook chapter on spin.