Walther Gerlach

Walther Gerlach was born on August 1, 1889 in Biebrich, a small town on the Rhine that has since been absorbed into Wiesbaden. His father was a physician, and the household assumed Walther would follow into medicine. He had other plans. In 1908 he enrolled at the University of Tübingen, fell in with the spectroscopist Friedrich Paschen, and by 1912 had finished a doctorate on the precise measurement of radiation. Paschen kept him on as an assistant, and Gerlach finished his Habilitation in 1916 while in uniform, juggling lectures with wartime service. The Great War sent him to Jena to work on wireless telegraphy under Max Wien, then to an artillery testing commission under Rudolf Ladenburg. He came out of the trenches a careful, methodical experimentalist who trusted apparatus more than theory.

Walther Gerlach (1 August 1889 – 10 August 1979) was a German physicist who co-discovered, through laboratory experiment, spin quantization in a magnetic field, the Stern–Gerlach effect. The experiment was conceived by Otto Stern in 1921 and successfully conducted first by Gerlach in early 1922. He was Nazi Germany's plenipotentiary of nuclear physics from December 1943 until his capture by US Army in May 1945.

The work that made his name came in early 1922 at Goethe University Frankfurt, where he had taken an extraordinary professorship the previous year. Otto Stern, eleven years his senior and already an established theorist-turned-experimentalist, had proposed an audacious test in 1921: shoot a beam of silver atoms through a non-uniform magnetic field, and the field gradient should sort the atoms according to the orientation of their magnetic moments. Classical physics predicted a continuous smear. The new quantum theory, in Sommerfeld’s formulation, predicted that the moments would point in only a discrete set of directions, and the beam would split. Stern conceived the experiment. Gerlach, the younger collaborator with the steadier hands, was the one who actually built it and made it work.

The apparatus was a nightmare of cranky vacuum pumps, hand-blown glass, and a magnet with knife-edge pole tips that had to be aligned to fractions of a millimeter. The silver atoms had to traverse a meter of imperfect vacuum without colliding with anything, and the deposit on the collector plate, when one finally appeared, was almost invisibly thin. The famous story is that Gerlach could not see his result until he developed a photograph and discovered that the silver had darkened where his own breath, and the smoke of the cigars he chewed through the long nights in the lab, had reacted with the metal. The sulfur in cheap cigar smoke turned silver to silver sulfide, which is black. The cigar smoke developed the experiment for him. Two clean spots, no smear in between, and the whole architecture of classical physics for atomic magnetism collapsed in an afternoon.

Stern had by then left for a chair in Rostock, so it was Gerlach alone who watched the plate emerge. On February 17, 1922, Wolfgang Pauli sent him a postcard with a single line of mock relief: “Jetzt wird hoffentlich auch der ungläubige Stern von der Richtungsquantelung überzeugt sein.” Now hopefully even the unbelieving Stern will be convinced of directional quantization. Gerlach and Stern published the result jointly in the Zeitschrift für Physik that same year. It is one of the rare experiments in twentieth-century physics that requires no theoretical reinterpretation. The beam splits. The picture is on the photographic plate. You can hold it in your hand.

Gerlach moved to Tübingen in 1925 as Paschen’s successor, then in 1929 to the chair at the Ludwig-Maximilians-Universität in Munich that Wilhelm Wien had vacated. There the trajectory of his life takes a darker turn. In December 1943, on the recommendation of the Nazi science administrator Rudolf Mentzel, Albert Speer appointed Gerlach head of the physics section of the Reich Research Council and plenipotentiary for nuclear physics, replacing Abraham Esau. For the last eighteen months of the war he ran the German uranium project, the Uranverein, working out of the Kaiser Wilhelm Institute in Berlin-Dahlem and later a school basement in Stadtilm. As late as December 1944 he was still assuring Martin Bormann that Germany held a “considerable advantage” in atomic research. American troops captured him in Bavaria in May 1945, and he spent the next six months interned at Farm Hall in England with nine other German nuclear scientists, his conversations secretly recorded by British intelligence. The transcripts catch him reacting to the news of Hiroshima with stunned disbelief.

He returned to Munich in 1946, resumed his professorship in 1948, served as rector, and helped found the Fraunhofer-Gesellschaft. In 1957 he was among the eighteen West German physicists who signed the Göttingen Manifesto refusing to work on nuclear weapons for the new Bundeswehr. He died in Munich on August 10, 1979, ten days after his ninetieth birthday.

For the quantum story, Gerlach is the man whose patience and cigar smoke made spin visible on a photographic plate, and whose two silent dots in 1922 told physics that nature counts angular momentum in discrete steps.