- Person
- Otto Stern
- Lived
- 1888–1969
- Field
- Experimental physics
- From
- German-American
The story
Early quantum theory made a strange prediction. In 1916 Arnold Sommerfeld and Peter Debye proposed that an atom's tiny magnet could only point in a few fixed directions in a magnetic field, an idea called space quantisation. Many physicists, including Debye himself, Max Born and Otto Stern, doubted it was real. Stern set out to test it directly, and on 26 August 1921 he submitted a paper describing how: a beam of silver atoms would be sent through an uneven magnetic field. If classical physics was right, the beam would smear into a band. If quantum theory was right, it would split into separate beams.
It was a difficult experiment. The beam apparatus was not much bigger than a fountain pen, the silver had to be heated in a tiny oven while the collector was cooled, and runs lasted hours. Stern worked out the design, and his Frankfurt colleague Walther Gerlach did much of the painstaking laboratory work, which he carried on largely alone after Stern took a post at Rostock in the autumn of 1921. Germany's post-war inflation made money scarce, and a gift from the New York banker Henry Goldman helped keep the work going.
On the night of 7 to 8 February 1922 an eight-hour run finally showed the beam neatly divided in two. A telegram to Stern read 'Bohr is right after all', and on 8 February Gerlach mailed Niels Bohr a postcard with a photograph of the split trace, calling it the experimental proof of space quantisation and congratulating him on the confirmation of his theory. Einstein called it the most interesting achievement in quantum physics at that point.
Even Stern was baffled. He later said he could not understand the two discrete beams at all, since explaining them needed both the new quantum mechanics and a magnetic electron, neither of which yet existed. Electron spin was proposed in 1925, and physicists gradually came to see that the split beam was a direct sign of it. The experiment became a textbook cornerstone of quantum mechanics.
Its descendants include nuclear magnetic resonance, optical pumping, the laser and atomic clocks. In 1992 the German Physical Society created the Stern–Gerlach Medal for excellence in experimental physics in honour of the two men.
Why it matters
The Stern–Gerlach experiment gave physicists hard evidence that quantum theory described real atoms, helping lay the foundation for modern physics, chemistry and electronics.
About Otto Stern
Physicist whose molecular-beam method proved space quantisation in the Stern–Gerlach experiment and measured the proton’s magnetic moment; Nobel Prize in Physics 1943.
More about Otto Stern
- Otto Stern showed whole atoms behave like wavesAround 1930 Otto Stern's Hamburg laboratory diffracted beams of helium atoms and hydrogen molecules off crystals, proving that entire atoms, not just electrons, travel as waves.
- Otto Stern's lab trained Rabi, opening a path to MRIIsidor Rabi learned the molecular-beam technique in Otto Stern's Hamburg lab, and built on it the magnetic resonance method behind NMR, MRI and atomic clocks.
- Otto Stern made molecular beams a precision toolIn 1920 Otto Stern measured the speed of silver atoms in a vacuum, turning molecular beams, first used by Louis Dunoyer in 1911, into a precise tool for studying single atoms.
- Otto Stern found the proton is not a simple particleIn 1933 Otto Stern, Otto Frisch and Immanuel Estermann measured the proton's magnetism and found it unexpectedly large, the first sign that the proton has inner structure.
Sources
- Stern and Gerlach: How a bad cigar helped reorient atomic physics Physics Today
- Otto Stern Wikipedia
- Otto Stern (1888–1969): The founding father of experimental atomic physics Annalen der Physik (arXiv)
Another act of tikkun olam. This is part of tikkun olam, the Jewish idea of repairing the world, and every one of us can add to it.