- Person
- Otto Stern
- Lived
- 1888–1969
- Field
- Experimental physics
- From
- German-American
The story
Theory in the early 1930s, based on Paul Dirac's equation for particles with spin, treated the proton as a simple point-like particle and predicted exactly how magnetic it should be: one nuclear magneton, the proton's equivalent of the electron's basic unit of magnetism. Measuring something so small, about two thousand times weaker than an electron's magnetism, was one of the most ambitious goals Otto Stern had set for his Hamburg laboratory in 1926.
In the spring of 1933, Stern and Otto Robert Frisch finally succeeded in deflecting a beam of hydrogen molecules with a magnetic field, a very demanding experiment even today. It was complicated by the fact that only one form of hydrogen, ortho-hydrogen, carries a nuclear magnetic moment, and by the small magnetism caused by the molecules' rotation. Stern recalled that theoreticians chided them for measuring something they thought they already knew.
The proton turned out to be far more magnetic than expected. Frisch and Stern's first estimate was 2 to 3 nuclear magnetons. Less than two months later Stern and Immanuel Estermann repeated the measurement and reported 2.5, with an error of about 10 per cent, consistent with today's accepted value of about 2.79. The result showed that the proton is not an elementary particle. Rabi's group at Columbia confirmed the unexpectedly large value, and in 1939 measured it precisely.
The measurement was made as the Nazis were forcing Jewish scientists out of German universities. Estermann and Frisch received their letters of dismissal on 23 June 1933, and Stern resigned a week later, so it was among the last results of his Hamburg laboratory. It earned a central place in his 1943 Nobel Prize citation, which honoured his discovery of the magnetic moment of the proton.
The puzzle of the proton's magnetism remained unsolved for some 30 years, until the quark model of the 1960s showed that protons are built from three smaller particles whose magnetic moments combine. Today the proton's magnetism is exploited in proton nuclear magnetic resonance, which chemists use to study molecules and which underlies the MRI scans used in hospitals.
Why it matters
The discovery opened the path toward understanding that protons are made of quarks, and the proton's magnetism is the basis of the MRI scans used in hospitals worldwide.
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 proved atoms obey quantum rules in 1922In February 1922 the Stern–Gerlach experiment split a beam of silver atoms cleanly in two, the first direct proof that quantum rules govern how atoms behave in a magnetic field.
- 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.
Sources
- Proton magnetic moment Wikipedia
- Otto Stern Wikipedia
- Otto Stern (1888–1969): The founding father of experimental atomic physics Annalen der Physik (arXiv)
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