- Person
- Hans Bethe
- Lived
- 1906–2005
- Field
- Theoretical physics
- From
- German-born American
The story
Hans Bethe earned his doctorate under Arnold Sommerfeld in Munich in 1928. In 1929 he moved to the Technische Hochschule in Stuttgart to work with Paul Ewald, and there he tackled a basic question: when a fast charged particle, such as a proton or an alpha particle, travels through a material, how quickly does it lose its energy?
Starting from Max Born's interpretation of quantum mechanics, Bethe produced a compact formula for these collisions. His paper, "On the Theory of the Passage of Fast Corpuscular Rays Through Matter", served as his habilitation thesis in 1930, the qualification for teaching at a German university. Bethe himself considered it his greatest paper. In 1930 he went to the Cavendish Laboratory in Cambridge on a Rockefeller fellowship, and at the request of Patrick Blackett, who was studying particle tracks in cloud chambers, he extended the formula to particles moving close to the speed of light. The relativistic version appeared in 1932.
The result, known as the Bethe formula, gives the average energy a charged particle loses per unit of distance as it passes through matter, a quantity called stopping power. In Bethe's theory the material is described by a single number, its mean excitation energy. In 1933 Felix Bloch showed how to estimate that number, so the result is often called the Bethe–Bloch formula. Later physicists added corrections for slower particles, and they are built into widely used programs that calculate the stopping power of protons and alpha particles.
The Nobel Foundation described Bethe's collision theory as simple and powerful, and noted that it gave nuclear physicists a tool for determining the stopping power of matter for fast charged particles. Physicists use it to work out how far radiation penetrates a material, which matters for designing particle detectors and shielding.
The same physics underlies proton therapy for cancer. Because a proton loses energy faster as it slows down, it deposits most of its energy near the end of its path, in a sharp maximum known as the Bragg peak. Doctors exploit this to concentrate the dose on a tumour while sparing healthy tissue around it.
Why it matters
A calculation made in 1930 remains part of the everyday toolkit of nuclear, particle and medical physicists nearly a century later.
About Hans Bethe
Refugee physicist who explained how the stars shine, led theory at Los Alamos, cracked the Lamb shift and spent half a century campaigning against the nuclear arms race; Nobel Prize 1967.
More about Hans Bethe
- Hans Bethe championed peaceful nuclear energyThroughout the 1980s and 1990s Hans Bethe campaigned for the peaceful use of nuclear energy, calling nuclear electricity 'a necessity, not merely an option'.
- Hans Bethe's 1931 method still solves quantum puzzlesWhile working with Enrico Fermi in Rome in 1931, Hans Bethe devised the Bethe ansatz, a method for exactly solving certain quantum many-body problems.
- 'Bethe's Bible' taught a generation nuclear physicsIn 1936–37 Hans Bethe, with Robert Bacher and Stanley Livingston, wrote three review articles so complete they became known as 'Bethe's Bible'.
- Hans Bethe explained how the stars shineIn 1938–39 Hans Bethe worked out the nuclear reactions that power the Sun and stars, work that won him the 1967 Nobel Prize in Physics.
- Bethe showed missile defence could be fooled by decoysIn 1968 Hans Bethe and Richard Garwin argued that missile defences could be defeated by simple decoys, an argument that helped lead to the 1972 ABM Treaty.
- Hans Bethe helped secure the 1963 Test Ban TreatyHans Bethe served on the President's Science Advisory Committee from 1956 to 1964 and helped bring about the 1963 treaty banning atmospheric nuclear tests.
- Hans Bethe cracked the Lamb shift on a trainIn 1947, on a train to Schenectady, Hans Bethe calculated the Lamb shift, showing quantum electrodynamics could work and opening modern particle physics.
- At 85, Hans Bethe helped solve the solar neutrino puzzleAt the age of 85, Hans Bethe wrote an important paper on the solar neutrino problem, helping establish that neutrinos change type on their way from the Sun.
Sources
- Hans Bethe Wikipedia
- Bethe formula Wikipedia
- Hans Bethe – Biographical NobelPrize.org
- Bragg peak Wikipedia
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