Hans Albrecht Bethe

Hans Albrecht Bethe
(1906)

German-born American theoretical physicist who helped to shape classical physics into quantum physics and increased the understanding of the atomic processes responsible for the properties of matter and of the forces governing the structures of atomic nuclei. He received the Nobel Prize for Physics in 1967 for his work on the production of energy in stars. Moreover, he was a leader in emphasizing the social responsibility of science.
Bethe studied physics at the University of Frankfurt and did research in theoretical physics at the University of Munich, where he obtained the doctorate in 1928. His doctoral thesis, on the theory of electron diffraction, remains of fundamental value in understanding observational data. His work on term splitting in crystals in 1929 showed how the symmetrical electric field by which an atom in a crystal is surrounded affects its energy states. In 1931 he worked with Enrico Fermi in Rome. He returned to Germany and served as a lecturer at the University of Tubingen until 1933. After a stay in Manchester, Eng., he emigrated to the United States and became, in 1934, a lecturer at Cornell University in Ithaca, N.Y., which remained his home. He was a professor there from 1937 to 1975, when he became professor emeritus.

In 1939 Bethe calculated the Sun's energy production, which results from the fusion of four hydrogen atoms (each of mass 1.008) into one helium atom (mass 4.0039). No direct fusion is possible, but Bethe showed that the probabilities of the four steps of the "carbon cycle" can account for the energy output. A carbon isotope of mass 12 reacts successively with three hydrogen nuclei (protons) to form the nitrogen isotope of mass 15; energy is produced through the fusion of a fourth hydrogen nucleus to release a helium nucleus (alpha particle) and the original carbon isotope.

Bethe became a U.S. citizen in 1941. At the beginning of World War II, Bethe had no U.S. clearance for military work. But, after reading in the Encyclop?dia Britannica that the armour-piercing mechanism of grenades was not well understood, he formulated a theory that became the foundation for research on the problem. His work, unpublished except in classified reports, illustrated his faculty for developing highly mathematical theories to the point that their numerical results could be compared with the actual measurements.

After working at the Massachusetts Institute of Technology on the development of radar, Bethe headed the Theoretical Physics Division of the Manhattan Project in Los Alamos, N.M. The development of the atomic bomb and the dropping of it on Hiroshima and Nagasaki created a strong feeling of social responsibility in Bethe and other Los Alamos physicists. He was one of the organizers and original contributors to The Bulletin of the Atomic Scientists. Moreover, he lectured and wrote on the nuclear threat in order to increase public awareness of it.

Bethe was awarded the Max Planck Medal in 1955 and the U.S. Atomic Energy Commission's Enrico Fermi Award in 1961. He became, in 1957, a foreign member of the Royal Society of London, as well as a member of the National Academy of Sciences in Washington, D.C.

The discovery of neutron stars led Bethe back to fundamental research in astrophysics in 1970. Although his main interest was in the rapidly developing subjects of atomic and nuclear processes, he also applied classical mathematical methods to the calculation of electron densities in crystals, the order-disorder states in alloys, the operational conditions of reactors, the ionization processes in shock waves, and the detection of underground explosions from seismographic records.

Bethe's later works include Elementary Nuclear Theory (1948), a discussion of the experimental evidence concerning the forces acting inside the atomic nucleus, and Intermediate Quantum Mechanics, 2nd ed. (1968), a theoretical description of atomic structure.

BIBLIOGRAPHY.
Jeremy Bernstein, Hans Bethe, Prophet of Energy (1980), describes Bethe's life and scientific research and examines America's energy problems. R.E. Marshak (ed.), Perspectives in Modern Physics (1966), a collection of essays written in Bethe's honour, contains a bibliography of his work.