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
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 His work, unpublished except in classified reports, illustrated his faculty for developing highly mathematical theories to the point that their numerical results could be compared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.
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