Hans Bethe
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Hans Bethe

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Hans Bethe

Name: Hans Bethe (1906-2005)

Occupation: Chief - Theoretical Division

Site: Los Alamos

Years on Project: 1942-1946

Hans Albrecht Bethe was born on July 2, 1906 in Strassburg, Germany (now Strasbourg, France). He received a Ph.D. in theoretical physics from the University of Munich in July 1928. In October 1933, he emigrated to England, where he briefly lectured at the University of Manchester. In February 1935 Bethe became an assistant professor at Cornell University, where he would remain for the rest of his career, then received a promotion to professor in summer 1937.

His war work took him first to the Radiation Laboratory at the Massachusetts Institute of Technology, working on microwave radar. After spending summer 1942 at the University of California-Berkeley on a design for the atomic bomb, Bethe was selected by J. Robert Oppenheimer to lead the theoretical division of Los Alamos. As theory chief, Bethe had to oversee and coordinate the work of the various theory groups, who had plenty to work on, including: creating a model for how neutrons diffuse through a critical mass, figuring out how to calculate the efficiency of nuclear explosions, determining critical masses and the limits of sub-critical ones, understanding how liquids and gases behaved in fractions of micro-seconds at immense temperatures and pressures, and designing an initiator.

At the end of the Second World War, Bethe worked on the United States' development of the hydrogen bomb, although he opposed the weapon's development and hoped that he would show that the H-bomb was impossible to build. Between 1956 and 1964, he served on the President's Science Advisory Committee, and in 1958 he headed a presidential study of nuclear disarmament. He helped to negotiate the 1963 Limited Test Ban Treaty with the Soviet Union, and acted as an informal advisor to Presidents Eisenhower, Kennedy, and Johnson. In his later life, he campaigned against President Ronald Reagan's proposed Strategic Defense Initiative missile system and advocated for the peaceful use of nuclear energy.

Bethe mainly worked on the theory of atomic nuclei. With Rudolf Peierls, he developed a theory of the deuteron in 1934. The next year, he resolved some contradictions in the nuclear mass scale. From 1935 to 1938, he studied the theory of nuclear reactions, predicting many reaction cross sections. This work led him to the discovery of the reactions which supply the energy in stars. The carbon-nitrogen cycle is the most important nuclear reaction in brilliant stars, while the Sun and fainter stars mainly use the proton-proton reaction. Bethe's main achievement in this vein was showing that other possible nuclear reactions. For this work, as well as his work on nuclear reactions in general, Bethe received the Nobel Prize in Physics in 1967.

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Image courtesy of Wikimedia Commons.


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