Bethe, Hans Albrecht

HE IS STILL ALIVE?

Bethe, Hans Albrecht (1906- ), German-born American physicist and Nobel laureate, noted for his contributions to theories of stellar energy production. Bethe was born in Strasbourg, Alsace-Lorraine (then a part of Germany), and educated at the University of Frankfurt and the University of Munich, from which he received a Ph.D. degree in 1928. Bethe taught physics at various universities in Germany from 1928 to 1933 and in England from 1933 until 1935, when he began his long association with Cornell University.

Beginning in 1943 he worked at Los Alamos, New Mexico, on the atomic-bomb project. After initial misgivings he took part in the later development of the hydrogen bomb. At the same time he continued his work for the peaceful use and international control of nuclear energy. A prime advocate of the partial test-ban agreement signed by the United States, the Soviet Union, and the United Kingdom in 1963, he became an opponent of the Strategic Defense Initiative, proposed by the United States in the 1980s.

Bethe was awarded the 1967 Nobel Prize in physics for his studies of the production of energy by the sun and other stars, which he postulated occurs through thermonuclear fusion, a long series of nuclear reactions by which hydrogen is converted into helium. He was naturalized a U.S. citizen in 1941.

born July 2, 1906, Strassburg, Ger. [now Strasbourg, France]

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 emphasizingthe 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 issurrounded 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 Tübingen until 1933. After a stay in Manchester, Eng., he immigrated 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 Encyclopædia Britannicathat 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 ofthe 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.

Paul P. Ewald

Additional reading

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.

Study of the Sun’s Energy

Hans Albrecht Bethe American physicist Hans Albrecht Bethe won the Nobel Prize in physics in 1967. He studied thermonuclear fusion, the process by which hydrogen is converted into helium. Fusion reactions in the Sun produce the Sun’s energy.© The Nobel Foundation

The Sun produces an enormous amount of energy. Scientists could not explain how something with the mass of the Sun could produce so much energy until they discovered nuclear fusion. The details of just how nuclear fusion changes hydrogen into helium nuclei were not known until discoveries in the field of elementary particles were made. Elementary particles are the tiny particles that make up all matter. The most familiar particles, the particles that make up atoms, are protons, neutrons, and electrons. Protons and neutrons are the main particles involved in nuclear fusion. Both types of particles are about the same size and mass, but protons have a positive electric charge, while neutrons are electrically neutral. New Zealand-born British physicist Ernest Rutherford discovered the proton in 1918. British physicist Sir James Chadwick discovered the neutron in 1932, and was awarded the 1935 Nobel Prize in physics for his discovery.

The first fusion reaction in a laboratory occurred in the early 1930s. In 1938 German-born American physicist Hans A. Bethe and American physicist Charles L. Critchfield demonstrated how a sequence of nuclear reactions, called the proton-proton chain, makes the Sun shine. Bethe was awarded the 1967 Nobel Prize in physics for his discoveries concerning energy production in stars.