HANS ALBRECHT BETHE (1906– 2005)

Hans Albrecht Bethe was born on July 2, 1906, in Strasbourg, Germany (now part of Alsace-Lorraine, France). He studied at the University of Frankfurt for two years beginning in 1924 and then went on to study at the University of Munich for about two-and-a-half years. Working under Professor Arnold Sommerfeld (1868–1951) in Munich, Bethe earned his Ph.D. in theoretical physics in July 1928. His doctoral thesis on electron diffraction still serves as an excellent example of how a physicist should use observational data to understand the physical universe. From 1929 to 1933, Bethe held positions as a visiting researcher or physics lecturer at various universities in Europe, including work with Enrico Fermi (1901–1954) at the University of Rome in 1931. With the rise of the Nazi Party in Germany, Bethe lost his position as a physics lecturer at the University of Tübinger in 1933 and became a scientific refugee. He left Germany, and emigrated to the United States after spending 1934 working as a physicist in the United Kingdom.

In February 1935, Cornell University offered Bethe the position of assistant professor of physics. The university promoted him to the full professor in the summer of 1937. Except for sabbatical leaves and an absence during World War II, he remained a physics professor at Cornell until 1975. At that point, Bethe retired, with the rank of professor emeritus. His long and very productive scientific career was primarily concerned with the theory of atomic nuclei.

In 1939, Bethe helped solve a long-standing mystery in physics and astronomy by explaining energy-production processes in stars like the Sun. Bethe proposed that the Sun’s energy production results from the nuclear fusion of four protons (hydrogen nuclei) into one helium-4 nucleus. The slight difference in the relative atomic masses of reactants and product of this thermonuclear reaction manifests itself as energy in the interior of stars. This hypothesis became known as the proton-proton chain reaction— the series of nuclear fusion reactions by which energy can be released in the dense cores of stars. Bethe received the 1967 Nobel Prize in physics for his “contributions to the theory of nuclear reactions, especially his discoveries concerning the energy production in stars.”

In 1941, Bethe became an American citizen and took a leave of absence from Cornell University so he could contribute his scientific talents to the war effort of his new country. His wartime activities took him first to the Radiation Laboratory at the Massachusetts Institute of Technology, where he worked on microwave radar systems. He then went to the Los Alamos National Laboratory in New Mexico and played a major role in the development of the atomic bomb under the Manhattan Project. Specifically, from 1943 to 1946, Bethe served as head of the theoretical group at Los Alamos and was responsible for providing the technical leadership needed to transform nuclear physics theory into functioning nuclear fission weapons.

Throughout the cold war, Bethe continued to serve the defense needs of the United States, primarily as a senior scientific advisor to presidents and high-ranking government leaders. In 1952, he returned briefly to Los Alamos to provide his theoretical physics expertise as the laboratory prepared to test the first U.S. hydrogen bomb. The following year, after the surprising successful atmospheric test of Andrei Sakharov’s hydrogen bomb by the Soviet Union, Bethe provided scientific guidance to senior U.S. government officials concerning the technical significance of the emerging Soviet thermonuclear weapons program. In 1961, the U.S. Department of Energy presented him with the 1961 Enrico Fermi award in recognition of his many contributions to nuclear energy science and technology. Yet, despite the important role he played in developing the United States’ powerful arsenal of nuclear weapons, Bethe is a strong advocate for nuclear disarmament and the peaceful applications of nuclear energy. Today, Bethe remains an active elder statesman of nuclear physics—a brilliant scientist whose work helped make nuclear and world history by unlocking the secrets of the atomic nucleus in the twentieth century.

Hans Heinrich Schaeder