Bethe, Hans Albrecht (1906- )

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BETHE, HANS ALBRECHT (1906– )

Hans Bethe, an only child, was born on July 2, 1906, in Strasbourg, when Alsace was part of the Wilhelminian empire. His father was a widely respected physiologist who accepted a professorship in Frankfurt when Hans was nine years old; his mother was a gifted musician who was raised in Strasbourg where her father had been a professor of medicine. The high school Bethe attended in Frankfurt was a traditional Humanistisches Gymnasium with a heavy emphasis on Greek and Latin. While there, he learned Latin and Greek, read Kant, Goethe, and Schiller, and also learned French and English and a good deal of science. Classes were from 8 A.M. to 1 P.M., six days a week, with much homework assigned daily.

Bethe's talents, particularly his numerical and mathematical abilities, manifested themselves early. By the time he had finished Gymnasium he knew he wanted to be a scientist and his poor manual dexterity steered him first into mathematics, and then into theoretical physics. In the fall of 1926, after completing two years of studies at the University in Frankfurt, Bethe went to Arnold Sommerfeld's seminar in Munich. Sommerfeld was a forceful and charismatic figure, and among his students were many of the outstanding theorists of their generation: Peter Debye, Paul Epstein, Paul Ewald, Max von Laue, Wolfgang Pauli, Werner Heisenberg, Gregor Wentzel, and Fritz London. In Munich, Bethe discovered his exceptional talents and his extraordinary proficiency in physics and Sommerfeld gave him indications that he was among the very best students who had studied with him.

Bethe obtained his doctorate in 1928 summa cum laude and became Paul Ewald's assistant in Stuttgart. Ewald—whose wife was the niece of a famous and influential reform rabbi—opened his home to the young Bethe and he became a frequent visitor. Ten years later he married Rose, one of Ella and Paul Ewald's daughters. After his brief stay in Stuttgart, Bethe returned to Munich to do his Habilitation with Sommerfeld.

During the academic year 1930-1931 Bethe was a Rockefeller fellow at the Cavendish in Cambridge and in Rome in Enrico Fermi's Institute. In 1932 he again spent six months in Rome working with Fermi. Fermi and Sommerfeld were the great formative influences on Bethe. Bethe's craftsmanship is an amalgam of what he learned from these two great physicists and teachers, combining the best of both: the thoroughness and rigor of Sommerfeld with the clarity and simplicity of Fermi. This craftsmanship is displayed in full force in the many "reviews" that Bethe has written.

By 1933 Bethe was recognized as one of the outstanding theorists of his generation. His book length Handbuch der Physik articles on the quantum theory of one- and two-electron systems and on the quantum theory of solids became classics as soon as they were published. In April 1933, after Adolf Hitler's accession to power, he was removed from his position in Tübingen because he had two Jewish grandparents. He went to England, and in the fall of 1934 he accepted a position at Cornell University and remained there for the rest of his career. At Cornell Bethe built a school of physics where he trained and influenced some of the outstanding theoretical physicists of their generation including Emil Konopinski, Morris Rose, Robert Marshak, Richard Feynman, Freeman Dyson, Richard Dalitz, Edwin Salpeter, Geoffrey Goldstone, Robert Brout, David Thouless, Peter Carruthers, Roman Jackiw, and John Negele.

In 1938 Bethe formulated the mechanism for energy generation in stars. This research grew out of his participation at the third Washington conference on theoretical physics in April 1938. The reaction had earlier been suggested by Carl von Weizsäcker as a possibility for energy generation and the production of deuterium in stars. The rate of this reaction in stars was calculated by Bethe and Charles L. Critchfield before the conference. Their conclusion was that the rate of such a reaction under the conditions in stellar interiors would be enough to account for the radiation of the sun, though for stars much brighter than the sun, other more effective sources of energy would be required. Until Bethe tackled the problem nucleosyn- thesis was conflated with the problem of energy generation. Bethe, on the other hand, separated the two problems. He advanced two sets of reactions—the proton-proton and the carbon cycle—that were to account for energy production in stars like the sun. The second depended on the presence of carbon in the star. At that time there was no way to account for the abundance of carbon in stars, that is, it was not at all clear what nuclear reactions in stars between elements lighter than carbon could produce this element. However, the presence of carbon in stars had been corroborated by their spectral lines in stellar atmospheres. Bethe accepted this fact and proceeded to compute the characteristics of stars nourished by the two cycles, and found that the carbon-nitrogen cycle gives about the correct energy production in the sun.

During World War II, Bethe worked on armor penetration, radar, and helped design atomic weaponry. He was a member of the Radiation Laboratory at the Massachusetts Institute of Technology from 1942 till the spring of 1943 when he joined Oppenheimer at Los Alamos and became the head of the theoretical division. Bethe is the supreme example why theoretical physicists proved to be so valuable in the war effort. It was his ability to translate his understanding of the microscopic world—that is, the world of nuclei, atoms, mole- cules—into an understanding of the macroscopic properties and behavior of materials, and into the design of macroscopic devices that rendered his services so valuable at Los Alamos and later on to industry. Bethe's mastery of quantum mechanics and statistical mechanics allowed him to infer the properties of materials at the extreme temperatures and pressures that would exist in an atomic bomb. Bethe, Fermi, and the other physicists on the Manhattan Project converted their knowledge of the interaction of neutrons with nuclei into diffusion equations, and the solutions of the latter into reactors and bombs.

After the war, Bethe became deeply involved in the peaceful applications of nuclear power, in investigating the feasibility of developing fusion bombs and ballistic missiles, and in helping to design them. He served on numerous advisory committees to the government including the President's Science Advisory Committee (PSAC) and was influential in getting the United States and the Soviet Union to sign a Nuclear Test Ban Treaty in 1963. In 1967 he won the Nobel Prize for his 1938 theoretical investigations explaining the mechanism of energy production in stars. Beginning in the mid-1970s Bethe collaborated with G. E. Brown, and this association resulted in exceptional productivity. He contributed importantly to the elucidation of supernovae explosions and to the solar neutrino problem. His most recent researches were concerned with the life cycle of supernovas and the properties of the neutrinos involved in the fusion processes in the sun. He has been and continued into his mid-nineties (at the time this article was written) to be an enormously productive scientist.

Silvan S. Schweber