Hans Bethe, in full Hans Albrecht Bethe (born July 2, 1906, Strassburg, Ger. [now Strasbourg, France]—died March 6, 2005, Ithaca, N.Y., U.S.), German-born American theoretical physicist who helped shape 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 of science.
Bethe started reading at age four and began writing at about the same age. His numerical and mathematical abilities also manifested themselves early. His mathematics teacher at the local gymnasium recognized his talents and encouraged him to continue studies in mathematics and the physical sciences. Bethe graduated from the gymnasium in the spring of 1924. After completing two years of studies at the University of Frankfurt, he was advised by one of his teachers to go to the University of Munich and study with Arnold Sommerfeld.
It was in Munich that Bethe discovered his exceptional proficiency in physics. Sommerfeld indicated to him that he was among the very best students who had studied with him, and these included Wolfgang Pauli and Werner Heisenberg. Bethe obtained a doctorate in 1928 with a thesis on electron diffraction in crystals. During 1930, as a Rockefeller Foundation fellow, Bethe spent a semester at the University of Cambridge under the aegis of Ralph Fowler and a semester at the University of Rome working with Enrico Fermi.
Bethe’s craftsmanship was an amalgam of what he had learned from Sommerfeld and from Fermi, combining the best of both: the thoroughness and rigor of Sommerfeld and the clarity and simplicity of Fermi. This craftsmanship was displayed in full force in the many reviews that Bethe wrote. His two book-length reviews in the 1933 Handbuch der Physik—the first with Sommerfeld on solid-state physics and the second on the quantum theory of one- and two-electron systems—exhibited his remarkable powers of synthesis. Along with a review on nuclear physics in Reviews of Modern Physics (1936–37), these works were instant classics. All of Bethe’s reviews were syntheses of the fields under review, giving them coherence and unity while charting the paths to be taken in addressing new problems. They usually contained much new material that Bethe had worked out in their preparation.
In the fall of 1932, Bethe obtained an appointment at the University of Tübingen as an acting assistant professor of theoretical physics. In April 1933, after Adolf Hitler’s accession to power, he was dismissed because his maternal grandparents were Jews. Sommerfeld was able to help him by awarding him a fellowship for the summer of 1933, and he got William Lawrence Bragg to invite him to the University of Manchester, Eng., for the following academic year. Bethe then went to the University of Bristol for the 1934 fall semester before accepting a position at Cornell University, Ithaca, N.Y. He arrived at Cornell in February 1935, and he stayed there for the rest of his life.
Bethe came to the United States at a time when the American physics community was undergoing enormous growth. The Washington Conferences on Theoretical Physics were paradigmatic of the meetings organized to assimilate the insights quantum mechanics was giving to many fields, especially atomic and molecular physics and the emerging field of nuclear physics. Bethe attended the 1935 and 1937 Washington Conferences, but he agreed to participate in the 1938 conference on stellar energy generation only after repeated urgings by Edward Teller. As a result of what he learned at the latter conference, Bethe was able to give definitive answers to the problem of energy generation in stars. By stipulating and analyzing the nuclear reactions responsible for the phenomenon, he explained how stars could continue to burn for billions of years. His 1939 Physical Review paper on energy generation in stars created the field of nuclear astrophysics and led to his being awarded the Nobel Prize.
During World War II Bethe first worked on problems in radar, spending a year at the Radiation Laboratory at the Massachusetts Institute of Technology. In 1943 he joined the Los Alamos Laboratory (now the Los Alamos National Laboratory) in New Mexico as the head of its theoretical division. He and the division were part of the Manhattan Project, and they made crucial contributions to the feasibility and design of the uranium and the plutonium atomic bombs. The years at Los Alamos changed his life.
In the aftermath of the development of these fission weapons, Bethe became deeply involved with investigating the feasibility of developing fusion bombs, hoping to prove that no terrestrial mechanism could accomplish the task. He believed their development to be immoral. When the Teller-Ulam mechanism for igniting a fusion reaction was advanced in 1951 and the possibility of a hydrogen bomb, or H-bomb, became a reality, Bethe helped to design it. He believed that the Soviets would likewise be able to build one and that only a balance of terror would prevent their use.
As a result of these activities, Bethe became deeply occupied with what he called “political physics,” the attempt to educate the public and politicians about the consequences of the existence of nuclear weapons. He became a relentless champion of nuclear arms control, writing many essays (collected in The Road from Los Alamos ). He also became deeply committed to making peaceful applications of nuclear power economical and safe. Throughout his life, Bethe was a staunch advocate of nuclear power, defending it as an answer to the inevitable exhaustion of fossil fuels.
Bethe served on numerous advisory committees to the United States government, including the President’s Science Advisory Committee (PSAC). As a member of PSAC, he helped persuade President Dwight D. Eisenhower to commit the United States to ban atmospheric nuclear tests. (The Nuclear Test Ban Treaty, which banned atmospheric nuclear testing, was finally ratified in 1963.) In 1972 Bethe’s cogent and persuasive arguments helped prevent the deployment of antiballistic missile systems. He was influential in opposing President Ronald Reagan’s Strategic Defense Initiative, arguing that a space-based laser defense system could be easily countered and that it would lead to further arms escalation. By virtue of these activities, and his general comportment, Bethe became the science community’s conscience. It was indicative of Bethe’s constant grappling with moral issues that in 1995 he urged fellow scientists to collectively take a “Hippocratic oath” not to work on designing new nuclear weapons.
Throughout the political activism that marked his later life, Bethe never abandoned his scientific researches. Until well into his 90s, he made important contributions at the frontiers of physics and astrophysics. He helped elucidate the properties of neutrinos and explained the observed rate of neutrino emission by the Sun. With the American physicist Gerald Brown, he worked to understand why massive old stars can suddenly become supernovas.
Bethe wrote the entry on the neutron for the 14th edition of Encyclopædia Britannica.