Hans Bethe (born July 2, 1906,Strassburg, Ger. [now Strasbourg, France]—died March 6, 2005,Ithaca, N.Y., U.S.) was a German-born American theoretical physicist who helped shapequantum 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 theNobel Prize for Physics in 1967 for his work on the production ofenergy in stars. Moreover, he was a leader in emphasizing the social responsibility ofscience.

Education

Bethe started reading at age four and began writing at about the same age. His numerical and mathematical abilities alsomanifested 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 theUniversity of Munich and study withArnold Sommerfeld.

It was in Munich that Bethe discovered his exceptional proficiency inphysics. Sommerfeld indicated to him that he was among the very best students who had studied with him, and these includedWolfgang Pauli andWerner Heisenberg. Bethe obtained a doctorate in 1928 with athesis onelectron diffraction in crystals. During 1930, as aRockefeller Foundation fellow, Bethe spent a semester at theUniversity of Cambridge under the aegis ofRalph Fowler and a semester at theUniversity of Rome working withEnrico Fermi.

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Early work

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 1933Handbuch der Physik—the first with Sommerfeld on solid-state physics and the second on thequantum theory of one- and two-electron systems—exhibited his remarkable powers of synthesis. Along with a review on nuclear physics inReviews of Modern Physics (1936–37), these works were instant classics. All of Bethe’s reviews were syntheses of the fields under review, giving themcoherence 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 theUniversity of Tübingen as an acting assistant professor of theoretical physics. In April 1933, afterAdolf 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 gotWilliam Lawrence Bragg to invite him to theUniversity 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 atCornell University, Ithaca, N.Y. He arrived at Cornell in February 1935, and he stayed there for the rest of his life.

Bethe came to theUnited States at a time when the American physicscommunity was undergoing enormous growth. TheWashington Conferences on Theoretical Physics were paradigmatic of the meetings organized toassimilate the insightsquantum 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 byEdward 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 1939Physical Review paper on energy generation in stars created the field ofnuclear astrophysics and led to his being awarded the Nobel Prize.

From atomic warrior to “political physicist”

DuringWorld War II Bethe first worked on problems inradar, spending a year at the Radiation Laboratory at theMassachusetts Institute of Technology. In 1943 he joined theLos Alamos Laboratory (now the Los Alamos National Laboratory) inNew Mexico as the head of itstheoretical division. He and the division were part of theManhattan Project, and they made crucial contributions to the feasibility and design of theuranium and theplutoniumatomic bombs. The years at Los Alamos changed his life.

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In the aftermath of the development of thesefission weapons, Bethe became deeply involved with investigating the feasibility of developingfusion 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 ahydrogen 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 theconsequences of the existence of nuclear weapons. He became a relentless champion of nucleararms control, writing many essays (collected inThe Road from Los Alamos [1991]). He also became deeply committed to making peaceful applications ofnuclear 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 thePresident’s Science Advisory Committee (PSAC). As a member of PSAC, he helped persuade PresidentDwight D. Eisenhower to commit the United States to ban atmospheric nuclear tests. (TheNuclear Test Ban Treaty, which banned atmospheric nuclear testing, was finally ratified in 1963.) In 1972 Bethe’scogent and persuasive arguments helped prevent the deployment ofantiballistic missile systems. He was influential in opposing PresidentRonald Reagan’sStrategic 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’sconscience. It was indicative of Bethe’s constant grappling withmoral 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 andastrophysics. He helped elucidate the properties ofneutrinos and explained the observed rate of neutrino emission by theSun. With the American physicist Gerald Brown, he worked to understand why massive old stars can suddenly becomesupernovas.

Bethe wrote the entry on theneutron for the 14th edition ofEncyclopædia Britannica.