A professor’s son from Strasbourg
Hans Albrecht Bethe was born on 2 July 1906 in Strasbourg, then part of the German Empire, the only child of Albrecht Bethe, a lecturer in physiology at the university, and Anna, née Kuhn. His mother was the daughter of Abraham Kuhn, a professor at the University of Strasbourg, and came from a Jewish background. Hans was raised a Protestant, like his father, and later in life became an atheist. To the Nazis, however, his Jewish ancestry would be decisive.
The family followed his father’s career, to Kiel in 1912 and to Frankfurt in 1915, where Albrecht Bethe headed the new Institute of Physiology. His schooling at Frankfurt’s Goethe-Gymnasium was interrupted by tuberculosis in 1916, and he spent time recovering at Bad Kreuznach and at the progressive Odenwaldschule before returning to the Gymnasium for his final years.
Sommerfeld’s student
Bethe entered the University of Frankfurt in 1924 to study chemistry, but proved a clumsy experimenter who ruined his lab coat with sulphuric acid. On the advice of the physicist Karl Meissner he moved in 1926 to Munich to study theoretical physics under Arnold Sommerfeld, just as Erwin Schrödinger’s papers on wave mechanics were arriving. He took his doctorate in 1928 with a thesis on electron diffraction in crystals. When he first met Wolfgang Pauli, Pauli told him that after Sommerfeld’s stories he had expected much better than the thesis; Bethe decided that, from Pauli, this was a compliment.
In Stuttgart in 1930 he wrote what he considered his greatest paper, on the passage of fast charged particles through matter; its Bethe formula is still used to calculate how radiation loses energy as it travels through materials. A Rockefeller fellowship took him to Cambridge and then to Enrico Fermi’s group in Rome, where in 1931 he devised the Bethe ansatz, a method for exactly solving certain quantum many-body problems that remains a pillar of mathematical physics. He also wrote two enormous review articles for the Handbuch der Physik, one of which laid out the foundations of what is now solid-state physics.
Dismissed by the Nazis
In 1932 Bethe became an acting assistant professor at Tübingen. In April 1933 the new Nazi government passed the Law for the Restoration of the Professional Civil Service, and because of his Jewish background he was dismissed. The experimental physicist Hans Geiger, his senior colleague, would not help him, but Sommerfeld at once restored his Munich fellowship and spent much of that summer finding places abroad for Jewish students and colleagues.
Bethe left Germany in October 1933 for a temporary lectureship at Manchester, where he lodged with his friend Rudolf Peierls, another refugee physicist, and Peierls’s wife Genia. Peierls drew him into nuclear physics; challenged by James Chadwick, the two worked out a theory of the photodisintegration of the deuteron on a four-hour train journey. After a term at Bristol he accepted an offer from Cornell University, and arrived in the United States in February 1935. He later wrote to Sommerfeld that he felt much more at home in America than he had ever been in Germany, as if he had been born there only by mistake.
After the Kristallnacht pogrom of November 1938 his mother, frightened to remain in Germany, was able to emigrate to the United States in 1939 under the French quota, thanks to her Strasbourg birth.
How the stars shine
At Cornell, with Robert Bacher and Stanley Livingston, Bethe wrote three long articles in the Reviews of Modern Physics in 1936–37 that summarised everything then known about the nucleus. Physicists called them “Bethe’s Bible”, and they served as the field’s textbook for years.
In March 1938 Edward Teller persuaded a reluctant Bethe to attend a small conference in Washington on the energy of the stars. By the end of the conference, working with Charles Critchfield, he had sketched the proton-proton chain that powers the Sun; back at Cornell he worked through the relevant reactions and found the carbon-nitrogen-oxygen cycle that dominates in hotter, more massive stars. His key achievement, as the Nobel Foundation put it, was the exclusion of other possible reactions. Needing $250 to release his mother’s furniture after her emigration, he first submitted the paper for a $500 prize from the New York Academy of Sciences, which it won, before publishing it in the Physical Review in 1939. It earned him 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 September 1939 he married Rose Ewald, daughter of his former mentor Paul Ewald, whom he had met while lecturing at Duke. They had two children, Henry and Monica. He became an American citizen in March 1941.
Los Alamos
Once cleared for secret work, Bethe joined the MIT Radiation Laboratory, where he invented a directional coupler for radar waveguides. In 1942 he took part in Robert Oppenheimer’s summer study at Berkeley on the design of an atomic bomb; he later said the fission bomb had to be built because the Germans were presumably building it. When Oppenheimer founded the Los Alamos laboratory in 1943, he made Bethe head of its Theoretical Division.
Bethe’s division calculated the critical mass of uranium-235, the efficiency of a nuclear explosion and, with Richard Feynman, a formula for the explosive yield. After the laboratory turned to the implosion design for plutonium, he worked on its hydrodynamics, on the neutron initiator and on radiation from the explosion. The Trinity test of 16 July 1945 confirmed his division’s calculations, and the same implosion design was used in the bomb dropped on Nagasaki.
The physicist of conscience
After the war Bethe opposed a crash programme to build the hydrogen bomb, and wrote that a war won with H-bombs would be remembered for its methods rather than its ideals. Yet after the outbreak of the Korean War he went back to Los Alamos and helped to develop it, hoping, he said, that he might still be a force for disarmament there, though sometimes he wished he were a more consistent idealist. In 1954 he testified on behalf of Oppenheimer at the security hearing that cost Oppenheimer his clearance, a stand that permanently damaged his old friendship with Teller.
For the next half-century Bethe was among the most respected scientific voices on arms control. He served on the President’s Science Advisory Committee from 1956 to 1964, headed a 1958 presidential study of disarmament and helped to bring about the 1963 treaty banning atmospheric nuclear tests. With Richard Garwin he argued in 1968 that a proposed missile-defence system could easily be defeated with decoys, and in the 1980s he opposed President Reagan’s Strategic Defense Initiative. In 1995, at eighty-eight, he called on scientists everywhere to cease and desist from work on nuclear weapons. He remained an advocate of nuclear power, and after Chernobyl concluded that the disaster revealed the failings of the Soviet system more than of nuclear energy itself.
The supreme problem-solver
Bethe’s science never stopped. In 1947, on a train from New York to Schenectady, he calculated the Lamb shift, the tiny difference between two energy levels of hydrogen that pre-war quantum electrodynamics could not explain. His three-page paper showed that the theory’s troublesome infinities could be tamed by renormalisation and opened the modern era of particle physics; Feynman called it the most important discovery in the history of the theory of quantum electrodynamics.
He went on to study nuclear matter, neutron stars and, from 1978 with Gerald Brown, the mechanics of supernova explosions. At eighty-five he helped explain the solar neutrino problem, and in his nineties he worked on the merging of neutron stars and black holes for the gravitational-wave observatory LIGO. He published important research in every decade of a career that spanned nearly seventy years. His former student Freeman Dyson called him “the supreme problem-solver of the 20th century”. Bethe loved hiking in the Alps and the Rockies, and collected stamps.
He also had a dry sense of humour. As a young postdoc in Cambridge he co-wrote a spoof paper that ‘derived’ the fine-structure constant from the temperature of absolute zero, mocking numerological physics; the authors had to apologise. In 1948 George Gamow added Bethe’s name, without asking, to a paper on the origin of the elements by Ralph Alpher and himself, so that the authors would read “Alpher, Bethe, Gamow”, like the first letters of the Greek alphabet. Bethe, one of the journal’s reviewers, saw the manuscript and simply struck out the words “in absentia” after his name. Of the hydrogen bomb he later said that Stanislaw Ulam was its father and Teller its mother, and that he himself was the midwife.
Death and legacy
Hans Bethe died of congestive heart failure at his home in Ithaca, New York, on 6 March 2005, aged ninety-eight, survived by his wife Rose and their children. He had spent his entire career at Cornell, apart from his war work and sabbaticals.
His honours included the Enrico Fermi Award, the National Medal of Science, the Max Planck Medal and the Bruce Medal. Cornell named one of its residential colleges after him, the American Physical Society created the Hans Bethe Prize, and the University of Bonn named its Bethe Center for Theoretical Physics in his honour. A refugee driven out of Germany because of his mother’s Jewish ancestry, he explained the light of the stars, helped to build the most terrible weapon of his age, and spent the rest of his long life trying to contain it.
Sources & further reading
Hans Bethe, Biographical, NobelPrize.org · Hans Bethe, Facts, NobelPrize.org · Hans Bethe, Atomic Heritage Foundation · Hans Bethe, Wikiquote · Hans Bethe, Wikipedia · Researched from the sources above and checked against them. Spotted an error? Tell us.
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