One of the unquestioned giants of physics and astrophysics, Hans Bethe, died on 6 March 2005, at the venerable age of 98, in his home town of Ithaca, New York. Seven decades of contributing to research and a Nobel Prize for his work on stellar hydrogen burning make a listing of his honors superfluous (besides being impossible in this space).
Bethe was born in Strassburg, in then German Alsass Lothringen, on 2 July 1906. His father, Albrecht Julius Bethe (1872-1954), taught physiology at the University, and his mother, Anna Kuhn (1876-1966), was a musician and writer. Both his grandfathers were physicians. He spent his youth in Strassburg, Kiel, and Frankfurt, and some time in sanatoria due to tuberculosis. Hans's first scientific paper, at age 18, was with his father and a colleague, on dialysis. His education and early career in Germany brought him into contact with many top stars in the quantum revolution.
Starting in Frankfurt in chemistry, Bethe soon switched to physics, taught there by Walter Gerlach and Karl Meissner, among others. In 1926, he successfully applied to join Arnold Sommerfeld's group in Munich, where he met one of his later long-term collaborators, Rudolf Peierls. Bethe considered his entry into physics to have come at an ideal time, with the new ideas of wave mechanics being developed and discussed right there; it was certainly also at an ideal place. His doctoral thesis was on the theory of electron diffraction by crystals, following the experimental work by Clinton Davisson and Lester Germer and the work on X-ray diffraction by Max von Laue and Paul Ewald.
The newly minted doctor went from there briefly to Frankfurt and then to Ewald in Stuttgart, where he felt at home academically and personally. In 1939, Bethe would marry Ewald's daughter Rose. Not much later, though, Sommerfeld recalled him to Munich, where Sommerfeld created a Privatdozent position for him. There he worked out the solution for a linear chain of coupled spins by what we now call the "Bethe Ansatz." Soon after his acceptance of an assistant professorship at Tübingen in 1932, he had to flee Hitler's Germany because his mother was Jewish. Bethe went to the Bragg Institute in Manchester, England, where he worked again with Peierls. In 1934, Cornell University unexpectedly offered him a position as part of R. Clifton Gibbs's expansion of the physics department; he accepted and stayed there for the rest of his life.
Right from the start, Bethe enjoyed America and its atmosphere very much. His first activity there was to write the "Bethe Bible": three articles in Reviews of Modern Physics to educate his colleagues in theoretical nuclear physics. Then he did the work that astrophysicists will still appreciate him most for, and which brought him the 1967 Nobel Prize.
Having worked with George Gamow's student Charles Critchfield (at Gamow's suggestion) on the proton-proton chain for nuclear fusion in the Sun (published in 1938), Bethe was initially a bit discouraged with Arthur Eddington's estimates of the Solar core temperature; their calculations did not agree well with the observed solar luminosity. However, at the Washington conference in 1937, he heard of Strömgren's new estimates of the solar interior, which brought his and Critchfield's theory into much better agreement with the data. Fairly soon after the meeting, Bethe also worked out the process whereby more massive stars must accomplish hydrogen fusion, in what we now call the CNO cycle.
Curiously, Bethe held up its publication briefly in order to compete for a prize for the best unpublished paper on energy production in stars. He did win, and used the money in part to bring his mother to the United States; eventually, the paper appeared in Physics Review in 1939, and founded a whole branch of astrophysics.
The war brought Bethe to the Manhattan project, of which he became one of the intellectual leaders. He ploughed through problems theoretical and practical by attacking them head-on and not allowing himself to be side-tracked by those who would deem the problem be much more complex and difficult, moving straight forward like an intellectual battleship ("The H.A. Bethe Way," as his collaborator Gerald E. Brown would dub the style). Bethe's involvement in the Project brought to light his abilities in the managerial and political arena, which he used later to much effect to influence the wider world; he was among those who fought hard during the Cold War to contain the impact of the terrible weapons he had helped invent. As his two children, Henry and Monica, were born, the war years also made him a family man. As his father did with him, he often took them on long walks, in the hills around Ithaca or further afield; he much enjoyed walking, and mountains.
Just after the war, during and following the June 1947 Shelter Island Conference, Bethe made another of his great contributions to physics—some might say his greatest. The experiments by Willis Lamb and Robert Retherford, on what came to be known as the "Lamb shift," were discussed, and during the meeting the assembled crowd (Richard Feynman, Julian Schwinger, and Hendrick Kramers among them) got stuck on the infinities of QED. During the train ride home, Bethe managed to compute the correct answer by realizing that the complex QED machinery could be bypassed, the H.A. Bethe Way.
His 1967 Nobel Prize spurred a brief revival of Bethe's interest in astrophysics, but his work in the following years continued to focus on nuclear physics and dense matter (and disarmament and nuclear power, of course). In 1978 he re-entered astrophysics with a bang: Bethe was losing interest in nuclear physics and, after a few years of trying, Gerry Brown lured him back to astrophysics during a stay at the Nordic Institute for Theoretical Physics (NORDITA). The refugee from Hitler and the refugee from McCarthy jointly attacked the problem of supernova collapse. Bethe had the crucial insight that the low entropy of massive stellar cores would cause them to collapse to well above nuclear density, contrary to prevailing opinion. With James Applegate and James Lattimer, they published their finding in the BBAL ('"babble") paper of 1979.
After that, astrophysics never quite left Bethe again, and with Brown (his "junior collaborator"), he took an interest in the fate of massive stars and black holes more generally. The series of papers on formation of black holes, gamma-ray bursts, and gravity-wave sources continued until close to his death. These papers are done very much the H.A. Bethe Way, often finding simple approximations to much more complicated work of others, and are quite straightforward.
An inevitable part of the Bethe-Brown collaboration was a January stay in California; during the 1999 edition I had the good fortune of becoming a small footnote to the great Bethe story. Gerry and Hans invited me to join them for a while, to discuss issues of binary star evolution and population synthesis. I have to admit to being rather taken aback by the way in which the 93-year old gave me a good intellectual runaround every day. And yet, as many others have commented, there was nothing facetious or overbearing in his manner: He made me feel like a valuable colleague and welcome guest. Good meals were an essential part of Hans's every day, and during a dinner prepared by Rose Bethe and Betty Brown, the old stories surfaced. I could not resist asking about the legendary story of Rose and Hans's evening walk under the stars. Hans, so says the story, tried to impress his fiancée by commenting that at that moment, he was probably the only person on Earth who understood why the stars shine. Hans grinned a bit sheepishly, but Rose roundly confirmed the story with a big smile. Not too impressed, she had replied: "That's nice." And so it was.