The sun is an ordinary yellow dwarf star that has been active for 4.6 billion years. Every second, five million metric tons of mass are converted into nuclear energy, equivalent to the detonation of 90 billion one-megaton hydrogen bombs. It is this constant blast of nuclear reactions that pushes energy to the surface, releasing it as light and heat.
Hans Bethe (1906-2005) did more than anyone else to solve the mystery of what makes the sun shine. His astonishing career spanned eight decades and included fundamental research in nuclear physics, astrophysics and many other fields. "If you know his work," said collaborator John Bahcall, "you might be inclined to think he is really several people, all of whom are engaged in a conspiracy to sign their work with the same name."
"The history of theoretical physics is a history dominated by 'off-scale' people," argues Silvan Schweber in "Nuclear Forces," borrowing a term from the laboratory. Most scientists analyze data within accepted frameworks; only a few can absorb complex data and produce a new synthesis of knowledge. "As a physicist," explains Mr. Schweber, "Bethe was a pragmatic, highly mathematically skilled problem solver." His way of theorizing combined thoroughness and rigor with conceptual lightness, simplicity and back-of-the-envelope insight.
Mr. Schweber, who worked briefly as a post-doc with Bethe in the 1950s, also wrote "In the Shadow of the Bomb" (2006), a fine study in parallel lives that contrasted the dangerous ambition of Robert Oppenheimer, the father of the atom bomb, with the workmanlike modesty of Bethe, whom Oppenheimer appointed head of the theoretical division of the Manhattan Project. Now Mr. Schweber returns to Bethe's youth to explain how "Bethe became Bethe." Though scholarly, "Nuclear Forces" is a highly readable account of a remarkable period in physics, tracing the future Nobel laureate through his formative years and up to the eve of World War II.
Born in July 1906 in Strasbourg, then a part of Germany, Bethe showed a childhood enthusiasm for science and mathematics. After university studies in Frankfurt, Bethe went to Munich, where in 1928 he earned a Ph.D. in theoretical physics under Arnold Sommerfeld, who had done pioneering mathematical work on quantum theory (among much else). At Munich, Sommerfeld created what he described as "a nursery of theoretical physics" that attracted talented young theorists, including the future definers of quantum mechanics Werner Heisenberg and Wolfgang Pauli.
The author plots Bethe's scientific progress against the backdrop of the cultural environment during the turbulent interwar years of German hyperinflation and political strife. Though Bethe never considered himself Jewish, he was dismissed from his university post in April 1933 because of his mother's Jewish roots. Most of Bethe's friends came from secular, assimilated Jewish families who under the Kaiser had expected, as Mr Schweber explains, that being "outstanding, creative, original and productive in mathematics or in the sciences—and recognized and accepted as such by the leading authorities and practitioners in these fields—was seen as trumping all supposed deficiencies, including Jewish roots." By the time the Nazis seized power in 1933, this was no longer true.
Sommerfeld, a patriotic German nevertheless disgusted by the treatment of his protégés, helped place many in England and America. Bethe secured a position at the universities of Manchester and Bristol and then, in 1935, went to Cornell, which he helped establish as a major center of physics that soon attracted the likes of Richard Feynman and Freeman Dyson. Despite the threat of war and the fallout from a broken engagement, Bethe later said that "the thirties were a very happy period." Before the end of the decade, he would make his most famous contribution to physics.
In 1938, Bethe attended a conference where he heard the Danish astrophysicist Bengt Strömgren challenge his colleagues to discover the chain of nuclear reactions that could generate the energy required to give rise to the observed luminosities of the sun and other stars. Bethe solved the problem in a matter of weeks, proposing two mechanisms by which different types of stars released the energy they radiated; he eventually worked out a six-step cycle in which carbon and nitrogen act as catalysts in producing a helium nucleus from hydrogen atoms. Even before the conference had ended, Bethe had worked out how a "proton-proton" chain-reaction mechanism could also build a helium nucleus from hydrogen.
The rates at which these two cycles occur in stars is sensitive to temperature, and Bethe concluded that for stars with internal temperatures greater than that of the sun, the carbon cycle predominates. For the sun and cooler stars, the proton-proton reaction is the driving force. The net result of the cycle of nuclear fusion is a 1% difference in mass that accounts for the energy released.
Bethe's insight into star power earned him a Nobel Prize in 1967 but would earlier make him crucial to the efforts to unleash atomic power. He regarded America's participation in the World War II as inevitable and desperately wanted to play his part. But he had to wait until he became a U.S. citizen in 1941 before being allowed to work on classified projects such as the development of radar at MIT. Shortly after, he renewed his acquaintance with Oppenheimer, whom he had first met in 1929 in Germany, and was appointed to lead the theorists at Los Alamos in explaining how the atomic bomb would work and produce its devastating effect. Bethe was 37, while the average age of those working on the bomb was 27.
After the war, Bethe initially refused to participate in the development of the hydrogen bomb, which relied on the fusion process he had described. He soon changed his mind. "If I did not work on the bomb, somebody else would—and I had thought if I were around Los Alamos I might still be a force for disarmament," he later wrote. "Sometimes I wish I were a more consistent idealist." As the years passed that is exactly what he became, in the best possible sense. On the 50th anniversary of Hiroshima he called "on all scientists in all countries to cease and desist from work creating, developing, improving and manufacturing further nuclear weapons—and, for that matter, other weapons of potential mass destruction such as chemical and biological weapons."
Of course, Mr. Schweber ends his fine book in September 1939, as Bethe is about to embark on his war work—which means that we must ask the author for more, please.
—Mr. Kumar is the author of "Quantum: Einstein, Bohr, and the Great Debate About the Nature of Reality."
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