Hans Bethe

Hans Bethe: The Man Who Explained Sunlight

In 1938 a thirty-two-year-old refugee at Cornell worked out where the light of the stars comes from. The CNO cycle — carbon catalysing the fusion of hydrogen into helium — settled the primary energy source of the heavier stars on the main sequence, and eventually won him a Nobel Prize. Four years later the same man was in the New Mexico desert, running the division that calculated how much of a city an atomic bomb would destroy. Hans Bethe spent the rest of his very long life trying to get the second discovery back under control.

Strasbourg to Sommerfeld

He was born on 2 July 1906 in Strasbourg, then part of German Alsace–Lorraine, to Anna Kuhn and Albrecht Bethe, a professor of physiology. He entered the University of Frankfurt in 1924 and took his doctorate at Munich in 1928 under Arnold Sommerfeld, the greatest supervisor of theoretical physicists of the age, with a thesis on the theory of electron diffraction in crystals. The productivity started immediately and never really stopped: the Bethe formula for the energy loss of charged particles passing through matter in 1930, the Bethe ansatz — a method for finding exact solutions of quantum many-body models that remains central to mathematical physics — in 1931.

In 1933 he was dismissed from the University of Tübingen because of his Jewish ancestry. He went to England, and in 1935 to Cornell University in Ithaca, New York, on a salary of $3,000. He stayed for seventy years. He married Rose Ewald on 13 September 1939, became an American citizen in 1941, and later wrote to Sommerfeld: "I am much more at home in America than I ever was in Germany. As if I was born in Germany only by mistake, and only came to my true homeland at 28."

Why the Stars Shine

Stellar nucleosynthesis was the problem of the decade, and Bethe solved the hard part of it. His account of the carbon–nitrogen–oxygen cycle explained how massive stars convert mass into radiation, and by extension why the sky is full of light rather than cinders. It was recognised with the 1967 Nobel Prize in Physics, nearly thirty years after the fact.

Los Alamos

From 1942 to 1945 Bethe headed the Theoretical Division at Los Alamos, the group responsible for turning physics into ordnance. His people calculated the critical mass of uranium-235, developed the theory of the implosion bomb, and worked on neutron initiators and the propagation of radiation. With Richard Feynman he devised the Bethe–Feynman formula for estimating the explosive yield of a fission weapon. His justification was the one most of his colleagues used, and he never dressed it up: "The fission bomb had to be done, because the Germans were presumably doing it."

The hydrogen bomb was different, and he was never comfortable about it. In 1968 he described his own reasoning with unusual candour: "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." Asked who fathered the thing, he gave the definitive answer: "Ulam is the father, because he provided the seed, and Teller is the mother, because he remained with the child. As for me, I guess I am the midwife."

Twelve Equations

Then, in 1947, came the calculation that colleagues still describe with something close to awe. Experimenters had found a tiny anomaly in the energy levels of hydrogen — the Lamb shift — that the existing theory could not accommodate. Bethe computed it in a paper three pages long containing twelve mathematical equations, showing that the infinities plaguing quantum electrodynamics could be handled and that the theory was fundamentally sound. Feynman called it "the most important discovery in the history of the theory of quantum electrodynamics." Paul Dirac called it "the most important calculation in physics for decades." Modern QED, the most precisely tested theory in science, begins there.

The Long Argument

Having helped build the weapons, Bethe spent five decades arguing against them. He campaigned against nuclear testing with Albert Einstein through the Emergency Committee of Atomic Scientists and worked on successive American administrations, helping to bring about the 1963 Partial Nuclear Test Ban Treaty and the 1972 Anti-Ballistic Missile Treaty. It was a peculiarly Bethe kind of activism — technical, patient, conducted through committee papers and yield calculations rather than manifestos, and effective for exactly that reason. He remained at Cornell throughout as the John Wendell Anderson Professor of Physics, publishing into his nineties.

Why Hans Is Called a Genius

The specific quality here is problem-solving capacity at industrial scale: the ability to take a physical situation nobody had reduced to numbers — a star, a shock wave, an anomalous spectral line — and produce the number. His doctoral student Freeman Dyson called him "the supreme problem-solver of the 20th century." The Lamb shift paper is the exhibit: three pages, twelve equations, the rescue of quantum electrodynamics, and two of the most eminent physicists alive calling it the most important calculation in decades. Very few people have contributed at the top level to solid-state physics, nuclear reaction theory, astrophysics, weapons design and quantum field theory; Bethe did all five.

The honest counter-case is that Bethe was not a revolutionary. He founded no framework and overturned no world-picture; he worked inside quantum mechanics as Born and Heisenberg had left it and inside quantum electrodynamics as Dirac had left it, and his signature achievements are computations within other people's theories, however brilliant. Sceptics of a different kind point at the moral ledger: he made the bomb work and then spent fifty years lobbying against its consequences, and his own explanation — that someone else would have done it anyway, and that he could be a force for disarmament from inside — is the standard defence of every weapons scientist who ever wanted it both ways. Bethe knew that. His refusal of paternity for the H-bomb, and his insistence on the word "midwife," reads like a man keeping his own accounts.

Legacy

The honours accumulated across six decades: the Henry Draper Medal in 1947, foreign membership of the Royal Society in 1957, the Franklin Medal in 1959, the Eddington Medal and the Enrico Fermi Award in 1961, the Rumford Prize in 1963, the Nobel Prize in 1967, the National Medal of Science in 1975, the Lomonosov Gold Medal in 1989, the Oersted Medal in 1993, the Bruce Medal in 2001, and the Benjamin Franklin Medal posthumously in 2005. He died at Ithaca on 6 March 2005, aged ninety-eight, having outlived nearly everyone he worked with and having explained, along the way, why there is daylight.