Hans Bethe

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Hans Bethe : biography

July 2, 1906 – March 6, 2005

A major talking point at the conference was the discovery by Willis Lamb and his graduate student Robert Retherford shortly before the conference began that one of the two possible quantum states of hydrogen atoms had slightly more energy than predicted by the Paul Dirac’s theory; this became known as the Lamb shift. Oppenheimer and Weisskopf suggested that this was a result of quantum fluctuations of the electromagnetic field. Pre-war quantum electrodynamics (QED) gave absurd, infinite values for this; but the Lamb shift showed that it was both real and finite. Hans Kramers proposed renormalization as a solution, but no one knew how to do the calculation.

Bethe managed to work it out on the train from New York to Schenectady. He arrived at a value of 1040 MHz, extremely close to that obtained experimentally by Lamb and Retherford. He did so by realising that it was a non-relativistic process, which greatly simplified the calculations. His paper, published in the Physical Review in August 1947 was only two pages long and contained just 12 mathematical equations, but was enormously influential. Hitherto, it had been assumed that the infinities meant that QED was fundamentally flawed, and that a new, radical theory was required. Bethe demonstrated that this was not necessary.

One of Bethe’s most famous papers is one he never wrote: the 1948 Alpher-Bethe-Gamow paper. George Gamow added Bethe’s name (in absentia) without consulting him, knowing that Bethe would not mind, and against Ralph Alpher’s wishes. As one of the Physical Review’s reviewers, Bethe saw the manuscript and struck out the words "in absentia".

Astrophyics

Bethe believed that the atomic nucleus was like a quantum liquid drop. He investigated the nuclear matter problem by considering the work done by Keith Brueckner on perturbation theory. Working with Jeffrey Goldstone, he produced a solution for the case where there was an infinite hard-core potential. Then, working with Baird Brandow and Albert Petschek, he came up with an approximation that converted the scattering equation into an easily-solved differential equation. This then led him to the Bethe-Faddeev equation, a generalisation of Ludvig Faddeev’s approach to three-body scattering. He then used these techniques to examine the neutron stars, which have densities similar to those of nuclei.

Bethe continued to do research on supernovae, neutron stars, black holes, and other problems in theoretical astrophysics into his late nineties. In doing this, he collaborated with Gerald E. Brown of the State University of New York at Stony Brook. In 1978, Brown proposed that that they collaborate on supernovae. These were reasonably well understood by this time, but the calculations were still a problem. Using techniques honed from decades of working with nuclear physics, and some experience with calculations involving nuclear explosions, Bethe tackled the problems involved in stellar gravitational collapse, and the way in which various factors affected a supernova explosion. Once again, he was able to reduce the problem to a set of differential equations, and solve them.

At age 85, Bethe wrote an important article about the solar neutrino problem, in which he helped establish the conversion mechanism for electron neutrinos into muon neutrinos proposed by Stabnislav Mikheyev, Alexei Smirnov and Lincoln Wolfenstein to explain a vexing discrepancy between theory and experiment. Bethe argued that physics beyond the Standard Model was required to understand the solar neutrino problem, because it assumed that neutrinos have no mass, and therefore cannot metamorphosise into each other; whereas the MSW effect required this to occur. Bethe hoped that corroborating evidence would be found by the Sudbury Neutrino Observatory (SNO) in Ontario, Canada, by his 90th birthday, but he did not get the call from SNO until June 2001, when he was nearly 95.

In 1996, Kip Thorne approached Bethe and Brown about LIGO, the Laser Interferometer Gravitational-Wave Observatory, which would be measuring the gravitational waves from merging neutron stars and black holes. Since Bethe and Brown were good at calculating things that could not be seen, could they look at the mergers? The 90-year old Bethe quickly became enthused, and soon began the required calculations. The result was a 1998 paper on the "Evolution of Binary Compact Objects Which Merge", which Brown regarded as the best that the two produced together.

Hydrogen bomb

After the war, Bethe argued that a crash project for the hydrogen bomb should not be attempted, though after President Harry Truman announced the beginning of such a crash project, and the outbreak of the Korean War, Bethe signed up and played a key role in the weapon’s development. Though he would see the project through to its end, in Bethe’s account he personally hoped that it would be impossible to create the hydrogen bomb. He would later remark in 1968 on the apparent contradiction in his stance, having first opposed the development of the weapon and later helping to create it:

As for his own role in the project, and its relation to the famous Teller-Ulam priority dispute, Bethe later said that: {}

In 1954, Bethe testified on behalf of Oppenheimer during the latter’s high-profile security clearance hearing. Specifically, Bethe argued that Oppenheimer’s stances against developing the hydrogen bomb in the late 1940s had not hindered its actual development, a topic which was seen as a key motivating factor behind the hearing. Bethe contended that the developments which led to the successful Teller-Ulam design were a matter of serendipity and not a question of manpower or logical development of previously existing ideas. During the hearing, Bethe and his wife also tried hard to convince Edward Teller against testifying. However, Teller did not agree, and his testimony played a major role in the revocation of Oppenheimer’s security clearance. While Bethe and Teller had been on very good terms during the pre-war years, the conflict between them during the Manhattan Project, and especially during the Oppenheimer episode, permanently marred their relationship.