Bethe , Hans Albrecht

(1906–) German–American physicist

Bethe, the son of an academic, was born in Strasbourg in France and educated at the universities of Frankfurt and Munich, where he obtained his doctorate in 1928. He taught in Germany until 1933 when he moved first to England and, in 1935, to America. In America he held the chair of physics at Cornell from 1937 until his retirement in 1975, serving in the war as director of theoretical physics at Los Alamos from 1943 to 1946.

Bethe soon established a reputation for his impressive knowledge of nuclear reactions. This was, in part, based on three review articles on nuclear physics he published in 1936 and 1937, which have been described as the first presentation of this field as a branch of science and which are sometimes known as ‘Bethe's Bible’.

In 1938 Bethe was invited by Edward Teller to contribute a paper on astrophysics for a conference he was organizing. Bethe at first pleaded ignorance of the subject but, under pressure from Teller, he finally agreed to search for a relevant topic. He noted that most astrophysicists seemed to be puzzling over the origin of the chemical elements. He therefore decided to consider another issue, namely, the sources of stellar energy. He managed to find, as he reported in his 1939 paper Energy Production in Stars, “…the only nuclear reaction which gives the correct rate of stellar energy production within the limits of the theory.”

Bethe was referring to the carbon cycle (or CNO cycle). The cycle begins with a hydrogen nucleus or proton (¹H) and a carbon-12 atom; it has six stages: ¹²C + ¹H -> ¹³N + γ¹³N ->¹³C + e⁺ + ν¹³C + ¹H -> ¹⁴N + γ¹⁴N + ¹H -> ¹⁵O + γ¹⁵O ->¹⁵N + e⁺ + ν¹⁵N + ¹H -> ¹²C + He⁴

Here, γ is a gamma ray, e⁺ a positron, and ν a neutrino. The net result of the cycle is to convert four protons (4¹H) into a helium nucleus (He⁴), while the carbon–12 atom remains available after step 6 to repeat the cycle once more. In the process 27 MeV (million electron volts) are released.

Although the amount of energy produced per cycle is modest, the large amount of stellar matter involved is sufficient to generate the enormous energies met with inside stars. Bethe's CNO cycle, however, gives no indication of the origin of the carbon–12 that starts the cycle. It was left to Fred Hoyle and his colleagues to resolve this issue in the 1950s. Bethe did, however, contribute, with Ralph Alpher to George Gamow's famous 1948 alpha-beta-gamma paper on the origin of the elements and the big bang. Unfortunately their paper advanced no further than the isotopes of hydrogen and helium.

Bethe has also played a significant part in public affairs. He quarreled in the 1940s with Teller on the need to build the hydrogen bomb and in the 1980s over the viability of the US Strategic Defense Initiative (known as ‘Star Wars’). In 1958 he served as a delegate to the Geneva Conference which negotiated the first test-ban treaty. He also worked for the 1963 ban on atmospheric testing. More recently, in 1992, Bethe called upon the US and Russia to reduce their nuclear arsenals to a thousand warheads each.

Bethe has also continued to work at physics in his retirement. He has collaborated with John Bahcall on several papers dealing with the solar-neutrino problem. He has also begun to tackle the problem of explaining how stars explode. Modern computer models, he complains, lead to moderate explosions when compared with the massive eruptions of a supernova.

For his earlier work on the theory of nuclear reactions, and for his contributions to astrophysics, Bethe was awarded the 1967 Nobel Prize for physics.