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Bethe also worked for the adoption of the Comprehensive Test Ban Treaty. Bethe worked on the problems of re-entry of missiles and rockets into the earth’s atmosphere, an area which typically goes under the heading of space-physics. After his work on the energy production in stars, Bethe worked on astrophysics problems off and on. In 1948 the article The Origin of Chemical Elements by R. Alpher, H. A. Bethe, and G. Gamow was published in Physical Review Letters. Actually the work was done by Alpher and Gamow, but Gamow so liked the sound of alpha, beta, gamma, that he sent the paper in with the authors listed as Alpher, Bethe (in abstentia), and Gamow. Bethe was the paper’s reviewer and his only change was to cross out the words ”in abstentia”. In the 1960s Bethe worked on white dwarf stars with R. Marshak, and in the 1970s with the discovery of neutron stars, he turned his attention to developing a theory of nuclear matter and the equation of state in neutron stars. This work led him to work on applying the theory of nuclear matter to finite nuclei which in turn led him to collaborate with G. E. Brown. In 1979, while visiting Brown in Copenhagen, Brown suggested they study the explosion mechanism of ”core-collapse” supernovae. That is the final fate of massive stars which can fuse elements all the way up to iron in their cores. These stars collapse and produce neutron stars or black holes, but they also expel the heavy elements made during their lifetime into the interstellar medium (which allows planets and life to exist). Bethe and his collaborators worked initially on the prompt mechanism of core-collapse whereby the incompressibility of nuclear matter creates a shock wave which expels the outer parts of the star and leaves behind either a neutron star or a black hole. But Bethe also wrote an influential article with J. Wilson interpreting one of Wilson’s numerical calculations and establishing the delayed mechanism, where the shock stalls due to lack of energy, but is re-invigorated by the deposition of energy from neutrinos released by the proto-neutron star and infalling matter. The exact mechanism of core-collapse is still a subject of active research. During the 1980s Bethe, Brown, J. Cooperstein, and students would spend January at either Caltech or Santa Barbara (to escape the harsh weather of Ithaca). That month was spent doing physics, cooking, and hiking in the nearby mountains. It is often said that Bethe had two passions: physics and hiking. With Brown, Bethe turned his attention to the formation of stellar black holes in the 1990s. In 1934, with Peierls, Bethe wrote an article on the possibility of observing free neutrinos. Not foreseeing that large neutrino fluxes would be available at nuclear reactors and particle accelerators, they were pessimistic about the chance of directly observing neutrinos (Reines and Cowan observed free neutrinos in 1956, Reines won the Nobel Prize in 1995). However, Bethe particularly followed the work on observing neutrinos from the sun that was pioneered by Ray Davis in the late 1960s. Recognizing an important paper of Mikheyev and Smirnov in 1986, Bethe wrote a Physical Review Letter showing how the work of Mikheyev and Smirnov (and previous work of L. Wolfenstein) could reconcile the theoretical predictions for a larger neutrino flux than was observed by Davis and later neutrino detectors. The most recent observations at the Sudbury Neutrino Observatory in 2001 has shown this interpretation to be correct. Bethe is responsible for training nearly four generations of physicists in solid state, atomic, nuclear, particle, and astrophysics, an immense legacy.

2 References: Perspectives in Modern Physics, Essays in Honor of Hans A. Bethe, R. E. Marshak and J. W. Blaker, eds., (New York: Wiley Interscience) 1966. Prophet of Energy: Hans Bethe, Jeremy Bernstein, (New York: Elesevier-Dutton) 1981. From a Life in Physics, H. A. Bethe et al., (Singapore: World Scientific) 1989. Selected Works of Hans A. Bethe With Commentary, World Scientifc Series in 20th Century Physics, Vol. 18, Hans A. Bethe, (Singapore: World Scientific) 1996.

Eddie Baron Homer L. Dodge Dept. of Physics and Astronomy University of Oklahoma 440 W. Brooks, Rm 100 Norman, OK 73019-2061 USA

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