Hans Bethe

Nobel prizewinner who worked on the atomic bomb and explained how the sun shines

Hans Bethe won the Nobel Prize for solving a riddle of nature that had puzzled humankind for millennia: what is it that makes the sun shine and the stars twinkle? It was just one of several fundamental contributions to science that placed him in the pantheon of 20th-century physics alongside friends and collaborators such as Robert Oppenheimer, Paul Dirac, George Gamow, Edward Teller, Enrico Fermi, Rudolf Peierls, Richard Feynman and Freeman Dyson.

Hans Albrecht Bethe, theoretical physicist: born Strasbourg, Germany 2 July 1906; Lecturer, Manchester University 1933-34; Lecturer, Bristol University 1934-35; Assistant Professor of Theoretical Physics, Cornell University 1935-37, Professor 1937-75; Head of Theoretical Physics Division, Los Alamos Atomic Scientific Laboratory 1943-46; consultant, Atomic Energy Commission 1945-64; Nobel Prize in Physics 1967; married 1939 Rose Ewald (one son, one daughter); died Ithaca, New York 6 March 2005.

Hans Bethe won the Nobel Prize for solving a riddle of nature that had puzzled humankind for millennia: what is it that makes the sun shine and the stars twinkle? It was just one of several fundamental contributions to science that placed him in the pantheon of 20th-century physics alongside friends and collaborators such as Robert Oppenheimer, Paul Dirac, George Gamow, Edward Teller, Enrico Fermi, Rudolf Peierls, Richard Feynman and Freeman Dyson.

Shaped in what he called the "golden times" of inter-war theoretical physics in continental Europe, he fled Nazism via Britain for the United States, where he played a pivotal part in making the first atomic bombs. Later he helped with negotiations for the first nuclear test ban treaty and became a sharp critic of President Ronald Reagan's "Star Wars", space-based defence scheme.

And for almost four decades he led a world-class school of physics at Cornell University. One of those rare people who might well have won the Nobel Prize more than once, he combined powerful scientific gifts with a very shrewd eye for a promising problem. No other career better defines the mainstream of physics from the 1930s to the 1960s, for at every stage the choices he made for himself matched the direction in which the discipline as a whole was moving.

Hans Albrecht Bethe was born in Strasbourg in 1906 at a time when the city was part of Germany. His mother was Jewish, but the family was not religious and he never considered himself a Jew. His father was a distinguished physiologist and the household had a scientific ambience so the young Hans, a sensitive boy, soon developed an interest in mathematics and physics. Preferring physics because it seemed to offer a greater range of solvable problems, he studied first at the University of Frankfurt am Main before switching to Munich to take his PhD under Arnold Sommerfeld, a professor of worldwide renown.

By the age of 22 it seems that his defining characteristics were in place. "Tall and heavily built, he spoke very slowly, with a deep and sonorous voice," wrote a fellow student of the time, noting also that "this tended to give his statements the air of great authority". He had an extraordinary facility for mathematical calculation - not mental arithmetic, but the often hugely complex algebra of the subject. He was also a gargantuan eater, gregarious without being rowdy, loyal to his friends and practical, in the sense that he always worked with hard experimental results and was less comfortable than many of his colleagues with the philosophical side of physics.

Impressive though he was in 1928, he let several years pass before he made his reputation. Scholarships and teaching jobs took him to several German universities and then abroad, both to Cambridge, where he worked briefly under Ernest Rutherford, and to Rome, where he met Enrico Fermi. Returning to Germany to teach at Tübingen University in 1932, he was dismayed to find young men in swastikas among his students, and within a year had lost his job because of his Jewish background.

His first refuge was Britain, where he worked under Lawrence Bragg at Manchester and then with Nevill Mott at Bristol. He liked England, and would later remark:

The English had a much healthier attitude toward life than the Germans. The mystical element in the life philosophy of many Ger-

mans had always repelled me, and still does. In England, everything was clear and simple. I was happy.

He had arrived at an opportune moment. Physics had experienced its annus mirabilis in 1932, with the discovery of the neutron and the splitting of the atomic nucleus (both at Cambridge), and this dramatically opened up the field of nuclear studies. Prompted by James Chadwick, the discoverer of the neutron, Bethe now brought his skill and authority to cataloguing the dozens of nuclear reactions that were being discovered, making sense of them and predicting others that might follow.

In 1935 he moved to Cornell University, at Ithaca, New York, where he would spend the rest of his career, and over the next three years he wrote, with various collaborators, the work that became known as "Bethe's Bible". Appearing as articles in Reviews of Modern Physics, it brought a necessary measure of order to the creative chaos that nuclear physics had become, and scientists the world over were grateful. It also set Bethe on the way to something even bigger. In 1938 he was among a group of theoreticians who attended a gathering of astrophysicists at the Carnegie Institution in Washington DC, where the growing knowledge of the character of stars was set out and the fundamental question asked: how do they burn?

It was a question that had baffled philosophers and astronomers going back to ancient Greece, and doubtless long before them, and it took Hans Bethe six weeks to come up with the answer. Delving into his knowledge of the nuclear reactions, and drawing on his personal grasp of the theory, he identified two distinct processes that could explain the known phenomena, one valid for our Sun and the cooler stars, and the other for brighter stars such as Sirius A.

Fusion, the hugely violent process by which two atomic nuclei merge into one, was the key: groups of nuclei were fusing in elegant, cyclical sequences, and releasing energy as they did so. Published as a paper entitled "Energy Production in Stars", this work was a landmark discovery in physics.

With the outbreak of war in Europe Bethe became, by his own account, "desperate" to help fight Nazism, and felt that his scientific abilities were his best weapon. First he undertook research for the US armed forces into armour penetration and shock waves, and when the United States entered the war in 1942 he joined the huge effort to extend the military uses of radar (first developed in Britain). Soon afterwards, however, came another call, to join the Manhattan Project. On behalf of the government, Oppenheimer was building a team to design an atomic bomb in a secret lab at Los Alamos, New Mexico, and Bethe's is believed to have been the very first name on his list. Bethe accepted, and was given the position of head of theoretical physics.

By his own account he believed the bomb impossible until he witnessed Fermi working on the first nuclear reactor in Chicago. "I then became convinced that the atom bomb project was real, and that it would work," he said later.

As a theorist Bethe always liked to stay in close touch with experimenters and Los Alamos was the perfect environment for him. Oppenheimer trusted his systematic approach to problems and desperately needed the stream of ideas and insights that he constantly produced. Bethe was, for example, one of the originators of the subtle and complex implosion technique used in the Nagasaki bomb, and it was he who, with a few cool calculations, calmed things when alarm broke out over suggestions that an atom bomb might ignite the whole of the Earth's atmosphere.

He lived at Los Alamos with his wife, Rose Ewald, a German professor's daughter he had known in Stuttgart and whom he had married in 1939. They were leading lights in the life of the camp, where Rose ran the housing allocations strictly on the basis of need rather than rank: the larger the family, the bigger the home. Perhaps it is relevant that their two children were born there (though for security reasons the address on the birth certificates was a PO box number).

Bethe attended the first test of an A-bomb in the desert in July 1945, and while his colleagues spoke of doom he appears merely to have registered relief that his parts of the weapon had worked. It was in keeping with his matter-of-fact approach.

Unlike Edward Teller Bethe had no enthusiasm for nuclear weapons, but unlike some others he felt no guilt either. He rejected in particular the notion that scientists should have tried to dictate whether or how the bombs were used against Japan - it was not for them, he thought, to set themselves above politics. Nor did he believe that scientists should opt out of moral matters. They needed to play their part and give advice, and Bethe did so in the years that followed, serving on a variety of government bodies and working as a consultant. Back at Cornell, meanwhile, he dedicated himself, with considerable success, to recreating the creative atmosphere of Los Alamos in civilian mode.

In the later 1940s Bethe was an H-bomb sceptic - as Rose put it, and he agreed, there was "enough of a bang" from A-bombs - but the first Soviet A-bomb and the outbreak of the Korean War changed his mind and he returned several times to Los Alamos for weeks at a time to give his advice.

In 1954, however, came an event that shook him deeply: the hearings, often described as McCarthyite, which ended with Oppenheimer's being stripped of his official security clearance and effectively disgraced. Bethe stood by his friend and testified passionately in his support, but to no avail.

By the later 1950s his government work had changed character, and he was advising those negotiating for a nuclear test ban treaty, an endeavour eventually crowned only with partial success - effectively the outlawing of atmospheric tests. Thereafter he wound down his government work and concentrated on Cornell, and science.

He remained an excellent talent-spotter with a gift for matching problems to students, and his department thrived. His personal influence was great, and he never ceased to contribute to fundamental science, notably with early work in the field of quantum electrodynamics and further researches into the atomic nucleus and astrophysics.

In 1967 he was awarded the Nobel Prize in Physics "for his contributions to the theory of nuclear reactions, especially for his discoveries concerning energy production in stars". Given the long interval since that discovery, he chose to construe it as an award "for a lifetime of quiet work in physics", and professed himself very proud and happy.

Even after his retirement from teaching in 1975 the scientific papers continued to flow until there were more than 300 in total, and as his fellow veterans of the "golden times" faded away he also became a sort of oracle for historians and those opposed to the nuclear arms race. His voice was an important one in the opposition to "Star Wars" in the 1980s, just as it was in support of a total nuclear test ban in the 1990s.

Among his regrets in those late years was a feeling that physics had lost its sense of wonder:

No one any longer pays attention to - if I may call it - the spirit of physics, the idea of discovery, the idea of understanding. I think it's difficult to make clear to the non-physicist the beauty of how it fits together, of how you can build a world picture, and the beauty that the laws of physics are immutable.

Brian Cathcart

Hans Bethe's contributions were crucial to understanding the physics of the energy generation in all types of main-sequence stars, writes Professor Malcolm Longair.

The idea that nuclear energy could power the Sun had first been proposed by Arthur Eddington in 1920 on the basis of the accurate measurements of the masses of the hydrogen and helium ions carried out by Francis Aston at the Cavendish Laboratory. As soon as quantum mechanics was discovered in the mid-1920s, these new concepts were applied to the problem of nuclear energy generation in the Sun.

Even at the high temperatures of stellar interiors, the Coulomb repulsion between protons and nuclei is so great that, according to classical physics, there was no way in which this energy source can be tapped. The solution of the problem had to await George Gamow's theory of quantum mechanical tunnelling in 1928. Only one year later, Robert Atkinson and Fritz Houtermans applied Gamow's theory to the physics of nuclear reactions in the hot central regions of stars. By considering the process of barrier penetration by a Maxwellian distribution of protons, they established that the nuclear reactions can take place at temperatures which are considerably smaller than might have been expected and explained why the luminosity of the stars should be a sensitive function of temperature.

By 1931, it had been established that hydrogen was by far the most abundant element in the stars and so Atkinson's objective was to account for the origin of the chemical elements by the successive addition of protons to nuclei. He argued that the process of forming helium by the combination of four protons was a very unlikely process and suggested instead that helium could be formed by the successive addition of protons to heavier nuclei which, when they became too massive for nuclear stability, would eject alpha-particles and so create helium.

This proposal was the precursor of the carbon-nitrogen-oxygen (CNO) cycle which was discovered independently by Carl von Weizsäcker and Hans Bethe in 1938. In this famous cycle, carbon acts as a catalyst for the formation of helium through the successive addition of protons accompanied by two beta-plus decays.

In the meantime, it had become possible to make estimates of the reaction rates for the simplest nuclear reaction, the combination of pairs of protons to form deuterium which can then combine with other deuterons to form helium-3 and helium-4. The first calculations were carried out by Atkinson in 1936 and were much refined by Bethe, with Charles Critchfield, in 1938, who combined Fermi's theory of the weak interaction with Gamow's theory of barrier penetration.

The crucial first reaction in the chain involves a weak interaction in which a positron and neutrino are released in what may be thought of as the transformation of one of the protons into a neutron. This reaction accounts for most of the energy release in the p-p chain. Bethe and Critchfield showed that this series of reactions could account for the luminosity of the Sun. In addition, they found that the rate of energy production of the p-p chain depends upon the fourth power of the central temperature of the star.

In 1939, Bethe worked out the corresponding energy production rate for the CNO cycle and found a very much stronger dependence, the reaction rate going as the 17th power of the temperature. He concluded that the CNO cycle was dominant in massive stars while the p-p chain was the principal energy source for stars with mass roughly that of the Sun or less.

These conclusions were confirmed by the much more detailed models of stellar structure that became available after the Second World War and, in particular, with the development of computer codes, which have converted the study of stellar structure into one of the most precise of the astrophysical sciences.