During the early 1950s, Bethe also played an important role in the development of the larger hydrogen bomb, though he had originally joined the project with the hope of proving it could not be made. Bethe later campaigned together with Albert Einstein in the Emergency Committee of Atomic Scientists against nuclear testing and the nuclear arms race. He influenced the White House to sign the ban of atmospheric nuclear tests in 1963 and the 1972 Anti-Ballistic Missile Treaty, SALT I. His scientific research never ceased even into the later years of his life. He is one of the few scientists who can claim a major paper in his field every decade of his career, which spanned nearly sixty years. Freeman Dyson called Bethe the "supreme problem solver of the 20th century."
Bethe left Germany in 1933 when the Nazis came to power and he lost his job at the University of Tübingen, moving first to England where he held a provisory position of Lecturer for the year 1933-1934 and in the fall of 1934, a fellowship at the University of Bristol. In England, Bethe worked with the theoretician Rudolf Peierls on a comprehensive theory of the deuteron. In 1929, Bethe made an important contribution to solid state physics and chemistry, with his formulation of the basic concepts of crystal field theory. His paper is regarded as the starting point for all serious later discussions of the topic. In 1930, he devised a formula for the energy loss of swift charged particles in matter called the Bethe formula, which is now as important as it was then.
In 1935 Bethe moved to the United States, and joined the faculty at Cornell University, a position which he occupied for the rest of his career. During 1948-1949 he was a Visiting Professor at Columbia University. At Cornell, Bethe became known as one of the leading theoretical physicists of his generation, and along with upcoming physicists such as cyclotron pioneer Milton Stanley Livingston, and later, after the war, experimentalist Robert R. Wilson and theoretician Robert Bacher, put Cornell on the world physics map. Together with Robert Bacher and Livingston, Bethe published a series of three articles which summarized most of what was known on the subject of nuclear physics until that time, an account that became informally known as "Bethe's Bible", and remained the standard work on the subject for many years. In this account, he also continued where others left off, filling in gaps from the older literature. From 1935-1938, he studied nuclear reactions and reaction cross sections, carbon-oxygen-nitrogen cycle, leading to his important contribution to stellar nucleosynthesis. This research was later useful to Bethe in more quantitatively developing Niels Bohr's theory of the compound nucleus.
When the war began, Bethe wanted to contribute to the war effort. Following the advice of the Caltech aerodynamicist Theodore von Karman, Bethe collaborated with his friend Edward Teller, then at George Washington University, on a theory of shock waves which are generated by the passage of a projectile through a gas. This work was later useful to researchers investigating missile reentry. Bethe also worked on a theory of armor penetration, which was immediately classified by the Army, making it inaccessible to Bethe, who was not an American citizen at the time.
During the summer of 1942 he served as part of a special session at the University of California, Berkeley at the invitation of Robert Oppenheimer, which outlined the first designs for the atomic bomb. Initially, Bethe was skeptical of the possibility of making a nuclear weapon from uranium. In the late 1930s, he wrote a theoretical paper arguing against fission, but was convinced by Teller to join the Manhattan Project. When Oppenheimer was put in charge of forming a secret weapons design laboratory, Los Alamos, he appointed Bethe Director of the Theoretical Division, a move that irked Teller, who had coveted the job for himself.
Bethe's work at Los Alamos included calculating the critical mass of uranium-235 and the multiplication of nuclear fission in an exploding atomic bomb. Along with Richard Feynman, he developed a formula for calculating the explosive yield of the bomb. After November 1943, when the laboratory had been reoriented to solve the implosion problem of the plutonium bomb, Bethe spent much of his time studying the hydrodynamic aspects of implosion, a job which he continued into 1944. In 1945, his work concerned working out the workings of the neutron initiator, and later on radiation propagation from an exploding atomic bomb.
During the project, Klaus Fuchs, who was leaking nuclear secrets to the Russians, was also in Bethe's division (often doing work which had originally been assigned to Teller). Like everyone else, Bethe had no knowledge that Fuchs was a spy.
When the first implosion atomic bomb was detonated in the New Mexico desert on July 16, 1945, at the Trinity test, Bethe's immediate concern was for its efficient operation, and not its moral implications and is reported to have commented: "I am not a philosopher."
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 relation.
During the 1980s and 1990s, Bethe campaigned for the peaceful use of nuclear energy. After the Chernobyl disaster, Bethe put together a committee of experts that analysed the incident, and concluded that a similar episode would not happen in any good US reactor, as the Russian reactor suffered from a fundamentally faulty design and human error also had significantly contributed to the accident. Throughout his life Bethe remained a strong advocate for electricity from nuclear energy.
In the 1980s he and other physicists opposed the Strategic Defense Initiative missile system conceived by the Ronald Reagan administration. In 1995, at the age of 88, Bethe wrote an open letter calling on all scientists to "cease and desist" from working on any aspect of nuclear weapons development and manufacture. In 2004, he signed a letter along with 47 other Nobel laureates endorsing John Kerry for President of the United States.
In 1967, Bethe was awarded the Nobel Prize in Physics "for his contributions to the theory of nuclear reactions, especially his discoveries concerning the energy production in stars". His postulate was that the source of this stellar nucleosynthesis was thermonuclear reactions in which hydrogen is converted into helium.
Bethe was also noted for his theories on atomic properties. In the late 1940s, he provided the first way out of the infinities that plagued the explanation of the so called Lamb shift. Although his calculation was a non-relativistic one, it was a definite starting point. This work provided the impetus for the pioneering later work done by Richard Feynman, Julian Schwinger and others which marked the beginning of modern quantum electrodynamics.
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 Brown of the State University of New York at Stony Brook. In his 80s, he wrote an important article about the solar neutrino problem in which he dealt with the conversion of electron neutrinos into muon neutrinos that was proposed to explain the discrepancy between theory and experiment. Physicist Kurt Gottfried says that he does not know anyone in the history of modern physics who has done work of such calibre in his 80s.
Bethe's hobbies included a passion for history and also stamp-collecting. About the latter, he wryly remarked that it was the only instance where all the countries in the world could coexist by each other's side in peace. He loved the outdoors, and was an enthusiastic mountain climber all his life. Bethe was also known for his great sense of humor, and once published a parody in 1931, On the Quantum Theory of the Temperature of Absolute Zero where he calculated the fine structure constant from the absolute zero temperature in Celsius units, causing a scandal in the scientific world. This second parody paper was intended to characterize a certain class of papers in theoretical physics of the day, which were purely speculative and based on spurious numerical arguments such as Sir Arthur Eddington's claim to have calculated the fine structure constant from fundamental quantities in an earlier paper. He has also, wrongly, been credited for allowing his name to be used in the Alpher-Bethe-Gamow paper in which he did not participate; in fact, George Gamow added Bethe's name without consulting him, and against Ralph Alpher's wishes.
Bethe died in his home in Ithaca, New York on March 6, 2005 . At the time of his death, he was the John Wendell Anderson Professor of Physics Emeritus at Cornell University. He also was, reaching the age of 98, the second-oldest Nobel laureate ever. Since his death, Cornell has announced that the third of five new residential colleges, each of which will be named after a distinguished former member of the Cornell faculty, will be named the Hans Bethe House. Bethe with his wife Rose, had two children, Henry and Monica.
Named after him