[6] During World War II, Bethe was head of the Theoretical Division at the secret Los Alamos National Laboratory that developed the first atomic bombs.

In 1947, he wrote an important paper which provided the calculation of the Lamb shift, which is credited with revolutionizing quantum electrodynamics and further "opened the way to the modern era of particle physics".

[16] Although his mother, the daughter of Abraham Kuhn, professor at the University of Strasbourg, had a Jewish background,[17] Bethe was raised Protestant, like his father;[18][19] and he became an atheist later in life.

The instruction in physics was poor, and while there were distinguished mathematicians in Frankfurt such as Carl Ludwig Siegel and Otto Szász, Bethe disliked their approaches, which presented mathematics without reference to the other sciences.

Because he was such a renowned scholar, Sommerfeld frequently received advance copies of scientific papers, which he put up for discussion at weekly evening seminars.

[45] Bethe accepted a request from Karl Scheel to write an article for the Handbuch der Physik on the quantum mechanics of hydrogen and helium.

Reviewing the article decades later, Robert Bacher and Victor Weisskopf noted that it was unusual in the depth and breadth of its treatment of the subject that required very little updating for the 1959 edition.

[44] His work on the handbuch articles occupied most of his time in Rome, but he also co-wrote a paper with Fermi on another new field, quantum electrodynamics, describing the relativistic interactions of charged particles.

[49] Bethe left Germany in 1933, moving to England after receiving an offer for a position as lecturer at the University of Manchester for a year through Sommerfeld's connection to William Lawrence Bragg.

[51] In 1933, the physics department at Cornell University, New York, was looking for a new theoretical physicist, and Lloyd Smith strongly recommended Bethe.

Bethe had already accepted a fellowship for a year to work with Nevill Mott at the University of Bristol for a semester, but Cornell agreed to let him start in the spring of 1935.

[54] Before leaving for the United States, he visited the Niels Bohr Institute in Copenhagen in September 1934, where he proposed to Hilde Levi, who accepted.

There were only 34 invited attendees, but they included Gregory Breit, Subrahmanyan Chandrasekhar, George Gamow, Donald Menzel, John von Neumann, Bengt Strömgren, Edward Teller, and Merle Tuve.

Bethe initially declined the invitation to attend, because the conference's topic, stellar energy generation, did not interest him, but Teller persuaded him to go.

Gamow and Carl Friedrich von Weizsäcker had proposed in a 1937 paper that the Sun's energy was the result of a proton–proton chain reaction:[66][67] But this did not account for the observation of elements heavier than helium.

[72] Bethe's graduate student Robert Marshak noted that the New York Academy of Sciences was offering a $500 prize for the best unpublished paper on the topic of solar and stellar energy.

[73][71] In 2002, at age 96, Bethe sent a handwritten note to John N. Bahcall congratulating him on the use of solar neutrino observations to show that the CNO cycle accounts for approximately 7% of the Sun's energy; the neutrino observations had started with Raymond Davis Jr., whose experiment was based on Bahcall's calculations and encouragement, and the note led to Davis's receiving a share of the 2002 Nobel Prize.

Following the advice of the Caltech aerodynamicist Theodore von Kármán, Bethe collaborated with his friend Edward Teller on a theory of shock waves that are generated by the passage of a projectile through a gas.

[81] After receiving security clearance in December 1941, Bethe joined the MIT Radiation Laboratory, where he invented the Bethe-hole directional coupler, which is used in microwave waveguides such as those used in radar sets.

[82] In Chicago in June 1942, and then in July at the University of California, Berkeley, he participated in a series of meetings at the invitation of Robert Oppenheimer, which discussed the first designs for the atomic bomb.

[87] Bethe's work at Los Alamos included calculating the critical mass and efficiency of uranium-235 and the multiplication of nuclear fission in an exploding atomic bomb.

[94] He later remarked in 1968 on the apparent contradiction in his stance, having first opposed the development of the weapon and later helping to create it: Just a few months before, the Korean war had broken out, and for the first time I saw direct confrontation with the communists.

While Bethe and Teller had been on very good terms during the prewar years, the conflict between them during the Manhattan Project, and especially during the Oppenheimer episode, permanently marred their relationship.

His paper, published in the Physical Review in August 1947, was only three pages long and contained just twelve mathematical equations, but was enormously influential.

[102] Bethe continued to do research on supernovae, neutron stars, black holes, and other problems in theoretical astrophysics into his late nineties.

[105][106] In 1996, Kip Thorne approached Bethe and Brown about LIGO, the Laser Interferometer Gravitational-Wave Observatory designed to detect the gravitational waves from merging neutron stars and black holes.

"My colleagues and I established," he explained "that the Chernobyl disaster tells us about the deficiencies of the Soviet political and administrative system rather than about problems with nuclear power.

[113] 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.

[114] In 2004, he joined 47 other Nobel laureates in signing a letter endorsing John Kerry for President of the United States as someone who would "restore science to its appropriate place in government".

[115] Historian Gregg Herken wrote: When Oppenheimer died, Oppie's long-time friend, Hans Bethe, assumed the mantle of the scientist of conscience in this country.