Hans Bethe, the nuclear physicist whose elegant calculations explained how stars shine and laid the foundation for development of both the atomic and hydrogen bombs, has died. He was 98.
Bethe, who received the Nobel Prize in Physics in 1967, died Sunday at his home in Ithaca, N.Y., Cornell University announced Monday.
A reluctant but crucial participant in the World War II effort to develop nuclear weapons, Bethe later became one of the country's most passionate and persuasive proponents of disarmament. He argued that the use of such weapons would cost not only countless lives, but "liberties and human values as well."
A brilliant, prolific and engaging theorist with an encyclopedic knowledge of nuclear physics, Bethe spent more than 60 years working with only a slide rule, a stack of blank paper and his enormous intellect, turning out page after page of mistake-free, complex calculations that fundamentally altered how scientists viewed the microscopic world of the atom.
"He was the last of the old masters," said astrophysicist Edward Kolb of the Fermi National Accelerator Laboratory. "He turned out classic paper after classic paper."
Bethe (pronounced BAY-ta) was the lone survivor of the remarkable group of mostly German physicists of the early 20th century -- a group that included Einstein, Dirac, Fermi and Heisenberg -- that deciphered the fundamental laws of matter and energy and set the stage for the remarkable technological developments of the last half of the century.
"There's no one of comparable stature alive today," Cornell physicist Kurt Gottfried told The Times in 2003. "I can't think of any physicist who was as productive at the frontier over such a long period."
Bethe was a young man literally fresh off the boat from Europe when he made his first mark on the then-infant field of nuclear physics. With two collaborators, he wrote three massive articles for the American Physical Society's Reviews of Modern Physics that, in effect, compiled everything then known about the field.
The 1936 articles, which later became known colloquially as "Bethe's Bible," clarified the theory of nuclear forces, the structure of nuclei and the theory of nuclear reactions. They were reprinted multiple times and served as the primary text in the field for decades.
"Bethe systematically laid the theoretical foundations for nuclear physics with such clarity and care that they could be used to support major applications: stars and, later, reactors and bombs," said MIT physicist Frank Wilczek.
Bethe received the Nobel Prize for his explanation of how the sun and other stars are able to pour forth so much energy over long periods of time. Before he began his seminal work in 1938, researchers had speculated that the light and heat emitted by stars were caused either by chemical reactions or the compression of gases by the bodies' massive gravity.
Calculations ruled out both models, however: The mass of the sun was sufficient to sustain such reactions only for a few tens of millions of years, not the billions that astronomers knew our local star had survived.
Bethe was introduced to the problem at the 1938 Washington Conference on Theoretical Physics, where astronomers speculated that perhaps nuclear reactions were involved.
He went back to his office at Cornell University after the meeting and within six weeks had solved the problem. Given the known temperature of the core of the sun -- about 20 million degrees Celsius (about 36 million degrees Fahrenheit) -- and the proportions of carbon, hydrogen and other elements in its interior, he deduced which nuclear reactions could be responsible for its radioactive fire.
He ultimately developed a six-step cycle in which carbon and nitrogen atoms act as catalysts for the conversion of four hydrogen nuclei (protons) into one helium nucleus. The very slight mass lost in the process is converted into large amounts of energy, according to Albert Einstein's famous equation, E = mc2.
Bethe later remarked with typical modesty that he found the answer by "looking through the periodic table [of elements] step by step. So you see, this was a discovery by persistence, not by brains."
One oft-told story about Bethe recounts the evening after he discovered the secrets of starlight. During a late-night stroll, his fiancee, Rose -- daughter of theoretical physicist Paul Ewald -- remarked on how beautiful the stars looked. He responded: "Yes, darling, and I'm the only one on Earth who knows how they do it."
Bethe and his Cornell colleagues then verified five of the six steps in the laboratory; the sixth was reproduced by researchers at Cambridge University. The entire six-step cycle takes 5.4 million years to complete.