Hans A. Bethe, the John Wendell Anderson Professor of Physics Emeritus, gave a Cornell audience on Oct. 28 an eyewitness account of the making of the atomic bomb at Los Alamos 51 years ago.
Delivering the Gemant Lecture in physics, Bethe gave a one-hour account of the work of the Manhattan Project to a standing-room-only crowd in Schwartz Auditorium, Rockefeller Hall. Bethe was director of the theoretical division for the Manhattan Project.
Describing fission of uranium, Bethe said "many physicists envisioned the idea of a chain reaction" and soon "realized that this might be a possibility for a weapon."
Leslie Groves, who led the Manhattan Project for the government, hired Robert Oppenheimer, a physicist, to direct the lab, Bethe recounted.
"Oppenheimer was chosen because he was the only one who could explain a chain reaction in a coherent, understandable way to the general," Bethe said. "Oppenheimer turned out to be a superb leader of the lab, which no one expected because all he had done was direct a few graduate students."
Still, Oppenheimer made one big miscalculation, Bethe said: "He thought 50 scientists would be enough to accomplish the goals. We started with 500 and ended up with 5,000."
Bethe said that using plutonium resulted in some worry: spontaneous fission, resulting in pre-detonation. "Can an accidental neutron start the chain reaction?" was the concern, he said. It was up to the G division, the "Gadget division," to make the detonators. They built a solid sphere, and the resulting plutonium ball was tested in the New Mexico desert -- at the Trinity test, July 16, 1945 -- and later dropped on Nagasaki, Japan.
The Theoretical Division, he said, had to calculate how much material would have to be compressed by the implosion to ensure success. "The test was a full success . . . High-speed cameras showed how the cloud developed over time.
"Three weeks later, the same kind of weapon was dropped on Nagasaki," Bethe said.
The hydrogen bomb dropped on Hiroshima, which used uranium as fissionable material, did not require a test. "We knew that would work," Bethe said, but the Los Alamos team did not have enough uranium from Oak Ridge National Laboratory to make that bomb first.
"So, Los Alamos accomplished its goal," Bethe said. "Two cities in Japan were destroyed. Seeing that destruction, many of us came to the conclusion that it should never be done again, and this is where I have been ever since."
The Gemant Lecture in physics is named for the Andrew Gemant Award. The 1995 award was given to Robert R. Wilson, Cornell professor emeritus of physics, by the American Institute of Physics. Part of the award includes a provision for a series of lectures of interest to the public as well as the local community of physicists.
Bethe, 90, won the Nobel Prize in physics in 1967 for describing the nuclear processes that power the sun.