nuclear bomb.
After the discovery of radioactivity and the relativistic equivalence of matter and energy, scientists realized that vast amounts of energy must be stored within the atom. Most insisted, however, that the enormous potential of atomic energy for industry and also for war could never be exploited in practice. More power would have to be fed into an atomic device than could be extracted; the threat of atomic weapons was consigned to science fiction. Increasing knowledge of the atomic nucleus did not affect this assumption. Once James Chadwick in 1932 had demonstrated the existence of neutrons, Leo Szilard inferred that a nucleus would not repel a neutron as it would a positive particle and might release more neutrons than it captured, leading to a chain reaction. But he was unable to pursue his ideas further.
Meanwhile astrophysicists suggested as early as the 1920s that the energy of stars came from the fusion of hydrogen atoms to form helium. Hans Albrecht Bethe and George Gamow worked out a convincing mechanism for the process in agreement with observation in 1937–1938. Their theory was of purely cosmological interest, since nobody imagined that humans could attain the temperatures required to set the process off. But then came the surprise discovery of fission at the end of 1938 by Otto Hahn and Fritz Strassmann, as interpreted by Lise Meitner and Otto Robert Frisch. Within a month it was widely understood that nuclear fission could release energy far in excess of any chemical combustion or explosion. To achieve the necessary chain reaction required obtaining a critical mass of potentially fissile material (that is, a large enough sample that the neutrons can participate in the chain before being lost to the environment).
Szilard persuaded Albert Einstein to inform President Franklin Delano Roosevelt about the possibility of a nuclear bomb. Roosevelt set up a scientific committee under the National Bureau of Standards to look into it. Meanwhile, atomic scientists were investigating the question in Germany, France, and Britain. Niels Bohr and John Wheeler had in 1939 proposed a mechanism for fission from which it followed that uranium 235 (U-235), although only about 0.7 percent of natural uranium, would be more suitable than the common isotope U-238. In March 1940 Frisch, now in England, and another refugee, Rudolf Peierls, completed their memorandum, “On the Properties of a Radioactive Super-Bomb,” which showed that the critical mass required for fission on a large scale was much less than had been presumed. The British government set up the MAUD committee to investigate; they reported in the summer of 1941 that a bomb would be feasible. They also suggested methods to enrich the proportion of U-235 in uranium. In Germany Werner Heisenberg, as head of the Kaiser-Wilhelm Institut für Physik, set out the main tasks of a nuclear energy program. For about a year German research stayed ahead of the British, who at first thought to develop their own nuclear weapon but soon felt obliged to accept the absorption of their program into America's.
Anglo-American research disclosed that the newly discovered plutonium (created by bombarding U-238 with neutrons) would probably also be fissile. The American government, through the National Defense Research Council, began to pump money into the nuclear program even before Pearl Harbor. In December 1941 what was to become the Manhattan Project began to take shape. In almost exactly a year Enrico Fermi and his team at Chicago achieved an atomic pile capable of a controlled chain reaction. Their accomplishment indicated that a plutonium bomb might be no more difficult to make than a uranium bomb. Huge factories in remote parts of the United States mushroomed into life—Oak Ridge in the Tennessee Valley (for separation of U-235) and Hanford in Washington state (for production of plutonium).
A third major laboratory, established at Los Alamos, New Mexico, under J. Robert Oppenheimer, assembled many outstanding nuclear scientists, including Bohr, Bethe, and Edward Teller, to design the bombs. They decided that uranium could be detonated by firing (with ordinary explosives) two subcritical masses together, but the plutonium would fizzle if made critical so slowly. Instead, a subcritical mass of plutonium would be made critical by suddenly increasing its density via an imploding charge outside it. War ended in Europe, and the failure of the German program was known, before the test in June 1945 of the plutonium bomb. The uranium bomb fell (without prior test) on Hiroshima and a plutonium bomb on Nagasaki in August. The shock of the invincible weapon forced the military leaders of Japan to capitulate.
The United States resumed testing in summer 1946. The Soviets, who had known about the Manhattan Project, resumed their own project, spurred on after August 1945 by fears that the Americans would use atom bombs to impose their supremacy over all. Helped by espionage (just how much is still debated), the Soviet Union exploded its own bomb in 1949. Shocked by this unwelcome surprise and revelations of past spying, the American government now pressed on to develop a thermonuclear device, the so-called H-bomb (for hydrogen bomb). Some scientists involved in the Manhattan Project, Teller for example, had even then wished to work on such a weapon, nicknamed the “super,” since the fission bomb would create the temperatures and pressures to make fusion feasible. Both the United States and the Soviet Union developed hydrogen bombs and tested them in the early 1950s. Meanwhile the British, feeling they had been shut out of a program to which their scientists had originally contributed much, decided in 1947 to make their own nuclear armory. They have been followed by a number of other states.
See also Kurchatov, Igor Vasilievich, and J. Robert Oppenheimer; World War II and Cold War.
Alwyn McKay, The Making of the Atomic Age (1984).
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Richard Rhodes, The Making of the Atomic Bomb (1986).
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John Newhouse, The Nuclear Age (1989).
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Lawrence Badash, Scientists and the Development of Nuclear Weapons (1995).
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Rachel Fermi and Esther Samra, picturing The Bomb (1995).
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Richard Rhodes, Dark Sun: The Making of the Hydrogen Bomb (1995).
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