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Franklin D. Roosevelt. The letter warned that “it may become possible to set up a nuclear chain reaction in a large mass of uranium,” leading to the creation of “extremely powerful bombs of a new type.” Einstein signed the letter, which was hand delivered to the president by a mutual friend. After British researchers concluded that such weapons could indeed be made andintelligence reports suggested that German physicists were trying to make them, the Manhattan Project was formed in 1942. Led by Leslie R. Groves, a brigadier general in the U.S. Army, it secretly gathered eminent scientists from Canada, Great Britain, and the United States, with the aim of creating atomic bombs.
    Conventional explosives, like TNT, detonate through a chemical reaction. They are unstable substances that can be quickly converted into gases of a much larger volume. The process by which they detonate issimilar to the burning of a log in a fireplace—except that unlike the burning of a log, which is slow and steady, the combustion of an explosive is almost instantaneous. At the point of detonation,temperatures reach as high as 9,000 degrees Fahrenheit. As hot gases expand into the surrounding atmosphere, they create a “shock wave” of compressed air, also known as a “blast wave,” that can carry tremendous destructive force. The air pressure at sea level is 14.7 pounds per square inch. A conventional explosion can produce a blast wave with an air pressure of1.4 million pounds per square inch. Although the thermal effects of that explosion may cause burns and set fires, it’s the blast wave, radiating from the point of detonation like a solid wall of compressed air, that can knock down a building.
    The appeal of a nuclear explosion, for the Manhattan Project scientists, was the possibility of an even greater destructive force. A plutonium core the size of a tennis ball had the potential to raise the temperature, at the point of detonation, totens of millions degrees Fahrenheit—and increase the air pressure tomany millions of pounds per square inch.
    Creating that sort of explosion, however, was no simple task. The difference between a chemical reaction and a nuclear reaction is that in the latter, atoms aren’t simply being rearranged; they’re being split apart. The nucleus of an atom contains protons and neutrons tightly bound together. The “binding energy” inside the nucleus is much stronger than the energy that links one atom to another. When a nucleus splits, it releases some of that binding energy. This splitting is called “fission,” and some elements are more fissionable than others, depending on their weight. The lightest element, hydrogen, has one proton; the heaviest element found in nature, uranium, has ninety-two.
    In 1933, Leó Szilárd realized that bombarding certain heavy elements with neutrons could not only cause them to fission but could also start a chain reaction. Neutrons released from one atom would strike the nucleus of a nearby atom, freeing even more neutrons. The process could become self-sustaining. If the energy was released gradually, it could be used as a source of power to run electrical generators. And if the energy was released all at once, it could cause an explosion with temperatures many times hotter than the surface of the sun.
    Two materials were soon determined to be fissile—that is, capable of sustaining a rapid chain reaction: uranium-235 and plutonium-239. Both were difficult to obtain. Plutonium is a man-made element, created by bombarding uranium with neutrons. Uranium-235 exists in nature, but in small amounts. A typical sample of uranium is about 0.07 percent uranium-235, and to get that fissile material the Manhattan Project built a processing facility in Oak Ridge, Tennessee. Completed within two years, it wasthe largest building in the world. The plutonium for the Manhattan Project came from three reactors in