Nuclear Weapons

German physicist Albert Einstein (1879–1955) did not know it at the time, but when he published his Special Theory of Relativity in 1905 he provided the world with the basic information needed to build nuclear weapons. One aspect of Einstin's work (embodied in the famous equation E= mc²) stated that the amount of matter of an object (i.e., its mass) is equivalent to a specific amount of energy. The exact amount of energy in an object equals its mass multiplied by the square of the speed of light. The speed of light is large—186,282 miles per second (300,000 km/sec)—so even a small piece of matter contains a vast amount of energy. A baseball-size sample of uranium-235, for example, can explode with as much energy as 20,000 tons of TNT—and this involves the conversion of only a tiny fraction of the uranium's mass into energy. One pound of explosive material in a fission weapon is approximately 100,000 times as powerful as one pound of TNT.

As World War II approached, two German chemists, Fritz Strassmann (1902–1980) and Otto Hahn (1879–1968), pointed a stream of neutrons at a sample of uranium and succeeded in splitting the nuclei of some of its atoms. This splitting of nuclei is termed nuclear fission. The energy released through nuclear fission was the source of power for the first atomic bomb, which was built in the United States by a large team of scientists lead by U.S. physicist J. Oppenheimer (1904–1967). This secret research and development program was termed the Manhattan Project.

The first atomic bomb was detonated in Alamogordo, New Mexico, on July 16, 1945. Three weeks later, on August 6, a United States bomber, the Enola Gay, dropped a four-ton atomic bomb containing 12 lb (5.4 kg) of uranium-235 on the Japanese city of Hiroshima. Seventy thousand people died as a direct result of the blast. Within two months, nearly twice that many were dead from blast injuries and radiation. Three days later, on August 9, a bomb containing several pounds of plutonium was dropped on Nagasaki. Thirty thousand people died in the seconds following the explosion, and more later. The Japanese surrendered the next day, ending World War II.

These first nuclear weapons were atomic bombs or A-bombs. They depended on the energy produced by nuclear fission for their destructive power. However, scientists like U.S. physicist Edward Teller (1908–) knew even before the first atomic bomb exploded that the fission weapons could be used to create an even more powerful explosive, now called a thermonuclear device, hydrogen bomb, or H-bomb. This weapon gets it power from the energy released when atoms of the hydrogen isotopes deuterium or tritium are forced together, a process called nuclear fusion. Starting a nuclear fusion reaction is even more complicated than setting off a fission atomic bomb; it requires such heat to initiate it that a fission bomb is used as a detonator to explode the fusion bomb. The United States tested its first hydrogen bomb on November 1, 1952. It exploded with the force of 10.4 megatons (millions of tons of TNT equivalent). Three years later, the Soviet Union exploded a similar device.

For the next 40 years, the United States, with its allies, and the former Soviet Union, with its allies, raced to build more nuclear weapons. Each side produced tens of thousands of nuclear weapons. The end of the cold war and the breakup of the Soviet Union in the early 1990s led to a significant decrease in the numbers of nuclear weapons in the world; however, the U.S. and Russia still possess many thousands of nuclear weapons.