Photo: Atomic bomb test explosion in Alamagordo, New Mexico, July 16, 1945. AP/Wide World Photos

In 1932, James Chadwick discovered the neutron, a small atomic particle with mass but no charge. This turned out to be an extremely useful tool for bombarding atomic nuclei. Two years later, Enrico Fermi bombarded uranium with neutrons, hoping that it would cause the uranium to emit a beta particle and become a new, artificial element above uranium in the periodic table. It seemed he had done this and in the process showed that slow-moving neutrons were more effective than high-energy neutrons for the task. Fermi won the Nobel Prize for his work in 1938. He was a committed antifascist and when he and his wife left Italy for the Nobel ceremony, it was for good. They settled in the United States.

Meanwhile, Fermi's work had produced some puzzling results and physicists Otto Hahn, Fritz Strassmann, and Lise Meitner took it up in Germany. They began to suspect that Fermi hadn't created a new element at all but might have actually split the uranium atom in two -- at that point a barely imaginable event. Before they could confirm this, Meitner -- who was an Austrian of Jewish descent -- had to flee the country when Germany annexed Austria in 1938. With the help of Niels Bohr, she got a position in Stockholm. From there, Meitner and her nephew Otto Frisch proved that the uranium atom had been split. Hahn published the results in January 1939 and Meitner and Frisch published the explanation a month later, introducing the term "nuclear fission." Using Bohr's liquid drop model of the nucleus, they suggested that when split, both halves of the atom would have a positive charge and would repel each other with powerful force.

The German government took little notice of the finding at first. But others felt the implications were immediately clear. Niels Bohr brought news of Meitner's discovery to the United States in 1939. Several scientists, realizing that fission could be used to build a devastating weapon, wrote to President Roosevelt to inform him. He immediately set up a committee to research the matter.

(Back in Germany, Hahn refused to do weapons research. He had worked on chemicals used as weapons in World War I. His advisers assured him it would bring a speedier end to the war, but he was horrified when he saw Russian soldiers who'd been gassed.)

By the end of 1941, British studies had outlined the materials requirements for an atomic bomb and uranium research was going on at about 12 American universities. In 1942 Fermi's team at the University of Chicago created a sustained chain reaction of fission for the first time. Also during 1942, the Manhattan District of the Corps of Engineers was formed to construct three secret "cities" for major portions of atomic bomb development. At Oak Ridge, Tennessee, a nuclear reactor and plant for separating uranium 235 from natural uranium was built. In Hanford, Washington, three reactors were built to extract plutonium (another element with atoms that could be split) from a non-fissionable type of uranium. Finally a lab for the design and construction of the bomb was built at Los Alamos, New Mexico. The cost of these Manhattan Project installations was $2 billion.

J. Robert Oppenheimer was made director of the Los Alamos lab, and in 1943 he gathered about 200 of the best scientists in the field to live and work there. They designed two bombs, one using uranium (called "Little Boy") and one using plutonium ("Fat Man"). By early 1945, the plants at Oak Ridge and Hanford had produced enough raw material for testing. On July 13, 1945, at a site called Trinity 100 km northwest of Alamogordo, a plutonium bomb was assembled and brought to the top of a tower. The test was postponed by thunderstorms. On July 16, the bomb was detonated, producing an intense flash of light seen by observers in bunkers 10 km away and a fireball that expanded to 600 meters in two seconds. It grew to a height of more than 12 kilometers, boiling up in the shape of a mushroom. Forty seconds later, the blast of air from the bomb reached the observation bunkers, along with a long and deafening roar of sound. The explosive power, equivalent to 18.6 kilotons of TNT, was almost four times larger than predicted.

Some of the Los Alamos scientists had circulated a petition asking President Truman to give Japan a warning and a chance to surrender before using the bomb. Some signed, some didn't, but the project remained a secret until the end.

Twenty-one days after the test, the B-29 bomber Enola Gay dropped the uranium bomb on Hiroshima, Japan. Three days later the plutonium bomb was used to bomb Nagasaki. The two bombs killed approximately 150,000 people when they fell. Earlier in the year, intense bombing of Tokyo with conventional bombs had killed about 100,000 people without causing Japan to surrender, but on August 15, 1945, Japan officially surrendered, bringing an end to World War II.

After the war, hydrogen bombs and other nuclear weapons continued to be developed by the United States and its former ally the Soviet Union. The competition between the ideologically different nations led to an "arms race" that shaped the postwar decades both economically and philosophically. Nuclear fission was also put to civilian use in power generators in several countries. They made cheap electricity and were a source of radioisotopes for other types of research, but came with their share of problems and dangers. Reactors are still in use, but in many places have fallen out of favor as an energy source.


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