The beginning of World War II changed everything for America's physicists. Suddenly, theoreticians who had spent their professional lives lost in abstract thought were called upon to put their theories to work creating practical applications. Suddenly, physics was relevant.
Historian Daniel Kevles has called World War II "the physicists' war," and with good reason. The wartime physics effort represented the largest and most expensive physics program in history, and physicists emerged from the war having created two of the century's greatest technological triumphs: radar and the atomic bomb. Both have been credited with winning the war for the Allied forces.
One of the two great physics projects of World War II, the development of radar, was already well under way by 1941. Some of the country's greatest physicists were gathered at the Massachusetts Institute of Technology's "Radiation Laboratory"–or "Rad Lab"–racing to create a working radar system. They assumed that this would be the great physics project of the war. They were wrong.
Three years before, the German physicists Otto Hahn and Fritz Strassman had made a startling discovery that would gravely impact the course of human events. They had no idea of this at the time, of course. All they knew was that when you bombarded a uranium nucleus with neutrons, you could split the nucleus and a release a huge amount of energy. Hahn and Strassman had discovered nuclear fission.
As it turned out, when one neutron struck the uranium nucleus, two or three new neutrons would be released, which meant, under the right circumstances, a chain reaction could be created with more and more neutrons. Thus more and more energy would be released at every step. The fissioning of a single uranium atom would release 200 million volts of energy.
The discovery had explosive possibilities if nuclear fission could be harnessed to create a powerful bomb, more powerful than any weapon mankind had seen before. The European physicists in the United States, many of whom had fled Hitler's Nazi regime, immediately realized the possibilities of nuclear fission and feared what would happen if Hitler's scientists realized them as well.