Once discovered, the basic physics underlying the bomb seemed almost simple: bombard a uranium nucleus, and the resulting split atom would release a huge amount of energy. But if the basic principle was simple, the physics entailed in the construction of the bomb itself was anything but. The physicists needed to figure out how to produce enough radioactive material, as there was not nearly enough to be found in nature. They needed to find a way to create a nuclear chain reaction they could control, and then they needed to figure out how to control the speed of a fast nuclear reaction, so that it would release energy quickly enough to create a super-powerful bomb but not so quickly that the bomb would explode too soon. And, of course, they needed to figure out what would happen after a nuclear explosion; there was a question at the outset, for example, whether such an explosion would vaporize the atmosphere and destroy all life on earth.
Perhaps the most difficult and most crucial problem that Oppenheimer and his team faced was the design of the bomb mechanism itself. How could they create a mechanism that would release the immense power of nuclear fission on command? In a triumph of theoretical and applied physics, the Los Alamos group eventually came up with two workable bomb designs, code-named "Little Boy" and "Fat Man."
Little Boy, the U-235 bomb, had the simpler design of the two, and the physicists were certain that it would work. The mechanism used a gun-assembly method, in which one subcritical mass of U-235 is fired at another subcritical mass of U-235. When the two join, they form a critical mass and ignite in a nuclear reaction. This is the type of bomb which was dropped over the Japanese city of Hiroshima.
Fat Man, the plutonium bomb, used a different method: the implosion method. In the Fat Man bomb, a subcritical sphere of plutonium is surrounded by explosives, and when the explosives fire, a shock wave compresses the plutonium into a critical mass, setting off a nuclear reaction. It was the Fat Man bomb that was dropped over Nagasaki, Japan. As this method was somewhat more complicated than the gun-assembly method, scientists were less sure it would work, and decided to test it out.
As the scientists were celebrating their breakthrough, foremost in their mind was the thought that they had done it, they had beaten Germany to the creation of the bomb. After all, the race against Germany had been the driving purpose of the Manhattan Project and the reason given by many of the physicists involved for why they were so passionately committed to the bomb effort.
But how real was the German threat? In 1944, as the Los Alamos physicists were readying themselves for a test of the nuclear bomb, the American government finally learned the status of the German nuclear program.