Now, the energy released in the splitting of one gram of U-235 is equivalent in power to the explosive force of 20 tons of TNT, or two old-fashioned blockbusters. Since we know from President Truman’s announcement following the bombing of Hiroshima that the wartime A-bomb “had more power than 20,000 tons of TNT,” it means that the atoms in an entire kilogram (1,000 grams) of U-235 or plutonium must have been split. In other words, after the A-bomb had reached a power of 20 tons of TNT, it had to be kept together long enough to increase its power a thousandfold to 20,000 tons. This, as we have seen, requires only ten more steps. It can also be seen that it is these ten final crucial steps that make all the difference between a bomb equal to only two blockbusters, which would have been a most miserable two-billion-dollar fiasco, and an atomic bomb equal in power to two thousand blockbusters. With the aid of these facts we are at last in a position to grasp the enormousness of the problem that confronted our A-bomb designers at Los Alamos and is confronting them again today. It can be seen that for a bomb to multiply itself from 20 to 20,000 tons in ten steps by doubling its power at every step, it has to pass successively the stages of 40, 80, 160, 320, and so on, until it reaches an explosive power of 2,500 tons at the seventh step. Yet it still has to be held together for three more steps, during which it reaches the enormous power of 5,000 and 10,000 tons of TNT, without exploding. Here was an irresistible force, and the problem was to surround it with an immovable body, or at least a body that would remain immovable long enough for the chain reaction to take just ten additional steps following the first seventy. There is only one fact of nature that makes this possible, or even thinkable—the last ten steps from 20 to 20,000 tons take only one tenth of a millionth of a second. The problem thus was to find a body that would remain immovable against an irresistible force for no longer than one tenth of a microsecond, 100 billionths of a second. This immovable body is known technically as a “tamper,” which pits inertia against an irresistible force that builds up in 100 billionths of a second from an explosive power of 20 tons of TNT to 20,000 tons. The very inertia of the tamper delays the expansion of the active substance and makes for a longer-lasting, more energetic, and more efficient explosion. The tamper, which also serves as a reflector of neutrons, must be a material of very high density. Since gold has the fifth highest density of all the elements (next only to osmium, iridium, platinum, and rhenium), at one time the use of part of our huge gold hoard at Fort Knox was seriously considered.