Free Computing with Quantum Cats by John Gribbin Page A

(neutrons) released by the fission (“splitting”) of one atomic nucleus trigger the fission of more nuclei, in a runaway chain reaction. Each “split” converts a little mass into energy, in line with Einstein's famous equation, and the overall result is the explosive release of a lot of energy as heat, light, and other electromagnetic radiation. But if the critical mass is not tightly confined, most of the neutrons escape and the material simply gets hot, rather than exploding. The first method the Los Alamos team considered for achieving the required result was to prepare a critical mass of uranium in two halves at opposite ends of a tube, and fire conventional explosives to smash one of them (the “bullet”) into the other. For obvious reasons, it was called the gun method, and was used in the Little Boy weapon dropped on Hiroshima. But this method could not be used with plutonium for technical reasons, not least the possibility that the plutonium, being more active than uranium, might “pre-ignite” just before the bullet hit the target, releasing its neutrons too gradually and causing the bomb to fizzle rather than explode. This was where von Neumann came in.
    Before von Neumann arrived in Los Alamos, another member of the team, Seth Neddermeyer, had suggested setting off explosives to produce shock waves which would squeeze a “subcritical” mass of plutonium to the point where it reached critical mass and exploded. But the idea had not beenfollowed up, and nobody had been able to work out exactly how to achieve the desired result. Edward Teller, a member of the Los Alamos team who is remembered as the “father of the hydrogen bomb,” later recalled how the problem was solved. 6 Von Neumann had calculated the kind of pressure that would be produced in a lump of plutonium if it was squeezed in the grip of a large number of explosive charges surrounding it and going off simultaneously. He discussed his results, which still seemed to fall short of what was necessary to produce a practical bomb, with Teller, who had worked in geophysics and knew that under very high pressures such as those at the center of the Earth even a substance like iron gets compressed to higher density than at the planet's surface. He pointed out that this compressibility would make the process von Neumann was describing even more effective, because the more the plutonium atoms were squeezed together the easier it would be for a chain reaction to take place. Von Neumann redid the calculation, taking account of compressibility, and found that the trick would work. After a great deal more work by many people, including von Neumann, the result was the Fat Man bomb dropped on Nagasaki, in which a hollow shell of plutonium was triggered into explosive fission by the firing of thirty-two opposing pairs of detonators to produce an inward compression. This process owed a great deal, in the days before electronic calculators, to von Neumann's ability at carrying out mathematical calculations; but it also highlighted the need for faster methods of carrying out such calculations, which could be used when there wasn't a von Neumann around to do the work. This became of crucial importance when von Neumann, who continued to spend two months each year visiting Los Alamos after the war, became involvedin the development of the hydrogen bomb, based on nuclear fusion, not fission: because even von Neumann couldn't do all the calculations on his own.
    The calculations for the Manhattan Project had been aided by the use of machines, supplied by the company International Business Machines (IBM), which correlated data using sets of punched cards. These were in no sense computers in the modern meaning of the word, but moronic machines, using mechanical switches, that could be set up to perform basic arithmetical operations. For example, a machine could take two cards punched with holes corresponding to two numbers (say, 7 and