News of Chadwicks discovery traveled quickly, from England to Den­mark to France to Germany, as experimentalists and theorists alike turned their rapt attention to the new particle and its interactions with the

nucleus. To those working in theoretical quantum mechanics, the pres­ence of a neutral particle in the nucleus made perfect sense. They had modeled the outer electron cloud as fixed stations of energy levels. As electrons were induced to jump from one level to another, the acceleration of the negative charge caused electromagnetic radiation in the form of light. The protons in the nucleus were obviously bound together by some strong force. It had to be stronger than the electromagnetic force that caused like charges to repel each other. They called it simply the strong nuclear force, and it has to depend on the neutrons. Alone, the protons lack enough force to hold themselves together.

The protons and neutrons making up the nucleus have their own abstract orbitals, as if they were swirling around in the tight, limited space at the center of the atom. Each orbital station in the nuclear orbit structure has an energy associated with it. The forces holding the nucleus together are millions of times more powerful than the forces holding electrons in orbit. Disturb the nuclear structure by knocking out a neutron, for example, and the nucleus has to reconfigure itself, with protons and neutrons jockeying for position and changing orbits. The severe change of energy status of a proton, with its positive electrical charge, causes a powerful electromag­netic pulse to radiate from the nucleus, in accordance with Maxwell’s equa­tions. Quantum mechanics thus explained the presence of gamma rays, the penetrating photon radiation produced in the beryllium experiment.

On Tuesday morning, September 12, 1933, Leo Szilard (1898-1964), a brilliant physicist from Hungary, happened to be in London, loung­ing in the lobby of the Imperial Hotel and reading the Times newspaper. The headlines were all about nuclear science. “BREAKING DOWN THE ATOM,” “TRANSFORMATION OF ELEMENTS,” “THE NEUTRON,” and halfway down the second column, a summary of a speech by Ernest Rutherford, “HOPE OF TRANSFORMING ANY ATOM.” There was a scientific meeting going on involving all the top scientists in England, and Szilard was acutely aware that he had not been invited. He started reading about Rutherford’s speech:

What, Lord Rutherford asked in conclusion, were the prospects 20 or 30 years ahead? . . . We might in these processes obtain very much more energy than the protons supplied, but on the average we could not expect to obtain energy in this way. It was a very poor and inefficient way of producing energy, and anyone who looked for a source of power in the transformation of the atoms was talking moonshine.

Rutherford was saying that nuclear power on an industrial scale is impractical and not worth thinking about. Szilard found such pronounce­ments bothersome. He tossed away the paper and wandered out onto the street, where he could think while walking. He was so put off by a scientist of such large reputation saying that something could not be done without having tried it, he tried to think of a counterargument.

He stopped at a traffic light on Southampton Row, at Russell Square, across from the British Museum in Bloomsbury. The light turned green, and just as he stepped off the curb to cross the street an idea flashed through his mind. Neutrons have no charge and are not constrained by the shielding effects of the electron or the proton. A neutron is free to hit the nucleus head on, if it is so directed. If an extra neutron wandered

into and was absorbed by an already heavily laden nucleus, such as in one of the heavier elements, it could render it unstable. The unstable nucleus would then vibrate itself apart, rending into two nuclei. These two nuclei would almost surely weigh less together than the original large nucleus, and the weight deficit would express as pure energy.

Furthermore, suppose the destruction of the nucleus includes a sin­gle neutron scattered out of the debris. That neutron could then bounce around until it hit another overloaded nucleus, and it would cause another nuclear breakdown. Some neutrons would fail to cause subsequent break­downs, just because there was only a finite probability of one hitting an adjacent nucleus. What if, instead of one free neutron from the break­down, there were two? If there were as many as two individual neutrons in the breakdown debris, then the process could be self-sustaining. It would be a chain reaction, in which energy was released by nuclear disintegra­tions in quantities millions of times greater than any chemical reaction.

By the time he reached the other side of the street, Szilard had outlined the nuclear power process. If such an element exists that will tear apart under neutron bombardment and will release free neutrons in excess of one per disintegration, then Lord Rutherford was wrong. Nuclear power on an industrial scale would be possible. He spent the rest of the day thinking of exploring the solar system and beyond with nuclear-powered rockets and of building weapons based on the severely concentrated energy of nuclear reactions. In 1934, his patent application for a nuclear power reactor was not granted, but the application document was assigned to the British Admiralty for security reasons. Leo Szilard would live to see his daydreams realized.