Niels Bohr, the Danish physicist who founded quantum mechanics, almost waited too long to escape from Europe during World War II. The German Army overran and occupied Denmark in April 1940, but Bohr felt fairly safe in spite of his Jew­ish heritage. He was head of the Institute of Theoretical Physics at the University of Copenhagen, and it would have seemed unusually severe if the Germans had arrested him. Denmark essentially collaborated with the German Army, maintain­ing an uneasy peace. However, in 1941, Bohr was visited by Werner Heisenberg from Berlin. They had a long walk and a private conversation that left Bohr with a feeling of dread that scientists in Germany were working on an atomic bomb. Although he would deny it after the war, Heisenberg seemed to hint at atomic weapons research back in Berlin, while probing Bohr for indications as to what the British and Americans were up to.

By 1943, the situation in Denmark had worsened, and on September 28 Bohr learned from the Swedish ambassador that he was to be arrested and deported to Germany within three days. Wasting no time, Bohr and his family walked through Copenhagen to the seaside and hid until nightfall in a gardener’s shed. A motorboat then ran them out to a fishing boat, which avoided minefields and German patrols and took them across the Oresund Channel to Limhamn, Sweden.

Sweden was relatively safe, but it was crawling with German agents, and there was real fear that Bohr would be assassinated now that he had escaped German con­trol in Denmark. He wanted to get to Britain, at least, where he could warn the Allies of an impending German nuclear weapon. He was flown out in a British Mosquito

to have miraculous qualities. The activation was much more intense when the experiment was performed on the wooden table.

It was a mystery worth further study. Fermi set up an experiment with a carefully machined piece of lead separating the neutron source and the silver target, but at the last moment, on a vague hunch, he substituted a scrap sheet of paraffin for the lead. The activation level increased dramati­cally over all previous experiments, and Fermi immediately knew what was taking place. The neutrons, barreling out of the neutron source at high speed, were slowed down to a crawl by elastic collisions with the hydrogen nuclei in the paraffin. Neutrons running slowly had more time to interact with the silver nuclei as they passed by, increasing the probability of being

twin-engine fighter-bomber, stripped of armaments but equipped with a compartment for a person to ride in a prone position where the bombs were normally kept. Bohr was strapped into a flight suit, with a parachute, a flight helmet with audio hookup, oxygen mask, and a handful of flares. They would be flying high, above 20,000 feet (6,100 m), to avoid German antiaircraft guns in Norway, and if they were caught by a fighter plane they were to open the bomb bay and drop Bohr into the ocean, in which case his flares would come in handy.

Unfortunately, Bohr had an unusually large head, and the standard issue flight helmet did not fit properly. He did not hear the pilot through the built-in headphones when he was told to turn on his oxygen after the plane made high altitude, and he passed out somewhere over Norway. The pilot could tell that something was wrong when he could get no verbal response from Bohr, and as soon as they were clear of Norway he dropped altitude and flew low over the North Sea. When they landed in England, Bohr was in fine shape, commenting that he had slept well during the flight.

From England, Bohr was flown to the United States, where he was taken to the top-secret atomic bomb laboratory in Los Alamos, New Mexico. Here he would add guidance, encouragement, and assistance to the theoretical work, as an expert in quantum mechanics. “An expert,” he commented, “is a person who had made all the mistakes that can be made in a very narrow field.” Upon seeing the extent of the American operation at Los Alamos, he was deeply impressed. Nothing of this magnitude had seemed possible anywhere in Europe. Although welcomed as a revered elder statesman of nuclear physics at the laboratory, he later confided to a friend, “They didn’t need my help in making the atom bomb." They seemed to have it well in hand.

captured by the target. In the original experiments, neutrons bouncing off the wooden table had been slowed down, again by collisions with hydro­gen nuclei in the wood. Hitting the heavy marble table top, the neutrons had not been slowed noticeably. To varying extents, the probability of neutron interaction would be increased as the speed of the particles was decreased, and this effect would apply to both absorption and fission. Fermi won the Nobel Prize in physics in 1938 for this discovery. Although he came very close to discovering fission, the Nobel Prize for that finding would go to Otto Hahn after World War II had ended.

At the subatomic level, all interactions of matter with matter are prob­abilistic in nature. If a freely traveling neutron flies close to a standing uranium atom, it does not necessarily do anything with the uranium, but there is a probability that it will be captured by its nucleus. The magni­tude of the interaction probability depends entirely on the speed of the neutron as it passes. Although an interaction can occur at any speed, it seems that the slower a neutron is traveling, the higher is its probability of interaction.

Neutrons set free in uranium fission events are most likely traveling with an energy of about 1 MeV. (Neutron speed is expressed as neutron energy, which is always expressed in electron volts. An MeV is a million electron volts.) There is a probability that a fast neutron can produce an additional fission by hitting a nearby uranium nucleus, but it is a low chance. Slow the neutron down to thermal speed, or 0.025 eV (electron volts), and the probability of fission increases 1,000-fold. The term thermal speed means the speed at which air molecules normally move at room temperature. To consider building a machine that will operate as a nuclear fission reactor using natural uranium, as it is mined, then all favorable probabilities must be maximized. All probabilities unfavorable to fission, such as unproductive neutron absorption or leakage, must be minimized.

In his experiments on a wooden table and with paraffin wax, Fermi had found that if a high-speed neutron hits a hydrogen atom at room tempera­ture, then the neutron and the hydrogen nucleus exchange momentum. The neutron slows to thermal speed and the hydrogen nucleus, which weighs about the same as a neutron, takes off at the speed of the original incoming neutron. This exchange between the energetic neutron and the room temperature hydrogen nucleus, or proton, would prove very impor­tant, as it is the mechanism by which the energy of fission can be trans­ferred to a working fluid and exploited as power. Use water as the working fluid in a reactor, and the fast neutrons slowing down in it make steam.

Fermi, a Roman Catholic, had married Laura Capon, the daughter of a Jewish officer in the Italian navy, and he felt that anti-Semitic laws being enforced by the Fascist government of Italy were threatening his family. He took his wife and children to Stockholm, Sweden, to accept the pres­tigious Nobel Prize, and they never returned to Italy, slipping away and shipping instead to New York City for a new life in the United States. The United States gained a Nobel laureate, and Europe lost one. Fermi began work at Columbia University upon his arrival, and in 1942 he transferred his work to the University of Chicago. At this carefully selected location in the Midwest, under strictest secrecy, Fermi and his team of scientists built the first working nuclear reactor, Chicago Pile 1, and physics and the world would never be quite the same.