Джеймс Глик - Genius - The Life and Science of Richard Feynman

He converted the equations into a form that would al ow a shortcut solution in terms of a minimum principle, now his favorite technique. He worked out a theorem for the spatial distribution of fissionable material—and discovered that the difference would not matter in a reactor as smal as this.

When enriched uranium final y began to arrive, the water boiler took form as a one-foot sphere inside a three-foot cube of black beryl ium oxide, sitting on a table behind a heavy concrete wal at the pine-shaded bottom of Omega Canyon, miles away from the main site. It served as the project’s first large-scale experimental source of neutrons

and the first real explosion hazard. For al the theorists, the elements of this first problem became leitmotivs of their time working on the bomb: the paths of neutrons, the mixing of esoteric metals, the radiation, the heat, the probabilities.

In the muddy weeks of April the population of scientists reached about thirty. They came and went through a temporary office in Santa Fe and disappeared from there into a void in the landscape. If they had seen their destination from the air, they would have understood that they were to be situated in a compound atop a flat finger of ancient lava, one of many radiating from the giant crater of a long-quiet volcano. Instead, their imagining of the place began with mysterious addresses: P. O. Box 1663 for mail, Special List B for driver’s licenses. Not al the procedures devised in the name of security helped al ay the suspicions of the local population. Any local policeman who pul ed over Richard Feynman on the road north of Santa Fe would see the driver’s license of a nameless Engineer identified only a s Number 185 , residing at Special List B , whose signature was, for some reason, Not required . The name Los Alamos meant hardly anything. A canyon? A boys’

school? When scientists reached the site they would see, as likely as not, a former professor standing outdoors and peppering a military construction crew with unwanted instructions. If Oppenheimer happened to be there to greet them, he would say from beneath the already famous hat,

“Welcome to Los Alamos and who the devil are you?” The first familiar face that Feynman saw belonged to his Princeton friend Olum—Olum was standing in the road with

Princeton friend Olum—Olum was standing in the road with a clipboard, checking off each truckload of lumber as it arrived. At first Feynman slept in one of a row of beds lined up on the balcony of a school building. Food was stil coming up from Santa Fe in the form of box lunches.

Amid the turmoil of construction, the concrete hardening in the open air, the noise of hand-held buzz saws everywhere, only the theorists had the equipment they needed to start work immediately—one blackboard on rol ers. Their true ground-breaking ceremony came on April 15. Oppenheimer gathered them together, along with the first few experimentalists and chemists, to learn official y what they had been told in hushed tones. They were to build a bomb, a weapon, a working device that would concentrate

the

neutron-spraying

phenomenon

of

radioactivity into a speck of space and time concentrated enough to force an explosion. As the lecture began, Feynman opened a notebook and wrote the cautionary words, “Talks are not necessarily on things we should discuss but things we have worked out.” Much was known to the teams from Berkeley and Chicago, or so it seemed.

The splitting of an ordinary uranium atom required a blow from a fast, high-energy neutron. Every atom was its own tiny bomb: it split with a jolt of energy and released more neutrons to trigger its neighbors. The neutrons tended to slow, however, dropping below the necessary threshold for further fission. The chain reaction would not sustain itself.

However, the rarer isotope, uranium 235, would fission when struck by a slow neutron. If a mass of uranium were

enriched with these more volatile atoms, neutrons would find more targets and chain reactions would live longer.

Pure uranium 235—though it would not be available in any but microscopic quantities for months—would make an explosive reaction possible. Another way to encourage a chain reaction was to surround the radioactive mass with a shel of metal, a tamper, that would reflect neutrons back toward the center, intensifying their effects as the glass of a greenhouse intensifies its infrared warming. A lanky Oppenheimer aide, Robert Serber, described the different tamper possibilities to his audience of thirty-odd men radiating an almost palpable energy of nerves. Feynman wrote quickly. “… reflect neutrons … keep bomb in …

critical mass … Non absorbing equiscattering factor 3 in mass … a good explosion …” He sketched some hasty diagrams. From nuclear physics the discussion was forced to turn to the older but messier subject of hydrodynamics.

While the neutrons were doing their work, the bomb would heat and expand. In a crucial mil isecond would come shock waves, pressure gradients, edge effects. These would be hard to calculate, and for a long time the theorists would be calculating blind.

Making a bomb was not like making a theory of quantum electrodynamics, where the ground had already been mined by the greatest scientists. Here the problems were fresh, close to the surface, and therefore—this surprised Feynman at first—easy. Beginning with the issues raised by the first indoctrination lectures, he produced a string of smal triumphs, gratifying by contrast with the long periods

of wandering in the dark of pure theory. There were compensating difficulties, however.

“Most of what was to be done was to be done for the first time,” an anonymous ghostwriter of the bomb’s official history wrote afterward. (The ghostwriter was Feynman, cal ed to this unaccustomed service by his former department head, Harry Smyth.) Struggling to sum up the problems of theoretical science at Los Alamos, he added

“untried,” and then “with materials which were for a long time practical y unavailable.” Materials —he could not bring himself to write uranium or plutonium after the euphemistic years of tubealloy and 49 . The wait for tubeal oy had been agonizing, for the theorists no less than the experimenters.

More mundane materials could be requisitioned—at the laboratory’s request Fort Knox delivered two hemispheres of pure gold, each the size of half a basketbal . Feynman, giving Smyth a tour one day, pointed out that he was absently kicking one of them, now in use as a doorstop. A request for osmium, a dense nonradioactive metal, had to be denied when it became clear that the metal urgists had asked for more than the world’s total supply. In the cases of uranium 235 and plutonium, the laboratory had to wait for the world’s supply to be multiplied a mil ionfold.

For now the only knowledge of these materials came from experiments on quantities so tiny as to be invisible.

The experiments were expensive and painstaking. Even getting an early measurement of plutonium’s density chal enged the team at Chicago. The first dot of plutonium

did not arrive at Los Alamos until October 1943. Trials with more comfortable quantities would have to wait; in the event, just one ful -size experiment would be possible. Most questions would have to be answered with pencil and paper. It soon became clear that theory at Los Alamos would be performed on a high wire without a net. The theoretical division was smal , just thirty-five physicists and a computing staff, charged with providing analysis and prediction for al the much larger practical divisions: experimental, ordnance, weapons, and chemical and metal urgical. Analysis and prediction—what would happen if… ? Theorists at Los Alamos had dispensed with the luxury of contemplating simple mysteries—the way a single atom of hydrogen emits a single packet of light in such and such a color, or the way an idealized wave might travel through an idealized gas. The materials at hand were not idealized, and the theorists, no less than the experimenters, had to poke about in the rubble-strewn territory of nonlinear mathematics. Crucial decisions had to be made before the experimenters could conduct trials. Feynman, in his anonymous account, listed the main questions: How big must the bombs be (the imploding sphere of plutonium or the gun device in the case of uranium)? What would be the critical mass and the critical radius for each material, the dimensions beyond which a chain reaction would sustain itself?