The Last Man Who Knew Everything

by David N. Schwartz

  However, as early as November 1942, doubts began to arise regarding the suitability of plutonium for use in a fission weapon. Those doubts only grew over time. Throughout the early part of 1944, Segrè had been working in a small shack located in a remote corner of the mesa, focused on analyzing the properties of the plutonium produced at Oak Ridge and Hanford. His research, which threatened to bring the entire plutonium project to a grinding halt, suggested that the plutonium produced in the heart of nuclear reactors was quite different from the plutonium produced in the cyclotron. Under the intense neutron bombardment over extended periods of time in the Oak Ridge reactor, some of the atoms of Pu-239 absorbed an additional neutron, creating the isotope Pu-240. That extra neutron threw the nucleus into turmoil, creating spontaneous fission. It was Fermi who, looking at Segrè’s data, suggested that Pu-240 was the culprit, prefissioning Segrè’s microsamples. At the power levels in the reactors at Oak Ridge and Hanford, Pu-240 could compose as much as 7 percent of the plutonium being produced. Spontaneous fission was going to be a major problem in using plutonium. As the news of Segrè’s findings was absorbed, it became clear that the only method of building a plutonium bomb would be the implosion method. A sphere of plutonium, even with 7 percent Pu-240, could be made sufficiently subcritical that spontaneous fission would not be a problem. If the subcritical sphere were then explosively compressed into a sufficiently dense sphere, it would become critical and a plutonium bomb would work.

  That was, however, a pretty big “if.” In order to achieve an effective critical mass, the sphere would have to retain a perfectly spherical shape from the initiation of the implosion through to the moment of criticality. Otherwise, the fission reaction would pass through the sphere unevenly, resulting in a misfire. To maintain the plutonium in a perfect sphere throughout the implosion, the shock waves of the explosion would have to arrive across the entire surface of the subcritical sphere at exactly the same time. There was very little room for error.

  To do this involved an unprecedented technical challenge. Imagine standing in a swimming pool, with one hand on a floating beach ball so that the ball is halfway submerged, the other holding a penny. When the water in the pool settles down and is perfectly calm, the penny is dropped. The penny creates a circular wave that travels on the surface of the water and strikes the beach ball. The front of the wave strikes the beach ball first, and then the rest of the wave breaks across the surface of the ball. A convex surface is striking a concave surface (or vice versa, depending on the point of view). The first moment they touch is at a single point on the surface of the ball closest to where the penny dropped. Blast waves behave like the waves created by the penny, emerging outward in circular fashion from the point of detonation. They begin to compress the subcritical sphere at the single point where the blast wave first touches the surface of the sphere, thereby deforming it almost immediately. Adding to this complexity is that the wave comes at the target sphere in three dimensions, unlike the wave in the pool. It is extremely difficult, but possible, to create simultaneous detonations all around the sphere at various equidistant points. However, the only way to make sure that the sphere retains its shape throughout the implosion is to shape the blast wave that emerges from each detonation so that it has the same shape as the sphere when it arrives there, microseconds after detonation. Scientists would need to reverse the shape of the wave and they would have to do it very quickly after the charge detonated. No one had ever done anything quite like this before. To do it at all, and with the requisite accuracy, would require an enormous amount of scientific and engineering brilliance.

  The Los Alamos reorganization involved the redeployment of personnel from the plutonium gun project to the implosion project. Research into implosion, which was under the auspices of a low-priority group headed by Navy officer William S. “Deak” Parsons, was transferred to a high-priority group headed by a flamboyant, Russian-born physical chemist named George Kistiakowsky, on secondment from Harvard. As part of the shake-up Oppenheimer named Fermi associate director of Los Alamos with overall responsibility for research and theory and for all special problems related to nuclear physics. This was an honorific title that gave him little administrative authority but that allowed him to poke his nose into issues as needed and as might interest him. He was also given direct responsibility over a special division—F Division, F for Fermi—under which a variety of projects not subsumed in other divisions were grouped. These included theoretical and experimental work on the “water boiler” project and on the “Super” project.

  The water boiler, a project particularly close to Fermi’s heart, was a high-intensity reactor that used powdered uranium enriched to 14 percent U-235 and mixed into ordinary water. Its location at a remote site code-named Omega in a canyon off one side of the mesa was an ideal place for Fermi to keep experimentally active, particularly when so much of his time was spent helping with other scientists’ projects. The water boiler ran at low power, but even so was sufficiently reactive, owing to the enriched uranium, that the water’s tendency to absorb neutrons could be ignored. The liquid was contained in a sphere one foot in diameter, with instrumentation surrounding it to measure neutron production and absorption, as well as control and safety features. Later configurations of the water boiler ran with increasingly enriched uranium. The water boiler was useful in the study of the critical mass of uranium. It also produced refined studies of neutron production, one of which, conducted by Fermi’s old friend Bruno Rossi, determined how quickly “prompt” neutrons emerge from fission reactions. Fermi’s Omega site team included L. D. P. King, Herb Anderson, and a young woman named Joan Hinton. King ran the project for Fermi. A Purdue-trained physicist, he worked closely with Fermi during these next few years. A graduate of Bennington and the University of Wisconsin, Hinton also became a daily colleague of Fermi at the Omega site. Segrè describes Joan as “very athletic,” perhaps because she could clamber down into the ravine that had been chosen to locate the water boiler. She was quite sympathetic to left-wing causes, and in 1948 as the revolution in China came to a close, she moved there and lived out the rest of her life under communist rule. For the time being, though, she was eager to serve as Fermi’s assistant.

  The Super was the fusion bomb (hydrogen bomb) project that had preoccupied Edward Teller practically every moment since early 1942 when Fermi first suggested the possibility. To his enormous frustration, Oppenheimer could not get Teller to work on anything else, either at Berkeley or at Los Alamos. Oppenheimer judged the likelihood of a real breakthrough on the Super too low to devote significant resources to it, but he wanted to find a way to keep the creative Teller happy and gainfully occupied, so he worked directly with Teller on the project. After the reorganization, Teller would become Fermi’s problem.

  A MAJOR PROBLEM PHYSICISTS HAD TO CRACK WAS THE MATTER OF critical mass. Given the extraordinary expense of producing U-235 and Pu-239, the project leadership required more than vague estimates. They required accurate calculations, based on theoretical considerations that were being explored for the first time. Fermi’s old friend Hans Bethe, the head of the Theoretical Division since the outset of Los Alamos, had been thinking deeply about this problem, as had many others, including, for example, a youngster from Queens, New York, named Richard Feynman, who had already annoyed military security with his penchant for breaking into locked safes and leaving “guess who?” notes. Bethe and company had been helped enormously by the arrival of a team from Britain, including Rudolph Peierls and his young protégé Klaus Fuchs who had been part of a parallel project, run by the British government since the spring of 1940, to explore the possibility of fission weapons.

  The story of the British project, known by the code name “Tube Alloys,” is fascinating and in many ways mirrors the Manhattan Project, although it started earlier and made significant progress before the US and British governments revealed to each other what they had been working on. As part of this project, German refugees Rudolf Peier
ls and Otto Frisch did important theoretical calculations regarding the critical mass of uranium 235 and, although the approximately one kilogram mass they calculated was actually too small for a true critical mass, their work indicated that the problem was not intractable. Fermi’s old student Bruno Pontecorvo was also involved, working on a plutonium production reactor project run by the British at Chalk River in Ontario, Canada. Pontecorvo had several meetings with Fermi in Chicago before Fermi arrived in Los Alamos, and they discussed various aspects of reactor design.

  Peierls spent time in Rome before the war and came to know Fermi well at Los Alamos. They worked together and lived one below the other in the cramped apartments built by Groves’s team. Their wives also hit it off and became fast friends. Peierls admired Fermi greatly, but he was also a subtle and observant critic. He noted that Fermi seemed deliberately to choose problems that could be radically simplified, that when he came to a stage in a problem where complex mathematics would be required to move forward, he “generally left them. He didn’t choose to go beyond that.” Peierls concedes that for Fermi “the range of things that seemed simple to him covered very many things which were complicated to all of us until he explained them,” but when a problem seemed like it would involve more work than he felt it was worth, he lost interest. This critique rings true.

  By the time Peierls came to Los Alamos under the cooperation agreement between the Manhattan Project and Tube Alloys, he had in tow a younger colleague named Klaus Fuchs. Fuchs was a member of the German communist party who left Germany in 1939 and moved to England. He and Peierls worked together, and Peierls brought the younger physicist with him to Columbia, and then to Los Alamos. The two of them worked in the Theoretical Division together under Peierls’s old friend Hans Bethe. From this vantage point, Fuchs was ideally placed to pass vital intelligence to the Soviets, for whom he had begun to spy several years earlier.

  The critical mass problem that preoccupied the Theoretical Division was amenable to brute force calculations. In an era prior to the ready availability of electronic computers, the most effective way of doing these calculations was to rely on slow, simple mechanical calculators, operated by teams of young women, called “computers,” who sat at their desks for eight-hour shifts of mind-numbing work. They were overseen by the exuberant young Feynman. An undergraduate at MIT before he was chosen by Oppenheimer for Los Alamos, Feynman had never before met Fermi. Feynman was mightily impressed with the Italian émigré, not because of Fermi’s reputation, which mattered little to him, but because of Fermi’s ability to interpret the results of calculations. Many years later he remembered an early encounter with Fermi:

  We had a meeting with him, and I had been doing some calculations and gotten some results. The calculations were so elaborate it was very difficult. Now, usually I was the expert at this; I could always tell you what the answer was going to look like, or when I got it I could explain why. But this thing was so complicated I couldn’t explain why it was like that. So I told Fermi I was doing this problem, and I started to describe the results. He said, “Wait, before you tell me the result, let me think. It’s going to come out like this (he was right), and it’s going to come out like this because of so and so. And there’s a perfectly obvious explanation for this—” He was doing what I was supposed to be good at, ten times better. That was quite a lesson for me.

  Feynman later engaged Fermi in an hour-long argument about a technical issue related to the water boiler, and when Fermi finally conceded that Feynman was right, the younger physicist regarded this as a sort of triumph.

  The respect between the two physicists was mutual. As a mark of that respect, Fermi naturally felt at ease teasing him. At Los Alamos Feynman one day picked up the phone. It was Fermi at the other end. He had just read a report that Feynman had produced and explained to Feynman that he considered the research too trivial to merit publication. He claimed the results were obvious even to a child. Feynman countered, “Only if that child is Fermi.” To which Fermi replied, “No, even an ordinary child.”

  THE CRITICAL MASS PROBLEM MAY HAVE BEEN DAUNTING, BUT the most challenging technical problem Fermi worked on involved calculations for the implosion device. Kistiakowsky’s team understood that high-explosive “lensing” would be required. Lensing is a technique that changes the shape of a blast wave through high-explosive material in the same way that an optical lens changes the shape of a light wave, by slowing it down. High-explosive material of differing densities through which the blast wave would travel at differing speeds, resulting in the proper shape of the wave just as it reached the subcritical plutonium sphere, would push the entire sphere inward at exactly the same time.

  Lensing required technical expertise in many areas. Kistiakowsky was perhaps the greatest expert in the world on high explosives—he loved blowing things up—but that was not sufficient. Other expertise was needed, particularly in the physics of optics. Luis Alvarez knew a great deal about optics from his work prior to the war and was drafted into the project. So did Ed Purcell from Harvard, another optics specialist. A Hungarian Jewish mathematician, however, would be the central figure to do the calculations required to structure the high-explosive charges around the plutonium sphere. His name was John von Neumann.

  Von Neumann is regarded by many as one of the greatest mathematicians of the twentieth century. Born in Budapest, he went to high school with the other Los Alamos Hungarians, Szilard, Teller, and Wigner. They all considered him the smartest of the bunch. By the age of eight, young “Johnny” was able to multiply eight-digit numbers by eight-digit numbers in his head, far faster than anyone could do it on paper. He had an idiot savant’s ability to calculate, but he was no idiot. He was highly social, at ease in groups, and a great storyteller. He was even shorter than Fermi and had an impish, mischievous look about him, which he reinforced with colorful but crude jokes. He also had an explosive temper and would erupt with anger frequently, certainly more frequently than the normally placid Fermi.

  He emigrated in 1933 when the Institute for Advanced Studies at Princeton offered him a tenured position. By the time the war started, he had contributed to virtually every area of mathematics and had, while dabbling in physics, published major work giving a formal mathematical basis for the quantum work of Heisenberg and Dirac.He joined the war effort early on and worked on conventional explosive shock waves before arriving as a consultant at Los Alamos. Fermi knew of his work, but the two had never met. At Los Alamos they were thrown together frequently, for long stretches. Fermi quickly recognized the Hungarian’s superior mathematical ability but always tried to outdo von Neumann when it came to calculating. He rarely succeeded. Bethe, Fermi, and von Neumann could often be found sitting together in a quiet room inside the throbbing heart of the Theoretical Division, challenging each other to solve complex integral equations related to pressure waves. Sometimes Oppenheimer would join them. Von Neumann usually left these other three brilliant physicists in the dust.

  Von Neumann’s mathematical abilities never ceased to amaze Fermi. Years later, returning from summer work at Los Alamos after the war, he regaled colleagues over lunch at the University of Chicago faculty club with a story of how von Neumann masterfully solved a particularly thorny mathematical problem. As Fermi’s young physics department colleague Courtney Wright recalls, Fermi observed of his own role in solving the problem: “You know, I felt like the fly who sits on the plow and says, ‘We’re plowing.’”

  Not every calculation was done on paper or in von Neumann’s head. Like the work on critical mass, work on the implosion device required a variety of mechanical calculators operated by the “computers.” Fermi enjoyed using them himself, so much so that one of the first things he did upon arriving at Los Alamos was write to Pegram at Columbia to send him the calculator he left behind when he moved to Chicago. Fermi also used the newest wave of IBM mechanical calculators that were driven by punch cards. This experience left a mark on him and inspired him after the war to become
one of the first physicists to use computers to simulate physical interactions. For von Neumann, the IBM machines inspired him in another direction and led him after the war to design the first programmable, fully electronic computer.

  FERMI SOON SETTLED INTO A FAIRLY INTENSE BUT REGULAR ROUTINE. After his traditional simple breakfast prepared by Laura, he would walk or bicycle to the highly secured Technical Area where the daily work on the bomb took place. The mornings were his alone, and he concentrated on any particular physics problems that were bothering him. He also tried to keep abreast of the myriad administrative duties involved in managing the wide range of scientific efforts under his supervision. The afternoons were for others and he opened his office door to all. It soon became clear that if a physicist or an engineer had a difficult problem to solve, approaching Fermi would almost always lead to a quick, clear solution. Segrè recounts a moment when there was a problem with a particular electric circuit. Fermi analyzed the problem, listed the characteristics of an electronics tube that would solve the problem, and a few hours later a tube with those characteristics had been found, inserted into the circuit, and the problem was solved. He was pulled into one meeting after another, to give advice and counsel.

  He also began to give lectures in physics to anyone who cared to attend. These became a regular series. The younger staff members particularly appreciated having the opportunity to break away from their work to hear one of the greatest physicists in the world lecture on neutron physics.