A second project that captured Fermi’s imagination during summers at Los Alamos was “Taylor instability.” An important concept in hydrodynamics, Taylor instability refers to what happens when the surface between two fluids of different density—oil and water, for example—is perturbed in some way. The complex interaction between the two fluids on a surface is extremely difficult to model mathematically, but the phenomenon becomes extremely important in certain types of events, including nuclear explosions. Fermi and Ulam published several papers on this crucial aspect of hydrodynamics.
The periods at Los Alamos gave Fermi time to stretch his mind in the company of a group of extraordinary physicists and mathematicians. It also provided time for relaxation and exercise. Fermi continued hiking and fishing around the Los Alamos area, habits acquired during the war. He also enjoyed playing tennis with anyone who was willing to accept his challenge, as Ulam did frequently.*
BY 1951, THE CYCLOTRON IN CHICAGO WAS UP AND RUNNING AT A then-impressive energy of 450 MeV, one of the most powerful cyclotrons in the world. Fermi had been preparing for this moment for some time. He summarized his preparations in lectures presented at Yale in the Silliman Lectures of April 1950, subsequently published in a short volume called Elementary Particles. In the lectures, he introduced a “statistical theory of pion production” providing a “plausible approximation” of high-energy collisions. When it turned out not to be an exact predictor, he resented the criticism, correctly pointing out that he had never intended it to be so. He also prepared a paper for presentation at the opening of the cyclotron in September 1951, an event attended by more than two hundred distinguished physicists from around the world. He had already conducted a few preliminary experiments and reported on these. The celebration gave him a chance to catch up informally with old friends.
He had thought carefully about what experiments he wanted to conduct and what might be interesting to explore. His main interest was an exploration of the “strong force,” the force that holds the nucleus of an atom together, by probing the nucleus with the pion, the particle Yukawa first suggested in 1935. In 1951, physicists believed the pion was responsible for the strong force and would interact in interesting ways with “nucleons”—that is, the protons and neutrons that dance together inside the nucleus. Importantly, the Chicago cyclotron was one of the only machines powerful enough to create pions with sufficient energy to probe the nucleus.
Fermi’s experiments, conducted with Anderson and a group of younger physicists, created a beam of pions by accelerating protons in the cyclotron to very high energies and then “smashing” them into a target. The protons stimulated the nuclei of the target to emit pions, and the pions were then used to probe other nuclei—in this case, hydrogen nuclei that consisted either of a simple proton or of a “deuteron,” a proton and a neutron. Pions hit these nuclei and bounced off—“scattering,” in physics terminology—and this scattering revealed interesting things about nucleons and their relationship to pions. The three different types of pions—positive, negative, and neutral—had slightly different masses, and keeping track of the way these different pions interacted with nucleons in the hydrogen nuclei told even more about nucleons.
In the process of studying these scattering patterns, Fermi realized that he produced a new particle, created when the proton in the nucleus was hit by a pion in a range of energies centering on 180 MeV. This was the first time protons were struck with such energy by a particle capable of exploring the strong force inside the nucleus. The particle, an extremely short-lived one, was at first known by its somewhat exotic quantum state and is now called the “delta plus plus” and fits quite neatly into the group theory framework of heavy particles (baryons) proposed by Murray Gell-Mann a decade later.
Physicists call this type of particle a “resonance.” Imagine that the nucleus is a radio station broadcasting at a number of different frequencies. The cyclotron is a radio receiver that can be tuned continuously to frequencies up and down the spectrum. When the listener starts tuning, only static might be audible, but then specific frequencies will come in and out of range, with clarity. In an analogous manner, the cyclotron produces pions that “tune in to” the nucleon at energies centering around 180 MeV, stimulating it to produce the new, somewhat exotic particle.
Analysis of these results suggested that protons were fairly complicated particles, which could be stimulated into a number of different states. Perhaps they were not “fundamental” in the sense of the muon or the electron. Perhaps they had an internal structure that could be further explored. It was an important insight that pointed the way to future developments in particle physics, developments that would entirely change the way these heavy particles were viewed. Fermi followed up these experiments with computer simulations at Los Alamos, aided by Metropolis, that were among the first particle simulations ever conducted.
Fermi wrote nine papers based on these experiments and computer studies. They were the last experimental papers he ever published. Herb Anderson writes that from mid-1953 onward, three Fermi graduate students—Jay Orear, Arthur Rosenfeld, and Horace Taft—ran cyclotron experiments that prevented Fermi from pursuing his own studies more thoroughly:
Through my illness [berylliosis], he lost a major supporter who was willing to help smooth the way and cater to his way of doing things. His new students, Rosenfeld, Orear and Taft asked his guidance and advice but wanted the work to be their own. So Fermi changed his role; he spent more and more time helping others by discussion and by frequently lending a hand in the experiment, but never again to the extent that would allow him to admit that the work was his own.
He goes on to suggest that Fermi’s decision to study the origins of cosmic rays with Chandrasekhar was a result of being sidelined by his graduate students.
Anderson was surely too critical of Fermi’s students. Fermi certainly did not need Anderson’s support to get time on the cyclotron. Fermi was the most prominent and powerful man at the institute and if he wanted to do an experiment on his own he certainly would have done so. He also had a long-standing fascination with the origins of cosmic rays dating back to 1947 and was happy to pursue conversations with Chandrasekhar without being forced to do so. What Anderson misses—perhaps through jealousy because Fermi was beginning to adopt a new, younger group of physicists to mentor—is that Fermi actually enjoyed working with his graduate students, encouraging their work, supervising their experiments on a big new instrument with such potential. Fermi justifiably had first crack at the cyclotron and by mid-1953, with two years of solid work under his belt, probably felt it was time to let his younger colleagues have a chance.
Anderson did have a jealous streak, as Richard Garwin found out. In late 1952, Garwin, who was as close to Fermi as anyone during this period, had a meeting with Anderson. The latter was rather direct: there was only room for one of them at the institute going forward. Garwin took that to mean Anderson and found a job at IBM’s Watson Lab in Yorktown Heights, New York. Anderson was rightly proud of his long-time collaboration with Fermi. Of all of Fermi’s collaborators in Rome and in the United States, Anderson probably worked with Fermi the longest. He was used to serving as a sort of gatekeeper, particularly for younger members of the team and resented it when physicists like Garwin or Rosenfeld developed their own special ties to the great man. Perhaps Anderson could be forgiven, though. He would eventually die of a disease contracted in a moment of heroism, when he dashed into a lab room ahead of Fermi and Zinn to put out a beryllium fire. In doing so, he spared both colleagues a similar fate.
FREEMAN DYSON, THE YOUNG THEORIST WHO DID SO MUCH TO reconcile the work of Feynman, Schwinger, and Tomonaga, provides a revealing coda to Fermi’s work on pion-proton scattering. Dyson was a junior professor at Cornell, responsible for supervising a small group of graduate students. They decided to tackle theoretical calculations of pion-proton interactions using the same technique successfully used to analyze quantum electrodynamics. The forces governing
the pion-proton interaction are much stronger than those of electrodynamics, but Dyson and his students did not consider this a problem and got results that were fairly similar to those Fermi achieved using the Chicago cyclotron. This was the work of several years, and when completed in the spring of 1953, Dyson took a bus from Cornell to Chicago to show Fermi what they had done.
Dyson was eager to show Fermi his work. They chatted for a while about personal matters and then Fermi turned to Dyson’s results. Dyson recalled Fermi’s judgment on the work in 2004, some fifty years later: “There are two ways of doing calculations in theoretical physics,” Fermi explained. “One way, and this is the way I prefer, is to have a clear physical picture of the process that you are calculating. The other way is to have a precise and self consistent mathematical formalism. You have neither.”
Dyson was understandably stunned and asked Fermi to elaborate. Fermi explained that the mathematical technique Dyson used was inappropriate for the problem he was trying to solve. When Dyson objected that his results came very close to the numbers that Fermi himself had measured in his 1951–1952 experiments, Fermi pointed out that the arbitrary number of parameters undermined Dyson’s calculations. “I remember,” Fermi replied, “my friend Johnny von Neumann used to say, with four parameters I can fit an elephant, and with five I can make him wiggle his trunk.” With that, Dyson made his way back to Cornell with the sad news that his work of several years had not passed Fermi’s test.
In retrospect Dyson was not bitter, but rather grateful that Fermi spared him from spending even more time on what was, really, a dead end:
Looking back after fifty years, we can clearly see that Fermi was right. The crucial discovery that made sense of the strong forces was the quark. Mesons and protons are little bags of quarks. Before Murray Gell-Mann discovered quarks, no theory of the strong forces could possibly have been adequate. Fermi knew nothing about quarks, and died before they were discovered. But somehow he knew that something essential was missing in the meson theories of the 1950s.… And so it was Fermi’s intuition, and not any discrepancy between theory and experiment, that saved me and my students from getting stuck in a blind alley.
THOUGH MUCH OF FERMI’S POSTWAR RESEARCH WAS PUBLIC, A significant portion of the summer work he pursued at Los Alamos throughout the period from 1946 through 1952 was classified, none more so than work on the hydrogen bomb. In this work, Fermi combined the roles of scientist and public policy adviser, the latter of which he found somewhat uncomfortable. Public policy was to become a major headache for Fermi and led to difficult, somewhat contradictory decisions. It also led to one of the most dramatic moments in his life, the defense of his old colleague J. Robert Oppenheimer against charges of disloyalty to the US government.
* A white dwarf star is the core remnant of a dead star in which the electrons and the nuclei are so compressed the atomic structure disintegrates, leaving the electrons in a degenerate state, compressed to the limit allowed by the exclusion principle. It is extremely dense, about two hundred thousand times the average density of the earth. National Aeronautics and Space Administration, “White Dwarf Stars,” Imagine the Universe!, revised December 2010, https://imagine.gsfc.nasa.gov/science/objects/dwarfs2.html.
* Ulam once won a match with Fermi, 6–4. Fermi refused to concede defeat. He pointed out that the difference between the two scores was less than the square root of the sum of the scores, 3.17. This is a shorthand method used by statisticians to determine whether a result is significant or within the limits of measurement error. Ulam found Fermi’s response at once ridiculous and adorable and continued to play tennis with his competitive friend. Adventures of a Mathematician, 164.
CHAPTER TWENTY-TWO
IN THE PUBLIC EYE
AS A CENTRAL FIGURE IN THE MANHATTAN PROJECT, FERMI could hardly have been surprised when he was nominated to serve on the first General Advisory Commission (GAC) to the AEC. His Manhattan Project colleague Robert Bacher, a commissioner himself, and another commissioner, Carroll Wilson, developed a list of scientists and engineers who could be relied upon to give expert advice to the AEC as it took control over the US programs on nuclear weapons and nuclear energy. The list contained all the usual suspects. Conant, Oppenheimer, Rabi, Seaborg, and Cyril Smith were also on the list, as were former MIT Radiation Lab director and newly appointed president of Caltech Lee DuBridge; Hood Worthington, an official at DuPont who worked on the Hanford reactors; and Hartley Rowe, another Los Alamos consultant. The inaugural meeting of the GAC was held in Washington the first week of 1947 and was convened there every several months for two or three days at a time.
Fermi underwent a full background check for this assignment, now under the auspices of the Atomic Energy Act of 1946, for the new “Q” clearance. His FBI file makes for interesting reading and is more thorough than the one prepared for him by the FBI in 1940. Those interviewed this time overwhelmingly supported granting him clearance, vouching for his trustworthiness and his brilliance as a physicist. Zinn described Fermi politically as an “ultra-conservative.” Norman Hilberry and Walter Zachariasen both considered him “the greatest living physicist,” which would probably have come as a surprise to Einstein and Bohr, both of whom were still alive. Hilberry went so far as to state that Laura’s family had been fervently anti-fascist, which was simply not true. These statements reflected less the mature judgment of those who were interviewed than an enthusiasm for ensuring that Fermi passed his background check.
Perhaps more fascinating, though, is the testimony of John Dunning, his former Columbia colleague and the builder of Columbia’s first cyclotron. Dunning started off by saying that he had no doubt as to Fermi’s personal integrity and loyalty to the United States, but then added that he was opposed to giving foreign nationals security clearances of any type. He pointed out that once someone had changed allegiance, that person might change it again. In this regard he pointed to Fermi’s decision to move from Italy to the United States in 1939. Incredibly, he also pointed to Fermi’s move from Columbia to Chicago as an example of changing allegiance, moving from one university to another. Surely, he knew that Compton ordered Fermi to move to Chicago, along with the entire pile project. He must have felt left behind as a result and whatever resentment he felt toward Fermi bubbled up during his interview with the FBI.
The FBI wisely ignored Dunning’s remarks, much as they had LaMer’s in 1940. That Fermi’s brain was the source of much of what was classified in atomic research did not seem to occur either to LaMer or to Dunning, or if it had, it was not enough to stifle their resentment. Fermi was granted a Q clearance, which he used both for GAC work and for summer work at Los Alamos.
NO MINUTES WERE TAKEN DURING THE FREE-WHEELING, SCIENTIFIC and technical discussions of the GAC, chaired by Oppenheimer. The topics were all highly classified, covering a range of issues relating to the development and refinement of the “conventional” nuclear arsenal. The hydrogen bomb remained a topic of intense interest, but so were the program on fission weapons and the continued development of nuclear reactors at Hanford and later at Savannah River, South Carolina, operated jointly by the AEC and by DuPont.
David Lilienthal, who headed the AEC from 1946 through 1950, was no stranger to the corridors of political power in Washington. Sophisticated though he was, the GAC scientists impressed him mightily, none more so than Fermi. In the first week of the GAC’s deliberations, Lilienthal had lunch “in a terrible little cafeteria in the War Department building” with Rabi and Fermi. “To have spent the day with Fermi,” he wrote that evening in his diary “is like saying that one spent the day with Copernicus or Galileo or the primitive who discovered fire.”
Fermi attended almost all of the meetings during his tenure on the GAC and participated when he had technical expertise or might have had strong feelings. For example, in policy deliberations exploring the choice of expanding the national laboratory program or strengthening existing programs, he leaned toward the latter option. He was out
spoken in his support for strengthening Los Alamos and, according to Segrè, thought that in a period of scarce resources the work at Los Alamos should have priority over the development of civilian nuclear power reactors. He also supported the building of a high-flux reactor for continued advanced research. Among the issues discussed at the GAC, the only official written record of his contributions relates to a series of critical meetings in late October 1949.
Overshadowing these meetings was the surprise detonation of the first Soviet fission test in August 1949. Like the launch of Sputnik eight years later, the first Soviet nuclear test raised alarm throughout the American national security establishment. There could be little doubt that brilliant Soviet physicists were also at work on a fusion weapon. President Truman and his national security advisers wanted advice on whether to launch an all-out effort to create a fusion weapon.
The technical problems were still as daunting as they were during the Manhattan Project: how to keep the device held together long enough to fuse a meaningful amount of hydrogen. These problems were discussed at length during a major classified symposium during the summer of 1946 at Los Alamos, attended by Fermi and many of his Manhattan Project colleagues, but the challenges remained. Teller and Ulam cracked the puzzle in 1951, but that was two years in the future, and in October 1949 many scientists, including Fermi, were still skeptical that a practical weapon was feasible.
Under significant political pressure to act, the AEC asked the GAC for its views. The question came to discussion and vote on October 29, 1949. The entire GAC voted unanimously against moving forward. The main report focused on three points: the hope that “the development of these weapons can be avoided”; a reluctance to see the United States take the initiative in this matter; and, finally, “that it would be wrong at the present moment to commit ourselves to an all-out effort toward its development.” They were also concerned that such an effort would divert resources and energy away from what was seen to be an equally important national security priority, the continued development of weapons based on nuclear fission. Given the uncertainties surrounding the feasibility of the hydrogen bomb, it seemed more responsible to focus on improving the existing fission stockpile.
The Last Man Who Knew Everything Page 34
