Enrico Fermi came to Harvard to give the Loeb Lectures in the fall of 1953. I was eager to meet him. I admired his work, of course, but I also thought there might be a distant family connection between us. My aunt had given me the impression that after Fermi’s arrival in the United States in 1939, she and members of the Fermi family had become the best of friends. When I ran into Fermi in the hallway of the Harvard physics building, I mentioned my aunt. Fermi gave me a chilly stare, and, without saying a word, walked away. Some years later, I described this encounter to someone who knew Fermi very well. He was not surprised.
During his visit, Fermi was persuaded to give an informal talk to a journal club formed under the guidance of Roy Glauber. Then a young assistant professor, Glauber would later win a Nobel Prize. He had gotten to know Fermi at Los Alamos during the war. I had hoped that Fermi would discuss the meson experiments being conducted at the University of Chicago. His talk went no further than describing an elementary problem in quantum theory. Most of us could have given the same lecture. With the exception of Paul Martin, we remained silent. Martin was the most brilliant of the graduate students; he objected to the approximations Fermi had made. Fermi gave a second lecture. Martin was still not satisfied. And a third. At that point, Martin gave up. Fermi would have continued until he had beaten Martin into submission.
Non simpatico.
The Pope of Physics is an account of Fermi’s life and times. Gino Segrè and his wife, Bettina Hoerlin, have written their account from the inside out; they knew a good many people who knew Fermi. Hoerlin’s father, Herman Hoerlin, worked with Fermi at Los Alamos, and Segrè’s uncle, Emilio Segrè, had been one of Fermi’s original collaborators in Rome. Both Segrè and Hoerlin could regard Fermi as a familiar presence.
Enrico Fermi was born in Rome on September 29, 1901. At the age of seventeen, after doing brilliantly in his entrance exams—sound waves, partial differential equations, Fourier transforms—Fermi was admitted to the Scuola Normale in Pisa.1 Italian physics was not at the time illustrious. Fermi was largely self-taught; he knew more physics than his professors.
After graduation, Fermi came to the attention of the physicist Orso Corbino, who happened to be connected to the political powers controlling the universities.2 Corbino recognized Fermi’s exceptional abilities, and, until his death in 1937, remained Fermi’s loyal pistone. Corbino arranged for Fermi to study at Göttingen. It was in Göttingen that David Hilbert presided over the world’s most important center of mathematical research, but Göttingen was a jewel with two facets. The eminences of theoretical physics were either in Göttingen or passing through it: Max Born, Werner Heisenberg, James Franck, Pascual Jordan, and many others.3
Fermi never had any interest in pure mathematics, and did little to cultivate the mathematicians.
He did little to cultivate the physicists either. It is not, in fact, clear what Fermi was doing or whom he was cultivating.
Curiously enough, one of Fermi’s early papers was purely mathematical.4 Consider a sphere. A great circle on the sphere is known as a geodesic. It can be the shortest distance between two points. In regions close to the geodesic, the sphere is locally flat. Curvature only becomes apparent globally. General relativity is a four-dimensional theory: three of space, one of time. Fermi considered an observer falling freely along a temporal geodesic; he is following the shortest distance between two points in time. It is possible to choose this geodesic so that it defines the spatial coordinates of the system at all points, and to choose it, moreover, so that its tangent vector defines the direction of time. Fermi gave an account of the local coordinates close to the temporal dimension, showing that they must be Euclidean. These coordinates still appear in relativity texts. There is no controversy about them.
The same cannot be said about one of Fermi’s other papers. In 1905, Albert Einstein published a brief paper entitled, “Does the Inertia of a Body Depend Upon Its Energy Content?”5 In it, he analyzed a situation in which a body emits an equal amount of radiation in opposite directions. In modern terms, we might think of these as light quanta, or photons. These quanta carry energy and momentum. Einstein argued that this energy is equivalent to the mass lost by the emitting object. The ensuing equation, E = mc2, is now famous.
How general is this equation?
In 1904, the brilliant Austrian physicist Fritz Hasenöhrl claimed that under some circumstances, E = ⅜mc2. Still another physicist, Max Abraham, argued that E = ¾mc2 when the electron’s self-interaction is taken into account.6 Fermi was convinced that Einstein’s formula was always correct. His paper was first published in Italian, translated into German, and reprinted in the Zeitschrift für Physik.7 It was read by many physicists. The consensus now is that while Fermi may have been right in that Hasenöhrl was wrong in this special case, E is not always equal to mc2.8
Abraham, at least, had a point.
There was little or no culture in Fermi’s family home, and, apart from what he had been taught in school, I doubt that he ever read a book of fiction or poetry for pleasure. It is interesting to compare him to Robert Oppenheimer, who read omnivorously in any number of languages, Sanskrit among them. Fermi never liked him very much. I was a postdoctoral fellow at the Institute for Advanced Study in Princeton between 1957 and 1959. Oppenheimer was then the director. From time to time, junior fellows would be summoned to his office and quizzed on what they had been doing. It was better to say you had been doing nothing than to tell Oppenheimer something incorrect or trivial. When I got my summons, I was not doing physics. I was reading Marcel Proust. I told Oppenheimer the truth. He gave me a wistful look. When he had been my age, he said, he had taken a bicycle trip around Corsica. At night he read Proust by flashlight. I found this endearing. Fermi would have found it irritating.
His time in Göttingen at an end, Fermi had no job. Corbino came to his rescue. He arranged for Fermi to teach a course at the University of Rome. By living at home, Fermi got by. He then accepted a lectureship in Florence, and, with Corbino’s help, in 1927, a professorship in Rome.9 That year, he married Laura Capon, the daughter of an assimilated Jewish naval officer.10 The physics department was located on the Via Panisperna. The Boys of Panisperna, as they became known, changed twentieth-century physics.11 Fermi was designated the Pope, and Corbino Padreterno, God Almighty, in recognition of his role in sustaining the group. Segrè’s uncle Emilio was Basilico, the legendary basilisk that could cause death with a single glance. Just who served as Beelzebub is not known.
Ettore Majorana was known as Il Gran Inquisitor, because of his brilliant and corrosive questions.12 He was the one member of the group that Fermi considered his intellectual equal. Fermi arranged for Majorana to spend some time in Germany with Heisenberg. The visit unhinged Majorana. He wrote an anti-Semitic letter to Segrè, and when he returned to Italy, entered into a period of self-isolation that lasted until 1937, when Fermi used his influence to get him a professorship at the University of Naples. In 1938, Majorana disappeared while taking a ferry from Palermo to Naples. His body was never found.13
The neutron afforded Fermi his first great triumph. James Chadwick correctly identified the neutron in 1932. It had been missed, as Majorana pointed out, by Irène Curie and Frédéric Joliot, who thought they had seen a very energetic massless radiation quantum. Majorana was sure the particle had to be massive.14 He was right. The work Fermi did on the neutron was both experimental and theoretical. The neutron is an unstable particle with a lifetime of a little less than fifteen minutes. Fermi constructed the first correct theory of its decay. Two of the particles the neutron decays into are very familiar: the proton and the electron.15 The third, casually suggested by Wolfgang Pauli, was a very light electrically-neutral particle. The Italians call the neutron the neutrone, so Fermi named Pauli’s particle the neutrino. The name stuck. Fermi’s theory has evolved over time, but one of the things that has remained is the insight that the electron and neutrino do not exist within the neutron before its decay. They are created by the process.16
Soon after the neutron was discovered, physicists realized that it was an ideal nuclear probe. Since it was electrically neutral, it could penetrate the nucleus of an atom without being repelled. Some nuclear collisions produce neutrons and these could be used as sources. Fermi and the Boys bombarded various elements with neutrons. They produced new, never-before-seen versions of matter, but they also made an accidental discovery that changed physics. They noticed that if their target element was on a wooden rather than a marble table, interactions with the neutrons were enhanced. At first Fermi thought this must be some kind of mistake, but then, one morning, he put paraffin in front of the target and got the same effect. He then went home for lunch and a siesta. By the time he returned to the laboratory, he understood everything.17
To help one appreciate Fermi’s insight, I need to say something about the quantum theory nature of the neutron. The neutron is both a wave and a particle. So is a baseball, but because of its mass and size this fact is unobservable. If someone gives you the task of breaking a window with a baseball, you increase your chances by throwing the ball harder. Precisely the opposite is true of the neutron. Its size, as measured by its wave length, is increased if it is slowed down, and as the wavelength of the neutron becomes comparable to the size of the nucleus, its interaction with a nucleus becomes more likely. The effect of the paraffin, or the wood in the table, was to slow the neutrons down. To moderate the speed of a neutron, one wants a collision with a particle of comparable mass. Paraffin is rich in hydrogen, which contains protons of mass comparable to the neutron. So paraffin acts as a moderator of neutron speeds. Moderators play an essential role in the design of nuclear reactors.
The Boys studied one element after another, coming eventually to uranium. Fermi had a clear idea of what he was going to find. He was sure the neutron would enter the uranium nucleus and produce an unstable uranium isotope, which would transform itself into a nucleus with an additional proton. This would be a new transuranic element. It would be produced in micrograms, but its radioactivity would give it away. When Fermi and the Boys did the experiment, they found novel radioactivity, and were sure it was from the transuranic element. Fermi’s result was published in Nature in 1934.18 Not long thereafter, a German chemist named Ida Noddack published a paper in a chemistry journal arguing that Fermi’s experiment had not considered all the possibilities. The nucleus might have broken up into several large fragments. In modern terminology, it might have fissioned. This paper became known to Fermi. He chose to ignore it.19 Was it because Noddack was a woman? I do not think that this was an important reason. If someone like Lise Meitner had made such a claim, it would have been taken very seriously. But Noddack did not give any mechanism for the fission. She did not even ask whether energy conservation allowed this to happen. When Meitner and her nephew Otto Frisch considered similar experiments done by Otto Hahn and Fritz Strassmann a few years later, the first thing they did was to study the energy balance.20
I once asked Emilo Segrè why the Boys did not discover fission. It was an accident, Segrè explained. Radioactivity from the bombarded uranium was overwhelming their detection equipment, so they introduced extra shielding around the uranium. When a nucleus like uranium fissions, the resulting charged energetic nuclear fragments leave an impact on any detector. But in this case, they were blocked.
There is a remarkable twist to the story. Fermi won the Nobel Prize in 1938. A note added to Fermi’s Nobel lecture reads:
The discovery by Hahn and Strassmann of barium among the disintegration products of bombarded uranium, as a consequence of a process in which uranium splits into two approximately equal parts, makes it necessary to reexamine all the problems of the transuranic elements, as many of them might be found to be products of a splitting of uranium.21
Noddack had been right after all.
Fermi was non-political, almost to the extreme. One has the impression he did not much care who ran the government so long as he was free to do his physics. Fermi joined the Royal Italian Academy, a creation of Mussolini which required members to wear an elaborate uniform at meetings.22 But it helped finance the physics that was all that mattered to Fermi.
I have the impression, from this book and elsewhere, that if his wife had not been Jewish, Fermi would have done what Gian Carlo Wick and others did and remained in Italy, hoping Mussolini would lose the war. Following Hitler, Mussolini introduced racial laws into Italy in the late 1930s. Fermi could see great Italian mathematicians being thrown out of universities;23 he began to wonder how safe his family was. He began a discreet search among American universities for a job. One was offered to him at Columbia. He asked them, as a matter of safety for his family, not to announce that this was a permanent job. He had his children baptized, so there would be no problems with their passports. Without being told anything, Niels Bohr guessed what Fermi was going to do. He told Fermi in confidence that he was going to win the 1938 Nobel Prize, realizing that this might provide an exit strategy along with some useful money. Fermi won $45,000, which was indeed very useful. The family had no problems getting to the United States and starting a new life.24 But fission changed everything.
Many physicists made the elementary calculation of how much energy would be liberated if one fissioned a kilogram of uranium. Most of them regarded the calculated amount as science fiction. One person who took it all seriously was the Hungarian-born polymath, Leo Szilard. Szilard had attached himself like a buzzing insect to the Columbia physics department. What he was buzzing about was the possibility of nuclear weapons.
In the early 1930s, Szilard invented the notion of a chain reaction: if a reaction produced a small amount of energy and a method of reproducing itself, this energy could be amplified. He even took out a patent. When fission was discovered, he immediately saw how his idea could be realized. Fission is neutron induced. If, in addition to the fission fragments, two or more neutrons are produced, these can in turn induce new fissions, and on and on. If this process were slow, it might be an interesting laboratory phenomenon, but it would not constitute an explosion.
How fast was it?
Uranium fission produces nearly three neutrons, on average. These travel at about a tenth of the speed of light, meaning that a whole kilogram could be fissioned in about a microsecond. This is indeed an explosion. The realization drove Szilard into high gear.
Szilard was desperately afraid the Germans might make a bomb before the U.S. He managed to persuade the Belgians not to sell the Germans uranium from the Congo. He also managed get American physicists to agree not to publish anything about fission.25
Fermi realized that in order to build a bomb, physicists would first have to create a device that could realize a self-sustaining chain reaction—a reactor, to use the current term. After some discussion, it was decided that this would be done at the University of Chicago.26 By June 1942, Fermi and his family had moved to Chicago.
Segrè and Hoerlin give an excellent account of how the first reactor was built and successfully tested on December 1, 1942.27 That this took place was largely due to Fermi’s genius for practical engineering physics. I would like to amplify one thing, which illustrates why the German project failed. The reactor needed a moderator. Graphite was chosen because it was plentiful and easy to work with. Szilard, who had joined the project, learned how graphite was produced by the National Carbon Company. He discovered that they added boron to help with structural stability. As Szilard knew, boron was a neutron absorber; all their graphite had to be boron-free. The Germans also wanted to use graphite as a moderator. One of their few remaining nuclear experimenters, Walther Bothe, took on the task of seeing how graphite reacted to neutrons. It never occurred to him to ask about boron. He simply concluded that graphite would not work. The German researchers then tried to use heavy water, but could never get enough. It was one reason why the program failed. Afterwards, they accused Bothe of having made a mistake. He did not make a mistake; his result was correct, given the graphite he used. No one felt they could question the result. The mistake was in discouraging the free flow of ideas.
Los Alamos began functioning in the spring of 1943 and the Fermis went there in August of 1944. Laura Fermi wrote a charming book about the experience, Atoms in the Family, parts of which were published in The New Yorker. Life in Los Alamos was difficult for the families; they had no idea why they were there. It probably helped create a rift between Fermi and his son Giulio, which never really healed.
The story of what Fermi did at the first test at Trinity in New Mexico is legendary.28 The assembled physicists tried to guess the bomb’s explosive power. To understand what Fermi did, one must understand the time sequence. I myself witnessed two tests in the summer of 1957. First, there is the light, brighter than a thousand suns. Then comes the supersonic shock wave, which makes your ears hurt. Then finally comes the noise. Fermi realized that if he could measure the strength of the shock wave he would have a measure of the strength of the explosion. He tore strips of paper and, when the shock wave arrived, let them drop. By seeing how far they went he was able to make a pretty accurate estimate of the strength of the explosion.
When the war ended, Fermi gave a mini-course on nuclear physics, which later morphed into a famous course he gave at the University of Chicago.
He also gave a classified lecture on the physics of Edward Teller’s then-version of the hydrogen bomb: the classical Super. There is some irony in this, since when Fermi first suggested to Teller, well before Los Alamos, that a nuclear fusion bomb might be possible, Teller thought the idea was crazy. He later changed his mind and became obsessed.29
Fermi’s lecture was a careful analysis of the physics, concluding with his feeling that there were still a great many uncertainties. One member of the audience was Klaus Fuchs, both a member of the British delegation and a Russian spy. Fuchs managed to turn over a copy of this lecture to the Russians. I think the main thing it did was to inform them of the status of the U.S. program. For many years, the lecture was classified but eventually the Russians released it and you can find it on the web.
After the war, Fermi was necessarily thrown into the political arena when it came to nuclear energy. He was a member of the General Advisory Committee of the Atomic Energy Commission. In 1949, they were asked to advise on a crash program to build the hydrogen bomb. Fermi and Isidor Isaac Rabi wrote an addendum to the report that read:
The fact that no limits exist to the destructiveness of the weapon makes its very existence and the knowledge of its construction a danger to humanity as a whole. It is necessarily an evil thing considered in any light. For these reasons we believe it important for the president of the United States to tell the American people and the world that we think it is wrong on fundamental ethical principles to initiate the development of such a weapon.30
This may well be the strongest political statement Fermi ever signed. They were right. The hydrogen bomb should never have been built.
Fermi now threw himself back into the new physics of high energy accelerators. One was built at the University of Chicago and was used to study the interactions of mesons and neutrons and protons.
Segrè and Hoerlin tell the story of Freeman Dyson’s encounter with Fermi.31 Dyson was then a professor at Cornell University, and had to provide dissertation problems for his students there. He decided to use a then-fashionable approximation method to try to reproduce Fermi’s results. I am quite familiar with the method, since I used it in my own thesis. Its virtue is that it is fairly easy to calculate with and seems to give sensible answers. Its flaw is that the terms you leave out may be as big or bigger than the ones you include. In any event, Dyson and his students got results that seemed to agree with Fermi’s experiment. Dyson then went to Chicago to see Fermi. It was not a long meeting. Fermi objected to the method and then asked how many free parameters Dyson had used to make the fit. Dyson said four. Fermi told him that John von Neumann had often said “with four parameters I can fit an elephant and with five I can make him wiggle his trunk.” That was the end of that.
There is a story they do not tell, which has a much happier conclusion. By the early 1950s, a variety of new particles had appeared, first in cosmic rays, and then in accelerators. No one had predicted them and they were very confusing. To help characterize them, Murray Gell-Mann introduced a new kind of charge which he called strangeness. Familiar particles like the neutron and the proton had zero strangeness, but others could have plus or minus one or two. In the production of particles, strangeness had to be conserved, but in their decay it could be violated. There was a set of mesons called kaons. They came in a charge plus and minus and in a neutral particle and an anti-particle. The K+ was assigned strangeness +1 while the K– was assigned the strangeness –1. The Ko had the same strangeness as the K+ and the anti-K0 had the same strangeness as the K–. It was a nice scheme and guided Gell-Mann’s later construction of particles using quarks.
Fermi asked whether a K0 might transform itself into an anti-K0 by the weak interaction and, if so, what the consequences would be. My own guess is that Fermi knew the answer but gave Gell-Mann the pleasure of working it out himself, which he did, together with Abraham Pais. What happens is that the two particles oscillate back and forth in time, which can be observed by studying their decays. The paper contains the acknowledgement “One of us, (M. G.-M.), wishes to thank Professor E. Fermi for a stimulating discussion.”32 A bit of an understatement.
By the early 1950s, it was clear to people who knew him that there was something wrong physically with Fermi. He was first assured by a doctor that he was all right, but by the fall of 1954, it was obvious that he had an inoperable cancer. Fermi was totally serene about his impending death. People who went to see him came away shaken.33 They could not believe that he was going to die. He did so on November 28.
