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Friday, June 30, 2017

Reading List: The Pope of Physics

Segrè, Gino and Bettina Hoerlin. The Pope of Physics. New York: Henry Holt, 2016. ISBN 978-1-62779-005-5.
By the start of the 20th century, the field of physics had bifurcated into theoretical and experimental specialties. While theorists and experimenters were acquainted with the same fundamentals and collaborated, with theorists suggesting phenomena to be explored in experiments and experimenters providing hard data upon which theorists could build their models, rarely did one individual do breakthrough work in both theory and experiment. One outstanding exception was Enrico Fermi, whose numerous achievements seemed to jump effortlessly between theory and experiment.

Fermi was born in 1901 to a middle class family in Rome, the youngest of three children born in consecutive years. As was common at the time, Enrico and his brother Giulio were sent to be wet-nursed and raised by a farm family outside Rome and only returned to live with their parents when two and a half years old. His father was a division head in the state railway and his mother taught elementary school. Neither parent had attended university, but hoped all of their children would have the opportunity. All were enrolled in schools which concentrated on the traditional curriculum of Latin, Greek, and literature in those languages and Italian. Fermi was attracted to mathematics and science, but little instruction was available to him in those fields.

At age thirteen, the young Fermi made the acquaintance of Adolfo Amidei, an engineer who worked with his father. Amidei began to loan the lad mathematics and science books, which Fermi devoured—often working out solutions to problems which Amidei was unable to solve. Within a year, studying entirely on his own, he had mastered geometry and calculus. In 1915, Fermi bought a used book, Elementorum Physicæ Mathematica, at a flea market in Rome. Published in 1830 and written entirely in Latin, it was a 900 page compendium covering mathematical physics of that era. By that time, he was completely fluent in the language and the mathematics used in the abundant equations, and worked his way through the entire text. As the authors note, “Not only was Fermi the only twentieth-century physics genius to be entirely self-taught, he surely must be the only one whose first acquaintance with the subject was through a book in Latin.”

At sixteen, Fermi skipped the final year of high school, concluding it had nothing more to teach him, and with Amidei's encouragement, sat for a competitive examination for a place at the elite Sculoa Normale Superiore, which provided a complete scholarship including room and board to the winners. He ranked first in all of the examinations and left home to study in Pisa. Despite his talent for and knowledge of mathematics, he chose physics as his major—he had always been fascinated by mechanisms and experiments, and looked forward to working with them in his career. Italy, at the time a leader in mathematics, was a backwater in physics. The university in Pisa had only one physics professor who, besides having already retired from research, had knowledge in the field not much greater than Fermi's own. Once again, this time within the walls of a university, Fermi would teach himself, taking advantage of the university's well-equipped library. He taught himself German and English in addition to Italian and French (in which he was already fluent) in order to read scientific publications. The library subscribed to the German journal Zeitschrift für Physik, one of the most prestigious sources for contemporary research, and Fermi was probably the only person to read it there. In 1922, after completing a thesis on X-rays and having already published three scientific papers, two on X-rays and one on general relativity (introducing what are now called Fermi coordinates, the first of many topics in physics which would bear his name), he received his doctorate in physics, magna cum laude. Just twenty-one, he had his academic credential, published work to his name, and the attention of prominent researchers aware of his talent. What he lacked was the prospect of a job in his chosen field.

Returning to Rome, Fermi came to the attention of Orso Mario Corbino, a physics professor and politician who had become a Senator of the Kingdom and appointed minister of public education. Corbino's ambition was to see Italy enter the top rank of physics research, and saw in Fermi the kind of talent needed to achieve this goal. He arranged a scholarship so Fermi could study physics in one the centres of research in northern Europe. Fermi chose Göttingen, Germany, a hotbed of work in the emerging field of quantum mechanics. Fermi was neither particularly happy nor notably productive during his eight months there, but was impressed with the German style of research and the intellectual ferment of the large community of German physicists. Henceforth, he published almost all of his research in either German or English, with a parallel paper submitted to an Italian journal. A second fellowship allowed him to spend 1924 in the Netherlands, working with Paul Ehrenfest's group at Leiden, deepening his knowledge of statistical and quantum mechanics.

Finally, upon returning to Italy, Corbino and his colleague Antonio Garbasso found Fermi a post as a lecturer in physics in Florence. The position paid poorly and had little prestige, but at least it was a step onto the academic ladder, and Fermi was happy to accept it. There, Fermi and his colleague Franco Rasetti did experimental work measuring the spectra of atoms under the influence of radio frequency fields. Their work was published in prestigious journals such as Nature and Zeitschrift für Physik.

In 1925, Fermi took up the problem of reconciling the field of statistical mechanics with the discovery by Wolfgang Pauli of the exclusion principle, a purely quantum mechanical phenomenon which restricts certain kinds of identical particles from occupying the same state at the same time. Fermi's paper, published in 1926, resolved the problem, creating what is now called Fermi-Dirac statistics (British physicist Paul Dirac independently discovered the phenomenon, but Fermi published first) for the particles now called fermions, which include all of the fundamental particles that make up matter. (Forces are carried by other particles called bosons, which go beyond the scope of this discussion.)

This paper immediately elevated the twenty-five year old Fermi to the top tier of theoretical physicists. It provided the foundation for understanding of the behaviour of electrons in solids, and thus the semiconductor technology upon which all our modern computing and communications equipment is based. Finally, Fermi won what he had aspired to: a physics professorship in Rome. In 1928, he married Laura Capon, whom he had first met in 1924. The daughter of an admiral in the World War I Italian navy, she was a member of one of the many secular and assimilated Jewish families in Rome. She was less than impressed on first encountering Fermi:

He shook hands and gave me a friendly grin. You could call it nothing but a grin, for his lips were exceedingly thin and fleshless, and among his upper teeth a baby tooth too lingered on, conspicuous in its incongruity. But his eyes were cheerful and amused.

Both Laura and Enrico shared the ability to see things precisely as they were, then see beyond that to what they could become.

In Rome, Fermi became head of the mathematical physics department at the Sapienza University of Rome, which his mentor, Corbino, saw as Italy's best hope to become a world leader in the field. He helped Fermi recruit promising physicists, all young and ambitious. They gave each other nicknames: ecclesiastical in nature, befitting their location in Rome. Fermi was dubbed Il Papa (The Pope), not only due to his leadership and seniority, but because he had already developed a reputation for infallibility: when he made a calculation or expressed his opinion on a technical topic, he was rarely if ever wrong. Meanwhile, Mussolini was increasing his grip on the country. In 1929, he announced the appointment of the first thirty members of the Royal Italian Academy, with Fermi among the laureates. In return for a lifetime stipend which would put an end to his financial worries, he would have to join the Fascist party. He joined. He did not take the Academy seriously and thought its comic opera uniforms absurd, but appreciated the money.

By the 1930s, one of the major mysteries in physics was beta decay. When a radioactive nucleus decayed, it could emit one or more kinds of radiation: alpha, beta, or gamma. Alpha particles had been identified as the nuclei of helium, beta particles as electrons, and gamma rays as photons: like light, but with a much shorter wavelength and correspondingly higher energy. When a given nucleus decayed by alpha or gamma, the emission always had the same energy: you could calculate the energy carried off by the particle emitted and compare it to the nucleus before and after, and everything added up according to Einstein's equation of E=mc². But something appeared to be seriously wrong with beta (electron) decay. Given a large collection of identical nuclei, the electrons emitted flew out with energies all over the map: from very low to an upper limit. This appeared to violate one of the most fundamental principles of physics: the conservation of energy. If the nucleus after plus the electron (including its kinetic energy) didn't add up to the energy of the nucleus before, where did the energy go? Few physicists were ready to abandon conservation of energy, but, after all, theory must ultimately conform to experiment, and if a multitude of precision measurements said that energy wasn't conserved in beta decay, maybe it really wasn't.

Fermi thought otherwise. In 1933, he proposed a theory of beta decay in which the emission of a beta particle (electron) from a nucleus was accompanied by emission of a particle he called a neutrino, which had been proposed earlier by Pauli. In one leap, Fermi introduced a third force, alongside gravity and electromagnetism, which could transform one particle into another, plus a new particle: without mass or charge, and hence extraordinarily difficult to detect, which nonetheless was responsible for carrying away the missing energy in beta decay. But Fermi did not just propose this mechanism in words: he presented a detailed mathematical theory of beta decay which made predictions for experiments which had yet to be performed. He submitted the theory in a paper to Nature in 1934. The editors rejected it, saying “it contained abstract speculations too remote from physical reality to be of interest to the reader.” This was quickly recognised and is now acknowledged as one of the most epic face-plants of peer review in theoretical physics. Fermi's theory rapidly became accepted as the correct model for beta decay. In 1956, the neutrino (actually, antineutrino) was detected with precisely the properties predicted by Fermi. This theory remained the standard explanation for beta decay until it was extended in the 1970s by the theory of the electroweak interaction, which is valid at higher energies than were available to experimenters in Fermi's lifetime.

Perhaps soured on theoretical work by the initial rejection of his paper on beta decay, Fermi turned to experimental exploration of the nucleus, using the newly-discovered particle, the neutron. Unlike alpha particles emitted by the decay of heavy elements like uranium and radium, neutrons had no electrical charge and could penetrate the nucleus of an atom without being repelled. Fermi saw this as the ideal probe to examine the nucleus, and began to use neutron sources to bombard a variety of elements to observe the results. One experiment directed neutrons at a target of silver and observed the creation of isotopes of silver when the neutrons were absorbed by the silver nuclei. But something very odd was happening: the results of the experiment seemed to differ when it was run on a laboratory bench with a marble top compared to one of wood. What was going on? Many people might have dismissed the anomaly, but Fermi had to know. He hypothesised that the probability a neutron would interact with a nucleus depended upon its speed (or, equivalently, energy): a slower neutron would effectively have more time to interact than one which whizzed through more rapidly. Neutrons which were reflected by the wood table top were “moderated” and had a greater probability of interacting with the silver target.

Fermi quickly tested this supposition by using paraffin wax and water as neutron moderators and measuring the dramatically increased probability of interaction (or as we would say today, neutron capture cross section) when neutrons were slowed down. This is fundamental to the design of nuclear reactors today. It was for this work that Fermi won the Nobel Prize in Physics for 1938.

By 1938, conditions for Italy's Jewish population had seriously deteriorated. Laura Fermi, despite her father's distinguished service as an admiral in the Italian navy, was now classified as a Jew, and therefore subject to travel restrictions, as were their two children. The Fermis went to their local Catholic parish, where they were (re-)married in a Catholic ceremony and their children baptised. With that paperwork done, the Fermi family could apply for passports and permits to travel to Stockholm to receive the Nobel prize. The Fermis locked their apartment, took a taxi, and boarded the train. Unbeknownst to the fascist authorities, they had no intention of returning.

Fermi had arranged an appointment at Columbia University in New York. His Nobel Prize award was US$45,000 (US$789,000 today). If he returned to Italy with the sum, he would have been forced to convert it to lire and then only be able to take the equivalent of US$50 out of the country on subsequent trips. Professor Fermi may not have been much interested in politics, but he could do arithmetic. The family went from Stockholm to Southampton, and then on an ocean liner to New York, with nothing other than their luggage, prize money, and, most importantly, freedom.

In his neutron experiments back in Rome, there had been curious results he and his colleagues never explained. When bombarding nuclei of uranium, the heaviest element then known, with neutrons moderated by paraffin wax, they had observed radioactive results which didn't make any sense. They expected to create new elements, heavier than uranium, but what they saw didn't agree with the expectations for such elements. Another mystery…in those heady days of nuclear physics, there was one wherever you looked. At just about the time Fermi's ship was arriving in New York, news arrived from Germany about what his group had observed, but not understood, four years before. Slow neutrons, which Fermi's group had pioneered, were able to split, or fission the nucleus of uranium into two lighter elements, releasing not only a large amount of energy, but additional neutrons which might be able to propagate the process into a “chain reaction”, producing either a large amount of energy or, perhaps, an enormous explosion.

As one of the foremost researchers in neutron physics, it was immediately apparent to Fermi that his new life in America was about to take a direction he'd never anticipated. By 1941, he was conducting experiments at Columbia with the goal of evaluating the feasibility of creating a self-sustaining nuclear reaction with natural uranium, using graphite as a moderator. In 1942, he was leading a project at the University of Chicago to build the first nuclear reactor. On December 2nd, 1942, Chicago Pile-1 went critical, producing all of half a watt of power. But the experiment proved that a nuclear chain reaction could be initiated and controlled, and it paved the way for both civil nuclear power and plutonium production for nuclear weapons. At the time he achieved one of the first major milestones of the Manhattan Project, Fermi's classification as an “enemy alien” had been removed only two months before. He and Laura Fermi did not become naturalised U.S. citizens until July of 1944.

Such was the breakneck pace of the Manhattan Project that even before the critical test of the Chicago pile, the DuPont company was already at work planning for the industrial scale production of plutonium at a facility which would eventually be built at the Hanford site near Richland, Washington. Fermi played a part in the design and commissioning of the X-10 Graphite Reactor in Oak Ridge, Tennessee, which served as a pathfinder and began operation in November, 1943, operating at a power level which was increased over time to 4 megawatts. This reactor produced the first substantial quantities of plutonium for experimental use, revealing the plutonium-240 contamination problem which necessitated the use of implosion for the plutonium bomb. Concurrently, he contributed to the design of the B Reactor at Hanford, which went critical in September 1944, running at 250 megawatts, that produced the plutonium for the Trinity test and the Fat Man bomb dropped on Nagasaki.

During the war years, Fermi divided his time among the Chicago research group, Oak Ridge, Hanford, and the bomb design and production group at Los Alamos. As General Leslie Groves, head of Manhattan Project, had forbidden the top atomic scientists from travelling by air, “Henry Farmer”, his wartime alias, spent much of his time riding the rails, accompanied by a bodyguard. As plutonium production ramped up, he increasingly spent his time with the weapon designers at Los Alamos, where Oppenheimer appointed him associate director and put him in charge of “Division F” (for Fermi), which acted as a consultant to all of the other divisions of the laboratory.

Fermi believed that while scientists could make major contributions to the war effort, how their work and the weapons they created were used were decisions which should be made by statesmen and military leaders. When appointed in May 1945 to the Interim Committee charged with determining how the fission bomb was to be employed, he largely confined his contributions to technical issues such as weapons effects. He joined Oppenheimer, Compton, and Lawrence in the final recommendation that “we can propose no technical demonstration likely to bring an end to the war; we see no acceptable alternative to direct military use.”

On July 16, 1945, Fermi witnessed the Trinity test explosion in New Mexico at a distance of ten miles from the shot tower. A few seconds after the blast, he began to tear little pieces of paper from from a sheet and drop them toward the ground. When the shock wave arrived, he paced out the distance it had blown them and rapidly computed the yield of the bomb as around ten kilotons of TNT. Nobody familiar with Fermi's reputation for making off-the-cuff estimates of physical phenomena was surprised that his calculation, done within a minute of the explosion, agreed within the margin of error with the actual yield of 20 kilotons, determined much later.

After the war, Fermi wanted nothing more than to return to his research. He opposed the continuation of wartime secrecy to postwar nuclear research, but, unlike some other prominent atomic scientists, did not involve himself in public debates over nuclear weapons and energy policy. When he returned to Chicago, he was asked by a funding agency simply how much money he needed. From his experience at Los Alamos he wanted both a particle accelerator and a big computer. By 1952, he had both, and began to produce results in scattering experiments which hinted at the new physics which would be uncovered throughout the 1950s and '60s. He continued to spend time at Los Alamos, and between 1951 and 1953 worked two months a year there, contributing to the hydrogen bomb project and analysis of Soviet atomic tests.

Everybody who encountered Fermi remarked upon his talents as an explainer and teacher. Seven of his students: six from Chicago and one from Rome, would go on to win Nobel Prizes in physics, in both theory and experiment. He became famous for posing “Fermi problems”, often at lunch, exercising the ability to make and justify order of magnitude estimates of difficult questions. When Freeman Dyson met with Fermi to present a theory he and his graduate students had developed to explain the scattering results Fermi had published, Fermi asked him how many free parameters Dyson had used in his model. Upon being told the number was four, he said, “I remember my old 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.” Chastened, Dyson soon concluded his model was a blind alley.

After returning from a trip to Europe in the fall of 1954, Fermi, who had enjoyed robust good health all his life, began to suffer from problems with digestion. Exploratory surgery found metastatic stomach cancer, for which no treatment was possible at the time. He died at home on November 28, 1954, two months past his fifty-third birthday. He had made a Fermi calculation of how long to rent the hospital bed in which he died: the rental expired two days after he did.

There was speculation that Fermi's life may have been shortened by his work with radiation, but there is no evidence of this. He was never exposed to unusual amounts of radiation in his work, and none of his colleagues, who did the same work at his side, experienced any medical problems.

This is a masterful biography of one of the singular figures in twentieth century science. The breadth of his interests and achievements is reflected in the list of things named after Enrico Fermi. Given the hyper-specialisation of modern science, it is improbable we will ever again see his like.

Posted at June 30, 2017 21:00