Enrico Fermi

• 1. Introduction

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  Enrico Fermi paced off the distance that strips of paper were blown by a blast wave on the 16th of July 1945, and from that simple measurement he calculated the yield of the first nuclear bomb explosion. He estimated ten kilotons of TNT. The actual figure was about 18.6 kilotons. This was the Trinity test, and the man dropping paper into the shockfront had already built the world's first artificial nuclear reactor three years earlier. He has been called the architect of the nuclear age. How did a boy who found a 900-page physics book at a market stall become one of very few physicists to excel in both theory and the laboratory? Why did the discovery that won him a Nobel Prize turn out to be something other than what he thought he had found? And what did it cost him to work, year after year, near a pile of uranium?

• 2. A Market-Stall Education

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  Enrico Fermi was born on the 29th of September 1901 in Rome, the third child of Alberto Fermi, a division head in the Ministry of Railways, and Ida de Gattis, an elementary school teacher. As a young boy he shared his brother Giulio's interests, building electric motors and playing with electrical and mechanical toys. Giulio died during an operation on a throat abscess in 1915, when the brothers were barely a year apart in age.

  At a local market in Campo de' Fiori, Fermi found a 900-page physics book, the Elementorum physicae mathematicae. Written in Latin by a Jesuit professor at the Collegio Romano, it laid out mathematics, classical mechanics, astronomy, optics, and acoustics as they were understood at the time of its 1840 publication. With a scientifically inclined friend named Enrico Persico, he built gyroscopes and measured the acceleration of Earth's gravity.

  In 1914 Fermi met Adolfo Amidei, a colleague of his father who walked part of the way home with Alberto after work. The young Fermi asked him a question about geometry, and Amidei gave him a book on projective geometry written by Theodor Reye. Two months later Fermi returned it, having solved every problem at the end, including some Amidei considered difficult. Amidei concluded that the boy was a prodigy, at least with respect to geometry, and noted that Fermi had such a good memory he could return books because he remembered their contents very well. That memory would soon carry him through one of the hardest entrance exams in Italy.

• 3. The Pisa Prodigy

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  Fermi graduated from high school in July 1918, having skipped the third year entirely. At Amidei's urging he learned German so he could read scientific papers published in that language, and he applied to the University of Pisa. The entrance exam included an essay on the theme Specific characteristics of Sounds. The 17-year-old Fermi used Fourier analysis to derive and solve the partial differential equation for a vibrating rod. After interviewing him, the examiner declared he would become an outstanding physicist.

  At Pisa, Fermi was advised by Luigi Puccianti, director of the physics laboratory, who said there was little he could teach Fermi and often asked the student to teach him instead. There were only three students in the department, Fermi, Franco Rasetti, and Nello Carrara, and Puccianti let them use the laboratory freely. They worked to produce a Laue photograph, an X-ray image of a crystal. Fermi initially chose mathematics, then switched to physics, remaining largely self-taught in general relativity, quantum mechanics, and atomic physics.

  During 1921 Fermi published his first scientific works in the Italian journal Nuovo Cimento, including a paper showing that a charge has a mass equal to U over c squared. He submitted his thesis on X-ray diffraction images in July 1922 and received his doctorate at the unusually young age of 20. Writing an appendix for an Italian edition of a book on Einstein relativity in 1923, he became the first to point out the enormous nuclear energy hidden inside the equation E equals mc squared. It does not seem possible, at least in the near future, he wrote, to find a way to release these dreadful amounts of energy. He added that this was all to the good, because the first effect of such an explosion would be to smash to smithereens the physicist who found a way to do it.

• 4. The Via Panisperna Boys

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  In 1926, at the age of 24, Fermi won a new chair at the Sapienza University of Rome, one of the first three in theoretical physics in Italy. It had been created at the urging of Orso Mario Corbino, the university's professor of experimental physics and a member of Benito Mussolini's cabinet. Corbino helped Fermi recruit his team, which gathered students such as Edoardo Amaldi, Bruno Pontecorvo, Ettore Majorana, and Emilio Segrè. They were nicknamed the Via Panisperna boys after the street where the Institute of Physics stood.

  Fermi married Laura Capon, a science student at the university, on the 19th of July 1928. They had two children, Nella, born in January 1931, and Giulio, born in February 1936. On the 18th of March 1929 Mussolini appointed Fermi a member of the Royal Academy of Italy, and that April he joined the Fascist Party. He later opposed Fascism when the 1938 racial laws threatened Laura, who was Jewish, and put many of his research assistants out of work.

  At the end of each day Fermi gathered his colleagues and graduate students to go over a problem, often from his own research. Foreign students began arriving in Italy, the most notable being the German physicist Hans Bethe, who came to Rome as a Rockefeller Foundation fellow. Their reputation grew on a 1932 paper on the interaction between two electrons. But it was a puzzle about a vanishing particle that would carry Fermi's name furthest into the foundations of physics.

• 5. The Ghost Particle

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  Physicists in the early 1930s were puzzled by beta decay, in which an electron is emitted from the atomic nucleus. To satisfy the law of conservation of energy, Wolfgang Pauli postulated an invisible particle with no charge and little or no mass, emitted at the same time. Fermi took up the idea in a tentative paper in 1933 and a longer one the next year, naming the postulated particle the neutrino. His theory, later called Fermi's interaction and then the theory of the weak interaction, described one of the four fundamental forces of nature.

  When Fermi submitted the paper to the British journal Nature, its editor turned it down, calling the speculations too remote from physical reality to be of interest to readers. Fermi saw the theory published in Italian and German before it appeared in English. His biographer David N. Schwartz found it strange that Fermi sought publication there at all, since Nature then published only short notes of that kind. The neutrino itself was detected only after Fermi's death, and his theory showed why it had been so hard to find.

  Fred L. Wilson, writing in a 1968 English translation, called this the first successful theory of the creation and annihilation of material particles. Previously, only photons had been known to be created and destroyed. When positron decay was later discovered, it slipped easily into Fermi's original framework, and the capture of an orbital electron by a nucleus, predicted on the basis of his theory, was eventually observed.

• 6. Slow Neutrons And A Mistaken Prize

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  In January 1934, Irène Joliot-Curie and Frédéric Joliot announced they had induced radioactivity by bombarding elements with alpha particles. Fermi decided to switch to experimental physics using the neutron, which James Chadwick had discovered in 1932. Because neutrons carry no charge, they were not deflected by the positively charged nucleus and needed no particle accelerator, which the Via Panisperna boys did not have. Fermi built a radon-beryllium source by adding 50 millicuries of radon gas to a bulb of beryllium powder, its strength declining with the 3.8-day half-life of radon.

  The team noticed that the experiment worked better on a wooden table than on a marble tabletop. Recalling that paraffin wax slowed neutrons, Fermi tried passing them through it, and the radioactivity induced in silver jumped a hundredfold. He traced the effect to hydrogen atoms, which slowed the neutrons through collisions, and slow neutrons were more easily captured than fast ones. He developed a diffusion equation to describe this, known as the Fermi age equation. In all, he induced radioactivity in 22 different elements, reporting the discovery in the Italian journal La Ricerca Scientifica on the 25th of March 1934.

  After bombarding thorium and uranium, Fermi concluded he had created new, heavier elements, which he called ausenium and hesperium. The chemist Ida Noddack suggested the experiments might instead have produced lighter elements, but her idea was not taken seriously, since fission was then thought improbable if not impossible. In 1938 Fermi received the Nobel Prize in Physics at the age of 37 for these new radioactive elements and for nuclear reactions brought about by slow neutrons. After collecting the prize in Stockholm he did not go home to Italy. He continued to New York City with his family in December 1938, driven primarily by the racial laws, and there applied for permanent residency.

• 7. The Italian Navigator

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  Fermi arrived in New York City on the 2nd of January 1939 and accepted a position at Columbia University. Soon after, Willis Lamb came back from Princeton very excited, reporting that Niels Bohr had leaked great news. The German chemists Otto Hahn and Fritz Strassmann had detected barium after bombarding uranium, which Lise Meitner and Otto Frisch interpreted as nuclear fission. For Fermi the news came as a profound embarrassment. The transuranic elements he had partly won the Nobel Prize for had not been transuranic at all, but fission products. He added a footnote to his Nobel acceptance speech, and Noddack was proven right after all.

  Leó Szilárd obtained 200 kilograms of uranium oxide from a Canadian radium producer, and he and Fermi designed a device to achieve a self-sustaining reaction. Fermi suggested using uranium oxide blocks with graphite as a moderator instead of water, which captured too many neutrons. By August 1941 he had assembled six tons of uranium oxide and thirty tons of graphite into a pile at Columbia. The plutonium work was then concentrated at the University of Chicago, where Fermi's team joined the new Metallurgical Laboratory.

  Fermi persuaded Arthur Compton he could build the reactor in the squash court under the stands of the University of Chicago's Stagg Field. Construction of Chicago Pile-1 began on the 6th of November 1942, and it went critical on the 2nd of December. Compton made a coded phone call to James B. Conant. The Italian navigator has just landed in the new world, he said. Is that so, Conant replied. Were the natives friendly? Everyone landed safe and happy, came the answer.

• 8. Reactors, Poison, And A Bomb's Yield

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  The air-cooled X-10 Graphite Reactor at Oak Ridge went critical on the 4th of November 1943, and Fermi was on hand in case something went wrong. The technicians woke him early so he could watch it happen. It produced the first small quantities of reactor-bred plutonium. Fermi became an American citizen in July 1944, the earliest date the law allowed.

  In September 1944 Fermi inserted the first uranium fuel slug into the much larger, water-cooled B Reactor at the Hanford Site. After 838 tubes were loaded the reactor went critical, but shortly after midnight on the 27th of September the power level began to drop, and by 06:30 the reactor had shut down. Fermi and John Wheeler traced the trouble to xenon-135, a fission product with a half-life of 9.1 to 9.4 hours that absorbed neutrons. DuPont had added 504 tubes to the original 1,500, an over-engineering the scientists had once called a waste of time. Fermi realized that loading all 2,004 tubes would let the reactor reach the required power.

  Oppenheimer persuaded Fermi to join Project Y at Los Alamos, where he led F Division, one branch of which worked on Edward Teller's thermonuclear Super bomb. He found out about the bombings of Hiroshima and Nagasaki from the public address system in the technical area. Fermi did not believe atomic bombs would deter nations from war, and he did not join the Association of Los Alamos Scientists. Earlier, in April 1943, he had raised with Oppenheimer the idea of contaminating the German food supply with radioactive byproducts, a proposal Oppenheimer wanted to pursue only if it could kill half a million people.

  Victor Weisskopf said Fermi always managed to find the simplest and most direct approach, with the minimum of complication. He disliked complicated theories, and though he had great mathematical ability, he would never use it when a job could be done more simply. His method of getting quick, approximate answers through back-of-the-envelope calculations became known as the Fermi method, and is widely taught. I can calculate anything in physics within a factor 2 on a few sheets, he said.

  His self-assurance earned him the name The Pope, whose pronouncements were taken as infallible in physics. He played tennis with considerable ferocity, and when climbing mountains acted rather as a guide. At the top of a mountain he once announced, Well, it is two minutes to two, let's all leave at two o'clock, and everybody got up faithfully. At a party at his house, when a colleague's wife was cutting the bread, Fermi took the knife from her hand, convinced his own method was superior. He had very few interests outside physics, and confessed that his interest in music was restricted to simple tunes.

  After the first Soviet fission bomb in August 1949, Fermi and Isidor Rabi wrote a strongly worded report opposing the hydrogen bomb on moral and technical grounds. With Stanislaw Ulam he calculated that the tritium needed for Teller's model would be prohibitive. In 1954 he testified on Oppenheimer's behalf at the hearing that ended in the denial of Oppenheimer's security clearance. Emilio Segrè called him the last person who knew all of physics of his day.

  Fermi underwent what was called an exploratory operation at Billings Memorial Hospital in October 1954, then returned home. Fifty days later he died of inoperable stomach cancer at his home in Chicago. He was 53. He had suspected that working near the nuclear pile involved great risk, but pressed on because he felt the benefits outweighed the danger to his personal safety. Two of his graduate student assistants who worked near the pile also died of cancer.

  A memorial service was held at the University of Chicago chapel, where Samuel K. Allison, Emilio Segrè, and Herbert L. Anderson spoke. His body was interred at Oak Woods Cemetery in a private graveside service presided over by a Lutheran chaplain. A synthetic element isolated from the debris of the 1952 Ivy Mike nuclear test was named fermium, making him one of 16 scientists who have elements named after them.

  Fermi was fond of pointing out that when Alessandro Volta worked in his laboratory, Volta had no idea where the study of electricity would lead. Near the end of his life Fermi voiced a similar uncertainty about nuclear technology. What we all fervently hope, he said, is that man will soon grow sufficiently adult to make good use of the powers that he acquires over nature.