A Mind Always in Motion: The Autobiography of Emilio Segrè
In 1926 Enrico Fermi was appointed a full professor of physics in Rome. He was only twenty-five years old, but he had already made several significant contributions to physics, the most important of which had to do with the statistical mechanics of particles like electrons. This was the first of several discoveries for which Fermi deserved a Nobel prize (although he did not receive one until 1938). He was, at the time, probably the only scientist in Italy who really understood modern physics. Until 1928 there was not even a text in Italian suitable for introducing graduate students to the subject. Fermi was determined to change all of that, and he began recruiting students who were not much younger than he was.
One of the early recruits was Emilio Segrè, whose posthumous autobiography. A Mind Always in Motion, has been published only recently, nearly four years after his death in 1989 at the age of eighty-four. Segrè wanted his book to appear posthumously because, as he writes, “I tell the truth the way it was and not the way many of my colleagues wish it had been.” He was not a man of much tact, as his book reflects.
Segrè was born into a prosperous, nonobservant Jewish family that had lived in Italy for centuries. His father owned a paper mill in Tivoli and, as a sort of unpaid service, looked after the nearby Villa d’Este, whose absentee owner, the Archduke Francis Ferdinand, wanted the buildings maintained at no cost to himself. One of the most attractive parts of Segrè’s book is his description of his seemingly idyllic childhood in Tivoli and then in Rome. He has particularly fond memories of his bachelor uncle Claudio, an engineer and member of the Italian academy, who encouraged Segrè’s budding interest in science, especially technical gadgets involving electricity. By the time Segrè reached high school he had begun to read Maxwell’s Theory of Heat and other works of advanced physics on his own. He found it difficult. “I had not yet learned,” he writes, “that in order to study physics, one has to use paper and pencil and work through the calculations as one goes along. Usually I read these books at school during boring classes that I disdained.”
After graduating from high school, Segrè entered the University of Rome intending to become an engineer and perhaps work in his father’s paper mill. He also discovered mountaineering and, with a group of fellow scientists—and without guides—completed a number of challenging climbs, including the so-called Italian route on the Matterhorn. It was through his climbing friends that Segrè first heard about the arrival of a “sort of genius” named Enrico Fermi. Segrè attended one of his lectures and was extremely impressed but nonetheless continued with his engineering, which he was finding increasingly distasteful. However, in the spring of 1927 he met Franco Rasetti, a young colleague of Fermi’s, who during several climbs persuaded Segrè to meet Fermi. Fermi, also a mountaineer, then came along with Segrè’s group on a hike, during which he quizzed Segrè on his knowledge of physics. While Fermi, then twenty-six, was only four years older than Segrè, there was never the slightest question then or afterward who was the teacher and who was the student.
Segrè made a favorable impression on Fermi and became his first graduate student in 1928. Perhaps the last physicist to master the entire field, which is now just too vast, Fermi worked both experimentally and theoretically in every branch of physics. It is little wonder that when Fermi’s group in Rome began giving themselves ecclesiastical nicknames, the wholly self-confident Fermi became known as the Pope. Rasetti was known as the Cardinal Vicar. Segrè writes that he was known as “the Prefect of Libraries, because I was interested in the library…however, I was also the Basilisk, because I was supposed to spit fire when mad.”
Fermi genuinely liked having students if they were bright enough, and began tutoring Segrè and Rasetti privately. Segrè says little about this, but I can imagine what that was like. When I was a graduate student at Harvard in the early 1950s, Fermi came to Cambridge to give a series of lectures. After one of them he gave a sort of private lecture for about a half dozen of us. He chose to talk about a standard problem in quantum mechanics which he had a novel way of looking at. After he finished his impromptu lecture one of us—bolder than the rest—challenged the rigor of Fermi’s demonstration. Fermi then gave a second lecture on the same subject with more rigorous mathematical proof. Each time our colleague interrupted, Fermi produced a new level of rigor. After a couple of these demonstrations, his questioner gave up. For his part, during a lecture by someone else, Fermi was capable of asking spontaneous questions which were so deep that they opened up entirely new avenues in physics.
After getting his degree in 1928, Segrè wanted to continue working with Fermi. Unlike most young researchers, Segrè was not in need of money since his father had agreed to support him. In the meantime Segrè was called upon to do his military service, which he did, not too disagreeably, as a second lieutenant in the anti-aircraft artillery stationed near Rome, which enabled him to keep in touch with the laboratory. After his service he rejoined Fermi’s group. By 1930 he was publishing his own experimental work and was able to spend a year as a traveling scientist in Holland and Germany. In Hamburg Segrè fell in love with a young German woman, an episode which he recounts by reprinting contemporaneous diary entries in his book. He considered marrying her, even though he knew she was a German nationalist who was becoming more and more committed to the Nazi movement, but he eventually broke off the relationship. Like many other upper-middle-class Jews at the time, members of Segrè’s family, especially among the older generation, had joined the Fascist Party in Italy. But Segrè knew that the German Nazi Party was something else.
Segrè’s really interesting work started in 1933 when Fermi began the work in nuclear physics for which he eventually won the Nobel prize. The neutron—the uncharged particle that complements the positively charged proton in the atomic nucleus—had been discovered a year earlier by the British physicist James Chadwick. It was ideal for use as a probe, since, being electrically neutral, it could penetrate deeply into the atomic nucleus without being repelled by the electrical force of the protons. Fermi began a series of experiments in which he bombarded one element after another with neutrons to see what would happen. The result was nuclear alchemy—the nuclei were transformed into radioactive isotopes which could decay into entirely different nuclei. Working up the periodic table, the group eventually came to uranium. They thought they had found the transformation of uranium into its neighboring nuclei. They had no idea—no one had any idea—that uranium could be made to fission, or split, into two light nuclei such as barium and krypton.
In 1934 the group made a very odd discovery. They found that if they did their experiments on a wooden table instead of a marble bench, the silver they were irradiating at the time became much more active. Somehow the presence of the wood enhanced the nuclear reaction. Puzzled, Fermi devised a test. He decided to put a filter between the neutron source and the target to see what would happen. His first choice as a filter was lead, but for reasons he could never explain later, at the last minute he chose to use paraffin. Miraculously, the neutrons filtered by the paraffin produced extraordinarily enhanced rates for the nuclear reactions they were inducing. At this point Fermi went home for lunch and his siesta. When he came back at three o’clock he had understood what had happened, and created a new branch of experimental nuclear physics.
What Fermi realized was that paraffin, in the language of the reactor physicists, acts as a “moderator.” The neutrons striking the carbon and hydrogen nuclei in the paraffin bounce off them, losing energy. They are slowed down and after a few collisions move about at the same speed as the molecules of paraffin. They have been, as physicists say, “thermalized.” What no one expected was that the slow neutrons would produce more effective collisions than fast neutrons. (As Fermi was later able to show, this unexpected result follows from the quantum theory of these collisions.) Having realized the efficacy of slow neutrons, he then repeated the original experiments, but now using slow neutrons.
Early in 1935 Fermi’s group did a slow neutron experiment with a uranium target—exactly what Otto Hahn and Fritz Strassmann did in Germany three years later in 1938, when they actually discovered fission. When I met Segrè at Columbia several years before his death, I asked him why Fermi’s group had failed to discover fission in 1935, a historical accident I found baffling. Segrè told me that in order to shield their detectors from unwanted radiation, his group had covered the uranium target with aluminium foil. This simple piece of foil had kept them from seeing the highly energetic pulses produced by the uranium fissions taking place.
When Segrè told me this anecdote I was stupefied. In view of Fermi’s genius for understanding the significance of experimental results, he would certainly have been led to the discovery of fission if he had seen unexpected energy pulses coming from uranium. A chemist named Ida Noddack had even suggested the possibility of fission in a speculative article sent to the Rome group in 1935, but Fermi and his colleagues dismissed her speculations, for they seemed inconsistent with some of the data on nuclear masses. Surely if they had seen these pulses they would have reexamined that data. The discovery of fission in 1935 would have meant that the race to build an atomic bomb might have started well before 1939. The Second World War could have been nuclear from the beginning, or, perhaps, the prospect of nuclear weapons could conceivably have prevented the war. When I suggested these possibilities to Segrè he did not seem much interested. What happened happened, and that was that. Indeed, while he describes the use of aluminum foil in his biography of Fermi he does not seem to find it interesting enough to include in his autobiography. Historical speculation, however startling, seems to have been of no interest to Segrè.
The Italian university system then required that aspiring professors begin their careers in the provinces. Some academic roads eventually led to Rome, but slowly, unless one was a Fermi. Segrè began his professional career in 1936 at the University of Palermo in Sicily. By this time he had married Elfriede Spiro, a German Jewish woman he had met in 1934, a year after her family had been forced to emigrate to Italy. Palermo was something of a backwater in physics, and Segrè set about to change that. He had already made two visits to the US with Fermi, and in the summer of 1936 he again returned, this time with the intention of visiting the University of California at Berkeley where Ernest O. Lawrence had constructed the first cyclotron, the machine that he had invented.