Emilio Segrè
Emilio Segrè; drawing by David Levine

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.

Much of the last part of Segrè’s book concerns his complex and often hostile relationships with other scientists, including Lawrence, J. Robert Oppenheimer, and the chemist Glen Seaborg. His relationship with Lawrence began amiably enough, and Lawrence gave him permission to take back to Palermo some radioactive detritus that had been manufactured haphazardly in the cyclotron. With this foil and some help from the chemist Carlo Perrier, Segrè made the one important discovery that was basically his own—the identification of a new element, one of the radioactive decay products of the molybdenum. The new element, eventually named “technetium,” turns out to have very important applications in the manufacture of radioactive medical drugs.

One may well wonder why Lawrence himself did not discover this element in his own molybdenum foils. Segrè delivers a somewhat harsh, but I think correct, judgment of Lawrence as an unexceptional scientist interested less in fundamental research than in building bigger and bigger cyclotrons. But Segrè gives Lawrence the credit he deserves for creating conditions under which others could do significant research. “Lawrence,” he writes,

was the chief of a great enterprise he had created from scratch, and he identified himself with the Rad Lab and its successes. Although no more than a mediocre scientist himself, he perhaps mildly looked down on his fellow scientists and their squabbles, which he disliked. He justly felt that their successes, if obtained at the Rad Lab, would always extend to him too.

In 1938 Segrè returned to Berkeley for what he thought was a summer visit. He ended up spending the next thirty-five years either in Berkeley or in Los Alamos. The Italian Manifesto della razza, or race manifesto, of 1938 prohibited Jews from holding jobs in universities, and Segrè’s position at Palermo disappeared. “Strangely, the first shock did not effect me very much emotionally,” Segrè writes. “The blow was not unexpected, and all in all I was too busy trying to rebuild my life to brood aimlessly.” Fortunately, his family had sent money abroad for just such a contingency, and Elfriede and their infant son were able to join him in Berkeley in October of that year, having waited out the worst of the Czechoslovakian crisis on an ocean liner anchored off Gibraltar.

Segrè’s family in Italy fared less well. His brother, Marco, hid in the hills behind Tivoli during the war, while his father, Giuseppe, took refuge in a papal palace in Rome under the protection of Monsignor Carinci, a high-ranking prelate. Segrè’s mother, Amelia, was captured and murdered by the Nazis in 1943, three months after the fall of Mussolini. Segrè learned of her death only in June 1944, after the liberation of Rome. His father died of natural causes in October of that year.

During the summer of 1938 Segrè met Oppenheimer and Seaborg, two men who would have an important part in his life over the following several years, and whom Segrè clearly disliked. I find Segrè’s characterization of Oppenheimer understandable but somewhat beside the point. He writes,

Oppenheimer and his group did not inspire in me the awe that they perhaps expected. I had the impression that their celebrated general culture was not superior to that expected in a boy who had attended a good European high school. I was already acquainted with most of their cultural discoveries, and I found Oppenheimer’s ostentation slightly ridiculous. In physics I was used to Fermi, who had a quite different solidity, coupled with a simplicity that contrasted with Oppenheimer’s erudite complexities. Probably I did not sufficiently conceal my lack of supine admiration for Oppenheimer, and I found him unfriendly, even if covertly, for a good part of my career, except when he wanted me to join his team at Los Alamos.

Unfortunately, Segrè does not explain what he means by the phrase “unfriendly, even if covertly,” but he was right about Oppenheimer’s “erudite complexities.” Still, he does not seem to appreciate that Oppenheimer, I.I. Rabi, and a tiny handful of other Americans, were responsible for bringing modern physics to the US in the late 1920s and early 1930s. Nor does he grasp that Oppenheimer was one of the greatest teachers of physics who ever lived, as well as a first-rate physicist. Among many other things, it was Oppenheimer, and his students, who in the late 1930s realized the possibility of gravitational black holes. Oppenheimer’s productive years, moreover, were cut short by the atomic bomb project, to which he devoted himself completely.1 One wonders if anyone else would have had the intellectual capacity to oversee such a vast and complicated project—not to mention the personal magnetism needed to manage such a collection of scientific prima donnas, including Segrè himself.

I imagine that Oppenheimer did not find Segrè’s work very interesting intellectually and, since he had even less tact than Segrè, he very likely made that clear one way or the other. But, in truth, I do not find Segrè’s work very interesting intellectually either. It is, of course, admirable that he discovered a new element. But how can one compare this to the discovery of the quantum theory which contains within its riches the explanation of why there is a periodic table of elements at all? Bohr, Dirac, Pauli, and the other creators of the quantum theory were Oppenheimer’s contemporaries and colleagues. If there was a tragic aspect of Oppenheimer’s life, it was that he knew that he had abilities equal to those scientists’, but that he could never quite match the level of their work. “Solidity,” Segrè’s word, may not be a bad description of what Oppenheimer lacked and men like Fermi and Bohr had in abundance.

To be fair, it must be said that Segrè himself knew that in the end, despite his Nobel prize, he would occupy a modest place among twentieth-century physicists. He recalls a conversation he once had with Fermi:

“Emilio, you could take all your work and exchange it for one paper of Dirac’s and you would gain substantially in the trade,”. Fermi once said to me. I knew this to be true, of course, but I answered: “I agree, but you could likewise trade yours for one of Einstein’s and come out ahead.” After a short pause, Fermi assented. I know of scientists who cannot resign themselves to being inferior to contemporaries, with dire consequences for their personalities and happiness.

Segrè’s relationship with Seaborg, seven years his junior, was another matter. The two men began collaborating on technetium isotopes as soon as Segrè arrived at Berkeley, but Segrè’s view of Seaborg soon darkened. In his book he writes,

What was unusual in Seaborg was the long term planning he diligently applied to everything. In 1941 he would say: in 1946 I shall be dean; in 1948, chancellor of the University of California [it actually took him until 1958]; in 1955, senator for California, and so on, and he never lost sight of his aims. In 1938 he always dressed in a blue suit, with a tie, differently from his colleagues, because he thought that these clothes would help him become a full professor, a small first step in the grand design. Ultimately, he devoted much effort to public service as chairman of the Atomic Energy Commission and many other organizations, receiving more than fifty honorary degrees and collecting pictures of himself with a number of presidents of the United States and other such figures.

Segrè makes it clear that he holds Seaborg, and even Seaborg’s wife, responsible for his not getting his fair share of the credit for the work that was done at Berkeley on transuranic elements like plutonium, for which only Seaborg and Edwin McMillan shared the Nobel prize in chemistry for 1951. Seaborg’s wife had been Lawrence’s secretary. Segrè suspects that she gave her husband some papers from Lawrence’s files that contained data that Segrè and his collaborator, Joseph Kennedy, had taken on plutonium. “After the war,” he writes, “I had started thinking that my work on the new chemical elements and on radiochemistry might bring me [the Nobel prize]. I saw Seaborg’s efforts at getting it on similar grounds, but I did not know how to stake my claim. I hoped that the Nobel committee would somehow split the award.” They didn’t, and Segrè writes that when he heard the news he was “deeply disappointed,” which I imagine is a considerable understatement.

In 1954, Segrè launched his own campaign for the Nobel prize, which would eventually result in similarly bitter feelings among those of his own collaborators who had been left out of the award. In physics, the Swedish Royal Academy can split the prize (which cannot be awarded posthumously) among no more than three people. Each year the Royal Academy sends out nominating ballots to a large number of physicists, some of whom clearly have more influence than others. In addition Academy members hold consultations with outside physicists. All deliberations and nominations are kept secret, usually for decades.

Segrè opened his almost desperate campaign to win the prize on a visit to Brazil, where he met an old friend, George de Hevesy, who had won the prize for chemistry in 1943 for his work on biological tracers using radioactive materials. Segrè writes,

We [he and Hevesy] were friends and I could speak freely to him. Thanks to his Swedish connections he knew many of the secrets of the Nobel Committee, and he told me that I had not been specifically nominated in the year 1951, which had automatically eliminated me. He advised me to try to interest Fermi. I did not do so because I knew perfectly well that Fermi could not be influenced in matters such as competitions and awards.

It doesn’t seem to occur to Segrè that there was something distasteful about his approaching Hevesy, or indeed anyone, to lobby for an award for himself.

Segrè finally won the prize for work he had done in 1955 on antimatter, whose existence the great British theoretical physicist P. A. M. Dirac had first suggested in 1931. Specifically, Dirac suggested the existence of the antiparticle to the electron, which is now known as the positron since it has a positive electric charge that exactly balances the electron’s negative charge. In the same year C. D. Anderson found the positron in cosmic rays. Over the next decades the theory of antimatter was clarified, leading to the prediction that for every particle there is an antiparticle with definite properties. The particle and its antiparticle must have exactly the same mass. They must have equal and opposite charges, and if they are unstable they must decay at the same rate. When a particle and an antiparticle meet they can annihilate each other, creating an assortment of particles. In this spirit it was assumed that the proton—and also the neutron—would have antiparticle counterparts. The antiproton would be an essentially stable object with a negative charge and the same mass as the proton. It could bind with antineutrons and create a universe of antimatter.

Physicists tend to give these particle masses in energy units, since Einstein’s relation E = mc2 means that mass and energy are essentially the same thing. The energy unit one uses here is the electron volt. In these units the electron has a mass energy of about a half million electron volts while the proton, the neutron, and their antiparticles have mass energies of about a billion electron volts. Creating an antiproton therefore requires an energy amounting to billions of electron volts.

Before the war, cyclotrons could produce proton energies in the thousand electron volt range. But after the war, new machines with even larger magnets raised the range to millions of electron volts. In 1954, a machine was constructed in Lawrence’s laboratory at Berkeley that could produce proton energies of 6.2 billion electron volts, approximately the minimum energy needed to produce an antiproton in a collision between two protons. To balance the various electric charges the simplest reaction that works is proton on proton producing three protons and an antiproton, which can be labeled p; symbolically p+p→p+p+p+p. The machine had been built to study just this reaction, and when the reaction was produced, a wag suggested that the machine should be given the Nobel prize.

Several scientists at Berkeley had the idea of doing this experiment and there was a great deal of infighting over which one of them would be allowed the necessary running time on the machine. Segrè does not say much about this in his book except to remark that “Several Berkeley groups started the hunt. My group had for some time studied the problem and prepared for it.” Segrè’s group consisted of himself and three other physicists—Owen Chamberlain and Clyde Wiegand, Berkeley graduates who had done important work on plutonium with Segrè at Los Alamos, and Tom Ypsilantis, who was a decade younger than the others but already an experimenter of great promise. Wiegand, five years older than Chamberlain, was an acknowledged electronics genius whose wizardry was essential to the success of the difficult experiment. The team detected about one antiproton per 100,000 events involving other particles.

The paper announcing the discovery was published in 1955 in the Physical Review under the title “Observation of Antiprotons.” It was signed, in alphabetical order, O. Chamberlain, E. Segrè, C. Wiegand, and T. Ypsilantis. The discovery was a logical candidate for a Nobel prize, but to whom? Segrè writes, “Needless to say, before the announcement I did not know if and how a prize given for the antiproton would be divided between Chamberlain, myself, Wiegand, and Ypsilantis, since the paper reporting the discovery had been signed by all four of us in alphabetical order.” If Segrè, one of the collaborators, had no clear way of making this decision then how could anyone else make it? But in 1959, for reasons that are still unknown, the Nobel committee awarded the prize in physics to Segrè and Chamberlain for the discovery of the antiproton, leaving the other two out.

Segrè’s reaction to this curious award seems to me to show an almost complete lack of understanding of the feelings of the people involved.

After the discovery of the antiproton and connected publicity, the moods of Owen and of Clyde separately darkened. Owen wanted to be more independent than he already was, which was hardly possible. [Why it was “hardly possible” is not explained.] He wanted to have his own group, but our group was so small that I felt that further splitting would impair its efficiency. Owen was then invited to go to Harvard…. On his return, he started a small separate group. Clyde, too, wanted to go it alone, and above all to work independently of me and Owen. Perhaps he wanted to show his personal prowess, although his ability was widely recognized, above all by me and his other colleagues in the group. It is possible that even Ypsilantis had similar wishes, but being younger, at the beginning of his career, and of a sunny disposition, he was less affected.

It never seems to occur to Segrè that Wiegand might have felt he had been unfairly used, although Segrè does apparently realize that something was not quite right with the Nobel committee’s choice. After describing the contributions of Wiegand and Ypsilantis, he writes, “Nor did I think that my contribution was as negligible as it perhaps then appeared to Owen and Clyde.” Indeed, Segrè never describes his own contribution to the experiment. His description of the experiment is oddly detached, in marked contrast to his recollections of the experiments in which he took part early in his career, where one can share some of his excitement as he shows what was involved in a scientific discovery. Segrè writes about the antiproton experiment as if it had been done by somebody else.

This seems to have been Wiegand’s view. In 1985, four years before his death, Segrè attended a symposium in Berkeley celebrating the thirtieth anniversary of the experiment. Segrè describes the occasion in a footnote. Wiegand, he writes,

gave a detailed account of the work performed by himself and Chamberlain. Among other things, he said that he and Chamberlain had started planning the experiment secretly, outside of regular working hours. He mentioned Ypsilantis only peripherally, in connection with the addition of a counter to the apparatus. He never mentioned me, as though I had not existed. I was saddened by this performance. It must represent Wiegand’s present state of mind; this must be his recollection of the experiment.

I know of only one example of a scientist who won a Nobel prize and apologized, in writing, to a collaborator who was left out. The collaborator was Max Born, whose posthumously published autobiography describes how during the mid-1920s, before Born was forced out of Göttingen by the Nazis, Heisenberg came to him with a paper on atomic mechanics he had just written. Born studied it and realized that Heisenberg had unknowingly used a branch of algebra involving what are known as matrices—arrays of numbers which obey certain algebraic rules. He and a student, Pasqual Jordan, then created something that is now known as “matrix mechanics”—a version of the quantum theory. Soon afterward they were joined by Heisenberg, and the three of them made several very important applications of the new mechanics. In 1932 Heisenberg was awarded the Nobel prize in Physics for this work and Born and Jordan were left out. Heisenberg felt so badly about this that he made a special trip to Switzerland so that he could mail an uncensored letter to Born, who was then living in England. This is what he wrote:

Dear Mr. Born,

If I have not written to you for such a long time, and have not thanked you for your congratulations, it was partly because of my rather bad conscience with respect to you. The fact that I am to receive the Nobel Prize alone, for work done in Göttingen in collaboration—you, Jordan and I—this fact depresses me and I hardly know what to write to you. I am, of course, glad that our common efforts are now appreciated, and I enjoy the recollection of the beautiful time of collaboration. I also believe that all good physicists know how great was your and Jordan’s contribution to the structure of quantum mechanics—and this remains unchanged by a wrong decision from outside. Yet I myself can do nothing but thank you again for all the fine collaboration, and feel a little ashamed.

With kind regards,

W. Heisenberg2

I have found much in Heisenberg’s life to reproach, but in writing to Born as he did, he showed an admirable generosity of spirit. One wishes that Segrè had been able to do the same.

This Issue

March 24, 1994