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 lingered on, conspicuous in its incongruity. But his eyes were cheerful and amused: very close together, they left room only for a narrow nose, and were gray-blue, despite his dark complexion.
The group of friends went to play in a park on the northern outskirts of the city just on the other side of what is now the Via Salaria, near a bend in the Tiber River. Fermi, who was a bit older than the others, took the role of leader and decided the group should play soccer. Laura explained she had never played the game before. Fermi assured her it would be fun and made her the goal keeper for his team. Laura describes what happened next:
There was an easy self-reliance in him, spontaneous and without conceit. Luck, however, was against him: at the height of the game the sole of one of his shoes came loose and dangled from the heel. It hampered his running, made him stumble and fall on the grass. The ball zoomed above his fallen body and sped toward the goal. It was up to me to save the day: while I was observing our leader’s predicament with more amusement than pity, the ball hit me in the chest. Stunned, I wavered, almost fell, recovered my balance. The ball bounced back into the field and victory was ours.
Our leader pulled out of his pocket a large handkerchief, wiped off the perspiration that streamed down profusely from the roots of his hair over his face, then sat down and tied his loose sole with a piece of string.
It was, she concluded, “the first afternoon I spent with Enrico Fermi, and the only instance in which I did better than he.” She would not see him again for some two years.
THE YEAR WENT BY QUICKLY, BUT FOR FERMI THE ACADEMIC YEAR was cut short by the death of his mother on May 8, 1924.
The family had been caring for her through a series of respiratory illnesses, so her passing was no surprise. Yet we can be sure it had an effect, though he would never speak directly about it. Giulio may have been her early favorite, but there is no reason to think that Ida and Enrico were not also close. Ida found some pride and a bit of respite from her grief in Enrico’s increasing and gratifying success. Segrè notes that Fermi in later years expressed admiration for his mother’s ability to make things for herself when she had to—a makeshift pressure cooker being an example. Fermi’s ability with his hands may have been inherited from this fragile, disciplined former schoolteacher. Her death was a blow, but it was a blow that he dealt with in typical fashion, that is, privately.
His father, Alberto, had been planning to move the family out of the city and to a modern but modest suburban area, Città Giardino Aniene, about five miles north of central Rome. He had bought a plot at what became Via Monginevra 17 and was in the process of having the home built when Ida passed away. The family moved there in 1925. It was an improvement over Via Principe Umberto 133, but by no means grand. Alberto and his surviving children, Enrico and Maria, called it home for the next few years. Alberto died in 1927 and Fermi married the following year, moving to his own apartment and leaving Maria installed at the family home.
TOGETHER CORBINO AND FERMI PLOTTED FERMI’S NEXT STEP. Corbino was aware of an International Rockefeller Foundation fellowship that might be available to Fermi for another overseas scholarship. In the interim, Fermi had made the acquaintance of a young Dutchman, less than a year his senior. The two events came together in a happy coincidence.
The Dutchman was named George Uhlenbeck. A graduate student in physics at the University of Leiden, in southern Holland about five miles outside of The Hague, Uhlenbeck had taken a year off and was working with the household of the Dutch ambassador to Rome, serving as a tutor in math and science to the ambassador’s two sons. Shortly after he arrived, Uhlenbeck received a note from his thesis adviser, Paul Ehrenfest, that a young physicist in Rome named Fermi had written an insightful paper on the ergodic theorem. This was exactly the kind of paper that would have appealed to Ehrenfest, a keen student of statistical mechanics who had been looking for ways to integrate it with the emerging quantum theory. Ehrenfest had an instinct that Fermi was a potential intellectual soul mate and urged Uhlenbeck to make Fermi’s acquaintance and report back.
The two young men got on famously. In 1962, Uhlenbeck recalled those first meetings with Fermi: “He was younger than I was, but he was in a sense a wonder child, like Pauli. Anyway, we had then a little seminar, with Pontremoli, Persico, and me, in this old building.… He was so much ahead of all three of us, that he was the one who talked all the time.”
The Dutchman and the Italian became lifelong friends.
Enthused at the prospect of working with Uhlenbeck’s colleagues in Holland, Fermi spent the rest of 1924 in Leiden on his Rockefeller fellowship and enjoyed his time immensely. Ehrenfest had enormous respect for his new young colleague. After the disappointment of Göttingen, Fermi was delighted to find his abilities appreciated. No doubt this had to do with the outgoing character of Ehrenfest and the team he had around him, including Uhlenbeck and another graduate student, Samuel Goudsmit. Uhlenbeck and Goudsmit were the Dutch equivalent of Fermi and Rasetti—extremely close friends who shared an impish sense of humor and a passion for their chosen profession. They even physically resembled their Italian counterparts: Uhlenbeck, tall and gaunt, towered over the shorter, sturdier Goudsmit. The two Dutch physicists were inseparable. Fermi found them engaging companions.
Much of Fermi’s success in Leiden was certainly due to Ehrenfest, and Fermi wrote to his sister Maria of his delight in getting to know the Dutch physicist, describing him as a “persona molto simpatica”—a very kind person. Ehrenfest, a close friend of Einstein and one of the participants in the philosophical dialogue between Bohr and Einstein over the meaning of quantum physics, was, in Einstein’s eyes, the best teacher of his generation. Indeed, Einstein visited Ehrenfest while Fermi was there, and Fermi met the great physicist for the first and only time. Fermi kept a group photo of Einstein, Ehrenfest, and a gathering of other colleagues; Fermi is not pictured, leading at least one scholar to presume that Fermi took the photo himself. Einstein clearly impressed the young man, who wrote an enthusiastic letter to Maria describing his encounter with the great scientist, in which the two spoke German since Fermi’s Dutch was not up to par. However, in later years, Fermi expressed annoyance at the level of adulation given Einstein by an adoring world. The creator of the theory of relativity was the only physicist about whom Fermi ever overtly expressed a sense of envy.
Ehrenfest shared Fermi’s deep interest in matters relating to statistical mechanics and probability. This shared intellectual passion formed the basis of a productive and influential relationship for both men. Ehrenfest also clearly appreciated just how special Fermi was, how much the young man had mastered, and how clearly he thought about complex physics problems. He was impressed and let Fermi know it.
Fermi got to work quickly and by November he presented a theoretical paper analyzing problems associated with the intensity of multiple spectral lines that had arisen during experiments at the University of Utrecht. In it he expressed his appreciation for an idea suggested by Ehrenfest. In contrast, none of his earlier Göttingen papers made any reference to ideas suggested by his German colleagues. The rest of his time at Leiden went quickly and by the beginning of January 1925, Corbino’s efforts resulted in a lectureship in Italy for his gifted protégé. This time, it was Florence. Fermi took on a teaching position at the university and rejoined his old friend Franco Rasetti, who had himself found a position teaching experimental physics.
He may not have known it, but in Florence Fermi was destined to make scientific history.
CHAPTER FOUR
QUANTUM BREAKTHROUGHS
THE BREAKTHROUGHS IN QUANTUM THEORY DURING 1925, including the one made by Fermi toward the end of the year, were brilliant. The characters involved were fascinating and the source of endless, often amusing anecdotes. To appreciate these achievements and how they affected Fermi’s
own work, it is important to understand the state of theoretical and experimental physics at the time and the contributions of two quantum theorists in particular—Wolfgang Pauli and Paul Dirac.
IT WAS MAX PLANCK IN THE 1890S WHO FIRST PROPOSED THAT ENERGY was not “continuous” but came in discrete, tiny “packets,” or “quanta.” (“Quantum” is the singular of quanta and gives the theory its name.) This was the only way Planck could explain the odd problems encountered by German engineers who were trying to build a more energy-efficient light bulb to compete with the American engineers at General Electric and Westinghouse. Building on Planck’s solution, Einstein later proposed that light was composed of such packets, which he dubbed “photons.” Further experimentation showed, rather confusingly, that photons appeared to display the characteristics of both particles and waves—a very strange duality that remains at the heart of quantum theory to this day.
The concept that energy seemed to come in discrete quanta was great news for scientists who were working with spectroscopes. For over a century it was known that when elements were isolated, heated, and the light produced put through a spectroscope (essentially a high-precision prism), the spectrum that resulted was not a continuous rainbow of light at all but consisted of distinct lines of color. Each element had its own unique pattern of lines, corresponding to different frequencies of color. That energy came in discrete packets and that spectra were not actually continuous suggested a connection between the two.
The early twentieth century had more than its share of geniuses. Planck and Einstein were two. A third, the Danish physicist Niels Bohr, had an idea based on the Planck-Einstein quantum work and on the experimental discoveries of a fourth genius, Ernest Rutherford, which suggested the underlying mechanism that produced these lines. Rutherford and his colleagues in Manchester, England, conducted elegant experiments to show that gold atoms seemed to have a very specific structure: they seemed to consist of a “cloud” of light, negatively charged matter surrounding a core of heavy, positively charged matter buried deep within that cloud. In 1897, Rutherford’s colleague J. J. Thomson isolated a negative particle, the electron, which led to the conclusion that the negative cloud consisted of electrons. Physicists began to call the heavy central core of the atom the “nucleus.” (Rutherford discovered the positive particle in the nucleus, the proton, in 1919. Neutral particles in the nucleus, called neutrons, were only discovered in 1932. They played an important part in Fermi’s story in due course, but when Fermi arrived in Florence, no one yet knew that they existed.)
With Rutherford’s discoveries in mind, Bohr began playing with a compelling idea. Perhaps electrons were confined to specific “orbits” around a nucleus and could not exist in the spaces between these orbits. All electrons had a minimum orbit, called a ground state, below which they could not go. Otherwise, an electron with negative charge would be attracted to a nucleus with positive charge and all matter would collapse into itself. Electrons could, though, “leap” from one orbit to another, stimulated by the absorption or emission of a particle of light, that is, Einstein’s photon. The leap from one orbit to another would correspond to the emission or absorption of a specific frequency of light, depending on the frequency of the electron’s orbit around the nucleus. Bohr imagined this movement to be similar to the way in which planets orbit the sun.
Bohr’s model was the work of a true genius and was perhaps the critical breakthrough (although in most important details it was wrong). Good as it was, though, it left some important questions unanswered. It seemed clear that when an electron was stimulated by a photon, it would absorb that photon and jump to a higher energy orbit, but it was not at all clear that it would always jump the same way. Of equal interest: What would determine whether it jumped back down to a lower energy level, thus emitting a photon? What determined the actual energy level of each orbit? These puzzles defied explanation, but they were reflected and magnified by some spectroscopic phenomena.
One such phenomenon was called the Zeeman effect, named after the Dutch physicist Pieter Zeeman who first observed it. In a magnetic field, the spectral lines produced by an element split. Another phenomenon was also puzzling: even without a magnetic field, some lines in a given spectrum are notably more intense—that is, they shine brighter—than other lines. Were there laws that determined why some lines would be brighter and others dimmer?
For physicists these puzzles were far from trivial. They were deeply disturbing. A full theory would be expected to explain all observed phenomena and to predict with precision any particular observation. The classical mechanics theory of Isaac Newton and the classical electromagnetic theory developed by James Clerk Maxwell each produced specific and precise predictions about the phenomena they addressed and were the standards against which physicists judged a new theory’s success. From this vantage point, quantum theorists were proving distinct failures.
Fermi was certainly aware of these problems. The nine months he spent in Göttingen exposed him firsthand to the frustrations of those struggling with these issues, including Born, Heisenberg, and Jordan. That he continued to keep abreast of their work is apparent in his work during his subsequent stint in Leiden, where he published his paper on the problems of predicting the intensity of spectral lines in November 1924. Fermi cited work by Sommerfeld, Heisenberg, and Born, so he certainly was aware of the progress being made during 1925. His own contribution came toward the end of the year, based largely on the work of a young theoretical physicist from Vienna named Wolfgang Pauli.
PAULI’S ENORMOUS GIFTS IN MATHEMATICS AND PHYSICS WERE recognized early and cultivated by those around him. He studied in Munich with the great theorist Arnold Sommerfeld, who famously said that by the time Pauli arrived in Munich there was little that Sommerfeld could teach him. Like Fermi, Pauli made his first contributions in relativity theory, writing a treatise on the subject at the age of sixteen. A subsequent treatment of the subject, written for a German encyclopedia on mathematics, established his international reputation while he was still at university.
Like Fermi, he was a child prodigy, and like Fermi he was short, about five-foot-five. In practically every other way he was Fermi’s antithesis. Physically, Fermi was solid but fit and unremarkable in appearance. Pauli was round, tending toward obesity, but he was darkly attractive, with soulful eyes and sensuous lips. Fermi was not a drinker; Pauli drank heavily and struggled with alcoholism throughout his life. Fermi habitually retired early; Pauli was a night owl, enjoying the sybaritic life at local cafés and cabarets wherever he happened to be. While at university in Munich, Pauli mixed with artists, writers, musicians, and other bohemians in the Schwabing district of the city. The most bohemian activity Fermi would indulge in was a hike in the mountains.
Fermi was gifted as both a theorist and an experimentalist. As a theorist Pauli was perhaps even more gifted than Fermi, but as an experimentalist he was a disaster. Physicists joked that if a piece of equipment wasn’t working properly, Pauli must be in the vicinity. Pauli also had a legendary mean streak, something Fermi completely lacked. Pauli gave new meaning to the phrase “acid-tongued.” He is said to have described a somewhat undistinguished colleague of his with the incredibly dismissive “So young and already so unknown.” He once derisively called Fermi a “quantum engineer.” Of a particularly murky and speculative theoretical paper, he famously exclaimed, “It’s so bad it’s not even wrong.” He delighted in calling his close friend Werner Heisenberg a “fool.” As a young man he informed the eminent British astrophysicist Arthur Eddington that work the older man was pursuing in general relativity theory was “meaningless for physics.” When Einstein came to lecture at the University of Munich, Pauli was the first to speak after the great man’s talk. “What Professor Einstein has just said is not really as stupid as it may have sounded,” he explained helpfully.
As noted previously, Fermi was not a philosopher and rarely showed any interest in intellectual or cultural matters beyond physics. He had little
time for religion or spirituality. Pauli was born Catholic and took his religion seriously, rejecting the opportunity to join his irreligious colleagues in disparaging religious beliefs. He had a deep mystical streak, which he indulged later in life in conversations and in a decade-long correspondence with the great psychoanalyst Carl Jung.
He was also a man of obsessions, and by the time he was doing a post-doc in Copenhagen under Niels Bohr, over the winter of 1922–1923, Pauli became obsessed with the “anomalous” Zeeman effect, a problem with which he would struggle over the next several years. At this time a colleague ran into him walking the streets of Copenhagen in a state of despair. “You look very unhappy,” his friend commented. “How can one look happy when he is thinking about the anomalous Zeeman effect?” came Pauli’s reply.
As mentioned above, the Zeeman effect involved the splitting of spectral lines in the presence of a magnetic field. Bohr’s model explained the normal tripling of spectral lines, but often more than three lines would appear—sometimes four, sometimes six. By early 1925, as a professor in Hamburg, Pauli developed a solution of sorts but wasn’t completely satisfied.
In the Bohr model, electron orbitals required three numbers to specify their location, their frequency, and their orientation. Location corresponded to their distance from the nucleus. Electrons could only be in certain specific orbitals and could not exist in the spaces between orbitals. The frequency related to the speed of their orbits. The orientation number referred to how the specific orbital oriented to the axis of the nucleus. Each one of these three numbers was a quantum “number” that, taken together, identified the quantum “state” of an electron.
Pauli realized that if a new quantum number was added to the mix, a number that could have only one of two opposite values, the anomalous Zeeman effect could be explained. He also posited—because the mathematics explained the phenomenon so beautifully—that no two electrons could share all four quantum numbers. This may not seem terribly subtle, but the implication is astonishing: two electrons can be in exactly the same place at the same time, moving at the same speed and in the same orientation, as long as their fourth quantum number is different. Thus was born Pauli’s “exclusion” principle.
The Last Man Who Knew Everything Page 7
