The Telescope in the Ice Page 4
But his crystal ball wasn’t quite as clear when it came to the constituents of the nucleus. The neutron, which we now know does “exist in the nuclei,” has two properties in common with the neutrino, electrical neutrality and a half unit of spin, but it weighs about the same as the proton, and is not emitted in beta decay. The two conundrums would require two particles and several advances in theory and experiment, which would tumble in over the next several years.
Pauli was astute enough to realize that he was groping in the dark:
I admit that my remedy may perhaps appear unlikely from the start, since one probably would long ago have seen the neutrons if they existed. But “nothing venture, nothing win,” and the gravity of the situation with regard to the continuous β-spectrum is illuminated by a pronouncement of my respected predecessor in office, Herr Debye [Peter Debye, who would win the Nobel Prize in Chemistry in 1936], who recently said to me in Brussels “Oh, it’s best not to think about it at all—like the new taxes.” One ought therefore to discuss seriously every avenue of rescue.—So, dear radioactive folk, put it to the test and judge.
By “radioactive folk,” he meant primarily Meitner and Geiger. And they responded favorably to the idea, at least as far as they could. They knew of no conflicting experimental evidence, but they knew of none that confirmed the idea either. This would be the case for another twenty-six years.
The Chinese American experimentalist Madame Chien-Shiung Wu, whom we shall meet, once observed that “later generations, having seen the triumphant success of the neutrino hypothesis, probably will never fully appreciate the courage and insight it took [in 1930] to put forth such an outlandish idea as the existence of an elusive particle.”
* * *
It seems fitting that such a strange and ghostly creation should have occurred to this man in the midst of a deep emotional crisis. For even though the neutrino was one of the first subnuclear particles identified, it has been baffling physicists ever since. Even now, going on a century since Pauli invented it, the little particle seems to be pointing the way to new physics beyond the standard model. In a letter that he wrote in 1958, two months before he died, he referred to the neutrino as “this crazy child of the crisis of my life (1930–1)—which also behaved crazily.”
* * *
In keeping with his cautious nature, its creator spoke guardedly of the “Pauli neutron” for the next couple of years. He was concerned that he may have built a castle in the sky. The British astronomer Fred Hoyle once told a story he had heard from the astronomer Walter Baade, who first met Pauli in Hamburg and became another of his lifelong friends. One evening in 1930 or 1931, so the story goes, perhaps on the very day Pauli wrote his famous letter, Baade was visiting him at his home in Zürich when Pauli declared, “I’ve done a terrible thing today, something which no theoretical physicist should ever do. I have suggested something that can never be verified experimentally.” Hoyle related that “Baade instantly made a bet, to be paid in champagne—Pauli’s favourite drink—that some day this neutrino would be detected.”
Pauli spent the summer of 1931 in the United States, lecturing in Chicago, Ann Arbor, and New York. In June, he presented “his” particle in public for the first time at a joint meeting of the American Association for the Advancement of Science and the American Physical Society in Pasadena, California. A New York Times article expressed the reception rather well: “Physicists will accept a third particle with reluctance. What with protons and electrons, they have reduced the atom to very simple terms. A third term adds complications that they dislike. Besides there is no experimental evidence of neutrons. They are like the ‘average man’ of the statisticians—a mere mathematical creation.” Indeed, many respected physicists would view the neutrino as nothing more than a useful mathematical trick for decades to come.
Pauli’s mental state continued to deteriorate. Although Prohibition was in effect in the United States, it seems that it wasn’t much of an obstacle. It was relatively easy to smuggle whiskey into Ann Arbor, for instance, which is close to the Canadian border, and at a dinner party in that town Pauli got so drunk that he fell down a flight of stairs and broke his arm. He made the long train ride to Pasadena in a cast that held it “up in the air like a traffic cop signaling,” and later joked that it was the only time he ever raised his hand in a “Heil Hitler” salute.
In late October, he traveled back across the Atlantic to Rome, in order to attend a conference organized by a new vital force in physics named Enrico Fermi. Samuel Goudsmit, one of the discoverers of spin, who also attended, later described this as “the first nuclear physics meeting.” And Pauli’s arrival, a day or two late, provided a benign example of the by then legendary “Pauli effect.” Goudsmit recalled that “he entered the lecture hall the very moment that I mentioned his name! Like magic! I remarked about it and got a big laugh from the audience.”
Pauli was haunted by bizarre coincidence all through his life. Things tended to break and accidents tended to happen when he was around, although an essential feature of the effect was that these accidents never inconvenienced Pauli himself in the least. One of his students, Marcus Fierz, writes that “even quite practical experimental physicists were convinced that strange effects emanated from Pauli. It was believed, e. g., that his mere presence in a laboratory produced all sorts of experimental mishaps.… For this reason, his friend [from the Hamburg days, fellow Nobel Laureate] Otto Stern, the famous artist of molecular beams, never let him enter his laboratory.… Pauli himself thoroughly believed in his effect. He once told me that he sensed the mischief already before as a disagreeable tension, and when the suspected misfortune then actually hit—another one!—, he felt strangely liberated and lightened.”
Perhaps the most famous example was the time a piece of equipment collapsed for no apparent reason in Nobel Laureate James Franck’s laboratory in Göttingen, Germany. Everyone thought Pauli was in Switzerland at the time, but when Franck wrote him a tongue-in-cheek letter, absolving him of the crime, Pauli revealed that he had been traveling to Copenhagen that day and his train had made a brief stop in Göttingen just at the time of the accident. Then there was the reception in his honor at which his friends tried to stage the effect by suspending a chandelier on a rope, planning to release it when Pauli entered the room. In the event, the rope got wedged in a pulley and the “accident” never happened.
* * *
Enrico Fermi was a year younger than Pauli. They had first met in Göttingen in 1923, when they had held fellowships together under Max Born. By 1930, Fermi had gathered a spirited group of young men around him, who were nicknamed the via Panisperna Boys, after the street in Rome where their institute was located. He had a practical turn of mind (quite unlike Pauli), was deeply engaged in the details—he is one of the few great theorists who also performed breakthrough experiments—and was quite open to new ideas.
He had invited Pauli to speak about his new particle at the conference, but Pauli “was still cautious and did not speak in public … only privately.” (“Horribile dictu,” he added, “I had to shake hands with Mussolini.”) He held useful discussions with Fermi, who “at once showed a lively interest in my idea and a very positive attitude to my new neutral particles,” and he engaged in the inevitable debate with Bohr, who “in complete contrast defended his idea that in beta decay energy was conserved only statistically.” Bohr was famous for his dogged debates.
Back home in Zürich, Pauli reached a new low of feverish drinking, partying, and brawling, and became so quarrelsome with his colleagues that he was threatened with dismissal from the ETH.
In desperation, he took the advice of the father he despised and sought the help of the psychologist, aka alienist, Carl Jung, who later recalled their first meeting:
And what shall we say of a hard-boiled scientific rationalist who produced mandalas in his dreams and in his waking fantasies? He had to consult an alienist, as he was about to lose his reason because he had suddenly become assailed
by the most amazing dreams and visions.… When [he] came to consult me for the first time, he was in such a state of panic that not only he but I myself felt the wind blowing over from the lunatic asylum.
Realizing nevertheless that he was dealing with an extraordinary individual with a mind “chock-full of archaic material,” Jung decided “to make an interesting experiment to get that material absolutely pure, without any influence from myself.” Suspecting that Pauli had trouble relating to the opposite sex, he assigned an inexperienced young woman to be his therapist and followed the course of his therapy from the background. (The young woman also had a disturbing first impression: Pauli was so overwhelmed with emotion during their first session together that he rolled around on the floor as he poured out his stories.)
* * *
Meanwhile, science marched on. In February 1932, at about the time Pauli entered therapy, James Chadwick discovered the neutron. The Pauli neutron now needed a new name, and it was supplied by the via Panisperna Boys. According to the contemporary Italian physicist Luisa Bonolis, “‘Neutrino,’ a funny and grammatically incorrect contraction of ‘little neutron’ (in Italian neutronino), entered the international vocabulary through Fermi.”
The particle explosion had begun. In August 1932, Carl Anderson from Caltech detected the positron in a shower of cosmic rays raining down from the sky.
* * *
Cosmic ray physics is an enormous and venerable field, which predates not only particle physics but even atomic and quantum physics. It also happens to provide the most comfortable home for neutrino astronomy. Up until the mid-1950s, when accelerators began to come on line, it served as the breeding ground for breakthroughs in nuclear and particle physics as well. Anderson’s discovery of the positron, which would have important implications for neutrino physics, and his subsequent discovery of the muon, which is intimately related to the neutrino, are prime examples.
Cosmic ray physics was born in 1912, when the Austrian physicist Victor Hess made the first measurements, at 17,400 feet in a hydrogen balloon, that gave proof to a pervasive “ionizing radiation,” constantly streaming into the atmosphere—and through it, into our bodies and our planet—from outer space. His “rays” are now known to consist mostly of protons and larger atomic nuclei—and now that we know how to detect them, neutrinos as well. It’s not possible to do astronomy with a charged cosmic ray particle, as interstellar magnetic fields will bend its trajectory as it flies through space, so that its incoming direction gives no clue as to the place it was born. Since the neutrino is uncharged, on the other hand, it travels in a straight line, like light, and can be used for astronomy.
Perhaps the greatest cosmic ray physicist of them all, the Frenchman Pierre Auger, once referred to the early pioneers as “mountaineers, mine workers, divers and air riders.” “The different places for carrying out measurements ridicule any description,” he wrote. “Thus also the amusing story I once heard a Russian physicist tell in a French lecture: ‘I have measured radiation in the sea and in high mountain ranges; I have measured it on the ground of lakes and in the highest atmosphere, in rock-salt and carbon mines and in deepest caverns. Finally I have measured it “en fer” (which means “in hell”).’ Of course, he wanted to say ‘dans le fer,’ ‘in iron.’”
Today’s practitioners still work in remote and far-flung places. There is a huge experiment on the Tibetan Plateau, and another at 13,500 feet on the flanks of a Mexican volcano. The Pierre Auger Observatory, named for the legendary pioneer, covers an area about the size of the state of Rhode Island on the vast Pampa Amarilla, a high plain in western Argentina.
* * *
Anderson’s positron was the first so-called antiparticle, anti in this case to the electron. It has the same mass and spin, but carries a positive electric charge. (And antimatter isn’t quite as exotic as it may sound. Abraham Pais points out that it is “as much matter as matter is matter.”) The positron also answered a question that had been raised by the so-called Dirac equation, which the young Paul Dirac had essentially dreamed up during the winter of 1927–28 and which many consider the most beautiful equation in all of physics.
Dirac was at first unsure how to interpret his creation, since it seemed to make the absurd prediction that an electron could have negative energy. But the math also worked if the particle in question had a positive electric charge, in which case it would have positive energy. After scratching his head for several years, he postulated the existence of a positively charged “anti-electron” in 1931, and Anderson’s positron left its first track in his cloud chamber less than a year after that. Anderson and Victor Hess shared the Nobel Prize in Physics in 1936.
In 1927, Dirac had made another theoretical contribution that would help the neutrino along, by applying the new quantum theory to the interaction of an atom with an electromagnetic field and thus proposing the first theory of “quantum electrodynamics.” Now, classically, light was thought to be a wave in the electromagnetic field; however, near the beginning of the century, Einstein had pointed out that it could also sometimes behave like a particle, the photon. By demonstrating in his new theory that the photon could appear and disappear spontaneously from the void, Dirac gave respectability to the general notion that other elementary particles might do the same. And when Chadwick’s neutron effectively banished the electron from the nucleus, some scientists began to suspect that the electron emitted in beta decay might be created spontaneously. And those few who took the neutrino seriously began to suspect that it might, as well.
* * *
Pauli later recalled that “a general clarification” took place at the Seventh Solvay Conference, which convened in Brussels in October 1933.
Owing to the recent burst of discoveries, there was a last-minute decision to focus this edition of these influential conferences on nuclear structure. Many important players in the beta ray saga were there, including Ernest Rutherford, Lise Meitner, James Chadwick, and Charles Drummond Ellis. Bohr and Pauli were accompanied by their fellow theorists, Schrödinger, Heisenberg, and Dirac, and two rising stars, Enrico Fermi and Rudolf Peierls, also joined in. (Peierls was studying with Pauli at the time.)
Marie Curie, the grande dame of radioactivity, was there (she would die of radiation-induced leukemia the following year), and so were her daughter and son-in-law, Irène and Frédéric Joliot-Curie, a couple that had led a star-crossed scientific career thus far. They had produced both the neutron and the positron in their laboratories before Chadwick and Anderson had found them, but had not realized in either case what they had done.
The Joliot-Curies’ luck began to turn in Brussels, when they presented the first glimmerings of perhaps the most momentous discovery of the twentieth century, the first step on the road to the discovery of nuclear fission, the “splitting of the atom” that would power the first atomic bombs thirteen years later. By bombarding thin sheets of aluminum and boron with alpha particles, that is, helium nuclei consisting of two protons and two neutrons, they had produced the first artificial radioactive substances: unstable isotopes of phosphorus and carbon. In keeping with their tradition of not quite knowing what they were up to, however, the Joliot-Curies didn’t actually realize they had produced these isotopes at the time of the conference—the discussions in Brussels would spur them on to this discovery less than three months later—but they did present evidence of great relevance to the neutrino: a new form of beta decay that produced a positron instead of an electron. And now that Chadwick had placed the neutron firmly in the nucleus, this made it possible to understand the two forms of beta decay as flip sides of the same coin.
Phosphorus stands two positions to the right of aluminum on the periodic table of the elements: it has two more protons in its nucleus. Thus the Joliot-Curies had succeeded in causing the aluminum nucleus to absorb both of the protons in an alpha particle. Their artificial phosphorous then decayed to silicon, which stands between aluminum and phosphorous on the table, emitting a positron as it did
so. We now know that in this form of beta decay, a proton changes to a neutron, and the decaying element therefore takes one step down on the periodic table—in contrast to the previously known process in which a neutron changes to a proton and the element takes a step up. Electric charge is conserved in each case, as the creation of a positron offsets the disappearance of a proton in the Joliot-Curies’ process, while the creation of the electron offsets the appearance of the proton in the original process.
Pauli’s last bit of clarification was provided by Charles Drummond Ellis, who put the final nail in the coffin of Niels Bohr’s alternative hypothesis for beta decay. Recall that Bohr had proposed that energy conservation might not hold for individual decays, but that it would hold on average, overall. This implied that the energy spectrum of beta-electrons would tail off at high energies, but have no upper limit. At the conference, Ellis and his student W. J. Henderson presented results demonstrating that the energy spectrum did have an upper limit, and right where it was expected to be, based on mass-energy arguments. This meant that the average energy of the electrons had to lie below this limit, so that energy was lost even on average unless at least one other particle was involved. There are those who argue that Ellis and Henderson discovered the neutrino with this experiment, and indeed by today’s standards they did. But Bohr was incredibly stubborn about this sort of thing. He didn’t give in for another three years.