The Telescope in the Ice Page 3
Born was not alone in considering Pauli a genius comparable to Einstein, and possibly even a greater scientist, though not as great a man. Pauli had no interest in the practical applications of science. He didn’t read newspapers. When he was offered a permanent position at the Institute for Advanced Study, for example, he turned it down and returned to the position at the Eidgenossische Technische Hochschule (ETH) in Zürich that he had held before the war, in the view that nuclear weapons research was casting a pall on American physics.
He was oblivious to the cutthroat competition of science; his passion was for clarity. Nor did he care for recognition; he usually found it irritating, although he did go out of his way to give credit to others. He made many important discoveries that are now attached to other people’s names, independently and frequently before they did, but rarely mentioned it. And he was unconstrained by the petty “publish or perish” mentality of most academics. His published output was relatively slim. But in his lifelong search for clear understanding, he wrote thousands of letters to the many giants who walked the world of physics in his day—and to experts in other fields, especially philosophy, psychology, and art history. His collected scientific correspondence comes to more than seven thousand pages. Often his most important ideas were first mooted in these letters, and sometimes he never went on to publish them in the scientific literature. The letters were copied, passed around, and studied, like teachings from on high.
Although Pauli is not generally associated quite as closely with the invention of quantum theory as are Bohr, Heisenberg, and Schrödinger, the letters demonstrate that he was crucially involved. No one understood the big picture as well as he did, and his colleagues respected him not only for this, but also for his caution and deep grounding in the classical traditions of physics. No less than Niels Bohr once referred to him as “the very conscience of the community of theoretical physicists” and “a solid rock in a turbulent sea” during the revolutionary years from 1925 to 1933 when quantum theory was born.
Charles Enz, his biographer and last assistant, writes that Pauli “was the critical analytical mind behind” the development of the theory: “Both Bohr and Heisenberg considered him the supreme judge.” They used him as the sounding board for their latest speculations—and braced themselves for biting comments when they did. Pauli was the first person with whom Heisenberg shared his famous uncertainty principle, in a fourteen-page letter that ended with, “I know very well this is still unclear in many points, but I have to write you in order to make it somewhat clearer. Now I await your merciless criticism.” He and Pauli had met in Munich, studying together under Sommerfeld, and remained friends for life. Their correspondence during the crucial period from 1925 to 1927, when Heisenberg invented quantum mechanics, indicates that much of his work was inspired by Pauli’s insights and suggestions.
Though his caustic wit was legendary, Pauli could be charming and funny as well, and the huge circle of friends he amassed in his short life demonstrates that they could see through his barbs to the kind and generous heart beneath. “We always benefitted by Pauli’s comments even if disagreement could temporarily prevail,” wrote Bohr in gentle memoriam after the younger man died. “If he felt he had to change his views, he admitted it most gracefully, and accordingly it was a great comfort when new developments met with his approval.”
Pauli’s “merciless criticism” was invariably directed at vague or shoddy thinking. His friend and fellow physicist Paul Ehrenfest nicknamed him “Scourge of God.” His most famous remark, tendered after reading a paper that he found especially wide of the mark, was, “Not only is it not right, it isn’t even wrong!”
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Professionally then, Wolfgang Pauli was thriving in 1930, the year the mysterious new particle began flitting about in his mind. He was world-renowned in physics circles and held a tenured professorship at the ETH. But his emotional life was on a different tack altogether.
He had moved to Zürich from Hamburg, where he had held various academic positions and made, as usual, a number of lifelong friends. His five years in that city may have been his most productive—he proposed the exclusion principle while he was there—but they were also the years when he began to lose his grip.
The town was famous for its night life, and he frequently availed himself of it with his friends. (“After the second bottle of wine or champagne,” he wrote in a letter, “I usually become good company (which I never am when sober) and can, under these circumstances, make a very good impression on my surroundings, especially if they are feminine.”). Unbeknownst to his companions, he often kept the party going after they went home, in the notorious red-light district of the city—which was named Sankt Pauli, believe it or not, another of Ehrenfest’s nicknames for him. He smoked and drank in the seedy bars, got involved in brawls, picked up women—the staid professor by day, the desperate libertine by night. Dr. Jekyll and Mr. Hyde.
When he moved to Zürich, he was more circumspect. He participated in the intellectual ferment and elegant night life of the city, rubbing shoulders in the salons with the likes of James Joyce, Thomas Mann, and the artists Max Ernst and Hermann Haller. But he continued to satisfy his darker urges with periodic visits to Hamburg and Berlin.
Pauli had been raised believing he was Christian; he was baptized Catholic. As a child he was not told that his father, an internationally renowned physician and professor of chemistry, had changed his name and converted from Judaism to Catholicism as a young man, in order to advance his career in the anti-Semitic world of Austrian academia. His mother was Catholic and part Jewish, and oddly, both parents converted to Protestantism when Pauli was about eleven. His mother was especially devout. It is unclear when he learned of his Jewish heritage—it was probably sometime in his teens or early twenties—but he sensed the lack of clarity and was upset by it all through his formative years.
During his final year in Hamburg, his father, who was an inveterate womanizer, left his mother for a young sculptor about his son’s age, and in November 1927, Pauli’s mother committed suicide. He received the offer from the ETH that very month.
A year and a half after his mother’s death, he left the Catholic Church and acknowledged his Jewish heritage. Six months after that, he married a German cabaret dancer, in an affair that was a disaster from the start: even before the marriage she announced that she was in love with another man, and she eventually left Pauli for that other man. Their union lasted less than a year, and the divorce was finalized on the twenty-sixth of November, 1930.
So, this thirty-year-old man had a lot on his mind in early December, as he pondered the problems of the nucleus.
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The first pertained to a common radioactive process known as beta decay, in which the nucleus of one element transmutes spontaneously into the nucleus of a different element and gives off an electron, which is also known as a beta ray. The conundrum was that beta decay seemed to be violating one of the most hallowed laws of physics, the principle of energy conservation.
In practice, this principle is something like balancing your checkbook: Any physical system that is involved in any sort of “transaction,” that is, changes in any way, must contain the same amount of energy after the transaction as it did beforehand. The energy may end up in different places, but it all has to go somewhere.
A specific example of beta decay would be the radioactive decay of carbon-14 into nitrogen-14. This is the process behind carbon dating, the method for determining the age of formerly living things, such as old pieces of wood or bone, which is widely used in archeology and geology. Carbon is the sixth element in the periodic table, which means that its nucleus contains six positively charged protons, and the “14” denotes atomic weight. This means, according to present knowledge, that along with its six protons, carbon-14 has eight neutrally charged neutrons in its nucleus, which adds up to a total of fourteen “nucleons.”
A significant contributor to the confusion in 1930 was the fact th
at the neutron had yet to be discovered. The primitive theory of the time held that the nucleus was made up of positively charged protons and negatively charged electrons, so that the carbon-14 nucleus, to stay with this example, would have comprised fourteen protons and eight electrons. It was known to have an electrical charge of six, so the electrons would have offset just the right amount of positive charge contributed by the protons.
The carbon-14 nucleus beta decays into nitrogen-14, an isotope of the seventh element in the periodic table, which, according to the old way of thinking, would have comprised fourteen protons—the same as in carbon-14—and seven electrons, one less than before. This took care of the change in electric charge, since the nitrogen nucleus has a charge of seven; and it all seemed to add up, since an electron is caught speeding away after the decay. Until you looked at energy.
In 1905, Einstein had demonstrated the equivalence of energy (E) and mass (m) with his famous equation E=mc2. (The letter c denotes the speed of light, which is a constant.) So, from an energy standpoint, before the decay we have simply the mass-energy of the carbon-14 nucleus, and afterward we have the mass-energies of the nitrogen nucleus and the electron, plus the so-called kinetic energy that the electron carries by virtue of its velocity. Since the masses of the nitrogen nucleus and the electron are fixed and add up to something less than the mass of the original carbon nucleus, the nuclear model of 1930 predicted that every electron emitted in a beta decay must emerge with the same kinetic energy, or velocity: just enough to make up the mass-energy difference between the one particle that existed before the decay and the two particles that existed after.
The problem was that the electrons emerged with a range, or spectrum, of energies. If they had all emerged with the highest energy in the range, everything would have been fine, but this was rarely the case. (And, in fact, we now know that it never actually happens.) A small amount of energy seemed to be disappearing somehow.
This problem had been festering for more than twenty years. Lise Meitner, an Austrian experimentalist with a background in theory, and Otto Hahn, an accomplished German radiochemist, had begun investigating the beta ray spectrum in 1907, expecting to find no spectrum at all. At first, they found what they wanted to—an uncharacteristic blunder for this superb experimental team. But it didn’t take long for them to identify some flaws in their methods, improve them, and in 1911 reveal the first confusing evidence that the electrons did emerge in a spectrum. Meitner, however, the theorist of the two, was not ready to accept her own result. She made various suggestions about problems with the new experiment or secondary processes in the nucleus that might modify an initially pure beam. Most people’s doubts were put to rest in 1914, when James Chadwick, working under the great Ernest Rutherford in the Cavendish Laboratory in Cambridge, England, completed what is now considered to be the first definitive experiment proving the existence of a spectrum. But Meitner continued to dig in her heels. This led to new experiments and other scientists joining the cause, including Charles Drummond Ellis, another Briton. The quest dragged on for another thirteen years, until 1927, when Ellis and his colleague William Wooster not only ruled out secondary processes but also proved that some energy was definitely going missing, because the average speed of the emerging electrons was too low to make up the mass-energy difference between the one nucleus that existed before the decay and the new nucleus and the electron that existed after. One test wasn’t enough to convince the entire community, however, especially Meitner. So it wasn’t until she and her assistant Wilhelm Orthmann confirmed and extended Ellis and Wooster’s result two years later, at the very end of 1929, that the physics community was forced to accept the fact that something fishy was going on in beta decay.
The atom had been providing so many surprises over the previous few decades that the architects of the new quantum theory, Niels Bohr in particular, were willing to question any classical truth. In a manuscript he sent to Pauli in mid-1929, Bohr suggested that the missing energy might indicate that the hallowed conservation law did not hold in the quantum realm.
This offended Pauli’s deep understanding of symmetry in the physical world. (Few laypeople realize the extent to which beauty and elegance motivate the theoretical physicist, and symmetry principles not only underlie much of that beauty, they are also among the theorist’s most powerful tools.) He didn’t see why electric charge would be conserved in beta decay, while energy, which was basically the central theme in Einstein’s highly successful theory of special relativity, would not. He responded to his mentor with characteristic candor. (Pauli had studied with Bohr at his institute in Copenhagen.) “I must say that your paper gave me very little satisfaction.… We really don’t know what is the matter here. You don’t know either.… In any case, let this [matter] rest for a good long time, and let the stars shine in peace!” Bohr never did publish the manuscript.
Pauli followed his own advice and let the matter rest, and as time went by, began to suspect that the problem of the missing energy might be related to a more recent puzzle in the existing nuclear model, having to do with spin. This is like the spinning of a top or the rotating of a planet, except that spin is an intrinsic property of elementary particles, like mass or electric charge. It’s as if they’re spinning all the time.
In 1924, when he had proposed the exclusion principle, Pauli had actually intuited the existence of spin before it was discovered. Bohr’s old quantum model of the atom, which was state of the art at the time, held that at most two electrons could fill each of the energy levels, or orbitals, surrounding an atomic nucleus. But this was simply a rule; he gave no underlying reason for it. Pauli supplied a reason, which turned out to be a new law of physics. In its simplest form, his exclusion principle states that no two electrons can exist in the same quantum state. And since two electrons were going into each of Bohr’s orbitals, Pauli deduced that the electron must have some property that had not yet been discovered. Believing it was counterproductive to use one’s classical mind to visualize the goings-on in the strange world of the quantum, however, he refused to make any claims as to what this property might be. He called it a “classically non-describable two-valuedness.” One year later, the Dutch physicists George Uhlenbeck and Samuel Goudsmit explained certain fine features of the emission spectrum of hydrogen by identifying this property as spin.
Particles behave differently depending on their spin, and like most things quantum-mechanical, this property comes in quantized units. Particles with half-integer spin, such as the electron, the proton, and the neutron, obey the exclusion principle. Those with integral spin, such as the photon or particle of light, do not: they like to get together. Spin also tied a nice ribbon around Bohr’s atomic model, since the electron, being spin one-half, will only have two spin states: up and down. An up spin will pair with a down spin in each atomic orbital, and the two will cancel each other out.
But the ribbon began to unravel early in 1929, when several experiments showed that the nitrogen nucleus had a total spin of one and did not obey the exclusion principle. This didn’t work with the proton-electron model of the nucleus, which called for the nitrogen nucleus to comprise fourteen protons and seven electrons—a total of twenty-one spin-one-half particles—for there is no way that an odd number of half-integral spins can be arranged to produce a total spin of one. Ten could point up, and ten could point down, for example, cancelling each other out, but the one that was left would then give a total spin of one-half.
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These two conundrums gestated in Pauli’s mind for about two years altogether, through the grief of his mother’s death and the anguish of his marriage. Some clarity must have been born from his divorce, however, for only eight days after it was finalized he wrote a witty “Open Letter to the Group of Radioactive Persons at the Conference of the District Society in Tübingen,” proposing a tentative solution to both.
He had been invited to this conference in the German city of Tübingen, but could “not appear p
ersonally,” he explained, “on account of a dance which takes place in Zürich on the night of 6 to 7 December.” A friend delivered the letter for him. It was addressed mainly to two experimentalists at the meeting whom he held in the highest regard, Lise Meitner and Hans Geiger, the inventor of the Geiger counter.
“Dear Radioactive Ladies and Gentlemen,” it began …
I have … hit upon a desperate remedy for rescuing the “alternation law” of statistics [the exclusion principle] and the energy law. This is the possibility that there might exist in the nuclei electrically neutral particles, which I shall call neutrons, which have spin-½, obey the exclusion principle and moreover differ from light quanta in not travelling with the velocity of light. The mass of the neutrons would have to be of the same order as the electronic mass and in any case not greater than 0.01 proton masses.—The continuous β-spectrum would then be understandable on the assumption that in β-decay, along with the electron a neutron is emitted as well, in such a way that the sum of the energies of neutron and electron is constant.
To clear up any confusion with the name, let’s just say that the particle he was describing is now known as the neutrino. In fact, it was an imperfect combination of the neutrino and the particle we now know as the neutron (and there are those who say he invented both). But be that as it may, Pauli got the neutrino side of the equation almost entirely correct: the missing energy in beta decay was being carried away by an unseen, lightweight, electrically neutral particle of spin one-half. Fifty years later, the Italian physicist Bruno Pontecorvo observed, “It is difficult to find a case where the word ‘intuition’ characterizes a human achievement better than in the case of the neutrino invention by Pauli.”
Pauli was suggesting that energy was being shared between his new, unseen particle and the kinetic energy of the electron. Some would power the electron as it sped away from the nucleus, the rest would go to the neutrino, the total amount would always be the same, but the fraction allotted to each would change randomly from one decay to the next. This would conserve energy for each individual beta decay and explain the continuous energy spectrum as well. By suggesting that an electrically neutral particle with spin one-half could “exist in the nuclei”—one for each electron—he was also placing an even number of particles in the nucleus and proposing a solution to the nitrogen anomaly.