The Telescope in the Ice Read online

  Lee and Yang were energetic collaborators. When they were seized with an idea they would talk it through in Chinese together for days on end—often quite loudly, according to their colleagues. They would break periodically to go off to their separate corners and do competing calculations, and then reconvene and pick up the discussion. One day, during an exchange in a restaurant on upper Broadway in Manhattan, across the street from Columbia, they realized that one way into the theta-tau puzzle might be to examine every experiment involving the weak force that had ever been done, to see what it might have to say about parity conservation. When they then went about doing so, they found that none had actually tested the law. Only eight days after Cowan and Reines sent their triumphant telegram to Wolfgang Pauli, Lee and Yang submitted a classic paper to the venerable U.S. journal Physical Review, suggesting that “one way out of the [theta-tau] difficulty is to assume that parity is not strictly conserved, so that the theta and tau are two different decay modes of the same particle,” which is to say, mirror images of each other. Observing that parity was conserved to a high degree of accuracy in experiments involving the strong and electromagnetic forces, they posed a challenge to the experimental community by suggesting some ways to test the parity of the weak force. The vast majority of physicists expected parity to survive.

  * * *

  The first experiment to yield an answer was based on straightforward beta decay. It was led by Chien-Shiung Wu, a close friend of Lee and Yang’s who also worked at Columbia and had also been raised in pre-communist China. Wu was a highly cultured, elegant, and attractive woman, internationally renowned for her precise and cautious experimental work.

  She settled upon the idea of using a magnetic field to line up, or polarize, the spins of radioactive cobalt-60 nuclei, which are basically tiny magnets, and observing the direction of the beta-electrons that they would emit. If the electrons sprayed out randomly in all directions, parity would be conserved; otherwise, it would not. It was necessary to cool the cobalt nearly to absolute zero in order to prevent the jostling that occurs at higher temperatures from knocking the nuclei out of alignment, so she collaborated with a team from the National Bureau of Standards in Washington on that aspect.

  After six painstaking months setting up her apparatus, Wu got her answer within minutes of starting the experiment: the beta-electrons emerged preferentially in the direction opposite the magnetic field. Parity was in its death throes.

  She was in constant communication with Lee and Yang, of course. On the third of January 1957, a Thursday, she called Lee to inform him of her results. Lee had initiated a tradition in the Columbia physics department of gathering for lunch on Friday at one of the many excellent Chinese restaurants near the Columbia campus. The next day, he shared the news with his colleagues … and set the mind of a fellow professor, Leon Lederman, to thinking. In what may be the shortest time between concept and result for a finding of such magnitude, Lederman and his colleague Richard Garwin designed a new method for testing the parity of the weak force by late that evening; had it up and running on the cyclotron at Columbia’s Nevis Laboratory, a few miles up the Hudson River, by two a.m.; and produced a definitive result, confirming Wu’s, by six a.m. the following Tuesday.

  A group at the University of Chicago that had been running a similar experiment for a few months came up with a second confirming result within days of Garwin and Lederman. Parity wasn’t just dead; it was annihilated.

  On the fifteenth of January, less than two weeks after Wu’s call to Lee, Columbia University took the unusual step of holding a press conference to announce a physics result (thus stealing the limelight from the Chicago group, who weren’t even mentioned). In a story that appeared on the front page of the New York Times, I. I. Rabi, the chairman of Columbia’s physics department, was quoted as saying, “In a certain sense, a rather complete theoretical structure has been shattered at the base and we are not sure how the pieces will be put together.” Such is the damage that the neutrino tends to leave in its wake.

  Lee and Yang were awarded the Nobel Prize later that year. Many believe Chien-Shiung Wu should have been honored as well.

  * * *

  Also in that eventful year, an elegant experiment at Brookhaven demonstrated that the neutrino is left-handed: it spins counterclockwise as it zips through space. And this brings us back to Ettore Majorana.

  One might conclude from Ray Davis’s null result with “reactor” neutrinos and Cowan and Reines’s positive one that the neutrino and its antiparticle are different—or, to put it another way, that the neutrino has an antiparticle. It turns out, however, that this is not necessarily true. Theory says that the chlorine-to-argon reaction that is the basis for Davis’s method is not sensitive to the difference between the particle and the antiparticle, but to the particle’s handedness: only a left-handed particle can initiate it. And eighty years after Majorana suggested that the neutrino might be its own antiparticle, the evidence still is not in. If he was right, then Cowan and Reines weren’t detecting antineutrinos; they were detecting right-handed neutrinos.

  As physics has evolved in the intervening decades, Majorana’s conjecture has taken on added meaning. Physicists now realize that it has implications not only for neutrino physics, but also for some of the most compelling unanswered questions in particle physics and cosmology.

  * * *

  Louis Pasteur once said that “in the field of observation, chance only favors the prepared mind.” Ray Davis and Fred Reines had prepared their minds equally well, but chance happened to favor Reines. Considering what was known about the neutrino at the time, this was really just a matter of luck.

  Davis, being a true and in fact visionary scientist, did not skip a beat. Once he proved to himself that his method could not detect “reactor” neutrinos (it took him several more years) he turned his attention to the Sun.

  * * *

  From his perch in Zürich, Wolfgang Pauli attended to the Glorious Revolution with great interest. Parity was one of his major obsessions, after all; he admitted to having a “mirror complex.”

  Four years before the Revolution began, even though, as he later wrote to Carl Jung, “there was not actually anything going on in the world of physics to justify focusing on that particular subject,” he had begun his own investigation into a deeper form of symmetry, which included not only parity (P); but also time reversal (T), running time backward; and charge conjugation (C), which is the act of converting every particle in a system into its antiparticle. (It is possible to run time backward on the atomic scale in the laboratory.) If a system remains unchanged by the act of mirroring all three of these properties at the same time, a transformation that Pauli called “strong reflection,” it would be said to have CPT symmetry or to be CPT invariant.

  He began investigating whether the fundamental equations of quantum mechanics and relativity obeyed this symmetry in 1952, and finally proved that they did near the end of 1954. To this day, no experiment has disproven Pauli’s CPT theorem. It is seen as his third great contribution to physics, after the exclusion principle and the invention of the neutrino. With parity’s downfall, and especially since “his” neutrino was so intimately involved, Pauli and his new theorem were “on everybody’s lips.” “To many physicists,” according to T. D. Lee, “CPT was a fixed point around which all else turned.”

  Remember that Pauli shared with Jung the notion of a mirroring between psychology and physics. “Now, ‘mirroring’ is an archetype,” he told an interviewer in 1957, after the news of parity violation had broken. “This has something to do with physics. Physics relies on a relation of mirror symmetry between mind and nature.… At that time [while he was working on the CPT theorem] I had vivid, almost parapsychological dreams about mirroring, while I worked mathematically during the day.… I would call that, for instance, a kind of synchronicity, since there are unconscious motives when one is engaged in something.”

  Not long after he finished writing his definiti
ve paper on the theorem, he experienced what he called “a very impressive dream”:

  I am in a room with the “dark woman,” and experiments are being carried out in which “reflections” appear. The other people in the room regard the reflections as “real objects,” whereas the Dark Woman and I know that they are just “mirror images.” This becomes a sort of secret between us. This secret fills us with apprehension.

  Afterward, the Dark Woman and I walk alone down a steep mountainside …

  In describing this dream to Jung in a long letter in 1957, Pauli reminds his friend of a “Chinese woman” who recurred in his dreams, whom he saw “as a special aspect—maybe a parapsychological one—of the ‘Dark Woman.’” In an earlier dream, he points out, the Chinese woman had “had a child, but ‘the people’ refused to acknowledge it.” He believes the “other people” in both dreams represent his own “conventional objections to certain ideas—and my fear of them.” The secret he shares with the Dark Woman, which fills him with apprehension, is the fact that there is “no symmetry of [objects] and reflections in this dream, since the whole point is about distinguishing between the two.” In other words, that parity is broken.

  This impressive dream occurred about a year and a half before Lee and Yang began to question the law of parity conservation and about two years before a Chinese woman proved that it wasn’t true. Pauli had met Madame Wu in Berkeley in 1941 and been “very impressed by her (both as an experimental physicist and as an intelligent and beautiful Chinese young lady).” He had not gone to the trouble of examining the fundamental nature of parity alone in his CPT investigations, since at the time he was sure it was conserved. He continued to hold that belief a year and a half later when the Glorious Revolution began to unfold, and when Lee and Yang put out the call for experiments to test parity, he firmly expected it to survive.

  When the results came in six months later, it took them several days, evidently, to cross the Atlantic. For, on the seventeenth of January, 1957, the very day that the New York Times announced parity’s downfall above the fold on its front page, Pauli nearly placed another of those bets that he was bound to lose: “I do not believe that the Lord is a weak left-hander,” he wrote to his student Victor Weisskopf, “and I am ready to bet a very large sum that the experiments will give symmetric results.”

  His day of reckoning came four days later, beginning with his morning mail. There he found a copy of the Times article, sent by another former student, along with two theoretical papers by Lee and Yang that explored the implications of parity violation. In the afternoon, he received a letter on the specifics of all three experiments from Valentine Telegdi, the leader of the Chicago group. By “coincidence,” Pauli was scheduled to give a talk on the history of the neutrino at the Zürich Society of Natural Sciences that evening. Witnesses say it was an excellent talk; he was quite excited. At the end, he broke the news of parity violation and offered some “reflections” on the problem and its importance.

  Despite his apparent bravado, the death of parity came as a shock to a man for whom symmetry held near-mystical significance. On the afternoon of his day of reckoning, he took a moment to write Madame Wu. “I congratulate you (to the contrary of myself). This particle neutrino, of the existence of which I am not innocent, still persecutes me.” And in his 1957 letter to Jung, he described being “very upset” after receiving the news, and behaving “irrationally for quite a while.”

  “Now after the first shock is over, I begin to collect myself,” he wrote to Weisskopf, six days after his reckoning. “It is good that I did not make a bet. It would have resulted in a heavy loss of money (which I cannot afford); I did make a fool of myself, however (which I think I can afford to do).… I am shocked not so much by the fact that the Lord prefers the left hand as by the fact that He still appears to be left-right symmetric when he expresses himself strongly. In short, the actual problem now seems to be the question: Why are strong interactions right-and-left symmetric?”

  There is nothing in the equations of physics that explains why the weak force should violate parity. The strong force does not, and neither do the electromagnetic force or gravity.

  As time went by, Pauli was calmed by the knowledge that strong reflection, CPT symmetry, still held, and his dreams reflected this gradual calming.

  * * *

  Wolfgang Pauli’s last piece of scientific writing was an essay about the history of the neutrino, based on the talk he gave that jarring evening in Zürich. He was greatly pleased not only that Cowan and Reines had proven his long-ago intuition true, but also at the excitement and new physics that his little particle was generating. Upon completing the essay, in September 1958, he sent a copy to Lise Meitner as a gift for her eightieth birthday.

  He died unexpectedly on December 15, a few months short of his own fifty-ninth birthday, not many days after being diagnosed with pancreatic cancer. This brilliant and unusual man was haunted by strange coincidence up until the moment of his death: another of his obsessions was what he and many others saw as the fundamental significance of the so-called fine-structure constant, a ratio of fundamental physical constants that is expressed as a simple number with the approximate value 1/137. Pauli once wrote that the “theoretical interpretation of its numerical value is one of the most important unsolved problems of atomic physics.” Charles Enz adds that “the number 137 also had an irrational, magic meaning for Pauli.”

  He was assigned to room 137 in the Red Cross Hospital in Zürich, and in that room he died.

  Part II

  The Dream of Neutrino Astronomy

  4. Wisconsin-Style Physics

  I’m waiting for ignition, I’m looking for a spark.

  Any chance collision and I light up in the dark.

  —PETER GABRIEL

  Fred Reines once told Francis Halzen that after he and Clyde Cowan showed “that the particle actually existed, literally everybody came up with the idea that one could do astronomy with neutrino beams.”

  Very few people put the idea in writing, however. It first appears in the literature in 1958, the year Wolfgang Pauli died, in the diploma thesis of one Igor Zheleznykh, a student of the respected particle theorist Moiseĭ Markov at Moscow State University. (Zheleznykh readily admits that it was his mentor’s idea.) Markov first presented it in public at a high-energy physics conference in Rochester, New York, in 1960, and at about the same time, Kenneth Greisen at Cornell, a former Manhattan Project physicist, proposed a similar idea at a conference in Berkeley.

  Although Markov and Greisen’s concepts were based on the same basic principle of operation, they had a fundamental difference that has led to two separate lines of experimentation over the years.

  In his talk in Rochester, Markov proposed “setting up apparatus in an underground lake or deep in the ocean in order to separate charged particle directions by Cherenkov radiation,” and in a journal article published in January of the following year, he and Zheleznykh expanded upon the idea: “All known particles with the exception of neutrinos are absorbed by scores of kilometres of [earth or rock] and thus entirely screened by the planet.… It is noteworthy that not only μ-mesons [muons] (from the reactions involving neutrinos) produced in the detector itself, but also the μ-mesons from the adjoining layers of the ground (‘the cushion’) are detected in the experiment.” This is a description of a telescope designed to observe up-going neutrinos that have passed all the way through the planet, and it works quite well for AMANDA and IceCube.

  In his remarkably prescient talk in Berkeley, meanwhile, Greisen prophesized the future development of “high-energy neutrino astronomy.… [T]he neutrinos will convey a type of astronomical information quite different from that carried by visible light and radio waves.” And at the end of a now-classic review article on cosmic ray showers, published in December 1960, he specifically proposed “a large Cherenkov counter, about 15 meters in diameter, located in a mine far underground.” Since Greisen’s concept has very poor angular resolut
ion, it’s more detector than telescope, but it would prove easier to put into practice than Markov’s and bear fruit much more quickly.

  Both of these visionaries emphasized the detection of μ-mesons, or muons, and they were ahead of the game in that respect as well, since there was still no proof in 1960 that muon neutrinos actually existed. They would be detected two years later by a team that included the same Leon Lederman who had played a role in the parity discovery. The experimental method, which required an accelerator that was very powerful for the time, had been conceived by Bruno Pontecorvo in Russia before it occurred to Lederman’s team, but the Soviets never built the accelerator, while the Americans did, and Lederman’s team went on to win the Nobel Prize in Physics in 1988—the first Nobel based on his own thinking in which Pontecorvo did not share.

  The principle of operation behind both Markov and Greisen’s concepts—a third method for detecting the neutrino, actually—is that when a muon neutrino collides with a nucleon and produces a muon through inverse beta decay, the newly born muon will speed away from the scene of its parent’s demise in nearly the same direction that the neutrino was going—much in the manner of a billiard ball hit head-on by a cue ball—and will emit a pale blue light known as Cherenkov radiation as it does so. By arranging a set of light detectors in some clever way in or around the specific volume of whatever clear medium they have chosen to monitor, neutrino enthusiasts can then determine the direction of the muon and its parent neutrino in turn. Both concepts are thus known as Cherenkov detectors.