by Close, Frank
With his mammoth detector, located in a South Dakota gold mine more than a mile underground, where it was shielded from cosmic rays, Davis managed to capture only one or two neutrinos a month. Everything known about the sun implied that he should be detecting one solar neutrino every few days. While this might seem like a small detail, it was in fact a serious worry. Many were convinced that Davis was trying the impossible—and proving them right.
By 1972 Davis had improved his experiment further, and at last managed to convince other scientists that he was indeed seeing solar neutrinos. However, there were still too few of them. Several theorists suggested that our understanding of the sun might be at fault. Bruno Pontecorvo wrote to Davis: “It starts to be really interesting! It would be nice if all this ends with something unexpected from the point of view of [neutrinos]. Unfortunately it will not be easy to demonstrate this even if nature works that way.”24
Pontecorvo was right. Years went by, and Davis accumulated more data, which consistently disagreed with the predictions of the standard solar model. The quest had reached an impasse. The arguments went on for twenty years, with theorists blaming the experiment, while experimentalists (and, to be fair, several theorists) doubted the accuracy of the solar models. Relatively little attention was paid to the possibility that the sun might be innocent and that the neutrinos were to blame.
Ironically, Bruno Pontecorvo had proposed the solution to the solar neutrino mystery in 1968, as soon as Davis’s first results appeared—and been ignored. Of course, he was not primarily interested in the sun. Instead he was focused on the nature of neutrinos, and his own insight that they come in different flavors.
OSCILLATING NEUTRINOS
One reason that it took so long to solve the solar neutrino mystery is that neutrinos can change their properties during the flight from the sun to the earth. Only those that retained the attributes they were born with showed up in Davis’s experiment, while those that had changed form escaped detection.
In 1968 Bruno Pontecorvo and a colleague, Vladimir Gribov, resurrected his idea of neutrino oscillations—the possibility that neutrinos can metamorphose from one variety to another. Whereas in 1957 Bruno had proposed that this might occur between neutrinos and antineutrinos, in 1968 he applied the idea to the electron and muon flavors of neutrino.25
The sun produces electron-neutrinos and Davis’s detector was sensitive to them, but it was blind to any muon-neutrinos. Thus any electron-neutrinos that metamorphosed into muon-neutrinos en route from the sun would be lost. This would explain why Davis detected fewer neutrinos than he expected.
The idea that neutrinos produced in the sun could change identity during their journey ran counter to everything in the textbooks. According to the standard theory of particle physics, this was impossible. Or at least it was impossible if, as scientists then believed, neutrinos were massless and traveled through space at the speed of light. However, Pontecorvo and Gribov realized that the laws of quantum mechanics allowed neutrinos to switch from electron to muon variety and back again, “oscillating” between one state and another—but only if neutrinos had some mass. It didn’t need to be large; in fact it could be trifling, thousands of times smaller than the mass of an electron. All that the theory required was the existence of two varieties of neutrinos, with slightly different masses.
In quantum mechanics, certainty is replaced by probability, which rises and falls like a wave. If two particles have the same energy, but slightly different masses, the associated waves have slightly different wavelengths, and interfere with each other as they move through space. Two sound waves whose frequencies differ slightly tend to mingle and produce a pulsing beat. The quantum waves associated with two neutrinos of slightly differing masses produce an analogous rise and fall in intensity. Only occasionally along the journey do the two waves match up in the precise form they started out in. At other points along the route, the initial electron-neutrino is a chimera, appearing sometimes as a muon-neutrino, and sometimes in its original form.
When the flow of these waves is interrupted—for example, by an atom of chlorine in a tank 4,500 feet beneath the hills of South Dakota—it is impossible to know which variety of neutrino will be revealed. If it is an electron-neutrino, Davis’s detector records the fact; if it is a muon-neutrino, it is invisible. All quantum theory can tell us is the probability of finding one or the other. In effect, if enough collisions occur, they will be divided roughly fifty-fifty between electron-neutrinos and muon-neutrinos.
Such quantum behavior is unfamiliar in the everyday world, but is illustrated in M. C. Escher’s Metamorphosis drawings. On the left side, a picture shows a collection of objects, which gradually change into a new form as you traverse the image. In the spirit of Escher, suppose there were some weird hybrid animal that could metamorphose between a cat and a dog. The dog transforms into a cat as it walks along the street. Halfway along the block the transformation is complete. The former dog (now a cat) continues on its way. By the end of the block it will be a dog once more. When you look at the dog-cat, what you see will depend on how far along the block you are.
Now suppose that you are not receptive to dog-cats, only to things that are one or the other: a dog or a cat. If you are near the start or the end of the block, you will most likely interpret the animal as a dog. If you are near the midpoint, you are more likely to interpret it as a cat. However, if your eyes are only capable of seeing dogs, and are blind to cats, you might conclude that the dog had disappeared.
So it is with neutrinos. In this analogy, the electron-neutrino is the dog and the muon-neutrino is the cat. The sun emitted a dog, and Davis’s detector was a dog-catcher. It recorded no reaction when a cat came along. According to this theory, there was nothing wrong with the sun; it was our understanding of neutrinos that was at fault.26
If one accepted Pontecorvo and Gribov’s theory of oscillating neutrinos, the apparent shortfall of solar neutrinos in Davis’s experiment could be understood. All that was required was to give up the myth that neutrinos were massless and traveled at the speed of light. However, few were prepared to do so at the time. The Russian duo’s idea was regarded as little more than a mathematical curiosity. Only after a variety of further experiments would Gribov and Pontecorvo be vindicated. That would take three decades and much would happen in the interim.
“TEN TO THE FIFTY-EIGHTH NEUTRINOS! ALL IN ONE GO!”
Physics seems to be a tradition in the Pontecorvo family. In addition to Bruno’s son Gil, two of his nephews have chosen the profession: Giuliana’s son Eugenio Tabet, and Gillo’s son Ludovico. Born in 1964, Ludovico, or Ludo, is an experimentalist, part of the ATLAS collaboration at CERN, which discovered the Higgs boson in 2012. This is the final piece in the Standard Model of particles and forces, whose origins can be traced back to Bruno Pontecorvo’s insights about muons and neutrinos in the 1940s.
In 1987, Ludo was a graduate student at university in Rome. By this stage the Soviet policy of glasnost allowed Bruno to visit Italy for extended periods, and he spent a lot of time in Ludo’s office during his visit that year. Ludo recalled Bruno’s intense excitement when his beloved neutrinos made one of the most remarkable contributions in the history of astronomy: “I arrived at my office where I was doing my thesis. Bruno was already there. I noticed a strange light in his eyes. He said, ‘Did you hear what happened today? Ten to the fifty-eighth neutrinos! All in one go!’27 I said, ‘What are you talking about?’ and Bruno replied, ‘There was a supernova.’ I felt he was living for that. [The first thing he said] was not ‘supernova’; he said, “‘Ten to the fifty-eighth all in one go!’”28
Bruno had suggested in 1958 that supernovas would produce neutrinos in vast amounts, capable of being detected on Earth. A supernova explosion releases more than a hundred times as much energy as the sun has put out in its entire life. Thus the breeze of neutrinos from the sun would be dwarfed by the hurricane from a supernova. The sun, however, is close by, whereas supernovas, thank
fully, are not. The one that excited Bruno in 1987 had actually occurred some 160,000 years earlier in the Large Magellanic Cloud, and it had taken that long for the shell of neutrinos to reach us. By this stage they were spread over a huge region, larger than the diameter of our galaxy.29 The blast wave passed through the earth and continued onward into deep space. A handful of these neutrinos were detected by experiments in underground caverns. From this sparse number, it is possible to deduce how much total energy the supernova released. The results confirmed what astrophysics already believed: a supernova occurs when a star implodes and becomes a neutron star. For the first (and, so far, only) time, humans had witnessed a star collapse by observing its neutrinos. Along with the study of solar neutrinos, this marked the beginning of a new science: neutrino astronomy.
SOLAR NEUTRINOS OBSERVED
In 1991 Bruno went to Paris again, and stayed at the Hôtel du Panthéon, located in the square by the mausoleum, near his old residence, the Hôtel des Grands Hommes. He visited the Curie Institute and discussed the neutrino experiments that French scientists were planning to conduct at CERN. By this stage the mystery of solar neutrinos was beginning to be resolved, and Pontecorvo’s ideas about oscillations were taken seriously enough that expensive experiments to test them were being designed. Bruno naturally was much in demand.
Meanwhile, after thirty years of pursuing solar neutrinos, Ray Davis was convincing the world that he was seeing some, but not enough. Gribov and Pontecorvo’s suggestion that electron-neutrinos were changing form en route from the sun was beginning to be viewed as the most likely explanation. One reason for this shift in thinking was that the scientific community’s understanding of neutrinos and their flavors had dramatically evolved in the interim.
Bruno’s original theory of neutrino flavor had matched two neutrinos, the electron and muon varieties, with the electron and muon themselves. Meanwhile, starting in the 1960s, the study of nuclear particles had revealed a deeper layer of reality—the quarks. Quarks, as it turns out, also come in pairs. The up and down varieties of quark are the constituents of protons and neutrons; strange and charm quarks form a second pair, which are found within strange and other exotic particles. These four types of quarks, when paired off this way, mirror the four types of leptons—the generic name for the electron, electron-neutrino, muon, and muon-neutrino, which are not affected by the strong nuclear force. Bruno’s proposal that there were two pairs of leptons reflected the existence of the two pairs of quarks.
In 1976, a third electrically charged analogue of the electron and muon turned up, known as the tau (the name comes from t as in third). This was soon followed by the discovery of another variety of quark, known as a bottom quark. Theorists anticipated that each of these would be revealed to be part of a pair, completed by a third neutrino (the tau-neutrino, confirmed in 2000) and a third quark (the top quark, discovered in 1995).
Although the third variety of neutrino was not discovered until after Bruno’s death, theorists were convinced of its existence from the 1970s onward. One reason was that cosmologists were best able to describe the formation of elements in the early universe if there were three varieties of neutrino. The other reason touched directly on the solar neutrino problem: Davis was seeing only about one-third as many neutrinos as astrophysicists expected. Pontecorvo and Gribov’s idea implied that if there were three rather than two alternate guises for an electron-neutrino to take on, then after its 150-million-kilometer trip from the sun, it would be an electron-neutrino one-third of the time, and one of the other varieties two-thirds of the time. Because Davis’s experiment was only sensitive to the electron type, he measured one-third as many neutrinos as had set out from the sun.
Davis’s shortfall, and the Gribov-Pontecorvo theory, began to appear more and more compelling.
THE FINAL PIECE IN THE SOLAR NEUTRINO ODYSSEY WOULD COME FROM an experiment conducted at the Sudbury Neutrino Observatory (SNO) in Ontario, less than two hundred miles from Chalk River, the laboratory where forty years earlier Bruno had conceived his ideas about neutrinos. The SNO team would eventually confirm his theory that solar neutrinos change form in their journey across space. Sadly, Bruno would not live to see this.
The main innovation at SNO was the use of heavy water, the very material that had been key to the nuclear reactor all those years before. However, this time the liquid was used as a detector of neutrinos rather than as a component of a reactor.
In Davis’s experiment, neutrinos that had changed to the muon or tau variety escaped detection. But when any variety of neutrino hits an atom of deuterium, it can bounce off the target and give enough of a kick that the deuterium nucleus breaks up. The recoil of its constituent proton and neutron can then be detected, and the collision of the neutrino can be inferred. The chance that a single neutrino will cause this to happen is given by Fermi’s theory, and the number of detected reactions can thus be used to calculate the total number of neutrinos that passed through the target. SNO’s results agreed with what the standard theory of the sun had predicted. The team was also able to count the number of collisions caused specifically by electron-neutrinos, as these both split the target atom and convert its neutron into a proton as well.30 The number of such events was one-third of the total, as suggested by Davis’s results. This confirmed that neutrinos are indeed changing form, from the electron type to the other two varieties, on their journey from the sun.
Although Bruno never returned to Canada after his move to Dubna, he was able to see the SNO exhibition at Expo ’92 in Seville, Spain. He had been nearby in Granada, at the Neutrino 92 physics conference, and had joined an excursion to the exposition in Seville. This was two years after construction had begun on the SNO observatory, but its completion was still several years in the future. Art McDonald, the leader of the project, escorted Bruno around the exhibition. He recalled that the occasion “was very nice for me,” because Bruno had been a prominent supporter of the project and was profoundly interested in its aims. For Bruno, “It was an opportunity to ask about Chalk River.”
IMAGE 17.1. Bruno balancing a stick, in defiance of Parkinson’s disease, c. 1990. (COURTESY GIL PONTECORVO; PONTECORVO FAMILY ARCHIVES.)
One memory from that day has stayed with Art ever since, as it has for others. This was a trick that Bruno performed. Always athletic and keen on sports, Bruno still enjoyed playing to the crowd, for example by balancing a stick on his foot or his nose. By this stage, Bruno was in the advanced stages of Parkinson’s disease, which caused him to tremble in his movements. Nonetheless, he got up to his old tricks. “Strange this disease,” he mused, as he balanced his walking stick on his foot without any trembling. The moment he took the stick back in his hand, however, the shaking returned.31
EIGHTEEN
PRIVATE BRUNO
THAT LIFE IN THE SOVIET UNION WOULD PROVE DISASTROUS FOR Marianne was obvious from the start; that Bruno too would suffer only became clear later. Cut off from friends and family, having passed through Stockholm without seeing her mother and siblings, Marianne’s only role in Russia was as an appendage to Bruno. He had a career there, restrictions notwithstanding; she had a vacuum to fill.
Marianne’s story encapsulates the Pontecorvo enigma. People who knew her before 1950 gave conflicting accounts, some recalling her as vivacious, an extrovert like her husband, while others found her to be withdrawn, morose, secretive. Several media reports in 1950 played up the latter image, implying that she was keeping secrets, aware of the hidden life of her husband, the spy. Marianne was even portrayed as a Mata Hari character, the leader of a communist cell. No source for this allegation was ever given, and there is nothing in the MI5 record to support it. One close relative insisted that she was never a political person and chose to follow Bruno, whom she loved. Gil confirmed this opinion: “I never thought of her as politically minded. I’ve heard claims that she was, but I doubt it.”1
Whatever her politics, Marianne missed Sweden terribly. She had left her homeland in
1939 at the start of the war, marrying Bruno just before they began their odyssey to North America as part of the tide of refugees fleeing the Nazis. It is now clear that she experienced frequent emotional crises, suffering highs and lows during their time in Canada and the UK. In the USSR these episodes developed into more serious problems.
Although the three boys were happy and in school, Marianne was beginning to suffer from a profound depression. A catastrophic collapse seems to have begun within her first five years in the USSR, around the time Gil was starting college.2 As we saw in Chapter 15, Marianne became almost pathologically averse to any form of social life, reading for hours while she lay on the bed and stared out the windows at the surrounding forest. Whether this was because the Pontecorvos had no social interaction with colleagues during this period and Marianne felt like an outcast, or because Bruno chose not to bring those colleagues home due to Marianne’s withdrawal, is impossible to know. Whatever the cause, it is certain that what began as shyness, withdrawal, and mild depression developed into a long-lasting and debilitating mental disorder.
At first she left home for short periods to undergo treatment in a clinic. At the end of the 1950s, however, Marianne had “a more serious crisis than previously” and began to spend considerable time in a psychiatric hospital.3 Bruno was now home alone with the boys, the head of an all-male household.
Even when Marianne was released from the clinic to spend time with her family, her condition was transparent. Friends and family who had known her before 1950, and who managed in later years to see the Pontecorvos in the USSR, noticed her decline. In the middle of the 1960s, Bruno’s niece Laura Schwarz visited Moscow with the Italian-Soviet Society. She recalled that Bruno had “the same smile” as always, but Marianne “seemed sick to me, sleepy, almost absent.”4 Laura Schwarz’s memory echoes those of Western colleagues, who had known the Pontecorvos in Canada. At Chalk River, as we saw, Bruno had been the “heartthrob of all the single women,” and Marianne was his “beautiful vivacious wife.”5 At a conference in Kiev in 1970, the Pontecorvos’ Canadian friends were shocked at how “downtrodden and discouraged” Marianne appeared, not at all like the vibrant woman of yore.6 A photograph from that time shows Bruno, confident and handsome in his shorts and sports shirt, while Marianne wears dark glasses at his side, her mouth pouting, her demeanor passive. Although Bruno was animated, reminiscing about happy times in Canada, Marianne didn’t say a word. As the physicist J. David Jackson put it, “Bruno had his physics to sustain him amidst the difficulties of Soviet life. Marianne evidently had nothing.”7 This period, which followed the death of her mother in 1967, must have been especially hard.
