The Telescope in the Ice Page 5
Putting all these new developments together, Pauli realized that both forms of beta decay presented another conservation problem, with spin, that seemed to require the emission of a neutrino: Remember that every particle involved in both forms of beta decay has spin one-half. To take the Joliot-Curies’ process as the example, if the proton in an unstable phosphorous nucleus changes to a neutron and ejects only a positron, an extra half unit of spin has been created: two half spins can add to one or zero, but not to the original one-half. But if a neutrino is also ejected, and its spin is one-half, then spin is conserved. “On this basis of this new situation,” Pauli wrote many years later, “my earlier caution in postponing publication now seemed superfluous.… I gave my ideas on the neutrino (as it was now called) in the discussion” at the conference.
The little particle wasn’t quite born, but one could say after nearly three years that it was finally conceived. And as it was conceived, its creator achieved psychological clarity as well.
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Pauli’s therapy with the young woman lasted five months, “and then for three months he was doing the work all by himself,” writes Jung, “continuing the observations of his unconscious with minute accuracy. He was very gifted in this respect.” In mid-1933, a few months before the Solvay Conference, he entered therapy with Jung himself.
Six months after the conference, Pauli entered into a second, more successful marriage, which lasted until the end of his life. His therapy ended a few months later, but his friendship with Jung endured. At the psychologist’s request, he continued to record his vivid and remarkable dreams—more than a thousand altogether. (“They contain the most marvelous series of archetypal images,” Jung exclaimed.) These dreams became the basis for some of Jung’s most important lectures and papers; however, the identity of the dreamer was always kept scrupulously anonymous. The truth was revealed about two decades after both men died.
They corresponded until the end of Pauli’s life, frequently meeting in Jung’s mansion on the shores of Lake Zürich for an evening’s conversation. Pauli’s psychological emergence (“individuation” in Jungian parlance) and his invention of the neutrino marked a turning point in his scientific life. He continued to make important contributions to pure physics, to be sure, but he evolved into more of a natural philosopher in the tradition of Newton, Kepler, and the ancient alchemists, who were of great interest to Jung.
Both the psychologist and the physicist viewed the traditional scientific approach to nature as being incomplete and together began searching for ways to extend it. One of these efforts was a joint investigation into what Jung once called “the no-man’s-land between Physics and the Psychology of the Unconscious … the most fascinating yet the darkest hunting ground of our times.”
Perhaps the most revolutionary feature of quantum mechanics is the famous “inseparability of the observer and the observed,” since it implies that there is no such thing as an “objective” reality. (Einstein found this consequence of the theory especially abhorrent.) What one observes depends upon how one observes it, and the act of observing inevitably changes the system being observed. But the observer in quantum mechanics is still quite detached: the inseparability is entirely physical, related only to how the measuring apparatus is set up and so on. Pauli suspected that the theory did not go far enough in this respect, “that the observer in present-day physics is still too completely detached.” He and Jung believed that mind and matter were mirror images of each other, “complementary aspects of the same reality … governed by common ordering principles”—those principles being Jung’s archetypes. I have already alluded to the profound significance and beauty of symmetry in physical law. Symmetry of all kinds, and mirror symmetry in particular, was one of Pauli’s obsessions.
In 1952, he and Jung published a book together, The Interpretation of Nature and the Psyche, comprising two monographs, one by each. Jung’s contribution was the famous (or infamous) “Synchronicity: An Acausal Connecting Principle,” in which he proposed a mechanism behind meaningful coincidence and certain paranormal phenomena, including telepathy. Pauli had a personal interest in acausal connections, of course, owing to his long experience with the Pauli effect, which he suspected to be a “synchronistic [manifestation] of a deep conflict between his rational and non-rational” sides. He had encouraged the hesitant psychologist to venture into this dangerous realm and made numerous contributions to the manuscript. His own monograph, “The Influence of Archetypal Ideas on the Scientific Theories of Kepler,” was an exploration of the role of the unconscious in scientific discovery. One learns to be very careful in trying to summarize Pauli’s extraordinarily precise and rigorous thinking, but I think it’s fair to say that he believed new discoveries become possible only as the collective mind becomes capable of visualizing or conceiving them and that this co-evolution, the leapfrogging between theory and experiment that I mentioned earlier, is driven by the emergence of archetypes. His invention of the neutrino just as the particle explosion began might serve as a good example.
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Once he committed his invention to print and gained his individuation, Pauli stepped back from the neutrino’s upbringing. His crazy child still had one big shock in store for him, however.
2. Infancy and Youth
The 1933 Solvay Conference provided a “general clarification” for Enrico Fermi as well. Upon returning to Rome, he dove in to the creation of a quantum theory for beta decay, completing it in December, less than two months after the conference ended.
Fermi took all the new ideas and ran with them: he assumed that the electron and neutrino could appear spontaneously, that protons and neutrons could exchange identity, and, agreeing with Pauli, that all the known conservation laws held true. Taking a cue from Heisenberg, who had suggested in Brussels (not quite accurately as it turned out) that the proton and neutron might be different states of the same fundamental particle, he guessed that the electron and neutrino might also be related—a conjecture that has resonated and continued to gain meaning ever since.
Although he didn’t employ a force per se, Fermi’s model for beta decay turned out to be the first rudimentary theory of the weak nuclear force. In other words, you could argue that he discovered the third fundamental force of nature, after gravity and electromagnetism. (Today there are four: the strong nuclear force was discovered soon thereafter.) Some also see his model as the first example of a modern field theory, and Francis Halzen, the guiding spirit of IceCube, points out that it could also be seen as the starting point for the standard model of particle physics, which didn’t come into being for another forty or fifty years.
On the ultimate test, predictive value, the theory passed with flying colors. Fermi used it to derive from first principles the curve that had bedeviled theorists and experimentalists alike for more than twenty years: the detailed shape of the electron energy spectrum in beta decay. Granted the neutrino had yet to be seen, but its intrinsic role in a theory that lined up so well with experiment made its existence hard to deny—although many continued to do so. When he submitted his paper on the theory to Nature, the world’s preeminent general science journal, it was rejected as containing “speculations too remote from reality to be of interest to the reader.” (Fifty years later, the journal’s editors admitted that this was perhaps the greatest editorial blunder they had ever made.) Fermi published it in three more specialized—and less visible—physics journals instead.
Employing his theory to estimate the mass of the neutrino, Fermi demonstrated that it must be “either zero or in any case very small with respect to the mass of the electron.” This statement has also turned out to be true, and, as usual with this strange particle, it only made the neutrino more elusive.
The theory also had implications for neutrino detection—and, therefore, neutrino astronomy. For it showed that beta decay could run in reverse: a free and invisible neutrino flying unheeding through space and time could pass close enough to a neutron or
proton to interact with it, change it to its counterpart, and produce a free electron or positron, which could then be detected. This process happens to be the basic principle of operation for IceCube.
Disappointingly, however, only three months after Fermi published his findings, Rudolph Peierls and another superb German theorist, Hans Bethe, showed that so-called inverse beta decay didn’t happen very often. They used Fermi’s theory to show that neutrinos at the energies common in beta decay could traverse about a thousand light years of water, on average—sixty-three million times the distance between the Earth and the Sun—without interacting, and therefore concluded that there was “no practically possible way of observing” the particle.
This sensible remark by two fairly intelligent individuals might stand as a cautionary tale on the dangers of prediction. Bethe and Peierls could not possibly have realized that the discoveries in physics over the next ten years would change this state of affairs dramatically—not to mention the entire human experience on this planet. Furthermore, as Peierls would admit about five decades later, they weren’t counting on “the ingenuity of experimentalists.”
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The via Panisperna Boys became the next set of midwives to the neutrino, even as it took a back seat to the neutron for the next decade or so, as Chadwick’s particle drove a plethora of new and literally earth-shattering discoveries.
Pretty much the moment the neutron appeared, Fermi and several other perspicacious individuals realized that it should be able to penetrate the nucleus more easily than the positively charged alpha particle can, since, being uncharged, it won’t be repelled by positively charged nuclear protons. Taking his cue from the Joliot-Curies, he and his boys set about bombarding every known scientific element with neutrons. Partway through, by complete serendipity, they discovered that “slow,” that is, low-energy, neutrons penetrate the nucleus more easily than fast ones do. And over the next several years, they succeeded in producing radioactive isotopes for every known element except the two lightest, hydrogen and helium. For this series of discoveries, in 1938, Fermi was awarded the Nobel Prize in Physics.
As it happened, he had just missed discovering nuclear fission. In fact, it is widely believed that the via Panisperna Boys induced the first manmade fission reaction while they were working with the heaviest known element at the time, uranium, but didn’t realize what they had done, because they misidentified the byproducts. The via Panisperna Boys weren’t quite as good at chemistry as they were at physics—and this may have been a blessing in disguise. As the gifted writer and theoretical physicist Jeremy Bernstein observes, “One can only imagine what might have happened if nuclear fission had been recognized in Fascist Italy in 1934.”
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Fermi won his prize at a difficult time. Among many other things, Mussolini allied with Hitler that year and began passing anti-Semitic laws in Italy. Ever the pragmatist, Fermi advised his wife Laura, who was Jewish, to invest what money they had in jewels, fur coats, and so on, and took her and their daughter with him when he accepted his prize in Stockholm. They then sailed directly for the United States, where they used the prize money and the selling of their finery as the nest egg to start a new life. The Fermis never resided in Italy again.
Many of the other scientists in the beta ray saga were also forced to flee: Pauli, Peierls, Lise Meitner. Meitner, a baptized, so-called assimilated Jew, was Austrian by birth and therefore protected from the anti-Semitic laws of Nazi Germany, where she lived and worked. But this thin platform of stability collapsed in March 1938 with the Anschluss, Hitler’s annexation of Austria. In a panic, she fled her home in Berlin and the Kaiser Wilhelm Institute of Chemistry, where she and Otto Hahn had been collaborating for a quarter of a century, and made a harrowing escape from Germany by train. Neils Bohr managed to find her a position in Stockholm, and in that unhappy exile she continued to collaborate with Hahn as best she could, through the mail.
This formidable pair had also been bombarding various elements with neutrons, and Hahn, who was probably the most accomplished radiochemist of his day, had taken a special interest in uranium, every isotope of which is radioactive. A few days before Christmas that fateful year, as Meitner enjoyed the holiday with friends in the west of Sweden, she received a letter from her collaborator, revealing that Hahn and his assistant, Fritz Strassmann, had discovered an isotope of barium among the breakdown products of a sample of uranium that had been bombarded with neutrons. Barium has an atomic number of fifty-six, whereas uranium’s is ninety-two. This was a transmutation of an entirely different order than the small movements on the periodic table that had previously been observed: the uranium nucleus had split nearly in half.
On Christmas Eve day 1938, while walking in a snowy wood with her physicist nephew, Otto Frisch, Meitner realized “that if you really do form two such fragments they would be pushed apart with great energy.” The sum of the masses of the breakdown products is so much smaller than the mass of the original uranium nucleus that an astounding amount of Einstein’s mass-energy is released. Frisch later calculated “that the energy from each bursting uranium nucleus would be sufficient to make a visible grain of sand visibly jump.” And since there are some 1021, or a billion trillion, nuclei in a single gram of uranium, this adds up to a very powerful explosion. In mid-January, Frisch named the process fission, by analogy with the binary fission of bacteria.
Not only does each individual fission of a nucleus produce an extraordinary burst of energy, but also, long before Hahn, Strassmann, and Meitner made their discovery, several far-seeing scientists realized that the splitting of a nucleus might also lead to a so-called chain reaction. When one neutron splits a single nucleus of one particular isotope of uranium, the fission products undergo beta decay and produce new neutrons, which happen to have the right speed or energy to split other nuclei, whose decay products produce more neutrons, which split more nuclei, and so on …
On December 2, 1942, a uranium pile (the heart of a modern-day nuclear reactor) engineered by Enrico Fermi underwent the first manmade, self-sustaining nuclear chain reaction in a squash court under the stands of the abandoned football stadium at the University of Chicago. Fermi then went on to become one of the principal architects of the first atomic bombs, which brought World War II to an end about two and a half years later.
And what does all this have to do with the neutrino? Well, each beta decay in a chain reaction also produces at least one of the ghostly particles. Thus a nuclear blast or a controlled nuclear pile produces so many neutrinos that it’s hardly worth attaching a number to it. “Zillions” probably gets it about right. The existence of such powerful neutrino sources laid the ground for detection.
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Other developments that took place during the war years also moved things along.
The first was a theoretical contribution by one of the more enigmatic figures in the history of physics, a thin, wealthy, pessimistic individual by the name of Ettore Majorana, who was one of the via Panisperna Boys.
The boys jokingly referred to themselves as a religious order in which the infallible Fermi played the role of pope and Majorana, grand inquisitor. Not unlike Wolfgang Pauli, he skewered any and all sloppy thinking. He had no need to work for a living, he hung around the Istituto out of a sort of bored and bemused interest, and his entire scientific output—slim, but extremely influential—was produced in a span of less than ten years. To say that he was unconventional would be an understatement. Not only was he an out-of-the box thinker, in 1938 he pulled a stunt that has turned him into a cultural icon and the subject of an enduring popular mystery in Italy: he boarded a ship with his passport and a large wad of cash and disappeared. Some believe he committed suicide, some that he took refuge in a Catholic monastery, and some, more recently, that he took up a second life under an assumed name in South America.
Majorana’s signal contribution to neutrino physics was to reveal another kind of mystery that also remains unanswered more t
han eighty years later. In a paper he published in 1937, the year before he disappeared, he presented a variation to the Dirac equation that suggested that the neutrino might be its own antiparticle. This conjecture may seem somewhat rarified, but it had down-to-earth implications for detecting the little particle, as we’ll see in a moment.
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The second development also occurred in 1937, and it came from the same Carl Anderson who had rocked the world by detecting the positron five years earlier. Again in a shower of cosmic rays, this time on top of Pike’s Peak in Colorado, he and his student Seth Neddermeyer detected the particle that is now known as the muon. This was a surprise, as it was unclear at the time what possible purpose the particle could serve. When Nobel Laureate I. I. Rabi heard about it, he quipped famously, “Who ordered that?”
At first, the mesotron, as it was originally known, seemed to fit the bill for a particle that had been postulated two years earlier by the Japanese theorist Hideki Yukawa. This was the proposed “field particle” that would carry or transmit the strong nuclear force, which binds protons and neutrons together in the nucleus. The analog in electromagnetism would be the photon, which carries the electromagnetic force. Yukawa had predicted his particle’s mass, and since the mass of Anderson’s mesotron fell in the right range, most physicists assumed it must be the one. Everything was going swimmingly until three Italians who had been conducting experiments in hiding during the war demonstrated that the mesotron could not possibly be Yukawa’s field particle, because it was unaffected by the strong force.
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At this point, the third great figure in the neutrino’s upbringing stepped to the fore, Bruno Pontecorvo, “a tall, broad-shouldered, handsome tennis champion from Pisa.” He had joined Fermi’s via Panisperna Boys as an undergraduate in 1931 and been working with the Joliot-Curies in Paris in 1938 when Mussolini had allied with Hitler. Being Jewish, he, like his mentor, had decided to flee with his family to the United States, an adventure that involved among other things escaping Paris on a bicycle as German troops approached the city, and riding it all the way to Toulouse.