Rob Hardy on books


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On the Trail of the Elusive Neutrino



Rob Hardy


You'd expect that physicist Ray Jayawardhana would be a fan of neutrinos. He has written Neutrino Hunters: The Thrilling Chase for a Ghostly Particle to Unlock the Secrets of the Universe (Scientific American / Farrar, Straus and Giroux). But you wouldn't expect him to dis the Higgs Boson. Oh, he doesn't really do that, but the Higgs is far more famous now since it was so diligently searched for and has been found. There is a big difference in the scientific aspects of the two particles, and not just because they have different physical characteristics. Jayawardhana's readable and intelligent book, aimed at the general reader and unencumbered by any equations except for E = mc2, shows why neutrinos are so interesting and so worthy of pursuit by the best minds and best budgets of physicists. Lay readers won't understand everything that is here, but that isn't the author's fault. Neutrinos will be understood someday, but no one understands them now, and that's the fun of the chase. 




The chase seems to be over for the Higgs Boson. The Large Hadron Collider near Geneva, Switzerland, was used for complicated experiments that found the particle that physicist Peter Higgs conjectured fifty years ago. It was a real milestone for physics, but it also seems to have been a dead end. Stephen Hawking was delighted at the discovery, but reflected about how it had been a pat confirmation: "But it is a pity in a way because the great advances in physics have come from experiments that gave results we didn't expect." The neutrino is no dead end; there are so many strange things about it that physicists will have plenty of roads to travel before they can map them all. It is, says Jayawardhana, "the most elusive and weirdest of all known denizens of the subatomic world," and his book is a good introduction as to why this should be. It also emphasizes how important understanding neutrinos could be in the future, so readers who try to keep up with physics will be ready to watch the show. 




A neutrino is an elementary particle, much tinier than the protons and even the electrons that make up the objects that are more familiar to us. It has no charge, and it has mass, but the mass is so infinitesimal that when its theoretical existence was first proposed, by Wolfgang Pauli in 1930, even some physicists who believed Pauli was right thought that the neutrino would never be detected. After all, a neutrino could zip through a block of lead one light-year long, and not interact with any of the lead atoms. You yourself are being pierced by neutrinos right now, billions of them while you read this sentence. There are lots of them; for every atom in the universe, there are a billion neutrinos. They were there seconds after the Big Bang, and they are produced at the Sun's core and in the radioactive decay of elements inside Earth and in the cataclysmic explosions as stars die. They aren't as weird as they could be; the report three years ago that they went faster than light made headlines, but was doubted at the time and has been subsequently refuted. If they had broken that speed barrier, they would have demolished our model of the cosmos, but they are raising other fundamental questions. 




Among the puzzles of neutrinos is that they have that vanishingly small mass. There are three different types (or "flavors") of neutrino. A neutrino can flip from one flavor to another (one of the reasons counting neutrinos accurately has proved to be so hard), but it can only do this if it has some mass to change. The problem is that according to the original understanding of the standard model of physics, neutrinos were assumed to have no mass. One physicist said, "The zero mass of neutrinos was taken as a self-evident truth by theorists. Nature has straightened us out." No one has been able to measure the neutrino's mass accurately yet; data suggest that it is at tops a millionth of the mass of a tiny electron. That teensy but non-zero mass has meant that something has to be adjusted within the standard model of physics. No one knows what the adjustment is going to be, but fundamentals will have to change. "That explains," writes Jayawardhana, "in a large part why neutrino physics has transformed from a sleepy backwater twenty years ago, when only a handful of scientists paid any attention, to a thriving hub of activity, with over a thousand researchers actively studying these shadowy particles." 




One of the reasons it takes so many searchers is that the wily neutrinos, although they are zipping around everywhere, interact only with great infrequency with anything (including detectors). Much of Jayawardhana's book is about neutrino detectors, which are quite unlike, say, a Geiger counter. For example, there is IceCube in the Antarctic. It finished construction in 2010, and consists of a grid of vertical strings of thousands of sensors, each of which is looking for a blue flash that indicates a neutrino has had a rare collision with a proton within the ice. The cables go 2,500 meters into the ice; a diagram in the book shows how they dwarf the 324 meter Eiffel Tower. Such detectors are often buried in caves or mines so that they can cut the outside clutter that comes from cosmic rays. The IceCube array has so far detected around thirty neutrinos that came from outside our solar system, the prey it is configured to find. Thirty little neutrinos, and the physicists are exultant about its success. 




Jayawardhana, who has visited Antarctica on scientific expeditions, has the most enthusiasm for IceCube, but there are plenty of other detectors now because the search is so important. One is the Cryogenic Underground Observatory for Rare Events in Italy, a mile beneath the Apennines. One of its requirements is that it be surrounded by a 3-centimeter lead lining to shield it from the natural radioactivity of the rock around it. But recently mined lead has the same sort of radioactivity. What to do? Use old lead. In this case, the physicists designing the detector arranged to get lead that sunk on a cargo vessel off the coast of Sardinia 2,000 years ago. You have to be finicky if you are hunting neutrinos, it seems. 




Jayawardhana has, in a brisk and bright book, summarized much of twentieth century physics, describing the efforts of scientists all around the world over the decades to imagine, understand, and capture a particle that their standard model did not originally acknowledge as existent. He gives capsule biographies of the physicists on the hunt, and they are respectively dull, zany, puritanical, hedonistic, reserved, and eccentric; the funny anecdotes here are welcome in emphasizing the human endeavor of scientific understanding. He reports that one researcher says that there may be practical value in harnessing neutrinos to send signals through the center of the Earth; such signals would get through milliseconds faster than signals that have to go around, and those milliseconds could make a lot of difference in, say, computerized stock trading. More seriously, he shows how understanding neutrinos might mean that we could comprehend how their mass affected the cosmology of the early universe, and why we have matter rather than antimatter. "Given all these exciting prospects," he writes, "it is no wonder that neutrino hunters are looking forward to the coming decade with great anticipation." Read the book and you can look forward, too. 




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