The measurements were sufficiently sensitive that Super-K could track the sun’s path across the sky and, from nearly a mile below the surface of the earth, watch day turn into night. And by tracing the exact path the electron traveled in the water, physicists could infer the source, in space, of the colliding neutrino. The sensors detect the occasional blue flash (too faint for our eyes to see) made when a neutrino collides with an atom in the water and creates an electron. Super-Kamiokande, or Super-K as it is known, began operating in 1996. The detector consists of 50,000 tons of water in a domed tank whose walls are covered with 13,000 light sensors. Hoping to detect neutrinos in larger numbers, scientists in Japan led an experiment 3,300 feet underground in a zinc mine. The monitoring continued for more than 30 years. Every few months, the scientists would flush the tank and extract about 15 argon atoms, evidence of 15 neutrinos. At the core of the experiment was a tank filled with 600 tons of a chlorine-rich liquid, perchloroethylene, a fluid used in dry-cleaning. In this experiment the scientists set out to observe neutrinos by monitoring what happens on the rare occasion when a neutrino collides with a chlorine atom and creates radioactive argon, which is readily detectable. The scientists had established that the proposed neutrino was in fact real study of the elusive particle accelerated.Ī decade later, the field scaled up when another group of physicists installed a detector in the Homestake gold mine, in Lead, South Dakota, 4,850 feet underground. The detector was tiny by today’s standards, but it still managed to spot neutrinos-three an hour. At a nuclear weapons laboratory in South Carolina in the mid-1950s, they stationed two large water tanks outside a nuclear reactor that, according to their equations, should have been making ten trillion neutrinos a second. “It is something no theorist should ever do.”Įxperimentalists began looking for it anyway. “I have done something very bad today by proposing a particle that cannot be detected,” Pauli wrote in his journal. So to account for that extra energy the physicist Wolfgang Pauli conceived an extra, invisible particle emitted by the nucleus. But in fact, scientists observed, the nucleus was losing more energy than detectors were picking up. When the nucleus of a radioactive atom disintegrates, the energy of the particles it emits must equal the energy it originally contained. In 1930, they created the concept to balance an equation that was not adding up. Physicists imagined neutrinos long before they ever found any. “Neutrinos may be able to tell us things that the more humdrum particles can’t,” says Kayser. In addition, neutrinos intrigue scientists because the particles are messengers from the outer reaches of the universe, created by violently exploding galaxies and other mysterious phenomena. And if physicists are ever going to fulfill their hopes of developing a coherent theory of reality that explains the basics of nature without exception, they are going to have to account for the behavior of neutrinos. Physicists study neutrinos in part because neutrinos are such odd characters: they seem to break the rules that describe nature at its most fundamental. “We don’t know where it’s going to lead,” says Boris Kayser, a theoretical physicist at Fermilab in Batavia, Illinois. It’s unclear what practical applications will come from studying neutrinos. These strangely beautiful devices are monuments to humankind’s resolve to learn about the universe. Enormous ones have been placed in gold and nickel mines, in tunnels beneath mountains, in the ocean and in Antarctic ice. So that neutrinos aren’t confused with cosmic rays (subatomic particles from outer space that do not penetrate the earth), detectors are installed deep underground. To capture these elusive entities, physicists have conducted some extraordinarily ambitious experiments. What’s more, neutrinos, unlike most subatomic particles, have no electric charge-they’re neutral, hence the name-so scientists can’t use electric or magnetic forces to capture them. Any instrument designed to do so may feel solid to the touch, but to neutrinos, even stainless steel is mostly empty space, as wide open as a solar system is to a comet. The problem for physicists is that neutrinos are impossible to see and difficult to detect. About 100 trillion neutrinos pass through our bodies every second. They come straight through the earth at nearly the speed of light, all the time, day and night, in enormous numbers. They’re among the lightest of the two dozen or so known subatomic particles and they come from all directions: from the Big Bang that began the universe, from exploding stars and, most of all, from the sun.
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