Origins ANTARCTICA, Scientific Journeys from McMurdo to the Pole
Ideas Tools Place Live Field Notes
  © Per Olof Hulth
  Looking into the camera as others prepare to lower AMANDA's optical modules into the ice.

A literary essay about AMANDA by Francis Halzen
page 3

IN THE PAST FOUR DECADES neutrino detectors of increasing scale and sensitivity have cropped up in odd spots around the globe: in an iron mine in Minnesota; underneath a mountain in Italy’s Apennine range; in an abandoned railway tunnel on the outskirts of Osaka, Japan. None of them are really telescopes, however: rather than tracking high-energy neutrinos to map deep space, as AMANDA does, they simply detect low-energy neutrinos from the sun.

One might think that one or two solar neutrino detectors would be enough. But a single mystery has continued to tantalize investigators. According to standard astrophysical theory, nuclear fusion inside the sun ought to spawn a stable number of neutrinos, and so physicists ought to detect a predictable number of them. Instead, month after month, in detector after detector, no more than half of the expected neutrinos are counted. Are the detectors faulty, or does solar astrophysics need some revision? Neither one, most physicists now say. The flux of solar neutrinos is too weak because the particles transform themselves en route to the earth. What were once electron neutrinos become their particulate cousins: muon neutrinos and tau neutrinos, which most neutrino detectors cannot detect.

The fact that neutrinos come in three "flavors" is old and undisputed. But the idea that they oscillate between those three flavors entails a fundamental rethinking of their nature—and perhaps of the universe itself. According to the standard model of elementary particle physics, neutrinos have no mass at all. Yet according to quantum theory, only particles with mass can oscillate between one flavor and another. By recent estimates, there are about 100 million times as many neutrinos in the universe as there are protons and neutrons combined. Even if the mass of each neutrino is no more than a tenth of an electron volt, their collective mass would be as great as that of all the visible matter in the universe.

"Neutrino oscillations have been discovered at least four times and undiscovered at least twice," notes the particle physicist Donald H. Perkins of the University of Oxford. Last June, however, physicists in Takayama, Japan, announced results that have silenced most of the remaining doubters. Their Super-Kamiokande detector, built in a working zinc mine a kilometer underground, incorporates more than 13,000 photomultipliers to survey 50,000 tons of water for telltale flashes of light. (A photomultiplier looks like a lightbulb and works like a lightbulb in reverse: light goes in and electricity comes out. But what a lightbulb! The photomultipliers in Super-K amplify signals by a factor of 100 million.)

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