How to turn a continent into a telescope

Of course, because they rarely interact with matter, neutrinos are very hard to spot. Compared with Gorham's project, the current crop of detectors is relatively small - a few cubic kilometers or less - so they find far fewer particles. With an Antarctic-sized detector, Gorham and his team are looking for bursts of radio energy that neutrino interactions give off as they smack into atoms deep in the ice.

This new approach raised eyebrows at first. The notion that neutrino interactions could trigger brief bursts of radio signals was first proposed by Russian physicist Gurgen Askaryan in the early 1960s. "But very few people believed the Askaryan effect was real," Gorham says. The idea languished until 2000, when a team led by Gorham and David Salzberg, a physicist at the University of California at Los Angeles, confirmed that the effect was real - and surprisingly strong.

The finding led to his project, called Antarctic Impulse Transient Array. ANITA could capture in 30 days the amount of data that would take its counterparts a decade to gather.

Funded through NASA and drawing on research and engineering assets from eight universities and NASA's Jet Propulsion Laboratory, the array consists of a set of 40 to 50 antennae clustered beneath a high-altitude balloon. The first full-scale launch of this payload, the size of a small school bus, is planned for December 2006. The balloon's path is expected to take it around the continent once every 15 days and over the thickest ice. At any one time, the balloon can track some 1.5 million cubic kilometers of ice.

Why go to all this trouble? Because neutrinos may offer clues to several astronomical mysteries, including the source of very high-energy cosmic rays.

Only 10 to 20 have been recorded over the past 30 years, says Paul Mantsch, an astrophysicist at the Fermi National Accelerator Laboratory in Batavia, Ill. One reason, scientists say, is that these particles are believed to run into the same photons from the big bang that limit traditional astronomy. Collisions are thought to break the cosmic rays into a shower of other particles - including neutrinos. Thus, neutrinos appear to be the only way of tracking these cosmic rays to truly distant sources.

Researchers have proposed a number of theories as to what generates these cosmic rays. But physicists have a hard time explaining how these mechanisms could accelerate cosmic rays to the high- energy levels observed, notes Steven Barwick, a physicist at the University of California at Irvine and the lead investigator for another neutrino observatory in Antarctica. Each theory carries its own prediction of the range of energy signatures that neutrinos from these processes should carry.

For particle physicists, neutrinos are valuable because they travel at energies no earthbound particle accelerator imaginable could generate. So they could represent a natural source of crash dummies, whose collision debris could help test theories about the origins and evolution of matter in the universe.

One puzzle involves gravity: Why is it so weak relative to the other three fundamental forces of nature? ANITA may only hint at the answers to these questions.

"We're only expecting a few dozen detections in one flight," Gorham says. "That's not a lot, but only a few years ago, people threw up their hands at the thought of trying to detect these neutrinos at all."

Yet researchers note that the detection of a handful of neutrino events in 1987 stimulated at least a decade's worth of study of supernovas. It's not unreasonable, they say, to see dozens of events inject fresh energy into astrophysics.

"It's a wonderful time to be a neutrino physicist," says Dr. Bahcall.

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