If a neutrino is so darn invisible, what's the plan, Stan?

Neutrinos may be shy, but they're not perfectly shy. Once in a great while, they actually hit an atom and produce a subatomic particle that makes light. [This page updated April 1, 1998].

But because the odds of a neutrino hitting a small detector are so dismal, physicists opted to make bigger targets. In principle, it's no different than a lottery player who "beats" a tiny chance of winning by buying hundreds of tickets.

Unfortunately, buying tickets to the neutrino lottery -- by building conventional telescopes large enough to gather a useful number of the zippy little characters -- could cost $10-billion. That's the kind of price tag that makes scientists turn creative: some of them have tried putting tubs of oil inside abandoned mines. Others have sunk strings of detectors in oceans. And in 1994, AMANDA, the Antarctic Muon and Neutrino Detector Array, started burying detectors in the Antarctic ice cap. Want the bare-bones facts on AMANDA?

To detect incoming neutrinos, the AMANDA researchers are using strings of photomultiplier tubes (defined). These instruments amplify the tiny light that results from a neutrino collision by one billion times -- and send the resulting data through a fiberoptic cable to the surface for analysis. The tubes are housed inside crush-proof glass spheres.

The photomultiplier tubes don't actually see neutrinos. Instead, they are designed to see light produced by subatomic particles called muons (defined), which form when a neutrino interacts with the nuclei of hydrogen and oxygen in the frozen water.

By gathering information from several detectors, the AMANDA researchers hope to figure out where the neutrino originated, and what kind of cosmic engine started it on its journey.

But because many false signals would originate in cosmic rays from overhead, this peculiar frozen telescope only reads signals coming through the earth -- in other words, it's the only telescope in the world that looks down. That allows it to use the Earth's mass to block spurious signals. (Only the highest energy neutrinos would be stopped by the Earth. Low-energy neutrinos could pass through a light-year-long hunk of lead. Think about that for a moment.)

The Antarctic ice is also ideal, because it has little natural radiation, and once the detectors are frozen in place, they are safe from damage. Still, it's tough to drill mile-long holes in ice, and string out delicate detectors where they will be placed under crushing pressures. What about working conditions at the South Pole? (Don't tell me. It's cold. But how cold? And what's that ice look like?)

In 1994, AMANDA started placing detectors under the ice at the South Pole 80 detectors were in place between 800 and 1,000 meters under the ice surface. By 1997, a second set of 296 detectors were placed between 1,500 and 1,900 meters deep in the ice. By 1999, a total of 462 detectors will be in place between 1,300 and 2,300 meters deep

That will complete AMANDA II, says Robert Morse, a University of Wisconsin-Madison physicists who works on the project, and serve as a shakedown cruise for Ice Cube, an ambitious project to insert about 5,000 instruments in a cubic kilometer -- more than 120 billion gallons -- of Antarctic ice. That project, with an estimated price tag of $20 million to $30 million, would emplace 4,800 detectors over five years, comprising easily the largest and wettest telescope in history.

As of this update, in April, 1998, the downward-pointing, no-mirror telescope has begun producing data. It is registering an average of one hundred "events" per second. Almost all of those, of course, are false alarms -- signs of cosmic rays coming from above rather than neutrinos from below.

However, about one signal per day is coming upward through the ice -- a sure sign of a neutrino. Researchers are now trying to make some sense of that data. For example, they are looking at neutrino arrivals that coincide with gamma ray bursts observed by satellite. Knowing that the gamma ray sources also produce neutrinos could help unravel the source of these mysterious, ultra high-energy cosmic riddles.

What else could the neutrino tell us about the universe?