IceCube, a telescope under construction at the South Pole, will search for neutrinos from the most violent astrophysical sources: events like exploding stars, gamma ray bursts, and cataclysmic phenomena involving black holes and neutron stars. The IceCube telescope is a powerful tool to search for dark matter, and could reveal the new physical processes associated with the enigmatic origin of the highest energy particles in nature. IceCube will encompass a cubic kilometer of ice and uses a novel astronomical messenger called a neutrino to probe the universe.
Neutrinos are produced by the decay of radioactive elements and elementary particles such as pions. Unlike other particles, neutrinos are antisocial, difficult to trap in a detector. It is the feeble interaction of neutrinos with matter that makes them uniquely valuable as astronomical messengers. Unlike photons or charged particles, neutrinos can emerge from deep inside their sources and travel across the universe without interference. They are not deflected by interstellar magnetic fields and are not absorbed by intervening matter. However, this same trait makes cosmic neutrinos extremely difficult to detect; immense instruments are required to find them in sufficient numbers to trace their origin.
Although trillions of neutrinos stream through your body every second, none may leave a trace in your lifetime. We actually use large volumes of ice below the South Pole to watch for the rare neutrino that crashes into an atom of ice. This collision produces a particle—dubbed a "muon"—that emerges from the wreckage. In the ultra-transparent ice, the muon radiates blue light that is detected by IceCube's optical sensors. The muon preserves the direction of the original neutrino, thus pointing back to its cosmic source. It is by detecting this light that scientists can reconstruct the muon's, and hence the neutrino's, path. The picture is radically complicated by the fact that most muons seen by IceCube have nothing to do with cosmic neutrinos. Unfortunately, for every muon from a cosmic neutrino, IceCube detects a million more muons produced by cosmic rays in the atmosphere above the detector. To filter them out, IceCube takes advantage of the fact that neutrinos interact so weakly with matter. Because neutrinos are the only known particles that can pass through the earth unhindered, IceCube looks through the earth and to the northern skies, using the planet as a filter to select neutrinos.
Since the 1950s scientists have built a compelling scientific case for doing astronomy and particle physics using high-energy neutrinos. The challenge has been one of technology to build the kilometer-sized observatory needed to do the science. Theorists anticipate that an instrument of this size is required to study neutrinos from distant astrophysical sources. Antarctic polar ice has turned out to be an ideal medium for detecting neutrinos. It is exceptionally pure, transparent and free of radioactivity. A mile below the surface, blue light travels a hundred meters or more through the otherwise dark ice. Frozen in the ice, IceCube not only will be the largest and most durable particle detector, but a real bargain at just 25 cents per ton!