Exploding stars on ice

What’s remarkable about the IceCube Telescope isn’t just that it will be taller than the Empire State building, the Chicago Sears Tower and Shanghai’s World Financial Center combined; rather it’s that it is being built to shed light on dark matter more than a kilometre below the surface of the South Pole. The IceCube, developed by the University of Wisconsin, will search for neutrinos, tiny subatomic particles created by exploding stars, gamma-ray bursts, and other phenomena like black holes. Though their size lets them stream through the universe largely undetected, neutrinos are among the most abundant particles in the universe, says principal investigator Francis Halzen. “The universe is filled with radiation, and most of it is light, and the second thing close behind is neutrinos.”

With a volume of one cubic kilometre, the IceCube will use an array of detectors embedded as deep as 1,400 m below the South Pole’s surface to spot the blue light emitted by the nuclear reaction of a single neutrino crashing into an ice atom. Studying the number and energy of these neutrinos will help scientists understand the sources of dark matter.

The South Pole’s ice—pure, transparent and undisturbed by air bubbles—is an ideal medium in which to detect neutrinos. But first researchers have to understand the “exquisite detail” of how light travels through the ice. “It’s formed out of layers that are a couple of centimetres thick. It’s a complicated medium,” says Helzen. “We’re doing this better and better, but we have to figure out how light propagates.”

Halzen says once the detectors are frozen, they remain untouchable for 25 years, the time it will take for that portion of the ice to migrate to the coast of Antarctica. “If a sensor breaks, that’s it. Fortunately, not much happens to them when they are sitting there.” The detector’s signals can be reprogrammed from his office, he notes.

With $272 million in funding, a team of 400 people and years of research, Halzen says the IceCube will be fully functional by December. “The first string was put in in 2005, and once the water freezes, we were able to take data from it,” he says. Then “we can possibly start to test some of the predictions that have been made.”