Why an Antarctic Neutrino Telescope Watches for Blue Light

Almost 100 holes melted near the South Pole are now being used as an astronomical observatory. Astronomers with the IceCube Neutrino Observatory lowered a long string knotted with basketball-sized light detectors into each vertical hole in the ice, turning a sheet of polar ice into instrument hardware rather than a place to put a telescope on top of it would be for optical astronomy. Here is the part worth holding on to: the detector is not really watching for neutrinos. It watches for blue light. That light shows up in the surrounding clear ice when a high-energy neutrino — one emitted by some powerful process out in the universe — collides with an ice molecule and frees fast-moving secondary particles. The neutrino itself is invisible; the collision is not. So the ice is both the target and the medium, and the array is a grid of bulbs waiting for the rare flash of something arriving from far away and finally hitting something.