Butterfly nets for ghosts

For decades physicists have suspected that neutrinos hold some of the universe’s darkest secrets. Determining their behavior and where they came from could tell rich stories of the early universe and potentially illuminate the curious nature of dark matter. Untold trillions of these tiny subatomic particles—some born soon after the birth of the universe, others born in the hearts of stars—have traveled unimaginable distances to pass through your body every second. So what does this mean for you? Not much, really. The nearly massless particles pass through almost all matter unabated, without leaving a trace. It’s this elusive nature that also makes them so difficult to detect and therefore study. Very occasionally, however, a neutrino collides into an atom, producing from the wreckage another particle—known as a muon—that can be detected (using special light sensors). At the IceCube Neutrino Observatory, a team of pioneering researchers has buried thousands of these sensors miles deep into the ice at the bottom of the Earth, all in an attempt to catch the rare neutrino that crashes into an atom of ice. By analyzing the specific path of this subatomic train wreck, the researchers can trace the neutrino’s path to its distant cosmic source. In this way, IceCube looks through the Earth and to the northern skies, using the planet as a filter to select neutrinos. “I like to say we’re building butterfly nets for ghosts,” says Francis Halzen, principle investigator of the project. “The ultra-transparent Antarctic ice itself is the detector. And a real bargain at just 25 cents per ton!”

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