For the first time, astronomers have traced a cosmic neutrino back to where it started its journey out in space. This is a big leap for astronomy: These elusive signals could carry information about the cosmos that we can't see otherwise.
Cosmic neutrinos come from high-energy sources, like hot stars or supernovas. But unlike the light we can see with telescopes and cameras, neutrinos usually don't even notice matter. They don't interact with magnetic fields, and they usually pass straight through stars or planets. This makes them really hard to spot.
But in certain rare cases, when neutrinos brush up against atoms, they can cause a tiny flash of visible light. That's what the IceCube neutrino detector in Antarctica watches for, and in September 2017, that's what it saw.
Telescopes on the ground and in orbit immediately started scanning the region of space the neutrino came from, searching for anything that might have been energetic enough to launch it. Together, they settled on a supermassive black hole in the center of a galaxy 3.7 billion light years away.
Scientists are excited to know exactly where the neutrino came from, because it could show us things even our most powerful telescopes can't see. Since neutrinos pass through just about everything, they could help us understand giant black holes, or the interior of the sun, or the inner workings of supernovas.
Moving forward, astronomers plan to look for more of these ghostly signals. And they have at least one good idea of where to look to capture more. The IceCube observatory has detected more than a dozen other neutrinos coming from the direction of that black hole.