(Every day until Christmas, I’ll be posting a science-related image.)

Day 11

Neutrinos are incredibly common particles in the Universe—trillions pass through your body each second—but despite that they’re hard to detect. This is because they don’t interact very strongly with anything: they are electrically neutral (their name means “little neutral one” in Italian, thanks to being named by Enrico Fermi), so they aren’t affected by electromagnetic forces or light. Their primary means of interaction is the weak nuclear force, a short-ranged force whose name suggests how wimpy it is. The best hope of detecting neutrinos is having them collide with an atomic nucleus.

For this reason, the IceCube detector in Antarctica uses a huge volume of pure ice near the South Pole. The experimenters drilled 86 holes to a depth of 2 kilometers in the ice, and lowered strings consisting of  60 detectors into the holes. These 5,160 detectors can pick up faint blue flashes of light that are produced when a neutrino strikes an atom. The collision creates a muon, a more massive and unstable cousin to electrons (I’m sure we all have such a relative). The muon is produced such that it moves faster than the speed of light in ice, so it makes shock waves, akin to a sonic boom. However, the shock waves in this case are blue light known as Čerenkov (pronounced “CHAIR-en-koff”) radiation. That’s the stuff the IceCube detectors are looking for; when several detectors are triggered, the researchers can tell where the neutrino came from, and even how much energy it had when it hit the ice.