By Phil Plait
[Why yes, I did have fun writing that title. Thank you for asking.]
Astronomers may have solved a mystery that started a few years ago. Or 9.5 billion years ago, depending on how you look at it.
Even better? This mystery involves, in no particular order: neutrinos, Antarctica, faster than light travel, bizarre radiation, the Fermi observatory, gamma rays, and a supermassive black hole with its barrel aimed right at us. That’s like astronomical mystery bingo right there.
OK, here’s how this played out.
In 2012, astronomers detected an extremely high-energy neutrino slamming into the ground. Neutrinos are a weird kind of subatomic particle, created by things like nuclear fusion in the Sun’s core, fission in nuclear reactors on Earth, stars exploding out in the Universe, and even when matter falls into a black hole. Neutrinos are very standoffish and don’t react to matter; they can easily pass through the entire Earth like it was completely transparent. Which to them it really is.
So detecting them is very difficult. But, it turns out, there’s a clever way to see them: Sometimes very high-energy neutrinos slam into ice molecules, creating a barrage of subatomic particles like shrapnel. These move so rapidly from the collision that they actually move faster than the speed of light through the ice (note: Before you start angrily writing comments, remember that the speed of light is the ultimate speed limit in a vacuum; light slows down when passing through solids, so the particles were moving faster than light can through the ice, but not faster than light can through a vacuum).
When they do this, they create a flash of energy that’s the equivalent of a sonic boom, but with light. I sometimes call this a photonic boom, but the technical term is Cherenkov radiation. That flash of blue light can be detected if the ice is clear enough. There are parts of Antarctic where that’s the case, and so scientists built IceCube; a series of detectors buried deep in the south polar ice.
In 2012 it detected a whopper of a neutrino (nicknamed Big Bird; all these events are named after Sesame Street characters to make it easier to keep track of them). The energy in that single neutrino was staggering beyond staggering: It contained 1000 trillion times as much energy as a photon of visible light. If that neutrino had hit someone they would have felt it. A blow from a single subatomic particle. Egads.
The mystery is a bit obvious: What the heck can create a neutrino with that kind of soul-crushing energy?
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