Cherenkov radiation is the blue light given off when a charged particle moves faster than light travels through a transparent medium like ice or water, even though it stays below the absolute speed of light in vacuum. Because light itself slows down substantially in such media, a fast-moving particle can exceed that local speed and produce a kind of photonic shockwave, analogous to a sonic boom. Detecting this faint flash lets physicists reconstruct the path and energy of the particle, and from that infer the energy and origin of the neutrino that produced it, which matters because neutrino interactions are so rare that only a huge detector has any chance of catching them.
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The neutrino-nucleon interaction cross section at these high energies is cited as roughly 10^-33 square centimeters, used to illustrate why detectors must be enormous.
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Light in ice and water is said to travel as much as 20 to 30 percent slower than in vacuum, which creates the speed gap a charged particle can exceed.
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The hosts contrast lower-energy solar and atmospheric neutrinos, around 10^5 to 10^6 electron volts, with the extragalactic neutrinos they are after, roughly ten orders of magnitude higher in energy.
631 words · auto-generated from the episode video
29:23This is Cherenkov radiation. This is the very famous blue light that is emitted in particle, actually in nuclear reactors. When you have nuclear reactor rods that are covered with water, those nuclear rods are emitting a bunch of particles that are moving at extremely high energies, right? Some of those energies lead to speeds that are faster than the speed of light in water. This is one of the way, way back we did a-- When we did our IQ-- Speed of the light in ice. Yeah exactly. So, oh, geez, we'll remember that meeting. Yeah, it was one of our first viral videos, actually. The idea is the following.
30:04So, the speed of light in a vacuum is the absolute speed limit of the universe, but light slows down in stuff because there's stuff that interacts with light. So as light bounces from one particle to the other, the phase velocity of light is actually slower than the actual velocity of light because of all of these interactions, right? In ice and in water, it can be as much as 20 to 30% slower. That gap is now accessible to particles because the speed of light in a vacuum is the absolute limit, but this is what the speed of light is doing in all of this media. So if a charged particle moves faster than the speed of light in the medium, but obviously
30:48slower than the absolute limit imposed by Einstein's relativity, you get something called Cherenkov radiation. In the same way that a fighter jet produces a sonic boom when it travels faster than the speed of sound, in this case, you're getting a light boom. The light boom! There you go, yeah. You get a light boom. It's a shockwave in the electromagnetic field. A photonic boom. A photonic boom, and that's the radiation that we are trying to catch. If we can catch Cherenkov light, then we can reconstruct what the particle trajectories were, and then from that, then reconstruct what the neutrino
31:31was, like how big the neutrino was in terms of energy and where it came from. Now, the probability is extremely tiny, okay? At these petaelectron volt energies, the neutrino nucleon cross-section, meaning like how often does the neutrino interact with quarks? That cross-section is on the order of 10 to the minus 33 square centimeters. That's like, you can imagine this is like the target that the neutrino has to hit on the quark to like get an interaction. So extremely small. On top of that, the flux itself is very small, and so you need
32:12a giant, giant detector to be able to achieve any hope of detecting these things, even though there's many of them coming through at the level that we wanna see, like the-- Yeah, the energies that we wanna see, there's not that many. There's not that many. When I say 60 billion coming through my fingernail, no, that's like 10 to the five, 10 to the six electron volts. Those are the earthly ones. Those are the earthly ones, the solar system-y ones. We want the extra galactic. We want the extra galactic ones that are like 10 orders of magnitude above the stuff that's going through my fingernail. And so, again, so now we know what a neutrino is, like how you would detect it because of this Cherenkov radiation.
32:52It kind of accesses this gap when the speed of light through a medium, like water or ice, is lower than the absolute limit. And so there's sort of a signal that goes off. There's a little blue, oh no, don't worry, we're not violating the laws of physics. Yeah, yeah, yeah, no, Einstein is still happy. Which one'd you know we're here?