When a charged particle travles faster than the speed of light in a (dielectric) medium (less than the speed of light in a vacuum, of course) it creates light, like a sonic boom in air. The particle polarizes the molecules it passes, and to realign themselves, the molecules emit light. When the particle is moving faster then light, constructive interference preserves the light - if not, destructive interference destroys it.

Causes radioactive things to glow blue underwater -Nifty

Named for the Russian physicist Pavel Alekseyevich Cherenkov, Cherenkov radiation is emitted by any particle travelling faster than the speed of light in the medium in which it is travelling. As it collides with particles, it slows down and the lost kinetic energy is released in the form of photons. When the particle slows down enough to be travelling at exactly the speed of light in the medium, it emits an optical boom, exactly analogous to the sonic boom emited by an object passing the speed of sound in its medium.

This radiation is emitted in a forward cone, and can be seen with the naked eye (as a blue glow) if there are enough particles creating it. The Cherenkov radiation emitted by even a single such particle can be detected by devices such as photomultiplier tubes, and is crucial for measurements made at neutrino observatories, such as SNO.

Cherenkov radiation has now been observed with particles that are moving at subluminal speeds. A team of researchers at the University of Michigan and the Max Planck Institute for Condensed Matter Research (in Stuttgart) has verified the finding experimentally.

(T. E. Stevens et al., Science 291, 627, 2001.) --pfs

First of all, I have actually seen cherenkov radiation in person. I went on a tour of the University of Virginia research nuclear reactor, the core of which simply sat at the bottom of a pool of water, so that you could stand at the edge and stare down at the core. You could see the unearthly blue glow of cherenkov radiation (produced by the beta particles escaping the core) all around the core. It was one of the coolest things I have ever seen. It was like being in a sci-fi film...without the bad acting. "If you have the means, I highly recommend it."

Cherenkov radiation is also used to detect other sorts of particles besides neutrinos. One such method involves using cherenkov radiation to detect high energy gamma rays. When the gamma rays enter the atmosphere they react with it causing a particle shower, which in turn creates cherenkov radiation. Then, an optical telescope, composed of a large parabolic dish of mirrors reflecting light to a bank of photomultiplier tubes is employed to collect the light. Using sophisticated computer analysis, background light can be filtered out and data about the original gamma ray can be extracted.

Ok, so the coolest thing relating to all this is the following: Usually cherenkov light produced by astronomical events is far far too faint to see with the naked eye; however, astronauts have reported strange flashes in their vision while in space. It has been speculated that they are seeing cherenkov light produced by high energy particles (gamma rays and cosmic rays), but unlike the case of the telescopes these are thought to be produced inside the eye and then collected by the retina. The water in the astronauts' eye is thought to be functioning like a particle detector! This happens to astronauts and not the rest of us because the atmosphere normally shields us (which is why the telescopes mentioned above work). There have apparently been experiments to test this hypothesis, but I am unaware of the results. It should be noted that it is probably fairly unlikely this is the correct explanation, but it would be cool.

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