> What I thought that wasn't possible? Or is this just the speed of light in water, so the particles are still moving slower than the speed of light in a vacuum

> What I thought that wasn't possible? Or is this just the speed of light in water, so the particles are still moving slower than the speed of light in a vacuum? Good question! It's the latter - the shining is due to Cherenkov radiation [0], "electromagnetic radiation emitted when a charged particle (such as an electron) passes through a dielectric medium at a speed greater than the phase velocity of light in that medium." [0] https://en.wikipedia.org/wiki/Cherenkov_radiation https://en.wikipedia.org/wiki/Cherenkov_radiation