Nothing travels faster than the speed of light through vacuum. But the index of refraction of water is 1.33 . So the speed of light through water is much slow

Nothing travels faster than the speed of light through vacuum. But the index of refraction of water is 1.33 . So the speed of light through water is much slower than through vacuum. And electrons can travel through water faster than light through water. The effect is similar to a massive object traveling faster than the speed of sound through air. https://en.wikipedia.org/wiki/Cherenkov_radiation https://en.wikipedia.org/wiki/Cherenkov_radiation Edit: I love that HN is so smart that a dogpile can form from all the people rushing in to explain Cherenkov radiation.