See You Space Cowboy...

Chevy Camaro SS

See, the speed of light is a universal constant, the c of e=mc². But that’s only in a vacuum—so much of physics is based first and foremost on activity in a vacuum—and the speed of light through matter is often significantly slower. Instead of 1 c, light moves through liquid water at about 0.75 c. That may still be ludicrously fast, but it’s still slowed down enough that charged particles like electrons can exceed that speed within the medium in question. Exceeding that speed does all kinds of weird, somewhat terrifying and very exciting things to the electromagnetic field of the medium, including the creation of a shockwave in a manner very similar to a sonic boom.