C, the letter "c", is used to represent the speed of light in equations. You know it, E=mc² , that's, energy is mass times the result of the speed of light times the speed of light . In a vacuum (not through air, or water, or anything else) the speed of light is 299,792,458 meters per second. Bloody fast eh! Whilst an atom is very very very small if you times it by the speed of light² that gives a lot, a fucking lot, of energy. Hence the atomic bomb going big boom! But, is that really the speed? 299,792,458 meters per second is the speed as measured with a round trip, the speed of light that went from a source, to something, and back again. But what if it's twice that speed on the outward journey and instant on the way back, we'd get the same answer. Huh? Watch this ... Oh, and WHY is, probably / almost certainly, the speed of light 299,792,458 meters per second - no-one knows, it just is .