why is "c" even in the equation? i was hoping someone could explain why light speed even enters the e=mc2 equation. energy equals mass times the speed of light squared. i get that, and what the formula predicts. but why the speed of light? i was never one to go into the mathematics of all of this. its just one of those things that you read and never really fully answer for yourself, i guess. i have read many books on relativity, and understand some things about it as a whole. but why is it that light-speed squared just happens to work out as the number that gives the right figure to multiply mass against? does it have something to do with the units that are used to calculate energy?