I have always had trouble appreciating E=mc^2 because I can't relate C^2 to a physical process, like velocity or acceleration. How should one imagine the dimensional characteristics of C^2? Along the same vein, is E=mc^2 a mathematical conversion only, or does C^2 describe an actual process by which mass can be converted to energy? e.g. spin 1 piece of matter around a cyclotron at a speed approaching C and another piece of matter in the opposite direction at a speed approaching C so that when they collide they will hit at C^2?