Unfortunately, there’s no intuitive way to understand why “the energy of the rest mass of an object is equal to the rest mass times the speed of light squared” (E=MC2). The answer Einstein came up with was , which is time as measured by clocks on the moving object in question. Notice that the energy and momentum here are not the classical (Newtonian) energy and momentum: and . The “proof” came when E=mc2 was tested experimentally (with particle accelerators ‘n stuff). So conservation of energy, momentum, and matter are all actually the same conservation law!