No equation is more famous than E = mc2, and few are simpler. Indeed, the immortal equation’s fame rests largely on that utter simplicity: the energy E of a system is equal to its mass m multiplied by c2, the speed of light squared. The equation’s message is that the mass of a system measures its energy content. Yet E = mc2 tells us something even more fundamental. If we think of c, the speed of light, as one light year per year, the conversion factor c2 equals 1. That leaves us with E = m. Energy and mass are the same.