MODEL RELEASED. Learning physics. 4-year-old boy holding a blackboard with the equation E=mc2 on it. This equation relates energy (E) and mass (m) by the square of a universal constant, the speed of light in a vacuum (c). The speed of light has a very large value, and this equation shows both that mass can be converted into energy, and that large amounts of energy can be obtained from small amounts of mass. This is the basis of nuclear power stations and atomic bombs.