E = mc2 is a formula developed by the physicist Albert Einstein that relates mass (amount of matter) and energy. In the formula, E stands for energy, m stands for mass, and c2 is a constant factor equal to the speed of light squared. The equation shows that mass is a form of energy and that a tiny amount of mass can be changed into large quantities of other kinds of energy. For example, a mass of one gram could produce about 25 million kilowatt-hours of energy for use as electric power. This energy could power a home for several thousand years.
The formula laid the basis for the application of nuclear energy. When Einstein announced it in 1905, scientists knew of no way to change mass into energy or energy into mass. But scientists now know of two ways to release this energy. Fission involves splitting certain heavy atoms into lighter ones. This process powers nuclear reactors and simple nuclear weapons. Fusion involves combining certain light atoms into heavier ones. This process keeps the sun and stars hot and bright and powers advanced nuclear weapons. In both fission and fusion, the atoms at the end of each process have less total mass than did the atoms at the beginning of each process. This missing mass, or mass defect, is converted to energy according to Einstein’s equation.
See also Einstein, Albert; Nuclear energy (Artificial fission).
