Relationship between mass (m) and energy (E) in Albert Einstein 's special theory of relativity , expressed E = m c 2 , where c equals 186,000 mi/second (300,000 km/second), the speed of light .
Whereas mass and energy were viewed as distinct in earlier physical theories, in special relativity a body's mass can be converted into energy in accordance with Einstein's formula. Such a release of energy decreases the body's mass (see conservation law ).