In nuclear processes, matter can be converted into energy. The relationship between a certain mass and the energy it is equivalent to is given by Einstein's equation, E=mc

^{2}.The energy released when matter is transformed into energy is equal to the product of the mass of matter transformed by the speed of light squared. On a microscopic scale, the transformation of energy into matter has also been observed.

We state the principle of conservation of matter and energy as follows: "The combined amount of matter and energy in the universe is fixed."