conservation of mass-energy

The principle of conservation of mass-energy states that the total energy of an isolated system remains constant. This means energy cannot be created or destroyed, only transformed from one form to another.1

Historically, mass and energy were considered separate. However, Albert Einstein's theory of special relativity revealed they are equivalent and interconvertible, expressed by the famous equation (E=mc^2). This means mass can be converted into energy, and vice versa.23

While mass conservation is a useful approximation for everyday chemical reactions, it is not valid in nuclear reactions, particle accelerators, or stellar processes like those in the Sun. In these instances, the conservation of mass-energy is the governing principle.341