The **rest energy** \(E_0\) of a particle is defined as \(E_0 = m_0c^2\) where \(c\) is the speed of light in vacuum. In general, only differences in energy have physical significance.

The concept of rest energy follows from the special theory of relativity that leads to Einsteinâ€™s famous conclusion about the equivalence of energy and mass. On the other hand, the concept of the equivalent Dirac invariant rest mass may be defined in terms of the self-energy corresponding to the product of a geometric matter current and a generalized potential as part of a single definition of mass in a unified geometric theory.