Relativistic Energy-Momentum

Relation

The invariant relation is

$$E^2=p^2c^2+m^2c^4.$$

For a particle at rest, this reduces to $E=mc^2$.

Four-Momentum

The four-momentum is

$$p^\mu=\left(\frac{E}{c},\mathbf p\right).$$

Its invariant norm gives the mass shell condition.