Four-momentum

Summary

The four-momentum of a particle is the four-vector that combines its energy and its momentum. For a particle of mass $m$ it is $p = m u$, where $u$ is the four-velocity. In an inertial frame its time component is $E/c$ and its space part is the relativistic momentum $\vec p$. The total four-momentum of an isolated system is conserved, which unites the conservation of energy and of momentum in one law.