4er-Impuls

${{p}^{\mu }}=m{\frac {d{{x}^{\mu }}}{d\tau }}=\gamma m\left({\begin{matrix}c&{{v}^{1}}&{{v}^{2}}&{{v}^{3}}\\\end{matrix}}\right)=\left({\begin{matrix}{\frac {E}{c}}&{{p}^{i}}\\\end{matrix}}\right)$ es gilt

${{p}^{\mu }}{{p}_{\mu }}={{m}^{2}}{{c}^{2}}$