u μ := d x μ d τ = γ ( c v 1 v 2 v 3 ) {\displaystyle {{u}^{\mu }}:={\frac {d{{x}^{\mu }}}{d\tau }}=\gamma \left({\begin{matrix}c&{{v}^{1}}&{{v}^{2}}&{{v}^{3}}\\\end{matrix}}\right)}