d τ = 1 c η μ ν d x μ d x ν = 1 − ( v c ) 2 d t {\displaystyle d\tau ={\frac {1}{c}}{\sqrt {{{\eta }_{\mu \nu }}d{{x}^{\mu }}d{{x}^{\nu }}}}={\sqrt {1-{{\left({\frac {v}{c}}\right)}^{2}}}}dt}
