Relativistic charged-particle action
= Relativistic charged-particle action
For metric signature $(-,+,+,+)$, a particle of mass $m$ and charge $q$ coupled to a four-potential has the reparametrization-invariant action
$$
S=-mc\int\sqrt{-\dot x^2}\,d\lambda
+q\int A_\mu(x)\dot x^\mu\,d\lambda.
$$