Relativistic particle
= Relativistic particle
{title2=$u^\mu u_\mu=c^2$}
A relativistic massive particle is described by a timelike worldline with proper-time four-velocity $u^\mu$. In signature $(+---)$ its normalization is $u^\mu u_\mu=c^2$ and its momentum is $p^\mu=mu^\mu$. The <relativistic charged-particle action> adds electromagnetic coupling, whose variation gives the <Lorentz force>.