Quote by Petr Mugver
[tex](d\tau)^2=dx^\mu dx_\mu[/tex]
is the proper time of the particle, that you can write
.

Just a minor error: Following the Riemannian lineelement for a general spacetime, this must have been
[tex](cd\tau)^2=dx^\mu dx_\mu[/tex]
because the LHS is just the square of the differential of proper length, [tex]ds^2.[/tex]
AB