# Lagrangian for relativistic massless point particle

I) OP's square root action is not differentiable along null-directions, which makes it ill-suited for a massless particle. So we have to come up with something else. The equation of motion for a scalar massless relativistic point particle on a Lorentzian manifold $$(M,g)$$ is

$$\tag{A} \dot{x}^2~:=~g_{\mu\nu}(x)~ \dot{x}^{\mu}\dot{x}^{\nu}~\approx ~0,$$

where dot denotes differentiation wrt. the world-line parameter $$\tau$$ (which is not proper time). [Here the $$\approx$$ symbol means equality modulo eom.] Thus a possible action is

$$\tag{B} S[x,\lambda ]~=~\int\! d\tau ~L, \qquad L~=~\lambda ~\dot{x}^2,$$

where $$\lambda(\tau)$$ is a Lagrange multiplier. This answer (B) may seem like just a cheap trick. Note however that it is possible by similar methods to give a general action principle that works for both massless and massive point particles in a unified manner, cf. e.g. Ref. 1 and eq. (3) in my Phys.SE answer here.

II) The corresponding Euler-Lagrange (EL) equations for the action (B) reads

$$\tag{C} 0~\approx ~\frac{\delta S}{\delta\lambda}~=~\dot{x}^2,$$

$$\tag{D} 0~\approx ~-\frac{1}{2}g^{\sigma\mu}\frac{\delta S}{\delta x^{\mu}} ~=~\frac{d(\lambda\dot{x}^{\sigma})}{d\tau} +\lambda\Gamma^{\sigma}_{\mu\nu}\dot{x}^{\mu}\dot{x}^{\nu}.$$

III) The action (B) is invariant under world-line reparametrization $$\tag{E} \tau^{\prime}~=~f(\tau), \qquad d\tau^{\prime} ~=~ d\tau\frac{df}{d\tau},\qquad \dot{x}^{\mu}~=~\dot{x}^{\prime\mu}\frac{df}{d\tau},\qquad \lambda^{\prime}~=~\lambda\frac{df}{d\tau}.\qquad$$ Therefore we can choose the gauge $$\lambda=1$$. Then eq. (D) reduces to the familiar geodesic equation.

References:

1. J. Polchinski, String Theory Vol. 1, 1998; eq. (1.2.5).