The motion of a photon in the gravitational field is described by a simple condition: the four vector has zero norm. My question is: is it possible to derive this dynamic from a Lagrangian? I was thinking to use the usual action but I have realized very soon that it applies only for massive particles.
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1Concerning the Lagrangian of a massless scalar particle, see http://physics.stackexchange.com/q/107921/2451 – Qmechanic Dec 24 '16 at 10:42