Originally posted by Uncle Al:
The path of a light ray in spacetime vacuum defines the metric. A photon is not an inertial frame of reference. Proper length and proper time are defined as the length of an object and the amount of time that passes in a comoving frame.
A time-like vector in Minkowski space-time selects a preferred frame (coordinate system) in which the t-axis points along the vector and spatial coordinates are orthogonal to it. In this special coordinate system, the t-component of the vector is called its proper length (or proper mass-energy when talking of a 4-momentum vector).
OTOH, a light-like vector points along one of the directions contained in the light cone. The light cone is unvariant under all Lorentz transformations. Thus, a light-like vector is simply unable to pick a preferred coordinate system. We cannot make any "proper" measurements of a photon's 4-momentum.
http://arXiv.org/abs/gr-qc/9909014
Amer. J. Phys. 71 770 (2003)
Phys. Rev. Lett. 92 121101 (2004)
falling light 
