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That's a very important point. As the analysis of the representation theory of the proper orthochronous Poincare group in the context of relativistic QFT reveals, the massless case is special, and the limit "##m \rightarrow 0##" is anything but trivial. That's also the mathematical reason for the fact that one must not think about photons as pointlike objects traveling with the speed of light wrt. any (inertial) reference frame.
The correct semiclassical point of view of the "photon", as usually treated in GR textbooks, is that this is in fact the eikonal approximation of Maxwellian electrodynamics. It describes the behaviour of wave vectors in the sense of geometric optics. The point-particle-photon picture can sometimes be a shortcut in deriving interesting things about em.-wave propagation (e.g., in the GR context the gravitational bending of light) but it must not be mistaken as a point-particle interpretation of photons. This was an erroneous point of view in the early days of the "old quantum theory", which is out of date for at least 97 years!
The correct semiclassical point of view of the "photon", as usually treated in GR textbooks, is that this is in fact the eikonal approximation of Maxwellian electrodynamics. It describes the behaviour of wave vectors in the sense of geometric optics. The point-particle-photon picture can sometimes be a shortcut in deriving interesting things about em.-wave propagation (e.g., in the GR context the gravitational bending of light) but it must not be mistaken as a point-particle interpretation of photons. This was an erroneous point of view in the early days of the "old quantum theory", which is out of date for at least 97 years!