mikehelland
commented
3 years ago With respect to relativity, the photon doesn't gain mass.
The geodesic followed by a photon should be finite, with a velocity of that begins at c, and eventually becomes 0 as HD increases.
A null geodesic should be thought of as a geodesic prototype. Each point on it marks the beginning of the actual geodesic, which has an ending at c/H away on the same prototype.
This may be a case for extensions to geodesics for photons:
"So you think photons slow down, which implies they change invariant mass. Basically they obtain restmass, they become a different kind excitation in terms of QFT. "