Ronald Held wrote:
In an inertial reference frame the speed of light should be constant. Anyone see a research grade paper that computed the backreaction of the EM vacuum of the geodesic of a photon?

If the velocity of a photon is constant, you could calculate it by calculating the average speed. If you know the velocity of the photon in every moment (or any moment, for that matter), by Heisenberg's uncertainty principle, the photon has to occupy the entire universe. Since the vast majority of photons are somewhere, their velocity vectors can't be constant. Of course, the change could be in both the direction and the size of the velocity vector, but suffice to say, the photon is not moving at a constant speed in a constant direction.
What's more, the individual photons can occasionally have a velocity larger than the speed of light, in fact they can move with
any speed shortly due to the uncertainty principle.
When you add up all points in time of the photon's journey, however, the average speed is exactly the speed of light if you subtract any interaction of the photon with other matter.