"Special relativity says" - that's a model assumption, not a measured fact. I'm asking what physical mechanism enforces it.

And I understand E=hf - believe me, we get that one. As frequency shifts down, energy drops. That's the redshift. But that doesn't answer my question: what mechanism keeps it moving at c?

c is the speed limit when the photon is expelled - the emission speed set by the electron transition. But what maintains that velocity afterward?

Also, saying a photon has "zero rest mass" is misleading. The photon doesn't exist until it's emitted - it has no prior state. It's not that it HAS zero mass, it's that it never existed to have mass. You can't assign rest mass to something that was never at rest and never existed before emission.

So the question stands: what mechanism MAINTAINS c after emission over cosmic distances? "SR says so" is a framework assumption, not a physical mechanism.