But a frame moving at light speed isn't.
So you do agree, "frame" can ONLY refer to something that is a moving aggregate of bound energy or inotherwords, matter or antimatter?
That is progress. Paddoboy, among others here, seem to be under the impression that a propagating photon is a FoR. Their only interest in relativity is to beat down anyone who goes against the teachings of someone like Minkowski. For all of the flaws in his version of relativity, at least he died in a timely manner.
Yet without pairs of rotational mode propagating photons (fundamental particles) matter itself doesn't exist, and neither do frames of reference and neither does inertia.
This is how energy gets what we call inertia. It is intimately involved with the other things we observe in relativity. Time dilation, and by means of extension, time itself depend on it. The passage of time continues for bound energy, or at least, on the outside. Otherwise, aggregate matter could not move, even at speeds closer to where we are, at rest or nearer to it than the speed of light.
If the speed of light or the speed of light squared were actually infinite, bound energy could not move and the universe would be static. It is because bound energy rotationally propagates faster than c but slower than infinity that time, motion, and inertia all become possible. The manifest graininess of time independent of the propagation of unbound photons makes sense. This is the part of the final exam Minkowski evidently failed. None of his math supports it.
This is a beautiful theory. What a shame it is that it was left to collect dust and never completed while physicists worked at developing fantasies like string theory. No one was interested, other than a tiny fringe who worked on developing boost matrices. Extension of relativity into explaining action at a distance and entanglement languished. Minkowsi is a big reason why. The Lorentz covariant model is fatally flawed and cannot be extended because it is pseudoscience.
For reasons I don't understand, relativity seems to attract a lot of folks interested in those. I only wish it attracted fewer. As for myself, I have no interest at all in pseudoscience. Science that is not extensible OR THAT HAS NOT CHANGED SUBSTANTIALLY IN OVER 100 YEARS to me is a red flag that WHAT YOU ARE DEALING WITH is pseudoscience. It is time for relativity either to move on or else get out of the way. It is blocking a large part of reasonable inquiry intended only to extend it into the 21st century.