[jeffreyH]

One more thing. A perfect vacuum should also be considered an absolute that can be directly related to absolute zero.

[PmbPhy]

No Mad. The 'curvature' described by a 'photon' passing the sun is seen as either 'c' varying due to SpaceTime 'curvature', or as the 'delay' being a result of the geometry, in which case 'c' still will be 'c'. If you go by the definition in where all rays path is the straightest one possible in a 'curved universe' then this 'delay' is explainable, as I think.
=

In some weird way this seems to me as a proof of testing a one way path of light?

Einstein explained it as refraction due to the varying speed of light, and he was right.

[mad aetherist]

It has to be remembered that c is an absolute value. In this case the absolute value is connected to rest mass and therefore inertia. Absolutes are named in physics because they can be approached but never reached. Media and fields pervade the universe and limit the photon's ability to reach this absolute. However, the value of c is fundamental to many mathematical models of the physical world.

Good wordage. The speed of light is never c, it is always a bit less or a lot less, it is c'.
Much of the time it is ok to have c in the equation, c being the max possible speed of light.  But if the equation needs the actual speed of light, which is never c, then the equation should use the expression c'.

[mad aetherist]

No Mad. The 'curvature' described by a 'photon' passing the sun is seen as either 'c' varying due to SpaceTime 'curvature', or as the 'delay' being a result of the geometry, in which case 'c' still will be 'c'. If you go by the definition in where all rays path is the straightest one possible in a 'curved universe' then this 'delay' is explainable, as I think.
In some weird way this seems to me as a proof of testing a one way path of light?

Einstein explained it as refraction due to the varying speed of light, and he was right.

I think that Einstein can be paraphrased to have said that there is no bending, it is a faux-bending, the bending is an illusion due to the bending of spacetime.
Likewise the slowing near mass is real, but the speed of light appears to an observer at that location to be constant, but this is a faux-constant, it is an illusion due to the bending of spacetime.
I think that thats how it is said to work.

[mad aetherist]

One more thing. A perfect vacuum should also be considered an absolute that can be directly related to absolute zero.

What if the vacuum contains em radiation?
What if the vacuum contains photons?
What if the vacuum contains neutrinos?
What if the vacuum contains electrons?
What if the vacuum contains spacetime?
What if the vacuum is near mass, in which case the vacuum contains bent spacetime?

What if the vacuum is midway tween two identical masses? -- is there spacetime in that vacuum? (or zero spacetime)(g being zero) -- or is there two lots of bent spacetime? (the bends cancelling & giving a nett zero bending, but there being two lots of bent spacetime in there anyhow).

What if the vacuum is near dark matter?
What if the vacuum contains dark matter?
What if the vacuum contains dark energy?

[yor_on]

Could you link that Pete? How did he think there? I always thought of refraction going out from a 'perfect vacuum' sort of, one without 'dust' etc. "  The index of refraction of a material is defined by the speed of light in vacuum c divided by the speed of light through the material v:      n = c/v "