Doesn't matter John, seen that argument(s) before but light is still defined as a constant. And it's one of the most tested facts existing as I know, as SR builds on it.

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Where it gets questioned is mostly from the aspect of assuming some sort of 'container universe', containing us all. Naively also assumed to be both 'infinite', as well as having some sort of boundaries, as in examples where you can walk out of a universe to the right, just to find yourself to appear at the left. Myself I'm not happy about those ones, I like the universe to be infinite and 'bound less' in all ways you can define it, assuming it to build itself up through the 'protocols' used, laws, constants, etc, defining it. That means that you won't find that sort of situation in 'my' idea of a

isotropic and homogeneous universe. Instead every other place you look out from, at the universe, it will look the same, no boundaries and no 'seams'. And it treats a inflation the same way as a expansion. Happening in 'all points'. In short, the 'protocol' (heh, some sf writer that likes that one? Feel free to use it) is the universe, and there is no way for me to step outside it, as the universe should be what communicate.

The difference is subtle, but there are no 'seams' in my sort of universe. Instead communication defines its limits. Which can be interpreted as where there is no communication my universe 'ends'. But on the other tentacle also should be understood as 'limits' doesn't exist, not for us at least inside it, if we by that means something defining 'seams' or the universes possible 'size', etc. With such a universe any definitions of a 'size' becomes questionable. A little like wanting to prove a universe to 'rotate', relative what?

But there are 'limits' if we by it mean constants, laws, rules, statistics etc. And 'c' is one of them. And another thing, the size of such a universe may be questionable, but scales, as QM use, is not, as far as I know. And that sort of scaling should hold true wherever you go, just as I would expect conservation laws to do. You can translate that one (scaling) into the one defining your local measurement of a length as belonging to a constant too. Your ruler never change its length, and the measurements (experiments) you make on a microscopic scale should be repeatable, as well as using a 'same scale', throughout a universe. Those are all ideal definitions naturally, as we consist of constituents of mass, acting and being acted on by mass. But Einstein has already defined the clock and the ruler ideally so I hope we can jump that one here.

also, as I use a strict local definition a black hole is communicative locally defined, in the sense that you can join it, on your own peril naturally, and so locally defined describe it (communicate with it). If we instead define communication as in the constant sending and receiving of information throughout a 'whole universe', a black hole may as per Hawking radiation, or may not, be 'communicative'. In the last definition where it is not it becomes a cosmic censorship, but it doesn't define a outside. Possibly one could look at it as another 'inside' though

if we look at how I defined it here, two out of three possibilities define it as communicative, so maybe it is? I don't know

Well, actually I think I do know, if I stay with strict locality you can go into a black hole and communicate with it. That the local definition I like, avoiding ideas of a universe as described through 'container models' needing higher dimensions.

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And yes, I think Einstein wanted a 'container universe'. He used 'dimensions' and as I understand kept searching for a way to define a fifth that would join observer dependencies into one perfect description of the 'real universe'. He also had a faith of sorts it seems, not in a 'God' creating men into his image, but of something more than just the universe, unless, maybe he thought of the universe as being just that? I don't know there but I wish I did.

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btw: I don't think Einstein used a geometric definition originally, he adopted it after some initial irritation from Minkowski

"By 1907 Minkowski realized that the special theory of relativity, introduced by his former student Albert Einstein in 1905 and based on the previous work of Lorentz and Poincaré, could best be understood in a four-dimensional space, since known as the "Minkowski spacetime", in which time and space are not separated entities but intermingled in a four dimensional space–time, and in which the Lorentz geometry of special relativity can be effectively represented. The beginning part of his address delivered at the 80th Assembly of German Natural Scientists and Physicians (21 September 1908) is now famous:

"The views of space and time which I wish to lay before you have sprung from the soil of experimental physics, and therein lies their strength. They are radical. Henceforth space by itself, and time by itself, are doomed to fade away into mere shadows, and only a kind of union of the two will preserve an independent reality."

Einstein himself at first viewed Minkowski's treatment as a mere mathematical trick, before eventually realizing that a geometrical view of space–time would be necessary in order to complete his own later work in general relativity"

And that geometrical way gives different 'paths of propagation' to a ray, depending on mass, acting and being acted on, as I understands it. Einstein used Algebra for SR, and I suspect he trusted more to that than the geometrical definition in his later works too, which may bring some light to him saying as he did in your citation. That is unless you can prove that a 'curved path' (geometrically defined), due to mass, can't be translatable into a longer path, equivalent to a longer time for the ray to reach the observer.