Questioning whether the speed of light has changed within a certain class of
theories is nonsense and this is not an opinion but an elementary
mathematical fact. Of course, one may e.g. question whether photons are
massive and whether this mass has changed, leading to a (wavelength
dependent) change of speed of light.

The recently proposed variable speed of light theories are legitimate
theories. However, they don't imply a changing speed of light. That's just
the name given to the theory, not its content.

The speed of light depends on the way you choose your units and is thus
completely arbitrary. It is not possible to measure the speed of light
before you define your units of space and time. If you use the speed of
light to relate your units of time to that of space, you find that the speed
of light is constant, regardless of your theory.

If a ''variable speed of light theory'' is confirmed by experiment, it is
still a matter of convention to say that the light speed has changed and not
some other dimensional constant. Only dimensionless combinations of
constants can be said to have changed independent of conventions.

This is the point that Duff made.

CMR wrote:

> With respect, I think it was Dawkins in the Blind watchmaker who quipped
> "your incredulity alone is no measure of reality" (I paraphrase..)
> You may well be correct that the speed of light is and has "always" been
> constant, but this remains a point of some controversy that only continued
> scientific research will settle (and never finally, at least according to
> Popper). Your opinion that questioning the "fact" of the SOL's constancy
> nonsense, is just that (an opinion not nonsense, that is). And such an
> opinion is less in the spirit of good science and more that of dogma, in
> own humble opinion. I submit that just because a position is held by a
> minority and is not yet well supported by empirical data does not
> automatically disqualify it or justify ridicule. Indeed, all currently
> favored theories, including Einstein's, were once so.

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