There are many other arguments as to the constancy of the speed of light (or more precisely, c). One thing I have been curious is what would the impact be on spectral measurements from distant luminous objects.
Basically, we can get the relative speed (at the time the observed) light was emitted based on a red shift in spectral absorption lines. Stars have predictable chemical composition, and expected elements have multiple absorption lines that all shift together.
Quantum field theory, which explains those absorption spectra, relies on electrical charge force carriers to move at c (I think). If c were to change, what impact would that have on spectral absorption lines? Presumably, electron shell energy levels would undergo a lot of changes.
(1) Different elements absorption patterns would shift differently, definitively proving that either c is constant, or else our observations of light from distant objects are nonsense.
(2) Absorption lines should all shift the same, making this argument say nothing at all one way or the other about c.
Anyone understand this well enough to comment?