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Is there any evidence to suggest that the speed of light (or its other properties) has been unchanged since the beginning?

The particular curiosity stems from the redshift observed in distant galaxies. A recent 60 Symbols Video discusses recent observations from JWST identifying very old galaxies that are perplexingly massive.

Our age estimates and characterization of the universe expansion rely on persistent properties of the photon across billions of years and close proximity to the big bang. If the energy of a photon decays after a few billion years traversing space, this could explain the redshift. Or if the speed of light has changed since the generation of the redshifted photons, this might also explain the redshift.

I trust the current models, but how safe is the assumption Light has been behaving the same way for all time? Do we have any evidence?

A McKelvy
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  • Did you do any search on your own for papers that discuss the topic? – FlatterMann Jun 01 '23 at 16:38
  • Yes Some: https://journals.aps.org/prd/pdf/10.1103/PhysRevD.59.043516

    But this question is sufficiently nuanced and the literature sufficiently complex that an expert synopsis is much more useful to me and others.

     – A McKelvy Jun 01 '23 at 16:42
  • To get super pedantic, the one-way speed of light is still unknown. We only know the average over two-way. – David S Jun 01 '23 at 18:28
  • @DavidS That's common lore on the internet, but I can measure the "one way speed of light" just fine with two synchronized clocks. Why? Because nature allows me to adiabatically move the two synchronized clocks apart without introducing an irreducible error. You can, of course, deny that, but you are basically ending up in the same philosophical position as a superdeterminist that way: you have to claim that nature is pulling the perfect con job on us all the time, every time. In this case quite literally. – FlatterMann Jun 01 '23 at 18:56
  • In any case, there is a good argument to be made that the speed of light is irrelevant as long as we are in a regime in which spacetime behaves geometrically. We have, so far, not been able to observe a deviation from that. That doesn't mean that spacetime has to be this way under all circumstances, but until we find that it isn't, this isn't even a topic for science. – FlatterMann Jun 01 '23 at 18:59
  • @DavidS the one-way speed of light is not "still unknown". Rather, if you understand what Special Theory of Relativity is saying, it cannot be known. It is a rather unfortunate choice of terminology. Veritasium covered it a while back but his treatment is mired in this terminology and the result is confusion. In short, if you can ever measure the "one way speed of light", then you will be able to find out which frame of reference is stationary, and thus violate relativity. – naturallyInconsistent Jun 02 '23 at 01:30

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