# Space expanding, or light slowing down? [duplicate]

This question seems a lot like one of those "Phylosoraptor" memes all over the Internet, and it might be very silly, but I've been thinking about this for a while.

Is it possible that space is not actually expanding but rather, the speed of light is decreasing throughout the entire Universe? ...as if light were traveling through a medium that would be changing its properties over time.

I guess there are several phenomenons that can be explained just as well by assuming that either space is expanding, either light is slowing down, but I do not know enough physics (and phenomenons that could contradict such a claim) in order to rule out this possibility.

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## marked as duplicate by Ben Crowell, BebopButUnsteady, Emilio Pisanty, Jim, DanAug 8 '13 at 2:53

Although many physicists are ignorant about it, the short answer is yes. I will hopefully give a thorough answer tomorrow. In the mean time you may want to read this and some articles by João Magueijo. – Ali Aug 3 '13 at 19:45
possible duplicate of Has the speed of light changed over time? – Colin McFaul Aug 3 '13 at 20:05
@Ali: I'm baffled by Albrecht and Magueijo's arxiv.org/abs/astro-ph/9811018 . They start by acknowledging, correctly, that it's meaningless to talk about variation in a dimensionful constant such as $c$. Then they go on to construct a theory of varying $c$. Mark M's correct answer here physics.stackexchange.com/a/34878/4552 also explains why it's meaningless to talk about varying $c$ as opposed to variation in a dimensionless constant such as the fine structure constant. – Ben Crowell Aug 4 '13 at 4:23
@BenCrowell That's a good paper! I suggest reading it carefully again. The point is when you do a measurement(e.g. length) you measure the dimensionless ratio between two dimensional things. Now, if this ratio is varying, which one of those would you say is changing? And as you can see, there is actually nothing(I mean theoretically) prohibiting variation in dimensional constants of nature; and they might have pleasant implications, where in this case they do. – Ali Aug 4 '13 at 6:27

While it is true that $c$ is constant as far as anyone can measure, and while part of the evidence comes from distant objects, this statement leaves something to be desired - namely how this measurement is done. It is in fact a nontrivial astronomical measurement, since you can't just go to a distant galaxy in the past and measure the speed of light there. Even spectroscopy runs into problems if all you can see is, say, the Balmer series. – Chris White Aug 3 '13 at 20:55