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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.

Jay Lorn
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  • 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 03 '13 at 19:45
  • possible duplicate of Has the speed of light changed over time? – Colin McFaul Aug 03 '13 at 20:05
  • Possible duplicates: http://physics.stackexchange.com/q/12805/2451 , http://physics.stackexchange.com/q/34874/2451 Related: http://physics.stackexchange.com/q/37629/2451 and links therein. – Qmechanic Aug 03 '13 at 20:07
  • @Ali: I'm baffled by Albrecht and Magueijo's http://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 http://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. –  Aug 04 '13 at 04:23
  • @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 04 '13 at 06:27
  • @Qmechanic Just a tiny question: as you are a moderator, if you mark a post as possible duplicate doesn't the state of the post instantly change to duplicate? – Ali Aug 04 '13 at 06:43
  • @Ali: Moderators are aiming at moderating in the interest of the majority of users and preferably by consensus. Therefore they try to let the community decide closures as much as it is practically possible. In particular, 3k+ users can cast closure votes. – Qmechanic Aug 04 '13 at 07:16

2 Answers2

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If the speed of light were changing, you would have to explain why distant objects are not blurred or distorted, since waves bend when the speed of the wave changes.

Zo the Relativist
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  • I think you are talking about refraction. But, both the bend and speed change might the result of medium change, instead of bend being the result of speed change, wrong? – Xfce4 Sep 23 '21 at 18:49
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Speed of light coming from a distant objects is constant - we have measured it.

Constantine
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    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. –  Aug 03 '13 at 20:55
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    @ChrisWhite: of course, the speed of light is peppered through all sorts of electromagnetic and QCD phenomena, though. In particular, star fusion would change pretty radically if the speed of light changed. – Zo the Relativist Aug 03 '13 at 21:32