# Why is speed of light a constant while distance in space is not?

Disclaimer: I asked this at Astronomy.SE, but got no answer whatsoever, so I am trying my luck here.

As you probably know current state-of-the art physics (i.e. gravitational waves, cosmic expansion) basically states that space itself is subject to expansion or contraction. Since there is no moving matter or energy involved, this might even happen at a "speed" faster than light.

So far, so good and obscure. What strikes me is the principle that the speed of light as a fundamental constant can only be expressed as a function of space-time. Where do we know that the one is constant but the other can suddenly be variable?

Is there any reason why the point of view of an expanding or contracting space is preferred over, say, a reduction in the speed of light or an increase in the "speed" of time? Is there any objective difference, a mathmatical model being a better fit or is it just the good old rubber metaphor being stretched (pun intended) too far?

In case the answer is: Both are equal w.r.t. current observations: How do we know that not both are actually variable?

• Re: "Is there any reason why the point of view of an expanding or contracting space is preferred over, say, a reduction in the speed of light or an increase in the "speed" of time?"...'any reason' covers a lot of territory. – IntuitivePhysics Mar 12 '16 at 22:31
• It has been mostly answered in this link: physics.stackexchange.com/q/2230 – Benjamin Mar 12 '16 at 22:31
• @choeger Actually the idea of a variable speed of light is/has been considered, even by Einstein himself, and nowadays in the context of the inflationary scenario, you may like to take a look at en.wikipedia.org/wiki/Variable_speed_of_light – udrv Mar 13 '16 at 4:56
• " this might even happen at a 'speed' faster than light": no. – Fabrice NEYRET Mar 13 '16 at 9:43
• So you have to bring in the machinery of general relativity. And since general relativity can already explain everything with a constant speed of light, you don't get anything out of making it vary. – knzhou Mar 14 '16 at 20:52

• It is not that clear to me what "equivalent formulation" means if $c$ is allowed to vary. The constancy of $c$ is the crux of special relativity. I'm sure there are theories out there in which $c$ is allowed to vary but I can not see how they could be called equivalent nor how they could be accepted since experiments for now tell us otherwise. – Jan Bos Mar 15 '16 at 0:09