Years ago, my brother and I had an argument where I was trying to convince him that nothing could travel faster than the speed of light. I was pursuing this in the context of Special Relativity. My main thought was to contrast the behaviour of light to the behaviour of baseballs thrown from train-cars, say. To me, this is an interesting and surprising feature, and I thought he'd be interested.

Instead, he turned the argument into "you don't really know that nothing can go faster than light". Over the years, we've gone back and forth where he'll claim that some new result proves that I was wrong. I've mostly been dismissive, since the cites are mostly pop-sci or will say something like there might be some 1 part in 10^17 discrepancy, or something.

So, recently, I sent him the Wikipedia article on Metric Expansion, and he shot back with "see, matter is traveling faster than the speed of light."

Is this an accurate statement, or does it hinge on interpreting "traveling"? I.e. one can make a distinction between traveling through space, and space itself expanding.

Was I wrong to say (again, according to GR not SR) "nothing can travel faster than the speed of light" (ignoring tiny fluctuations, and according to currently known physics)?

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    $\begingroup$ You won't be wrong. But working with non flat spaces, you have to add "locally". You can always choose locally flat coordinate frame in which nothing can move faster than "c". I guess you have encountered an analogy with an inflating balloon. Two points there are not really traveling, they move relative to each other simply because there is constantly appearing new "space" between the two points $\endgroup$
    – Kosm
    Feb 12, 2016 at 3:26
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    $\begingroup$ I'll specify what "locally" means. Imagine a photon traveling through a warped space from A to B. Then imagine some other particle traveling through exactly the same warped space, again, from A to B. Then the particle cannot come to the point B faster that the photon. But if the photon traveled through an expanding space, and the particle - through the static, then it may come in less time. $\endgroup$
    – Kosm
    Feb 12, 2016 at 3:45
  • $\begingroup$ @Kosm when you write "it may come in less time", does "it" refer to the photon or to the particle? $\endgroup$ Feb 12, 2016 at 9:35
  • $\begingroup$ to the particle. It can "outrun" the photon, but the photon will have covered longer distance (points A and B are the same!), since it moved through an expanding space, so its not quite fair to say "outrun". $\endgroup$
    – Kosm
    Feb 12, 2016 at 13:35
  • $\begingroup$ Possible duplicates: physics.stackexchange.com/q/26549/2451 and links therein. $\endgroup$
    – Qmechanic
    Feb 17, 2016 at 17:38