It is often said that faster than light travel would violate causality.

However, because the universe is expanding, there are actually distant stars that move away from us at a speed greater than the speed of light.

Why is this allowed, even though it could violate causality?

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    $\begingroup$ This question is similar to physics.stackexchange.com/questions/90440/… and physics.stackexchange.com/questions/60519/… and probably related questions therein $\endgroup$
    – innisfree
    Jun 17 '14 at 7:50
  • $\begingroup$ @innisfree It is indeed related, but I am asking about how it can be reconciled with causality, while the question is about the speed of light. I have some idea of what the answer would be (which is also given on the wikipedia page) and I think it is quite different from the questions you've linked to. $\endgroup$
    – Superbest
    Jun 17 '14 at 7:57
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    $\begingroup$ I don't think innisfree's suggestions are duplicates because they address the question of how superluminal expansion is possible. This question is specifically why it doesn't violate causality. $\endgroup$ Jun 17 '14 at 8:16

If you only take into account expansion of space, then there is no way to violate causality, because information still cannot travel faster than light. If two galaxies drift apart faster than light, it is not even possible for signals emitted from one galaxy to reach the other. They have crossed each other's Hubble radius.


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