Speed Limit of causal influence

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Theory of relativity restricts the speed of causal influences to the speed of light. But is it a proven science fact or merely a presumption? Could it be proven wrong by observations? Or may be there are already observations (such as the quantum -"spooky action at a distance") that are contrary to this presumption. What impact will it have on our current understanding of things if it is proven wrong?
 

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  • #2
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'Proven science fact' is a gray area.... there are only degrees of certainty.
It is very well established (mostly theoretically, but also by an extensive and telling lack of exceptions in observation) that the speed of light is an absolute limit. "Speed of casual influence" also gets a little tricky at times, a better thing to consider is the "speed of information transfer." There are no examples of this exceeding the speed of light, even the example of quantum entanglement ("spooky action at a distance") cannot be used to carry information---although the philosophical questions of its effects on causality are still in dispute.

If it was proven wrong (which I wouldn't expect to happen), it would be one of the biggest shocks in scientific history, presumably leading to one of the biggest paradigm shifts. If information could be conveyed faster than the speed of light, it might not have any direct impacts per se, but it would suggest that lots of our theories are completely off (e.g. relativity [both], quantum field theory, basically everything).
 
  • #3
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But is it a proven science fact or merely a presumption?
There are lots of experimental results consistent with information transfer < c. There are no experimental results consistent with information transfer > c. That is as proven as anything can get in science.
 
  • #4
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The theoretical particle the tachyon travels faster than the speed of light, but instead of sending information to something else later in time it is also theorized to travel backwards in time since it travels FTL. So, then the partical would arrive to some location before it was sent. Then in a sense it would still be impossible to send information FTL at a later point in time.
 
  • #5
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There are lots of experimental results consistent with information transfer < c. There are no experimental results consistent with information transfer > c. That is as proven as anything can get in science.
Aren't all the experimental successes of special and general relativity evidence that spacetime is locally Minkowsian, and thus that the speed of light has to be the absolute limit? I think that evidence is much more persuasive than the mere fact that we haven't seen information transfer greater than the speed of light. We haven't seen information transfer taking place at .9999c either, as far as I know.
 
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Aren't all the experimental successes of special and general relativity evidence that spacetime is locally Minkowsian, and thus that the speed of light has to be the absolute limit? I think that evidence is much more persuasive than the mere fact that we haven't seen information transfer greater than the speed of light. We haven't seen information transfer taking place at .9999c either, as far as I know.
No..the principle of the speed of causal influences is something different to the principle of the speed of light. the two principle could be equivalant only if we add third principle that causal influence must exist by means of an object move between the two sides of the influence.
GR and SR will not be affected at all if the principle of the speed of causal influences is proven wrong by observations while SR and GR will collapse if the principle of the speed of light is proven wrong by observations .
Einstein use the principle of the speed of light to build SR and GR but he use the principle of causal influences only (as one way ) to destroy Newtonian theory of gravity.
 
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We haven't seen information transfer taking place at .9999c either, as far as I know.
Well, the index of refraction for air is 1.0002, so we see information transfer taking place at .9992 c every time we look around :smile: But in principle I agree.
 
  • #8
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Well, the index of refraction for air is 1.0002, so we see information transfer taking place at .9992 c every time we look around :smile:
Why do those experimenters have to be so precise?:grumpy:
 
  • #9
DrGreg
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GR and SR will not be affected at all if the principle of the speed of causal influences is proven wrong by observations
Not necessarily. If causal influences can travel faster than light relative to the "emitter", and if relativity is correct, then someone could causally influence their own past, giving rise to potential grandfather paradoxes and the like. See tachyonic antitelephone for how to do it. So if faster-than-light influences were ever discovered, that might cause big problems for relativity. Fortunately no such influences have yet been found.
 
  • #10
bcrowell
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Not necessarily. If causal influences can travel faster than light relative to the "emitter", and if relativity is correct, then someone could causally influence their own past, giving rise to potential grandfather paradoxes and the like. See tachyonic antitelephone for how to do it. So if faster-than-light influences were ever discovered, that might cause big problems for relativity. Fortunately no such influences have yet been found.
This kind of thing is fairly complicated and not settled theoretically. We don't know if the chronology protection conjecture holds. CTCs can exist even in a spacetime where the local maximum speed of causality is as predicted by SR. Naked singularities in GR (such as the big bang) are acausal, and we don't know if cosmic censorship holds. It may be that our universe simply doesn't have the kind of orderly causality we would like, and that's just something we'll end up having to accept.
 
  • #11
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There are lots of experimental results consistent with information transfer < c. There are no experimental results consistent with information transfer > c. That is as proven as anything can get in science.
Do we have the right kind of technology / measurement devices to measure something faster than c?
 
  • #12
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Sure. What would make you think that we don't?
 
  • #13
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The theoretical particle the tachyon travels faster than the speed of light, but instead of sending information to something else later in time it is also theorized to travel backwards in time since it travels FTL. So, then the partical would arrive to some location before it was sent. Then in a sense it would still be impossible to send information FTL at a later point in time.
I think the statement which I hear so often 'information will arrive before it is sent' is incorrect. Our current 'measuring stick' is light for both distance and time. If we find 'something' that travels faster than light, we would be using a new measuring tool to measure time and causality will not be violated.

I think it is safe to say special relativity breaks down for velocity above c.
 

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