5 ms·

> Literally any method of getting from A to B faster than light would, has a causality paradox. Pretty much every explanation I've seen as to why this would os

> Literally any method of getting from A to B faster than light would, has a causality paradox. Pretty much every explanation I've seen as to why this would ostensibly be so would - when applied consistently - consider the average thunderstorm to be full of causality violations because you can see the lightning before you can hear it. It shouldn't matter how long it takes me to go from Point A to Point B. As long as I arrive at Point B after I departed from Point A, then causality should be preserved. Doesn't matter if n=1s or 100s or 10000s or 0.0000000000000000000000001s. As long as n is positive, then where, pray tell, was causality violated? And it shouldn't matter if someone else sees me arrive before one sees me depart, for the same reason why it shouldn't matter if the light from a flash of lightning arrives before the sound does. That is: just because you perceive something out-of-order does not mean it was actually out-of-order.

bitdizzy 6y ago
https://en.m.wikipedia.org/wiki/Tachyonic_antitelephone https://en.m.wikipedia.org/wiki/Tachyonic_antitelephone
- yellowapple 6y ago
  That doesn't answer the question. The hypothetical existence of a tachyonic antitelephone presupposes that superluminal communication causes backward time travel. The question is why superluminal communication (allegedly) causes backward time travel, and by what mechanisms. And no, just because some arbitrary frame of reference is slow to receive information doesn't mean the events creating that information have scrambled causality. Just like how just because I heard lightning strike B before lightning strike A doesn't mean B struck before A.
  - bitdizzy 6y ago
    it's not like a thunder storm. The wikipedia page has a worked out example. If you can go back and forth between 2 places FTL you can arrive before you left and interfere with your departure. You are more convinced by a metaphor than the actual theory of relativity. That can't be helped.
    - yellowapple 6y ago
      > The wikipedia page has a worked out example. If you can go back and forth between 2 places FTL you can arrive before you left and interfere with your departure. That example has multiple assumptions that seem to make said example only relevant to a very specific case, rather than to superluminal communication in general (least of all to approaches specifically designed to avoid those assumptions): 1. Alice and Bob are themselves moving at relativistic speeds relative to each other while communicating. That doesn't seem relevant for cases where they are not doing so - in particular when they are communicating across a region of spacetime crafted specifically such that they are stationary relative to each other. 2. Alice's signal to Bob and Bob's reply to Alice are both somehow themselves moving through normal spacetime at a speed greater than c - i.e. the messages are tachyonic (hence: tachyonic antitelephone). Again, when discussing mechanisms of warping spacetime such that no >c movement within any local frame of reference is actually necessary, bringing up tachyons doesn't seem especially relevant. > You are more convinced by a metaphor than the actual theory of relativity. That can't be helped. That can be helped, specifically by explaining why the metaphor is wrong, and where it actually contradicts general or special relativity.
      - bitdizzy 6y ago
        The name tachyonic antitelephone is a bit unfortunate because it need not involve tachyons. All it takes is the ability to send a message outside of your light cone. Along the path actually taken need not be the case, we are reasoning about the endpoints of the communication independent of the means. In particular it does not matter whether you use a warp drive or not. The violation occurs relative to reference frames outside of the warp bubble where special relativity fully applies. Here, explicitly looks like a worked out example: http://exvacuo.free.fr/div/Sciences/Dossiers/Time/A%20E%20Everett%20-%20Warp%20drive%20and%20causality%20-%20prd950914.pdf http://exvacuo.free.fr/div/Sciences/Dossiers/Time/A%20E%20Ev... You can create instances of FTL travel or communication that don't violate causality. But the ability to perform such FTL tricks implies the possibility of constructing a closed timelike curve so long as you're talking about a localized phenomenon like a warp drive or tachyon where special relativity holds outside of a negligible area. If you're messing with the global geometry of spacetime then you can make very weird things happen and I would not be surprised if there were such a geometry that allows FTL communication in a limited way that doesnt allow CTCs. But that would not like our universe or a warp drive, a specifically localized situation.
AnimalMuppet 6y ago
Here's the problem: Event A at location X at time T1 causes event B at location Y at time T2 > T1. If I can travel FTL, I can find a frame of reference where T2 < T1 - that is, where the effect comes before the cause. This leaves a few possibilities, and they're all bad: - There is no real cause and effect. - Cause and effect are real, but effects can come before causes. - The laws of physics are not the same in all frames of reference. Our current understanding/belief is that some things really do cause other things, that causes always precede effects, and that the laws of physics are the same in all frames of reference.
- yellowapple 6y ago
  > If I can travel FTL, I can find a frame of reference where T2 < T1 How? Sure, it's possible that the light from an event happening at T2 arrives before the light from an event happening at T1, but that says nothing about when those events actually happened - just like how if we hear one lightning strike before another, that doesn't mean the one we heard first actually happened first. That is: this notion that all superluminal travel somehow magically necessitates the possibility of backwards time travel doesn't seem to be logically sound, and seems to be one of these things that people keep repeating without bothering to actually explain why that is. Specifically, this "FTL necessitates backwards time travel" argument seems to depend on some notion that light is infinitely fast (because the only way to go back in time is to have a speed greater than infinity - i.e. one where the t component in d/t is negative), which is empirically false (we can measure light's speed, and it's clearly finite).
  - SideburnsOfDoom 6y ago
    > How? > this notion that all superluminal travel somehow magically necessitates the possibility of backwards time travel doesn't seem to be logically sound. There is literally decades worth of physics that answers your how, and explains the soundness of it, I would urge you to study the subject in well, any depth at all before naively dismissing it as "unsound". I mean, who's more likely wrong: Physics since Einstein's 1905 papers, or guy on the internet who's been thinking about it for 10 minutes, and has questions with basic misconceptions in them. Asking for answers in the comments is a very poor way to approach a subject like this. All you'll get are quick responses and displays of ignorance.

bitdizzy 6y ago

https://en.m.wikipedia.org/wiki/Tachyonic_antitelephone https://en.m.wikipedia.org/wiki/Tachyonic_antitelephone

yellowapple 6y ago
That doesn't answer the question. The hypothetical existence of a tachyonic antitelephone presupposes that superluminal communication causes backward time travel. The question is why superluminal communication (allegedly) causes backward time travel, and by what mechanisms. And no, just because some arbitrary frame of reference is slow to receive information doesn't mean the events creating that information have scrambled causality. Just like how just because I heard lightning strike B before lightning strike A doesn't mean B struck before A.
- bitdizzy 6y ago
  it's not like a thunder storm. The wikipedia page has a worked out example. If you can go back and forth between 2 places FTL you can arrive before you left and interfere with your departure. You are more convinced by a metaphor than the actual theory of relativity. That can't be helped.
  - yellowapple 6y ago
    > The wikipedia page has a worked out example. If you can go back and forth between 2 places FTL you can arrive before you left and interfere with your departure. That example has multiple assumptions that seem to make said example only relevant to a very specific case, rather than to superluminal communication in general (least of all to approaches specifically designed to avoid those assumptions): 1. Alice and Bob are themselves moving at relativistic speeds relative to each other while communicating. That doesn't seem relevant for cases where they are not doing so - in particular when they are communicating across a region of spacetime crafted specifically such that they are stationary relative to each other. 2. Alice's signal to Bob and Bob's reply to Alice are both somehow themselves moving through normal spacetime at a speed greater than c - i.e. the messages are tachyonic (hence: tachyonic antitelephone). Again, when discussing mechanisms of warping spacetime such that no >c movement within any local frame of reference is actually necessary, bringing up tachyons doesn't seem especially relevant. > You are more convinced by a metaphor than the actual theory of relativity. That can't be helped. That can be helped, specifically by explaining why the metaphor is wrong, and where it actually contradicts general or special relativity.
    - bitdizzy 6y ago
      The name tachyonic antitelephone is a bit unfortunate because it need not involve tachyons. All it takes is the ability to send a message outside of your light cone. Along the path actually taken need not be the case, we are reasoning about the endpoints of the communication independent of the means. In particular it does not matter whether you use a warp drive or not. The violation occurs relative to reference frames outside of the warp bubble where special relativity fully applies. Here, explicitly looks like a worked out example: http://exvacuo.free.fr/div/Sciences/Dossiers/Time/A%20E%20Everett%20-%20Warp%20drive%20and%20causality%20-%20prd950914.pdf http://exvacuo.free.fr/div/Sciences/Dossiers/Time/A%20E%20Ev... You can create instances of FTL travel or communication that don't violate causality. But the ability to perform such FTL tricks implies the possibility of constructing a closed timelike curve so long as you're talking about a localized phenomenon like a warp drive or tachyon where special relativity holds outside of a negligible area. If you're messing with the global geometry of spacetime then you can make very weird things happen and I would not be surprised if there were such a geometry that allows FTL communication in a limited way that doesnt allow CTCs. But that would not like our universe or a warp drive, a specifically localized situation.

AnimalMuppet 6y ago

Here's the problem: Event A at location X at time T1 causes event B at location Y at time T2 > T1. If I can travel FTL, I can find a frame of reference where T2 < T1 - that is, where the effect comes before the cause. This leaves a few possibilities, and they're all bad: - There is no real cause and effect. - Cause and effect are real, but effects can come before causes. - The laws of physics are not the same in all frames of reference. Our current understanding/belief is that some things really do cause other things, that causes always precede effects, and that the laws of physics are the same in all frames of reference.

yellowapple 6y ago
> If I can travel FTL, I can find a frame of reference where T2 < T1 How? Sure, it's possible that the light from an event happening at T2 arrives before the light from an event happening at T1, but that says nothing about when those events actually happened - just like how if we hear one lightning strike before another, that doesn't mean the one we heard first actually happened first. That is: this notion that all superluminal travel somehow magically necessitates the possibility of backwards time travel doesn't seem to be logically sound, and seems to be one of these things that people keep repeating without bothering to actually explain why that is. Specifically, this "FTL necessitates backwards time travel" argument seems to depend on some notion that light is infinitely fast (because the only way to go back in time is to have a speed greater than infinity - i.e. one where the t component in d/t is negative), which is empirically false (we can measure light's speed, and it's clearly finite).
- SideburnsOfDoom 6y ago
  > How? > this notion that all superluminal travel somehow magically necessitates the possibility of backwards time travel doesn't seem to be logically sound. There is literally decades worth of physics that answers your how, and explains the soundness of it, I would urge you to study the subject in well, any depth at all before naively dismissing it as "unsound". I mean, who's more likely wrong: Physics since Einstein's 1905 papers, or guy on the internet who's been thinking about it for 10 minutes, and has questions with basic misconceptions in them. Asking for answers in the comments is a very poor way to approach a subject like this. All you'll get are quick responses and displays of ignorance.