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B FTL signal and causality

  1. Jan 30, 2016 #1
    Dear PF Forum,
    I've tried to googled how causality can happen by FTL communcation (I use communication, because if it "travels", I'm having trouble calculating ##\gamma## imaginer). But from all examples that I've found it can violate causality if one of the party is moving relative to the other?
    FTL Communication.jpg
    I can 'understand' that picture above about violated causality if one of the party is moving.
    Is there an example about violating causality if one of the party is NOT moving?
    Thank you very much.
  2. jcsd
  3. Jan 30, 2016 #2

    Vanadium 50

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    In relativity there is no such thing as "not moving".
  4. Jan 30, 2016 #3
    I mean if Party A and B are co moving. Their distance is the same all the time. So if A sends an FTL signal to B and B replies, there's no causality violation?
    [EDIT: Party A and B are co moving]
  5. Jan 30, 2016 #4


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    If all FTL signals are FTL in one given frame, you will not have causality violations. You need FTL signals in at least two frames to produce causality violation - because one of them is then time reversed in the other frame. If you posit FTL signals that violate Lorentz invariance (e.g. there is a preferred frame in which you can send signals any direction and speed, but in other frames you cannot), then you can avoid causality violations.
  6. Jan 30, 2016 #5
    Thanks a lot.
  7. Jan 31, 2016 #6
    Thanks for the answer.
    So concerning this picture again.
    FTL Communication.jpg
    This the ST diagram for someone (red) who sends FTL signal to the other party (blue) who is moving relatif to red.
    But it doesn't have to be signal and parties, right?
    It could be a very fast rocket who travels from red, and at some point it slows down so its relative speed to (supposed) red (party) is say,... 0.6c then it travels back to red?
    Then if the rocket travels FTL, when the rocket arrive back at spot Red, it will be at red spot wordline in the past?

    Thanks for any replies.
  8. Jan 31, 2016 #7


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    Yes, if a rocket could somehow travel FTL in two different frames, it could end up in the causal past of where it started.
  9. Jan 31, 2016 #8
    Dear PF Forum,
    I'm sorry if I ask again.
    I still don't understand about this causality violation.
    There are three parties: Blue, Blue, Green
    B1, B2 and G.
    B1 and B2 are co moving.
    G travels at v.
    The speed of FTL signal is instaneous

    Please consider the left diagram
    1: At E2, if B2 sends an FTL signal to B1, when/when will the signal arrive at?
    A: E1?
    B: E3?

    Please consider the right diagram
    2. At F2, if G sends an FTL signal to B1, when/where will the signal arrive at?
    A: F1?
    B: F3?

    3. At E2, there's an FTL signal (never mind who sends it) travels to B1, when/where will the signal arrive at?
    A: E1
    B: E3

    4. At F2, an FTL signal travels to B1, when/where will the signal arrive at?
    A: F1
    B: F3.

    This SR things really confuses me. Perhaps if I'm given a simple answer, I will take the conclusion myself.
    There can't be FTL signal. But if there is, I'd like to know how it is impossible.

    Train - Platform.jpg

    This is the train - platform animation.
    In this example, there are two answers. Both train and platform don't agree about event red and blue. But there is only one signal (RED) that arrives in the middle of the train, and one signal (BLUE) that arrives in the middle of the train. This at least I can "understand" (or grab)

    But in those FTL example, there will be two signals?
    One arrives at B1 "now", and the other one arrives at B1 "past",

    What I want to know in this post is this:
    5: Not that FTL CAN send signal in the past, but will there be two reply signals if we send FTL signal?

    Thank you very much for the answer.
  10. Jan 31, 2016 #8


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    It really depends on your model of FTL - which, since there is no hint of it, all theories about it are speculation. Going with what seems to be your model, clarified: an FTL signal travels on the spacelike geodesic 4-orthgonal to the emitter's world line at the point of emission. Then B2's signal would arrive at E1. G's signal emitted at the same place and time would arrive at E3. This is because B2 and G are in relative motion, so their FTL signals travel on different spacetime paths.
    per the theory under consideration, at F3.
    You can't say 'never mind who sends it'. Because "travels spatial distance instantly" is a frame dependent statement. My formalization of your model effectively states that this statement is (travels instanty) is true in the rest frame of the emitter. Thus the emitter must be specified and is crucial. Thus this question is counterfactual - even to your proposed model, ant the possible questions have been answered in (1) and (2).
    Same answer as for (3).
    Not in the proposed model - the spacetime paths of FTL signals depend on the rest frame of the emitter.
  11. Jan 31, 2016 #9
    I'm sorry. Hevan't I said that my FTL is instaneous?
  12. Jan 31, 2016 #10
    Thank you very much PAllen.
  13. Feb 1, 2016 #11


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    Yes. The problem is that this means that the emission and reception events are simultaneous. But simultaneous according to who? You haven't answered that, and that there is no obvious answer to that question is basically why we assume it's impossible. PAllen has provided two sensible answers. One is in #4, where he allows simultaneous communication in one frame only, which is to say that there is one frame in which communication is instantaneous independent of the motion of the emitter. This does not allow causality violations because all FTL communication is represented as parallel lines on the Minkowski diagram. Even though I can receive your message before you sent it in some frames, my reply must be sent after I received it and the parallel lines requirement means that you must receive it after your initial sending. However, we've picked one frame out as special, which is not a very comfortable thing to do in relativity.

    The other model he suggests is in #8 where he says the emission and receipt is simultaneous in the rest frame of the emitter. This can lead to paradoxes since the worldline of my reply can cross your original signal and arrive at you before you sent your original signal.

    Which of these models you choose to apply is up to you. There's no evidence of the actual existence of FTL communication from which to be able to make any kind of real decision.
  14. Feb 4, 2016 #12


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    I'm not going to go over your scenario in detail, but the following are well known and accepted:

    a) The math of GR allows 'warp drive' solutions which have the feature that locally (e.g. within the warp bubble) matter never exceeds c, but there is no upper bound on how quickly, in the frame of two stars far apart, how quickly a ship in the bubble gets from one to the other
    b) These solutions always do allow for causality violations and can be used to construct closed timelike curves where you can visit your own past. Basically, you can always construct a warp analog of the FTL signals in two frames that was explained to you ad-nauseum above, leading to causality violation.
    c) They require manipulating large amounts of negative energy, and there is, at present, no hint of an ability to acquire and manipulate negative energy in quantitity. There are quantum systems (e.g. the Casimir effect) that involve very small amounts of negative energy that no one knows how to extract and manipulate. A small number of scientists are pursuing this possibility.
    d) The combination of (b) and (c) leads most scientists to discount the possibility that warp drive will ever exist in our universe, but that is a judgement of future
    possibility, not a proof. As a side note, there are calculations that the GR allowed warp drives would normally destroy the origin of travel, the destination, and the traveler due to incredibly extreme radiation. That is, computations seem to show that a warp bubble sufficient to carry a rocket will, as it approaches a target planet, release enough radiation to vaporize the planet.

    [[Moderator's note: deleted comments about Star Trek, off topic.]]
    Last edited by a moderator: Mar 14, 2016
  15. Mar 14, 2016 #13
    Dear PF Forum,
    After sometimes, I'm intrigued to calculate what is the minimum FTL speed so that we can go back in the past.
    So if Green travels 0.6c, what FTL speed that a signal from Blue has to travel to Green and return to Blue so that the signal comes at the same time Blue sends the signal?
    I find that this is the formula.
    V is Blue and Green relative speed
    W is the speed of the FTL
    ##W = \frac{1+\sqrt{1-V^2}}{V}##
    Is this the formula? Just in case anybody has bothered him/her self to calculate it before.
    For V = 0.6c W is 3
  16. Mar 14, 2016 #14
    Perhaps they use this "warp" thing so that non scientists will think that it make sense. Frankly, I've watched star trek for 15 years and I just realized that warp will cause time travel to the past when I created this thread and thought it a little longer.
    So what about my calculation phinds?
    ##W = \frac{1+\sqrt{1-V^2}}{V}##
    Is this correct that to go back in the past, we should travel in W speed to chase someone traveling at V and go back to our location.
  17. Mar 14, 2016 #15


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    I don't know about the math of it all and don't care because I consider it impossible in practice. My point was that you said they invited a "serious scientist" and I was wondering how you thought a "serious scientist" would have or could have contributed to the nonsensical notion that the time-span effects of FTL don't exist.
  18. Mar 14, 2016 #16


    Staff: Mentor

    Done. Star Trek "science" is not science, and Star Trek scenarios cannot be analyzed using the actual laws of physics.
  19. Mar 14, 2016 #17


    Staff: Mentor

    To the extent that there is an actual question about actual physics in this thread, it has been answered. Thread closed.
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