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FTL, avoiding time travel issues?

  1. Jul 8, 2014 #1
    After reading about everything I can find on the subject while researching for a story, I've come to the conclusion that wormholes (or "jump" drives or gates) are the only potential "class" of FTL mechanisms which are even remotely (not a problem for fiction) plausible while also not opening the door to time travel / causality problems that are just ignored for the sake of telling the story.

    I'm not entirely sure about the door being closed, so that's what I'm looking for some feedback on.

    As far as I can work out, the time travel problem only manifests with a wormhole if you can accelerate one end relative to the other after creating it. Is this an accurate assessment? More to the point, if (because... scifi handwaving reasons) the ends must be fixed in space relative to one another, does the time travel boogyman still rear his head?

    I'm coming up with other technobabbleish restrictions as well to eliminate as many of the social and economic repercussions that I don't want to write about as possible (while leaving those I do intact), but like many, I want the "fun" of FTL travel while avoiding that pandoras box of paradoxes and time travel without simply ignoring it.
  2. jcsd
  3. Jul 8, 2014 #2
    Most stories that use FTL accept that that will be their weakest point on the "hardness" scale. You can still have a relatively hard sci-fi story while using FTL. You can go the route of the "jump" or some such thing, forget the technological details, and just move on with the story. It's a pretty established plot tool and most readers will accept this and move on.

    You could also do the "mysterious FTL alien object" thing, like the hypernet gates used in The Lost Fleet series by (pen name) Jack Campbell, which is otherwise fairly hard science fiction as far as the spaceflight goes. Basically, humans discover a hypernet that has already existed (relic of a past alien civilization? Something else? Up to you.) and make use of it.

    Or, if you want to go the wormhole route, forget the techno-jargon. Your characters wouldn't be discussing it usually anyway (unless there was a problem with it). When I drive to the store with a friend, I don't say, "Friend, let's transport ourselves to the store using this automobile machine which utilizes a synchronously ignited hydrocarbon fuel and air mixture in a reciprocating piston chamber to drive the wheel mechanism!" I just say, "Let's drive to the store", the how isn't important to either of us. Though, if your plot points rest on a problem with the mechanism, for instance, "Oh no, the wormhole was supposed to be coupled in space-time with our entry point, but instead an unstable gravitational distortion caused us to wind up in an unknown time and region of space!!!" then you'll have to work it all out in a plausible way.
  4. Jul 8, 2014 #3
    I am going to avoid the jargon as much as possible, I just don't want the system to be open to too many "why didn't they" or "what if" scenarios when it comes to that particular problem. That's where the question about the wormholes/jumpgates comes from.

    Everything I've been able to find so far only brings up the paradox if you can open the gate and then move one end of it off at relativistic speed to take advantage of time dilation. If the ends must remain fixed (or relatively so, heh), or a "jump drive" is used, I haven't found any potential time travel loopholes.

    I've read Lost Fleet and enjoyed it myself, especially his treatment of the tactical repercussions of light speed delays once arriving in system.
  5. Jul 9, 2014 #4
    As far as i know (but i can be wrong), in every pop SF FTL method, (Alcubierre warp drive, negative mass, wormhole, hyperspace jump) your local speed wont be FTL, if spacetime is warped, than meter and second wont be the same as in another reference frame.
  6. Jul 9, 2014 #5
    I think this is only (practically) true if the travelers traverse normal spacetime and can share information with observers during the journey; that's what I'm trying to nail down. Would there be repercussions for causality if instantaneous travel were possible?

    The closest I've found is http://www.theculture.org/rich/sharpblue/archives/000089.html which is four parties each with ansibles, each pair at rest in one frame moving relativistically vs. the other pairs frame. In each frame, each party is outside the others light cone.

    It seems that even in this case if the wormholes/gates are still "safe" due to the made up "little/no relative motion" rule, and a gateless "jump drive" in concept seems no different.

    Edit: Reading the diagram and example closer, it seems that the made up rule doesn't actually fix this situation. Maybe I'll have no choice but to ignore it entirely, but I hope not.
    Last edited: Jul 9, 2014
  7. Jul 9, 2014 #6
    I expelled air from my lungs in a spastic fashion in order to express my sense of amusement.
  8. Jul 10, 2014 #7


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    OK, I'm confused. These mathematicians from Hungary and U of Sheffield claim FTL doesn't imply time travel.

    So in that case the OP can do whatever method he wants.:wink:

    Last edited: Jul 10, 2014
  9. Jul 11, 2014 #8
    an idea

    why don't you try to make the story original to the best of your ability and the time travel/science/techinical stuff can be based on hard core real scientific theories, the old and the new and try to make it as realistic as you can while keeping the story lively and based on real true logic, you might want confused mathew from youtube to review your story before showing it to the public. I would try to not borrow any elements from other sci-fi stories. I too have written a little sci-fi plot with a screenplay and plot sequels but I think people would rather think it is bad or a joke, I don't know, I've never showed anyone and most of what I have on my work is stuck in a old floppy disk that I can't get my computer to pull up.
  10. Jul 14, 2014 #9
    Or, at least the theory would make a good explanation as to why FTL works in-universe. I wonder how a 3+1 dimension mink-diagram would look. I can barely handle the 2D-versions...

    I simply and neatly made a universe with an preferred frame of reference for a story I've been working on (cue Stewies voice...), the explanation, if asked for, why the rest of physics still behave pretty much like in our own universe would be anything but simple. Good I have the power of authorship to prevent the characters from asking those questions!
  11. Jul 14, 2014 #10


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    I was confused because everyone was telling me that FTL necessarily means time travel, and here are some scientists who deny that.
  12. Jul 15, 2014 #11
    Are you sure wormholes are better than jump drives?

    Should FTL be able to detect Lorentzian ether wind?
  13. Jul 15, 2014 #12
    Personally, I dont see difference, if FTL is time travel why would be any method different from others... otherwise i always failed to understand, how could an IMAGE of the past receive the signal. I now i'm not the brightest scientist...
    Now I'm really glad that now i can be able to refer to some fellow hungarians in my SF book. :)

    "Should FTL be able to detect Lorentzian ether wind? "

    According to wiki someone thought the weak point of the Alcubierre warp drive is that if it works FTL it generates strong Hawking radiation that kills the crew... if that happens to be true (provided that we could reach FTL in the first place whether it is casual or not...), the wormhole or jump drive is better.
    Last edited: Jul 15, 2014
  14. Jul 15, 2014 #13
    Time travel does not exist ! Time is a measurement of change. No matter how fast you go, you can only get there quicker from the "now". I will continue to argue until someone convinces me otherwise that time in itself is not a "thing" but a ruler. Therefore you can not go back or forward "in time". Time is relative to the local body, so you could go somewhere where time is "slower" than in this local body and spend 1 earth year there (if you could track earth time !) and then come back to earth and everyone on earth "aged" more than a year. But this is still not time travel. You will never be able to do the opposite,"back in time".
  15. Jul 15, 2014 #14
    There is nothing in SR or GR that says that FTL is impossible, or even that time travel is impossible. That's not the right way to think about the problem. What they say is that nothing with mass can go *exactly* the speed of light (doing so leads to infinite energy) and in SR, the reason you can never get there is easy to visualize as a sort of zenos paradox. If c is a constant and there is no preferred inertial frame, then if you spend the energy required to accelerate to 99% the speed of light relative to me -- in your own frame, you're at rest relative to light (c is constant), and you must spend the same amount of energy to get to c in your new frame. Repeat ad nauseum.

    As for FTL and time travel, one is just a side effect of the math for the other. If you ignore the energy requirement and put a velocity greater than c into the equations, t becomes negative. This implies travel backwards in time, which if correct puts causality at risk and opens the door to paradoxes.

    My question WRT wormholes and other such instantaneous methods of travel is just me trying to cover my bases. What I'm trying to do is avoid situations within the context of the fiction where a reader asks "why didn't they just do _______ here?" where _______ is some time-travel side effect of the fictional FTL method being employed. It's obvious that if the travel takes place through the intervening space at some fixed (or variable) velocity > c, that the paradoxes are inescapable.

    Wrapping my head around potential problems where the intervening space is not traversed and the travel is instant is proving more difficult, especially if I assume that some things (like taking one end of a wormhole on a relativistic journey) is impossible, and that the inertial frame used before and after the 'jump' is the same -- i.e. no jumping from A to B where B is in motion relative to A.
  16. Jul 16, 2014 #15
    Actually square root (negative value) is a complex, not a negative value.

    But no one said you should ignore Lorentz transformations.

    http://curious.astro.cornell.edu/question.php?number=575 [Broken]

    That one claims, that distant galaxies can receed FTL while their local speed is below light, because the universe itself is expanding.

    The idea of SF space travel is that we will be able do something similar, with spacetime warp, negative mass, wormhole or jump drive. The local speed will be below light.

    Well, people with a bigger knowledge will analyse that paper chasrob sent, you either accept it in your universe or not. If you dont, wormholes are also able to take you out of the light cone...
    In that case, you should rather write a near future SF.
    Last edited by a moderator: May 6, 2017
  17. Jul 16, 2014 #16


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    An expert I am not, but the paper's conclusions are interesting, especially to SF writers! Rather, those who don't want to wrestle with the problems of time travel. My story for instance doesn't involve that; I have enough handwaving with the FTL:cry: Speaking of which:

    Pssst! Can you-all keep a secret? While looking into FTL I came across this on arXiv. Something to do with adding a constant, ε2, to prevent diverging of the equations--or something:wink:--and the next superluminal regime has a max speed of 5e26 c! According to my calculator, that's across the visible universe in a millionth part of a nanosecond! Luckily my protag has reaction times equal to the Planck time:tongue: And there are even speedier regimes after that!

    I plan on using this in my tale, so do keep this quiet.:wink:
    Last edited: Jul 16, 2014
  18. Jul 16, 2014 #17
    Right, sorry. Complex/imaginary. I haven't had time to look at the paper yet.
  19. Jul 16, 2014 #18
    Well the paper is interesting, I'll have to take more time later to read it more carefully, but closing remarks contain this paragraph which is 'worrying' for lack of a better term.

    If two observers are disagreeing on what direction time is flowing, that seems to indicate that they are traveling backwards in time with respect to each other. The paper seems to be saying that the paradoxes are still unresolved, but it's not the concept of FTL alone that is responsible. This paper is only a week old, so future developments and reviews may shed more light on it (or invalidate it).

    From a fictional standpoint it does seem like a good foundation to mention in passing along with a fictional future scientist who answered the new questions.. Thanks for the link!
  20. Jul 16, 2014 #19


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    ArXiv is not peer reviewed, so I suppose anything posted there has to be taken with a grain of salt. Depending on the author of the paper of course. But, it is fertile ground for SFnal ideas
    Last edited: Jul 16, 2014
  21. Jul 24, 2014 #20


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    Whoops, my calculator let me down again.:biggrin: It'll take you nearly 6 whole nanoseconds to cross the visible universe at that velocity.
  22. Nov 14, 2014 #21
    huh.. is there any way to increase something's mass?

    if there is, then you could take it, and do a "bugee jump thing" and launch it around/through your craft, depending on weather your craft was ring or stick shaped.

    if you did that enough, you could, in theory, get to c1
  23. Nov 16, 2014 #22
    Yes, if you per E=mc2 add a lot of energy in the form of velocity an object would become heavier, relativistically speaking. But it still wouldn't reach, c, since that would require adding infinite energy.
  24. Nov 20, 2014 #23


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    Just use the Mass Effect relays, you will be fine ;)
  25. Nov 20, 2014 #24
    But they were blow up, right?
  26. Nov 25, 2014 #25


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    I think the standard way of making a time machine out of a wormhole is for one end of the wormhole to be moving relative to the other, or for two wormholes to exist, such that a time loop can be formed. If you purport that wormhole ends cannot be accelerated by any means, and all wormholes must be stationary relative some "special" frames, then you can prevent time travel. (It's not enough that they don't accelerate.) For example, maybe you will stipulate that the wormholes must be co-moving with the cosmic microwave background. The expansion of the universe provides a sense of absolute time to the universe.
    Last edited: Nov 25, 2014
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