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The Possibility of Paradox-Free Time Travel 421

relliker writes in with word of a paper up on the ArXiv by Seth Lloyd and co-workers, exploring the possibility that "postselection" effects in non-linear quantum mechanics might allow paradox-free time travel. "Lloyd's time machine gets around [the grandfather paradox] because of the probabilistic nature of quantum mechanics: anything that this time machine allows can also happen with finite probability anyway... Another interesting feature of this machine is that it does not require any of the distortions of spacetime that traditional time machines rely on. In these, the fabric of spacetime has to be ruthlessly twisted in a way that allows the time travel to occur. ... Postselection can only occur if quantum mechanics is nonlinear, something that seems possible in theory but has never been observed in practice. All the evidence so far is that quantum mechanics is linear. In fact some theorists propose that the seemingly impossible things that postselection allows is a kind of proof that quantum mechanics must be linear."
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The Possibility of Paradox-Free Time Travel

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  • Primer (Score:5, Interesting)

    by kylben ( 1008989 ) on Sunday July 25, 2010 @03:01PM (#33022614) Homepage
    The movie "Primer" had an interesting take on avoiding paradoxes. [] (entire movie online)
  • Other issues exist (Score:4, Interesting)

    by AnAdventurer ( 1548515 ) on Sunday July 25, 2010 @03:14PM (#33022704)
    And if you travel outside of your light-cone? (other then math breaking down)
  • Re:Dress it up! (Score:2, Interesting)

    by maxwell demon ( 590494 ) on Sunday July 25, 2010 @03:15PM (#33022718) Journal

    I don't think the authors expect to see post-selection time machines any time soon. However such work may enable us to understand better the structure of quantum mechanics. By exploring what is and isn't possible in theory, we get guidelines in which directions it makes sense to look for adaptions of quantum theory (e.g. for quantum gravity), and which directions are better avoided.

  • Re:Dress it up! (Score:1, Interesting)

    by Anonymous Coward on Sunday July 25, 2010 @03:17PM (#33022732)

    but spending valuable time "researching" time travel, before we can even explain what time or even gravity is, seems like skipping over the hard work

    Except that most of the progress in science involve poking at theories around the edges, and seeing where it unravels.

    I imagine that, if you were in 1900, you would be criticizing Einstein "Albert, it's no use day-dreaming about what would happen if you could ride a beam of light! We don't know what light is made of, let alone how you would attach a seat to it!"

    The point is, these sorts of crazy thought experiments allow you to see where the theories are thin or fall apart. Hell - it's in TFS. If this scientist is correct, this sort of time-travel holds implications about how the world behaves, but only if quantum mechanics is non-linear. Unless I'm mistaken, what will happen is a bunch of researchers will poke and prod at the theory, and come up with a simple, doable experiment (almost certainly *not* involving time travel in any sort of "Back to the Future" sense, and probably not involving anything bigger than a proton). "If results are A, we prove that QM is non-linear. If results are B, we can conclude that it *is* linear." Anytime theory departs from intuition, it's a great time to test the theory. e.g. the double split experiment: "Quantum mechanics seems to require that the particle go through both slits simultaneously! That's nonsense! I'll just do an experiment to show those quantum mechanics people how nature *really* works ..."

    The ultimate result of Lloyd's work isn't going to be an H.G. Wells-style time machine, it'll be a new, better understanding of the nature of physics.

  • by bar-agent ( 698856 ) on Sunday July 25, 2010 @03:23PM (#33022772)

    As far as I can tell, the article is saying that if you impose a condition in the present, you cause the past to change so that it matches. This process of imposing a condition must affect the quantum mechanical properties of whatever you are checking, similar to a quantum computer.

    So basically, if your granddad rigs up a machine that kills him depending on the quantum state of a particle, and then he leaves that particle in an indeterminate quantum state until he has your dad and your dad has you, and then you collapse that particle's waveform into the state that would have killed him, he will have died back then. And somehow paradox is avoided.


  • idea of time travel (Score:4, Interesting)

    by phantomfive ( 622387 ) on Sunday July 25, 2010 @03:34PM (#33022860) Journal
    I can accept FTL travel as maybe possible, but time travel seems farfetched to me. It means that every single state that the universe has ever been in is preserved (somewhere) in it's exact state. We're not talking about the awareness of the state being preserved on the speed-of-light boundary away from the location of the state, it's the actual state, in a way that can be modified and changed. Does this even seem reasonable? How could all that be stored?

    Not only that, it means that a change in one of those states will instantly change every subsequent state. So when you travel back, everything will be different. This is really hard to believe.
  • Pretty much, yep. (Score:4, Interesting)

    by khasim ( 1285 ) <> on Sunday July 25, 2010 @03:43PM (#33022904)

    Essentially being a causeless effect, or rather an effect that causes it's cause never to have occurred?

    Pretty much, yep. Because you are now existing at a prior point in the chain of causality. So you've already accepted either:
    1. circular causality (and a supreme janitor who cleans up the past to keep the future tidy) (and how would you tell the difference) or

    2. effect without cause (because you exist prior to your parents giving birth to you).

  • Re:Dress it up! (Score:3, Interesting)

    by maxwell demon ( 590494 ) on Sunday July 25, 2010 @03:44PM (#33022910) Journal

    The concept of time travel itself shows misunderstanding of what time is. It basically posits an extra fifth dimension that's the "real" time, and treats our time as a dimension just like one of the three spatial dimensions.

    No. That may be a common misconception in SF time travel, but it's not the essence of the idea of time travel. The misconception which leads to this "hidden meta-time" is the idea of an universal, absolute time. But for our real universe, we already know since Einstein that there's no absolute time, and two different observers may disagree about how much time was between two events, and may even disagree about the temporal order of causally unrelated events. Time travel is nothing than a logical extension of this concept, where observers also disagree about the temporal order of causally related events. There's absolutely no meta-time needed.

  • Re:the other angle (Score:4, Interesting)

    by icannotthinkofaname ( 1480543 ) on Sunday July 25, 2010 @04:08PM (#33023052) Journal

    I think it's the first one, but only because I'm pretty sure it can be rephrased thus:

    "He proved that paradox-free time travel is possible through postselection of quantum teleportation by postselecting the condition that quantum mechanics is non-linear."

    However, I believe this phrasing assumes that the probability of quantum mechanics being non-linear is nonzero, so if I just divided by zero, I apologize.

  • by goombah99 ( 560566 ) on Sunday July 25, 2010 @04:12PM (#33023080)

    Yesterdays SMBC had a good point:

    You can not have any motivation or objective if you are going to travel otherwise the act of time travel is a paradox. SMBC put it thusly: if you are travelling to change some outcome, and you succeeded, you would not have had the motivation to time travel to make that change.

    SMBC's conclusion was that only nitwits have the capacity to time travel and the fact that there seem to be so many confirms that time travel must be going on right now.

    But another way to say this is, you can only choose objectives that either already happened in your past or are inevitable no matter what you do.

    For example, You could however travel with the objective of sinking the Titanic, but not the objective of preventing the sinking. If you saved the titanic, it would never occur to you to try to save the titanic.

    For example, If your objective was to save Abe Lincoln and you succeeded, then it never would have occurred to your pre-travel self that you needed to go back and save abe lincoln.

    What all this adds up to I think is that time travel is still forbidden but observational time travel-- gathering information-- is not forbidden.

    THere is an interesting proof regarding the computability of any proposition by David Wolpert that shows time travel is forbidden unless the information you gain by doing so is probabilistic or faulty. That is he proves rigorously that it is not possible to answer any arbitrary true/false question about the past with perfect fidelity. Thus time travel that preserves information with fidelity is forbidden. Error prone time travel is however allowed.

  • by ceraphis ( 1611217 ) on Sunday July 25, 2010 @04:31PM (#33023224)
    Yes, but what if with the act of time travel with motivation you also create an alternate reality where the titanic didn't sink or abe lincoln survived? Is there some rule where you must be attached to your original timeline?
  • by OrangeCatholic ( 1495411 ) on Sunday July 25, 2010 @04:34PM (#33023244)

    >Does this even seem reasonable? How could all that be stored?

    Yep. People who believe in parallel universes don't seem to comprehend the vast amount of data that would need to be stored to make such a thing possible. Every electron twitch - boom, new universe, with all of its state intact, loaded into a new memory location far away from the previous one.

    I guess with time travel they are saying all the previous states are still stored. Pretty close to the same thing. You would need a whole other universe whose job is to calculate every combination of ours.

    The universe is big, but not arbitrarily powerful where you can just ascribe all sorts of amazing abilities, like the ability to remember every state it's ever been in. Too much.

  • by tius ( 455341 ) on Sunday July 25, 2010 @06:33PM (#33023990)

    I think the problem is that everyone assumes that time is essential to processes within the universe. There's nothing that I've ever read or studied that suggests reasonably that this assumed correlation is actually real.

    Consider that if time is real, and not fundamental to processes (i.e. the correlation is not causal) then it is entirely possible to travel back in time and kill your grandfather without self destructing. This is because in the stream of processing your existence, all the important stuff has already occurred in the process dimension.

    But looking around this idea it seems more likely to me that time is an illusion as far a real dimension is concerned. It strikes me more as an analog to temperature; i.e. it's a statistical like reference. Do we really experience the passing of time or do we really just have a sense of passing of process?

    How do we measure time? By change in processes. You can pull in relativity and still see that the effects on time are purely due to the real effects on the spacial dimensions. The relative dimensional changes in space lead to changes in the processes used to measure time.

    I'm not sure if this is true, but if it were it eliminates the absurdity of the grandfather paradox.

  • Re:If (Score:2, Interesting)

    by maxwell demon ( 590494 ) on Sunday July 25, 2010 @06:54PM (#33024076) Journal

    Not necessarily. If there needs to be specific infrastructure at the arriving time, then you'll not see time travelers before that specific infrastructure was installed, which likely isn't before setting up that infrastructure.

    For example, imagine a special stabilizer field which must be at your destination if you don't want to disintegrate as soon as you re-enter spacetime. Then any time before invention and deployment of those stabilizer fields won't be possible destinations for time travelers (to see what happens if you try anyway, look to Tunguska :-)).

  • by PagosaSam ( 884523 ) on Sunday July 25, 2010 @07:30PM (#33024278)
    You know, that a funny thing about infinities. You have room for everything. ;-)
  • Re:Time Cube? (Score:3, Interesting)

    by thepotoo ( 829391 ) <> on Sunday July 25, 2010 @10:45PM (#33025276)
    Hmm, I think Gene Ray just failed the Turing Test. We should put up some horribly unaesthetic statue in his honor.
  • by blincoln ( 592401 ) on Sunday July 25, 2010 @11:04PM (#33025442) Homepage Journal

    But often this definition is too strict, and "FTL" proponents would be happy with arriving someone else faster than it would take light to travel that same journey, without getting caught up in the details of whether you were actually travelling faster than light at any point. There are various possible loopholes - teleportation, moving space itself, using a "hyperspace" to travel in, and so on.

    Assuming that the theory of relativity is more or less accurate (specifically regarding the ability of observers moving at relativistic speeds to end up with perceptions of "simultaneous" that are literally different (as opposed to only appearing different due to light-speed lag)), there does not seem to be a way to get around the speed of light without introducing the ability to violate causality. So (again, assuming the theory is essentially correct), either you believe that the past cannot be altered (and therefore no information or objects can move faster than light), or you believe that FTL travel/communication is possible, but as part of the bargain accept that it's also possible to send messages and/or objects backwards in time (e.g. I can send today's winning lottery numbers to myself yesterday).
    This definitely applies to teleportation. I have to imagine it applies to the use of wormholes as well, since the mechanism doesn't actually seem to matter. If information can get from one point in the universe to another faster than it would get there traveling at lightspeed, causality is broken or relativity is wrong in some way.
    I would love to see FTL technology, but to me Occam's Razor says that if it were possible to send information into the past, the galaxy (or even the universe) would be overrun with an advanced race that had repeatedly used that ability to obtain the best possible technology in essentially zero time.

  • Re:Pretty much, yep. (Score:3, Interesting)

    by BiggerIsBetter ( 682164 ) on Monday July 26, 2010 @04:02AM (#33026960)

    You would still have an effect due to a cause. It may be a strange route, but the cause is still you killing your grandfather, regardless of how the timeline plays out. It causes a problem from the logic perspective obviously, but it doesn't magically become different because its hard to wrap your head around.

    Indeed. Folks who think time is a straight line that we can both only forward (or perhaps back) are akin to the folks who thought the world was flat. The timeline of your matter doesn't have to be the same as the timeline of other matter. You exist for whatever reason, and you go back in time (or rather loop back) then kill your grandfather, so you've altered their future. You'll still exist in your timeline, and why wouldn't you?

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