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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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  • Caution about ArXiv (Score:5, Informative)

    by vsage3 ( 718267 ) on Sunday July 25, 2010 @02:46PM (#33022482)
    I have neither the capacity nor the will to vet the paper, but it should be noted that ArXiv is not peer reviewed. While experimentalists use it as a place to publish pre-prints of their papers and will typically only put them up after the papers have been accepted, but theorists use the medium as a substitute for publishing and so many wacky and untrue claims get put up there.
  • Re:Just like magic (Score:1, Informative)

    by Anonymous Coward on Sunday July 25, 2010 @02:50PM (#33022516)

    If you see it without comments it'll be red.

  • Quantum Leap (Score:2, Informative)

    by thetoadwarrior ( 1268702 ) on Sunday July 25, 2010 @03:54PM (#33022966) Homepage
    Clearly no one has watched Quantum Leap. You can only time travel within your own life. Time travel is so far off that we won't see anyone traveling back in time yet.
  • Re:Huh ? (Score:3, Informative)

    by CODiNE ( 27417 ) on Sunday July 25, 2010 @03:55PM (#33022978) Homepage

    Grandfather paradox: Go back in time, shoot your grandfather... now your father wasn't born, you weren't born. Which means your grandfather doesn't get shot, so you get to be born, etc... the universe flip-flops your time travel forever, the record is skipping for eternity.

    I personally like Primer and it's time machine concept. You could go back to yesterday and kill yourself, that version of you would die but you would not disappear. Also if you kept the yesterday you from going in the time machine you now have 2 of yourself that you're stuck with. No paradoxes, it runs more a computer program or a flowchart.

    Grandfather paradox requires a sort of feedback loop in time where the universe seemingly gets stuck in a cycle until things resolve themselves.

  • Re:the other angle (Score:2, Informative)

    by maxwell demon ( 590494 ) on Sunday July 25, 2010 @04:00PM (#33023002) Journal

    Neither. Assuming he's right (and the Technology Review article correctly reproduced his claims; I haven't actually read the arXive article yet), he proved that you cannot have non-linear quantum mechanics without time travel. Given that some people try to resolve the measurement problem by adding nonlinearities, that's certainly an interesting result.

  • by Anonymous Coward on Sunday July 25, 2010 @04:17PM (#33023116)

    "...many wacky and untrue claims get put up there."

    This is demonstrably untrue. Since the mid-90s ArXiv has been the standard way in which theorists communicate their papers to a wider audience. Although you're technically correct that it's not peer reviewed in the traditional sense, it does have quite a strict authentication and author endorsement procedure that filters out 95% of the garbage that would appear there if it were open to all. In addition, reading a paper's abstract and looking at the names and institutions of the authors is enough to determine whether a paper is obvious garbage. Reading it and going through the arguments is then enough to determine its value conclusively. The paper in question passes all of those initial tests easily.

    Really, ArXiv works astonishingly well and is an excellent resource.

  • by maxwell demon ( 590494 ) on Sunday July 25, 2010 @04:30PM (#33023216) Journal

    I can accept FTL travel as maybe possible, but time travel seems farfetched to me.

    You can't have one without the other.

  • by maxwell demon ( 590494 ) on Sunday July 25, 2010 @05:21PM (#33023558) Journal

    Citation needed.

    David Deutsch, Quantum mechanics near closed timelike lines, Phys. Rev. D 44, 3197–3217 (1991)

  • by Dwonis ( 52652 ) * on Sunday July 25, 2010 @05:44PM (#33023684)

    And if the past and future already exist, what the hell does energy even mean? No, I say, fuck all that.

    I'm pretty sure that quantum mechanics experiments have taught us that the universe doesn't care what you fuck.

  • by Anonymous Coward on Sunday July 25, 2010 @05:49PM (#33023722)

    Am I the only person who has noticed the Authors name?

    Seth Lloyd = Sith Lord

    I think we should be very cautious of these findings.

    OP was wrong, wasn't Seth Lloyd, was Christopher Lloyd. Obviously.

  • by John Hasler ( 414242 ) on Sunday July 25, 2010 @08:47PM (#33024706) Homepage

    > Special relativity is known to be wrong.

    [Citation needed]

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

    by OrangeCatholic ( 1495411 ) on Sunday July 25, 2010 @08:52PM (#33024732)

    Thanks, I've actually read that page before. The traincar example is excellent.

    But there's no example of observer 1 seeing A happen before B, and observer 2 seeing B happen before A. They say it can happen, and there's a nice colorful diagram that makes no sense, but they don't explain it.

    Let's try this: Let's say all stars die at exactly 6b years. First we would see our own supernova, then 6b years later, we would see the supernova of a star 6b lightyears away. Conversely, people at that star would see theirs first and ours second.

    But so what? That's not time inverting itself, that's a lag in transmission. People have known about lag since they used horses to deliver the mail.

  • by JohnFluxx ( 413620 ) on Sunday July 25, 2010 @09:15PM (#33024824)

    There seem to be a few misconceptions from the people that have replied.

    If we take "FTL" to strictly mean travelling faster than light travelling nearby, then in any macroscopic ordinary sense special relativity doesn't let you do this. There are a few exceptions - for example if the curvature of space is such that it can no longer be considered flat, such as near extreme black holes. Or for extremely short periods of time.

    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.

    Scifi tends to call travelling in "hyperspace" as FTL despite at no point actually travelling faster than light.

    This sort of thing is acceptable in SR, as long as whatever method use always uses a fixed frame of reference (e.g. you can't move hyperspace itself and you have an upper speed limit in hyperspace, or that you can only teleport instaneously from the point of view of the microwave background radiation etc)

Civilization, as we know it, will end sometime this evening. See SYSNOTE tomorrow for more information.