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Causal paradoxes - how to solve them?

  1. Chronology protection

    3 vote(s)
    25.0%
  2. Self-consistency

    4 vote(s)
    33.3%
  3. Parallel universes

    2 vote(s)
    16.7%
  4. Other

    3 vote(s)
    25.0%
  1. Sep 12, 2007 #1

    Demystifier

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    Under certain conditions, the theory of relativity suggests that closed causal loops can be formed, which seems to allow the causal paradoxes (like killing your grandfather and so). Namely, under certain conditions general relativity allows nontrivial spacetime topologies (like wormholes), while special relativity under certain conditions allows superluminal signaling (e.g. by tachyons). There are basically 3 types of solutions of such paradoxes:

    1. Chronology protection:
    The true laws of physics conspire so that such exotic topologies and superluminal velocities are not really possible. The problem is to explain it with the presently known physical laws.

    2. Self-consistency:
    Only self-consistent solutions are physical. It is related to the block-time view of the universe, in which time does not "lapse", but simply is, just like space. It is very much in spirit with the relativistic view of time. The problem is that it confronts with the subjective experience of time and the notion of "free will".

    3. Parallel universes:
    Seems too speculative and unscientific, but it seems that many people still like it.

    So, what is your (favored) choice?
     
  2. jcsd
  3. Sep 12, 2007 #2

    George Jones

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    I gave a little nore background in post #8 of this thread.

    I hope no stong gust of wind comes along, as it might cause me to fall off the fence.
     
  4. Sep 12, 2007 #3
    It doesn't matter much, since nobody can experience a time travel.

    Say, you time travel to London year 1777. Well, there was no You in the real 1777 London, so even if you'd somehow get yourself to a place that seemingly is 1777 London, it really is not.
     
  5. Sep 12, 2007 #4
    Well I disagree. I voted for self-consistency, which would claim that you could 'go back' to London 1777 only if you were already in London in 1777. You may think that that's impossible since you weren't even born yet, but so long as you are able to get there then you were there already.

    I was actually discussing the idea of self-consistency and free will with my roommate the other day (he was a religion and psych major as an undergrad). He claimed that this does ruin the idea of free will. I think it doesn't -- it's not that you would be forced to do what you already did if you travel back in time so much as you've already made those decisions. In that small time frame, you did exercise free will, so revisiting that time frame simply reveals to you the decisions you've already made. If you can't do something it's not due to a lack of free will, but instead due to what you've already done with your free will.

    If I rephrase that one more time it will only make less sense than it already does.
     
  6. Sep 12, 2007 #5

    JesseM

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    How do you know there was no you in the real 1777 London? If the self-consistency option is correct, then any trip you made back in time was part of history all along, even before you decided to make it.

    I think all three options are logically consistent, but realistically I'd have to go with chronology protection, from what I've read when physicists try to incorporate quantum physics into general relativity's time travel scenarios they always tend to find plausible reasons that time travel would become impossible...look at this article on analyses of time travel from string theory, for example.
     
  7. Sep 12, 2007 #6
    avoiding paradoxes

     
  8. Sep 12, 2007 #7

    JesseM

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    There are no problems with causality in SR unless you allow for the possibility of faster-than-light particles or "tachyons". But in GR there are various solutions which seem to allow for time travel into the past (at least if you ignore quantum effects and allow for violations of the weak and null energy conditions), like traversable wormholes.
     
  9. Sep 13, 2007 #8

    Demystifier

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    It is not that simple. In some cases, if you start with some arbitrarily chosen initial condition (initial "free will"), then further deterministic evolution necessarily breaks self-consistency. So, you are not free to choose the initial condition even the "first" time you attempt to do so. Nevertheless, in my opinion this is still not in contradiction with free will. It only means that free will, if exists, does not work in terms of initial conditions, but in a nonlocal way as a choice of the global self-consistent solution. (I would like to know what your religious roommate would think about such a modified notion of free will.)
     
  10. Sep 13, 2007 #9

    pervect

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    One possibility that has been discussed before is the possibility that GR is actually just some "funky fields on a Minkowski space-time. Since there would be an underlying trivial topology, I believe there wouldn't be any time travel in this scenario.

    I gather there are some consistency problems with this idea, though, in spite of the fact that it appears to work locally. I'm not sure I understand the arguments completely, in thinking about the issue I ask questions like "Is the magnitude of these fields finite or infinite at the event horizon of a blackhole".
     
  11. Sep 13, 2007 #10
    None of the above. Rather, I give up the postulate that space-time should be modelled as a
    semi-Riemannian manifold and adopt a model of space-time where closed causal loops are impossible
    by construction. Yes, this is radical, and not compatible with GR.
     
  12. Sep 13, 2007 #11
    Well I'm in the "Other" Category as well. However, I do not favor Space-Time as a valid interpretation of Special Relativity.

    I prefer to apply SR to a more Classical expectation of an Absolute Space along with an Absolute Time to avoid causal paradoxes. Not only would this mean GR & QM are incomplete, but our current Classical assumptions of how we observe both Space and Time are likely not complete either. Do not know if that makes me more traditional or radical, certainly not a typical ‘Local Realist’.
    RB
     
  13. Sep 14, 2007 #12
    Hmmm, well I will first have to make sure I understand this myself :tongue:

    By the way, I didn't quite say that my roommate was religious, just that he was a religion major in college. However, both he and I are in fact practicers of and believers in religions.

    Also, do we know how you voted?
     
  14. Sep 14, 2007 #13

    Demystifier

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    I voted for self-consistency. In fact, I have presented by arguments in more detail here
    http://xxx.lanl.gov/abs/gr-qc/0403121 (Found.Phys.Lett. 19 (2006) 259-267)
     
  15. Sep 14, 2007 #14

    Demystifier

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    I like this possibility. Still, there remains a possibility of superluminal signals.
     
  16. Sep 14, 2007 #15

    Demystifier

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    I like this view. But the problem then is to explain why our current fundamental theories are invariant under relativistic transformations of space and time coordinates.
     
  17. Sep 14, 2007 #16

    Demystifier

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    Well, this is actually a version of 1. Chronology protection.
     
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