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Time travel into the past is logically possible

  1. Sep 9, 2003 #1
    Time travel into the past is logically possible, provided that your presence in the past only causes things to happen that have, in fact, already happened. In other words: if time travel is restricted to only one universe (as in general relativity), then there is only one version of the universe that actually happened -- it's never the case that one thing happens in the first "draft" and another in the second "draft".

    So, it's not possible to go back in time and kill your grandfather, thereby causing yourself not to have been born. If you try this, you will be stopped by freak accidents, such as tripping on a banana peel.

    In other words: if you look at all initial conditions, and time-evolve them forward, then the vast majority will lead to inconsistencies (blatant ones as in the grandfather case, or subtler ones such as a blade of grass being in the wrong place). The only ones left with time travel involve amazing coincidences.

    My question to all of you is: does this make time travel extremely improbable? The paper Bananas Enough for Time Travel argues it doesn't, but I'm not sure I'm convinced. I think to solve this question, you would have to know how to assign probabilities to various boundary conditions in General Relativity, which I don't.

    So, are there bananas enough for time travel?
    Last edited by a moderator: Feb 5, 2013
  2. jcsd
  3. Sep 9, 2003 #2
    Re: Bananas

    Wasn't this "fulfilling of the past" idea originally concieved by Kurt Godel? I have two problems with such a concept:

    1) It requires predestination.

    2) It requires backward time travel (obviously), and there is logical flaw with traveling to any time before the time you start traveling.
  4. Sep 9, 2003 #3
    Re: Re: Bananas

    What comes out of a time machine depends on events that will happen in the future. In that sense, the future already has to be "fixed" (though not by anything outside the universe). I don't see this as a problem, since this is true in general relativity (or for that matter, Newtonian mechanics), or at least the obvious way to look at the theory (block universe, spacetime manifold, and so on).

  5. Sep 9, 2003 #4
    Re: Re: Re: Bananas

    Yes, my problem with the predestination thing is mainly just personal preference.

    In my thread, "Why we can't 'go back'", I used the following circumstance to explain what I believe is a logical flaw in the concept of backward time travel (please note: in my analogy, the minute is the smallest increment of time. This is for the purpose of convenience, and says nothing about the analogy's practicality in actual time) :

    Let's say that the time traveller get's into his time machine at 5:00 P.M. He then pushes the button (or pulls the lever, or however he starts the thing ) at 5:01 P.M. and the time machine starts to go backward in time. Well, now, the time before 5:01 is 5:00, so this should be the first point in time that he reaches, but here is where the problem begins. At 5:00, he was not traveling - he was still "strapping himself into" the time machine. So, how is it that he is now traveling through time, at a point in time before he ever began travelling in the first place?

    IOW, he would have to be traveling before he ever starts traveling. Not really logical is it?
  6. Sep 11, 2003 #5
    You are assuming that the time traveler is staying in our 3D plane. If our 'experienced' universe exists as a 3D hyperplane within four spatial dimensions, then moving our time traveler along that 4'th dimension axis, before traveler start backwards on the time axis, then back (into our 3D hyperplane) at the termination of the journey will circumvent your logical problems.

    This does assume a 4th spatial dimension. When making a logical argument, it's good for you to make your assumptions explicit.
    Last edited: Sep 11, 2003
  7. Sep 11, 2003 #6


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    Likewise if a traveler procedes FTL from his base to a remote station moving away from the base at high sublight velocity, comes to rest at that station, and then returns FTL to his base, he comes back to a date in his original time line earlier than his departure, and depending on the distances and speeds, maybe quite a bit earlier. But he does not need to traverse his original timeline backwards to do so.
  8. Sep 11, 2003 #7
    I'm not that familiar with the implications of Einsteins relativity equations. That said, the last time I played with them it seemed you got a negative it (along an imaginary time axis), not just a negative t. If this is the case, doesn't it imply that FTL travel forces a negative delta on a time axis that's orthoganol to our normal time axis?
  9. Sep 13, 2003 #8
    Not at all. No spacial dimension is free of time. The two-dimensional creature is as much "under the laws of time" as the ten-dimensional one.
  10. Sep 13, 2003 #9
    But this is not logical. If a person arrives at a time that is before the time that he left, then he is not arriving at all, since he's never left... .

    Help, please.
  11. Sep 14, 2003 #10
    selfAdjoint is right:

    http://sheol.org/throopw/tachyon-pistols.html [Broken]

    This does lead to inconsistencies, so again this will either not be possible or there will be banana peels preventing people from traveling FTL in different reference frames.
    Last edited by a moderator: May 1, 2017
  12. Sep 14, 2003 #11
    The author of that illustration (about the tachyon pistols) missed one very valid point: B wouldn't just be outraged that A fired before 8 seconds were up, but also that he (B) was hit in the back before A ever fired (at least, that is how it happened in B's reference frame). At least, that's how it seems to me (please tell me if I'm wrong).
    Last edited by a moderator: May 1, 2017
  13. Sep 15, 2003 #12
    If 'No spatial dimension is free of time" then that implies time isn't orthoganol to the spatial dimensional axes, therefore time isn't a dimensional axis. Since it is defined that way, I don't agree. If you imply that all spatial dimensions are not outside of time, then you are correct, but neither is time outside of any of the other spatial dimensions - that is the definition of a dimensional space. Theoretically (and this is only considering the math, not real physics) translation can occur within any of the dimensions and not any or all of the others - each dimensional axis is orthoganol to all others.

    It is possible to have two time axes (theoretically), such that change along one time axis is unchanged yet continues along another. It seems that the relativity formulas imply such a dimensional axis at velocity's higher than c.
    Last edited: Sep 15, 2003
  14. Sep 15, 2003 #13
    Irrelevant to my argument. It was stated that "going back in time was impossible, because the time traveler would run into 'things' in the real environment" (paraphrased). I solved this by moving the traveler outside the three spatial dimensional plane containing these 'things' which would be run into. I didn't take the traveler outside of the time axis - I couldn't since he is required to travel 'backwards' along such an axis if he is to appear prior to when he left.
    Last edited: Sep 15, 2003
  15. Sep 15, 2003 #14
    Though it certainly may have causal implications, I fail to see your reason(s) for saying it's illogical. The person departed at one point in time, and arrived at a different point in time. This happens all the time, with the caveat that it is usually the case that the arrival point is after the departure point.
    Last edited: Sep 15, 2003
  16. Sep 15, 2003 #15


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    At 4 seconds in B's proper time, he is hit, turrns instantly and looks at A. What he sees is A at A's 2 seconds, according to the Lorentz transform. A is there pointing the gun at him because in A's frame, actually 8 seconds have passed. A's action is time shifted in B's frame. And this is real, not an illusion.

    But the idea you have is a good one. By a little rearrangemnt and the kind offices of a friend, B can feel himself shot by A before the duel ever starts.
  17. Sep 15, 2003 #16
    Not at all, in fact it implies quite the opposite. If there were a spacial dimension that was free of time then it would not be orthogonal. That's why I disagreed that one could leave the 3 spacial dimensions that we are used to and thus travel back in time, since there is no spacial dimension that is free of time.

    But that's what I said.
  18. Sep 15, 2003 #17
    Ah, I see. I guess I hadn't sen the objection that "he'd run into things", though I disagree that this would happen, even in our own dimensions (since nothing that used to exist in the past still exists now, it's the past, everything has "moved on").
  19. Sep 15, 2003 #18
    Then apparently you haven't tried to conceptualize it. Let me help you out (again, in my illustrations, the minute is the smallest incrimant of time).

    If I tell someone, at 4:30, that I plan to leave for Detroit, Michigan at 5:00, and then call them at 4:31 to tell them that I have arrived (even as I stand right beside them still, since it is not yet 5:00, and so I will not yet depart), a great infraction of logic appears to have occured, doesn't it?
  20. Sep 16, 2003 #19
    You are mis-stating my argument. I didn't say anything about leaving 3 dimensions, only leaving (our) three dimensional hyperplane. In a five dimensional space (four spatial and one time) our three dimensional universe (that we percieve) would be a three dimensional hyperplane, which could be left, just as a two dimensional plane can be left via motion along the (perpendicular) third dimension.

    That is not how I interpreted your statement.

    As I reiterate, I was responding, with a plausible scenerio, regarding colliding with objects in our three dimensional hyperplane when attempting travel back along the time axis.
  21. Sep 16, 2003 #20
    I have been considering the implications of time travel for forty years. I will override my initial impression and assume you are not attempting to be condescending. I would kindly ask you to avoid the appearance of such condescension in the future.

    A lot of very intelligent writers and scientists have considered time travel and have not come to the same conclusions you have. Asimov, Cramer, and Hawkings are few that come to mind. It's funny that I haven't seen any of them mentioning the illogical aspects that you have. Causality and paradox problems, yes, logical contradictions in the travel itself, no. Considering they have not come to the conclusions you have, perhaps some respect for this side of the argument is warranted, at least in any apparent emotional content.

    Your example may seem a violation of common sense, but isn't a violation of logic.

    You seem to imply that having two of you in existence, at the same point in time is illogical. It isn't when you consider that the two of you in existense would not be at the same point in three space, nor the same point in experiential time - the time-traveler would be older.

    This may be outside our normal experience, true, but not illogical. You have failed to show even one violation of logic in time travel, except thru unspoken assumptions.

    Obviously, the time traveler's experiential time doesn't follow the standard time axis, otherwise time travel becomes meaningless and the universe can be seen as simply a four dimensional object (3D spatial+ 1D time), with our experience being the only thing giving the arrow of time it's direction. True time travel seems to imply a second time axis.

    If you wish to show that time travel is illogical, then giving common sense examples will most likely fail you, given time travel is outside common experience, and not within typical 'common sense'.
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