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Deductive Logic on the subject of Time Travel.

  1. Nov 20, 2003 #1
    When I say "Time Travel" in the title, I'm referring to traveling into the past. I just wanted to see if I could establish deductive validity for my assumption that it is not just impossible, but non-sensical, to travel backward in time.

    ("P" stands for "Proposition" and "C" stands for "Conclusion")

    Here we go:
    P1: I did not exist in 1776.
    P2: I exist now in a reality that includes P1 as being true.
    P3: I go back in time (using whatever means) to 1776.
    C: I did exist in 1776! ...

    But, then, I have invalidated P1 and P2, and even (by extension) P3, so my conclusion violates all the propositions...is this logically sound?

    Any and all helpful criticism is appreciated.
  2. jcsd
  3. Nov 20, 2003 #2


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    from c it can be concluded that you no more life in the present time therefor p1 isnt right anymore and so is p2, instead of them you can propse the premises which come from the conclusion (the first conclusion becomes a premise):
    p1:I exist in 1776
    p2: i dont exist in 2003.
    c: i must have gone back in time (-;
  4. Nov 21, 2003 #3
    Re: Re: Deductive Logic on the subject of Time Travel.

    I'm sure you already know that this doesn't follow, lqg. Ben Franklin existed in 1776, but he didn't go back in time.

    No, I seriously need a criticism of my reasoning on the matter, since it is supposed to be "possible" to travel back in time.

    BTW, I always "exist in the present time" (as you put it), since the very term "exist" is in the present tense. I don't think P1 can be invalidated.
  5. Nov 21, 2003 #4


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    Re: Re: Re: Deductive Logic on the subject of Time Travel.

    but the present time is changing for you when youre travelling in time, if you travell in time the present is ofcourse subjective the past becomes your present and the present becomes your future which you already know it (so you can say the future is forseeable).

    i think your reasoning is true the conclusion contradicts the premise.
  6. Nov 21, 2003 #5
    Re: Re: Re: Re: Deductive Logic on the subject of Time Travel.

    Yes, this is all true, and it is the reason why P2 is phrased as it is. I exist now, in a reality that is contingent with P1.

    Thanks for the support, I can't seem to find a flaw either...I hope someone else can.
  7. Nov 21, 2003 #6


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    We can circumvent this contradiction if we assume parallel universes (which is the standard way to get around most time travel paradoxes, such as the matricide paradox). If we assume parallel universes, then we get something like this:

    P1: I did not exist in 1776 in universe A.
    P2: I exist now in a reality that includes P1 as being true (although I think this one is a little redundant, if we are already assuming P1 to be true).
    P3: I go back in time (using whatever means) to 1776.
    C: It is logically inconsistent for me to still be in universe A; therefore I must be in a different universe, call it B, with a history/timeline very similar to A's.
  8. Nov 22, 2003 #7
    You guys ever heard of this theory ----> Present conditions affects past

    Sounds illogical isn't it? Its always a forward motion of ------> the present conditions affect the future.

    Your thoughts?
  9. Nov 22, 2003 #8
    Re: Re: Deductive Logic on the subject of Time Travel.

    Hmm...I suppose this could work, but wouldn't adding an extra Universe violate Occam's Razor?

    Also, Stephen Jay Gould (along with many others) is in favor of the idea (the name of which I've forgotten...temporary brain fart) that changing even the smallest thing in history can dramatically change the series of events that happen afterward. He believes that the course of history is occurance acting on the otherwise probabilistic/chance-governed nature of reality. I (currently) agree with him on this point, since the alternative seems like determinism.

    Oh yeah, the phrasing of P2 could seem redundant, I suppose, but I couldn't think of a better way to phrase it at the time.
  10. Nov 22, 2003 #9
    I guess it's not unscientific, but I do think it may be illogical...my first attempt at a proof:

    P1: I exist in the present (slightly redundant in itself, since "exist" is in the present tense).
    P2: P1 is true only because of the previous occurances which lead up to my having been born and living as I've lived.
    P3: Because of P1 and P2, any change in the past, caused by me, stops me from existing, which stops me from changing the past, which allows me to exist again...which will go on in circles.
    C: The idea of my changing the past is self-contradictory/paradoxical.
  11. Nov 24, 2003 #10
    I find time travel an indeed very stupid idea
    because, wouldn't your memory of what you have perceived
    now and in the past flow back to the
    surroundings through your five senses
    when you travel to the past?!
    So you would not even know that you had travelled
    back in time becasue you can't bring
    your present consciousness to the past.

    Sounds logical right?

    For example:
    Imagine that you are living in negative time zone
    (equivilant to travelling back in time)
    and you are gliding at a speed of 10m/s for -10s
    (because you are in negative space-time).

    So the distance travelled= (10m/s)*(-10s)= -100m
    Conclusion: If you are travelling back in time,
    everything that you did or possess would act
    in the reverse manner! (including your brain!)
  12. Nov 24, 2003 #11
    Ok, so you're talking about regressive backward time travel. Yeah, I agree, it is completely illogical.
  13. Nov 24, 2003 #12
    Isn't this where I came in, Mentat?

    If you went back in time to 1776 (P3) then that would invalidate P1. The rest would still be valid except the inclusion in P2 that P1 is true. It would also be true that in the present time of P2 the fact that you went back to 1776 (P3); (C) would already be in your history which would again make P1 invalid. P1 can never be true if you ever go back in time to 1776. P1 and P2 are true only if you never go back in time to 1776. It isn't a paradox as P1 and P3 are mutally exclusive. One or the other but not both can be true.
  14. Nov 24, 2003 #13
    Okay im only 15 but i question your idea. Because isn't your memory just chemicals in your brain which your brain reconize as memorys? if thats right when you travel back in time then those chemicals would be the same therefore your memory would be the same? right? Anyways maybe we shouldn't go back in time unless the idea of multiple (and i mean lots!) realitys or timeflow(brain just went dead) right? or we might do one of those weird paradox things where we **** up the universe:D
  15. Nov 24, 2003 #14


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    Re: Re: Re: Deductive Logic on the subject of Time Travel.

    Maybe, maybe not. From where we currently stand, it seems to violate Occam's Razor. But suppose one day we find that time travel into the past is possible. Well, if this occurs, we know that the traditional paradoxes that are associated with time travel must be false, otherwise we would have a logically inconsistent system (much like the one you created). At this point, then, it would be more reasonable to assume parallel universes to explain how time travel into the past is possible without raising all those paradoxes.

    That sounds more like he is describing history as chaotic, as opposed to non-deterministic. In any case, I agree that history probably works like that. No doubt that going back to 1776 would greatly change the future course of history. But the important idea is that that greatly changed future happens to another universe, and thus is not in contradiction with the history we have observed in our own universe.
  16. Nov 25, 2003 #15
    That line of thought ignores the fact that if some one were to go back in time and do something that would alter history and thus the present and future history, it would already be in our history that that had happened.
    Going back in time may or not be possible in one universe, I think not; but, changing past history is logically impossible. As I said, any change would already be part of our history and not a change at all. What has happened in the past happened and that would include someone going back and effecting what was then happening.
    Multiverses is another thing. If some one where to go back and do something that would start another universe in which his doing whatever he did would already be in its history. That doesn't make any sense at all to me; but then I don't believe in multi-universes.
    Here I agree with Mentat, that that would violate Occam's Razor. Not tha Occam's Razor is an inviolate Law; but, that it is not necessary, elegant or beautiful but messy, redundant and over complicated.
  17. Nov 25, 2003 #16


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    Well, I see what you are saying about the "already would have happened" part. I agree with that. Sometimes it is easier to say "change," however, to draw attention to how things in our respective universes are similar (or identical) up until the point when a time traveler from one enters the other at a certain point in time.

    But say someone traveled back to 1776. It would not have already happened in our universe, since in our universe A no one did go back to 1776. That happened in universe B.

    Then again, Occam's Razor is not really relevant here. We aren't talking about whether time travel is actually possible, only if it could be logically possible. Thus, it suffices to create a logically consistent scenario where time travel is logically possible. Whether that logical possibility is then a physical actuality is a question of empirical investigation, which is a separate issue.
  18. Nov 25, 2003 #17
    Then again "logically" speaking in a multiverse scenario, it would have already happened in B's history. We can't get around it. No matter which history or which unverse's past we go to, It had to have already happened, been recorded and experienced in that universe's history. It is either that or a split occurs in which one universe's past was not visited and changed and ther other was visited and changed but no one would know it because that universe's history would already contain and record the visit and "change" but for them it would not be a change in there history because their history already includes the "change". Only some supra spatial being outside of space and time would be able to see the change by noticing the split.
    Simply put, logic will not allow the future to effect any change in the past as it would have already been included in its history and thus is not a change. No way around it. I hate to be a bubble buster because I have always loved sci-fi time travel stories but logically it can't happen.
    We can't go back and save Lincoln or assassinate Hitler because if we do it would already be part of our history and it would be imperitive
    that we do go back and save or assassinate. This would make the universe's present determined by it past, eliminating free will.
    So instead of going back and killing your grandfather to change history all you have to do is not go back and save lincoln in a universe where you did go back and save lincoln; but in that universe you would not have that option or ability. No matter what you tried or tried not to do you would be forced to go back and save lincoln because that is already your history. And on and on and on ad nausium.

    Okay, I'll go along with that but then I have shown how it is logically impossible. Logic will not only not allow a paradox but it won't allow a messy, ugly, redundant multiverse either.
    It is not a Law of logic that can't be violated as a physical law but that if it were violated then it wouldn't be logical. Our universe has so far as we know is logical. It is cosmos not chaos.
    Last edited: Nov 25, 2003
  19. Nov 25, 2003 #18
    Re: Re: Deductive Logic on the subject of Time Travel.

    I don't understand what you are saying. If P3 is true, then both P1 and P2 also have to be true (since my existence is kind of required for "me" to travel back in time :wink:).

    Besides, my point is that I can establish P1 and P2 as being true (by scientific and other such proofs) but they would invalidate P3, and thus I can never travel back in time...does that mean that it is illogical to postulate time travel?
  20. Nov 25, 2003 #19
    Re: Re: Re: Re: Deductive Logic on the subject of Time Travel.

    "At that point it would be reasonable"? Does that mean that at this point it is not reasonable? Are the "extra" postulates really "extra", since there is no indication (now) that other Universes exist?

    Here's a question for you, with regard to that "extra Universe" idea: How can one ever get to another Universe? This is not an irrelevant question of possibility in practice, I'm talking about possibility in principle. If there is another Universe*, then there is nothing seperating that Universe from ours (not space, not time, not anything), and thus there is (logically) also nothing connecting us, which precludes our ever traveling there, doesn't it?

    *In these contexts, I will deign to refer to "Universe" in the common sense of what astronomers refer to as "local universes", and not in the "set of all that exists" sense.
  21. Nov 25, 2003 #20
    On the one hand, I agree with you that you are just constructing a possible-in-principle framework, in response to my request for all possibilities to be presented.

    On the other hand, the problem with Occam's Razor is a real one, in this case, since yours could (note: "could", not necessarily "will") become an ad hoc argument, since you would first add the postulate of an extra Universe. Then postulates of how (even in principle) a connection could be established between Universes, and so on.
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