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Why we can't go back .

  1. Mar 21, 2003 #1
    Why we can't "go back".

    I explained the paradox of backward time travel, in the old PFs, and I would like to do so again.

    Here is why it is impossible to travel backwards in time (and it doesn't matter how far backwards):

    As soon as you travel to a time that is before the exact time when you started traveling, you create paradox. If I start traveling at 5:00 A.M. (I know that using minutes, as a way of measuring exact time, is crude, but it should get the point across), and I travel back to 4:59, I have yet to start traveling. But, if I have yet to start traveling, how did I get to 4:59? The answer: I didn't, it's impossible. Unless someone can prove me wrong, it makes no sense to imply that I can start traveling, after having already arrived at my destination.
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  3. Mar 21, 2003 #2
    Some people have suggested the "Pretzel Time" idea - which basically states that if (in the future) I travel back in time, then my future was already going to be in the past. I disagree with this idea.

    I'm not going to get into a discussion of free will (that's for the Philosophy forum, and not really relevant to my argument).

    However, there is the question of whether there are an infinite amount of "Mentats" in any given place. Because, if not, then there is no way that I am involved in some form of "loop time traveling", as the "Pretzel Time" idea will imply.

    You see, if it is part of my future, to go back into the past, then it must be determined to be so. But I would thus, have to have been there, at that point in time. Then, in a universe where I was already there in 1987, I was about to be born the next year - and the "Mentat" that is born in 1988 will eventually travel back to my (the Mentat in 1987) time. We might, perhaps, meet each other, or maybe not, it's not really important; but both of us know that, soon, another one of us (the one that will be born next year) will pop up (of course, this is not true, the "third" Mentat would appear at exactly the same time as the other two - and so would the infinite "other" Mentats).

    If this point needs clarification, just ask. I realize that it is a bit of a strange concept.
  4. Mar 21, 2003 #3
    People have also drawn up the "Different T dimensions" argument. This argument basically states that there are many (possibly infinite) different time dimensions; and that a person could travel back in time, but would be traveling, not to that point in time in his/her original T dimension, but in another (identical) one.

    My side of this is that it is demonstrably true that every point on the T dimension, corresponds to every point on the 3 spacial dimensions that we know of. I would have to assume that this same reasoning applies to the rest of the spacial dimensions (but I can't know for sure). And, if this is true of all of the spacial dimensions, you cannot travel to a dimension that is "outside" of the one T dimension.
  5. Mar 21, 2003 #4
    Say an electron interacts with some photons and flips around in time at 5:00; to observers like you and me it looks like two different (forward-in-time-travelling) particles, an electron and positron, annihilating in a flash of photons. However, that positron was really only the electron travelling backwards through time (after the flipping around in time). At 4:59 we therefore see the electron at two different stages of its life, one in which it's going forward and one in which it's going backwards. Can there be no positron because at 4:59 it hasn't started travelling backwards through time yet? Of course not, positrons are very real.

    The analogy Feynman originally used to describe that whipping around in time for particles and their antiparticles was:

    Since Feynman's interpretation of antimatter has been out there for over 50 years and apparently works well enough and is consistent, I would think it as least shows that travelling backwards through time isn't blantantly paradoxical.
  6. Mar 21, 2003 #5


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    You don't get an infinite number of Mentats looking from a non-Mentat perspective:...

    In 1987, a fully grown Mentat appears in the universe.

    In 1988, a baby Mentat was born.

    In {future date}, the younger Mentat (which is the same age and has the same knowledge as the older Mentat did when he appeared) vanishes from the universe.

    This "timeline" is perfectly consistent with you (instantly) travelling back in time, but yields no more than 2 Mentats at any instant.

  7. Mar 21, 2003 #6
    First off, that means that it's still impossible for me to travel back in time; because, even if I had an anti-Mentat, he would travel back in time, not me.

    I will continue this response later, I have to go now...
  8. Mar 21, 2003 #7
    First off, "he" would be you.

    Second, I thought you were arguing that time travel into the past in general is paradoxical and impossible. Hence:

    Was I incorrect in that?
  9. Mar 22, 2003 #8
    Alright, to complete my response...

    I would like to point out that Relativity states that an object's movement through time is inversely proportional to it's motion through space. Thus, the positron would have to be going much faster than the speed of light, and this isn't possible.
  10. Mar 22, 2003 #9
    Let's imagine that I was born in 1988, and then later traveled back to 1987 (because that was what I was predestined to do - my time was "looped"). Now the universe has two of me - as of 1988, that is - and the one that was born on 1988 (in a universe where there are two of that same person) travels backward...

    Now, I'm not sure (anymore) but it seems as though this Mentat (the one that comes from a universe of two Mentats) should meet the other Mentat (that appeared in 1987), and so now the universe has three Mentats, and so on...
  11. Mar 22, 2003 #10
    Hmmm, I know Feynman gave lecture called "The reason for antiparticles" in the '80s (they have it at Amazon and I'm considering buying it) in which he demonstrates:

    "If we insist that particles can only have positive energies, then you cannot avoid propagation outside the light cone. If we look at such propagation from a different frame, the particle is traveling backwards in time: it is an antiparticle. One man’s virtual particle is another man’s virtual antiparticle.”

    So, as I said above, apparently his ideas work very well. And time travel looks possible (if only at the quantum level).
  12. Mar 22, 2003 #11
    You admit that both Mentats are the same person but you're not seeing what that means: the Mentat that exists in the past beside his younger self is the time traveler that the younger one will grow into. So you won't meet a third one unless you travel back to 1987 a second time. It seems to me that the number of one way time trips n creates the possibility of only n+1 Mentats existing in one slice of time (though of course it needn't be that many; you could go back to ancient Greece and never run into another version of yourself).
  13. Mar 23, 2003 #12
    Alright, I guess I withdraw the refutation of the Pretzel time idea. I was wrong.

    However, Zefram, are you saying that Feynman ignores Special Relativity? That's a pretty good indication that he's wrong.
  14. Mar 23, 2003 #13
    Or that special relativity is wrong...
  15. Mar 23, 2003 #14
    You've got to be kidding.
  16. Mar 24, 2003 #15


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    I don't see how there would be a third Mentat and so on. It's possible for the time-travelling Mentat to meet the new-born Mentat in 1988. But that's it. Where does the third Mentat come from? The new-born Mentat will eventually grow-up and travel back in time to 1987. When he travels back in time one Mentat will exit from the loop while another one will be introduced in 1987. So it seems there are only 2 Mentats in the loop.
  17. Mar 24, 2003 #16


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    Re: Why we can't "go back".

    Not sure if I understand why it's a paradox. If you travel back to 4:59, wouldn't you meet the other you who is about to embark on a time-travel?
  18. Mar 24, 2003 #17
    Re: Re: Why we can't "go back".

    How many of you are there, les? If, by traveling back ward in time, you can create as many of you as possible, what's the point of cloning :wink:?

    Seriously, if I go back in time, without chaning my position in space, I could not see the "other me" (even if such a thing could exist), because we would be occupying the same space.
  19. Mar 24, 2003 #18
    Why do we assume that "the past" is a place that exists?

    While I wish I had some math to support my hypothesis, I simply do not see any evidence that "the past" is a place that exists. And it follows, of course, that travel to a place that does not exists is impossible.

    I firmly believe that time is an effect of expansion. Time is what appears to happen after one universe is replaced by the next. A single frame of a movie film has no "time". Time is only perceived when you look at successive frames of the film, one right after the other. It is not time that exists. It is the ability for our universe to allow for motion and/or action that is real. Time is an artifact of this process.
  20. Mar 24, 2003 #19


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    Feynman was a strong proponent of relativity, having been involved in the creation of relativistic quantum mechanics. I think this is what he got the Nobel for.

    Anyway, he certainly demonstrated that our notion of time travel needs re-thinking. As I see it, time travel - at the quantum level - is not just possible, it is reality.
  21. Mar 25, 2003 #20
    Why restrict the arrow of causality to past-->future? As stated earlier, quantum processes are time symmetric, that is, they can proceed both ways. Some macroscopic entropic phenomena have localized anentropy (Prigogine), suggesting there an effect analogous to time reversal. Hawking claims that a "big crunch" following the big bang would maintain time direction, but if the universe is indeed isolated, given enough time quantum interactions will influence a repetition of its overall temporal history or overtake macroscopic physics as a whole.

    The topology of spacetime, however, by its very nature may forbid the paradoxical interchange of past and future. Perhaps the present is a singularity that disallows relativity (except for "fuzzy" quantized spacetime) continuity of travel back in time along the time line.
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