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Grandfather Paradox

  1. Aug 19, 2010 #1
    Just an amateur (and hardly even that)... so take it easy on me... but I have some questions about the Grandfather Paradox. Assuming time travel to your past were otherwise possible, doesn't that imply that each moment in time is its own unique physical state existing simultaneously? Presumably each moment, and therefore each physical state is linked. That would be by cause and effect, right? If I travel back in time and knock off my grandfather, the ultimate effect is that I am never born to knock off my grandfather. But wouldn't that change have to percelate through the infinite number of moments in time via the same chain of cause and effect linking the physical states? In which case, it would never catch up to me because I am still moving forward at the same pace, right?
  2. jcsd
  3. Aug 19, 2010 #2
    well, assuming you could travel through time, that would mean time is not a one-way line, which means, just as you did, the effect of killing your grandfather would fallow the physical links (as though it where a shockwave) back to you. Im not sure if I explained this enough, but I am convinced the universe would then be doomed to an enertiny of repeating history between the moment of your grandfathers death and the moment you traveled back in time
  4. Aug 19, 2010 #3
    But I am constantly moving forward even as I go back in time, right? So isn't possible that the ripple simply never catches me?

    Using a clock ticking off the seconds... if I am at second 30 when I go back to second 1 and kill my grandfather, I am still moving ahead on the clock as I exist.. second 31, 32, 33.. etc. The moment I kill my grandfather, the ripple starts moving through the seconds, but it is only going as fast as I am. When I am at second 60, the ripple is at second 31. It never catches my physical state.
  5. Aug 19, 2010 #4
    The moment you kill your grandfather, the entire timeline of the future changes in less than an instant. I say less because that time technicly never existed, so it couldnt have changed to begin with. so since the wave is infinitly fast, it would exist in all times after the moment your grandfather died. I only see your idea plausible if you kill him WHILE moving backwards in time, and not stopping untill you get so far back, you couldnt possibly live long enough to be in the time of his death. though this would mean you couldnt have kids, or change anything in the world, as it would be undone the moment your grandpa dies.
  6. Aug 19, 2010 #5
    The act of killing your grandfather in the past would cause the universe to split into another universe. Meaning you didn't come from that specific universe in the future, you came from another universe/multiverse.

    In the universe you came from, specific events lead to other specific things. Pretty much the act of traveling back in time in the first place would instantly cause the universe to split because something happen in your universe that wasn't supposed to happen, resulting in another universe.

    Though this is based off http://en.wikipedia.org/wiki/Many-worlds_interpretation
  7. Aug 19, 2010 #6
    so the question is, which universe will you live in? is killing your grandfather a way to travel between universes or will you descover what its like to have your atoms torn apart into incredibly small pieces so that it can fit through the fabric of space?
  8. Aug 19, 2010 #7
    That depends on which theory of the universe(s) you hold to be true.

    If you hold that there is only one universe, that by going back in time and killing your grandfather, then that is indeed impossible, as you cannot accomplish something if you never existed to accomplish it.

    On the other hand, if you hold to the many-universese theory, then at each juncture of decision, a new and now separate universe unfolds, then at the moment you went back in time, your counterpart in another universe decided not to and two universes were born from the original - one in which you went back, and one in which you didn't. Furthermore, the moment you killed your grandfather, yet another universe was split off. In fact, it's not a many-universe theory, but a multi-verse, as per http://www.imdb.com/title/tt0267804/" [Broken]. It's a good flick, if you haven't seen it, yet.

    Then there's an intermediate theory, that of several universes, but only with splits as generated by cataclysmic, spacetime-ripping events, such as going back and killing one's own grandfather. And this intermediate theory allows for reintegration of the two universes, provided no paradoxes occur. The split might only cover the paradox, and may only last for a moment, jumping over the paradox.

    All of this is conjecture, of course, as we have no direct evidence, of course, for a multiverse.
    Last edited by a moderator: May 4, 2017
  9. Aug 19, 2010 #8
    Putting aside the multi-verse theory and focusing on a single universe for this issue (or maybe the way I am thinking implies multiple universes, but from the standpoint of time)... I guess my question really comes down to what it means to be able to travel back in time. If I go into the past, what is the physical state? It's not like I am rewinding all of the particles in the universe to some previous existence and the present that I came from no longer exists, right? That would seem to flip cause and effect around, at the very least. It would have to be that the past exists simultaneously with the present, perhaps through an infinite string of moments, and I somehow jump to it. It is a separate physical state though. Now if this is correct, then the physical states would seem to be linked by cause and effect. Anything I do in the past would have to travel the cause and effect links like everything else. But since I am still moving forward, isn't it possible that whatever I do in the past never catches up to me?
  10. Aug 19, 2010 #9
    I think the paradox is over exaggerated. The atoms in your body dont 'remember' that they were born from somebody. I think you could kill your grandfather and then go on living in the past. So what if you never got born in the new timeline, the atoms in your body dont know or care about that.
  11. Aug 19, 2010 #10
    This argument can't really be settled because all of the theories involving the nature of the universe including Single Universe and Multi-Verse Theory are only theories. We can't really test them until we have a Time-Travel device.... assuming a Time-Travel device is actually possible. There's far too much unknown about the nature of the universe in terms of what we don't observe.
  12. Aug 19, 2010 #11
    True.. but that's what makes arguing about it so much fun, right?
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