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Re: time-space Why the paradoxes of time? (CONFUSED)

  1. Feb 20, 2006 #1
    I think this might be a very simple question, but I found no info on the net or in my chicken head...

    I've read M Kaku and Hawkings, and they write about a lot of paradoxes if time travel was possible (killing your parents, rewriting history, etc etc)

    I might be overlooking something, but if we travel thru time, there would be NO earth.. the earth has moved throughout space, AND throughout time... So if we travel back iin time, there won't be ANY Earth...

    My picture is this:
    If Mark and Lisa have a date at 12:00 at the park, but Marks gets stucked in traffic, and gets there 6 hours left, the very impatient Lisa would have already left.. She is ~somewhere~ else..

    Likewise if we travel back in time, everything would be ~sometime~ else, even if we're in the same "place", the Earth is at a different "time"...

    Thanks for the help.
  2. jcsd
  3. Feb 20, 2006 #2
    I think that in reaching this conclusion you're assuming some sort of 'universal time,' or at least a universal simultenaity.

    Picture a 3d box, the vertical axis being time. Let's align the box such that the earth is along the central vertical of the box, if I let time move forward I will see the earth move up towards the top if the box.

    If I understand you correctly, you're saying that if someone who was on the earth when it was near the top of the box were to time travel back to the bottom of the box they would find that there is no earth, since the earth is now at the top of the box.

    It's a nice idea, I like it, but I don't think it'll work. In relativity we are mostly concerned with what are called events, an example of an event would be a light pulse going off or an object reaching a particular point. We can think of the earth as being a whole series of events, every moment in time there is an event of "the earth is here", so that if we look at the whole collection of events the earth sweeps out a vertical tube in our box. So I think you could actually think of the earth as being a sort of four dimensional tube structure. At any given time when you look at the earth it's spherical, but that's because you're only looking at a three dimensional cross section of its true four dimensional structure.
  4. Feb 20, 2006 #3


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    This seems really confusing--why would the earth move forward in time if you move back in time? I think he was just talking about the fact that the earth is moving around the sun in its orbit, so if you assume some sort of absolute space and say that time travel means travelling through time without travelling through space, then the earth would have been in a different position in absolute space in the past.

    There are a few assumptions here that would be wrong according to modern physics. The first is the assumption of absolute space, the idea that there's a single point in space in the past that's at "the same place" as where I am in the present--in reality, different reference frames have different definitions of what the same point in space is at different times. For example, if I'm driving by you at 50 mph, in your rest frame you'd say that an hour ago the car was 50 miles behind the point in space it is now, but in my rest frame I'd say that an hour ago the car was in exactly the same position it is now, since in my rest frame the car is standing still while you are the one moving at 50 mph. So there's no single position in the past that's at "the same position I am now", this depends on which reference frame you use, and all reference frames are equally good according to modern physics.

    The other wrong assumption is that time travel would just involve disappearing from one time and reappearing in another, the way it usually works in science fiction movies like Back to the Future or the Terminator. But all the proposals for how time travel might be possible according to general relativity involve continuous paths with no sudden breaks, like travelling through a wormhole in such a way that you can enter one mouth in the present and exit the other mouth in the past. Obviously in this case, where you exit is simply a matter of where the two wormhole mouths are placed, if both are travelling along with the earth than you can enter and exit next to the earth. But in any case, even if you travelled to the past and found yourself far from the earth in space, as long as your distance from the earth in light-years was smaller than the amount of time in years you had travelled into the past, there would always be the potential to race back to earth in a rocket and arrive before you departed, so the potential for "grandfather paradox" type situations would still be there.
  5. Feb 20, 2006 #4
    Didn't occur to me as being a problem, I guess I figure that if you've mastered moving through time then moving through space as well should pose no great difficulty.

    Edit: BTW, in my example the earth didn't move forward in time when I moved back, it just stayed where it was.
  6. Feb 20, 2006 #5


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    Oh, I missed the fact that the earth was already at the top of the box when you left--that makes more sense, maybe that is actually what jose was talking about.
  7. Feb 20, 2006 #6
    Actually my doubt is related with what dicerandom said, his analogy of the box is better than the park thingy...:P

    Thanks for the explanation, however, I'm afraid that I still don't have the picture clear, even if the Earth is 4D... a 3D tube... shouldn't it move thru the "box"...
  8. Feb 20, 2006 #7


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    There's no movement in 4D, everything is just frozen in place. As an analogy, imagine you had some pieces of string frozen in a block of ice, and then you sliced the block into a series of thin horizontal cross-sections, took a picture of each one, and used the pictures as frames in a movie. When you played the movie the cross-sections of the strings might appear to be moving around, but this would just be because the pieces of string were not all straight and vertical in the ice, so the cross-sections of a given string would be in different positions in different horizontal slices. Now just imagine the block of ice as spacetime, with time as the vertical dimension and space as the horizontal dimensions, so each horizontal slice represents space at a particular moment in time. The strings aren't moving within the block, there's only movement when you break up the 3D block into a series of 2D slices, and similarly the tubes corresponding to the earth or the traveller aren't moving in 4D spacetime, it's only when you break it up into a series of 3D moments that you see movement.
  9. Jun 29, 2008 #8
    Exactly, it's all relative. :)

    But which do you subscribe to: the self-consistent theory or the multiple timeline/parallel universes theory?
  10. Jun 29, 2008 #9


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    Personally my guess is that in quantum gravity, time travel will turn out to be impossible...there seem to be hints of this in both semiclassical gravity and string theory, see here and here.
  11. Jun 29, 2008 #10
    I have a hard time seeing any paradoxes with CTCs. Classically what happens after an event is fully determined by the causal structure of spacetime, e.g. an incomming lightcone.
  12. Jun 29, 2008 #11


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    In spacetimes with CTCs the causal structure is a little more complicated since some event A can lie in both the past and future light cone of some other event B. And there can be situations where there are multiple self-consistent outcomes even if you fix the conditions outside the Cauchy horizon where CTCs can occur...see the discussion in the Novikov self-consistency principle article from wikipedia:
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