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Returning to the past?

  1. Sep 27, 2012 #1
    Ok to begin this, I just want to say this: I"m pretty sure more than 50% of this world wants to return back to the past especially people who are getting old. They will want to go back to the days when they are young. Things which look impossible MIGHT turn possible, back in the 90"s I bet people wouldnt even think about having touch screen devices or other professional technology which we have now. Now please don't complain, I'm just asking if this is possible, people have dreams, which can come true. That"s what I want to achieve so I just want to ask a fill questions. We have also seen the past, we"ve been their before and experienced it.

    1) Is it possible to return back to the past?
    2) Even if we actually return, are we going to go back young again? Is this possible? Or are we staying with the same body we have now?
    3) if we use the effect of parallel universe would this be better if it actually exist?

    Most of you will probably tell me to enjoy your life, stay with it, stop thinking about it etc, but once we all get old I think it's too late.

    Thx to all who answered this
     
  2. jcsd
  3. Sep 27, 2012 #2
    I've never seen anything in physics that suggests this is possible. Conceptually it is absurd imo (specifically the with regard to the way you describe the scenario).

    Considering the "twin paradox". You traveling to the future is coming from the past to your new hosts.
     
  4. Sep 27, 2012 #3

    PeterDonis

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    There are solutions to the Einstein Field Equation that contain closed timelike curves (CTCs), which is the technical way of saying "return back to the past". So as far as the best current theory we have is concerned, the answer is "in principle, yes". However, AFAIK nobody thinks those solutions are physically reasonable, so the real answer is probably "no".

    However, even if it's probably not actually possible, the solutions I referred to above can be used to answer "what if" questions like the next one you pose:

    The solutions I referred to above, with CTCs in them, involve you continuing to travel "forward" in time, as you perceive it, but eventually coming back to a region of spacetime that you've already been in once (and which is therefore in your "past"), similar to how you can travel around the Earth in the same direction and eventually return to your starting point. So just taking that at face value, you would still have the same body; traveling into your past by this method would not make you any younger.

    However, there's a snag in this: the aging of your body involves increasing entropy, but if you could travel around a CTC and end up in the same region of spacetime again, there would have to be two "copies" of you in that region of spacetime, with different entropy. By repeating the process again and again, there could be an unbounded number of copies of you in that same region of spacetime, all with different entropy. This doesn't seem consistent with the second law of thermodynamics (and it's one reason why many physicists don't think solutions with CTCs in them are physically reasonable).

    The solutions I referred to above don't say anything about this, so I can't answer it.
     
  5. Sep 27, 2012 #4

    mfb

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    You cannot use the time machine over and over again, unless you are immortal, have infinite space to occupy and have some way to dump entropy (otherwise you die from overheating within hours anyway).

    The scenario says that you travel - and that "you" is the old body, with its memories and its ageing effects.
     
  6. Sep 27, 2012 #5

    PeterDonis

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    Which isn't ruled out by the Einstein Field Equation.

    Which also isn't ruled out by the EFE; in fact, in the current "best fit" EFE solution used in cosmology, our universe is spatially infinite.

    Which was part of my point about the second law; but actually, just increasing entropy by itself doesn't necessarily generate any heat. That topic probably deserves a separate thread, though (and probably not in this forum since it's more about thermodynamics than relativity).
     
  7. Sep 27, 2012 #6

    PAllen

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    A minor point: a material body following CTC doesn't represent travel to the past so much as an a-causal loop (to a third party: object appears out of nowhere; splits; merges; ceases to exist). However, in a manifold allowing CTC, there are 'almost' CTC spiral curves that do represent travel to the past over and over again with increasing age.
     
  8. Sep 27, 2012 #7
    I say you can travel to times in the past only if you go far enough away from your present position that your new timeline could never cause anything to happen that would intersect with your previous timeline.
     
  9. Sep 27, 2012 #8

    PAllen

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    You can say what you want, but the discussion here is about what established scientific theories say (in principle).
     
  10. Sep 27, 2012 #9

    berkeman

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