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Is this idea of time travel consistent with relativity?

  1. Nov 6, 2009 #1
    This is the explaination I give people when they ask about time travel. I am only an undergrad so I just want to ask the more experienced people here if this is correct. Thanks...

    Time travel in the context of Einstein's relativity: So we all know that time travel into the future is easy (well given a very fast spaceship anyway). You don't even need a wormhole. The acceleration (not the velocity) causes time dilation so you can accelerate to near the speed of light and travel 10 light years away and come back. The end result is you end up 20 years in Earth's future even though it only took seconds or minutes for you. So given a wormhole, you could keep one end on Earth and take the other end with you for that same ride. Now you have gates A and B next to each other. There is no cosmic censorship keeping this from happening since it does not violate causality. If I go through wormhole B, I find myself 20 years in the past but there is only the one gate A there and I am at the same time I started. So you can't keep going back in time to kill your grandfather or something. You can keep going through wormhole A though and traveling into the future at 20 year increments.

    That is only given relativity, if we throw quantum mechanics into the mix things get messy. Anyway, let us consider an alien civilization that is highly advanced and exists a few billion years ago that have all this technology. Say they leave one wormhole at their home and take the other one on a similar ride but 2 billion light years away and back. So they end up in our time and they decide to check out Earth. If we go through their wormhole we end up 4 billion years in the past and we can go to Earth and just go through our entire history right? (Using the easy form of future time travel in increments) Well according to strict relativity yes, but in order to maintain causality the time traveling versions of ourselves would have existed in our past and we should see evidence of this in the fossil record right now. But if we consider quantum mechanics, then we (the time travelers) won’t see our history as it really happened, we will see a completely different series of events since each event is just a random outcome of some number of wave function collapses (however you define that collapse, Copenhagen or MWI). So after 4 billion years you might not even end up with an Earth with life. The only way we can see what happened in our past is to go through wormholes that were made at different times in our past. But as soon as you go through it that universe starts differentiating so you only have a small amount of time to check out your past (this depends on the event you are looking at, the age of dinosaurs is a long age but the time to assassinate Hitler is much shorter). Again none of this violates causality. Even if you travel to see your grandfather before you were born and kill him. This is because as soon as you go through the wormhole that person is not exactly the same person as your grandfather, even though he looks and sounds the same.
  2. jcsd
  3. Nov 11, 2009 #2
    Hello, can anyone verify this?
  4. Nov 11, 2009 #3
    What? Verify what happens when you go through a wormhole? No. :p
  5. Nov 11, 2009 #4


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    Time dilation in special relativity is a function of velocity, not acceleration. It is true that if two observers cross paths twice at different points in spacetime, and one has moved inertially between the two meetings while the other accelerated at some point during the journey, than the one who accelerated will always have aged less.
    Cosmic censorship deals with naked singularities, maybe you're thinking of the chronology protection conjecture?
    If your grandfather didn't have your parents until after the date wormholes were taken on the relativistic trip and brought back together with a time difference between them, then you certainly could meet your grandfather before your parents were born. Even if this type of time travel were possible, it doesn't necessarily mean you could succeed in killing him, perhaps the Novikov self-consistency principle would apply and it would be impossible to change the past. But grandfathers aside, this type of time travel is certainly the kind that would "violate causality" in the sense meant by physicists, where it is no longer true that we can assign a temporal order to events in such a way that causes always precede effects...for example, you could wait until the results of a lottery drawing were announced, then travel back one day before that and tell your younger self to buy a ticket with those numbers...even if there is no paradox created here, the cause of your younger self buying the ticket with those numbers lies in his own future, so this is a causality violation. And this form of time travel would definitely contradict the "chronology protection conjecture" (the most common idea about how chronology protection would be enforced in the case of wormholes involves virtual particles which build up in a feedback loop as you try to move the two ends close enough to allow for backwards time travel, either destroying the mouths before they can be moved close enough or causing a repulsive effect...see here and here for a discussion, and for a more detailed discussion read the last chapter of Kip Thorne's book https://www.amazon.com/Black-Holes-Time-Warps-Commonwealth/dp/0393312763).
    Again, you misunderstand what "causality" means, time travelers from Earth today going back to Earth 4 billion years ago would automatically be a causality violation, even if something like the Novikov self-consistency principle held sway and this trip did not lead to any "changes" to history.
    Since we don't have a theory that combines general relativity with quantum mechanics it's not at all clear what such a theory would say about the GR solutions that seem to allow backwards time travel, like spacetimes involving wormholes. Probably the most popular hypothesis is that quantum gravity will rule out these loopholes and chronology protection will hold true. But if backwards time travel is possible in quantum gravity, there's no reason to think it would necessarily lead to the possibility of changing the past, it might be that even though there are multiple possible histories as in the MWI, the Novikov principle applies to each individual history so time travel does not allow anyone to visit a different history than their own.
    Last edited by a moderator: May 4, 2017
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