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Is SR-allowable time travel future-only ?

  1. Feb 4, 2010 #1
    Is SR-allowable time travel "future-only"?

    Relativity theory allows for the possibility of time travel to the future, according to time dilation as illustrating by the twin paradox.

    Wikipedia says, "it is currently unknown whether the laws of physics would allow backwards time travel."

    My question is, doesn't relativity theory remove any uncertainty as to the possibility of past-ward time travel? What unknowns in SR give room for its possibility?
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  3. Feb 4, 2010 #2
    Re: Is SR-allowable time travel "future-only"?

    In a way, it does disallow it, but SR doesn't describe the universe perfectly.

    In SR, backwards time travel can be represented as a worldline that goes in a loop. But since we know that no physical object can have such a worldline, this is not possible.

    But another aspect is interaction between ordinary objects and tachyons (objects travelling faster than light). SR is a kinematical theory saying that no object can be accelerated up to or above the speed of light. However, it doesn't say that no tachyons exist. You can read about tachyons here:


    If you can interact with tachyons, then you can influence events in the past.

    But, if spacetime contains closed time like curves, then things are a bit different. I.e. if the topology of spacetime is such that a worldline can make a loop without every violating the rule of not accelerating past the speed of light, then it becomes a more complicated issue, because then relativity in itself doesn't rule it out, and people start to talk about quantum mechanics and entropy.

    Usually, the existence of closed timelike curves is seen as unphysical, and such a spacetime manifold will not be taken as seriously as a possible spacetime of the universe.

  4. Feb 4, 2010 #3
    Re: Is SR-allowable time travel "future-only"?

    Generally speaking the emergence of CTCs in a theory is a real problem. Even the famous "wormhole" is never constructed (in this universe's history) in such a fashion as to be traversable.

    Just think of it this way: You can always travel into your own future with the classic Time Dilation 'Planet of The Apes' scenario. Maybe not what you meant, but otherwise travel into the future is not possible in SR/GR. Traveling from the future to the past IS. Maybe a fine distinction.

    You could travel into the past in SR (if you ignore all modern thinking on the issue) if you achieved geometries in space-time that would have to contain a negative curvature. While it's possible on paper to imagine such geometries in line with SR, it is hard to imagine that they could exist in nature without the universe being radically different than we observe it to be.

    Personally I like Stephen Hawking's view, which is that any attempt to construct such a 'machine' would result in arbitrarily high energy densities and collapse (destruction) of the 'machine'.

    This should be clear however: this is about a human being in THIS universe as described by SR and GR. QM and its interpreations have many more (conflicting) views.
  5. Feb 4, 2010 #4
    Re: Is SR-allowable time travel "future-only"?

    How would the bold-faced sentence be pictured in explicit terms of SR? I don't see anything in SR being capable of taking us to the future by any means! In GR, the most optimistic view over this issue is on the basis of finding a http://en.wikipedia.org/wiki/Closed_timelike_curve" [Broken] put forward by Hawking which states that at large scales time travel is by no means possible to occur. Most physicists believe that the possibility of time travel even through this way is too beyond our technology and science! Quite a while ago, Hawking himself believed that if the universe changed its phase of expansion to contraction, then the arrow of time would possibly be reversed so the dream of time travel could come true because then we would go through time backwards and see what we did in the past. But Penrose proved him wrong and forced him to go back to his old-fashioned idea of microscopic possibility of time travelling!

    The last thing to say, if I were you, I would go and study everything but this stuff!

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  6. Feb 4, 2010 #5

    George Jones

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  7. Feb 4, 2010 #6
    Re: Is SR-allowable time travel "future-only"?

    Did you just not read the words immediately following, which were, "If you ignore all modern thinking on the issue". Did that sound like I was endorsing the possiblity to you, really? No.

    Edit: To breakdown the sentence... "You could travel into the past in SR (if you ignore all modern thinking on the issue)..." and "While it's possible on paper to imagine such geometries in line with SR, it is hard to imagine that they could exist in nature without the universe being radically different than we observe it to be." Again... where on earth does it seem like I'm endorsing the possiblity?
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  8. Feb 4, 2010 #7
    Re: Is SR-allowable time travel "future-only"?

    Looks like you are really angry at me! The point of my early post was that I actually wanted to say "No" to the reality of your claim since I didn't get what exactly you were up to say by "If you ignore all modern thinking on the issue"!! I don't get what this even has something to do with a paper job of revealing a possibility (you call it imagination, but I find it so delusive here since on one hand I don't even know how mathemtics of SR is able to get us such picture in which we can travel through time backwards, and on the other hand I got to say imaginations have shown us they could come true someday if assuming there is one that you are aware of in this case!) It's only, if possible, up to GR to decide whether we can go to past times or not! Period!

  9. Feb 4, 2010 #8
    Re: Is SR-allowable time travel "future-only"?

    I'm not angry, but I get the sense that English is not your first language, and you missed the point of what I was saying, or just parroted part of it. Again, perhaps this is a language-barrier issue, which I certainly don't hold against you.
  10. Feb 5, 2010 #9
    Re: Is SR-allowable time travel "future-only"?

    That has nothing to do with language and I think you are trying to digress the main discussion! You say

    "You could travel into the past in SR (if you ignore all modern thinking on the issue) if you achieved geometries in space-time that would have to contain a negative curvature. While it's possible on paper to imagine such geometries in line with SR, it is hard to imagine that they could exist in nature without the universe being radically different than we observe it to be."

    Could you please explain to me (who has no knowledge of GR!);

    1- What does "negative (scalar) curvature" have to do with time travelling and why positive curvature can't do this job?

    2- How, again, can we assume that SR allows us to travel in time backwards?

    Completely believable and I also like that view!

    Yes. But not all human-related stuff has entered in these sciences to make every dream come true for people! (My own idea)

  11. Feb 5, 2010 #10
    Re: Is SR-allowable time travel "future-only"?

    The question of negative curvature (like the classic "surface of a saddle" that is so often referenced) would allow for travel into the past without the creation of structures such as ERBs that only emerge in GR. As I was saying, it would be a radically different universe than the one who's history we seem to be a part of. Without that negative curvature travel in SR is only possible in the context of the Twin Paradox and other Time Dilation effects, or Tachyons (probably a mathematical artifact).

    From a GR point of view (where my knowledge primarily lies, not in SR), the perfect example is AdS space embedded with (p,q) where q is >/= to 1, but that is definitely in GR, not SR.

    I don't quite understand your last sentence however. I believe you're pointing out that everything we want to be true about physics isn't always true. If that is the case, I agree with you completely. If not, I'd love for you to rephrase it so I might understand.

    When it comes to time travel/FTL in general however outside of the realm of SR, and in the universe we actually live in, we agree. Chronology Protection, conjecture or not, really does make sense and would expain the lack of temporal tourists and CTCs. The notion of infinite transit for some particles in the CTC causing an effect like the approach of a massive body to 'c' requiring arbitrarily high amounts of energy to accelerate seems sensible. Probably untestable, but fascinating.

    Hmmmm... could you have an SR 'Alice' Universe? I realize you'd need GR to get there, but the notion of no universal definition of charge seems doable under SR.
  12. Feb 5, 2010 #11


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    Re: Is SR-allowable time travel "future-only"?

    The claim that one can travel only to the future makes sense only if future can be distinguished from the past. However, the theory of relativity alone does not distinguish the future from the past. Instead, one must introduce a time arrow that points from the past to the future, which defines a preferred direction. On the other hand, a preferred direction seems to be in conflict with the theory of relativity. Nevertheless, there is no conflict, as explained in
    http://xxx.lanl.gov/abs/gr-qc/0403121 [Found.Phys.Lett. 19 (2006) 259]
    It turns out that travel to the past is impossible, but the reason for that has nothing to do with relativity. Instead, it has to do with thermodynamics.
  13. Feb 5, 2010 #12
    Re: Is SR-allowable time travel "future-only"?

    I've read similar arguments in the context of CPT symmetry and breaking, as well as a general examination of why we percieve a particular arrow of time being linked to thermodynamic processess.
  14. Feb 5, 2010 #13


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    Re: Is SR-allowable time travel "future-only"?

    It seems to me that you're mixing up two different issues here: causality and the arrow of time.

    Causality in SR can be expressed by saying that if observer 1 measures time t1 between events A and B, then for any other observer, 2, the signs of t1 and t2 agree. You don't need an arrow of time in order to express this.

    I think you can also have an arrow of time while violating causality, e.g., with CTCs.

    But there are those who have made the case that CTCs aren't a big problem, e.g.: http://authors.library.caltech.edu/6469/

    [EDIT] Removed mistaken word "proper" above.
    Last edited: Feb 5, 2010
  15. Feb 5, 2010 #14
    Re: Is SR-allowable time travel "future-only"?

    Ahhh, Kip Thorne. I respect the hell out of him, but I think this is yet another defense of wormholes. I respect the notion that there may be a huge number of 'safe' trajectories as described in the paper, but what if human beings were able to use such a wormhole? Immidiately you have causality issues seperate from the issue of microscropic self-interaction.

    The concept of CTCs that don't ACT like CTCs is fine in theory, but find me something without Kip Thorne's name on it that talks about traversable wormholes and CTCs as anything but a possible footnote.
  16. Feb 5, 2010 #15


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    Re: Is SR-allowable time travel "future-only"?

    I thought the Barcelo-Visser paper, http://arxiv.org/abs/gr-qc/0205066 , linked to from your #8 in that thread was very interesting. Basically they argue that classical scalar fields violate all known energy conditions, and allow (unstable) traversable wormholes. Because the result is *classical*, we can't wriggle out of it by imposing a high-energy cutoff, or by saying that semiclassical gravity is suspect. My personal reaction is that it would be really fun if the LHC failed to observe the Higgs. Presumably all hell would break loose among particle physicists, but if it leaves us with the ability to disbelieve the existence of scalar fields, then maybe it would be comforting for non-particle physicists.
  17. Feb 5, 2010 #16
    Re: Is SR-allowable time travel "future-only"?

    Seems like we are gently transmigrating into a very different universe than the one SR creates. I was wondering if you could show me some article, paper or book regarding how negative curvature can create a universe where people go to past times and I believe strongly that time travel cannot ever be found in SR (meaning that all those scenarios e.g. twin paradox, tachyons and time dilation would never let one travel in time backwards; maybe if the question was "is travelling forwards in time possible" one would answered in the affirmative due to the twin paradox). An argument concerning time travel is given in this http://en.wikipedia.org/wiki/Time_travel#cite_ref-Gott_20-0" and for this reason the whole scenario has been put aside by physicists and myself. But now I'm probably getting the idea of your sentence "if you ignore all modern thinking on the issue" which can be this big paradox rising in SR version of time travel and why I'm telling you that SR is incapable of giving us an opportunity to see our past is this, right? Hope that you are no longer angry at me!

    Mine too lines in GR, and of course there are lots of examples which are left abandoned because of all those annoying paradoxes. One of the interesting possibilities is Alcubierre drive that GR offers for those who like to travel in time which is similar to the idea of a "tachyonic time machine" because it makes use of FTL (faster than light) scenario, thus again serious problems such as VoC (violation of causality) or violation of energy conditions in gigantic amounts happen to exist in this case, too! (For more info read http://www.arxiv.org/abs/gr-qc/9702026 and http://www.arxiv.org/abs/gr-qc/9905084).

    I mean that, for example, Higgs bosons have not yet been observed but they exist in theory and maybe they are going to remain on papers for good. The dilation of "biological ageing" has not ever been shown to be true in twin paradox experimentally though we are used to making example of a twins, one on the Earth and the other on a spaceship travelling at the speed of .9c so after 40 years that the flying brother comes home he is way younger than his brother! But this, too, may rest in theories forever.

    But I don't want to be much of a pessimist when it comes to possibility of time travel. I think someday one of us will show how to do it! Don't get me wrong!

    Let me stay with my GR 'Alice' Universe! :)
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  18. Feb 5, 2010 #17
    Re: Is SR-allowable time travel "future-only"?

    Causality has nothing to do with the arrow of time in general. This means that it would be imaginable to find a relation between them, as Stuckelberg in formulating the TCP symmetry of QFT puts forward, for anti-particles which appear to travel backwards in time and be detected before their emission, causality seems to be violated, but this is a matter of convention in physics and your claim is not generally correct!

  19. Feb 5, 2010 #18
    Re: Is SR-allowable time travel "future-only"?

    Personally I'm very much a pessemist when it comes to time travel/FTL. The only way I can imagine it working is if human beings changed ourselves in such a fashion as to no longer be human. Either we'd have to keep track of multiple times and histories, or construct wormhole that somehow (pure fiction I think) acts as 'tuner', delivering you to a distant place FTL, but adjusting for time as if the trip were made at c.

    I don't think the universe works that way however. I think it's a vast universe of what are essentially islands.

    By the way, it's an important distinction to make that while yes, many textbooks already have the Higgs in there, it's not the Boson but the MECHANISM which has such faith in the scientific community. Granted, that has led to a pretty high confidence that the Higgs Boson will be found, but a failure to do so would still leave the door open for underlying means by which mass arises. Of course, it would be an experimental and practical kick in the beans to not find it. :rofl:

    Finally, and most importantly, YES, I do see what you mean about SR, and I am on the same page with you. Again, I really never was angry with you, I'm just a little harsh sometimes. You're correct however, I don't see any way with just SR, and that is the title of the thread. I should have given the matter more careful consideration and remembered the issues of gravity and multiple framees GR brought. Ah well, I live I learn.
  20. Feb 5, 2010 #19
    Re: Is SR-allowable time travel "future-only"?

    In fact, Hawking Radiation can be described in terms of a single particle in which the emission is time-reverse of the other half of the formerly 'virtual' pair falling into the singularity and past the EH.

    I have to say, I still remember how shocked I was when I first learned that Parity necessarily symmetric. It's odd at first, and definitely has lead to a lot of anthropic thinking.
  21. Feb 6, 2010 #20
    Re: Is SR-allowable time travel "future-only"?

    Very interesting.

    So this defines time's arrow, the thermodynamic arrow of time, "TA", as the gradient of entropy density (a scalar field, the magnitude of the entropy density vector field), and "physical TA" as the component of TA tangent to a curve. Nikolic raises the possibility that this curve needn't be timelike. What would it mean for TA to be spacelike?

    Later he writes "physical time is defined globally, as a coordinate with respect to which the total entropy increases." And "for observers that are able to measure entropy in large regions of spacetime, it is natural to divide the spacetime into appropriately large regions and to assign the entropy to each region as the average entropy of this region."

    So now, and please correct me if I've misunderstood, we have the following concepts, "TA" (local, invariant, a gradient), "physical TA" (local, frame-dependent, a tangent vector), coordinate TA (regional), "physical time" (global, the average direction of increase of entropy). I suppose, for a timelike curve, "physical TA" is the direction of proper time, and "coordinate TA" the (frame-dependent) direction of coordinate time.

    At the bottom of page 4, what does "a constraint on the boundary condition" mean?
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