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Quantum Entanglement and time travel

  1. Nov 22, 2006 #1
    I'm not buying this for reasons of paradoxes, but Brian Greene is saying that time travel backwards is possible.

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    what do you guys think?
     
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  3. Nov 22, 2006 #2

    selfAdjoint

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    This is a way of expressingit for maximum gee-whiz value (why am I not surprised?), but consider it's only another facet of this point: QM contains an irreversable componennt, which appears in the wave function formalism as collapse, but also appears in other formulations. And nobody has ever been able to prove that's a necessary component. Seems that all current formulations of QM, to say nothing of all those wild and crazy interpretations, are incomplete.
     
  4. Nov 22, 2006 #3
    I was under the impression that when particles 'communicate' or interact nonlocally, that no information is being transferred. If this is definitely true, would this have any bearing on a QM theory of time travel?

    Also, the arrow of time seems to irreversibly flow forwards, however, what the hell do I know.
     
    Last edited: Nov 22, 2006
  5. Nov 22, 2006 #4
    The physical nature of what's being 'measured' by detectors (or emitted by emitters) in quantum experiments is unknown. That is, nobody knows what's being transferred from emitters to detectors (or if it is also being transferred from detectors to emitters, or from detectors to detectors, or whatever). To call it 'nonlocal' (or 'local' for that matter) is probably not a good idea. But, if you must call it something, then 'acausal' and 'alocal' would seem to be fitting candidates (since qm is all about correlations between various sorts of events -- both emission and detection, and various combinations thereof). As far as can be determined, there is no ftl interaction between spatially separated detectors, filters, emitters, etc. in quantum experiments -- even though there are some ways to sort of 'back into' the idea that there is.

    Is there a quantum theory of time travel??
    I think that most physicists would agree with the idea that the arrow of time flows irreversibly forward. When people like Brian Greene talk about the possibility of backward time travel, and ftl or instantaneous causation at a distance, and quantum weirdness, etc., it should be taken with great skepticism.
     
  6. Nov 22, 2006 #5
    There's nothing to prove. Backward time travel is a meaningless idea. Think about it. What is Greene talking about? Do you think it makes any sense to entertain the idea that the motions of some region of the universe for some interval can somehow be rewound and rerun like you would do with a vhs tape or a dvd?
     
  7. Nov 22, 2006 #6

    JesseM

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    Although the article conflates Greene's comments with Cramer's retrocausality experiment, my guess is that Greene wasn't talking about quantum physics at all, but rather about general relativity, which does theoretically allow time travel in certain unusual circumstances, like in the neighborhood of a traversable wormhole (though it is quite possible that when quantum effects are taken into account, these loopholes will be closed).
     
  8. Nov 22, 2006 #7

    JesseM

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    Backwards time travel has nothing to do with "rewinding" anything, it has to do with a worldline that loops around and revisits a portion of spacetime it's already crossed through. It's important to think of these things in terms of relativity's view of spacetime as a 4-dimensional continuum in which past, present and future events all coexist, rather than the intuitive view that there is a single objective "present" and that things in the past have "ceased to exist" or that things in the future "don't yet exist".
     
  9. Nov 23, 2006 #8

    Demystifier

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    That is exactly my point too. :smile:
     
  10. Dec 8, 2006 #9
    A geometric interpretation of relativity theory is one way of looking at it. But it doesn't provide a physical understanding of why backward time travel is a rather silly idea.

    Define the universe (or some region thereof) as some set of objects. Any particular configuration of the set of objects is a time of the set of objects.

    If the universe is expanding, then it is physically impossible for any universal scale configuration to be reproduced. But the capability to reconfigure very large scale configurations of objects is what would be needed in order to 'revisit' those configurations of objects (or, iow, travel backwards in time).

    Revisiting the past would require rewinding a configuration of objects in the sense that it would involve a repositioning of those objects -- and even if the universe isn't expanding, it would still be an impossible task.

    [/QUOTE]
    I disagree. The intuitive view is better for understanding some things.

    If you want to translate some data from one reference frame to another, then, yes, the definitions and conventions of relativity theory facilitate this in an unambiguous manner.

    But, if you want to understand why backward time travel is a nonsensical idea, then using notions of a four-dimensional spacetime, etc., is not the most promising way to proceed.
     
  11. Dec 8, 2006 #10
  12. Dec 9, 2006 #11

    George Jones

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    I'm going to expand on JesseM's comments by reposting stuff that I've posted elsewhere.

    Going backwards in time means going into the past while traveling forwards in time.

    Let p be an event in spacetime, Event q is in the (chronological) past of p if there exists a future-directed timelilke curve from q to p.

    Suppose that event p is on the worldline of an observer, and that there is an event q is in the past of p such that a future-directed timelike curve from p to q. Then, it is possible for an observer to travel into his own past.

    Joining the future-directed timelike curve form p to to q with the future-directed timelike curve from q to p, shows that this is completely equivalent to the existence of a closed timelike curve.

    Its certainly allowed by general relativity, as there are numerous solutions to Einsten's equations that have closed timelike curves.

    How does one deal with the paradoxes associated with time travel? Also as mentioned (in another thread), Matt Visser has written interesting stuff about this. He talks about four possibilies:

    1. Radically rerwite physics from the ground up;

    2. Permit time travel, but also invoke consistency constraints;

    3. Quantum physics intervenes to prevent time travel;

    4. the Boring Physics Conjecture, where we assume (until forced not) that our particular universe is globally hyperbolic, and thus doesn't have closed timelike curves.

    In the past 4. was often assumed, but since global hyperbolicity is a very strong global condition and Einstein's equations are (local) differential equations, many physicists have moved to 2. and 3. Stephen Hawking likes 3., for example, and has formulated the Chronology Protection Conjecture, "It seems that there is a Chronology Protection Agency wich prevents the appearance of closed timelike curves and so makes the universe safe for historians."

    This roughly states that near a chronology horizon (horizon at which spacetime becomes causally ill-behaved), expectation values of stress-energy tensors for quantum fields blow up, thus preventing (by wall-of-fire barriers) physical objects from crossing chronology horizons. There seems to be some semi-classical evidence for this conjecture, but a more refined analysis by Kay, Radzikowski, and Wald muddies the picture a bit. Their analysis shows that the semi-classical stress-energy tensor is ill-defined, but not necessarily infinite, at a chronology horizon.

    This may be just an indication that the semi-classical theory breaks down at chronology horizons, and that full quantum gravity is needed for definitive predictions.
     
  13. Dec 9, 2006 #12
    selfAdjoint:” This is a way of expressingit for maximum gee-whiz value (why am I not surprised?), but consider it's only another facet of this point: QM contains an irreversable componennt, which appears in the wave function formalism as collapse, but also appears in other formulations. And nobody has ever been able to prove that's a necessary component. Seems that all current formulations of QM, to say nothing of all those wild and crazy interpretations, are incomplete.”

    Seems that all current formulations of relativistic QM…
    May be Careful know the answer?
     
  14. Dec 9, 2006 #13

    vanesch

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    I don't think that, in a strictly GR universe, there is any problem with CTC. Indeed, given that in a strict GR universe, all time evolution is deterministic, then "on the second round" one cannot make any "different decisions" than the first time one went by a certain event. This would imply, for instance, that there cannot be any different "memory" state "the second time around". The (deterministic) decisions will be identically the same. In other words, if you meet your grand-dad 100 years ago along such a curve, you must be in such a state that you don't know specifically that it is your granddad, and that you will do anything else than you "did the first time around".
    The local state of a local spacelike foliation on a CTC cannot be different as a function of the "loop number" and hence, things must evolve in such a way that there is no paradox.
     
  15. Dec 9, 2006 #14

    George Jones

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    Any spacetime that has CTCs is not globally hyperbolic, and so does not possess a Cauchy surface necessary for the complete specification of initial data - initial-value problems are not well-posed in spacetimes that have closed timelike curves.
     
  16. Dec 9, 2006 #15

    JesseM

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    I think you're still not understanding how the geometrical view sheds light on closed timelike curves in GR--nothing is being rewound or repositioned! To see how the geometric "block time" view works, imagine spacetime as a literal block of ice, with some pieces of string embedded in it to represent worldlines. Now imagine slicing this block up into a stack of very thin cross-sections, like slicing meat at a deli counter. Each cross-section of the block will contain cross-sections of all the strings, which will just look like dots embedded in a 2D sheet. If we were to take pictures of each cross-section in succession, and then run them together as frames in a movie, we'd see the dots moving around over time, corresponding to particles moving around in space.

    Now, time travel in GR does not mean that the configuration of dots in the movie must return to a copy of their configuration in an earlier frame of the movie. Equivalently, it does not mean that a later cross-section of the ice looks identical to an earlier cross-section. Instead, returning to thinking about the whole block of ice before it was sliced into sections, a CTC should be thought of as a piece of string that loops around and intersects an "earlier" part of itself. From our perspective viewing the ice as a whole, nothing is changing, it's just a static configuration of strings embedded in the ice with one of them happening to form a loop. You could even imagine the block of ice being cone-shaped, so that successive cross-sections would be larger and larger, representing the expansion of space; contrary to what you suggested above, there is no notion of a past state having to be recreated when the universe is larger, since again, it's just a string which loops around and revisits a section of the cone closer to the tip where the cross-section is smaller.

    Similarly, if you can vaguely imagine standing outside spacetime as a whole, it would just look like a static curved 4D surface with various worldlines embedded in it, and CTCs would just be worldlines that form a loop. This picture really only makes sense in terms of the "block time" view, thinking in terms of the view that time "really flows" will just get you confused.
    Well, the intuitive view has caused you to misunderstand the idea of CTCs in GR, so at least in this situation it doesn't seem very helpful.
    Backwards time travel might be problematic for other reasons, but it's definitely allowed in GR (though a theory of quantum gravity may change this), and your arguments for why it's nonsensical don't work, for the reasons I tried to explain above.
     
  17. Dec 9, 2006 #16

    JesseM

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    Something I wondered about--if a spacetime is "not globally hyberbolic", does that automatically imply it contain CTCs, or are there examples of spacetimes with no CTCs that are not globally hyberbolic for other reasons? (maybe a spacetime could have a naked singularity but no CTCs?) And my understanding is that if a spacetime is globally hyperbolic, that means it can be "foliated" into a series of spacelike hypersurfaces, while a spacetime that's not globally hyperbolic can't be, is that right?
     
  18. Dec 9, 2006 #17

    George Jones

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    No, see your own answer below! :biggrin:

    Right. For example, consider Minkowski spacetime with the positive x-axis (of some inertial frame) removed. This spacetime is not globally hyperbolic, but it contains no CTCs.

    A spacetime that has CTCs has not only a Cauchy horizon, but also a chronology horizon.

    Yes, a globally hyperbolic spacetime M can be written as a product TxS of "time" and "space".

    I'm not sure about this.
     
  19. Dec 9, 2006 #18
    Suppose [tex] (M,g_{ab}) [/tex] is foliated by spatial hypersurfaces [tex] \Sigma_t [/tex] and that [tex] M [/tex] can be mapped to [tex] \Sigma \times R [/tex] then it is easy to see that any past and future inextendible timelike curve will cross [tex] \Sigma_t [/tex] exactly once ; hence the result. As a rule of thumb, you do not want CTC's expecially not in the asymptotically observable region, but they do occur already for simple systems such as rotating rods as far as I remember (Will Bonnor has done lots of work on that).
     
    Last edited: Dec 9, 2006
  20. Dec 9, 2006 #19
    Moving forward or backwards in time could easily be considered IF you think of time as a River with a meander developing into an Oxbow Lake. As you go down the flow of time on your riverboat as it goes into or out of the meander that is becoming an Oxbow you could swing across the developing cutoff point (Call this the ‘wormhole’ to deposit yourself into the flow of time (the river current) greatly separated from the position the riverboat in that flow.

    BUT here’s the problem – there is no reason to expect the river boat to still in that part of the river, it only exists in the time and place you left it. The riverboat (representing your world and universe as you knew it) is lost, as you are now floating in a new part of the stream of time.

    In a similar fashion in our 3D world if you hold a 2x4 in your hand a see that it is 2” in z and 4” in y experience has shown us there is no reason to believe we will find the same to be true about the 2x4 at any position in x. Or that the 2x4 will still even exist at any position in x as in could end leaving a void or allowing something else, like a brick (or some other world/universe) to take that position.
     
    Last edited: Dec 9, 2006
  21. Dec 9, 2006 #20

    vanesch

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    Yes, that's why I said "locally" (in a sufficiently small environment so that there's a "local" foliation, using, for instance, the eigentime parameter of the CTC around a small piece of it). Imagine a CTC, for instance, which takes about 400 years (eigentime). Over a few minutes (and probably even several years), we can have a local foliation around the curve which looks "normal enough". However, over a period of 400 years, it is of course not possible.
    What you would, for instance, have, I presume, is that if a clock was sent on a CTC, that it would not have a memory state that allowed it to register its eigentime in such a way that it could find out "how many loops" it had executed, because when it "came by event P", then the neighbourhood of event P (which can be locally foliated in order to determine the dynamics and the memory state of the clock) will each time determine exactly the SAME memory contents of the clock. So it "cannot remember" its own past far enough in the past to know its "previous passage by event P", simply because its memory state is part of the local environment of P. So no matter how it "locally evolved" on its curve, it will be in a state, around P, such as not to remember its previous passage through P. Or am I wrong ?
    I think that the "paradox" of CTC (killing our grandpa when he was 9 years old) appears because we implicitly allow for our memories, decisions and so on, NOT to live on a deterministic block universe, and hence allow ourselves erroneously to "be in a different memory state" the second time we come by the same event on a CTC. But our memory states being fixed by the neighbourhood of that event just as well, we won't know it, that we came by the second time.
     
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