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Time Travel

  1. Oct 23, 2006 #1
    Anyone have any good reads they want to recommend?
  2. jcsd
  3. Oct 24, 2006 #2


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    On the popular level, "Black holes and time warps - Einstein's outrageous legacy" has some material about time travel. Also, there are a few articles by Robert Forward (I don't recall the titles) and John Cramer's "Alternate views" column (which you can find online).


    If you want serious papers, you can try the following, if you can find them. (Most of them are quite advanced)

  4. Oct 24, 2006 #3

    George Jones

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    At what level?

    pervect gave a number of good technical references.

    For an excellent, non-technical reference, have a look at the second edition of https://www.amazon.com/gp/reader/0387985719/ref=sib_dp_pt/002-1086803-0800840#reader-link" by Paul Nahin. This is a wonderful book that is written for the educated layperson.

    Physicist (and relativist) Kip Thorne wrote a foreword for the second edition of this book, and here's a quote from this foreword: "It now is not only the most complete documentation of time travel in science fiction; it is also the most thorough review of serious scientific literature on the subject - a review that, remarkably, is scientifically accurate and at the same time largely accessible to a broad audience of nonspecialists."

    In a few posts in this http://groups.google.ca/group/sci.p...2afb?lnk=st&q=&rnum=2&hl=en#ca7fd4ed9d282afb", I attempted to explain a wormhole time machine like one in pervect's fourth technical referenece.

    The spacetime diagram in my first post looks better when Show original under show options is used.
    Last edited by a moderator: Apr 22, 2017
  5. Oct 24, 2006 #4


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    I should probably add the "billiard ball" paper, which addresses a billard ball version of the Grandfather paradox. The "Grandfather paradox" is a causal loop where someone goes back in time and kills his own grandfather). The billiard ball version involves a billiard ball whose future version travels back in time and obstructs the original from entering a wormhole.

  6. Oct 24, 2006 #5

    Meir Achuz

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    H. G. Wells "The Time Machine" should convince you not to try.
  7. Oct 24, 2006 #6
    If you get the Science Channel, they have a couple episodes on time travel. One is called TIME TRAVEL as fate would have it. It's pretty decent. Get's into parallel universes and the Grand Father paradox, amongst many other things. It's only 30 min.

  8. Oct 26, 2006 #7
    Hi. it's probably too late for me to post this but I'm bored so might as well. Before you go searching for the secret of time travel you should know that every modern book on theoretical physics contains at least one chapter on time travel and every one says the same thing. It's blah, blah, blah, more energy than is availllable iin the entire universe...blah, blah, blah, find a way to reposition a black hole, blah, blah, blah. Some physicists even reject (it can be rejected) the idea that time travel is possible all together. So are there any good time travel references? Not really, unless you're bored.:grumpy:
  9. Oct 26, 2006 #8
    Just have your deflector emit chromoton particles and enter the tear in the space time continuum that forms in front of the ship --

    Sorry, just saw First Contact on TV the other day.
  10. Nov 13, 2006 #9


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  11. Nov 29, 2006 #10
    For the sake of the discussion, time travel is possible.
    Could a person go back in time with the intention, for example, of killing Hitler, hoping to prevent the war? Obviously he can, but could he succeede?
  12. Nov 29, 2006 #11
    The grandfather paradox comes to mind, but there is more to it than that.
  13. Nov 29, 2006 #12
    Well assuming time travel IS possible, it is fair to speculate upon what restrictions or requirements are imposed on such travel.
    I would assume what ever the tool or mechanism (Worm hole etc.) the following limitation would apply:

    Based on the time as measured in local vacuum; that is not on a “slow time caused be a large Mass. And the MBR as a local fixed “preferred reference frame”.
    That any movement forward or backwards in time by an amount “T”.
    Would result in an associated one way displacement of spatial location by a minimum of twice T times the speed of light (2cT).

    With this limitation there would be no possibility of undoing Hitler’s work or invoking the Grandfather Paradox.
    However others would remain that would say even this sould not be possible.
    Last edited: Nov 29, 2006
  14. Nov 29, 2006 #13
    Correct, he couldn't do anything to stop the war. Could he do something that would change it?
  15. Nov 29, 2006 #14


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    There's no need to invent totally fictitious laws of physics (introducing a local preferred reference frame would be seen as very implausible, for one thing), which we aren't supposed to do in this forum anyway. Better to stick to known laws of physics like general relativity, which already suggest at least a possibility of time travel. Most physicists who analyze time travel or "closed timelike curves" using GR (like Kip Thorne, for example) assume that there is only a single unchangeable spacetime, which would mean you could never change anything in the past, although you might play a role in causing events that were already part of your history all along. There have also been some attempts to apply QM to these situations to reach the same conclusion that only self-consistent histories are possible (see the Novikov self-consistency principle, for example), although I've also read that the physicist David Deutsch argues that the many-worlds interpretation of QM would imply that time travel would take you to a different alternate "world", meaning you could change the past (but Stephen Hawking argued in a lecture that even if the MWI is true, time travels would stay in a single self-consistent 'world'--see his comments in http://www.hawking.org.uk/lectures/warps3.html [Broken]).
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  16. Nov 30, 2006 #15
    There is no difference between stopping and changing.
    Defining a common “local” preferred frame based on the MBR for any speed object can be done using SR, not implausible at all based on current science. What cannot be done is defining “A Single” preferred frame that would be common for any location in the universe – the fact of the Hubble Constant scientifically demonstrates that the local frame at some distant location cannot be the same frame as our nearby location.

    What is implausible is this threads premise being used for the “sake of the discussion” that “time travel is possible”.
    Just as the speculations[\b] of time travel within the Theories of GR QM MWI, etc. even by notables like Thorne, Novikov, Deutsch, or Hawking are just that, speculations and not a part of demonstrated science.
    Which is why those theories must still be considered theories and NOT “known laws of physics” not yet at the level of say the Conservation Laws of momentum, mass & energy.

    Even the best of them GR & QM have unresolved questions and doubts still being tested, including the idea that time travel is not ruled out by those theories, that alone to me means they could likely be incomplete theories, and thus not ready to be describe as part of the definitive “laws of physics”.
  17. Nov 30, 2006 #16


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    I can see this discussion is going downhill since I last visited it....

    True statements: nobody except Ron Mallet is seriously working on an actual time machine.

    Important notes: Papers challenging Mallet's approach have been published. They apparently have not convinced Mallet of the error in his approach, but they've convinced (for example) me. (See the Wikipedia article on Mallet for more details or look for his name in old PF posts).

    False statements: "Blah blah blah, more energy than is available in the entire universe" - this is total technobabble, with no informational content, and it does not represent the level of serious work that has been done on the topic. For instance, you'll not find the above remarks or anything resembling them in anything I quoted, even the popular works.
  18. Nov 30, 2006 #17


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    If you are imagining the local laws of physics would work differently in that frame than they would in any other local frame in that region, that would be a major violation of known physics, akin to re-establishing the existence of the aether. And if the local laws of physics don't work differently in that frame than in any other, then the frame isn't "preferred" as physicists use the term.
    My point was that this forum certainly allows discussion of the theoretical predictions of well-confirmed theories such as general relativity, but it doesn't allow discussion of phenomena which aren't predicted by any mainstream theory. And what is your basis for judging time travel any more "implausible" than, say, the idea that black holes have event horizons, which is also a theoretical prediction of GR for which there is no direct experimental evidence?
  19. Nov 30, 2006 #18
    He could not have stoped it, because if he did, why would he go there. If he had, by accident killed a general, he could change the war, but not to any extent that would have taken away his reason for trying to stop it.
  20. Dec 1, 2006 #19

    Chris Hillman

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    Plausibility of event horizons vs. time travel

    I wish to avoid "debunking" in this forum, but I will attempt to cautiously inject what I hope everyone here will agree is a reasonable "reality check".

    Few astronomers would agree that there is no (more or less) direct observational evidence for event horizons. Although you could probably quibble endlessly over what is "direct" versus "indirect" observational evidence (I assume "experimental" was a slip), most astronomers have felt for quite some time that the evidence that black holes exist (indeed are ubiquitous) in nature is overwheliming; see for example http://arxiv.org/find/astro-ph/1/au:+Rees_M/0/1/0/all/0/1, particularly surveys like http://www.arxiv.org/abs/astro-ph/0401365

    This does not imply at all that further observations which tend to confirm or challenge the existence of event horizons in nature are without interest; far from it, as the papers of Rees demonstrate! I might also mention that various groups are trying to lay the foundations for the theory of optical and acoustic analogs of black holes, which might one day allow experimental (as in, "laboratory experiments") studies of effects such as Hawking radiation which we expect to be associated with event horizons.

    Most physicists do seem to regard "time travel" (in the sense in which this term has recently been employed in various speculative papers in gravitation physics) as implausible, but this remains an area of active research. See http://arxiv.org/find/gr-qc/1/ti:+AND+curves+AND+Closed+timelike/0/1/0/all/0/1 and http://arxiv.org/find/gr-qc/1/ti:+AND+time+machine/0/1/0/all/0/1.

    Chris Hillman
  21. Dec 1, 2006 #20


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    Yes, I should have said "empirical".
    There is certainly plenty of evidence that very dense objects that fit the profile of black holes exist, but the existence of an event horizon around such a dense object is itself a theoretical prediction of GR, not something there is currently any clear observational evidence for. The second link you provide says:
    Of course there is plenty of observational evidence that general relativity is an accurate theory in other circumstances, so I suppose you could count that as indirect evidence for any given prediction it makes, but then time travel is also a theoretical prediction of GR.
    They do regard it as implausible, but not because they doubt that general relavity allows it (the technical term would be 'closed timelike curves'), it's more a matter of hunches about what a future theory of quantum gravity will look like, based partly on semiclassical attempts to incorporate QM into GR.
  22. Dec 1, 2006 #21

    Chris Hillman

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    Hi, JesseM,

    It is not my intent to quibble over semantics (what is "clear"?). I hope and assume that you are familiar with the implications of observations of things falling into a "black hole candidate" (if you prefer) and apparently vanishing without a trace (if the objects in question had some kind of surface, we'd expect some kind of flash as the infalling material impacts the surface). If not, Martin Rees has laid out the case from observational evidence that astrophysical objects featuring event horizons do exist in nature in many readable survey papers, and I hope you will seek out these or comparable surveys (see the link I already provided to the arXiv search tool and the additional citation provided below).

    Indeed there is, but I was speaking about more or less direct observational evidence for event horizons specifically. This is a somewhat different issue from the issue of comparing gtr with its competitors, since many of these also predict the existence of event horizons. If you want to argue about this evidence, however, you should probably take it up with Martin Rees!

    Well, it depends upon what you mean by "theoretical prediction". Everyone agrees, of course, that CTCs occur in many specific vacuum solutions to the EFE, most notably in the deep interior of the Kerr vacuum. So in this sense one could say that gtr predicts that CTCs " can occur". But most researchers in classical gravitation would probably say that this viewpoint is too naive to be really useful.

    An analogy might help: you can write down a Newtonian potential which describes a gravitostatic solution which models a uniform density disk with a pencil resting on the disk, "standing on the tip of its nose", but is it a "prediction" of Newtonian gravitation that if you stand a pencil on its tip, you can expect it to remain in that state? Most of us would say: "no, because this solution is unstable".

    Another analogy: you can write down a two body solution in Newtonian gravitation, featuring two particles with equal but opposite masses, in which the system spontaneousy accelerates away. Should such an odd scenario be counted as a "prediction" of Newtonian gravitation? Most of us would say: "no, because the boundary conditions assumed in this solution are unphysical".

    Ultimately, when dealing with any classical field theory of gravitation, we should consider such questions as: "what constitutes reasonable boundary conditions?", "what energy-momentum tensors constitute admissible source terms?", "what features are stable under small (linear, nonlinear) perturbations?", and so on. We shouldn't expect simple answers to any of these questions, and indeed these questions have remained vexed in the case of gtr (and similar theories) from the very beginning of the subject, despite a huge amount of work in the past forty years or so.

    Getting back to the original point, a sophisticate would argue that only those features of "acceptable" exact solutions (occuring as models of realistic astrophysical scenarios) which possess sensible source and which are "nonlinearly stable" should be considered to be features "predicted" by gtr (or its competitors). Unfortunately, the solution space of gtr remains somewhat mysterious. For example, it was only fairly recently that such basic solutions as Minkowski vacuum and de Sitter lambdavacuum were shown to be nonlinearly stable (and even these theorems are weaker than one would like). But what one really wants here would be nonlinear stability results for a solution exhibiting an event horizon, which (AFAIK) presently appears out of reach. These issues do NOT neccessarily have anything to do with what a quantum theory of gravitation might look like, by the way--- they are crucial and longstanding issues which have lain at the foundation of gtr (and its classical competitors) from the beginning.

    You should probably read an earlier survey by Rees, "Astrophysical Evidence for Black Holes", in the book edited by Robert Wald, Black Holes and Relativistic Stars, University of Chicago Press, 1998. (If you look for earlier arXiv papers by Rees, I believe you will find a electronic version of this paper, but I am too lazy to look for it now.)

    In the abstract you quoted, Rees remarks that it is not true that "we have evidence for black holes with the precise Kerr metric", and of course he is right about that, but this is a quite different issue from the existence of event horizons in nature! One issue he is probably thinking of there is that unlike the situation for the Schwarzschild vacuum, which is easily matched to the huge and fascinating family of static spherically symmetric perfect fluid solutions (or rather, the subfamily featuring zero pressure surfaces at a finite Schwarzschild radius), all "known" sources for the Kerr vacuum appear to be unphysical--- or at the very least, highly implausible--- and it turns out that an exact solution once regarded as the most promising candidate, the Walhquist fluid, cannot be matched to the Kerr vacuum after all. Indeed, the Kerr exterior vacuum may not match ANY rotating perfect fluid ball, which may indicate yet another instance in which gtr is a victim of its own success, in the sense that a more realistic theory is neccessarily more stringent in what counts as "admissible" boundary conditions, and this may imply that some idealizations familiar from less demanding theories may not longer be admissible.

    OK, now I've raised a bunch of new issues in classical gtr, but I hope my main point is becoming more clear: from what I've seen so far, I doubt that we really disagree on the essentials.

    To sum up: science is subtle and hard. If you expect it not to be subtle or hard, you will get confused in discussions which attempt to deal with genuine scientific issues, rather than mere caricatures.

    Chris Hillman
  23. Dec 2, 2006 #22
    All you need do is read the second of two sentences of the paragraph you quoted me from (the one you did not quote) to see I made no assumption that physics laws would change. Also, establishing a “LOCAL” preferred frame from the MBR [much different than a LR universal aether preferred frame] is not an invention of mine. Just what do you think scientists use to establish the idea of “The Great Attractor”, there is a continuing active scientific effort to find it and explain that one.

    Continuing research to find/explain evidence of speculations like event horizons are listed by other posters here. If you think either GR or QM are “well confirmed” enough to define time travel as possible I completely disagree. Nether theory has demonstrated that such a possibility implied by those theories is real OR in the alternative developed their theory to show why within the theory that time travel should be consider to be impossible (something more than just arbitrarily saying “let us assume time travel is impossible”). Until one can be well defined or explained to do one of those things I do not think it unreasonable to consider both of them “incomplete”. Even if some may consider one of them to be so, I doubt you can find anybody that considers both GR and QM to be complete. That was my point, and all of that is part of current science.
  24. Dec 2, 2006 #23


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    I just read a really great book telling exactly how to build a time machine!

    Unfortunately, it won't be published for another 50 years.
  25. Dec 2, 2006 #24


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    Here is something more (perhaps) more interesting to argue about. Does the so-called DWBF aproach (see for instance http://xxx.lanl.gov/abs/astro-ph/0006423 and http://math.ucr.edu/home/baez/RelWWW/group.html [Broken] "Does GTR admit a reinterpretation as funny fields on a Minkowski space-time" allow one to formulate a different theory of gravity that is locally equivalent to GR (but not globally equivalent) which does not have time machines (because the underlying theory admits only trivial topologies)?
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  26. Dec 2, 2006 #25

    Chris Hillman

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    Background for future discussion of backgrounds

    Hi, pervect,

    There have been several long and contentious threads in sci.physics.research on this topic over the years, to which I tried to contribute a note of sanity (unfortunately even in 1999, not everyone at s.p.r. had enough geometrical sophistication to grasp the relevance of what I was trying to tell them, which was partially responsible for the contentiousness, quite apart from the War of Strings). Try these:



    (Hit the "thread index" button on those pages; I linked to the first post in each thread.)

    Chris Hillman
    Last edited: Dec 2, 2006
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