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Time Travel using the gravitational field of a circulating light beam

  1. Dec 12, 2006 #1

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    Last edited: Dec 12, 2006
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  3. Dec 12, 2006 #2

    Danger

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    Your attachment hasn't been approved yet, but going by the title of the thread, the answer is 'no'.
     
  4. Dec 12, 2006 #3
    There's a link now.
     
  5. Dec 12, 2006 #4

    Danger

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    Unfortunately, between the time of your first post and now, I've had two very large hot buttered rums (to fight off the effects of my once-per-decade cold) and half a dozen beers (to cool me off from the effects of the hot buttered rums). I therefore cannot, at this point, understand your link. :redface:
    It's certainly not within my province to dispute a physics professor, but something strikes me as being a bit 'off'. I'll take another look at it tomorrow, but I still probably won't understand it.
     
  6. Dec 13, 2006 #5

    JesseM

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    See this paper by Ken D. Olum and Allen Everett, which has some basic objections to the claim. As summarized on the wikipedia entry on Mallett, the most basic objection is that "Mallett's spacetime contains a singularity even when the power to the laser is off, and is not the spacetime that would be expected to arise naturally if the circulating laser were turned on in previously empty space." And in addition, there's also this:
    (A quick search shows this proposal was also discussed a while ago on this thread.)
     
    Last edited: Dec 13, 2006
  7. Dec 13, 2006 #6
    That's a pretty strong counter.
     
  8. Dec 13, 2006 #7

    JesseM

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    So I was flipping through a copy of Mallett's book Time Traveler: A Scientist's Personal Mission to Make Time Travel a Reality today at my local Borders, and I think I found something that may shed light on this issue of the singularity. In a paragraph on pp. 167-168, Mallett mentions that when trying to model the effects of circulating light using GR, he was not able to figure out a way to directly model the effects of confining the light to a circle using a fiber optic cable, so he chose to include a "line source" in his solution to act as a "geometric constraint" to force the light to move in a circle. Presumably this line source is a 1D singularity? If so, it seems like a pretty huge leap to treat "light moving in a circle due to passing through a fiber optic cable" as equivalent to "light moving in a circle in a vacuum due to curved spacetime around a line source", since the only thing they have in common is the light moves in a circle in both cases! But then, it was also a pretty huge leap for him to treat "light slowed down due to passing through a medium" as equivalent to "the constant c decreased in the equations of GR", which he uses to bring the energy requirements down.

    It seems intuitively obvious that neither of these equivalences are very plausible, but I'm not sure exactly how you'd prove it. Since the reason light bends/slows down when passing through substances would be understood in terms of quantum physics, I guess to rigorously show these assumptions of his are wrong you'd need an analysis which incorporated QM and GR...but since the energies of an ordinary laser aren't very great I assume it wouldn't require quantum gravity, couldn't you model the effects of light passing through some substance in a GR context by using quantum field theory in curved spacetime?
     
    Last edited: Dec 13, 2006
  9. Dec 14, 2006 #8

    Garth

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    In Mallett's own words:
    (Emphasis mine)

    I notice he is also asking for contributions. Surely, if built sometime in the future, the time machine could be used to send back the winning lottery numbers?

    Garth
     
  10. Dec 14, 2006 #9

    JesseM

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  11. Dec 14, 2006 #10
  12. Dec 14, 2006 #11

    pervect

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    I would say no, that this analysis is for a finite length. It looks to me like he analyzes an infinitely dense but finite in length "pencil" of light, and that he's only interested in the "frame dragging" in the center of the ring and doesn't make any claims about time travel in this paper.
     
  13. Dec 14, 2006 #12

    JesseM

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    Thanks pervect--and can you tell what assumption he makes to keep the light moving in a circle? Does he use the "line source" that I mentioned earlier?
     
  14. Dec 14, 2006 #13

    pervect

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    As nearly as I can tell, Mallett doesn't analyze the effect of whatever is needed to make the light beam reflect. He assumes that there are mirrors, but he does not have any mechanism for keeping them in place. (When the beam reflects off the mirors, there will physically have to be some structure to hold them in place due to the changing in momentum of the light beam).

    I think that this basically means that Mallett's stress-energy tensor is going to violate some conservation laws, i.e. [itex]\nabla_{\mu}T^{\mu\nu}[/itex] isn't going to be zero like it should be. IMO his idealized beams also do not diffract properly as they would if they were actually solutions of Maxwell's equation.

    I am not sure how severe these faults are because the paper is only a weak field analysis, so some degree of approximation is OK. The gravitational effect of stresses is tiny under normal conditions. My intuition is that the gravitational effect of the stresses won't be significant but it would be nice to see a more formal and rigorous analysis of this point. Similar remarks could be made about the beam profile - the actual beam profile would probably be close to a Gaussian beam (actually even that's an approximation, the paraaxial approximation, rather than an exact solution) and I think that the mirrors would have to be curved to maintain this beam profile. But I'm not sure if it would make much different in the end result. It would have been nice if the paper itself would have addressed some of these points.
     
  15. Dec 15, 2006 #14

    pervect

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    I think I can come up with a pretty good argument to justify at least some of the approximations in the paper. Basically, the problem has been linearized, so one can use the principle of superposition of sources. (Note that this will only apply to the weak field linear analysis, the lack of the ability to use supperposition is what makes the nonlinear analysis so hard).

    So we can add in the a physical support structure to hold the mirrors in place and analyze its effect separately. This will certainly add fields to the problem, but I think we can argue that it shouldn't add any frame-dragging, which is the point of what Mallet is computing.

    Even for a disk with strain, the field generated by the stationary disk should be the gradient of some scalar potential function (like Newtonian gravity). This should mean no frame dragging due to the support structure.
     
    Last edited: Dec 15, 2006
  16. Dec 18, 2006 #15

    JesseM

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    So today I went back to Borders and copied down the paragraphs in his book where he discusses the "line source" in his time travel solution (which, again, I presume is the same as the singularity that Olum and Everett mention as being present in his solution even when the light beam is turned off). He mentions that although the frame-dragging caused by a circulating laser can be modeled using approximations since the energy is low (what I think is known as the 'weak-field approximation), closed timelike curves would only arise at much higher energies and so he would need to find an exact solution to Einstein's field equations. On pp. 167-168 he comments on the difficulty of this, writing:
    Later, on p. 173, he comments that CTCs disappear when the line source is present but the circulating light is removed:
    So my question here is, when he talks about combining solutions in GR, and says that "the solution really contained two solutions: one for the circulating light and one for the static source", is he referring to some technical procedure for combining multiple solutions into one and decomposing single solutions into combinations of several? Or is he just making the totally handwavey argument that, since the CTCs are present with the circulating light + line source but absent with just the line source, that means it was exclusively the circulating light that had "produced" the CTCs, so the line source was not important? If the latter, this seems like obviously faulty logic, since the curvature of spacetime in a solution where light was moving in a helix due to a central 1D singularity would presumably be quite different than the curvature of spacetime in a solution where light was moving along a similar path due instead to passing through a medium like a fiber optic cable or photonic crystal, with no central singularity present.
     
    Last edited: Dec 18, 2006
  17. Dec 18, 2006 #16

    pervect

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    I gather the paper Mallet is referring to is http://prola.aps.org/abstract/PRD/v31/i2/p416_1, but I don't have access. It seems to have been well received, though.

    Reading it might shed more light on what he's doing (it sounds like it could be "Schwarzschild surgery" often used to construct wormhole solutions). But I dont see how he expects to recover the solution without the line source from the solution with the line source. Since the problem is nonlinear, one cannot say that the total solution is the solution for the line source plus the solution for the light beam.
     
  18. Dec 18, 2006 #17

    Chris Hillman

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    A curved analog of the Bonnor beam?

    I too would advise extreme caution in accepting such claims uncritically. While I wish to avoid "debunking" in this forum, perhaps I may be permitted a general observation worth bearing in mind when reading press releases and popular press news items (often based on press releases or press conferences, only very rarely upon an intelligent reading of an actual paper). To wit:

    One should never accept uncritically some physicist's claim to have discovered a "new exact solution"! (Unless you know the literature well enough to know that a particular author really does know what he is talking about.)

    Experts on exact solutions know that:

    1. Many published "exact solutions" turn out not to be solutions at all; see for example http://www.arxiv.org/abs/gr-qc/9809013 (fortunately, this phenomenon is now fairly rare for papers published in the best journals, but eprints fairly often make false claims of this nature.)

    2. Of those which are the real McCoy, many are not new; see the eprint just cited and the monograph by Stephani et al., Exact Solutions of Einstein's Field Equations.

    3. False claims of vacuum, perfect fluid, electrovacuum, null dust, and many other important types of solution are easily debunked by computing the Einstein tensor wrt a suitable frame field, but it is not easy to give a precise yet general definition of "exact solution of the EFE"! In such cases, experience and geometrical insight coupled with good judgement solidly grounded in physical intuition is often essential.

    4. Subtle local versus global issues can lead to further difficulties in deciding whether or not some Lorentzian spacetime (possibly equipped with tensor fields modeling nongravitational phenomena) are acceptable as exact solutions in gtr.

    5. Singularities of various kinds are very often overlooked, misinterpreted, or misunderstood by newbies.

    I have not seen the papers in question (that I recall; I don't usually read Physics Letters A or Foundations of Physics Letters, which often publish papers which have been rejected from journals I do read!), but the Wikipedia article http://en.wikipedia.org/wiki/Ronald_Mallett offers what appear to be quotations from the abstracts of the papers in question (unfortunately, the links to go Mallett's home page; the published papers appear not to be available on-line) does offer what appear to be quotations from the abstracts of two Phy. Lett papers. (All the usual caveats about the "instability by design" of Wikipedia articles apply, of course; unless you know that a particular version has been "vetted" by someone whose knowledge and good judgement you have good reason to trust, you should "read defensively".)

    Based on this and on long experience solving the EFE, I strongly suspect that Mallett has misinterpreted whatever he found, and that his model is probably not useful for analyzing any experiment which might be performed in a real laboratory. I could get into an absurd amount of detail about why I was immediately suspicious, but I doubt this would be worthwhile unless all participants have studied (at the very least) the following specific versions of some further Wikipedia articles:

    http://en.wikipedia.org/w/index.php?title=Exact_solutions_in_general_relativity&oldid=45119659

    http://en.wikipedia.org/w/index.php?title=Null_dust_solution&oldid=41389091

    http://en.wikipedia.org/w/index.php?title=Bonnor_beam&oldid=47130395

    Please note the last article cited above describes an model of a straight light beam, the so-called Bonnor beam, which is a genuine null dust solution of the EFE. (I was principle author of all three articles, but am no longer participating in Wikipedia.) Mallett is claiming to present a similar model of a curved light beam, which may help to explain the relevance of mastery of the Bonnor beam, null dusts, and exact solutions generally.

    [NOTE, added 19 Dec 2006: it seems that in fact Mallett was claiming to present a model of a spinning beam, not a curved beam; see followup below where I noted that this correction does not reduce the force of my off-the-cuff objections.]

    (The issues I have in mind are closely related to the suspicious singularity mentioned by other posters. The Bonnor beam solution, significantly, lacks such a singularity; in fact, this is a homogeneous null dust inside the beam and an axisymmetric vacuum pp-wave outside the beam.)

    It is true that closed timelike curves (CTCs) exist in many genuine solutions of the EFE (such as the Taub-NUT vacuum or the Goedel lambadust), but this must be interpreted with great caution. Most experts currently believe that these are unphysical features resulting from pushing an idealization too far (e.g. extending a local solution found assuming some symmetry Ansatz), or from employing inappropriate boundary conditions (easily overlooked until one tries to pass from a local solution to a global solution), or both. It is also true that there is a large literature on a topic called "time travel", but this term has a (rather loose) technical meaning not easily explained to lay audiences. Note too that this work is highly speculative and cannot be easily assessed without extensive knowledge of the physical and mathematical background.

    Hmm... just noticed a critical paper http://arxiv.org/abs/gr-qc/0410078 whose authors have also contributed to the above mentioned literature on so-called "time travel", so their criticism cannot be put down to lack of imagination or unwillingness to consider new ideas! At a glance it appears that my guesses above about what might have gone wrong were at least partially correct. I highly recommend to anyone interested in this topic a review paper on exact solutions coauthored by Bonnor (of the beam); see the references in the Wikipedia article on Exact solutions cited above.

    It might also help to point out that Mallett's claims have previously been discussed in PF: see https://www.physicsforums.com/archive/index.php/t-42834.html.
     
    Last edited: Dec 19, 2006
  19. Dec 18, 2006 #18

    Chris Hillman

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  20. Dec 18, 2006 #19

    JesseM

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    The links go to pdf files of these papers which he has on his home page...do you just mean that these files may not be the same as the published versions?
    Yes, this is the same paper I brought up earlier which argues against his solution on the basis of that central singularity. But did you read my post quoting Mallett's book? It appears he put this "line source" singularity in intentionally, justifying it as a "geometric constraint" in lieu of actually incorporating into his solution the fiber optic cable or photonic crystal which is supposed to get the light moving in a circle. Do you think this argument might make sense, or is it as fishy as it looks?
     
  21. Dec 18, 2006 #20

    Chris Hillman

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    Be cautious where you surf

    Hi, JesseM,

    Let's just put it like this: a wise surfer will avoid untrusted websites. In fact, unless Physics Forums scans attached images or pdfs using the latest version of a utility such as clamscan http://sourceforge.net/projects/clamav/, I'd avoid downloading the attachment offered by the OP, who might not have checked the file he uploaded.

    I am trying to avoid "debunking" in this forum, but you can probably infer from my previous post how much effort I would be inclined to put into checking the details.

    I think we should let this drop now.
     
    Last edited: Dec 19, 2006
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