Time Travel using the gravitational field of a circulating light beam

[Chaos' lil bro Order]

I saw this paper on time travel. Does anyone think its plausible?http://www.physics.uconn.edu/~mallett/main/research_activities.htm

 

[Danger]

Your attachment hasn't been approved yet, but going by the title of the thread, the answer is 'no'.

 

[Chaos' lil bro Order]

Your attachment hasn't been approved yet, but going by the title of the thread, the answer is 'no'.

There's a link now.

 

[Danger]

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.
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.

 

[JesseM]

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:

Another objection by Olum and Everett is that even if Mallett's choice of spacetime were correct, the energy required to twist spacetime sufficiently would be huge, and that with lasers of the type in use today the ring would have to be much larger than the observable universe. Mallett agrees that in a vacuum the energy requirements would be impractical, but notes that the energy required goes down as the speed of light goes down, so he argues that if the light is slowed down significantly by passing it through a medium (as in the experiments of Lene Hau where light was passed through a superfluid and slowed to about 17 metres per second) the energy would be attainable.[2] However, the physicist J. Richard Gott argues that slowing down light by passing it through a medium cannot be treated as equivalent to lowering the constant c (the speed of light in a vacuum) in the equations of general relativity, saying:

One has to distinguish between the speed of light in empty space, which is a constant, and through a medium, which can be less. Light travels more slowly through water than through empty space but this does not mean that you age more slowly while scuba diving or that it is easier to twist space-time underwater.

The experiments done so far don't lower the speed of light in empty space; they just lower the speed of light in a medium and should not make it easier to twist space-time. Thus, it should not take any less mass-energy to form a black hole or a time machine of a given size in such a medium.[3]

(A quick search shows this proposal was also discussed a while ago on this thread.)

 

[cesiumfrog]

the most basic objection is that "Mallett's spacetime contains a singularity even when the power to the laser is off

That's a pretty strong counter.

 

[JesseM]

So I was flipping through a copy of Mallett's book https://www.amazon.com/dp/1560258691/?tag=pfamazon01-20 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?

 

[Garth]

In Mallett's own words:

3. CONCLUSION

It has long been known(3, 4) that the van Stockum solution for the exterior metric of an infinitely long rotating dust cylinder contains closed timelike lines. The present paper has shown closed timelike curves also occur for an infinitely long circulating cylinder of light. This model also shares some of the same limitations as the van Stockum solution in that the metric is not asymptotically flat. Bonnor,(4) however, has emphasized that certain aspects of an infinitely long rotating dust cylinder may be shared by a long finite one. This may also apply to a long but finite circulating cylinder of light.

(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

 

[JesseM]

Good catch Garth. I notice Mallett also has another paper which purports to analyze the more specific situation of a ring laser:

http://www.physics.uconn.edu/~mallett/Mallett2000.pdf

Can anyone tell if the metric he uses to analyze it here also assumes an infinitely long cylinder of light?

 

[Jheriko]

a copy of Mallett's book https://www.amazon.com/dp/1560258691/?tag=pfamazon01-20

The title of this book tells me all I need to know.

 

[pervect]

Good catch Garth. I notice Mallett also has another paper which purports to analyze the more specific situation of a ring laser:

http://www.physics.uconn.edu/~mallett/Mallett2000.pdf

Can anyone tell if the metric he uses to analyze it here also assumes an infinitely long cylinder of light?

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.

 

[JesseM]

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.

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?

 

[pervect]

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. \nabla_{\mu}T^{\mu\nu} 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.

 

[pervect]

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.

 

[JesseM]

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:

Since it can be written in a single line, Einstein's gravitational field equations look deceptively simple. However, when taken out of the highly compressed tensor calculus notation, they represent a set of ten extremely complex equations. To do the calculations, I reached back to my experience of finding an exact solution for the Einstein field equations for the strong gravitational field of an evaporating black hole in an inflationary universe. In that case, it had been necessary for me to combine two solutions of Einstein's equations to form a new solution. I had combined the Vaidya black hole solution with the de Sitter cosmological solution to produce the Vaidya-de Sitter solution. My experience with this technique proved to be of significant value in dealing with the present problem.

I decided to dispense with trying to model mathematically either an optical fiber or a photonic crystal. Instead, for the sake of generality and to keep the light beam on a cylindrical path, I elected to use a geometric constraint. This constraint was represented by a static (nonmoving) line source. Light naturally wants to travel along a straight line. The only purpose of the line source in my calculations was to act as a general constraint to confine the circulating light beam to a cylinder. (Set up experimentally, the line source could look like wrapping a piece of string around a maypole, with the string being the light beam and the maypole serving as the line source.) The light beam itself would be conceived of as a massless fluid flowing in only one direction around the cylinder. This meant that the solution really contained two solutions: one for the circulating light and one for the static source.

Later, on p. 173, he comments that CTCs disappear when the line source is present but the circulating light is removed:

As my results indicated, when I turned off the light flow, the time loops disappeared while the line source around which the light had circulated remained. The closed loops in time had been produced by the circulating flow of light, and not by the non-moving line source. When it came to translating my theory into the design for a circulating-light time machine, it was clear the circulating flow of light would be the machine's on-and-off switch. Per my results, in order to achieve closed loops in time, the light source had to be on.

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.

 

[pervect]

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 don't 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.

 

[Chris Hillman]

A curved analog of the Bonnor beam?

I saw this paper on time travel. Does anyone think its plausible?http://www.physics.uconn.edu/~mallett/main/research_activities.htm

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.

 

[Chris Hillman]

Yet another paper by Mallett

Hi, pervect

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.

This is a different paper, I think; see http://www.arxiv.org/abs/gr-qc/0010006

 

[JesseM]

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)

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?

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.

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?

 

[Chris Hillman]

Be cautious where you surf

Hi, JesseM,

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?

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.

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?

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.

 

[JesseM]

Let's just put it like this: a wise surfer will avoid untrusted websites.

But the files are on Mallett's own website! Surely he's not going to but up a bogus version of the paper on his site? (and I'm not sure if you're suggesting something worse than an incorrect version of the paper, like a virus, but I think the risk of getting a virus by downloading a pdf file from a website is vanishingly small).

 

[Chris Hillman]

Two corrections

Hi, JesseM,

But the files are on Mallett's own website! Surely he's not going to but up a bogus version of the paper on his site? (and I'm not sure if you're suggesting something worse than an incorrect version of the paper, like a virus, but I think the risk of getting a virus by downloading a pdf file from a website is vanishingly small).

You misunderstood the nature of my concerns, but I must decline to explain further. My silence on any further guessing (which I would discourge) should not be interpreted as an affirmation, since I have now dropped this matter. Please take no offense! Nothing personal here, it just that discussions of the type you seek must be restricted for obvious reasons.

I should correct something I wrote earlier, referring to the Bonnor beam (an exact null dust solution):

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.

I still haven't seen the original paper (but looking at the arXiv eprint I mentioned, it is clear that this concerns a different topic entirely). However, a glance at the critical paper by Olum and Everett cited above shows that in fact Mallett was trying to concoct a "spinning beam" (think of an infinite cylinder spinning around its axis of rotational symmetry), not a "circular beam" (think of a torus).

Unfortunately, this ambition is subject to the same fundamental objections, some mathematical (the defining property of pp wave spacetimes, such as the Bonnor beam null dust, rules out the wave vector null congruence required for a spinning beam) and some physical (in particular, roughly speaking one must provide for some physical interaction causing the photons to circulate rather than simply run along straight line paths).

(These objections are not entirely independent of each other, needless to say.)

 

[JesseM]

Chris, I really have no idea what you might be hinting at--it seems odd to say something like "a wise surfer will avoid untrusted websites" when you give no clue as to why this is "wise" or what makes Mallett's website "untrusted." But no, I don't take offense, and if you're unwilling to elaborate I'll drop the issue.

 

[pervect]

For what it's worth:

I'm still with Mallet's critics on this one, not Mallett. Mallet's remarks in his book don't clarify his paper any for me. I'm assuming that the version of the paper I saw on the web was the original (or at least sanctioned by Mallett) - I don't have the resources or the inclination to track down the original published paper.

Note that if someone's reputation, livelihood, or at they very least a great deal of grant money :-) is "on the line", I haven't studied the matter sufficiently closely to where I'd really want to comment. But since none of the above is at stake, and this is a public forum dedicated to discussions of physics, I'll stick with my original opinion, which I expressed in a lot greater detail in the original (quite old) thread. The new information in Mallett's book hasn't changed anything from me - I still don't see what he thinks he's up to.

 

[Chaos' lil bro Order]

So its malarky it would seem.

 

[Chris Hillman]

Please don't misunderstand

Hi, Chaos,

So its malarky it would seem.

That depends upon what you mean by "malarky", and what claims you wish to characterize as such! I don't wish to be misunderstood. Please note:

1. The views expressed in this thread address specific claims in specific papers,

2. Olum and Everett critique specific claims made in R. W. Mallett, Found. Phys. 33 (2003): 1307 in their eprint http://www.arxiv.org/abs/gr-qc/0410078); in brief, they argue that the phrase "exact solution" requires qualification, and that Mallett's arguments (that time travel and CTCs should occur in a proposed laboratory experiment) are vitiated by his gross misinterpration of the "solution" and by some other rather elementary errors made in this paper,

3. In some of his posts above, pervect's comments referred to a completely different paper coauthored by Mallett with Mbonye, http://www.arxiv.org/abs/gr-qc/0010006, which appears to me (and I think also to pervect) "unobjectionable upon casual reading"; the Vaidya null dust is a genuine exact null dust solution of the EFE, and perturbations of this spacetime model by gravitational radiation would be a perfectly reasonable thing to study (the perturbed spacetime would not be an exact solution, but such approximations can yield invaluable insight),

4a. The currently current version of the Wikipedia article http://en.wikipedia.org/w/index.php?title=Ronald_Mallett&oldid=95361327 cites R. W. Mallett, Phys. Lett. A 269, 214 (2000), as well as the later paper already mentioned, R. W. Mallett, Foundations of Physics 33, 1307 (2003), and quotes (apparently) "blurbs" from Mallett's home page; the earlier paper apparently concerns a purported "toroidal variant" of the Bonnor beam, while the later paper apparently concerns a puported "spinning variant" of the Bonnor beam; the Bonnor beam is a genuine exact null dust solution of the EFE but the two variants apparently have a very different character and would probably not be characterized as solutions by careful physicists, and almost certainly would not be given the interpretation offered by Mallett,

4b. As an example of assessing a given version of a WP article, note thathttp://en.wikipedia.org/w/index.php?title=Ronald_Mallett&oldid=95361327 has been mostly written http://en.wikipedia.org/w/index.php?title=Ronald_Mallett&action=history by WP User: Hypnosifl http://en.wikipedia.org/wiki/User:Hypnosifl; I happen to be largely unfamiliar with this particular user's contributions, but from http://en.wikipedia.org/wiki/Special:Contributions/Hypnosifl one can quickly see that this user has recently edited a number of articles related to this topic, and a crude impression of this user's WP contribs can be quickly gained from edit counters such as http://tools.wikimedia.de/~interiot/cgi-bin/Tool1/wannabe_kate (give it a few minutes to do its work); however, ultimately a reliable assessment of the accuracy of the article (and the contribs from the various contributors) would require a line by line study of recent edits to the article, and a close reading of Mallett's papers by someone who knows the literature well, who has sufficient technical ability, insight and judgement to render a fair assessment; I have made no attempt to do this work; please note that this kind of effort would rarely be expended by anyone with the required expertise!,

5. I myself expressed initial concerns about specific claims based upon what pervect and others wrote here and upon the purported quotations in "currently recent" versions (CRV) of the Wikipedia (WP) article; I later read the critical eprint by Olum and Everett, but I still haven't see the original two papers on an alleged "exact solution" allegedly describing a spinning light beam,

6. The concerns raised "semi-independently" by myself, pervect, and Olum and Everett appear to address similar points, which tends to support my contention that these concerns should be "obvious at a glance to anyone familiar with the literature". If you like, the sentences quoted in the CRV of the WP article cited above "For the strong gravitational field of a circulating cylinder of light, I have found new exact solutions of the Einstein field equations for the exterior and interior gravitational fields of the light cylinder. The exterior gravitational field is shown to contain closed timelike lines" and "This creates the foundation for a time machine based on a circulating cylinder of light" have been characterized (if you will) as "malarky",

7. The sentence "In Einstein's general theory of relativity, both matter and energy can create a gravitational field. This means that the energy of a light beam can produce a gravitational field" (also quoted in the same v. of the same WP article) is correct; see for example my "approved version" of the WP article on the Bonnor beam null dust solution. I would characterize the sentence "In the weak gravitational field of a unidirectional ring laser, it is predicted that a spinning neutral particle, when placed in the ring, is dragged around by the resulting gravitational field" as "suspicious", simply based upon the above summarized criticisms of the later paper (on, apparently, a spinning light beam); I'd have to think about this to decide whether the proposed effect is indeed a prediction of gtr, but if so, off the cuff I doubt that it would impossible to measure this in a laboratory experiment.

OK, I've written far more than this topic deserves; you can probably see how hard it is to avoid being sucked into a long peroration if one bothers to utter a word of warning about some "fringe science" topic which has been popularized in the pop science literature. And I hope this particular one has now been laid gently but firmly to rest!

However, I hope this lengthy series of caveats might be valuable for anyone trying to get some idea of some of the more elementary considerations which one should bear in mind when one is told that a paper by A has been critiqued by B, or in reading a WP article.

 

[Chaos' lil bro Order]

Fantastic post Chris,

I appreciate your summaries and references, its nice to read a well formatted on topic reply.

 

[Chris Hillman]

Hi, Chaos,

Fantastic post Chris,

I appreciate your summaries and references, its nice to read a well formatted on topic reply.

Thanks!

Here's a question for anyone who knows the answer: I have feeling that this topic will arise again, and I'd like to conventiently bookmark this and some of my other longer posts. I see that PF has some facility for blogging; has anyone tried to use this to store links to posts he/she expects to cite in the future? Is there any limit on the allocated memory? (I do know about the PF search engine, but I expect that it would be more convenient to keep a list.)

 

[Chaos' lil bro Order]

Hi, Chaos,



Thanks!

Here's a question for anyone who knows the answer: I have feeling that this topic will arise again, and I'd like to conventiently bookmark this and some of my other longer posts. I see that PF has some facility for blogging; has anyone tried to use this to store links to posts he/she expects to cite in the future? Is there any limit on the allocated memory? (I do know about the PF search engine, but I expect that it would be more convenient to keep a list.)

I believe there are some exceptional posts that administrators have made 'sticky'. They appear at the top of the forums for users to easily refer to and read. If you PM an administrator they may 'sticky' it for you.