Can Light Create a Time Machine?

[Ivan Seeking]

Summary of Research Activity

Gravitational Field of Circulating Light Beams

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. My current research considers both the weak and strong gravitational fields produced by a single continuously circulating unidirectional beam of light. 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 [R. L. Mallett, "Weak gravitational field of the electromagnetic radiation in a ring laser," Phys. Lett. A 269, 214 (2000)

http://temporology.bio.msu.ru/EREPORTS/mallett.pdf .

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 [R. L. Mallett, "The gravitational field of a circulating light beam," Foundations of Physics 33, 1307 (2003)]. The presence of closed timelike lines indicates the possibility of time travel into the past. This creates the foundation for a time machine based on a circulating cylinder of light.

Popular articles on this research are given, for example, by
http://www.villagevoice.com/issues/0216/edlim.php
or http://www.walterzeichner.com/thezfiles/timetravel.html .

An audio presentation of an invited lecture entitled "A Brief History of Time Travel" given at the Boston Museum of Science on April 5, 2002 can be found at:

http://streams.wgbh.org/forum/forum.php?lecture_id=1203

Additional information can also be obtained by doing a google search under Ronald L. Mallett.

The Learning Channel aired an hour long documentary on December 3, 2003 featuring some of my current time travel research.[continued]

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

Does anyone know the latest on this?

 

[Ivan Seeking]

For his main page.
http://www.phys.uconn.edu/faculty/mallett.html

 

[gonzo]

I have a different view on early time travel, that this reminds me of. Not the exact mechanics, but the first use if a mechanic ever becomes even partially feasable.

Most of what I've seen dwells on sending large objects through time. I've seen theoretical calculations for using negative energy to keep a wormhole open, but then it is pointed out the absurd amount of energy it would take to open even the tiniest of holes, even if it was possible.

It seems to me the initial best use would be to send a single photon a very very very short distance backwards in time. You would then have in effect a compute that could perform infinite calculations instantly. Though it does seem like it would get quite hot or require a lot of energy or something.

Just a thought anyway.

 

[Evo]

I posted this back in April. You don't ever read my posts.

Ivan, are you familiar with Ronald Mallett? I saw a documentary about his research into time travel on a special that TLC did back in December. It was very interesting. For the first time I started thinking this could be a form of "time travel" that might work, but it is not the type of time travel that we are familiar with from books and films. Power is the main drawback people see to this working.

Basically here is the gist of what he's thinking, this from an article I will also post a link to.

Why you haven't met someone from the future

However, putting Ronald's theory into practice presents plenty of problems. For example, the temperature of the ring would have to be close to absolute zero (-273°C), so humans would find it difficult to use. It would also be impossible to travel back to a time before the machine was switched on. This explains why people from the future haven't visited us - we are yet to build a time machine for them to exit from.

Ronald hopes that travellers from the future may be able to overcome these difficulties and use the rings of light that we construct today as portals to our time.

http://www.abc.net.au/science/slab/wormholes/default.htm

This is an excerpt from a "layman's explanation" of Mallet's paper.

Nearly 50 years later, Mallett may be on the verge of building the world's first operative "time machine," though it will bear little relation to that of Wells, or to the DeLorean sports car of the movie Back to the Future, or the Tardis of Dr. Who or any of the other hundreds of time traveling mechanisms that have been imagined since Wells first took a crack at it.

Mallett's machine, as laid out in his May 2000 paper in Physics Letters entitled "Weak gravitational field of the electromagnetic radiation in a ring laser," is based on Einstein's formulation that light and matter are both forms of energy.

We know that matter can bend space-time and according to Einstein's theory, matter and light are both forms of energy. So why can't light bend space-time?

This fall, with UConn colleague Dr. Chandra Raychoudri, Mallett will begin work on building a "ring laser"--basically, a device that will create a circulating light beam, perhaps within a photonic crystal that will bend the light's trajectory and slow it down.

Then, a neutron particle will be sent into the space in the center of the beam. In short, the beam--perhaps two beams in one model, with the light traveling in opposite directions--is expected to twist the space-time inside the circle into a loop.

Think of a spoon stirring thick gravy in a pot and creating a vortex, only the vortex in this case is the fabric of space-time twisting, with past, present and future, circling one another so that the future precedes the past.
Then--and while this might not seem very exciting--a neutron, a small particle of matter--will be sent into the center of the beam. If its spin is affected, then it is being affected by warped space-time.

In a further experiment that Mallett has considered, two identical samples of a radioactive substance could be put into the center of the ring, one going in the direction of the beam. The other in the opposite direction. Since radioactivity decays at a measurable rate, it would be possible to measure, in effect, the time that both particles had experienced within the beam. If the time proves to be different, then time will have been measurably altered.

Eventually, says Mallett, "what would be neat is if you saw another neutron in there that you hadn't introduced yet." In essence, the same neutron "visiting itself from the future."

So you've moved a neutron. So what?

What Mallett will have shown--if it works--is that the fabric of time itself can be altered by light, and a thing can be moved into the past. If it works for a neutron, in theory, it would work for you and me.

http://www.walterzeichner.com/thezfiles/timetravel.html

Mallett's paper "Weak gravitational field of the electromagnetic radiation in a ring laser"

http://temporology.bio.msu.ru/EREPORTS/mallett.pdf

 

[pervect]

I'm rather skeptical about Mallet's time machine.

The basic problem is this - the purported time machine doesn't have the regions of negative energy density that one needs, for instance, to form a wormhole, so there are theorems that suggest that Mallet's scheme should not work. I would tend to believe that Mallet probably made too many simplifications in his strong field analysis (ignoring the stress-energy tensor of the mirrors needed to make the light beam swirl would be my guess).

To quote from Steve Carlip on spr

msg id: <aa1iho$3oa$1@woodrow.ucdavis.edu>

http://www.google.com/groups?safe=images&ie=UTF-8&as_umsgid=%3Caa1iho%243oa%241%40woodrow.ucdavis.edu%3E&lr=&hl=en

If this is the claim, it seems to
contradict Hawking's ``chronology protection'' result
(Phys. Rev. D46 (1992) 603), which shows that creation of
closed timelike curves from a compact region of spacetime
requires that the weak energy condition be violated. In
other words, Mallett's claim (at least in your inferred form)
requires negative energies, which you're not going to get
from classical light beams.

 

[sol2]

Scepticism is always good eh?

Ronald L. Mallett
Department of Physics, 2152 Hillside Road and UniÍersity of Connecticut, Storrs, CT 06269, USA Received 19 January 2000; accepted 3 April 2000 Communicated by P.R. Holland

Abstract The gravitational field due to the circulating flow of electromagnetic radiation of a unidirectional ring laser is found by solving the linearized Einstein field equations at any interior point of the laser ring. The general relativistic spin equations are then used to study the behavior of a massive spinning neutral particle at the center of the ring laser. It is found that the particle exhibits the phenomenon known as inertial frame-dragging. q2000 Elsevier Science B.V. All rights reserved.
http://temporology.bio.msu.ru/EREPORTS/mallett.pdf

 

[Ivan Seeking]

I posted this back in April. You don't ever read my posts.

I read it.

I was just too busy to follow up at the time. He seems to be gaining support.

 

[pervect]

Scepticism is always good eh?

I don't know, is it?

To amplify on my point a bit:

http://prola.aps.org/abstract/PRD/v46/i2/p603_1

Chronology protection conjecture

S. W. Hawking
Department of Applied Mathematics and Theoretical Physics, University of Cambridge, Silver Street, Cambridge CB3||9EW, United Kingdom

Received 23 September 1991

It has been suggested that an advanced civilization might have the technology to warp spacetime so that closed timelike curves would appear, allowing travel into the past. This paper examines this possibility in the case that the causality violations appear in a finite region of spacetime without curvature singularities. There will be a Cauchy horizon that is compactly generated and that in general contains one or more closed null geodesics which will be incomplete. One can define geometrical quantities that measure the Lorentz boost and area increase on going round these closed null geodesics. If the causality violation developed from a noncompact initial surface, the averaged weak energy condition must be violated on the Cauchy horizon. This shows that one cannot create closed timelike curves with finite lengths of cosmic string. Even if violations of the weak energy condition are allowed by quantum theory, the expectation value of the energy-momentum tensor would get very large if timelike curves become almost closed. It seems the back reaction would prevent closed timelike curves from appearing. These results strongly support the chronology protection conjecture: The laws of physics do not allow the appearance of closed timelike curves.

Given that Mallett's time machine is finite, Hawking's theorem would seem to apply. It's difficult to see how Mallett's result can be correct unless Hawking's result is in error, unless there is a violation of the weak energy condition. But Mallett doesn't mention any violation of the weak energy condition, there's certainly nothing that should violate it in a cylinder of rotating light. So I have to remain extremely skeptical unless I see this point addressed. BTW, the link you quoted for Mallet's paper doesn't work - and a link to his strong field analysis would be more interesting than the weak field analysis, anyway.

http://arxiv.org/PS_cache/gr-qc/pdf/0204/0204022.pdf

has some more info on chronology protection. The second part of Hawking's analysis, the part about infinite stress-energy tensors at the CTC, may be open to challenge with a quantum analysis and some rather special conditoins (see the above URL) but nobody's challenged the first part of his analysis that I've seen.

 

[Ivan Seeking]

Okay, I think I know what Mallett is using to ditch this argument: The Many Worlds Theory.

Does this sound right; not the theory, but using the MWT to escape this issue of Chronology Protection? In the audio presentation in my first link, right at the end he takes a question and uses the MWT to provide an answer.

 

[Evo]

The TV show had an interview with David Deutsch, so you may be correct.

 

[Ivan Seeking]

So, if this is correct, might this qualify as a test of the MWT? That would be interesting in itself.

 

[PRodQuanta]

How do you figure Ivan? You've got my attention. Maybe a link. Or just your thoughts.

Paden Roder

 

[Evo]

There was an article in Scientific American, March, 2004 "The Quantum Physics of Time Travel" with Deutsch. I haven't read it, but he discusses CTCs - Closed Timelike Curves

There is a paper by Deutsch

"Quantum Mechanics Near Closed Timelike Curves", Physical Review D, Vol.
44 No 10, pp 3197-3217, November 15, 1991.

I won't even pretend to understand this stuff. Just enough sinks in so that I have no clue what is being discussed.

But I've been reading "Fabric of Reality" because I think Deutsch is cute.

What is your opinion on Deutsch?

 

[elkoweb]

John titor said that a time machine would be invented by 2007 if i remember correctly. hmmmm

 

[humanino]

I don't understand : how the linearized approximation can be valid with a strong field ?

I also a agree with pervect : the positivity conditions of Penrose and Hawking should prevent storage of negative energy.

I totally agree with the fact that one cannot go back further than the lightcone defined by the event where the machine is switched on. This is always the case in what I have read. Besides, I wish time machine could be constructed, but I think scepticism is always the natural first reaction of a scientist.

There is a poll right now on the interpretations of QM. If you backup MWT, please vote. They need your help right now, Niel's up ahead

________
EDIT : sorry, forget the first question

 

[Ivan Seeking]

What is your opinion on Deutsch?

He has been one of my favorites ever since he proved the existence of crop cylinders; finally putting the "cylinder nuts" in the mainstream.

Quote from Deutsch
Moreover, I will stake my scientific reputation on the proposition that these cylinders not only existed, but were no accident: they were placed there by intelligent beings - and later removed to a destination I can only guess at.

http://www.qubit.org/people/david/UFO/UFO.html

Something else of interest that popped up while I was looking:

I show how a minor modification of the Alcubierre geometry can dramatically improve the total energy requirements for a `warp bubble' that can be used to transport macroscopic objects. A spacetime is presented for which the total negative mass needed is of the order of a few solar masses, accompanied by a comparable amount of positive energy. This puts the warp drive in the mass scale of large traversable wormholes. The new geometry satisfies the quantum inequality concerning WEC violations and has the same advantages as the original Alcubierre spacetime.

http://xxx.lanl.gov/abs/gr-qc/9905084

I don't understand : how the linearized approximation can be valid with a strong field?

I'm confused on this point. His paper only addresses weak fields.

 

[Evo]

He has been one of my favorites ever since he proved the existence of crop cylinders; finally putting the "cylinder nuts" in the mainstream.


http://www.qubit.org/people/david/UFO/UFO.html

I have some of those in the field behind my house!

Here is some information with references to some of Deutsch's papers.

http://plato.stanford.edu/entries/time-travel-phys/

 

[Evo]

I really felt Mallett may be onto something. I am disppointed I haven't been able to find more recent information on how things are going. I was afraid that we might not have the ability to create the right conditions. I wish you could have seen the show, it went into a lot of detail.

 

[Ivan Seeking]

How do you figure Ivan? You've got my attention. Maybe a link. Or just your thoughts.

Paden Roder

I'm just connecting the dots.

Here is more interesting information.

http://philsci-archive.pitt.edu/archive/00000845/00/TTdraft.pdf

 

[Ivan Seeking]

I really felt Mallett may be onto something. I am disppointed I haven't been able to find more recent information on how things are going. I was afraid that we might not have the ability to create the right conditions. I wish you could have seen the show, it went into a lot of detail.

I'm trying to contact him. He may have something to say.

I just saw the show. Its running again on The Science Channel. It wasn't clear to me that Deutsch addressed any of these issues.

 

[Ivan Seeking]

What is the weak field limit referenced by Carlip?

 

[pervect]

What is the weak field limit referenced by Carlip?

As I understand it, the main issue is that one needs to violate one of the weak energy conditions (this has nothing to do with weak fields, it has to do with a precise definition of "negative energy density") to generate a closed timelike curves. Mallett claims to be generating a CTC via purely classical means, with classical light beams. But these classical light beams would have a positive energy density, and there's no sign of a negative energy density (i.e. a violation of the averaged weak energy condtion) anywhere in his apparatus. Thus there shouldn't be any way to generate a CTC with said apparatus classically. I've only seen the abstracts of Mallet's papers, but he doesn't seem to be relying or using quantum mechanics in his formulas from what I've seen.

Secondary objections are the usual problems with CTC's and back reactions.

http://prola.aps.org/abstract/PRD/v46/i2/p603_1
has the specific objection raised.

 

[humanino]

We must be aware of something : the energy conditions of Penrose and Hawking are designed to ensure causality. They are not theorems. But, if time travel is possible, one might have to give up causality (of course, that would be a cataclysm for my securing scientific certitudes ) So is this an acceptable argument ? I mean : if one guy performs time travel, we will have to face it. The energy conditions are not proven by anything else than, precisely, CTC do not occur !

 

[pervect]

Yes, but Mallett claims (as I understand it) to be able to design a time machine simply by using a classical light beam, without using any form of negative energy / exotic matter / quantum effects.

I'm not trying to prove that the energy conditions are never violated when quantum mechanics is taken into consideration. Considering that the Casimir force exists and has been measured, that would be difficult. What I am saying is that a purely classical analysis that purports to create a time machine without negative energy densities, and with finite sources, is probably in error. If Mallett's solution requires an infinite cylinder, like Tippler's, it will obviously be a bit difficult to construct :-).

Gott's "cosmic string" time machine also falls vitctim to the theorem mentioned. (I believe that Gott, like Tipler, used an infinite source, mainly to simplify the calculations which were otherwise intractible). But at the time Gott published, this theorem wasn't known, in fact it appears likely that Gott's work helped inspire Hawking's work. Nowadays we know better.

 

[Ivan Seeking]

Well, I'm not sure what to think. First of all, Dr. Carlip has always answered my emails. We have corresponded a bit over the last six years but apparently he did not care to comment on this one again. Obviously he may just be busy but it may be dicey territory as well. Mallett never responded either. Since his website is still touting his time machine concept, I assume that he has not backed down from his position. Finally, a review of the pop literature seems to indicate that the objections made are recognized; at least to some extent. So from what I can see we don't have a case of an obvious, undisputed error.

...Mallett says. "It's a technological problem. I'm not saying it's easy, but we're not talking about exotic technology here; we're not talking about creating wormholes in space."...Last month, Mallett gave his first talk on the idea at the University of Michigan at the invitation of astrophysicist Fred Adams, who accepts that the theoretical side of Mallett's work stands up to scrutiny. "The reception was cautious and sceptical," Adams admits. "But there were no holes punched in it, either. The solution is probably valid." [continued]

http://www.spacedaily.com/news/timetravel-01a.html

Am I misunderstanding this, or does the phrase "we're not creating wormholes here" speak to some objections made here earlier? Does this speak to issues of the weak field limit?

Next, in the Chonology Protection Conjecture we find this statement which addresses the shell of zero ellapsed time about the time reversed interior region of a time machine:

A classical photon placed on this fountain will circulate around the
fountain infinitely many times; in effectively zero “elapsed” time. On
each circuit around the fountain there is generically a nontrivial holonomy
that changes the energy of the photon. For a past chronology horizon,
which expands as we move to the future (as defined by someone outside
the chronology violating region) this provides a boost, a net increase in
the photon energy for each circuit of the fountain. The photon energy
increases geometrically, reaching infinity in effectively zero time

Can anyone explain to me what a nontrivial holonomy is and why it changes the energy of the photon?

 

[pervect]

Can anyone explain to me what a nontrivial holonomy is and why it changes the energy of the photon?

I'll save you a potentially long read, and point out up front that the question you probably really want to know the answer to is why the holonmy isn't trivial. Because if the holonmy was trivial, the energy wouldn't change, and if it isn't, it does.

But now I'll fill in some of the missing details, though global methods aren't my strongest point. (That might actually help some, because I'll be sloppy enough to perhaps be comprehensible, rather than be correctly rigorous and opaque. At least that's what I hope to accomplish.)

http://www.maths.adelaide.edu.au/people/mmurray/line_bundles/node8.html

has an explanation of them, which is a bit on the technical side. There's probably people on the math forum who can give more detail, or perhaps less detail would be better :-).

But from the above URL, a holonomy involves transporting a vector around a loop, and watching what happens to it. The result for a specific loop is a map from the vector you started out with, to the vector you get after going around the loop.

It's an invertible map, because you can go around the loop in the opposite direction to get your original vector back. Thus, it's fair to call this mapping a group (groups are associative and have inverses).

Now for this to make sense we need to describe what parallel transport is
There's some discussion of parallel transport at

http://math.ucr.edu/home/baez/gr/parallel.transport.html

The most intuitive definition is to use some variant of Schild's ladder, IMO. The simplest variant is this - if you have a way of measuring distances on a curved surface (a metric), you have the ability to construct a parallelogram by making the opposite sides of equal length (because you hae a metric, you have a way of measuring length).

Given this construction of a parallelogram, you can say that the opposite sides of a parallelogram are parallel.

This allows you to define the notion of "parallel transport" along a very small distance.

Now to parallel transport a vector along a curve, all you have to do is take the limit for a very large number of transports over smaller and smaller distances.

Now we've been talking about transporting arbitrary vectors, but we could be more specific - we could talk about transporting the energy-momentum 4-vector around a loop, and asking what happens to it.

If it doesn't change, we have a especially simple holonomy group, the trivial group. But if it does change, we have a non-trivial group.

So the bottom line is - that by saying that the holonomy group is non-trivial we are just saying that a classical particle traveling around the CTC will gain or lose energy.

So probably the question you really want to know the answer to is why the holonomy group isn't trivial. And I'm afraid I'm not sure I know the answer to that one :-(. But I'll point out that in general, on curved surfaces (such as the Earth's surface), the holonmy isn't trivial, vectors do change orientation when you transport them around a loop as per Baez' example.

 

[selfAdjoint]

So probably the question you really want to know the answer to is why the holonomy group isn't trivial. And I'm afraid I'm not sure I know the answer to that one :-(. But I'll point out that in general, on curved surfaces (such as the Earth's surface), the holonmy isn't trivial, vectors do change orientation when you transport them around a loop as per Baez' example.

You've got the idea. If the "curvature" of the geometry, as expressed by the curvature tensor, is nonzero, then the parallel transport around a closed curve will not be zero (pretty much that IS the definition of curvature, via the connection and the covariant derivative). This is the same as saying the parallel transport from one point to another is path-dependent, so if you exponentialte and integrate, to get the holonomy, you won't get a unique result.

 

[bino]

http://www.usatoday.com/news/science/wonderquest/2001-06-20-time-travel.htm

 

[humanino]

Can a circulating light beam produce a time machine?

In a recent paper, Mallett found a solution of the Einstein equations in which closed timelike curves (CTC's) are present in the empty space outside an infinitely long cylinder of light moving in circular paths around an axis. Here we show that, for physically realistic energy densities, the CTC's occur at distances from the axis greater than the radius of the visible universe by an immense factor. We then show that Mallett's solution has a curvature singularity on the axis, even in the case where the intensity of the light vanishes. Thus it is not the solution one would get by starting with Minkowski space and establishing a cylinder of light.

 

[pervect]

Thanks, humanio - that's pretty much what I was expecting, but it's nice to see that I was right...

 

[bino]

Mallett's machine, as laid out in his May 2000 paper in Physics Letters entitled "Weak gravitational field of the electromagnetic radiation in a ring laser," is based on Einstein's formulation that light and matter are both forms of energy.
We know that matter can bend space-time and according to Einstein's theory, matter and light are both forms of energy. So why can't light bend space-time?
This fall, with UConn colleague Dr. Chandra Raychoudri, Mallett will begin work on building a "ring laser"--basically, a device that will create a circulating light beam, perhaps within a photonic crystal that will bend the light's trajectory and slow it down.
Then, a neutron particle will be sent into the space in the center of the beam. In short, the beam--perhaps two beams in one model, with the light traveling in opposite directions--is expected to twist the space-time inside the circle into a loop.
Think of a spoon stirring thick gravy in a pot and creating a vortex, only the vortex in this case is the fabric of space-time twisting, with past, present and future, circling one another so that the future precedes the past.

 

[pervect]

Unfortunately, Mallet did not do the calculations correctly. His most serious error (IMO) was assuming that the light swirled, without providing a mechanism to make it swirl. What he wound up finding was a space-time in which light would swirl around a line singularity (similar to a cosmic string) - rather than flat space-time with light being made to swirl via the use of mirrors or other optics, which is the case that can be generated in the laboratory.

To quote from the paper Humanio cited:

Unfortunately, it appears that the metric of \cite{Mallett} is not the metric
that one would get by starting from Minkowski space and establishing a
circulating cylinder of light. It is true that it is almost
everywhere a solution to Einstein's equations with Eq.\
(\ref{eqn:source}) as a source, but at the origin $$\rho = 0$$ there is
a line singularity. For example, the trtr component of the Riemann
tensor is
$$\be R_{rtr}{}^t = \frac{1}{8\rho^2} \ee$$
and so diverges at the origin. This is not a coordinate artifact, as
we can see by taking the scalar
$$\be R^{\alpha\beta\gamma\delta} R_{\alpha\beta\gamma\delta} =\frac{3}{4\alpha \rho^3} \ee$$
which is also divergent. Worse yet, this divergence has no dependence
on $$\lambda$$, and so persists if one takes the limit in which the
source intensity $$\epsilon$$ goes to 0.

Thus it appears that the metric of \cite{Mallett} describes a cylinder of
light circulating in a spacetime which is pathological even without
the light. It is thus unlike the spacetime of van Stockum
\cite{vanStockum} studied by Tipler \cite{Tipler:ctc}, which does go
to Minkowski space as the source is removed.

 

[bino]

Prof. Mallet theorized in 2000 that if a powerful laser light were bent into a
ring, it would create a region at its center where space-time curves back on
itself so severely that someone proceeding into the future would wind up back when he started, in his own past. In 1991, Princeton University astrophysicist J. Richard Gott theorized that cosmic strings, thinner than an atomic nucleus but infinitely long and more massive than a galaxy, could warp space-time enough to create these paths to the past, called closed timelike curves.

But it is a 1989 discovery, by Caltech’s Kip Thorne and colleagues, that has done the most to get the physics of time travel into reputable scientific
journals. They theorized that general relativity permits wormholes - tunnels
that cut across a curved region of space-time, connecting here to there and
now to then. Earlier calculations suggested that wormholes don’t stay open
long enough to serve as practical time machines, but Prof. Thorne showed that, with enough negative energy, they can be propped open.

 

[gptejms]

Unfortunately, Mallet did not do the calculations correctly. His most serious error (IMO) was assuming that the light swirled, without providing a mechanism to make it swirl. What he wound up finding was a space-time in which light would swirl around a line singularity (similar to a cosmic string) - rather than flat space-time with light being made to swirl via the use of mirrors or other optics, which is the case that can be generated in the laboratory.

I glanced through Mallet's paper and a web page on cosmic strings and made the following observation(tell me if I am wrong)--the latter contains the off-diagonal element g_{r,theta} which causes r,theta to trace out a circle(section of a cone).The former contains the off-diagonal element g_01(and g_02) which causes x-t(or y-t) to curve and trace out a circle--allowing the possibility of a particle going back in time.

 

[pervect]

Digging around, I can't find the "Foundations of Physics" paper by Mallet which is the only one to talk about CTC's. (I'm not positive I ever had it). I do have another of Mallet's papers, but it's not of much use to resolve this particular question.

In general, though, I think that you [gptejms] are on the right track. A cosmic string would not have the correct geometry to bend light in a circle.