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Danger said:Your attachment hasn't been approved yet, but going by the title of the thread, the answer is 'no'.
(A quick search shows this proposal was also discussed a while ago on this thread.)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]
That's a pretty strong counter.JesseM said:the most basic objection is that "Mallett's spacetime contains a singularity even when the power to the laser is off
(Emphasis mine)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.
JesseM said:a copy of Mallett's book https://www.amazon.com/dp/1560258691/?tag=pfamazon01-20
JesseM said: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?
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 said: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.
Later, on p. 173, he comments that CTCs disappear when the line source is present but the circulating light is removed: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.
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.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.
Chaos' lil bro Order said:I saw this paper on time travel. Does anyone think its plausible?http://www.physics.uconn.edu/~mallett/main/research_activities.htm
pervect said: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.
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?Chris Hillman said: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)
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 said: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.
JesseM said: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?
JesseM said: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?
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 said:Let's just put it like this: a wise surfer will avoid untrusted websites.
JesseM said: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 said: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.
Chaos' lil bro Order said:So its malarky it would seem.
Chaos' lil bro Order said:Fantastic post Chris,
I appreciate your summaries and references, its nice to read a well formatted on topic reply.
Chris Hillman said: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.)
The circulating light beam creates a powerful gravitational field that can warp the fabric of spacetime, allowing for time travel to occur.
In theory, anyone can travel through time using the gravitational field of a circulating light beam. However, the technology and resources required are currently beyond our capabilities.
Although the concept of using a circulating light beam for time travel has been explored in scientific research, it is currently considered to be purely theoretical and has not been achieved in real life.
There are many potential consequences of time travel using the gravitational field of a circulating light beam, including altering the course of history and creating paradoxes. It is a highly complex and unpredictable process.
One limitation of using a circulating light beam for time travel is that it requires an immense amount of energy and precise calculations to create a strong enough gravitational field. Additionally, there is currently no way to control or predict where and when the time traveler will end up.