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In this article it is claimed that certain type of string theory called EGBd allows for traversable wormhole solutions that do not require exotic matter.

What is this EGBd model and how it fits in the grand scheme of things of string theory? is this fringe science or is serious? (i know that there might be fringe authors in ArXiv). But my main concern is how general or realistic is this result?

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What is this EGBd model and how it fits in the grand scheme of things of string theory?

The EGBd (dilatonic Einstein-Gauss-Bonnet) model is a low-energy approximation to a particular variant of string theory. This approximation is roughly similar to General Relativity (GR). In GR the geometry of empty space is solely determined by a particular mathematical representation of the curvature of spacetime. In EGBd the geometry of empty space is determined additionally by a different mathematical representation of the curvature of spacetime (the Gauss-Bonnet term) and by a scalar field (the dilaton field).

Background: The Gauss-Bonnet term is precisely zero, if spacetime is assumed to have 4 dimensions. It was shown a few years ago that in 5 dimensions the non-zero Gauss-Bonnet term mimics exotic matter (needed to prevent wormhole collapse) and permits traversable wormholes (in 5D).

This paper purports to show that a EGBd allows traversable wormholes in 4 dimensions.

My problem with the paper is that it assumes that the kinetic energy of the dilaton field is manifestly negative. This is contrary to the manifestly positive kinetic energy of the dilaton field in the usual Gauss-Bonnet-dilatonic low-energy approximation to string theory. It shouldn't be too surprising that a theory that is similar to GR except that it contains a field with negative kinetic energy (and a probably zero or negligibly important Gauss-Bonnet term) would permit traversable wormholes in 4D (because the negative-energy matter needed to hold open wormholes in built right into the theory).

is this fringe science or is serious?

It's serious, but unimportant. Assuming there are no mistakes in the paper, it's upshot is this: If a theory for which there is not a shred of experimental evidence (string theory) is true, then traversable wormholes are possible in 4D without the need of a separate source of exotic (negative-energy) matter.

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  • $\begingroup$ I would upvote, but your last sentences claim that strings allow wormholes, but that the paper uses a wrong sign kinetic term for the dilaton. Is the paper correct or not (I didn't read it)? $\endgroup$
    – Ron Maimon
    Dec 25, 2011 at 7:29
  • $\begingroup$ That's the reason for my qualification, "Assuming there are no mistakes in the paper". It would take a lot of effort to determine whether their chosen Lagrangian, which they claim to be "motivated by low-energy heterotic string theory" is in fact derived from that theory. Although I'm not a string theorist, I think that if there was something as bizarre as a negative-energy scalar field in some version of low-energy string theory, it would have been adduced in just about any discussion of wormholes and exotic matter. But I've never heard of such a thing. $\endgroup$
    – Belizean
    Dec 25, 2011 at 9:39
  • $\begingroup$ The kinetic term sign is sometimes funky depending on the frame convention for the metric. This is someplace where it is easy to screw up, so it is important to check carefully. I glanced at the paper, but I couldn't muster the willpower to sit down check, because I am almost certain it's not right, since this would lead to causality violation. It is also good to remember that string theory has a good amount of experimental evidence, in that GR is true and string theory is the only really plausible quantum version one can imagine so far that actually works. $\endgroup$
    – Ron Maimon
    Dec 25, 2011 at 13:43
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    $\begingroup$ @RonMaimon causality violation should not be a stand-alone reason to dismiss it, since causality violation counter-intuitive behaviour is attributable at the possibility of building NP-complete computers out of CTC. NP-complete computers would seem almost as oracles to us. But even if that was a reason for automatic dismissal, space-like signal transmission does not equate to causality violation if we accept some form of chronology protection mechanism, as quantum field feedbacks $\endgroup$
    – lurscher
    Dec 2, 2012 at 18:50
  • $\begingroup$ @lurscher: Yes, but it's much more speculative than what people normally think is acceptable for a reasonable conjecture--- it's about as likely as OPERA neutrinos, which is not impossible but really really unlikely. $\endgroup$
    – Ron Maimon
    Dec 2, 2012 at 20:41

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