Traversable wormhole without negative energy (paper)

[MTd2]

I'd like to know your views. Is it too good to be true? Or is there any obvious blunder?http://arxiv.org/abs/1509.07854

NUT wormholes
Gérard Clément, Dmitri Gal'tsov, Mourad Guenouche
(Submitted on 25 Sep 2015)
We show that supercritically charged black holes with NUT provide a new setting for traversable wormholes. This does not require exotic matter, a price being the Misner string singularities. Without assuming time periodicity to make Misner strings unobservable, we show that, contrary to expectations, geodesics do not stop there. Moreover, since there is no central singularity the space-time turns out to be geodesically complete. Another unpleasant feature of spacetimes with NUTs is the presence of regions where the azimuthal angle φ becomes timelike, signalling the appearance of closed timelike curves (CTCs). We show that among them there are no closed timelike or null geodesics, so the freely falling observers should not encounter causality violations. Considering worldlines of charged particles, we find that, although these can become closed in the vicinity of the wormhole throat for large enough charge-to-mass ratio, the non-causal orbits are still disconnected from the distant zones. All these findings support our feeling that wormholes with NUTs deserve to be taken seriously. Integrating the geodesic equations completely, we demonstrate the existence of timelike and null geodesics connecting two asymptotic regions of the wormhole, such that the tidal forces in the throat are reasonably small. We discuss bounds on the NUT charge which follow from the Schwinger pair creation and ionization thresholds and speculate that such NUT wormholes could be present in some galactic centers.

 

[bcrowell]

The first challenge for me was to parse "with NUT." In the main text of the paper, this is amplified to "space-times endowed with a Newman-Unti-Tamburino (NUT) parameter." Casually scrolling through the paper, I don't see where they ever say what they mean by this parameter. Clearly it has something to do with the Taub-NUT solution, but it's not clear to me what the connection is.

 

[MTd2]

They define it as n, here:

"In the case of a non-vanishing NUT charge, n =/= 0, the Brill solution (2.1) is not singular at r = 0, as can be checked by computing the quadratic curvature invariants" , right above eq. 2.5, p. 5

 

[MTd2]

I invited the authors to discuss their papers here.

 

[bcrowell]

This was interesting, and I spent some time studying the paper. It seems to me that the authors' claims are extravagant and not supported by their calculations. They claim it can be a real astrophysical object at center of galaxies, but to me it seems extremely implausible for it to form naturally, for two reasons: (1) to make $b^2>0$ you need a huge amount of charge (astronomical bodies are normally electrically neutral), and (2) you need the Misner string, which makes the spacetime not asymptotically flat.

 

[MTd2]

The authors will answer under the nickname of NUTW.

 

[NUTW]

Yes, the charge must be superextremal to evade formation of the horizon, so there is a danger of pair creation.
There are some extimates in the text, classical picture looks good for the scale of the centers of galaxies.
Charging mechanism is needed of course, this paper was not intended to develop a detailed theory.
The same for Misner strings, their possible sources have to be investifated. At this level our primary goal was
to attract attention to this novel type of wormholes.

 

[NUTW2]

It is not a claim, only a suggestion. Also, the charge may be purely magnetic (not electric), and supported by the wormhole (no source, except for a Dirac string). The spacetime would not be strictly be asymptotically flat when the Misner string extends to infinity, though it is not clear how this would be observed.
In the case of a Wheeler-Misner wormhole evoked near the end of the paper, topologically a torus with a point removed, the Misner string would be closed and the spacetime asymptotically flat.

 

[MTd2]

Considering the FRW model, there is no asymptotic flatness (though close). The string should close on itself after going round in a 3-sphere slice of the universe.

 

[NUTW2]

Considering the FRW model, there is no asymptotic flatness (though close). The string should close on itself after going round in a 3-sphere slice of the universe.

That's a possibility. But then the net NUT charge should vanish.

 

[bcrowell]

What seems much more remarkable to me is this earlier paper by the same authors:

Clement et al., "Rehabilitating space-times with NUTs," http://arxiv.org/abs/1508.07622

This is about the Taub-NUT spacetime, a well-known and physically well-motivated exact solution to the Einstein field equations that has been known for 50 years. Everyone believed for all those years that this spacetime was geodesically incomplete, but it turns out to be geodesically complete. Wow!