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Towards traversable wormholes from forcefree plasmas
by Nabil Iqbal, Simon F. Ross
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Submission summary
Authors (as registered SciPost users):  Nabil Iqbal · Simon Ross 
Submission information  

Preprint Link:  https://arxiv.org/abs/2103.01920v1 (pdf) 
Date submitted:  20210325 13:34 
Submitted by:  Iqbal, Nabil 
Submitted to:  SciPost Physics 
Ontological classification  

Academic field:  Physics 
Specialties: 

Approach:  Theoretical 
Abstract
The nearhorizon region of magnetically charged black holes can have very strong magnetic fields. A useful lowenergy effective theory for fluctuations of the fields, coupled to electrically charged particles, is forcefree electrodynamics. The low energy collective excitations include a large number of Alfven wave modes, which have a massless dispersion relation along the field worldlines. We attempt to construct traversable wormhole solutions using the negative Casimir energy of the Alfven wave modes, analogously to the recent construction using charged massless fermions. The behaviour of massless scalars in the nearhorizon region implies that the size of the wormholes is strongly restricted and cannot be made large, even though the force free description is valid in a larger regime.
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Reports on this Submission
Anonymous Report 3 on 2021521 (Invited Report)
 Cite as: Anonymous, Report on arXiv:2103.01920v1, delivered 20210521, doi: 10.21468/SciPost.Report.2951
Strengths
1) Very well written paper
2) Exceptionally clear
2) Very novel idea
3) Clear description of calculations
Weaknesses
1) The mechanism they study unfortunately doesn't lead to large traversable wormholes
Report
This paper attempts to construct large traversable wormholes using standard physics arising from charged particles interacting with a strong magnetic field, in regimes where a particular low energy effective theory is useful. The problem of constructing large wormholes without invoking physics beyond the standard model is of great interest, and the approach taken here is a novel and interesting mechanism, certainly worthy of exploration.
This is a very well written paper, that is exceptionally clear and well organized  and overall very enjoyable to read. I therefore recommend the paper for publication.
Anonymous Report 2 on 2021518 (Invited Report)
 Cite as: Anonymous, Report on arXiv:2103.01920v1, delivered 20210518, doi: 10.21468/SciPost.Report.2933
Report
Constructing traversable wormholes in the low energy theory of our universe is an important and exciting problem. This paper proposes an interesting new idea to achieve this by using collective light modes arising from the forcefree electrodynamics description (which has proven useful in astrophysical setups). At the end, the simple setup described in the paper doesn't lead to a consistent wormhole solution. Nonetheless, the idea is interesting and deserves further study. The nearlyAdS2 perspective on these collective modes is also of some interest. The analysis is clear and convincing. Therefore, I recommend this paper for publication in SciPost Physics.
That said, I would like to ask the authors some minor clarifications/questions:
1) Is it be possible to clarify a little bit the physical setup, to give some intuition to a reader who doesn't know much about FFE? If I understand correctly, the Alfven modes are supposed to describe coherent excitations of the plasma and EM field, so does this mean that the hypothetical wormhole must be filled with plasma? Does this has any physical significance?
2) The Alfven wave modes are supposed to follow the magnetic field lines in closed loops threading the wormhole. A question that I haven't seen addressed is the following: is it clear that the FFE approximation is also valid in the outside region, far from the black holes? Shouldn't this put a constraint on the outside distance d?
3) In [1], a large magnetic field was crucial to have a large number of Casimir modes. This doesn't seem to be the case here where it's only needed for the FFE to be valid. So could we repeat the analysis in a more realistic astrophysical setup? For example by replacing the RN black hole with nearextreme Kerr in a strong magnetic field?
Anonymous Report 1 on 202151 (Invited Report)
 Cite as: Anonymous, Report on arXiv:2103.01920v1, delivered 20210501, doi: 10.21468/SciPost.Report.2864
Report
The paper studies the possibility of making travesable wormholes in 4d Einstein gravity by using Casimir energy of Alfeven waves of forcefree electrodynamics(FFE). Although the idea of using Casimir energy is not new, and ultimately authors encountered serious obstacles to realizing this idea, FFE near black holes is an interesting topic which recently drew some attention in the literature. Also taking into account that the paper is clearly written and authors discuss various physical effects, I recommend the paper
for publication with major revisions.
Here is the list of questions I would like the authors to address:
1) FFE seems to be an emergent phenomena when standard electrodynamics interacts with matter. The authors considered the correction to the (total) stressenergy tensor from Alfven waves. Does this take into account the stressenergy tensor of underlying matter?
2) It seems that eventually authors impose simple periodic boundary conditions on matter. Is Section 4 necessary then?
3) The conclusion of the paper seems that it is hard to sustain a wormhole with FFE in 4d. In higher dimensions the black hole story is well studied(AdS2 near horizon, 2d Casimir energy, etc), _except_ the dispersion relation for Alfven waves. It would be nice if the author could comment on this.
4) The lengthy calculation in Section 5.2 results in the condition $m^2 < 1/(R l)$. The authors explain this result as: "...AdS kinematics imply that the relevant mass scale is the geometric mean...". I think it would be very helpful for readers to see this kinematic estimate explicitly. Naively, in eq. (41) mass is responsible for $1/(\cos^2 \sigma)$ potential, which is cutoff at $\sigma\pi/2 \sim R/l $. Since $\omega \gtrsim 1$, the effect of the mass is negligible for $m l \lesssim 1$.