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Entanglement islands in higher dimensions
by Ahmed Almheiri, Raghu Mahajan, Jorge E. Santos
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Submission summary
Authors (as registered SciPost users):  Raghu Mahajan 
Submission information  

Preprint Link:  https://arxiv.org/abs/1911.09666v2 (pdf) 
Date submitted:  20200114 01:00 
Submitted by:  Mahajan, Raghu 
Submitted to:  SciPost Physics 
Ontological classification  

Academic field:  Physics 
Specialties: 

Approaches:  Theoretical, Computational 
Abstract
It has been suggested in recent work that the Page curve of Hawking radiation can be recovered using computations in semiclassical gravity provided one allows for "islands" in the gravity region of quantum systems coupled to gravity. The explicit computations so far have been restricted to black holes in twodimensional JackiwTeitelboim gravity. In this note, we numerically construct a fivedimensional asymptotically AdS geometry whose boundary realizes a fourdimensional HartleHawking state on an eternal AdS black hole in equilibrium with a bath. We also numerically find two types of extremal surfaces: ones that correspond to having or not having an island. The version of the information paradox involving the eternal black hole exists in this setup, and it is avoided by the presence of islands. Thus, recent computations exhibiting islands in twodimensional gravity generalize to higher dimensions as well.
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Reports on this Submission
Anonymous Report 2 on 202046 (Invited Report)
 Cite as: Anonymous, Report on arXiv:1911.09666v2, delivered 20200406, doi: 10.21468/SciPost.Report.1611
Strengths
1. Generalises the doubly holographic calculations showing the emergence of an "island" from [14] to higher dimensions. A useful and valuable goal.
2. Technically precise and clearly presented.
Weaknesses
1. Little discussion of the qualitative differences between twodimensions and higherdimensions
Report
The paper uses tools from numerical relativity to calculate the Page curve of an equilibrating black hole in a doubly holographic theory with 3+1 dimensions. It is a natural and important followup to the previous work on the subject (e.g. [1215]), which mostly focussed on twodimensional (or effectively twodimensional) theories (although note the discussion of higherdimensions and greybody factors in [12]). Twodimensional gravity theories are more analytically tractable but are obviously less similar to reality.
Since the main technical difference between twodimensions and higherdimensions is the existence of greybody factors, it would be nice to have more discussion about how these effects can be seen in the doubly holographic description, where they are not particularly manifest. Ideally it would be nice to have a calculation where some parameters of the black hole can be dialed to change the greybody factors, so that its effect on the growth of the doublyholographic HRT surface can be seen. This may be impractical, of course. Regardless, some more discussion would be nice.
Requested changes
1. It would be good to see an explicit comparison of the results in the present paper with the results about higherdimensions and greybody factors from [12] (presumably the authors would note that in [12] the location of the QES was only calculated up to unknown O(beta) corrections in the retarded time  it would be nice to see this explicitly commented on however).
2. Some discussion how greybody factors affect the equilibration process and how this can be seen in the doubly holographic description.
Anonymous Report 1 on 2020212 (Invited Report)
 Cite as: Anonymous, Report on arXiv:1911.09666v2, delivered 20200212, doi: 10.21468/SciPost.Report.1504
Strengths
1 Clear, wellmotivated, and interesting goal (find island in higher dimensional BH setup).
2 Good, logical explanation of method and results.
Weaknesses
1 Does not discuss the existing higher dimensional analysis in their reference 12, by Penington.
2 Unclear why the RS brane is allowed to have RT surface dynamically end on it and yet be considered part of the `boundary.’
Report
This is a thorough, clear analysis that answers an interesting question. The authors have done a service for the field by writing this up.
Two comments:
There could be some improvement in how these results are related to prior work. In particular, in reference [12], Penington argues for the existence of these islands in general dimensions. While he does make simplifying assumptions (spherical symmetry), his arguments are relevant to the goal of this paper and should be acknowledged.
How are we supposed to understand the seeminglyinconsistent treatment of the RS brane? Sometimes it acts like part of the "bulk’’ (because the RT surface is allowed to end on it at a dynamical location), and other times it acts like part of the "boundary’’ (because its degrees of freedom are understood to be entangled with the "true boundary’’ degrees of freedom). Is one of these interpretations more correct, e.g. that it should only be understood as part of the bulk (in which case how do we understand its entanglement with the "true boundary’’)? Or perhaps is the answer unknown, and you are simply demonstrating that treating the RS brane this way gives an interesting answer?
Requested changes
1 Add a sentence or two about Penington’s higherdimensional island arguments. If what you are doing is more impressive for certain reasons, mention!
2 Add a couple sentences or a footnote about how we should think of the RS brane. As part of the bulk, part of the boundary, or somehow both?
Author: Raghu Mahajan on 20200527 [id 840]
(in reply to Report 1 on 20200212)
Thank you very much for the report! We have addressed your requested changes as follows:
1 We have added a new section 4.3 comparing our work to Penington's
2 We have added some discussion about the fundamental microscopic description of the system under study, and its two gravitational avatars. This is on top of page 5.
Author: Raghu Mahajan on 20200527 [id 841]
(in reply to Report 2 on 20200406)Thank you for the report. We have addressed the requested changes as follows:
We have added a new section 4.3 that includes a discussion of results of reference [12]. We have also added some comments on greybody factors.