Talk:AdS/CFT correspondence/GA1

GA Review

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Reviewer: SPat (talk · contribs) 00:11, 14 September 2013 (UTC)

Initial remarks

I'm starting a review. On first read, I'm highly impressed by your ability to convey such a technical topic to a fairly wide audience while still not omitting too many details. I have some specific issues with style and presentation that I'll get to over the next couple of days; and I haven't had much time to look at the references; but overall, good job! SPat ^talk 00:11, 14 September 2013 (UTC)

Detailed comments

Layout

The basic layout seems to be Overview/Background->Applications->History. Is there a reason you've put history at the end? I believe the norm is to put history up front (right after the lead).

Overview

I think your goal here is to explain the main ideas behind the correspondence. However, to make things accessible you spend a fair amount of effort on introducing the basic terminology which end up looking like digressions. I think one way to solve this would be to split this section into two; a "Background" section describing to set up the terminology (holography/quantum gravity/general relativity/...) and an "Overview" section, where you actually describe the AdS/CFT correspondence.

• A holographic description of physics

Do you think a picture to illustrate the idea of a hologram would help? I can't think of anything in particular, but something that would illustrate the notion of depth being captured in a two-dimensional image. I'm sure commons will have something helpful. Also, you may not want to do it here itself, but you should provide some details about what exactly you mean by "equivalent formulation"

• Quantum theory and gravity

Again, would an illustration for compactification help?

• Gravity in anti-de Sitter space

This section, I think, has an issue in that you spend too much time making the reader familiar with the idea of non-Euclidean spaces at the expense of actually explaining AdS itself. I'll take a leap and say that majority of readers who come to this article will have at least heard of the idea of non-Euclidean geometry, and it might be enough to mention the non-flatness of space in a couple of sentences at most. Instead, I think it is much more important to explain what exactly an AdS space is, what (a sufficiently simplified version of) the metric tensor in this geometry look like, what is the importance of the negative Ricci scalar is, etc.

"In anti-de Sitter space, it is possible to define a notion of "boundary" of spacetime at infinity." This sentence needs a lot more explanation and details, as it pretty much seems to be the crux of the matter. Maybe a mathematical example would help. Also I don't know if the fact that this boundary is conformal is trivial or not - might be worth explaining.

It's probably going to be helpful to use atleast a few images describing the AdS geometry (some suggestions) The Escher picture, great as it is, is more of a distraction at the moment, I think. (Sorry if I'm appearing too picky about images!)

• The idea of AdS/CFT

I'd consider this subsection to be the heart of the article, and I think it needs a major expansion - maybe make it an entire section instead of a subsection.

"...can be regarded as the "spacetime" for a special type of quantum field theory called a conformal field theory." If you haven't done so earlier, this is a good place to explain what exactly a CFT is. For the "dictionary", is it possible to give a specific example of a correspondence? Maybe you could put that in the subsequent "Examples" subsection, but if possible you could take a specific (simplified?) example like AdS5/N=4SYM and write out some correspondences ("X-field in AdS goes to Y-interaction in CFT"). I don't know if it's actually possible to do something like that, I just think it would be helpful to add if you can.

(I've to leave now, will continue later...) SPat ^talk 19:47, 14 September 2013 (UTC)

Thank you very much for these comments! I don't know if you want me to start making changes now or wait until you finish reviewing. In any case, when you're ready, I will make the following changes to the article:

1. I will make a separate "Background" section combining material from the subsections entitled "A holographic description of physics" and "Quantum theory and gravity". This will include a more detailed discussion of the notion of duality in physics and (hopefully) some more pictures.

2. This will be followed by an "Overview" section explaining the geometry of AdS space and including a longer discussion of the idea of AdS/CFT. It is unfortunately not very easy to give a precise definition of AdS space without taking a lengthy detour with lots of mathematics. As explained in the anti-de Sitter space article, it is usually defined by taking a hyperboloid in an ambient space with two timelike directions and then passing to the universal cover. Instead of trying to explain all this, I think I'll try to make my own version of the picture in this article of a stack of Escher prints. This would be much more accessible, and it would help to address your concerns about pictures. I will also say more about conformal field theory and the notion of conformal boundary.

I was thinking of an explanation similar to the one given in the AdS article, in particular, emphasizing the fact that it is always a negative curvature. SPat ^talk 22:26, 15 September 2013 (UTC)

When I was writing the article, I decided to put the "History" section near the end because I felt that the reader would need to first have some understanding of the topic. The thinking was that it would make more sense to the reader after he or she understands what QCD is about and how N=4 SYM is related to string theory via AdS/CFT. Please let me know what you think about this. I'll move it if you think it's necessary, though I'm not sure whether it belongs before or after the "Background" section…

Thanks again for the feedback. Polytope24 (talk) 22:46, 14 September 2013 (UTC)

(Thanks for the quick response! I'll continue with the rest of my review and get back to your reply later) SPat ^talk 22:18, 15 September 2013 (UTC)

• Examples of the correspondence

Mostly ok. Maybe you could spend a couple of lines each explaining the newly introduced CFTs - the N=4 super Yang Mills, (2,0)-theory, ABJM superconformal field theory? It's very abstract as it stands.

Applications to quantum gravity

• A non-perturbative formulation of string theory

Is it factually accurate to say perturbative QFT was developed by Feynman? If not, it might suffice to say something like "...Richard Feynman and others".

"...and there are many problems for which one requires a non-perturbative formulation." An example? Isn't gravity the canonical example here? (I might be wrong about that last bit)

"The problem of developing... ...the AdS/CFT correspondence." [Maldacena 1998] doesn't seem to directly imply this. Maybe you could include a secondary reference?

• Black hole information paradox

Mostly ok. I didn't quite understand how AdS/CFT solves the problem though - maybe you could use an explanation similar to the one given in [Maldacena 2005]?

Applications to quantum field theory

• Nuclear physics

It seems the main arguments of this section rely on two primary sources, [Kovtun et al 2001, Luzum et al 2008]. You need to replace/supplement these with secondary sources. In other words, you need secondary sources that says something like "AdS/CFT calculations predicted a lower bound for a quantity which was experimentally confirmed at RHIC". Also, some details about the calculations would be nice.

• Condensed matter physics

As far as I understand, AdS/CMT doesn't say anything about Bose-Einstein Condensates in general. From what I've heard, AdS/CMT is mainly useful in cases where the CMT involve strongly-correlated systems, like in high-Tc superconductors. I think you should point out why people are using holographic methods in particular for condensed matter applications. Also, I'm not sure the superfluid-mott transition is the only known application of AdS/CMT - I vaguely remember reading about someone who calculated optical conductivity for a superconductor using AdS/CMT. Sachdev,McGreevy and co. have written dozens of articles about this - it might be useful to get a broader picture of the field than the one in [Sachdev 2013].

• Criticism

Looks pretty good.

History

The first two paragraphs seem to be a history of string theory in general and not AdS/CFT in particular. I'd suggest you condense these to a couple of sentences or so. But the rest of the section definitely needs major expansion - I'd say more than double. Is the large-N QCD stuff t'Hooft's only contribution to holography? I believe he also did some work on holographic cosmology and Hawking radiation. You should definitely say something about Susskind, and Stephen Hawking and Hawking radiation definitely deserve a mention. Are there any biographical accounts of Maldacena's first paper? That might be interesting to add. Also, Son's QCD stuff and Sachdev's CMT stuff should be discussed as important applications.

My theory about history sections is that they're the ones non-technical audiences will focus on, and so we should emphasize historical details as much as possible. That is things like the broad questions in the field, the dramatis personae, and locations/collaborations etc. This is also why I prefer to have them at the beginning. It's fine even if you end up repeating some stuff later in the article - as long as you re-word it properly.

• Most of the history section seems to be a repetition of things said earlier in the article. Maybe the history section could be collapsed to a simple list of sentence-long references further explained in the other sections? — Preceding unsigned comment added by 88.192.19.110 (talk) 16:04, 8 December 2013 (UTC)

Summary

Whew - I've put up a huge list of suggestions for improvement. I'm not saying all of them need to be incorporated to achieve GA status. However, I think the couple of points you do need to address are A) providing sufficient details in the main "Overview" section so that things don't end up looking too hand-wavy. (My interpretation of WP:TECHNICAL is that mathematical details should not be included if they act as a digression, but should definitely be included if they help in illustrating arguments.) B) you need expand and re-structure the history section so that it can function as a stand-alone piece. As I mentioned earlier, many non-technical readers will only read that one section and we shouldn't leave them empty handed. C) most of the references are ok, but you need to be careful about using secondary and not primary sources for citing important statements.

Let me know if you need any clarifications, and congratulations on the good job so far! SPat ^talk 22:18, 15 September 2013 (UTC)

Thanks for the very thoughtful review. Should I get started on revising the article now? Polytope24 (talk) 04:09, 16 September 2013 (UTC)

Sure, go ahead. Tip: to keep a track of things, it might be helpful if you respond to my comments immediately below that comment. SPat ^talk 14:44, 16 September 2013 (UTC)

Revised article

Alright. I just finished some major revisions to the article. In particular, I have made the following changes:

1. I've created a new "Background" section in which I review the basic ideas of quantum gravity, string theory, and quantum field theory.

2. The "Overview" section has been significantly expanded. I've added new pictures and more explanation of the geometry of AdS space. As I was saying before, it's a little complicated to give a precise definition of AdS space (i.e., you can't just write down a metric on R^n or something). In order keep the article reasonably accessible, I therefore decided to explain (2+1)-dimensional AdS space as a stack of hyperbolic disks, following Maldacena 2005. This should be accessible to non-experts, and it gives a pretty good idea of what AdS space actually is.

3. I've clarified parts of the section entitled "Applications to quantum gravity", following your suggestions.

4. I've found secondary sources supporting the claims in the section entitled "Applications to quantum field theory", and I removed the reference to Bose-Einstein Condensates.

5. I have significantly expanded the "History" section with lots of details about the Thorne-Hawking-Preskill bet, the holographic principle, and Son's work. I condensed the discussion of the early history of string theory but also explained why this is relevant to AdS/CFT. I still think it makes sense to put the "History" section near the end of the article since AdS/CFT is such an abstract idea which requires so much background to understand. Nevertheless, the section is non-technical enough that it can be read independently of the rest of the article.

Please let me know if you have any questions about the choices I've made. Thanks again. Polytope24 (talk) 01:29, 18 September 2013 (UTC)

I am, unfortunately, very busy at the moment and haven't had time to look carefully. But you (and other editors) have put in a very impressive effort - especially with the "Overview" and "History" sections. It looks like you're mostly good to go - I'll look over the references etc. one last time before signing off sometime in the next couple of days. One minor style point: per MOS:IMAGES, staggering of images is good, but may be a problem if the text gets sandwiched between images, especially for readers using small screens. Maybe you could fix that? SPat ^talk 11:47, 19 September 2013 (UTC)

Okay, I made come changes that should help with this. Polytope24 (talk) 15:54, 19 September 2013 (UTC)

 Done It may not be perfect, but it easily satisfies all GAC. Cheers! SPat ^talk 23:10, 19 September 2013 (UTC)