# Padmanabhan holographic gravity (Paris last week)

1. Jul 25, 2005

### marcus

Last week there was the Einstein Centenary conference in Paris and Padmanabhan was one of the invited speakers (with Gerard 't Hooft, Carlo Rovelli, Brian Greene, Abhay Ashtekar...)

I was reading a French blogger Fabien Besnard just now, to see what of interest he had to report from the Paris conference.
http://math-et-physique.over-blog.com/

Besnard seemed especially impressed by Padmanabhan's talk, which he said was about the "Constante cosmologique et gravité holographique" and in reporting it he gave this link to a recently published paper

http://fr.arxiv.org/abs/gr-qc/0412068
Holographic Gravity and the Surface term in the Einstein-Hilbert Action

"Certain peculiar features of Einstein-Hilbert (EH) action provide clues towards a holographic approach to gravity which is independent of the detailed microstructure of spacetime. These features of the EH action include: (a) the existence of second derivatives of dynamical variables; (b) a non trivial relation between the surface term and the bulk term; (c) the fact that surface term is non analytic in the coupling constant, when gravity is treated as a spin-2 perturbation around flat spacetime and (d) the form of the variation of the surface term under infinitesimal coordinate transformations. The surface term can be derived directly from very general considerations and using (d) one can obtain Einstein's equations just from the surface term of the action. Further one can relate the bulk term to the surface term and derive the full EH action based on purely thermodynamic considerations.

The features (a), (b) and (c) above emerge in a natural fashion in this approach. It is shown that action Agrav splits into two terms $S+\beta E$ in a natural manner in any stationary spacetime with horizon, where E is essentially an integral over ADM energy density and S arises from the integral of the surface gravity over the horizon. This analysis shows that the true degrees of freedom of gravity reside in the surface term of the action, making gravity intrinsically holographic. It also provides a close connection between gravity and gauge theories, and highlights the subtle role of the singular coordinate transformations."

It seems to me that we have been hearing about "the holographic universe" for roughly 20 years now, it is associated in my mind with Gerard 't Hooft, and also with Susskind: IIRC there was a SciAm article co-authored by Susskind about this not long ago.

but even though holography may be a common idea, Besnard thought Padmanabhan was adding some new insight. he used the English word "WOW", approvingly I think.

The article might interest some at PF. If anyone has a response, putting the Padmanabhan article into larger perspective, I'd be glad to hear it.

2. Jul 25, 2005

### marcus

<<Ce dernier point est développé ici. Les grandes idées sont les suivantes : 1) beaucoup de solutions des équations d'Einstein ont des horizons des événements 2) en conséquence l'action d'Einstein-Hilbert s'étend à des régions qui sont inaccessibles à certains observateurs 3) mais on peut exprimer une action équivalente contenant uniquement un terme de surface ! 4) on utilise un principe variationnel très différent de ce qu'on fait habituellement : on déplace infinitésimalement l'horizon 5) si ça vous rappelle vos cours de thermo c'est normal : ce qu'on fait c'est simplement calculer le changement d'entropie due à un travail infinitésimal sur une membrane, les équations d'Einstein sont en fait équivalentes à TdS=dE+PdV !
Wow... Je suis très loin de comprendre tout ça, d'autant plus que l'article est très dense, mais j'encourage vivement sa lecture. L'idée qui est derrière tout ça est que les quantités qui apparaissent en relativité générale sont en fait des grandeurs "thermodynamiques", dont la relation aux variables de la gravité quantique seraient de même nature que la relation entre la vitesse des particules d'un gaz et la température de ce gaz, autrement dit le lien entre la gravité quantique et la relativité générale serait du même ordre que celui qui unit physique statistique et thermodynamique. Ceci donne encore un peu plus de crédit à l'idée "d'atomes d'espace-temps", telle quelle peut apparaître par exemple en gravité quantique à boucles.
Bonne lecture ! >>

3. Jul 25, 2005

### Canute

I wish I understood a quarter of that, it sounds interesting. Without wishing to divert the discussion I should just mention that the holographic model has been around since the Upanishads, not just for twenty years. (In case you're interested the 'Jewel Net of Indra' is a good visual metaphor findable online).

Last edited: Jul 25, 2005
4. Jul 25, 2005

### ohwilleke

From www.systranbox.com

5. Jul 25, 2005

### marcus

Hi ohwilleke, thanks for providing the machine translation. here is another, done by hand for comparision (the machine got pretty close and by hand has its own errors and awkwardness)

"...This last point is developed here. The main ideas are the following:
1) many solutions of the Einstein equations have event horizons
2) consequently, the Einstein-Hilbert action extends to regions which are inaccessible to certain observers
3) but one can express an equivalent action containing only a surface term!
4) a variational principle is used quite different from the usual one: one displaces the horizon by an infintesimal amount
5) if that reminds you of your thermodynamics course, this is normal: what we're doing is simply calculating the change in entropy that results from an infinitesimal amount of work applied to a membrane, the Einstein equations are, in fact, equivalent to TdS=dE+PdV !

Wow...I am far from understanding all that, all the more so because the article is quite dense, but I strongly encourage reading it. The idea behind it all is that the quantities appearing in general relativity are actually "thermodynamic" values, whose relation to the variables of quantum gravity will be of the same nature as the relation between the speed of particles in a gas and the temperature of the gas.

to put it differently the connection between quantum gravity and general relativity will be on the same order as that linking statistical physics and thermodynamics. This gives still more credibility to the idea of "atoms of space-time" which appears, for example, in Loop Quantum Gravity.

Last edited: Jul 25, 2005
6. Jul 25, 2005

### ohwilleke

Good translation. Were I feeling more ambitious I would have done one (I did take many years of French). But, I'm glad I didn't as I'm sure that yours is better done than mine would have been.

The blogger IMHO really does a good job of summing up the holography concept.

7. Jul 25, 2005

### marcus

not a sure bet. please try doing one, then we can argue about merits

BTW you compliment the blogger Besnard. I like his style too. he writes the clear and simple essay: orderly and not overcrowded. his French is easier to read than some people's English

Just as a sample of Padmanabhan, here is the opening paragraph of his paper that Besnard was noticing.

Last edited: Jul 25, 2005
8. Jul 25, 2005

Staff Emeritus
I started to read the paper, but right away I ran into this even handed dissing of the grails of both string theory and LQG.

So not only is the stringists' vaunted graviton a meaningless sideshow, but the LQGers' beloved diff invariance may be a blunder!

Well, back to the paper.

9. Jul 25, 2005

### Berislav

I have been thinking about the implications of this for the last few days (before I heard about this paper). Especially how this relates to the geometry and topology of the particles themselves, which is of course important for higher-order diagrams. Maybe some constraints can be placed on a particle's geometry if we associate an observable with it? Namely the eigenvalues should never allow for the existance of an event horizon, even in areas inaccesible to observers, because otherwise particle would evaporate. For instance, in QED the S-matrix for two electrons and one photon is zero thus allowing a singe virtual photon to "enter" the inner event horizon (Hawking style) without being absorbed via electromagnetic processes.

Thank you for the information, marcus. It will be useful in my amateur attempts at research.

10. Jul 25, 2005

### marcus

dammit Berislav, do not be so modest! we all think you are brilliant
and going on to be a pro.

don't say "amateur attempts", simply say, without pretension,
"it will be useful in my research" (a highschooler or entering college student is certainly allowed to have research interests, right?)

no, I take that back, either way is OK, say it either way

11. Jul 25, 2005

### Berislav

Thank you very much for your compliments, marcus! Your confidence makes me very happy.

12. Jul 25, 2005

### marcus

I am not complimenting, I am acknowledging something. but this doesnt matter. either way of talking (modest or not modest) is fine.

let's see if we can get somewhere with this Padmanabhan paper
(as before I interrupted)

13. Jul 25, 2005

### Berislav

From the paper, p. 3, below eq. (10)
Very interesting! The surface term ($A_{sur}$) is non-vanishing even in the Newtonian limit?! That, I think, adds credence to this thermodynamic approach.

14. Jul 25, 2005

### Spin_Network

But My handwaving was ignored here:https://www.physicsforums.com/showthread.php?t=56909
? ?

The date and my statement says it all

15. Jul 25, 2005

### marcus

You prophetic soul!
You posted that 15 December of last year!
So you are the same person who used to sign "Wave_Hands_Particle", and I remember you from even before that.
Several times you have found papers which at the time I overlooked and then for one reason or another I or somebody else encountered them and we started discussing the paper, forgetting that you had started a thread about it weeks earlier. Well well.

You surely deserve to gloat a little, and to be congratulated Spin_Network

here is the 15 December post
God, it's noisy here, and I am making about 60 percent of the uproar. I will quiet down. BTW Berislav just made a point a couple of posts back.

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16. Jul 25, 2005

### Spin_Network

Thanks marcus..but you have just posted a number of papers elswhere..which no doubt have to be read!..again many thanks.

17. Jul 25, 2005

### marcus

that is largely just library-keeping. nothing urgent, certainly nothing visionary. I just have to keep that library thread updated or else I lose track, more articles get lost.
a new Lee Smolin article just appeared. but I think we mostly all know the points he makes in it, so it basically could just be a review or something to reference.

Let's get back to the Padmananbhan topic if we can! selfAdjoint was reading the article, and also Berislav. You recommended it months ago. We should all concentrate and really see if it is good or not.

18. Jul 25, 2005

### Spin_Network

Ok, I have to dig out the paper I have printed here, its amoungst at least several hundred, or it may be I have a word-document on my other comp, thats where I store at least 2,000 of my fav pre-print papers. Having allready read the paper at its appearence, I no doubt have commented it on another forum, but I know that forum is no longer a viable source(superstringtheory.com), so I will do a manual search at home, and get back pronto.

19. Jul 27, 2005

### Mike2

Are gauge theories a quantum mechanical, QFT, or classical approach to physics? Thanks.

20. Jul 27, 2005

### marcus

There are gauge theories in several branches of physics. The method isnt restricted to one branch. A possible specific question here would be what kind of gauge theory is he talking about?

Mike, here is my post #1 of this thread. You will see that the passage you asked about comes from the abstract written by Padmanabhan, summarizing his article:

I have bolded the part of Padmanabhan's summary that you asked about. since it is a summary of the article, to find out what it means one needs to look at the article and find where he expands on that part of the condensed summary.

Mike, I believe the answer to your question is "all of the above"
is that satisfactory?

Last edited: Jul 27, 2005