Padmanabhan holographic gravity (Paris last week)

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marcus

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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 [itex]S+\beta E[/itex] 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.
 

marcus

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In case anyone is interested (and likes reading the occasional bit of french) here's what Fabien said about this article:
<<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 ! >>
 
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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).
 
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ohwilleke

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From www.systranbox.com

This last point is developed here. The great ideas are as follows: 1)
much of solutions of the equations of Einstein horizons of the
consequently events have 2) the action of Einstein-Hilbert extends to
areas which are inaccessible to certain observers 3) but one can
express an equivalent action containing only one term of surface! 4)
one uses a variational principle very different from what one usually
does: the horizon infinitésimalement is moved 5) if that points out
your courses of thermo to you it is normal: how one does is simply to
calculate the change of entropy due to an infinitesimal work on a
membrane, the equations of Einstein are in fact equivalent to
TdS=dE+PdV! Wow... I am very far from including/understanding all
that, more especially as the article is very dense, but I encourage
his reading highly. The idea which is behind all that is that the
quantities which appear in general relativity are in fact of the
"thermodynamic" sizes, whose relation with the variables of quantum
gravity would be of comparable nature that the relation between the
speed of the particles of a gas and the temperature of this gas, in
other words the bond between quantum gravity and general relativity
would be of the same order as that which links physical statistics and
thermodynamics. This gives still a little more credit to the idea "of
atoms of space time", just as it is can appear for example in quantum
gravity with loops. Good reading!
 

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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.

Good reading!"
 
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ohwilleke

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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.
 

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feeling more ambitious I would have done one (I did take many years of French)... yours is better done than mine would have been.
...
not a sure bet. please try doing one, then we can argue about merits :smile:

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.

arXiv:gr-qc/0412068

If we treat the macroscopic spacetime as analogous to a continuum solid and the unknown microscopic structure of spacetime as analogous to the atomic structure [1], then it is possible to gain some important insights into the possible nature of quantum gravity.

First of all, we note that the macroscopic description of a solid uses concepts like density, stress and strain, bulk velocity etc., none of which can even be usefully defined in the microscopic description. Similarly, variables like metric tensor etc. may not have any relevance in quantum gravity.

Second, the quantum theory of a spin-2 field (“graviton”) will be as irrelevant in quantum gravity, as the theory of phonons in providing any insight into the electronic structure of atoms.

Third, the symmetries of the continuum description (e.g., translation, rotation etc.) will be invalid or will get strongly modified in the microscopic description. A naive insistence of diffeomorphism invariance in the quantum gravity, based on the classical symmetries, will be as misleading as insisting on infinitesimal rotational invariance of, say, an atomic crystal lattice.
 
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selfAdjoint

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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.


If we treat the macroscopic spacetime as analogous to a continuum solid and the unknown microscopic structure of spacetime as analogous to the atomic structure
[1], then it is possible to gain some important insights into the possible nature of quantum gravity. First of all, we note that the macroscopic description of a solid uses
concepts like density, stress and strain, bulk velocity etc., none of which can even be usefully defined in the microscopic description. Similarly, variables like metric tensor
etc. may not have any relevance in quantum gravity. Second, the quantum theory of a spin-2 field (“graviton”) will be as irrelevant in quantum gravity, as the theory
of phonons in providing any insight into the electronic structure of atoms. Third, the symmetries of the continuum description (e.g., translation, rotation etc.) will be
invalid or will get strongly modified in the microscopic description. A naive insistence of diffeomorphism invariance in the quantum gravity, based on the classical symmetries,
will be as misleading as insisting on infinitesimal rotational invariance of, say, an atomic crystal lattice. In short, the variables and the description will change in an
(as yet unknown) manner. It is worth remembering that the Planck scale (1019 GeV) is much farther away from the highest energy scale we have in the lab (102 GeV)
than the atomic scale (10−8 cm) was from the scales of continuum physics (1 cm).
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.
 
1) many solutions of the Einstein equations have event horizons
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.
 

marcus

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Berislav said:
Thank you for the information, marcus. It will be useful in my amateur attempts at research.
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
 
marcus said:
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
Thank you very much for your compliments, marcus! Your confidence makes me very happy.
 

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Berislav said:
Thank you very much for your compliments, marcus! Your confidence makes me very happy.
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)
 
From the paper, p. 3, below eq. (10)
In the Newtonian limit this leads [itex]P=-2g[/itex]
Very interesting! The surface term ([itex]A_{sur}[/itex]) is non-vanishing even in the Newtonian limit?! That, I think, adds credence to this thermodynamic approach.
 
marcus said:
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 [itex]S+\beta E[/itex] 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.
But My handwaving was ignored here:https://www.physicsforums.com/showthread.php?t=56909
? :rolleyes: ?

The date and my statement says it all :biggrin: :approve:
 

marcus

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

The date and my statement says it all :biggrin: :approve:
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
Wave's_Hand_Particle said:
Here is a pretty amazing paper:http://arxiv.org/abs/gr-qc/0412068

Quote:This procedure will throw light on several peculiar features of gravity (which have no explanation in the conventional approach) and
will provide a new insight in interpreting general coordinate
transformations.

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

marcus

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you have just posted a number of papers elswhere..which no doubt have to be read!...
that is largely just library-keeping. nothing urgent, certainly nothing visionary. :smile: 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.
 
marcus said:
that is largely just library-keeping. nothing urgent, certainly nothing visionary. :smile: 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.
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.
 
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marcus said:
It also provides a close connection between gravity and gauge theories, and highlights the subtle role of the singular coordinate transformations."
Are gauge theories a quantum mechanical, QFT, or classical approach to physics? Thanks.
 

marcus

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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:

marcus said:
Last week there was the Einstein Centenary conference ...

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 [itex]S+\beta E[/itex] 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."

...
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" :smile:
is that satisfactory?
 
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selfAdjoint

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Mike2 said:
Are gauge theories a quantum mechanical, QFT, or classical approach to physics? Thanks.
Gauge theoriws are a form of QFT. They are often called Gauge Field Theories. QED is a global gauge theory and Yang-Mills theory is a local gauge theory. In a gauge theory the physics is invariant under the operations of some group, in a global gauge theory the action happens everywher (in QED it is a change in phase of the waves). In a local gauge theory the operations happen as you move from place to place.
 

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selfAdjoint said:
Gauge theoriws are a form of QFT. They are often called Gauge Field Theories. QED is a global gauge theory and Yang-Mills theory is a local gauge theory. In a gauge theory the physics is invariant under the operations of some group, in a global gauge theory the action happens everywher (in QED it is a change in phase of the waves). In a local gauge theory the operations happen as you move from place to place.
I found the part in Padmanabhan where he talks about the analogy between gravity and non-Abelian gauge theories. It is on page 8, second paragraph or so, around eqn. (29).

He also talks about classical electrodynamics at that point, if I am not mistaken.

It looks to me as if he is drawing an analogy between classical (unquantized) gravity, say with ashtekar connection variables, and classical (unquantized) field theory. I could be mistaken, of course, but I think the analogy exists at the classical level as well as quantum level
 
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marcus said:
I found the part in Padmanabhan where he talks about the analogy between gravity and non-Abelian gauge theories. It is on page 8, second paragraph or so, around eqn. (29).

He also talks about classical electrodynamics at that point, if I am not mistaken.

It looks to me as if he is drawing an analogy between classical (unquantized) gravity, say with ashtekar connection variables, and classical (unquantized) field theory. I could be mistaken, of course, but I think the analogy exists at the classical level as well as quantum level
Marcus, it is early days , but I believe that the whole premise is based on emergence?..for instance the 'analogy' of emerging actions of say, Solid to Liquids? R Laughlin paper has some relevance.

The Holographic Principle in this light is a virtual process embedded into a background, 'real' field? A hologram for instance is 'fixed' inside a 'holding-frame'..glass for instance? ..the hologram does not move, if you rotate the glass, you get different observations of the Hologram, the hologram appears to be dynamic, yet fixed! Observer frame dependant!

Do microscopic quanta project 'virtual' energies into the surrounding space that encompasses the area from below the planck length?..does the rotation of an extended field, cause a local effects by projecting virtual particles that interact with the 'local' field?

Now I have not found an important paper, but it seems 'twistor theory' may be able to shed some light?..as an example, if one has a 'rubber' mass, fixed to a location, and turns the rubber, it twists at one location and spreads outwards. I am just trying to convey an important correlation to how far of effects can appear local, so if one has a large wheel(like a ships steering helm), made from 'rubber', you can turn it 720%, the strain energy appears at the centre, the 'twists' appear local to the strain, but not at the extreme edge of the wheel.

The consequence of this is that the flexible rubber loses its ability to 'flex', and has become a solid at its core, to imagine this, hitting a rubber in its 'normal' state, produces low-energy, resinates at a low frequency that does not travel much farther than the local hitting point, one can say that there is a high 'absorbtion', but put the rubber under enormous stress, and the resanation travels further, it does not absorb into local field because the field is already maxxed out.

What this got to do with the Holographic Principle?..stay tuned for some Relativistic Implications for, gauge actions and field effects!

P.S just found this, so it may be of use to conform some light into discussions:http://uk.arxiv.org/abs/hep-th/0507260
 
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marcus

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My first awareness of Thanu Padmanabhan, or contact with any of his writing, was when selfAdjoint started this thread

https://www.physicsforums.com/showthread.php?t=44414

here was the orig. post
String Gravitons yield GR. NOT

This paper does a lot of testing of different kinds, and concludes that the string theorists assertion that the graviton reproduces the physics of GR in flat spacetime is a myth.
it brings up an interesting question. the original paper was not published but the SAME MESSAGE has since appeared elsewhere.

why was the original paper not published? was this by Padmanabhan's choice? did it have anything to do with the main message and supporting arguments, or was it for some other reason?

There was report of a rumor that something might be wrong with the idea and that particular paper may not have been passed by the referees. But we do not have any clear evidence of this. In any case peer review did not block the idea.

Padmanabhan's stature is such that I tend to suspect that he can publish anything he wants to have published. Therefore if he writes something and doesnt publish it, then (I would suspect absent evidence to the contrary) stemmed at least partially from his own decision. Now it seems (from this more recent paper) that he has not changed his mind!

So this is an unresolved issue for me, still needs sorting out.
 
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marcus

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here's the orig. paper
http://www.arxiv.org/abs/gr-qc/0409089


and here is a quote from the more recent "Holographic gravity" paper,
http://arxiv.org/gr-qc/0412068 [Broken]

--quote from page 7--
...Thus the full Einstein-Hilbert Lagrangian is non-analytic in [itex]\lambda[/itex] because the surface term is non-analytic in [itex]\lambda[/itex] ! It is sometimes claimed in literature that one can obtain Einstein-Hilbert action for gravity by starting with a massless spin-2 field [itex]h_{ab}[/itex] coupled to the energy momentum tensor [itex]T_{ab}[/itex] of other matter sources to the lowest order, introducing self-coupling of [itex]h_{ab}[/itex] to its own energy momentum tensor at the next order and iterating the process.
It will be preposterous if, starting from the Lagrangian for the spin-2 field... and doing a honest iteration on [itex]\lambda[/itex], one can obtain a piece which is non-analytic in [itex]\lambda[/itex] (for a detailed discussion of this and related issues, see [24]). ...The nonanalytic nature of the surface term is vital for it to give a finite contribution on the horizon and the horizon entropy cannot be interpreted in terms of gravitons propagating around Minkowski spacetime. Clearly, there is lot more to gravity than gravitons.
--end quote--

the reference [24] is to the orig. paper "From Gravitons to Gravity: Myths and Reality"
 
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