- #1

marcus

Science Advisor

Gold Member

Dearly Missed

- 24,738

- 785

## Main Question or Discussion Point

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

"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)

The features (a), (b) and (c) above emerge in a natural fashion in this approach. It is shown that action A

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.

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 A

_{grav}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.