# Interpretation of the Einstein-Hilbert action

+ 5 like - 0 dislike
18 views

Everyone knows the famous Einstein-Hilbert action $S_{EH} = \int d^4x \sqrt{-g} R$. I'd like to know if, after we first explicit the Ricci scalar in terms of the metric, it could be possible to interpret each one of the terms as a kinetic or potential term, and even more read what could be the free propagator of the graviton, and the graviton-graviton vertexes. If yes, what are the form of those vertexes (how many? are some of them derivatives couplings? how many gravitons per vertex?)

Thanks.

This post imported from StackExchange Physics at 2014-03-22 17:22 (UCT), posted by SE-user toot
To clarify: are you asking what the Feynman rules for GR (based on the EH action, and known to be perturbatively unrenormalizable) would look like?

This post imported from StackExchange Physics at 2014-03-22 17:22 (UCT), posted by SE-user twistor59
In some way: yes.

This post imported from StackExchange Physics at 2014-03-22 17:22 (UCT), posted by SE-user toot
The second derivative terms in R are perfect derivatives, the first derivative terms are a form of pseudo-stress-energy and they have kinetic-potential interpretation. The perturbation theory terms are straightforward to work out but practically hopeless--- there are too many vertices that are too complicated. Here supergravity and string theory are essential--- several graviton scattering amplitudes were computed from string theory first. There is a lot of recent literature on this, look up Dixon's recent papers.

This post imported from StackExchange Physics at 2014-03-22 17:22 (UCT), posted by SE-user Ron Maimon

+ 3 like - 0 dislike

The only reference I seem to have to this material is a review article by Duff[*] which states some results of calculations performed using the quantities:

$$\tilde{g}^{\mu\nu}\equiv\sqrt{g}g^{\mu\nu}$$ $$\tilde{g}_{\mu\nu}\equiv\frac{1}{\sqrt{g}}g_{\mu\nu}$$

The graviton field $h^{\mu\nu}$ is defined by a perturbation about flat space $$\tilde{g}^{\mu\nu}=\eta^{\mu\nu}+\kappa h^{\mu\nu}$$ together with the corresponding quantity $$\tilde{g}_{\mu\nu}=\eta_{\mu\nu}-\kappa h_{\mu\nu}+\kappa^2h_{\mu\alpha}h_{\alpha\nu}+...$$ (here $\kappa=\sqrt{16\pi G}$). The free graviton momentum space propagator (after some gauge fixing choices) looks like $$D_{\mu\nu\rho\sigma}(p^2)=\frac{1}{p^2}(\eta_{\mu\rho}\eta_{\nu\sigma}+\eta_{\mu\sigma}\eta_{\nu\rho}-\eta_{\mu\nu}\eta_{\rho\sigma})$$

There are expressions for 3-point, 4-point etc vertices which look rather complicated.

ETA: I found this online reference. The treatment discussing the propagator is around equation (65) onwards. I suspect that there will be much more detail in the original papers of Feynman and de Witt, but I don't have access to them.

[*] M.J.Duff "Covariant Quantization" in Quantum Gravity-an Oxford Symposium. ed Isham, Penrose, Sciama. OUP 1975

This post imported from StackExchange Physics at 2014-03-22 17:22 (UCT), posted by SE-user twistor59
answered Apr 24, 2012 by (2,490 points)

 Please use answers only to (at least partly) answer questions. To comment, discuss, or ask for clarification, leave a comment instead. To mask links under text, please type your text, highlight it, and click the "link" button. You can then enter your link URL. Please consult the FAQ for as to how to format your post. This is the answer box; if you want to write a comment instead, please use the 'add comment' button. Live preview (may slow down editor)   Preview Your name to display (optional): Email me at this address if my answer is selected or commented on: Privacy: Your email address will only be used for sending these notifications. Anti-spam verification: If you are a human please identify the position of the character covered by the symbol $\varnothing$ in the following word:p$\hbar$ysi$\varnothing$sOverflowThen drag the red bullet below over the corresponding character of our banner. When you drop it there, the bullet changes to green (on slow internet connections after a few seconds). To avoid this verification in future, please log in or register.