General Relativity from the String Theory Point of View

Question

I have a hard time understand the statement that

When you only look at the classical limit or classical physics, string theory exactly agrees with general relativity

Because from what I know, String Theory assumes a fixed space time background (ie, all the strings and membranes interact in a fix background, and their interaction gives rise to fundamental particles that we observe), but General Relativity assumes that the space time background is influenced by what is in it and the interaction between them.

Given that both have very different assumptions, what do string theorists mean when they say string theory agrees with general relativity in a classical limit? Or more specifically, how is string theory--a fixed spacetime background theory--reconciled with the general relativity on dynamic spacetime background part? I can understand a fixed, static spacetime in the context of changing, dynamic spacetime background, but I cannot understand a changing, dynamic spacetime in the context of a fixed, static spacetime background.

This post imported from StackExchange Physics at 2014-03-07 16:32 (UCT), posted by SE-user Graviton

Comments

Polyakov, gravitons are not fermions-- what? I am not fermions? :-)

This post imported from StackExchange Physics at 2014-03-07 16:32 (UCT), posted by SE-user Graviton

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@Graviton: one approach to string theory is the perturbative expansion on a flat spacetime. However this does not constitute string theory, just one method of doing calculations with it.

This post imported from StackExchange Physics at 2014-03-07 16:32 (UCT), posted by SE-user John Rennie

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@JohnRennie what do you mean by your comment? Of course is string field theory string theory too ... ;-)!

This post imported from StackExchange Physics at 2014-03-07 16:32 (UCT), posted by SE-user Dilaton

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@Dilaton, Graviton: Sad to hear that your brother "Photon" and all but 1 of your bosonic cousins (the bosons born to Ramond and Ramond) were killed by worldsheet parity. At least, photon is reborn as your second cousin (Born to Neveu-Schwarz the bacterium). One of the Gravitino twins and the Dilatino twins were already killed by the cruel GSO Projection (1 Dilatino/Gravitino was born 2 Ramond and Neveu Schwarz, the other was born to Neveu Schwarz and Ramond). At least some new gauge bosons and gauginos were also born to help you recover from the tragedy : ( .

This post imported from StackExchange Physics at 2014-03-07 16:32 (UCT), posted by SE-user Dimensio1n0

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Possible duplicates: physics.stackexchange.com/q/1073/2451 , physics.stackexchange.com/q/5815/2451 , physics.stackexchange.com/q/54317/2451 and links therein.

This post imported from StackExchange Physics at 2014-03-07 16:32 (UCT), posted by SE-user Qmechanic

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@BenCrowell, so can we say that in string theory there is nothing like dynamic, curvy spacetime, because everything is done in the background of static, flat splacetime?

This post imported from StackExchange Physics at 2014-03-07 16:32 (UCT), posted by SE-user Graviton

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This answer to the other question explains pretty well how general relativity works in string theory and clarifies the background (in)dependence issue.

This post imported from StackExchange Physics at 2014-03-07 16:32 (UCT), posted by SE-user Dilaton

Answer 1

UPDATE: I have written a more complete answer here: How do the Einstein's equations come out of string theory?

If you take the Polyakov (gravitons are bosons) Action:

$$S_P=-\frac{T}{2}\int \sqrt{\pm h}h^{\alpha\beta}\partial_\alpha X^\mu\partial_\beta X^\nu g_{\mu\nu}\mbox{ d}^2\xi$$

And take the gravitational terms of a somewhat "effective" spacetime action, you get

$$S_{G}=\lambda\int\left(R+\ell_s^2R_{\mu\nu\rho\sigma}R^{\mu\nu\rho\sigma}\right)\mbox{ d}^D x$$

Where we neglected terms of order $\ell_s^4$ and greater. Since $\ell_s$, the string length, is very small, this is approximately, (if we let ${\ell_s\rightarrow0}$)

$$S_{EH}=\lambda\int R\mbox{ d}^D x$$

The generalised (n-dimensional) EH Action. This is what is meant by string theory going to GR at the classical limit, the Polyakov Action simply goes down to the EH Action.

Edit: Also see JoshPhysics's answer here: In what limit does string theory reproduce general relativity?. The method I stated here is possible in principle, but is much more commplicated than the JoshPhysics's answer there. In his answer, he simply uses the Beta functional, $$\beta^G_{\mu\nu} = \ell_s^2 R_{\mu\nu}+\ell_s^4R_{\mu\nu}R_{\mu\nu\rho\sigma}R^{\mu\nu\rho\sigma}+...$$ Then, setting the LHS to 0 to preserve conformal invariance:

$$R_{\mu\nu}+\ell_s^2R_{\mu\nu}R_{\mu\nu\rho\sigma}R^{\mu\nu\rho\sigma}+...=0$$

For weak gravity, all terms except the first vanish, so that

$$R_{\mu\nu}=0$$

Comments

This is correct, but the main detail missing is the Ward identity--- this is derived in chapter 2 of Green Schwarz Witten--- a variation in the metric is equal to a graviton insertion, and diffeomorphisms don't count, so you get a Ward identity.

This post imported from StackExchange Physics at 2014-03-07 16:32 (UCT), posted by SE-user Ron Maimon

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All the conclusions are correct, but your next-to-leading order equation is incorrect. The second term is quadratic in the Riemann tensor, and has the form $\frac{\alpha^{\prime}}{2} R_{\mu \rho \sigma \tau} R_{\nu}^{\phantom{a} \rho \sigma \tau}$. See Green, Schwarz, Witten, for example.

Answer 2

First of all, the statement is by design that perturbative string theory reproduces perturbative quantum-gravity+Yang-Mills at low energy, for perturbation about any solution to the supergravity equations of motion (what user "dimension10" mentions is one part of the statement that perturbative string theory around such backgrounds is consistent to start with). Notice that this perturbative nature is not some secret bug, but is so by the very nature of what perturbation theory is, in whichever context. (See also http://ncatlab.org/nlab/show/string+theory+FAQ#BackgroundDependence).

Moreover, the way in which this works in not new to string theory but is the time-honored process of effective quantum field theory (see there for the historic examples): you write down some scattering amplitudes that you are interested in for one reason or another, and then you look for a quantum field theory that reproduces these scattering amplitudes in some low energy regime. Once found, this is the given effective quantum field theory which approximates whatever theory your scattering amplitudes describe at possible high energy.

Next you play this game with the string scattering amplitudes which are defined by summing up correlation functions of some 2d super-conformal field theory of central charge -15 over all possible Riemann surfaces with given insertions (your asymptotically in- and outgoing states). Next you ask if there is an ordinary quantum field theory such that it's perturbative scattering amplitudes coincide with these at low energy. Turns out that this is a higher dimensional locally supersymmetric Einstein-Yang-Mills theory, which is hence the effective field theory that describes the perturbative dynamics of strings at low energy.

See on the nLab at String theory FAQ -- How is string theory related to the theory of gravity?

This post imported from StackExchange Physics at 2014-03-07 16:32 (UCT), posted by SE-user Urs Schreiber

Comments

+1. Which surprises me is that Quantum laws appears at a "fundamental" level (String theory), but arise also at the effective theory level (Yang-Mills). This is something strange, after all, if Quantum is fundamental, it should appear only at a fundamental level. And if Quantum laws are only effective laws, Quantum should not appear at a fundamental level.

This post imported from StackExchange Physics at 2014-03-07 16:32 (UCT), posted by SE-user Trimok

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The principle of quantum physics underlies all of this. Within quantum physics there are fundamental and there are effective theories. Think of a simple example of the quantum mechanics of a neutron in a potenial well (say as in arxiv.org/abs/1207.2953). Fundamentally the neutron is a highly complicated QFT bound state, but neverthess its center of mass point motion is described by simple quantum mechanics.

This post imported from StackExchange Physics at 2014-03-07 16:32 (UCT), posted by SE-user Urs Schreiber