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Firstly, General Relativity states that Spacetime is dynamic and is consonant with the distribution of matter/energy. How does String Theory predict gravity, when it is background dependent, that is it starts off with a fixed, and presumably flat space-time?

Secondly, String Theory predicts the existence of a graviton, but seeing we don't have a quantum theory of gravity, how do we even know that this is the correct quantization of gravity? In other words how do we verify that the graviton should have the properties that String theory says it does, that is what grounds do we have for believing in the properties of the graviton?

Thirdly, how do we go from the existence of gravitons to the curvature of space-time? When we talk about gravitons aren't they already existing in some space-time framework?

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In general relativity the gravitational field is given by the metric tensor in space-time. The metric tensor is the solution to Einstein’s field equations. It is a symmetric tensor with ten degrees of freedom. So fundamental excitation in quantum gravity must have spin two. In string theory the oscillations of the closed string includes a symmetric tensor state that can be identified with gravity as well as anti-symmetric tensor (The Kalb Ramond field) and a scalar field (the dilaton). To get Einstein’s GR field equations you have to find an effective low energy action that is fully compatible with the quantum conformal symmetry of the original closed string theory describing the massless bosonic excitations at the classical level. So the GR field equations are just the field equations obtained from the low energy massless bosonic action of closed string theory. This way you can even generate corrections to GR by going to higher level terms in the double curvature and string interaction expansion. This process of obtaining effective gravity equations is well described at an easily understandable level in Maurizio Gasperini’s String Cosmology book. In that book is also explained how to describe strings in general curved backgrounds. About you third point. gravitons are usually described in the context of linear GR. In this description the metric tensor of space-time is separated in two parts the background metric and a fluctuating part that is then quantized. So you can visualize a graviton as a quantized oscillation describing the fluctuation of space time with respect to the background metric. So you can not say that the graviton is propagating in space time, the graviton is a quantized fluctuation of the space-time itself, not just propagating in it. In the context of string theory you can visualize the graviton as closed string states present in regions of fluctuating curvature of space-time. This is analogous to thinking about photons and electromagnetic waves. The EM description is valid when the number of photons is high. You can say that a region in the wave with large amplitude is rich in photon states. Gravitons closed string states and curvature can be seem the same way. A region of space where curvature is high is rich in closed string states that are a part of spacetime itself. This is a simplified and rough version of what gravity waves are in string theory.

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  • $\begingroup$ Nice answer, and thanks for the reference, Gasperinis book looks like exactly the one I need to look at. $\endgroup$ – Mozibur Ullah Jun 20 '12 at 22:17
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Someone -- Richard Feynman apparently [nope, I was wrong! see comment by @RonMaimon below] -- decided at some point to see if the massive success of QED theory could also be applied to gravity. Attempting to represent gravity as a force mediated by bosons (force particles) produced spin-2 carriers for the gravitational force, what we now call gravitons. When loops in early versions of string theory predicted similarly spin-2 particles, it was (and is) assumed that the two concepts are basically the same.

Alas, if any truly elegant reconciliation exists between Einstein's decidedly non-quantum curved-space model of gravity and the attempts to make gravity a matter of particle exchanges, I've never heard of it. I think the quantum model of gravity sort of won by default, perhaps in part because it provided a new and interesting set of mathematical issues to pursue.

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    $\begingroup$ It wasn't Feynman--- Pauli and Fierz analyzed linearized GR to see that it's spin 2 in the 1930s. The gravitons are required by the fact that other things are quantum, so that gravity has to exist in superpositions, otherwise we could violate the uncertainty principle with gravity. $\endgroup$ – Ron Maimon Jun 14 '12 at 0:57
  • $\begingroup$ Ron, thanks! I think you beat out Lawrence Krauss in Quantum Man with that bit of trivia, since that was my main source for the comment. I was surprised to see Feynman had had anything to do with quantum gravity. $\endgroup$ – Terry Bollinger Jun 14 '12 at 1:04
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    $\begingroup$ This is not trivia! Pauli and Fierz worked out spin-2 gravity, and popular writers don't know anything. Feynman wanted to work out perturbative GR after QED. He started in the 1960s, but it was hard, and he switched to Yang-Mills as a test case on Gell-Mann's advice. He discovered ghosts in 1963. The main contributors then became Veltmann and t'Hooft in 1970s, who analyzed quantum gravity after gauge theory and discovered many interesting things that are still central, like boxes and the counterterm-divergence correspondence. This is ancient (pre-string) history, and it is not important. $\endgroup$ – Ron Maimon Jun 14 '12 at 3:18
  • $\begingroup$ @maimon: If I undesrtand you, Pauli & Fierz showed if Gravity was represented by some QFT, it must be a spin 2 field. How does one go back the other way, from a QFT with a spin 2 field, or a string theory (is there a spin 2 particle in the spectrum of the bosonic string, or do we need supersymmetry) to a curves space-time? $\endgroup$ – Mozibur Ullah Jun 19 '12 at 8:24

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