LQC and string theory combined in one theory

[Haelfix]

But isn't the claim for string theory to reproduce GR more than that, since that only reproduces the vacuum Einstein equations. Matter minimally coupled to gravity comes from the string excitations, which are perturbative, so is it still true that the whole Lagrangian of GR pops out if one discusses non-vacuum GR?

"it seems that any acceptable quantum theory of gravity, whatever its ultimate formulation, is likely to reduce to a perturbative string theory in the appropriate limit."

Whether you get any matter at all is going to depend on the nature of the Stringy geometry and details of the moduli. Of course once you are far enough away from the Planck scale you can treat all the matter as an effective field theory and it will act like a stress energy term sourcing the Einstein equations.

As far as Lee's quote. Well, no one knows if String theory is correct or not, but of course whatever final theory of gravity is the truth, it will have to at the very least explain and subsume all the perturbative results from semiclassical gravity in the correct regime. That was my point earlier. We know a few things about perturbation series and gravity, and this does constrain our options.

 

[marcus]

But this is still a pure argument.

You quantize string theory on flat spacetime and get back (as a consistency condition) the Einstein equations. But you are still on flat spacetime; spacetime hasn't become fully dynamical in that setup. It's is still a consistency condition for the classical background.

I do not doubt that this is a hint towards some underlying truth but it is definately not this truth itself.

It is not necessariy a flat backround; but the formalism is on-shell only and space-time is not dynamical, this is correct. Indeed no one claims that this is the final answer!

Good. So in the end the "bootstrap" breaks and does not go the last way to recovering GR. But gets a strong suggestion of it.
And Suprised is right that the fixed background does not have to be flat. There is work using fixed but curved backgrounds.
We've had other discussions, over the years, that brought these same points out.
I appreciate your clarity and frankness, Suprised, in this instance.