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Locally, General Relativity is Lorentz invariant. Globally, it is not. e.g. Galaxy's sufficiently far apart are travelling more than the speed of light relative to each other. Although no particles in a small region of space will pass each other above the speed of light.

Presumably string theory (like particle field theories) is locally and globally Lorentz invariant. So how can a globally Lorentz invariant theory (string theory) produce a globally Lorentz non-invariant theory (General Relativity) ?

Presumably the non-locality of strings themselves is something to do with it?

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    Are you sure ST would be globally Lorentz invariant? Compare to QFTCS. – J.G. Sep 23 '18 at 21:31
  • @J.G. Aren't all quantum field theories of particles? After all string theory is a theory of an infinite ladder of particles with increasing masses. If there is a get-out-clause for string theory. What is it? QFTCS is just putting the theory on a curved space-time which you could do with any theory even without gravity. Assume you calculate string theory on a flat background. –  Sep 23 '18 at 21:33
  • Ask yourself what makes GR so unlike SR. – J.G. Sep 23 '18 at 21:41
  • "Galaxy's sufficiently far apart are travelling more than the speed of light relative to each other." No, not at all, see here. GR is Lorentz invariant and I'm not really sure what you're getting at with "global" Lorentz invariance. – knzhou Sep 23 '18 at 21:44
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    Well anyway, doing more research I gather that perturbative string theory can't reproduce GR in any meaningful way. We need non-perturbative methods. –  Sep 23 '18 at 21:46
  • Have a look https://arxiv.org/abs/gr-qc/0410049 – anna v Sep 24 '18 at 04:54
  • What do you mean by globally Lorentz invariant? – MBN Sep 24 '18 at 13:55
  • @MBN. I probably just mean the space-time is Minkowski. Most field theories can be defined on a Minkowski background. –  Sep 24 '18 at 19:23
  • String theory doesn't require flat spacetime. – MBN Sep 25 '18 at 16:11
  • @mbn no but it is a possible background on which the perturbation series can be expanded –  Sep 25 '18 at 18:49
  • It is the same in GR. You can use the Mikowski spacetime as a background. – MBN Sep 26 '18 at 10:15
  • Yes but gravitons on a minkowski background cant account for the expansion of spacetime on galactic scales. –  Sep 26 '18 at 21:47

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