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I'm asking this to get yet another lesson in the inability of QFT and GR to cohabit. Many people believe GR must yield to quantization. The question here is as to why the activity of the vacuum cannot be imagined as having some sort of baseline and the gravitational force having some origin in this baseline. And, of course then, that there could exist variations from this baseline on a more fundamental level than the wavelike disturbances of QFT. The variations I intend may have to do with types of virtual particle-antiparticle pair eruptions favored at a particular location, tendency toward shorter or longer lifespans of such events, etc.

Is it just that it is impossible to say anything meaningful about gradings in the living vacuum, and that such hypothetical gradings, however dynamically conjured, could not ever come to mimic aspects of a GR like picture?...due to subtle stuff like violation of basic physical principles and such.

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    $\begingroup$ I'm having a hard time following your question. What do you mean by "activity," "strength of activity," and "baseline?" In the second sentence you seem to be describing the Einstein field equations...? Why is the second sentence phrased in the negative? The Einstein field equations would simply be the classical limit of a theory of quantum gravity, just as Maxwell's equations are the classical limit of QED. $\endgroup$
    – user4552
    Commented Jul 28, 2011 at 1:10
  • $\begingroup$ @Ben: I believe OP means that from the point of view of GR, vacuum is a calm place whereas in QFT there are fluctuations generating virtual pairs. One has to reconcile this somehow via fluctuating background (and it's not obvious how). As for quantization: by this one means "classical theory -> quantum theory" through replacement of functions on phase space by observables. Not all quantum systems are obtained this way and it's very probable that QG is one of them, i.e. you need truly quantum-theoretical approach. It will of course also contain a classical limit but that's something else... $\endgroup$
    – Marek
    Commented Jul 28, 2011 at 4:10
  • $\begingroup$ My impression from following efforts of quantizing gravity over the years is that the difficulty comes from the spin 2 graviton that needs to be exchanged which cannot fit in a re normalizable perturbative expansion a la QED etc. $\endgroup$
    – anna v
    Commented Jul 28, 2011 at 6:51
  • $\begingroup$ @anna: I don't believe spin 2 is a problem. It's possible to include spin 2 particles in other ways (including supersymmetry, modifying Lagrangian, etc.). Rather, the problem lies in the precise form of the GR Lagrangian. Nor is the non-renormalizability itself a problem: by now we understand that effective theories (such as Fermi's theory of weak interactions) need not be renormalizable to be perfectly usable upto some scale. It just means they break up at higher scales... $\endgroup$
    – Marek
    Commented Jul 28, 2011 at 7:12
  • $\begingroup$ @Marek you are referring to supergravity? It dates me, but I have the impression that they do not have computable diagrams, and that only in string theories the situation between QFT and gravity is resolved, but any enlightenment is welcome. $\endgroup$
    – anna v
    Commented Jul 28, 2011 at 12:02

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