Instantons and Non Perturbative Amplitudes in Gravity

Question

In perturbative QFT in flat spacetime the perturbation expansion typically does not converge, and estimates of the large order behaviour of perturbative amplitudes reveals ambiguity of the perturbative expansion of the order $\exp(-1/g^2)$ where $g$ is the expansion parameter. This ambiguity in turn is related to the existence of asymptotically Euclidean classical solutions (instantons) which contribute to these correlation functions and whose contribution resolves the ambiguity in the perturbative expansion and allows for a non-perturbative completion of the theory.

All this well-known stuff is a prelude to my question about gravity. Naively all the statements about the perturbative expansion still hold, at least if one can resolve the problems arising from non-renormalizability of the theory (in other words define the individual terms in the series). Optimistically, perhaps for $N=8$ SUGRA that should be possible. This brings to mind the question of the existence of instantons, namely:

Do non-trivial asymptotically Euclidean solutions exists in theories of gravity?

Now, there are well-known objects that are called "gravitational instantons", but those are not asymptotically Euclidean. Rather they are asymptotically locally Euclidean - they asymptote to a quotient of Euclidean flat space. The difference means that these objects do not actually contribute to correlation functions (or more to the point S-matrix elements) around flat spacetime. My question is whether objects that do contribute exist in some (perhaps unconventional) theories of gravity.

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Comments

What is "the perturbative expansion" of gravity? There is no single obvious quantum field theory of gravity. Ordinary perturbative QFT builds on a background Minkowski spacetime, so this is not going to work here.

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My question is about properties of the perturbative expansion of gravity theories around Minkowski spacetime, this is a well-defined procedure and I ask a well-defined (perhaps no so interesting) question about it.

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I agree that this can only be interesting if there are field theories where the perturbative expansion can be made sense of. There are some indications that N=8 SUGRA in four dimensions might be perturbatively finite which is what brought this question to mind.

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Is there a reason why you're looking specifically for instantons? My understanding was that in field theory there are various renormalon divergences that aren't necessarily related to semiclassical instantons. Also, I don't see why semiclassical solutions localized in space but not time can't contribute to this sort of thing. It seems like black holes should be relevant for the S-matrix in a gravitational theory, right?

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@Matt, there could be other ways to obtain non-perturbative amplitudes but instantons are the easiest one. As for solitons, in my mind their Euclidean action is infinite due to time translation invariance so they do not contribute to the partition function. This should include static solutions with or without horizons.

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In other words, black hole have temperature so they obey boundary conditions different from what you need for S-matrix elements, or zero temperature correlators.

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[Hi @Moshe: this is not directly related to the question: but if you think your earlier questions from Physics SE might be more suited here, maybe you might like to repost the ones you think are most appropriate here? ... The more level-appropriate questions we get the more likely the Beta stage succeeds. - cheers]

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@UGP Not a bad idea to do that on some systematic level, but this is a discussion for meta, and I am not sure how that can be achieved. As for my own questions, I think I only had a couple there. I am confused about many things so generating questions should not be a problem...

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[@Moshe: You are quite correct. Do you want me to post a question along these lines? ... As for reposting: I'm simply asking if you'd like to manually repost the ones you think are level-appropriate here. (And delete **only those** from PSE.) That would be a great thing to get our Beta into another gear. .. Anyhow, let me know either way.] -cheers

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@UGP, I honestly don't remember these questions and I'm not sure if I want to delete the good discussion over there. Maybe you can clarify the issue on meta.

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[@Moshe: Sorry for posting again. ... (I'm assuming your PSE account is the following: http://physics.stackexchange.com/users/566/moshe): the level of the questions asked there and the one you ask here are nearly identical. This poses a bit of a problem: if the community votes down the proposal [to request all members with PSE questions to migrate and then *delete* all level-appropriate questions from PSE] then it creates a major problem for us: SE admins will eventually request us to merge with PSE (- the very reason Joe suggested this site in the first place). ... I was hoping to find a...

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... way to circumvent this inevitable problem by asking each member individually to repost the relevant ones here. ... If you can assist us that would be the most helpful for us. - regards ... ps: I don't want to ask this on Meta if there's no reason to directly.]

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Re the first comment above asking "What is "the perturbative expansion" of gravity?". Something that's often forgotten is: there is no problem with treating gravity as an effective QFT and computing perturbative effects at relatively low energies with that. A nice review is _Introduction to the Effective Field Theory Description of Gravity_ http://arxiv.org/abs/gr-qc/9512024 This is independent of your belief about the UV completion of gravity.

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@Moshe: but semiclassical black holes evaporate, and the standard lore is that high-energy scattering in gravitational theories is dominated by black hole production. So I don't see how they can be irrelevant for the S-matrix.

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@Matt, thanks, that may be an efficient way to think about it. I’ll do that when I have more time.

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Answer 1

The answer is yes in dimensions where there exists an exotic sphere. So, the answer is yes in dimensions 7,8,9,10,11,13,14,15... (In 4 dimensions the existence of such an exotic sphere hinges upon the resolution of the smooth 4-dimensional Poincare conjecture.) The logic as to why this is the case is as follows...

For any Euclidian Yang-Mills instanton I there always exists an "anti-instanton" -I such that the instanton I when "widely" separated from the anti-instanton -I yields a guage field I - I that is homotopic to the trivial gauge field A=0.

As I - I is homotopic to the trivial gauge field A=0, one must include I - I in path integrals. In such path integrals I may be centered at x and -I may be centered at y. If x and y are very distant, then this produces, by cluster decomposition, the same result as an isolated instanton I at x. This is why instantons play a role in path integrals.

Applying this logic to gravity one wishes to find an instanton J and an anti-instanton -J such that J - J is diffeomorphic to the original manifold. If there exists such a pair, then J should be interpreted as an instanton and -J as an anti-instanton.

The set of exotic spheres form a group under connected sum. Hence, for any exotic sphere E there exists an inverse exotic sphere -E such that the connected sum of E and -E is the standard sphere.

Consider now a manifold M of dimension n=7,8,9,10,11,13,14,15... As M is of this dimension, there exists an exotic sphere E of dimension n and an inverse exotic sphere -E such that the connected sum of E and -E is the standard sphere. As the connected sum of the standard sphere and M is diffeomorphic to M, these exotic spheres can be interpreted as instantons in n dimensions vis-a-vis our above argument.

This logic was first presented in section III of Witten's article Global gravitational anomalies.

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Comments

Thanks! I am collecting quite the reading list here already...

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