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In the concluding section of this post user Chiral Anomaly states following:

On the other hand, since any stable marriage of quantum theory and gravity (in the sense of general relativity, not just Newton) is necessarily highly non-local, even in its causal structure, the door seems to be open for non-quantum theories, whatever that means.

May he/she might have answered it at cost of intuition so can anyone tell me what mathematically more tight statement would be.

Before this statement, he preludes that asymptotic structure may be necessary for the meaningfulness of quantum gravity. But what is bugging me here are the three terms and interception with each other non-local, causal structure, non-quantum.

  • If we 're going to turn off locality ain't we throwing away continuity equation?
  • If we have a non-local theory how are we going to maintain causality in our theory? Though this point may be wrong cause as I know we force locality in QFT by defining $[\hat{O}(x),\hat{O}(y)]=0$ if $(x-y)^2<0$ signature is $(+---)$ and locality is there in QFT since we're using local Lagrangian, at the expense of local gauge. And when we calculate commutator of green function using local Lagrangian suprisingly the causality is satisfied (we also have to set the statistics of the particle for the commutator to vanish in the spacelike region)
  • The third term non-quantum theory (might be bad because of language issue), though sounds pretty fancy doesn't make sense(at least for me) cause we put any theory under the label of quantum which doesn't satisfy classical mechanics. So what theory are we going to put under the label of non-quantum theory?
aitfel
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  • such a theory would only be non-local on incredibly small scales. people expect locality/causality to emerge at longer distances. also a minor comment re second point: local operators in axiomatic QFT are smeared (convolved against smooth functions with support in the neighborhood of the operator), but still obey causality – 4xion Jul 20 '20 at 18:23
  • Chances are that Chiral Anomaly has bought the argument of AMPS that the black hole information paradox quantum states that either (a) physics is non-unitary or (b) physics is non-local. He's then calling non-unitary theories non-quantum. (Or maybe he's been listening to somebody else who's doing this, and possibly even getting what they say wrong.) – Peter Shor Jul 20 '20 at 18:33
  • @PeterShor I don't follow the logic of your inferences. Why AMPS? Why not just the holographic principle? or why not explicit examples like AdS/CFT aka string theory? Pretty much all serious work on quantum gravity leads to the conclusion that the usual concept of spacetime (e.g. locality, causal structure) is only an approximation. That's all I meant by non-local. The lack of a prescribed causal structure (i.e. prescribed spacetime background) is part of what makes quantum gravity so hard. This is well-known. – Chiral Anomaly Jul 21 '20 at 00:45
  • @PeterShor Regarding "non-quantum": Our current best examples of quantum gravity models still rely on a special type of asymptotic spacetime structure, and it's been that way for a few decades now. In particular, according to Witten and to Banks (independently), quantum gravity in asymptotically de Sitter spacetime seems to be problematic. Maybe relaxing conventional spacetime isn't enough; maybe we need to relax something else, too, to remove the dependence on asymptotic structure. Don't know. That's all I meant by "non-quantum," nothing more specific than that. No pet theory, and no AMPS. – Chiral Anomaly Jul 21 '20 at 00:45
  • @ChiralAnomaly Thanks! your comments were sufficient for me at my level. I just have one more favor can you please put those comments in the answer body so that I can close this question since comments get deleted and I want someone in future may find the answer helpful. – aitfel Jul 21 '20 at 12:37

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I'll try to clarify.

If we 're going to turn off locality ain't we throwing away continuity equation?

Any viable theory of quantum gravity should be consistent with the fact that classical general relativity is normally an excellent approximation — such a good approximation that we have not yet directly observed any problems with it. In particular, the conventional idea of locality (mathematically expressed using things like a metric with Lorentzian signature and hyperbolic equations of motion) should be an excellent approximation under normal conditions. Only under extreme conditions, beyond anything experiments have directly explored, do we expect those conventional ideas to break down. Exactly what fundamental concepts should replace the conventional ones is still an open question.

So we won't really be "throwing away" the conventional theories, just like we didn't throw away Newton's theory of gravity when general relativity came along. General relativity works better, but it still reduces to Newton's theory (to an excellent approximation) under most practical conditions.

If we have a non-local theory how are we going to maintain causality in our theory?

That's one of the fundamental questions that the whole quantum-gravity community is asking, or they might ask it this way: "How will quantum gravity ensure that the usual concept of local causality is recovered under the appropriate approximations?" That's one of the issues that makes quantum gravity such a difficult and mysterious subject.

...we put any theory under the label of quantum which doesn't satisfy classical mechanics.

Well, I guess "non-quantum" was a pretty poor choice of words.$^*$ I certainly didn't mean reverting to classical mechanics. I only meant we don't know yet which principles will need to be modified to make quantum gravity work without special asymptotic conditions (if that's even possible), and maybe I went too far when I suggested that even quantum theory itself might need to be modified in some not-yet-conceived way. I didn't have any specific proposal in mind, nor have I seen any such proposal that I think is even mildly compelling. I was only leaving room for possible surprises, because we don't know yet (at least I don't know) where quantum gravity research will ultimately lead.

$^*$ I edited that answer to change the wording of that part.

Chiral Anomaly
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