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Comment by eigenket | Hacker News Reader

I attended a talk by Jonathan Oppenheim a while back on this subject, and my understanding based on that is that in his model something (very) roughly like this happens. You put your massive object in superposition, it interacts with space-time and "tries" to put space-time in superposition but since space-time is fundamentally classical (in his model) what happens is space-time ends up in a probabalistic mixture of the different states rather than a superposition. Then the interaction between space-time and the massive object end up pushing the object from the superposition state you tried to produce into a mixed state just like the space-time.

Essentially from the point of view of the massive object, the interaction with space-time acts as some decoherence process.

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> the interaction with space-time acts as some decoherence process.

Well, the question is, can we use gravity in the same way that we use quantum processes, i.e., in a coherent way? If we can, e.g. using it to establish entanglement, then the theory you describe cannot hold.

Thats definitely a good question, and one that people have thought about before. I think its amazingly difficult to test it experimentally, even compared to something like directly measuring to see whether a massive object in superposition is forced to decohere faster than we expect, which is already incredibly difficult.

I was taught that choosing a QM interpretation is a matter of taste. Am I understanding correctly that we do have proposed quantum gravity experiments that can falsify various QM interpretations, it's just that they are all very hard to execute?

I don't know about quantum gravity experiments but there are already (proposed) experiments that don't involve gravity which might help distinguish between different interpretations:

https://www.scientificamerican.com/article/this-simple-exper...

This isn't about a QM interpretation, rather it's about a non-quantum model of gravity with some features that would make it distinguishable in principle from any quantum gravity.

Edit: I maybe semi take this back, it actually may be the case that this particular weird model of gravity would provide a way to distinguish between the interpretations of QM because of how grossly it violates the usual assumptions of QM. It seems too implausible for me to really dig into but I don't entirely grok how they propose melding the proposed inherent but non-quantum randomness with the rest of quantum physics in a way that still makes things line up well in the face of observation/collapse (or the equivalent in many-worlds). They might lean enough on assumptions about how that happens to break the observational equivalence?

> I was taught that choosing a QM interpretation is a matter of taste

You were taught wrong

> Then the interaction between space-time and the massive object end up pushing the object from the superposition state you tried to produce into a mixed state just like the space-time.

So we'd be evolving from a pure to a mixed state?

Seems like this would show up as an equivalence principle violation, no?

That isn't obvious to me - the equivalence principle works fine in classical GR, I'm not sure why it would break in Jonathan's model, but I'm very far from qualified to speak authoratively about his work!

Spacetime observing massive objects.

I'm quite happy to admit that I don't know the classical-quantum CQ programme well, and certainly not well enough to hazard a genuinely informed opinion that might take the form "it's a candidate for a fundamental theory" or "it's a candidate for a better EFT". I think I can say it isn't obviously not one of those, though. Also, I'm not ready to sloganize the work beyond "it's complicated".

A good entry point to CQ is Oppenheim's [hep-th] https://arxiv.org/abs/1811.03116

"... invariant under spatial diffeomorphisms ... consistent ... completely positive, norm preserving, and linear in the density matrix ... the metric remains classical even when back-reacted upon by quantum fields ... the dynamics here, while stochastic, can leave the quantum state pure -- it is rather the classical degrees of freedom, which gain entropy."

It's a heavy couple dozen pages (part V is a mountain for mountaineers (I am nowhere near the summit) and the foothills are also hard work, though most of it shouldn't pose technical comprehensibility problems anyone who's done QFT (GKSL is prominent [short intro <https://arxiv.org/abs/1906.04478>], and the Lindbladian is the generator of time-translations) and who has encountered Lagrangian and Hamiltonian formulations of gravitation, especially if they've glanced at Birrell & Davies. A crash read or refresher of Wald's QFTCS https://arxiv.org/abs/gr-qc/9509057 might be useful if like me you've mostly spent time in one of the two fundamental silos and want a bit more meat than what's in the first couple pages of Oppenheim 2018's part I. The general relativistic constraint equations are also important, and there is a good overview at https://link.springer.com/article/10.1007/s41114-020-00030-z

The meat, for me, is the potential for a better approximation than semiclassical gravity when quantum fluctuations are large: "[even if not fundamental, CQ gives] the ability to consistently study back-reaction effects in cosmology and black-hole evaporation ... [but] care should be taken, since an effective theory might violate our assumptions of Markovianity or complete positivity at short time scales or when the gravitational degrees of freedom have not fully decohered.", "This theory serves as a sandbox in which to understand issues around the quantisation of the gravitational field. After all a probabilty density \rho and the Liouville equation have a lot in common with the wave function \psi and the Heisenberg equations of motion. [and the rest of that paragraph just before (eqn 2)]".

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