A Post-Quantum Theory of Classical Gravity

Speaker: Jonathan Oppenheim

Quantum Information Society, University of Oxford

[danielgoldsmith]

Thanks for a very interesting talk Prof Oppenheimer.

[Achrononmaster]

@35:00 I thought the essence of Feynman's argument was that through very delicate procedures a superposition of a single mass could be *_amplified_* up to a detectable "clink & clank" at a detector. It's not the same as superposition of the entire spacetime per se, but that there must be an amplitude for the gravity field to be in two states. But that's perfectly fine for gravity wave disturbances and the like, provided gravity is not a graviton particle. You have to quantize gravity according to Feynman's argument only if the gravitational field is not spacetime, but a boson instead. At least that's imho. As with Young's experiment and all the rest, it's fine if electromagnetism is always a wave, waves can "be in two places at once", but not if it is actually a photon. The whole problem in QM is the matter waves (not the gravity waves) are probability amplitudes, not actual _physical_ field amplitudes, so if you consider them *real* then they have to decohere magically somehow to deposit a particle.
Gravity will be fine and classical if it is not actually the graviton. I think this was Feynman's implicit assumption. He wanted gravitons moving on flat spacetime. I think he understood the essential absurdity of superposition of all of spacetime. Maybe? I'm not totally sure of course!

[caralladas]

Now everybody's talkin about this and the video has only 500 views!

[Achrononmaster]

@1:13:00 just a comment for JO if he's following: non-commutative algebra is not a hallmark of quantum mechanics. The Clifford algebra for ordinary 3-space or 4-spacetime or any D>1 is non-commutative, we talk there about the bivector and pseudoscalar graded structure. What is the hallmark of QM then? I'd tell you it is, (1) entanglement structure (has to be non-trivial topology in spacetime, not merely non-commutativity), and (2) taking ħ>0. Having ħ>0 manifest is absolutely critical, without it the non-commuting generators have no bite, you'd still have perfectly precise measurements possible in principle, so Heisenberg would not be a finite non-zero constraint, hence you'd be doing CM or classical statistical mechanics.
The interesting thing is how (1) and (2) could be related. Generalized probability theory (GPT) seems to suggest they are tightly related. Entanglement implies continuous transforms exist between pure states and the converse, and I believe (but not sure) that implies ħ>0, or that perfectly precise measurements for incompatible observables is not possible, which is near enough to the same statement, no? (For a dirtbag physicist. lol.)

[Achrononmaster]

@1:01:00 Check me, but I don't think Lorentz covariance is required for composite systems. The Lagrangians or states must be Lorentz invariant, not the whole system. Lorentz covariance is violated all over the place, just by sticking a particle of matter in empty spacetime. It's the processes governing the particle's time evolution, and whatnot, which are supposed to be Lorentz covariant, as well as respect other exact symmetries, not the physical system itself.
Or think of it this way:" the symmetry principles are saying certain laws are universal, and homogeneity and isotropy of space are one such set. But any matter field in spacetime that's not a perfect homogeneous isotropic fluid, breaks this symmetry. Which is what we absolutely must do if we want to live! Life is all about violating symmetries as much as possible. Perfect symmetry is perfect death so-to-speak. However, physics is _defined_ by processes and laws that respect the perfect symmetries, which means life is not completely subjective and a chaotic hellscape Dormammu maelstrom.
Think of it another way: the laws, like Navier-Stokes, governing fluid motion are so perfect there'd be no interesting fluid motion, since we'd just quote the governing equations and say "done with that!" What makes fluid mechanics non-boring are the initial value/boundary conditions, which are the source of all variety and interest. Without IV/BC you are lifeless abstraction.

[Achrononmaster]

@21:00 this is where I have a bit of a problem. Entanglement is well-known to be monogamous. So you cannot entangle two planets. You can only entangle the composite parts pair-wise. A superposition of a single planet with itself is only possible via pairwise entanglement. (This is distinct to sum-over-histories, which is related but a different phenomenon). If you are serious about quantum gravity I think this sort of gedankenexperiment is not quite right. You do not want to consider matter being in superposition, that's just ordinary QM, rather what you need is an entire spacetime to be entangled with the entire spacetime. Globally. To my mind this makes no sense, so (to my mind) there is no "quantum gravity" proper, but it's much simpler than Oppenheim lays out.
If you can fabricate a gravitational instanton or soliton, then it's spin=2, a graviton, and that *_can_* be quantized, same as for a photon, but it's not "quantum gravity". Gravity does not have to ever consider such instantons, they disperse too rapidly, are too weakly interacting, and only maybe at the early universe you might use them to explain some effects, but it is completely different to having all of spacetime in a superposition. People get thoroughly confused about these two distinct meanings to "quantized gravity". A graviton is not really quantum gravity, because not all of gravity is gravitons. 100% minus ε of gravity is gravity waves and curvature (+ torsion), not instantons.

[Achrononmaster]

@58:00 singularities are not "unphysical". If Nature has singularities then they're physical. The idea is that our mathematical models do not handle singularities well (or not at all). People are always confusing, deliberately or otherwise, the mathematics for the physics --- from Pythagoras to Bohr right through to Tegmark, and beyond. Heed the likes of Feynman, *_the mathematics is not the physics_* people.

[ZhanMorli]

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