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For some reason, you are trying to make it seem like I'm saying that the author is making a quantum theory (i have no idea why you think im saying this), I'm no

For some reason, you are trying to make it seem like I'm saying that the author is making a quantum theory (i have no idea why you think im saying this), I'm not. > He's presenting a purely classical model which displays some features (in particular charge quantization) also seen in quantum mechanical ones. Compared to other physicists who just outright state that there is no compatibility… You can ignore it, but it is an area for further study in regards to non time orientability for others.

T-A 6y ago
> For some reason, you are trying to make it seem like I'm saying that the author is making a quantum theory No, I am pointing out that you do not understand the paper you misquoted as evidence that "Not all physicists think they [GR and QM] are incompatible". The author would not have written a paper trying to take a small step toward a solution if he didn't think that there is a problem.
- cinquemb 6y ago
  The author acknowledges in the paper where the overlaps lie wrt to EM (coupled to GR) and QM using a classical model that treats particles as an asymptotically flat spacetime manifold with a region of non trivial topology where time is not orientable (but manifold subtracting out the worldtubes of said particles is both space and time oreintable). Just because he does not say explicitly "[GR and QM] are compatible", does not mean opposite. If you aren't interested in the that U(1) bundle described in the paper is from utilizing time oreintability from a classical standpoint and not formed from the complex phase of a quantum mechanical wave-function, that's on you.
  - T-A 6y ago
    > Just because he does not say explicitly "[GR and QM] are compatible", does not mean opposite. It also doesn't mean what you are claiming. To have a meaningful discussion about the incompatibility of GR with QM, you first need to know how they are incompatible. Reproducing classical electromagnetism with the Kaluza-Klein trick from 1919 [1] has nothing to do with it. QM deals with unit vectors in a state space, operators acting on those vectors, and observables whose values can not all be known simultaneously [2]. The hole left by the observer's limited knowledge about the state of the system is filled by an intrinsic randomness which does not exist in classical theories. GR and other classical theories have only observables, their evolution is deterministically determined and the state of the system can in principle be fully known at all times. We have known for a long time that nature does not actually work like this [3]. If you want to claim that GR is compatible with QM, you have to resolve the above contradiction. What people who actually understand the problem have been trying to do for a long time is: 1) Quantize GR. This is the obvious first thing to try; you treat the problem as analogous to the quantization of classical electromagnetism and hope to create a quantum version of GR, the gravitational equivalent of QED. When you try that, it turns out you can't make the theory renormalizable - the infinities caused by loops in Feynman diagrams can not be brought under control. Trying to fix that leads to things like loop quantum gravity [4]. Weinberg came up with the alternative idea of asymptotic safety, which tries to neuter the problem with a fixed point in the renormalization group flow [5], but proving that it actually works is another story. 2) Come up with a deterministic (i.e. classical) theory underlying QM. Naive approaches involving hidden variables (i.e. trying to explain away our limited knowledge of quantum states as ignorance rather than truly fundamental) died with Bell's theorem. A few brave souls, notably t'Hooft [6] are still trying more sophisticated approaches (I think it would be nice if superdeterminism [7] got more attention). 3) Write down a more fundamental theory which produces both GR and the other known interactions in the appropriate limits (thermodynamic, large number of quanta). Since the other known interactions are empirically known to be quantum mechanical, it is generally believed that this theory will have to be quantum mechanical too. String theory is the obvious example. In principle, you could also imagine a classical (i.e. deterministic) theory along the lines of (2), but I can't think of any example which comes close to actually doing the job. [1] https://en.wikipedia.org/wiki/Kaluza%E2%80%93Klein_theory https://en.wikipedia.org/wiki/Kaluza%E2%80%93Klein_theory [2] https://en.wikipedia.org/wiki/Quantum_mechanics#Mathematical_formulations https://en.wikipedia.org/wiki/Quantum_mechanics#Mathematical... [3] https://en.wikipedia.org/wiki/Bell%27s_theorem https://en.wikipedia.org/wiki/Bell%27s_theorem [4] https://en.wikipedia.org/wiki/Loop_quantum_gravity https://en.wikipedia.org/wiki/Loop_quantum_gravity [5] https://en.wikipedia.org/wiki/Asymptotic_safety_in_quantum_gravity https://en.wikipedia.org/wiki/Asymptotic_safety_in_quantum_g... [6] https://arxiv.org/abs/1405.1548 https://arxiv.org/abs/1405.1548 [7] https://en.wikipedia.org/wiki/Superdeterminism https://en.wikipedia.org/wiki/Superdeterminism

T-A 6y ago

> For some reason, you are trying to make it seem like I'm saying that the author is making a quantum theory No, I am pointing out that you do not understand the paper you misquoted as evidence that "Not all physicists think they [GR and QM] are incompatible". The author would not have written a paper trying to take a small step toward a solution if he didn't think that there is a problem.

cinquemb 6y ago
The author acknowledges in the paper where the overlaps lie wrt to EM (coupled to GR) and QM using a classical model that treats particles as an asymptotically flat spacetime manifold with a region of non trivial topology where time is not orientable (but manifold subtracting out the worldtubes of said particles is both space and time oreintable). Just because he does not say explicitly "[GR and QM] are compatible", does not mean opposite. If you aren't interested in the that U(1) bundle described in the paper is from utilizing time oreintability from a classical standpoint and not formed from the complex phase of a quantum mechanical wave-function, that's on you.
- T-A 6y ago
  > Just because he does not say explicitly "[GR and QM] are compatible", does not mean opposite. It also doesn't mean what you are claiming. To have a meaningful discussion about the incompatibility of GR with QM, you first need to know how they are incompatible. Reproducing classical electromagnetism with the Kaluza-Klein trick from 1919 [1] has nothing to do with it. QM deals with unit vectors in a state space, operators acting on those vectors, and observables whose values can not all be known simultaneously [2]. The hole left by the observer's limited knowledge about the state of the system is filled by an intrinsic randomness which does not exist in classical theories. GR and other classical theories have only observables, their evolution is deterministically determined and the state of the system can in principle be fully known at all times. We have known for a long time that nature does not actually work like this [3]. If you want to claim that GR is compatible with QM, you have to resolve the above contradiction. What people who actually understand the problem have been trying to do for a long time is: 1) Quantize GR. This is the obvious first thing to try; you treat the problem as analogous to the quantization of classical electromagnetism and hope to create a quantum version of GR, the gravitational equivalent of QED. When you try that, it turns out you can't make the theory renormalizable - the infinities caused by loops in Feynman diagrams can not be brought under control. Trying to fix that leads to things like loop quantum gravity [4]. Weinberg came up with the alternative idea of asymptotic safety, which tries to neuter the problem with a fixed point in the renormalization group flow [5], but proving that it actually works is another story. 2) Come up with a deterministic (i.e. classical) theory underlying QM. Naive approaches involving hidden variables (i.e. trying to explain away our limited knowledge of quantum states as ignorance rather than truly fundamental) died with Bell's theorem. A few brave souls, notably t'Hooft [6] are still trying more sophisticated approaches (I think it would be nice if superdeterminism [7] got more attention). 3) Write down a more fundamental theory which produces both GR and the other known interactions in the appropriate limits (thermodynamic, large number of quanta). Since the other known interactions are empirically known to be quantum mechanical, it is generally believed that this theory will have to be quantum mechanical too. String theory is the obvious example. In principle, you could also imagine a classical (i.e. deterministic) theory along the lines of (2), but I can't think of any example which comes close to actually doing the job. [1] https://en.wikipedia.org/wiki/Kaluza%E2%80%93Klein_theory https://en.wikipedia.org/wiki/Kaluza%E2%80%93Klein_theory [2] https://en.wikipedia.org/wiki/Quantum_mechanics#Mathematical_formulations https://en.wikipedia.org/wiki/Quantum_mechanics#Mathematical... [3] https://en.wikipedia.org/wiki/Bell%27s_theorem https://en.wikipedia.org/wiki/Bell%27s_theorem [4] https://en.wikipedia.org/wiki/Loop_quantum_gravity https://en.wikipedia.org/wiki/Loop_quantum_gravity [5] https://en.wikipedia.org/wiki/Asymptotic_safety_in_quantum_gravity https://en.wikipedia.org/wiki/Asymptotic_safety_in_quantum_g... [6] https://arxiv.org/abs/1405.1548 https://arxiv.org/abs/1405.1548 [7] https://en.wikipedia.org/wiki/Superdeterminism https://en.wikipedia.org/wiki/Superdeterminism