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The problem is that it doesn't act like a mass distribution, it acts as two non spatially overlapping possibilities. I think that the only way to save classical

The problem is that it doesn't act like a mass distribution, it acts as two non spatially overlapping possibilities. I think that the only way to save classical gravity would be superdeterminism. If quantum states correspond to anything other than our ignorance, i.e. if superposition states are actual physical states of reality, then gravity will need to be quantum.

eigenket 3y ago
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.
- bowsamic 3y ago
  > 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.
  - eigenket 3y ago
    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.
    - skinner_ 3y ago
      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?
      - codethief 3y ago
        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-experiment-could-challenge-standard-quantum-theory/ https://www.scientificamerican.com/article/this-simple-exper...
      - cornel_io 3y ago
        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?
      - bowsamic 3y ago
        > I was taught that choosing a QM interpretation is a matter of taste You were taught wrong
- consilient 3y ago
  Seems like this would show up as an equivalence principle violation, no?
  - eigenket 3y ago
    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!
- codethief 3y ago
  > 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?
- cryptonector 3y ago
  Spacetime observing massive objects.
- raattgift 3y ago
  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 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 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 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 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)]".
meroes 3y ago
Do you know if Bohmian mechanics with a particle ontology (still with non-local "hidden" variables ofc) or GRWf has potential to save classical gravity as well?
- jostylr 3y ago
  To my knowledge, it does not. There are various items to address with only item 4 being currently with no clear direction: 1) Bohmian mechanics seems to require some kind of simultaneity. Various proposals have been put forth for making natural foliations, possibly using the wave function to do so, that allow one to evaluate the positions of all the particles at a given time so as to know which configuration point to use in obtaining the velocity of the particle. This is possible, but it does not feel philosophically satisfactory yet. GRWf does not require a foliation to be relativistic which is a nice feature. 2) Quantum field theory naturally models particle creation and annihilation. Most QFTs are presented in a mathematically incoherent way. While conclusions can be made via renormalization, etc., driving an actual dynamics is tricky from that stuff. By not doing perturbations but rather defining the operators by taking into account the shifting of the probability from one sector of n particles to another of n+1 particles, QFTs can actually be made to make mathematical sense. This has been accomplished in some of the simpler models. It is not a problem whatsoever to define a Bohmian evolution where particles appear and disappear in a probabilistic fashion. The difficulty is purely in having a properly defined wave function evolution and that is on its way to being solved. 3) One needs to define wave functions, their evolution, and the particle evolutions in a curved space-time. This is not a problem whatsoever. It is very easy to translate and interpret what we need into differential geometric language. QM has issues with the usual observables/operators translating (such as a momentum operator), but since these are derived concepts in Bohmian mechanics, no fundamental difficulty arises. 4) In general relativity, the mass distribution is part of the evolution of the space-time metric. The mass is based on where the particles are. To date, the wave function tells the particles what to do, but the particles do not have any impact on the wave function. The wave function is impacted by the space-time metric. Also, there are some suggestions that mass might be entirely a part of the wave function and not associated with the particle, i.e., the particles are really just undecorated points moving about. Basically, gravity and the wave function need to work it out and the particles will then be guided by both as the space-time metric is what takes the gradient of the wave function or, for Dirac style motion, something analogous to a square root of the metric is floating around. Bohmian mechanics does have the advantage that it just has to be concerned with the evolution of the particle lines and not having to figure out how to define various observables. In other words, a known space-time metric interfaces just fine with Bohmian mechanics, but figuring out how to evolve the space-time metric is still an open question. At the current time, I am not aware of any novel ideas coming from a Bohmian or GRWf point of view towards resolving the problem of gravity. If interested in a good discussion of all these things and much, much more, I highly recommend the recent book Foundations of Quantum Mechanics by Roderich Tumulka which covers what the title says, but also discuss Bohmian mechanics and other interpretations.
nocoolnametom 3y ago
When you say superdeterminism are you referring to something like Pilot Wave theory, where what appear to our measurements as probabilistic yet random interactions are merely expressions of a more complex yet non-random underlying system that we cannot, as yet, measure? (I don't even know if that's the proper description of the hypothesis.)
- xmcqdpt2 3y ago
  Superdeterminism is simply the idea that all quantum experiments results could have been known before performing them, assuming a perfect knowledge of the state of the universe. It's one of those things that could be true and explain all of QM but also is kind of a cop out. "Of course your two detectors are giving correlated results, they were tightly coupled 13.8 billion years ago and now they are forever linked like all things."
  - consilient 3y ago
    > Superdeterminism is simply the idea that all quantum experiments results could have been known before performing them, assuming a perfect knowledge of the state of the universe. That's just determinism. Superdeterminism additionally posits that the results of those experiments are all correlated so as to make it appear to us as if local hidden variable theories were false. Pick the settings on two polarimeters for a Bell-type experiment by measuring the spins on photons emitted 10 billion years ago from two different galaxies, and you'll find (or rather, won't find, because it's being hidden from you) that they were arranged, long before the Earth was formed, just so as to trick you. The universe is conspiring against us, and the whole scientific project is a farce. That, or superdeterminism is false.
    - zmgsabst 3y ago
      Alternatively, non-local correlations (in the style of anyons) persist from the early universe. And we’re just measuring those.
  - zmgsabst 3y ago
incompatible 3y ago
These distributed "particles" are passing through both slits simultaneously before somehow transferring their energy at a particular position at the target screen. If gravitation is non-quantum, I suppose I'd expect that a gravitational measurement would show it passing through both slits, half of its total mass through each.

eigenket 3y ago

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.

bowsamic 3y ago
> 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.
- eigenket 3y ago
  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.
  - skinner_ 3y ago
    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?
    - codethief 3y ago
      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-experiment-could-challenge-standard-quantum-theory/ https://www.scientificamerican.com/article/this-simple-exper...
    - cornel_io 3y ago
      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?
    - bowsamic 3y ago
      > I was taught that choosing a QM interpretation is a matter of taste You were taught wrong
consilient 3y ago
Seems like this would show up as an equivalence principle violation, no?
- eigenket 3y ago
  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!
codethief 3y ago
> 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?
cryptonector 3y ago
Spacetime observing massive objects.
raattgift 3y ago
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 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 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 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 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)]".

meroes 3y ago

Do you know if Bohmian mechanics with a particle ontology (still with non-local "hidden" variables ofc) or GRWf has potential to save classical gravity as well?

jostylr 3y ago
To my knowledge, it does not. There are various items to address with only item 4 being currently with no clear direction: 1) Bohmian mechanics seems to require some kind of simultaneity. Various proposals have been put forth for making natural foliations, possibly using the wave function to do so, that allow one to evaluate the positions of all the particles at a given time so as to know which configuration point to use in obtaining the velocity of the particle. This is possible, but it does not feel philosophically satisfactory yet. GRWf does not require a foliation to be relativistic which is a nice feature. 2) Quantum field theory naturally models particle creation and annihilation. Most QFTs are presented in a mathematically incoherent way. While conclusions can be made via renormalization, etc., driving an actual dynamics is tricky from that stuff. By not doing perturbations but rather defining the operators by taking into account the shifting of the probability from one sector of n particles to another of n+1 particles, QFTs can actually be made to make mathematical sense. This has been accomplished in some of the simpler models. It is not a problem whatsoever to define a Bohmian evolution where particles appear and disappear in a probabilistic fashion. The difficulty is purely in having a properly defined wave function evolution and that is on its way to being solved. 3) One needs to define wave functions, their evolution, and the particle evolutions in a curved space-time. This is not a problem whatsoever. It is very easy to translate and interpret what we need into differential geometric language. QM has issues with the usual observables/operators translating (such as a momentum operator), but since these are derived concepts in Bohmian mechanics, no fundamental difficulty arises. 4) In general relativity, the mass distribution is part of the evolution of the space-time metric. The mass is based on where the particles are. To date, the wave function tells the particles what to do, but the particles do not have any impact on the wave function. The wave function is impacted by the space-time metric. Also, there are some suggestions that mass might be entirely a part of the wave function and not associated with the particle, i.e., the particles are really just undecorated points moving about. Basically, gravity and the wave function need to work it out and the particles will then be guided by both as the space-time metric is what takes the gradient of the wave function or, for Dirac style motion, something analogous to a square root of the metric is floating around. Bohmian mechanics does have the advantage that it just has to be concerned with the evolution of the particle lines and not having to figure out how to define various observables. In other words, a known space-time metric interfaces just fine with Bohmian mechanics, but figuring out how to evolve the space-time metric is still an open question. At the current time, I am not aware of any novel ideas coming from a Bohmian or GRWf point of view towards resolving the problem of gravity. If interested in a good discussion of all these things and much, much more, I highly recommend the recent book Foundations of Quantum Mechanics by Roderich Tumulka which covers what the title says, but also discuss Bohmian mechanics and other interpretations.

nocoolnametom 3y ago

When you say superdeterminism are you referring to something like Pilot Wave theory, where what appear to our measurements as probabilistic yet random interactions are merely expressions of a more complex yet non-random underlying system that we cannot, as yet, measure? (I don't even know if that's the proper description of the hypothesis.)

xmcqdpt2 3y ago
Superdeterminism is simply the idea that all quantum experiments results could have been known before performing them, assuming a perfect knowledge of the state of the universe. It's one of those things that could be true and explain all of QM but also is kind of a cop out. "Of course your two detectors are giving correlated results, they were tightly coupled 13.8 billion years ago and now they are forever linked like all things."
- consilient 3y ago
  > Superdeterminism is simply the idea that all quantum experiments results could have been known before performing them, assuming a perfect knowledge of the state of the universe. That's just determinism. Superdeterminism additionally posits that the results of those experiments are all correlated so as to make it appear to us as if local hidden variable theories were false. Pick the settings on two polarimeters for a Bell-type experiment by measuring the spins on photons emitted 10 billion years ago from two different galaxies, and you'll find (or rather, won't find, because it's being hidden from you) that they were arranged, long before the Earth was formed, just so as to trick you. The universe is conspiring against us, and the whole scientific project is a farce. That, or superdeterminism is false.
  - zmgsabst 3y ago
    Alternatively, non-local correlations (in the style of anyons) persist from the early universe. And we’re just measuring those.
- zmgsabst 3y ago

incompatible 3y ago

These distributed "particles" are passing through both slits simultaneously before somehow transferring their energy at a particular position at the target screen. If gravitation is non-quantum, I suppose I'd expect that a gravitational measurement would show it passing through both slits, half of its total mass through each.