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I’m a quantum physicist and yeah you’re totally right. It’s already obvious that a quantum theory of gravity is needed because we need a way to talk about super

I’m a quantum physicist and yeah you’re totally right. It’s already obvious that a quantum theory of gravity is needed because we need a way to talk about superpositions of spacetime curvatures. Either QM is wrong in general or we need a way to treat spacetime that is in a quantum superposition Also my old supervisor actually suggested an experiment to observe this gravity induced entanglement but it would require extremely low temperatures to not degrade due to thermal noise

phkahler 3y ago
>> It’s already obvious that a quantum theory of gravity is needed because we need a way to talk about superpositions of spacetime curvatures. Is that because of things like the double slit experiment they mention? A particles could be monitored via its gravitational effect on spacetime to determine which slit it went through. What if the particles mass behaves as a mass distribution in such an experiment? Does that save classical gravity?
- bowsamic 3y ago
  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?
  - 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.
H8crilA 3y ago
What would such an experiment look like? If it makes it easier, perhaps an experiment that's even harder (or much, much harder) to realize but easier to explain.
- bowsamic 3y ago
  It's pretty simple. You basically have two suspended mirrors on wires and you shoot lasers at each mirror. Due to gravity there will be an interaction between the mirrors. There are then two things you can probe. First, the correlation between the two reflected lasers, and if you can achieve the extreme temperature requirements, entanglement between the two reflected lasers caused by the gravity. Now, the former doesn't really imply that gravity is quantum, it just shows that quantum correlations can be mediated by gravity, but this may also be true classically, however, the much more difficult to show entanglement would definitely require quantum gravity. The first step is still good though because no one has yet managed to demonstrate quantum correlations via gravity. The experiment would also have two mirrors two turn the laser beams into cavities, increasing the interaction strength. The setup would look a bit like this: _ _ ! ! ---|-----| |-----|--- where | are mirrors, ! are suspension wires, and - is the laser beam
  - H8crilA 3y ago
    Those reflected beams would at best be very very weakly entangled, right? I'm not sure what's the name for it, but if you arranged the quantum state of the two beams into a 2x2 matrix then the determinant would be just a tiny bit non-zero.
    - bowsamic 3y ago
      Yes
  - raattgift 3y ago
    You might also be interested in Bose et al. (2017), "Spin Entanglement Witness for Quantum Gravity" https://journals.aps.org/prl/abstract/10.1103/PhysRevLett.119.240401 https://journals.aps.org/prl/abstract/10.1103/PhysRevLett.11... aka [hep-th] https://arxiv.org/abs/1707.06050 https://arxiv.org/abs/1707.06050 which proposes a way to "certify gravity as a quantum coherent mediator, through simple spin correlation measurements".
codethief 3y ago
> It’s already obvious that a quantum theory of gravity is needed because we need a way to talk about superpositions of spacetime curvatures. But it doesn't seem to be that way, at least not necessarily, and that's what the article is about. What makes it obvious to you that there is no room for unification theories (of GR and QM) that don't involve a quantized version of gravity? Note that I'm not claiming gravity is not quantized. I'm just saying it cannot be ruled out yet.
- bowsamic 3y ago
  The only way it "doesn't seem to be that way" is if we determine that it's actually wrong that QM tells us that we can have entanglement between spatially separated objects. If QM is right about this, then we need quantum gravity. So either QM is wrong in some sense and/or we need to limit ourselves to a specific interpretation, or gravity must be quantum

phkahler 3y ago

>> It’s already obvious that a quantum theory of gravity is needed because we need a way to talk about superpositions of spacetime curvatures. Is that because of things like the double slit experiment they mention? A particles could be monitored via its gravitational effect on spacetime to determine which slit it went through. What if the particles mass behaves as a mass distribution in such an experiment? Does that save classical gravity?

bowsamic 3y ago
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?
- 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.

H8crilA 3y ago

What would such an experiment look like? If it makes it easier, perhaps an experiment that's even harder (or much, much harder) to realize but easier to explain.

bowsamic 3y ago
It's pretty simple. You basically have two suspended mirrors on wires and you shoot lasers at each mirror. Due to gravity there will be an interaction between the mirrors. There are then two things you can probe. First, the correlation between the two reflected lasers, and if you can achieve the extreme temperature requirements, entanglement between the two reflected lasers caused by the gravity. Now, the former doesn't really imply that gravity is quantum, it just shows that quantum correlations can be mediated by gravity, but this may also be true classically, however, the much more difficult to show entanglement would definitely require quantum gravity. The first step is still good though because no one has yet managed to demonstrate quantum correlations via gravity. The experiment would also have two mirrors two turn the laser beams into cavities, increasing the interaction strength. The setup would look a bit like this: _ _ ! ! ---|-----| |-----|--- where | are mirrors, ! are suspension wires, and - is the laser beam
- H8crilA 3y ago
  Those reflected beams would at best be very very weakly entangled, right? I'm not sure what's the name for it, but if you arranged the quantum state of the two beams into a 2x2 matrix then the determinant would be just a tiny bit non-zero.
  - bowsamic 3y ago
    Yes
- raattgift 3y ago
  You might also be interested in Bose et al. (2017), "Spin Entanglement Witness for Quantum Gravity" https://journals.aps.org/prl/abstract/10.1103/PhysRevLett.119.240401 https://journals.aps.org/prl/abstract/10.1103/PhysRevLett.11... aka [hep-th] https://arxiv.org/abs/1707.06050 https://arxiv.org/abs/1707.06050 which proposes a way to "certify gravity as a quantum coherent mediator, through simple spin correlation measurements".

codethief 3y ago

> It’s already obvious that a quantum theory of gravity is needed because we need a way to talk about superpositions of spacetime curvatures. But it doesn't seem to be that way, at least not necessarily, and that's what the article is about. What makes it obvious to you that there is no room for unification theories (of GR and QM) that don't involve a quantized version of gravity? Note that I'm not claiming gravity is not quantized. I'm just saying it cannot be ruled out yet.

bowsamic 3y ago
The only way it "doesn't seem to be that way" is if we determine that it's actually wrong that QM tells us that we can have entanglement between spatially separated objects. If QM is right about this, then we need quantum gravity. So either QM is wrong in some sense and/or we need to limit ourselves to a specific interpretation, or gravity must be quantum