7 ms·

(Not a quantum physicist. Please correct me if I am misunderstanding.) From what this article says, his assumption is that gravity is classical, but "fuzzy" or

(Not a quantum physicist. Please correct me if I am misunderstanding.) From what this article says, his assumption is that gravity is classical, but "fuzzy" or probabilistic: you can't precisely measure the gravitational field of a sufficiently small object. In the last years we've seen progressively bigger objects being put in quantum superposition. The theory from this article is incompatible with this process continuing indefinitely. When and if we create a sufficiently big object that we are able to entangle it with something else via gravitational interaction, this would immediately disprove this theory. So the good news is that this theory is clearly falsifiable, possibly even without creating a particle accelerator the size of the solar system.

bowsamic 3y ago
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?
        3y ago
- 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
contravariant 3y ago
> possibly even without creating a particle accelerator the size of the solar system. Sure, you just need to entangle two objects big enough to exert a noticeable amount of gravity on one another but somehow do not interact gravitationally with the rest of the set up. Anyway, let's try a cat sized object first, then we'll finally know if Schroedinger had a point.
- eterevsky 3y ago
  If they are close enough to each other and far enough from everything else, then why not. The smallest objects for which we measured their gravitational interaction weighed just 90 mg (https://arstechnica.com/science/2021/03/researchers-measure-the-gravity-exerted-by-a-90-milligram-object/ https://arstechnica.com/science/2021/03/researchers-measure-...). The biggest object put in quantum superposition weighed around 1 mcg (https://physics.aps.org/articles/v16/s45 https://physics.aps.org/articles/v16/s45).
  - feoren 3y ago
    So 90,000 times smaller. At first glance you made it look 90x smaller because I couldn't tell if "mcg" was a typo for "mg" or not (without clicking the link). I've never seen anyone use "mcg" to mean "microgram" before and I feel like it's misleading. Use "ug", or "μg" if you're feeling fancy.
    - twic 3y ago
      Use of "mcg" for micrograms is common practice in medicine. I always assumed it was because it's easier to type than μg, but it seems a medical body recommends it because μg is too easily mistaken for mg: https://www.ismp.org/sites/default/files/attachments/2017-11/Error%20Prone%20Abbreviations%202015.pdf https://www.ismp.org/sites/default/files/attachments/2017-11...
      - marcosdumay 3y ago
        > μg is too easily mistaken for mg Is that some joke about medical writing? Anyway, I just noticed that table doesn't have rules for nanogram. There is also no mega-anything. (On a serious parenthesis, I think I actually understand their rationale; but changing the abbreviation of only one of them is still confusing.)
GoblinSlayer 3y ago
Even for small particles superposition can span arbitrarily large distances. I think it will be difficult for classical gravity to gloss over such distances. And the rationale is strange: information paradox is pure math unrelated to reality, it doesn't warrant a new theory.

bowsamic 3y ago

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?
        3y ago
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

contravariant 3y ago

> possibly even without creating a particle accelerator the size of the solar system. Sure, you just need to entangle two objects big enough to exert a noticeable amount of gravity on one another but somehow do not interact gravitationally with the rest of the set up. Anyway, let's try a cat sized object first, then we'll finally know if Schroedinger had a point.

eterevsky 3y ago
If they are close enough to each other and far enough from everything else, then why not. The smallest objects for which we measured their gravitational interaction weighed just 90 mg (https://arstechnica.com/science/2021/03/researchers-measure-the-gravity-exerted-by-a-90-milligram-object/ https://arstechnica.com/science/2021/03/researchers-measure-...). The biggest object put in quantum superposition weighed around 1 mcg (https://physics.aps.org/articles/v16/s45 https://physics.aps.org/articles/v16/s45).
- feoren 3y ago
  So 90,000 times smaller. At first glance you made it look 90x smaller because I couldn't tell if "mcg" was a typo for "mg" or not (without clicking the link). I've never seen anyone use "mcg" to mean "microgram" before and I feel like it's misleading. Use "ug", or "μg" if you're feeling fancy.
  - twic 3y ago
    Use of "mcg" for micrograms is common practice in medicine. I always assumed it was because it's easier to type than μg, but it seems a medical body recommends it because μg is too easily mistaken for mg: https://www.ismp.org/sites/default/files/attachments/2017-11/Error%20Prone%20Abbreviations%202015.pdf https://www.ismp.org/sites/default/files/attachments/2017-11...
    - marcosdumay 3y ago
      > μg is too easily mistaken for mg Is that some joke about medical writing? Anyway, I just noticed that table doesn't have rules for nanogram. There is also no mega-anything. (On a serious parenthesis, I think I actually understand their rationale; but changing the abbreviation of only one of them is still confusing.)

GoblinSlayer 3y ago

Even for small particles superposition can span arbitrarily large distances. I think it will be difficult for classical gravity to gloss over such distances. And the rationale is strange: information paradox is pure math unrelated to reality, it doesn't warrant a new theory.