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I think there's a bit of language problem here. In the way physicists use the word in 2024, making gravity "quantum" means something very different to just havi

I think there's a bit of language problem here. In the way physicists use the word in 2024, making gravity "quantum" means something very different to just having discrete "quanta" particles. It means a theory with stuff like superpositions of classical states,interference and entanglement. It also means having a theory which can consistently interact with other quantum stuff. The standard through experiment goes something like this. Take a particle with some mass (maybe am electron, maybe a bowling ball), general relativity tells us what its mass does to space-time around it. Now take that particle and put it in a superposition of being located at point A and point B (you can do this if the particle is quantum). What the heck is it doing to spacetime now? GR doesn't tell us, because GR can't deal with quantum particles. Naively it seems like space-time should also be in a superposition, but we tried to make theories like that and they didn't really work.

kromem 3y ago
Yes, I'm familiar with the thought experiments. My favorite to date was Zych, et al Bell’s theorem for temporal order (2019). But it still seems a bit like putting the cart ahead of the horse given the amount of resources dedicated to the search when the smallest gravitational field measurements to date (or at least as of 2021) were on a 92mg object. And while in theory the largest mass to be put in a quantum superposition to date was 16mg, the degree of separation in the vibrational superposition is still well below an order of magnitude gap from a difference detectable with current gravitational measurement. For a field where behavior changes in relationship to measurement and the persistence of information, there's not currently any measurable discrepancy between GR and QM. It's literally only in gedankenexperiments there's a conflict. Maybe the difficulty in preserving coherence for larger objects relates to their inadvertently leaving measurable information in spacetime? Perhaps we'll find there's a fundamental limit on gravitational measurement that happens to be right around the limit at which we can maintain coherence. It just seems like the kind of quest that's premature when we can't measure a conflict existing outside of our imaginations, and where the mystery will likely resolve almost immediately after we can actually measure the conflict in the first place (should we ever be able to do so). So yes, if we end up with an entire planet in a quantum superposition near two different spaceships shooting at each other, things might get weird. But at least currently that 'if' seems to be doing a lot of heavy lifting.
- eigenket 3y ago
  I don't think its premature when we have stuff like the model of Oppenheim et. al. which appears to predict stuff which we can observe from astronomical data. This paper [1] also discusses some experimental tests of their model which are not too technologically insane (you don't have to put a planet in superposition). Pars of the parameter space of their model are already ruled out by stuff we already know and more can be excluded with a combination of deriving better theoretical bounds (their analysis is quite coarse to simplify things) and better experiments. I don't think its very meaningful to talk about any "measurable discrepancy between GR and QM" in any case - we simply don't have any predictions in many cases where both QM and GM are very relevant. General relativity doesn't tell you want happens when you put a particle in superposition - its not that it tells you something and we expect some differences from that in reality. [1] https://arxiv.org/abs/2203.01982 https://arxiv.org/abs/2203.01982

kromem 3y ago

Yes, I'm familiar with the thought experiments. My favorite to date was Zych, et al Bell’s theorem for temporal order (2019). But it still seems a bit like putting the cart ahead of the horse given the amount of resources dedicated to the search when the smallest gravitational field measurements to date (or at least as of 2021) were on a 92mg object. And while in theory the largest mass to be put in a quantum superposition to date was 16mg, the degree of separation in the vibrational superposition is still well below an order of magnitude gap from a difference detectable with current gravitational measurement. For a field where behavior changes in relationship to measurement and the persistence of information, there's not currently any measurable discrepancy between GR and QM. It's literally only in gedankenexperiments there's a conflict. Maybe the difficulty in preserving coherence for larger objects relates to their inadvertently leaving measurable information in spacetime? Perhaps we'll find there's a fundamental limit on gravitational measurement that happens to be right around the limit at which we can maintain coherence. It just seems like the kind of quest that's premature when we can't measure a conflict existing outside of our imaginations, and where the mystery will likely resolve almost immediately after we can actually measure the conflict in the first place (should we ever be able to do so). So yes, if we end up with an entire planet in a quantum superposition near two different spaceships shooting at each other, things might get weird. But at least currently that 'if' seems to be doing a lot of heavy lifting.

eigenket 3y ago
I don't think its premature when we have stuff like the model of Oppenheim et. al. which appears to predict stuff which we can observe from astronomical data. This paper [1] also discusses some experimental tests of their model which are not too technologically insane (you don't have to put a planet in superposition). Pars of the parameter space of their model are already ruled out by stuff we already know and more can be excluded with a combination of deriving better theoretical bounds (their analysis is quite coarse to simplify things) and better experiments. I don't think its very meaningful to talk about any "measurable discrepancy between GR and QM" in any case - we simply don't have any predictions in many cases where both QM and GM are very relevant. General relativity doesn't tell you want happens when you put a particle in superposition - its not that it tells you something and we expect some differences from that in reality. [1] https://arxiv.org/abs/2203.01982 https://arxiv.org/abs/2203.01982