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> I don't understand what you mean. If I make a prediction in curved spacetime... Perhaps "make a risky prediction that could be falsified if spacetime was in

> I don't understand what you mean. If I make a prediction in curved spacetime... Perhaps "make a risky prediction that could be falsified if spacetime was in fact not curved" is a better phrasing. Spacetime is spacetime. It's curved. > Perturbation theory is applicable to any QFT problem; its first major successes were predicting the Lamb shift and the shift in the magnetic moment of the electron due to quantum field effects. Oof, looks like I badly underestimated the extremes of scale that are involved here. It turns out you need to use perturbation theory to approximate the solution for something as complex as a single atom of hydrogen. I guess "macro" was an awful choice of word, but that means non-perturbative QFT is even less feasible than I thought for stuff at macro (to say nothing of relativistic) scales.

pdonis 6y ago
> Perhaps "make a risky prediction that could be falsified if spacetime was in fact not curved" is a better phrasing. That doesn't seem to me to be any better. If spacetime is curved, and you make a prediction assuming curved spacetime, you can test that prediction in curved spacetime. The prediction isn't assuming flat spacetime to begin with, so what would it even mean to say it could be falsified if spacetime were not curved? > that means non-perturbative QFT is even less feasible than I thought for stuff at macro (to say nothing of relativistic) scales It means no such thing. Non-perturbative QFT is commonly used in condensed matter physics, which deals with objects of macroscopic size. In fact, IIRC it was condensed matter physicists who first developed non-perturbative QFT methods; the particle physicists adopted them later. As for "relativistic scales", you are aware that relativity has measurable effects even on atomic scales, right? Look up why gold has the color it does, for example. For that matter, our best current understanding of why nucleon masses are what they are has to take relativistic effects into account.
- guscost 6y ago
  We're talking past each other, I'm just looking for any example of a QM theory predicting something that overlaps with GR, which doesn't rely on a kludge like the renormalization in Loop Quantum Gravity, and therefore hasn't hit a dead end (with LQG this is the "renormalization problem"). I don't think such an example exists - String Theory seems to be in even worse shape, but it does have a much bigger scope. From re-reading your earlier comments it sounds like you think that the final theory will not be built on QM/QFT/Standard Model foundations, and will be different enough that QM and GR will remain incompatible approximations forever. I should have addressed that idea before, and you might be right. Anyway, here we go: > The prediction isn't assuming flat spacetime to begin with, so what would it even mean to say it could be falsified if spacetime were not curved? That's how the whole process works, your theory makes a risky prediction (spacetime is curved, therefore we should see X) and then you go check. If spacetime is actually flat, you're gonna see Y instead! > Non-perturbative QFT is commonly used in condensed matter physics, which deals with objects of macroscopic size. In fact, IIRC it was condensed matter physicists who first developed non-perturbative QFT methods; the particle physicists adopted them later. Pretty cool, when was that? Do you need a fast computer to do that? But it's not relevant to the rest of this, unless this is a good example where GR also predicts something that those objects do (riskily, anything that simplifies to Newtonian dynamics doesn't count). > As for "relativistic scales", you are aware that relativity has measurable effects even on atomic scales, right? Look up why gold has the color it does, for example. For that matter, our best current understanding of why nucleon masses are what they are has to take relativistic effects into account. Yes, I've heard of Dirac. I think you're confusing special and general relativity, and/or I should have used "gravitational scales" instead.
  - pdonis 6y ago
    > I'm just looking for any example of a QM theory predicting something that overlaps with GR The most famous example is of course Hawking's prediction that black holes emit thermal radiation and will eventually evaporate. This prediction has not been tested yet, but it is testable. Another example would be the recent test of whether the Newtonian gravitational potential affects the quantum phase of neutrons (it does). This doesn't test any relativistic effects of gravity, but it does provide evidence that gravity is an interaction in the quantum sense. > which doesn't rely on a kludge like the renormalization AFAIK Hawking's prediction did not make any use of renormalization, nor do the more recent treatments of the same thing. > when was that? The 1960s, IIRC. > I should have used "gravitational scales" instead That makes it clearer what you meant, yes.
    - guscost 6y ago
      > The most famous example is of course Hawking's prediction that black holes emit thermal radiation and will eventually evaporate. Ahhh you're right, that's a gravitational-scale thing! But even a rotating black hole has a tiny state vector, which is the only reason that model has any chance of working. Good luck extending this approach to an ordinary pebble, asteroid, or planet. Also the compression-of-quantum-state contributes to the black hole information paradox (a result of Hawking's theory), right? Has that actually been resolved, and until it is, isn't that essentially an incompatibility with [some stuff that I can't talk about intelligently]? > Another example would be the recent test of whether the Newtonian gravitational potential affects the quantum phase of neutrons (it does). This doesn't test any relativistic effects of gravity, but it does provide evidence that gravity is an interaction in the quantum sense. Sounds pretty cool, but I doubt I'm going to be able to discuss this meaningfully. Is this related to the Higgs boson? That's the force carrier for gravitation in the Standard Model, if I understand correctly.
  - pdonis 6y ago
    > your theory makes a risky prediction (spacetime is curved, therefore we should see X) and then you go check. If spacetime is actually flat, you're gonna see Y instead So basically your issue here is that QFT doesn't predict that spacetime is curved? Or it doesn't make a prediction either way about whether spacetime is curved or flat? This amounts to saying "we don't yet have a good theory of quantum gravity". Yes, this is true. > it sounds like you think that the final theory will not be built on QM/QFT/Standard Model foundations I am skeptical about whether string theory will turn out to be the quantum gravity theory we are searching for, and string theory is the only candidate I'm aware of that is a QFT of any sort. So yes, if string theory doesn't turn out to be right, I think we will be in the position you describe. > and will be different enough that QM and GR will remain incompatible approximations forever Only in the same sense that, for example, Newton's laws and Maxwell's Equations are incompatible approximations. I don't see that sort of thing as a problem.

pdonis 6y ago

> Perhaps "make a risky prediction that could be falsified if spacetime was in fact not curved" is a better phrasing. That doesn't seem to me to be any better. If spacetime is curved, and you make a prediction assuming curved spacetime, you can test that prediction in curved spacetime. The prediction isn't assuming flat spacetime to begin with, so what would it even mean to say it could be falsified if spacetime were not curved? > that means non-perturbative QFT is even less feasible than I thought for stuff at macro (to say nothing of relativistic) scales It means no such thing. Non-perturbative QFT is commonly used in condensed matter physics, which deals with objects of macroscopic size. In fact, IIRC it was condensed matter physicists who first developed non-perturbative QFT methods; the particle physicists adopted them later. As for "relativistic scales", you are aware that relativity has measurable effects even on atomic scales, right? Look up why gold has the color it does, for example. For that matter, our best current understanding of why nucleon masses are what they are has to take relativistic effects into account.

guscost 6y ago
We're talking past each other, I'm just looking for any example of a QM theory predicting something that overlaps with GR, which doesn't rely on a kludge like the renormalization in Loop Quantum Gravity, and therefore hasn't hit a dead end (with LQG this is the "renormalization problem"). I don't think such an example exists - String Theory seems to be in even worse shape, but it does have a much bigger scope. From re-reading your earlier comments it sounds like you think that the final theory will not be built on QM/QFT/Standard Model foundations, and will be different enough that QM and GR will remain incompatible approximations forever. I should have addressed that idea before, and you might be right. Anyway, here we go: > The prediction isn't assuming flat spacetime to begin with, so what would it even mean to say it could be falsified if spacetime were not curved? That's how the whole process works, your theory makes a risky prediction (spacetime is curved, therefore we should see X) and then you go check. If spacetime is actually flat, you're gonna see Y instead! > Non-perturbative QFT is commonly used in condensed matter physics, which deals with objects of macroscopic size. In fact, IIRC it was condensed matter physicists who first developed non-perturbative QFT methods; the particle physicists adopted them later. Pretty cool, when was that? Do you need a fast computer to do that? But it's not relevant to the rest of this, unless this is a good example where GR also predicts something that those objects do (riskily, anything that simplifies to Newtonian dynamics doesn't count). > As for "relativistic scales", you are aware that relativity has measurable effects even on atomic scales, right? Look up why gold has the color it does, for example. For that matter, our best current understanding of why nucleon masses are what they are has to take relativistic effects into account. Yes, I've heard of Dirac. I think you're confusing special and general relativity, and/or I should have used "gravitational scales" instead.
- pdonis 6y ago
  > I'm just looking for any example of a QM theory predicting something that overlaps with GR The most famous example is of course Hawking's prediction that black holes emit thermal radiation and will eventually evaporate. This prediction has not been tested yet, but it is testable. Another example would be the recent test of whether the Newtonian gravitational potential affects the quantum phase of neutrons (it does). This doesn't test any relativistic effects of gravity, but it does provide evidence that gravity is an interaction in the quantum sense. > which doesn't rely on a kludge like the renormalization AFAIK Hawking's prediction did not make any use of renormalization, nor do the more recent treatments of the same thing. > when was that? The 1960s, IIRC. > I should have used "gravitational scales" instead That makes it clearer what you meant, yes.
  - guscost 6y ago
    > The most famous example is of course Hawking's prediction that black holes emit thermal radiation and will eventually evaporate. Ahhh you're right, that's a gravitational-scale thing! But even a rotating black hole has a tiny state vector, which is the only reason that model has any chance of working. Good luck extending this approach to an ordinary pebble, asteroid, or planet. Also the compression-of-quantum-state contributes to the black hole information paradox (a result of Hawking's theory), right? Has that actually been resolved, and until it is, isn't that essentially an incompatibility with [some stuff that I can't talk about intelligently]? > Another example would be the recent test of whether the Newtonian gravitational potential affects the quantum phase of neutrons (it does). This doesn't test any relativistic effects of gravity, but it does provide evidence that gravity is an interaction in the quantum sense. Sounds pretty cool, but I doubt I'm going to be able to discuss this meaningfully. Is this related to the Higgs boson? That's the force carrier for gravitation in the Standard Model, if I understand correctly.
- pdonis 6y ago
  > your theory makes a risky prediction (spacetime is curved, therefore we should see X) and then you go check. If spacetime is actually flat, you're gonna see Y instead So basically your issue here is that QFT doesn't predict that spacetime is curved? Or it doesn't make a prediction either way about whether spacetime is curved or flat? This amounts to saying "we don't yet have a good theory of quantum gravity". Yes, this is true. > it sounds like you think that the final theory will not be built on QM/QFT/Standard Model foundations I am skeptical about whether string theory will turn out to be the quantum gravity theory we are searching for, and string theory is the only candidate I'm aware of that is a QFT of any sort. So yes, if string theory doesn't turn out to be right, I think we will be in the position you describe. > and will be different enough that QM and GR will remain incompatible approximations forever Only in the same sense that, for example, Newton's laws and Maxwell's Equations are incompatible approximations. I don't see that sort of thing as a problem.