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Sorry, I meant direct tests of gravitational waves. I think that claim is accurate.

jeremyjh 4y ago
Why is that one thing particularly important? The point is that general relativity had immediately testable predictions. It wasn't just math.
- spekcular 4y ago
  General relativity predicts gravitational waves. For the theory to be correct, gravitational waves need to exist. You haven't fully confirmed the theory experimentally unless you confirm that consequence.
  - mr_toad 4y ago
    General relativity predicts singularities. By your argument we haven’t confirmed GR experimentally until we’ve observed a singularity.
  - jeremyjh 4y ago
    That doesn't mean that he didn't have ideas for testable predictions of the model. It implied all kinds of things, some of which he had not even thought of (like Black Holes). Its quite a lot different to propose a model with some novel predictions that you can validate, and to propose a model that can never be tested in any form at all.
    - spekcular 4y ago
      Some predictions of the model were tested soon after Einstein published the theory. Others, like gravitational waves, were not. Yet people still believed in gravitational waves long before they were empirically confirmed. I don't see any possible basis for that belief (at the time) other than arguments based on non-empirical reasons like consistency and parsimony. (That is, it would be strange if other predictions of the model worked out but this one didn't.) Yet it is exactly this kind of theoretical reasoning that Hossenfelder, taken literally, seems to reject (in the absence of experimental data). I'll also note that any other solution of the black hole information paradox has to be consistent with the the rest of what we know about physics, and any future empirical observations. So it can't be completely untethered from reality; it makes testable claims in this way. Further, it's not clear such solutions can never be directly tested, or will never be found to imply novel testable consequences.
  - mannykannot 4y ago
    This is a useless way of looking at things - even the task of just listing all the implications of a theory is probably unbounded, and if, at any point, you assume the correctness of another theory... If you are only maximally skeptical in some matters, then you are being inconsistent.
    - spekcular 4y ago
      Sure. But this is a huge, incredibly novel prediction. It's not some trivial matter.
      - hither_shores 4y ago
        You're double-counting. Starting from Newtonian mechanics, gravitational waves are indeed a huge, incredibly novel prediction. So is gravitational lensing. So is any other consequence of a coupling between the metric and the stress-energy tensor - because that's the real novel prediction. Once you grant that the metric varies smoothly over space and time, gravitational waves are pretty mundane.
        raattgift 4y ago
        I don't think one could say it was obvious, if that's what you mean by mundane. Einstein for many years went back and forth on the issue of whether gravitational plane waves could be observed even in principle (sometimes accepting his initial findings, sometimes accepting Eddington's point that gravitational plane waves were unphysical artifacts of the approximation to the full theory Einstein had to use for calculational tractability, sometimes arguing that plane waves were physical but unstable, and several variations on all of these). It wasn't until shortly after his death that it became widely accepted that gravitational radiation from a binary could couple to mass at a distance (notably Pirani 1956, reprinted <https://sci-hub.se/https://link.springer.com/article/10.1007/s10714-009-0787-9 https://sci-hub.se/https://link.springer.com/article/10.1007...>, the two paragraphs after eq (2.17) on the 9th page of the linked pdf, p. 1223[1], and the three paragraphs after eq (3.7) on the 12th&13th pdf page, pp 1226-1227 being particularly interesting), which in turn provoked Feynman's sticky bead argument and the work by Bondi, Synge and others. I don't understand the point your comment's parent's user has been trying to make in this conversation, but the entire enterprise of Einstein's theory of gravitation did not in its early days depend sensitively on the details (or even physicality) of gravitational radiation. Indeed, it was not widely accepted during his lifetime that gravitational waves could be measured even in principle. And even then it was almost twenty years before the first relativistic binary (Hulse-Taylor) was discovered and a further six or seven before it became clear that gravitational radiation is a useful test of various theories of gravitation. <https://ui.adsabs.harvard.edu/abs/1982ApJ...253..908T/abstract https://ui.adsabs.harvard.edu/abs/1982ApJ...253..908T/abstra...> (PDF freely available via link in top left): "With the exception of general relativity and the Brans-Dicke theory, none of [these alternative gravitational] theories predicts even the proper sign of orbital period change due to emission of gravitational radiation, let alone the proper magnitude". I would insert the words "local, minimal" before your "coupling between the metric and the stress-energy tensor". c, whatever its value, is relevant, and there are plenty of gravity theories with non-minimal couplings (NMDC) and spooky action at a distance (https://www.phy.olemiss.edu/~luca/Topics/ft/nonlocal.html https://www.phy.olemiss.edu/~luca/Topics/ft/nonlocal.html is a "zoo"), which tend to have different post-Newtonian effects. I would almost add "unique" as a qualifier too, but e.g. Brans-Dicke is still technically viable. - -- [1] Of particular (pardon the pun) interest to anyone who has read Synge's five point curvature detector.

Why is that one thing particularly important? The point is that general relativity had immediately testable predictions. It wasn't just math.

spekcular 4y ago
General relativity predicts gravitational waves. For the theory to be correct, gravitational waves need to exist. You haven't fully confirmed the theory experimentally unless you confirm that consequence.
- mr_toad 4y ago
  General relativity predicts singularities. By your argument we haven’t confirmed GR experimentally until we’ve observed a singularity.
- jeremyjh 4y ago
  That doesn't mean that he didn't have ideas for testable predictions of the model. It implied all kinds of things, some of which he had not even thought of (like Black Holes). Its quite a lot different to propose a model with some novel predictions that you can validate, and to propose a model that can never be tested in any form at all.
  - spekcular 4y ago
    Some predictions of the model were tested soon after Einstein published the theory. Others, like gravitational waves, were not. Yet people still believed in gravitational waves long before they were empirically confirmed. I don't see any possible basis for that belief (at the time) other than arguments based on non-empirical reasons like consistency and parsimony. (That is, it would be strange if other predictions of the model worked out but this one didn't.) Yet it is exactly this kind of theoretical reasoning that Hossenfelder, taken literally, seems to reject (in the absence of experimental data). I'll also note that any other solution of the black hole information paradox has to be consistent with the the rest of what we know about physics, and any future empirical observations. So it can't be completely untethered from reality; it makes testable claims in this way. Further, it's not clear such solutions can never be directly tested, or will never be found to imply novel testable consequences.
- mannykannot 4y ago
  This is a useless way of looking at things - even the task of just listing all the implications of a theory is probably unbounded, and if, at any point, you assume the correctness of another theory... If you are only maximally skeptical in some matters, then you are being inconsistent.
  - spekcular 4y ago
    Sure. But this is a huge, incredibly novel prediction. It's not some trivial matter.
    - hither_shores 4y ago
      You're double-counting. Starting from Newtonian mechanics, gravitational waves are indeed a huge, incredibly novel prediction. So is gravitational lensing. So is any other consequence of a coupling between the metric and the stress-energy tensor - because that's the real novel prediction. Once you grant that the metric varies smoothly over space and time, gravitational waves are pretty mundane.
      - raattgift 4y ago
        I don't think one could say it was obvious, if that's what you mean by mundane. Einstein for many years went back and forth on the issue of whether gravitational plane waves could be observed even in principle (sometimes accepting his initial findings, sometimes accepting Eddington's point that gravitational plane waves were unphysical artifacts of the approximation to the full theory Einstein had to use for calculational tractability, sometimes arguing that plane waves were physical but unstable, and several variations on all of these). It wasn't until shortly after his death that it became widely accepted that gravitational radiation from a binary could couple to mass at a distance (notably Pirani 1956, reprinted <https://sci-hub.se/https://link.springer.com/article/10.1007/s10714-009-0787-9 https://sci-hub.se/https://link.springer.com/article/10.1007...>, the two paragraphs after eq (2.17) on the 9th page of the linked pdf, p. 1223[1], and the three paragraphs after eq (3.7) on the 12th&13th pdf page, pp 1226-1227 being particularly interesting), which in turn provoked Feynman's sticky bead argument and the work by Bondi, Synge and others. I don't understand the point your comment's parent's user has been trying to make in this conversation, but the entire enterprise of Einstein's theory of gravitation did not in its early days depend sensitively on the details (or even physicality) of gravitational radiation. Indeed, it was not widely accepted during his lifetime that gravitational waves could be measured even in principle. And even then it was almost twenty years before the first relativistic binary (Hulse-Taylor) was discovered and a further six or seven before it became clear that gravitational radiation is a useful test of various theories of gravitation. <https://ui.adsabs.harvard.edu/abs/1982ApJ...253..908T/abstract https://ui.adsabs.harvard.edu/abs/1982ApJ...253..908T/abstra...> (PDF freely available via link in top left): "With the exception of general relativity and the Brans-Dicke theory, none of [these alternative gravitational] theories predicts even the proper sign of orbital period change due to emission of gravitational radiation, let alone the proper magnitude". I would insert the words "local, minimal" before your "coupling between the metric and the stress-energy tensor". c, whatever its value, is relevant, and there are plenty of gravity theories with non-minimal couplings (NMDC) and spooky action at a distance (https://www.phy.olemiss.edu/~luca/Topics/ft/nonlocal.html https://www.phy.olemiss.edu/~luca/Topics/ft/nonlocal.html is a "zoo"), which tend to have different post-Newtonian effects. I would almost add "unique" as a qualifier too, but e.g. Brans-Dicke is still technically viable. - -- [1] Of particular (pardon the pun) interest to anyone who has read Synge's five point curvature detector.