6 ms·

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.
deleted 4y ago
[deleted]
mannykannot 4y ago
You can make the claims of your original post (and arguably more effectively) without the subjective (and rather idiosyncratic) view that GR was in some sort of indeterminate level of credence until the detection of gravity waves.
- spekcular 4y ago
  Sorry, I wrote poorly. I don't mean to say that GR or gravitational waves were in some sort of indeterminate level of credence. I mean to say that our credence in gravitational waves in the absence of experimental confirmation is justified by exactly the kinds of non-empirical evidence that Hossenfelder rejects. So it's clear that she's being too restrictive in what she believes is worthwhile theoretical physics.

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.

deleted 4y ago

[deleted]

mannykannot 4y ago

You can make the claims of your original post (and arguably more effectively) without the subjective (and rather idiosyncratic) view that GR was in some sort of indeterminate level of credence until the detection of gravity waves.

spekcular 4y ago
Sorry, I wrote poorly. I don't mean to say that GR or gravitational waves were in some sort of indeterminate level of credence. I mean to say that our credence in gravitational waves in the absence of experimental confirmation is justified by exactly the kinds of non-empirical evidence that Hossenfelder rejects. So it's clear that she's being too restrictive in what she believes is worthwhile theoretical physics.