6 ms·

> But MOND has so many problems The fact that the results happen to fit some MOND model(s), while interesting, doesn't really mean much: these results could be

> But MOND has so many problems The fact that the results happen to fit some MOND model(s), while interesting, doesn't really mean much: these results could be 100% correct and all of MOND a pipe dream at the same time. The crucial question is, is the analysis correct?

sigmoid10 3y ago
If the results are correct, it means that extreme weak field gravity laws are actually in need of modification. I'd be very surprised if any existing variant of MOND got it right, but it would still mean that the premise of MOND is correct, which would be a huge deal.
- V1ndaar 3y ago
  > premise of MOND is correct Well, typically the term MOND refers to very specific modifications of Newtonian gravity and doesn't even take into account GR. So I'm not sure I would agree with the statement in general terms (note, I haven't read the paper so far, so I don't know what they claim). There's _plenty_ of extensions of GR though that modify gravity. That in itself isn't really that surprising. Scalar-Tensor theory and the like. And there's a reason these are usually not called MOND. Some go by "MOG" for modified gravity.
  - sigmoid10 3y ago
    The paper actually mentions a specific form of MOND called AQUAL [1]. The original version of MOND was a very specific, simple modification of Newton's laws, but over the decades it has become more of a synonym for all attempts at modifying gravity at long distances. Especially since all of these attempts modify the weak field regime, which is equivalent to modifying Newton's law of gravity. [1] https://en.m.wikipedia.org/wiki/AQUAL https://en.m.wikipedia.org/wiki/AQUAL
  - AnimalMuppet 3y ago
    This particular proposed modification is for very weak accelerations. That's where Newtonian gravity is least corrected by GR.
- naasking 3y ago
  > I'd be very surprised if any existing variant of MOND got it right I'm not sure why this would be surprising, MOND has made many more successful predictions than DM.
  - sigmoid10 3y ago
    That is simply not true. Like, it's literally the opposite of the truth.
    - naasking 3y ago
      Don't confuse post-hoc curve fitting with predictions: From galactic bars to the Hubble tension: weighing up the astrophysical evidence for Milgromian gravity, https://arxiv.org/abs/2110.06936 https://arxiv.org/abs/2110.06936
      - sigmoid10 3y ago
        You should read up on how DM is fitted vs how MOND is fitted. But neither preceded the data in the way you imply and DM is vastly more successful at predicting all observations with a single fit.
        naasking 3y ago
        What I know is that MOND predicted the high redshift galaxies we're now seeing 20 years ago, and DM did not: https://tritonstation.com/2022/01/03/what-jwst-will-see/ https://tritonstation.com/2022/01/03/what-jwst-will-see/ There are many instances like this as outlined in the paper I linked.
MattPalmer1086 3y ago
But if the analysis is correct, it does mean that general relativity will need to be different to the current formulation and that dark matter is unlikely to be the (entire) explanation for observed galactic rotations.
- naasking 3y ago
  No, some form of dark matter could have interactions that produce MOND dynamics in the correct regime. This is compatible with GR. For instance, various superfluid dark matter proposals that are being researched.
  - MattPalmer1086 3y ago
    My understanding is that dark matter cannot explain this motion in wide binaries, which is why these are interesting observations.
    - naasking 3y ago
      Dark matter is not one theory, but a class of theories. The point of superfluid dark matter proposals is that it reproduces MOND dynamics, therefore it would explain these observations.
      - MattPalmer1086 3y ago
        The paper states: "First of all, the gravitational anomaly in the dynamics of binary stars cannot be attributed to dark matter because the required amount is absurd, and thus there is no way to save the standard theory of gravity."
        naasking 3y ago
        Which applies to classical dark matter theories, not ones that naturally reproduce MOND dynamics.
taylodl 3y ago
If these observations are confirmed, then Newton is wrong (which we already knew), GR is wrong, and MOND is wrong. I don't think the solution is going to be tweaking GR - it's going to be something radically different. This could be so big we might even uncover something causing us to re-think QFT and then put all the forces on equal footing. That would be huge, because where we are now we're stuck. We've been looking for hints for how to move forward, this could be it.

sigmoid10 3y ago

If the results are correct, it means that extreme weak field gravity laws are actually in need of modification. I'd be very surprised if any existing variant of MOND got it right, but it would still mean that the premise of MOND is correct, which would be a huge deal.

V1ndaar 3y ago
> premise of MOND is correct Well, typically the term MOND refers to very specific modifications of Newtonian gravity and doesn't even take into account GR. So I'm not sure I would agree with the statement in general terms (note, I haven't read the paper so far, so I don't know what they claim). There's _plenty_ of extensions of GR though that modify gravity. That in itself isn't really that surprising. Scalar-Tensor theory and the like. And there's a reason these are usually not called MOND. Some go by "MOG" for modified gravity.
- sigmoid10 3y ago
  The paper actually mentions a specific form of MOND called AQUAL [1]. The original version of MOND was a very specific, simple modification of Newton's laws, but over the decades it has become more of a synonym for all attempts at modifying gravity at long distances. Especially since all of these attempts modify the weak field regime, which is equivalent to modifying Newton's law of gravity. [1] https://en.m.wikipedia.org/wiki/AQUAL https://en.m.wikipedia.org/wiki/AQUAL
- AnimalMuppet 3y ago
  This particular proposed modification is for very weak accelerations. That's where Newtonian gravity is least corrected by GR.
naasking 3y ago
> I'd be very surprised if any existing variant of MOND got it right I'm not sure why this would be surprising, MOND has made many more successful predictions than DM.
- sigmoid10 3y ago
  That is simply not true. Like, it's literally the opposite of the truth.
  - naasking 3y ago
    Don't confuse post-hoc curve fitting with predictions: From galactic bars to the Hubble tension: weighing up the astrophysical evidence for Milgromian gravity, https://arxiv.org/abs/2110.06936 https://arxiv.org/abs/2110.06936
    - sigmoid10 3y ago
      You should read up on how DM is fitted vs how MOND is fitted. But neither preceded the data in the way you imply and DM is vastly more successful at predicting all observations with a single fit.
      - naasking 3y ago
        What I know is that MOND predicted the high redshift galaxies we're now seeing 20 years ago, and DM did not: https://tritonstation.com/2022/01/03/what-jwst-will-see/ https://tritonstation.com/2022/01/03/what-jwst-will-see/ There are many instances like this as outlined in the paper I linked.

MattPalmer1086 3y ago

But if the analysis is correct, it does mean that general relativity will need to be different to the current formulation and that dark matter is unlikely to be the (entire) explanation for observed galactic rotations.

naasking 3y ago
No, some form of dark matter could have interactions that produce MOND dynamics in the correct regime. This is compatible with GR. For instance, various superfluid dark matter proposals that are being researched.
- MattPalmer1086 3y ago
  My understanding is that dark matter cannot explain this motion in wide binaries, which is why these are interesting observations.
  - naasking 3y ago
    Dark matter is not one theory, but a class of theories. The point of superfluid dark matter proposals is that it reproduces MOND dynamics, therefore it would explain these observations.
    - MattPalmer1086 3y ago
      The paper states: "First of all, the gravitational anomaly in the dynamics of binary stars cannot be attributed to dark matter because the required amount is absurd, and thus there is no way to save the standard theory of gravity."
      - naasking 3y ago
        Which applies to classical dark matter theories, not ones that naturally reproduce MOND dynamics.

taylodl 3y ago

If these observations are confirmed, then Newton is wrong (which we already knew), GR is wrong, and MOND is wrong. I don't think the solution is going to be tweaking GR - it's going to be something radically different. This could be so big we might even uncover something causing us to re-think QFT and then put all the forces on equal footing. That would be huge, because where we are now we're stuck. We've been looking for hints for how to move forward, this could be it.