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An article on HN a couple months ago https://news.ycombinator.com/item?id=26442021 https://news.ycombinator.com/item?id=26442021 claims that our theory of gravi

An article on HN a couple months ago https://news.ycombinator.com/item?id=26442021 https://news.ycombinator.com/item?id=26442021 claims that our theory of gravity is perfectly fine, we just haven't been taking into account the proper relativistic corrections at galactic scales. This seems much more plausible and elegant of an explanation to me.

rich_sasha 5y ago
I meant to comment there but never got round to it: we have now found galaxies apparently without dark matter. They behave exactly as you’d expect from direct measurements. So either somehow that explanation doesn’t apply there (but why?) or dark matter is real, and just somehow absent from some galaxies. Link: https://www.space.com/19-galaxies-missing-dark-matter.html https://www.space.com/19-galaxies-missing-dark-matter.html
- XorNot 5y ago
  Also specifically artifacts like the bullet nebula, where observationally we can see gravitational lensing totally removed from the colliding gases in the X-ray spectrum - exactly what would be expected if dark matter had been separated from the object by it's inability to collide with itself via normal mechanisms.
  - TrispusAttucks 5y ago
    FTA: Then there are collisions of galaxy clusters at high velocity, like the bullet cluster or the el gordo cluster. These are difficult to explain with particle dark matter, because dark matter creates friction and that makes such high relative velocities incredibly unlikely. Yes, you heard that correctly, the Bullet cluster is a PROBLEM for dark matter, not evidence for it. Yes, you heard that correctly, the Bullet cluster is a PROBLEM for dark matter, not evidence for it.
    - pavel_lishin 5y ago
      Does "friction" refer to something different here? I thought that Dark Matter didn't even interact with itself in any meaningful sense, and therefore had no friction.
      - Keysh 5y ago
        Friction in this context refers to "dynamical friction", where massive objects -- e.g., dense cluster cores -- moving through a medium of many small masses -- e.g., DM particles -- will experience gravitational drag and slow down. https://en.wikipedia.org/wiki/Dynamical_friction https://en.wikipedia.org/wiki/Dynamical_friction
    - SAI_Peregrinus 5y ago
      Except that the core thing that makes dark matter dark is that it doesn't interact via the electromagnetic force, and friction is a result of the electromagnetic interaction. So that statement is nonsense.
      - dnautics 5y ago
        No that's not right. Dark matter itself causes an entirely gravitational "friction". If you have a noninteracting particle over time a bunch of them get slingshotted to very high velocity on accident which bleeds the net momentum of the rest of the system. If you've ever written n-body simulators you would see this.
        gus_massa 5y ago
        I think you are correct, but I like freebees, ... Do you have an online demo? Is the maximal speed of the slingshotted particle something like twice the speed of the big objects? Do they escape or they just form a cloud that gets hotter? For this subject, it's important that the simulation conserves energy. Most naive numeric simulations don't do that, and even symplectic simulations increase/decrease the total energy slowly.
- devoutsalsa 5y ago
  Dr. Becky does a good job of describing how this could be possible in this video => https://www.youtube.com/watch?v=gMDKDs9C4lo&t=742s https://www.youtube.com/watch?v=gMDKDs9C4lo&t=742s
- cjohansson 5y ago
  Sounds like the DM theory is great for postdictions but not for predictions, like you can't know in advance whether it is applicable or not
abnry 5y ago
But surely someone would have worked out the math and people would agree that the relativistic explanation fits the data?
542458 5y ago
My understanding is that while MOND seems simpler on the surface, it a) is hard to make match observed data (you end up needing dark matter anyways to make it work, and it still falls apart on things like the bullet cluster) and b) breaks all sorts of other things in physics that don’t look like they need to be broken (you have to staple new stuff to the side of relativity to keep it all consistent, in some cases breaking conservation). Dark matter looks/sounds hacky and broken, but in general matches real-world data better than the competing theories.
- Blammar 5y ago
  The OP presentation is saying that dark matter matches some things and MOND matches other things. Both together seem to explain everything.
  - deleted 5y ago
    [deleted]
Maursault 5y ago
> we just haven't been taking into account the proper relativistic corrections at galactic scales Dark Matter was first postulated because galaxy rotation curves strongly suggested if there was not unseen matter, galaxies would fly apart. But these observations of galaxies were each of a galaxy in isolation. So in essence, Dark Matter is a fudge to explain the observation that galaxies are not flying apart. Since, other observations that can't be explained have been lumped into Dark Matter. But it turns out, Dark Matter is unnecessary to explain galaxy rotation curves.[0] Galaxies are never isolated. They come in clusters and superclusters. [0] https://www.youtube.com/watch?v=PL0ewiwqoTw https://www.youtube.com/watch?v=PL0ewiwqoTw
- ravi-delia 5y ago
  The scope of observations goes a little further than rotational curves. The Bullet Cluster, for instance, clearly shows mass where there is no visible matter. Plus, there are lots of smaller galaxies that behave just as our normal theories prescribe, which dark matter (or the lack thereof) can explain and other theories have trouble with.
  - bjornsing 5y ago
    So? The relativistic effects of mass currents (that the parent is talking about) could put “apparent mass” where there is none, no?
    - ravi-delia 5y ago
      In principle, yes. In practice, these proposals match the spin curve very well and the bullet cluster not so much. That isn't a knock down argument; the bullet cluster offers oddities for dark matter too. However OP implied that because galactic rotation works without dark matter, and dark matter was first posited to deal with galactic rotation, we don't need dark matter. That's not right.
      - bjornsing 5y ago
        I find it a bit hard to believe that the mass current calculations for the bullet cluster have even been made. How can I convince myself that they have?
Sniffnoy 5y ago
This paper is just wrong, lots of other people have also done this calculation and shown that it's off by multiple orders of magnitude. Robin Hanson claims the particular error is the paper's assumption of zero pressure: https://www.overcomingbias.com/2021/03/what-holds-up-a-north-pole-of-dust.html https://www.overcomingbias.com/2021/03/what-holds-up-a-north...
- escape_goat 5y ago
  I have no doubt that Robin Hanson is a brilliant man, and I am aware that he has an extremely strong physics background for a professor of economics, but it would seem prudent at this point for those of us who are not domain experts to have somewhat Bayesian anticipations regarding the cross-disciplinary adventurism of economics professors. People read his blog. If this thing is indeed a ball, there will be plenty of astrophysicists happy to pick it up and run with it. This doesn't mean that you're wrong that the paper is wrong, but Dr. Hanson should absolutely not be our referent for certitude on the matter.
  - throwaway64643 5y ago
    > If this thing is indeed a ball, there will be plenty of astrophysicists happy to pick it up and run with it. This reminds me of what happened with Sir Atiyah and his 'proof' on Riemann hypothesis few years ago. Sir Atiyah, despite of being a well-known mathematician (but in a different discipline), got absolute silence from researching community and experts when he proposed his proof on Riemann hypothesis and connection of fine structure constant of physics to mathematics. If experts stay silent, if your work gets no follow-up, it's likely that what you said is wrong or has no value to people.
    - deleted 5y ago
      [deleted]
  - Sniffnoy 5y ago
    Robin Hanson isn't the one saying it's wrong. A number of other physicists (e.g. Garrett Lisi as mentioned in the post) have said it's wrong, on the basis that the calculation is easy and when they do it they get a very different result. Hanson is just the one who claims to have found the particular error that the paper made, because nobody else thought it was worth bothering to find the particular error in a paper that got such an obviously wrong answer.
Keysh 5y ago
This seems to suffer from the usual problem of non-astronomers not understanding the breadth of the problem they think they're trying to solve. That is, they've vaguely heard that the problem is in the rotation of disk galaxies, and so they propose solutions that depend on the rotation (and maybe also on the flattening). But the "dark matter" problem occurs for basically all galaxies, including things like elliptical and dwarf spheroidal galaxies that have no bulk rotation at all. (And also disk galaxies where a significant fraction of the stars and gas are counter-rotating.) The same applies to groups and clusters of galaxies.
- bjornsing 5y ago
  > And also disk galaxies where a significant fraction of the stars and gas are counter-rotating. So? Have you calculated the relativistic effects of those incredibly complex mass currents and come to the conclusion that GR is not a sufficient explanation for the motion? I’m sure there’s a high risk that GR can’t explain everything, but it sure is frustrating that people tend to grasp for these adhoc hypotheses without exhausting GR first.
teamonkey 5y ago
It's not the first time something like this has been proposed... and found fault with. https://inspirehep.net/literature/690135 https://inspirehep.net/literature/690135 > Recently a new model of galactic gravitational field, based on ordinary General Relativity, has been proposed by Cooperstock and Tieu in which no exotic dark matter is needed to fit the observed rotation curve to a reasonable ordinary matter distribution. We argue that in this model the gravitational field is generated not only by the galaxy matter, but by a thin, singular disk as well. The model should therefore be considered unphysical.

rich_sasha 5y ago

I meant to comment there but never got round to it: we have now found galaxies apparently without dark matter. They behave exactly as you’d expect from direct measurements. So either somehow that explanation doesn’t apply there (but why?) or dark matter is real, and just somehow absent from some galaxies. Link: https://www.space.com/19-galaxies-missing-dark-matter.html https://www.space.com/19-galaxies-missing-dark-matter.html

XorNot 5y ago
Also specifically artifacts like the bullet nebula, where observationally we can see gravitational lensing totally removed from the colliding gases in the X-ray spectrum - exactly what would be expected if dark matter had been separated from the object by it's inability to collide with itself via normal mechanisms.
- TrispusAttucks 5y ago
  FTA: Then there are collisions of galaxy clusters at high velocity, like the bullet cluster or the el gordo cluster. These are difficult to explain with particle dark matter, because dark matter creates friction and that makes such high relative velocities incredibly unlikely. Yes, you heard that correctly, the Bullet cluster is a PROBLEM for dark matter, not evidence for it. Yes, you heard that correctly, the Bullet cluster is a PROBLEM for dark matter, not evidence for it.
  - pavel_lishin 5y ago
    Does "friction" refer to something different here? I thought that Dark Matter didn't even interact with itself in any meaningful sense, and therefore had no friction.
    - Keysh 5y ago
      Friction in this context refers to "dynamical friction", where massive objects -- e.g., dense cluster cores -- moving through a medium of many small masses -- e.g., DM particles -- will experience gravitational drag and slow down. https://en.wikipedia.org/wiki/Dynamical_friction https://en.wikipedia.org/wiki/Dynamical_friction
  - SAI_Peregrinus 5y ago
    Except that the core thing that makes dark matter dark is that it doesn't interact via the electromagnetic force, and friction is a result of the electromagnetic interaction. So that statement is nonsense.
    - dnautics 5y ago
      No that's not right. Dark matter itself causes an entirely gravitational "friction". If you have a noninteracting particle over time a bunch of them get slingshotted to very high velocity on accident which bleeds the net momentum of the rest of the system. If you've ever written n-body simulators you would see this.
      - gus_massa 5y ago
        I think you are correct, but I like freebees, ... Do you have an online demo? Is the maximal speed of the slingshotted particle something like twice the speed of the big objects? Do they escape or they just form a cloud that gets hotter? For this subject, it's important that the simulation conserves energy. Most naive numeric simulations don't do that, and even symplectic simulations increase/decrease the total energy slowly.
devoutsalsa 5y ago
Dr. Becky does a good job of describing how this could be possible in this video => https://www.youtube.com/watch?v=gMDKDs9C4lo&t=742s https://www.youtube.com/watch?v=gMDKDs9C4lo&t=742s
cjohansson 5y ago
Sounds like the DM theory is great for postdictions but not for predictions, like you can't know in advance whether it is applicable or not

abnry 5y ago

But surely someone would have worked out the math and people would agree that the relativistic explanation fits the data?

542458 5y ago

My understanding is that while MOND seems simpler on the surface, it a) is hard to make match observed data (you end up needing dark matter anyways to make it work, and it still falls apart on things like the bullet cluster) and b) breaks all sorts of other things in physics that don’t look like they need to be broken (you have to staple new stuff to the side of relativity to keep it all consistent, in some cases breaking conservation). Dark matter looks/sounds hacky and broken, but in general matches real-world data better than the competing theories.

Blammar 5y ago
The OP presentation is saying that dark matter matches some things and MOND matches other things. Both together seem to explain everything.
- deleted 5y ago
  [deleted]

Maursault 5y ago

> we just haven't been taking into account the proper relativistic corrections at galactic scales Dark Matter was first postulated because galaxy rotation curves strongly suggested if there was not unseen matter, galaxies would fly apart. But these observations of galaxies were each of a galaxy in isolation. So in essence, Dark Matter is a fudge to explain the observation that galaxies are not flying apart. Since, other observations that can't be explained have been lumped into Dark Matter. But it turns out, Dark Matter is unnecessary to explain galaxy rotation curves.[0] Galaxies are never isolated. They come in clusters and superclusters. [0] https://www.youtube.com/watch?v=PL0ewiwqoTw https://www.youtube.com/watch?v=PL0ewiwqoTw

ravi-delia 5y ago
The scope of observations goes a little further than rotational curves. The Bullet Cluster, for instance, clearly shows mass where there is no visible matter. Plus, there are lots of smaller galaxies that behave just as our normal theories prescribe, which dark matter (or the lack thereof) can explain and other theories have trouble with.
- bjornsing 5y ago
  So? The relativistic effects of mass currents (that the parent is talking about) could put “apparent mass” where there is none, no?
  - ravi-delia 5y ago
    In principle, yes. In practice, these proposals match the spin curve very well and the bullet cluster not so much. That isn't a knock down argument; the bullet cluster offers oddities for dark matter too. However OP implied that because galactic rotation works without dark matter, and dark matter was first posited to deal with galactic rotation, we don't need dark matter. That's not right.
    - bjornsing 5y ago
      I find it a bit hard to believe that the mass current calculations for the bullet cluster have even been made. How can I convince myself that they have?

Sniffnoy 5y ago

This paper is just wrong, lots of other people have also done this calculation and shown that it's off by multiple orders of magnitude. Robin Hanson claims the particular error is the paper's assumption of zero pressure: https://www.overcomingbias.com/2021/03/what-holds-up-a-north-pole-of-dust.html https://www.overcomingbias.com/2021/03/what-holds-up-a-north...

escape_goat 5y ago
I have no doubt that Robin Hanson is a brilliant man, and I am aware that he has an extremely strong physics background for a professor of economics, but it would seem prudent at this point for those of us who are not domain experts to have somewhat Bayesian anticipations regarding the cross-disciplinary adventurism of economics professors. People read his blog. If this thing is indeed a ball, there will be plenty of astrophysicists happy to pick it up and run with it. This doesn't mean that you're wrong that the paper is wrong, but Dr. Hanson should absolutely not be our referent for certitude on the matter.
- throwaway64643 5y ago
  > If this thing is indeed a ball, there will be plenty of astrophysicists happy to pick it up and run with it. This reminds me of what happened with Sir Atiyah and his 'proof' on Riemann hypothesis few years ago. Sir Atiyah, despite of being a well-known mathematician (but in a different discipline), got absolute silence from researching community and experts when he proposed his proof on Riemann hypothesis and connection of fine structure constant of physics to mathematics. If experts stay silent, if your work gets no follow-up, it's likely that what you said is wrong or has no value to people.
  - deleted 5y ago
    [deleted]
- Sniffnoy 5y ago
  Robin Hanson isn't the one saying it's wrong. A number of other physicists (e.g. Garrett Lisi as mentioned in the post) have said it's wrong, on the basis that the calculation is easy and when they do it they get a very different result. Hanson is just the one who claims to have found the particular error that the paper made, because nobody else thought it was worth bothering to find the particular error in a paper that got such an obviously wrong answer.

Keysh 5y ago

This seems to suffer from the usual problem of non-astronomers not understanding the breadth of the problem they think they're trying to solve. That is, they've vaguely heard that the problem is in the rotation of disk galaxies, and so they propose solutions that depend on the rotation (and maybe also on the flattening). But the "dark matter" problem occurs for basically all galaxies, including things like elliptical and dwarf spheroidal galaxies that have no bulk rotation at all. (And also disk galaxies where a significant fraction of the stars and gas are counter-rotating.) The same applies to groups and clusters of galaxies.

bjornsing 5y ago
> And also disk galaxies where a significant fraction of the stars and gas are counter-rotating. So? Have you calculated the relativistic effects of those incredibly complex mass currents and come to the conclusion that GR is not a sufficient explanation for the motion? I’m sure there’s a high risk that GR can’t explain everything, but it sure is frustrating that people tend to grasp for these adhoc hypotheses without exhausting GR first.

teamonkey 5y ago

It's not the first time something like this has been proposed... and found fault with. https://inspirehep.net/literature/690135 https://inspirehep.net/literature/690135 > Recently a new model of galactic gravitational field, based on ordinary General Relativity, has been proposed by Cooperstock and Tieu in which no exotic dark matter is needed to fit the observed rotation curve to a reasonable ordinary matter distribution. We argue that in this model the gravitational field is generated not only by the galaxy matter, but by a thin, singular disk as well. The model should therefore be considered unphysical.