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My hangup with MOND is still general relativity. We know for a fact that gravity is _not_ Newtonian, that the inverse square law does not hold. Any model of gra

• MattPalmer1086 2y ago
  MOND isn't pitched as an improvement to GR. It was always a Newtonian theory - it's in its name! There are relativistic versions of MOND, for example, TeVeS [1], but they all still have some problems. [1] https://en.m.wikipedia.org/wiki/Tensor%E2%80%93vector%E2%80%93scalar_gravity https://en.m.wikipedia.org/wiki/Tensor%E2%80%93vector%E2%80%...
  • samsartor 2y ago
    TeVeS is definitely interesting, but it still has problems like you said. AFAICT gravitational wave observations are particularly bad for TeVeS theories. TeVeS isn't dead, but if dark matter theories are criticized for being patched up post-hoc, that standard should also apply to modified gravity.
    • gliptic 2y ago
      The weirdest thing about TeVeS IMO is that it adds additional fields that warp spacetime, so how is it not a dark matter theory?
      • MathMonkeyMan 2y ago
        For the fields to be considered particles, they have to be freely propagating in space. TeVeS adds a vector field, a scalar field, and some lagrange fields that are part of their coupling. The degrees of freedom aren't consistent with one or more particles.
• ajross 2y ago
  To be fair, there are relativistic generalizations of MOND, in the sense of relativistic theories that simplify to MOND dynamics in the low energy case. My understanding (this not being my field) is that they're sort of kludgey and non-calculable and that no one takes them very seriously. All the "real work" on MOND is just done using the classical stuff. And yeah, that seems like pretty terrible cheating. It's one thing to hang a big theory on a single conjecture, but you still need to be trying to prove the conjecture.
• meindnoch 2y ago
  >We know for a fact that gravity is _not_ Newtonian, that the inverse square law does not hold [citation needed] The consensus is that gravity - outside of extreme mass/energy environments - works just as Newton described it to many many decimal places. Emphasized part added because people in the replies thought that I literally think that General Relativity is somehow wrong. Don't be dense. All I'm saying is that gravity at galactic scales works as Newton described it. General Relativity has extremely tiny effect at those scales.
  • hobs 2y ago
    When you say "outside of" - that's the thing where it doesn't hold. It's interesting and not even wrong to say "these rules work in these contexts" but as far as I can tell we're looking for the scenario invariant rules.
  • samsartor 2y ago
    https://en.wikipedia.org/wiki/Tests_of_general_relativity#Perihelion_precession_of_Mercury https://en.wikipedia.org/wiki/Tests_of_general_relativity#Pe... is the example I learned in school. You don't need to be around a black hole for GR to suddenly switch on. Newtonian gravity is an approximation. A perfectly acceptable one in many contexts, but still measurably incorrect.
    • meindnoch 2y ago
      Nobody said that general relativity is "switched on" around black holes. But ok, let me put it this way: outside of extreme energy/mass environments, gravity is described by Newton's law of gravitation with very high precision. If you look very hard, you may notice differences on the order of 10e-MANY. But for all intents and purposes, gravity is Newtonian in 99.99999% of the universe.
      • DiogenesKynikos 2y ago
        The inaccuracy of the Newtonian theory of gravity is large enough that it was already noticed by astronomers in the mid-1800s.
• haccount 2y ago
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• throwawaymaths 2y ago
  > My hangup with MOND is still general relativity. Fwiw, we know for a fact also that for edge cases GR is wrong because it doesn't agree with quantum mechanics (unless QM is wrong), so it's maybe not right to take GR as gospel, especially for a theory that only seems to also change GR in edge cases, and the only reason why "it doesn't agree" might amount to "the math is hard and the physicists haven't put enough work in yet" To wit, accepting a mond-ified GR is probably not going to change how GPS works so the claim that "GR has withstood the test of time and engineering" is not a totally solid refutation of MOND
  • mort96 2y ago
    Well this doesn't seem like such a conundrum. We know for sure that ND is wrong because it predict things incorrectly which GR predicts correctly. We know GR is wrong because it is incompatible with any form of QM and we know some form of QM is more or less correct. Essentially, GR and ND are both wrong, but ND is more wrong than GR.
    • codethief 2y ago
      > we know some form of QM is more or less correct This is a wild take, given all the issues QM and QFT have.
      • mort96 2y ago
        Sure, a better way of saying it is tat we observe natural phenomena which are explained by something like QM and for which we have no other explanation. I think people got the idea.
    • RandomThoughts3 2y ago
      > We know GR is wrong because it is incompatible with any form of QM and we know some form of QM is more or less correct. It’s not really about one model being correct. GR is not a perfect model because its predictions don’t match what’s observed on the scale where QM gives predictions which do. "Wrong" is overall a poor way of thinking about models. People would like a model which is both general and elegant, not simply a model which is "right". A large and very general model with a lot of parameters which can be well tuned to fit all the observations we have would be "correct" but I am not convinced it would be very useful.
• twothreeone 2y ago
  GR says spacetime is curved by mass, right. So what's the basis for explaining the curvature of space (which can be measured, e.g., LIGO) in MOND?
  • MathMonkeyMan 2y ago
    MOND has nothing to say about the curvature of spacetime, since MOND is Newtonian (MOdified Newtonian Dynamics). It goes back to "F=ma and gravity is a force" and modifies the rules so that gravity grows weaker faster at a certain scale. The fact that MOND fits a lot of the data troubled cosmologists, because they know that a General Relativistic theory is needed to explain pretty much the rest of gravity. TeVeS is an extension to General Relativity that reduces to MOND in the non-relativistic limit. For comparison, General Relativity reduces to Newtonian gravity in the non-relativistic limit. The non-relativistic limit is when speeds and spacetime curvature are small.
    • Gooblebrai 2y ago
      How does MOND deal with the effects of time dilation and length contraction? Do we have to go back to Newton's time where there's a universal time?
      • MathMonkeyMan 2y ago
        I don't know if Newtonian gravity can be reconciled with Special Relativity. First thought is "no, that's why Einstein arrived at General Relativity." But I'm not in the field, so I don't know.
  • oneshtein 2y ago
    > GR says spacetime is curved by mass, right. Wrong. GR says that gravitation can be modeled as acceleration.
    • mog_dev 2y ago
      General Relativity states that mass-energy curves spacetime, and objects follow the straightest possible paths (geodesics) through this curved geometry. The equivalence principle relates gravity and acceleration, but it's not the main description of gravity in GR.
• phkahler 2y ago
  IANA physicist but everywhere I look I see the same mistake being made. The shell theorem does not apply to disks or galaxies. I see the same simplification in the most advanced writings. Namely 1) matter out to a radius can be treated as a point mass in the center and 2) we can ignore gravity from mass outside a radius because it all cancels. These simplifications work for spherical shells or solids of uniform density. They do not apply to disks or rings (galaxies). Period.