4 ms·

MOND doesn't actually get rid of dark matter, it just means you only need ~20% as much of it. If it did fix dark matter it'd very possibly be the more popular t

MOND doesn't actually get rid of dark matter, it just means you only need ~20% as much of it. If it did fix dark matter it'd very possibly be the more popular theory. The issue is that galaxies-- according to the dark matter paradigm-- don't have a precisely predictable amount of dark matter. There are galaxies that seem to have vastly different quantities of dark matter than other galaxies that despite the fact that their ordinary mass distribution looks very similar. That is, different galaxies need different amounts of correction from Newtonian Dynamics in ways that are seemingly random, and therefore cannot be incorporated into MOND. So you still need some dark matter to fix this problem.

DougN7 7y ago
This has always felt a little wrong to my layman mind. It’s like saying 4+2=7. Oh it doesn’t match? Well, invent some magic stuff to fix the difference (4+2+magic=7) so we don’t have to change what we believe. I guess I don’t understand why it’s not easier to doubt what we think we know than invent something we don’t have evidence for. Is dark matter’s existence a settled thing?
- AnimalMuppet 7y ago
  I am also a layman, but I'll take a stab at it. Here's a galaxy with a certain amount of mass, distributed a certain way. It has a certain rotational velocity (as a function of radius from the center). Here's a different galaxy that has just about the same mass and mass distribution, but it has a different rotational velocity. That leaves a very few options: - Our measurements are wrong. - Rotational velocity is not a function of mass. - Or, there's more mass than we can see. People have carefully checked the first, but it's still possible. The second seems really weird: Rotational velocity depends on mass plus the roll of 2d6?!? That leaves dark matter as the explanation that seems the most reasonable.
  - DougN7 7y ago
    Aren’t the first and third the same thing - we didn’t measure it all? To just invent something that can’t be explained to make us feel better sounds a whole lot like religion. [edit] Also, thank you for your reply. It helps
    - AnimalMuppet 7y ago
      One is, either we didn't correctly measure the mass (which we measure by the light emitted), or we didn't measure the rotational velocity correctly (which we measure by doppler differences between the different sides of the galaxy). The third is that there is mass that doesn't emit light at all. (I think it's a bit stronger than that. I think that they think that it doesn't interact with light at all, not just not emit it. But I forget how they think they know that, and I'm not certain that they think they know it at all.) > To just invent something that can’t be explained to make us feel better sounds a whole lot like religion. They don't invent it to make themselves feel better. They invent it so their equations work. (Of course, they feel better when their equations work...)
      - dogma1138 7y ago
        Dark matter doesn’t interact with any of the forces other than gravity and possibly the weak force. Not interacting with the electromagnetic force is the same thing as not emitting light. But this also means that DM cannot be cold baryonic matter. MOND or any other modifications of Newtonian Gravity or GR aren’t random. In the case of MOND (and TEVES) Gravity is modified but it’s modified in a manner that is consistent with both macro and micro observations it’s not adding a random dice throw. The reason why people went after MOND and the rest is that Newtonian Gravity is surprisingly accurate even without using PPN. Now the big problem with MOND and most of the other modifications is the speed of gravitational waves that pretty much shut down much of the discussion. That said I’m not entirely sure they had the wrong idea at this point I think it’s more likely we don’t understand something about gravity or space time than there is a new class of matter out there. That said the fact that we have both found clusters which apparently more dark matter than usual and those which seem to completely lack DM is still a good reason to continue searching for DM. However I really don’t like how MOND proponents were treated especially pre-LIGO.

DougN7 7y ago

This has always felt a little wrong to my layman mind. It’s like saying 4+2=7. Oh it doesn’t match? Well, invent some magic stuff to fix the difference (4+2+magic=7) so we don’t have to change what we believe. I guess I don’t understand why it’s not easier to doubt what we think we know than invent something we don’t have evidence for. Is dark matter’s existence a settled thing?

AnimalMuppet 7y ago
I am also a layman, but I'll take a stab at it. Here's a galaxy with a certain amount of mass, distributed a certain way. It has a certain rotational velocity (as a function of radius from the center). Here's a different galaxy that has just about the same mass and mass distribution, but it has a different rotational velocity. That leaves a very few options: - Our measurements are wrong. - Rotational velocity is not a function of mass. - Or, there's more mass than we can see. People have carefully checked the first, but it's still possible. The second seems really weird: Rotational velocity depends on mass plus the roll of 2d6?!? That leaves dark matter as the explanation that seems the most reasonable.
- DougN7 7y ago
  Aren’t the first and third the same thing - we didn’t measure it all? To just invent something that can’t be explained to make us feel better sounds a whole lot like religion. [edit] Also, thank you for your reply. It helps
  - AnimalMuppet 7y ago
    One is, either we didn't correctly measure the mass (which we measure by the light emitted), or we didn't measure the rotational velocity correctly (which we measure by doppler differences between the different sides of the galaxy). The third is that there is mass that doesn't emit light at all. (I think it's a bit stronger than that. I think that they think that it doesn't interact with light at all, not just not emit it. But I forget how they think they know that, and I'm not certain that they think they know it at all.) > To just invent something that can’t be explained to make us feel better sounds a whole lot like religion. They don't invent it to make themselves feel better. They invent it so their equations work. (Of course, they feel better when their equations work...)
    - dogma1138 7y ago
      Dark matter doesn’t interact with any of the forces other than gravity and possibly the weak force. Not interacting with the electromagnetic force is the same thing as not emitting light. But this also means that DM cannot be cold baryonic matter. MOND or any other modifications of Newtonian Gravity or GR aren’t random. In the case of MOND (and TEVES) Gravity is modified but it’s modified in a manner that is consistent with both macro and micro observations it’s not adding a random dice throw. The reason why people went after MOND and the rest is that Newtonian Gravity is surprisingly accurate even without using PPN. Now the big problem with MOND and most of the other modifications is the speed of gravitational waves that pretty much shut down much of the discussion. That said I’m not entirely sure they had the wrong idea at this point I think it’s more likely we don’t understand something about gravity or space time than there is a new class of matter out there. That said the fact that we have both found clusters which apparently more dark matter than usual and those which seem to completely lack DM is still a good reason to continue searching for DM. However I really don’t like how MOND proponents were treated especially pre-LIGO.