4 ms·

MOND, MOND, MOND…

by earthboundkid 7y ago

MOND, MOND, MOND…

earthboundkid 7y ago
I can believe in dark matter. I can believe in dark energy. I cannot believe in both… MOND, MOND, MOND…
- AnimalMuppet 7y ago
  You sound like a cheerleader. But science isn't done by seeing which side cheers loudest...
  - anyfoo 7y ago
    I am reminded of the Steady State model of cosmology. It was a contestant to the Big Bang theory, appearing cleaner and somewhat more sensible. Go read up on it, it relies on a really neat principle. But we all know how this turned out: Overwhelmingly data supported Big Bang, and Steady State is now just an episode in science history. So while I really like MOND and wish for it to be the answer, I won’t be surprised if it’s not.
- Tomminn 7y ago
  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...)
- 8bitsrule 7y ago
  This abstract, "Newtonian explanation of galaxy rotation curves based on distribution of baryonic matter" uses an argument (not DM or MOND) I've not seen before. https://arxiv.org/abs/1406.2401 https://arxiv.org/abs/1406.2401 (It says that 'A large number of likely galaxy mass distribution profiles should in fact produce flat or accelerating rotation curves similar to those observed in reality.') Older (2007) abstract: Kepler's third law (works for the solar system) might not apply because of galactic mass distribution. http://www.marmet.org/cosmology/nonkeplerian/galaxy.html http://www.marmet.org/cosmology/nonkeplerian/galaxy.html Five years old; didn't get a chance to find a counter-argument, but worth mentioning.

MOND, MOND, MOND…

earthboundkid 7y ago

I can believe in dark matter. I can believe in dark energy. I cannot believe in both… MOND, MOND, MOND…

AnimalMuppet 7y ago
You sound like a cheerleader. But science isn't done by seeing which side cheers loudest...
- anyfoo 7y ago
  I am reminded of the Steady State model of cosmology. It was a contestant to the Big Bang theory, appearing cleaner and somewhat more sensible. Go read up on it, it relies on a really neat principle. But we all know how this turned out: Overwhelmingly data supported Big Bang, and Steady State is now just an episode in science history. So while I really like MOND and wish for it to be the answer, I won’t be surprised if it’s not.
Tomminn 7y ago
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...)
8bitsrule 7y ago
This abstract, "Newtonian explanation of galaxy rotation curves based on distribution of baryonic matter" uses an argument (not DM or MOND) I've not seen before. https://arxiv.org/abs/1406.2401 https://arxiv.org/abs/1406.2401 (It says that 'A large number of likely galaxy mass distribution profiles should in fact produce flat or accelerating rotation curves similar to those observed in reality.') Older (2007) abstract: Kepler's third law (works for the solar system) might not apply because of galactic mass distribution. http://www.marmet.org/cosmology/nonkeplerian/galaxy.html http://www.marmet.org/cosmology/nonkeplerian/galaxy.html Five years old; didn't get a chance to find a counter-argument, but worth mentioning.