4 ms·

> Should we presume that it must be an invisible ball of matter that gravitates but we have no other way of detecting except through the same effect we use to i

> Should we presume that it must be an invisible ball of matter that gravitates but we have no other way of detecting except through the same effect we use to infer its existence, or should we tweak Newton's laws of physics? Are these substantially different? Of course using “must” isn’t appropriate, but spouting off the best theory you have at the time seems okay to me. Hopefully no one is saying that we cannot possibly ever find another better theory the same way Einstein’s was better than Newton’s or that we shouldn’t ever look for one.

2muchcoffeeman 4y ago
I guess the other side effects might be different right? E.g. we talk about relativistic mass. But when they did experiments it was found that no other attributes that come with increased mass was found. So the answer is right but there is something off with our understanding.
throwawaymaths 4y ago
> Hopefully no one is saying that we cannot possibly ever The OP article might as well. The thrust of the article's argument is, "heaven forbid we add third parameter to MOND" meanwhile, draco, fornax, and Andromeda have wildly different dark matter densities so that's three parameters right there.
- pdonis 4y ago
  > draco, fornax, and Andromeda have wildly different dark matter densities so that's three parameters right there. No, it isn't. The distribution of stress-energy is a single free parameter in GR; there is no law in the theory that says the density of any particular kind of stress-energy has to be the same in different galaxies. So adjusting the distribution of stress-energy to match the observed behavior in different galaxies does not require adding a new parameter for each galaxy; it only requires adjusting the single parameter that already exists, in different regions of spacetime.
  - deleted 4y ago
    [deleted]
  - throwawaymaths 4y ago
    > the single parameter that already exists, Ok I concede. It is a single parameter that contains a millions of subparameters, at least one for each galaxy. Against three parameters, each of which is a single number. But it is three parameters! That's 3x more parameters! Moreover, we had a problem and we had to increase the number of parameters by 50%. So yeah One parameter is much, much better, especially since we have never changed that it's one parameter.
    - pdonis 4y ago
      > It is a single parameter that contains a millions of subparameters, at least one for each galaxy. No, it's a single parameter with a distribution in space and time. It's the same one we already have to vary in space and time anyway based on our observations of light. Dark matter just means adjusting the same distribution further based on observed motions even if we can't see the source of gravity that is producing the motion. If MOND actually gave the choice of adjusting a single parameter in place of having to adjust the distribution of matter and energy, that would be one thing. But, as I noted in another comment upthread, once you take into account that the theory of gravity we are using is actually General Relativity, not Newtonian gravity, there is no such single parameter anyway. Relativistic versions of MOND actually end up looking just like dark matter: they have to invent new fields, new forms of energy, for which we have no other evidence. There is no way to get the desired effect in GR by just tweaking a parameter in the law of gravity.
      - throwawaymaths 4y ago
        > distribution You're right. I was wrong. It doesn't have millions of subparameters. It is a single parameter, a vector with countably infinite dimension.
        andrewflnr 4y ago

2muchcoffeeman 4y ago

I guess the other side effects might be different right? E.g. we talk about relativistic mass. But when they did experiments it was found that no other attributes that come with increased mass was found. So the answer is right but there is something off with our understanding.

throwawaymaths 4y ago

> Hopefully no one is saying that we cannot possibly ever The OP article might as well. The thrust of the article's argument is, "heaven forbid we add third parameter to MOND" meanwhile, draco, fornax, and Andromeda have wildly different dark matter densities so that's three parameters right there.

pdonis 4y ago
> draco, fornax, and Andromeda have wildly different dark matter densities so that's three parameters right there. No, it isn't. The distribution of stress-energy is a single free parameter in GR; there is no law in the theory that says the density of any particular kind of stress-energy has to be the same in different galaxies. So adjusting the distribution of stress-energy to match the observed behavior in different galaxies does not require adding a new parameter for each galaxy; it only requires adjusting the single parameter that already exists, in different regions of spacetime.
- deleted 4y ago
  [deleted]
- throwawaymaths 4y ago
  > the single parameter that already exists, Ok I concede. It is a single parameter that contains a millions of subparameters, at least one for each galaxy. Against three parameters, each of which is a single number. But it is three parameters! That's 3x more parameters! Moreover, we had a problem and we had to increase the number of parameters by 50%. So yeah One parameter is much, much better, especially since we have never changed that it's one parameter.
  - pdonis 4y ago
    > It is a single parameter that contains a millions of subparameters, at least one for each galaxy. No, it's a single parameter with a distribution in space and time. It's the same one we already have to vary in space and time anyway based on our observations of light. Dark matter just means adjusting the same distribution further based on observed motions even if we can't see the source of gravity that is producing the motion. If MOND actually gave the choice of adjusting a single parameter in place of having to adjust the distribution of matter and energy, that would be one thing. But, as I noted in another comment upthread, once you take into account that the theory of gravity we are using is actually General Relativity, not Newtonian gravity, there is no such single parameter anyway. Relativistic versions of MOND actually end up looking just like dark matter: they have to invent new fields, new forms of energy, for which we have no other evidence. There is no way to get the desired effect in GR by just tweaking a parameter in the law of gravity.
    - throwawaymaths 4y ago
      > distribution You're right. I was wrong. It doesn't have millions of subparameters. It is a single parameter, a vector with countably infinite dimension.
      - andrewflnr 4y ago