MOND is the name for one model that changes the theory of gravity and some slight variations on it. It is not the name for all models that change the theory of gravity. Essentially all MOND models have the same threshold acceleration adjustable parameter a0 and are just extensions of the original toy-model MOND.PeterDonis said:
Is there are more general name for that class of models?ohwilleke said:
I usually call models that don't use dark matter particles to explain "dark matter phenomena" either "gravity based models" or "modified gravity based models".PeterDonis said:
I would remind you that when Planck invented his constant, he did it the way you are saying no one does it. He simply chose a one-parameter solution to the ultraviolet catastrophe. He had no model in mind for doing that, he always regarded it as a mathematical trick. The model came later. I actually think that historical sequence is more common than you imagine (consider for example the Rydberg series of energies in the hydrogen atom, discovered 35 years before quantum mechanics.) I'm suggesting that what MOND looks like today might end up like what the Rydberg series was in 1890.PeterDonis said:
Choosing the one-parameter solution is choosing a model. The fact that Planck just considered it a "mathematical trick" does not mean it isn't a model. A model is an algorithm for making predictions. What the person using it thinks of it is irrelevant.Ken G said:
No, a physical interpretation of the model that physicists could accept (and not all of them even then--IIRC Planck never did accept it) came later.Ken G said:
This is an example of a first, very crude model being replaced by a better one later on.Ken G said:
Yes, that's possible, but it doesn't mean MOND as it is today is not a model. It is. Possibly a very crude one that will end up being replaced by a better one later on. Or possibly just a mistaken one that will end up being discarded. We'll see.Ken G said:
Definitely.PeterDonis said:
This is no longer a "question mark" for anyone who has actually studied and understood the most recent papers that use several different statistical analysis methods. This new version (v3) from Chae performs yet a 3rd type of analysis, different from the earlier "acceleration-plane" and "stacked velocity profile" analyses. All 3 analyses agree well with each other.ohwilleke said:
I've noticed that Bannik's paper begins to receive ridicule, not least from the fact that he refuses to engage Chae or Hernandez in constructive debate even after they have carefully pointed out problems in his analysis method (e.g., he uses velocity cell sizes that are incompatible with the error bounds on the data).ohwilleke said:
McGaugh talks about this on his recent blog entry, titled Rotation Curves Still Flat After A Million Light Years.Mistele et al said:
McGaugh has read them and understood them, and he currently takes an inconclusive position on their results. The conclusions of Chae may be consistent, but AFAIK none of the at least three groups looking at wide binaries have reached the same result looking at that data.strangerep said:
This tightens but doesn't completely rule out primordial black hole (PBH) parameter space for some asteroid sized PBHs. The allowed range is discussed at:pinball1970 said:
Er,... which "three groups" are you referring to?ohwilleke said:
The point was in relation to the claim "Nobody picks the simpler parametrization (how would you even do that anyway?) and then says, ok, let's pick the model that has that." That's exactly what Planck did, he found the simplest possible parametrization, without worrying about anything else. I'm saying that if you call every mathematical expression a model, then the distinction you are drawing (that people don't start with parametrizations they start with a model they like) does not exist, because parametrizations and models are the same thing. But it doesn't matter the semantics of "model," it matters that sometimes we make progress by finding nothing more than the simplest possible parametrization, which is more or less what MOND is.PeterDonis said:
Yes, that's what I'm saying. The issue is not what we will call a model, that's just definitions. The issue is the way progress is made, which sometimes starts with a simple parametrization and later leads to physical interpretations. Those physical interpretations often involving adding a lot of explanatory scaffolding to the original parametrization, and can also extend the parametrization into a broader context. That might be what eventually happens to MOND, where what now just looks like a parametrization emerges from something akin to how the Rydberg series emerges from the Schroedinger equation, whereby the latter applies in a lot of places the former does not. MOND is waiting for its Schroedinger equation, and if it finds it, it will likely be taken seriously more broadly than it is now.PeterDonis said:
It seems like you are agreeing with me; you just don't like calling this process "picking a model with the simplest parameterization". I can't understand why you object to that phrasing, but in any case, what Planck did is an example of what I was describing.Ken G said:
Again, you are agreeing with me here, since this is what I said might happen with MOND. Or it might not. We'll see.Ken G said:
PeterDonis said:
Perhaps we are saying the same thing then, it sounded like you were contracting what I said. But that's what I said, it was an example of what Planck did. I'm merely wondering if MOND is in a similar boat at the moment-- picking a simple parametrization, and waiting for the Schroedinger equation to come along later.PeterDonis said:
I'm not disputing any particular meaning for the word "model," that is of no consequence to me. At issue is simply the paths progress can take, which sometimes flow from looking for the simplest parametrization first, and then later trying to understand why it worked-- if it did. In other words, it is often not necessary to decide what model one prefers, on any kind of physical grounds or any philosophical basis at all, one can simply follow the parametrization that seems to be getting more bang from its buck than would be expected if it was a dead end. Some MOND followers are seeing reasons to think the simple a_o type parametrization is getting such bang, at least to a surprising degree. So that could be that phase of the process.PeterDonis said:
I certainly agree that none of us have the necessary crystal ball to know where it will go, the issue is not to predict the future but rather to frame the status of the present. I think the main error to avoid is to try to decide if one prefers a solution involving a new parametrization of gravity, versus a solution that involves invisible particles. There is no reason to prefer anything, we just look under all the streetlights first, preferences are simple biases. An individual has to decide where to put their energy, that's true, but people mistake preferences for evidence sometimes. It's like some people pointing at the Bullet Cluster and other people pointing at other clusters that behave differently, when does evidence get confused with preference?PeterDonis said: