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Fitting an elephant with four non-zero parameters

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Re: Fitting an elephant with four non-zero parameters

#111

Post by [deleted] »

andrewflnr wrote: post #84
GuB-42 wrote: post #64

Earlier quoted context omitted.

The dark matter theory is falsifiable. Sure we can't see dark matter (it doesn't interact electromagnetically), but we can see its effects, and it has to follow the laws of physics as we understand them today. It is actually a satisfying theory with regard to the Occam razor. We don't have to change our laws of physics to explain the abnormal rotations of galaxy, we just need "stuff" that we can't see and yet interac…

> For the Occam razor, adding some random term to an equation is not really better than adding some invisible but well characterized stuff... You're being too kind. It's worse. Especially when (in my understanding anyway) that added term doesn't even explain all the things dark matter does.

[deleted]

Re: Fitting an elephant with four non-zero parameters

#112

Post by danbruc »

pharmacy7766 wrote: post #28

One parameter is enough: https://aip.scitation.org/doi/10.1063/1.5031956

The number of parameters is just the wrong metric, it should be the amount of information contained in the parameter values, their entropy, Kolmogorov complexity or something along that line.

Re: Fitting an elephant with four non-zero parameters

#113

Post by jampekka »

XorNot wrote: post #109
nyssos wrote: post #69

Earlier quoted context omitted.

> What can't be fit by declaring the amount of dark matter that must be present fits the data? Tons of things - just like there are tons of things that can't be fit by declaring the amount of electromagnetically-interacting matter that must be present fits the data. You can fit anything you like by positing new and more complicated laws of physics, but that's not what's going on here. Dark matter is ordinary mass gra…

You also have to do other things like not break General Relativity. Which MOND does: it creates huge problems fitting into GR. Whereas dark matter as just regular mass that interacts poorly by other means does not.

There are modified gravity theories that are compatible/extensions to GR, e.g the f(R) gravity theories.

Nobody probably believes MOND as such is some fundamental theory, rather as a "theory" it's sort of a stepping stone. Also MOND is used often interchangeably (and confusingly) with modified gravity theories in general.

Re: Fitting an elephant with four non-zero parameters

#115

Post by jampekka »

Dylan16807 wrote: post #97
jampekka wrote: post #62

Earlier quoted context omitted.

There will be no Copernicus if everybody just studies epicycles. E.g. there are massive resources put into the desperate WIMP hunt that could be used for finding new theories.

I don't see how those resources are fungible with each other.

Research funding is very competitive and scarce.

Re: Fitting an elephant with four non-zero parameters

#117

Post by Retric »

adgjlsfhk1 wrote: post #100
adrian_b wrote: post #87

Earlier quoted context omitted.

If we add an arbitrary amount of dark matter everywhere, to match the observed motions of the celestial bodies, that adds an infinity of parameters, and not even a enumerable one. This obviously can match almost anything and it has extremely low predictive power (many future observations may differ from predictions, which can be accounted by some dark matter whose distribution was previously unknown), so it is a much…

the reason this isn't true is that by the hypothesis of dark matter, it follows gravity but not electromagnetism. as such it only fits distributions recoverable from evolving gravity. e.g. if we require a certain distribution today, it fixes the distribution at all other points in time, and we can use light speed delay to look into the past to verify whether the distributions have evolved according to gravity.

All observations of individual galaxies occur at a specific point in time. We can’t use light speed delay to see the evolution of individual galaxies only completely different galaxies at some other point in time. As such each galaxy gets its own value for the amount of dark matter.

At minimum this is a ~200 billion parameter model, and more if you’re looking at smaller structures.

Re: Fitting an elephant with four non-zero parameters

#119

Post by karmakaze »

adrian_b wrote: post #90
nyssos wrote: post #71

Earlier quoted context omitted.

> Sure we can't see dark matter (it doesn't interact electromagnetically), but we can see its effects Even this is granting too much: "seeing it" and "seeing its effects" are the same thing . No one has ever "directly seen", in the sense that internet DM skepticism demands, anything other than a photon.

"Seeing" is indeed a poorly chosen word. The problem with dark matter is that there does not exist any second relationship from which to verify its existence, like in the case of normal matter, which takes part in a variety of interactions that lead to measurable effects, which can be compared. The amount and the location of dark matter is computed from the gravitational forces that explain the observed movements of…

> The problem with dark matter is that there does not exist any second relationship from which to verify its existence.

This is exactly it! Dark matter is strictly defined by its effects. The only 'theory' part is a belief that it's caused by yet to be found particle that's distributed to fit observations. Take all the gravitational anomalies that we can't explain with ordinary matter, then arbitrarily distribute an imaginary 'particle' that solves them: that's DM.

The problem is that the language used to talk about DM is wrong. It's not that DM doesn't interact with EM, or the presence of DM is causing the galaxies to rotate faster than by observed mass. These are all putting the cart before the horse. What we have is unexplained gravitational effects being attributed to a hypothetical particle. If we discovered a new unexplained gravitational property, we would merely add that to the list of DM's attributes rather than say "oh then it can't be DM".

Re: Fitting an elephant with four non-zero parameters

#120

Post by karmakaze »

andrewflnr wrote: post #83
karmakaze wrote: post #43

Earlier quoted context omitted.

That's how I feel about dark matter. Oh this galaxy is slower than this other similar one. The first one must have less dark matter then. What can't be fit by declaring the amount of dark matter that must be present fits the data? It's unfalsifiable, just because we haven't found it, doesn't mean it doesn't exist. Even worse than string/M-theory which at least has math.

Dark matter is constrained by, among other things, dynamical simulations. For instance, here's an example of reproducing real world observations, that previously didn't have great explanations, using simulations with dark matter: https://www.youtube.com/live/8rok8E_tz8k?si=Q7vmQYpZr_6K7--m . And that's not even getting into the cosmology that has to (and mostly does) fit together.

Interesting that you should link that video. Its title card says "Angela Collier". Here's a more recent video by the physicist[0].

Re: where it says "using simulations with dark matter", we can't simulate DM because it doesn't have any properties beyond our observations. All we do is distribute amounts of it to match observations. It could be "Dyson spheres with EM shields" and the results would be the same.

[0] https://www.youtube.com/watch?v=PbmJkMhmrVI

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