Mond - gravity as two separate forces?

[Fred Blame]

TL;DR Summary

Mond as standard Gravity plus an additional force.

Is it possible that MOND might be standard gravity plus an additional force that is inversely proportional to distance. One that is also a property of matter?

I ask, because viewing this way begs the questions: If two, then why not more, and if more then perhaps a series with a hope of definition.

Possible contenders would be nuclear forces and a force that is independent of distance.

 

[PeroK]

Is it possible that MOND might be standard gravity plus an additional force that is inversely proportional to distance. One that is also a property of matter?

Lots of things might be possible. You could do a search of the literature on MOND to see whether anyone is reseaching anything like this.

I ask, because viewing this way begs the questions: If two, then why not more, and if more then perhaps a series with a hope of definition.

Why not, indeed? But, equally, why?

Possible contenders would be nuclear forces and a force that is independent of distance.

That I doubt. In any case, as far as I know, there is no theoretical basis for MOND (and certainly not a quantum mechanical basis). It's a theory designed to fit the data, rather than a fundamental theory in itself.

 

[Fred Blame]

What I wrote is purely speculative, and unlikely to lead anywhere. However, unlikely is not impossible. So, yes, I doubt too, but doubt should not stop speculation, just conviction.

As to quantum mechanics, I see no reason to go there until we first understand the forces involved. If MOND is right, then we are really not sure. If exotic dark matter is right, then the same.

 

[PeroK]

What I wrote is purely speculative, and unlikely to lead anywhere.

You ought to read the PF rules. Personal speculation is not allowed!

 

[Fred Blame]

It's speculation, but not personal speculation. It a genuine question as to what is possible, (which you have answered in the affirmative.) plus an explanation as to why I asked.

I honestly expect somebody to ether explain why MOND can't reasonably be though of as two forces, or tell me where to find articles on the topic. Isn't that why this forum is here, so that we can get direction?

Finally, I can think of a good reason for it to be wrong, I just don't know if it's true - That there is a better fit to the evidence if the component of standard gravity is actually phased out as the modified gravity takes over.

 

[Fra]

It's speculation, but not personal speculation.

I personally don't start pondering in this end of things so I have no meaningful contextual comments, but fwiw here is at least and arxiv paper entertaining some of ideas you mention, but I don't have any meaningfull comments on their approach.

An Alternative to Particle Dark Matter​

"We propose an alternative to particle dark matter that borrows ingredients of MOdified Newtonian Dynamics (MOND) while adding new key components. The first new feature is a dark matter fluid, in the form of a scalar field with small equation of state and sound speed. This component is critical in reproducing the success of cold dark matter for the expansion history and the growth of linear perturbations, but does not cluster significantly on non-linear scales. Instead, the missing mass problem on non-linear scales is addressed by a modification of the gravitational force law. The force law approximates MOND at large and intermediate accelerations, and therefore reproduces the empirical success of MOND at fitting galactic rotation curves. At ultra-low accelerations, the force law reverts to an inverse-square-law, albeit with a larger Newton's constant. This latter regime is important in galaxy clusters and is consistent with their observed isothermal profiles, provided the characteristic acceleration scale of MOND is mildly varying with scale or mass, such that it is ~12 times higher in clusters than in galaxies. We present an explicit relativistic theory in terms of two scalar fields. The first scalar field is governed by a Dirac-Born-Infeld action and behaves as a dark matter fluid on large scales. The second scalar field also has single-derivative interactions and mediates a fifth force that modifies gravity on non-linear scales. Both scalars are coupled to matter via an effective metric that depends locally on the fields. The form of this effective metric implies the equality of the two scalar gravitational potentials, which ensures that lensing and dynamical mass estimates agree. Further work is needed in order to make both the acceleration scale of MOND and the fraction at which gravity reverts to an inverse-square law explicitly dynamical quantities, varying with scale or mass. "

-- https://arxiv.org/abs/1409.0012

/Fredrik

 

[Fred Blame]

Fra. My thanks.

I suppose my original question was badly written. Perhaps I might have asked if breaking the force into standard gravity plus modified gravity made the maths more, or less elegant.

If that my first reaction is right, that article suggests elegance is in no way involved, which might be a good point to give up. At least with that particular approach.

 

[Vanadium 50]

Why stop at two? Why not ten? I can always add, subtract and rearrange terms.

 

[Fred Blame]

Vanadium. Why? Because there are always an infinite number of ways to explain anything. The trick is to find the simple that explains much. By all means stop at ten....if those ten equations sum to less complexity than the original one. But let's face facts, they won't. With the original "standard gravity plus one fudge factor" two just might.

 

[Vanadium 50]

This sounds a lot more like "I like it better" than "it explains something".

Of course you are free to like it better, but you can surely appreciate why that is an unconvincing argument.

 

[Fred Blame]

Vanadium

Not really. If the choice is between a simple explanation and a complex one that, on examination says exactly the same thing... I would imagine that most would be convinced to go for simple.

Not that this is the case with the problem that standard gravity and observed mass don't alone explain what is going on with galaxies. There we have a choice of fudge factors, and Occam's razor is our best guide.

 

[ohwilleke]

TL;DR Summary: Mond as standard Gravity plus an additional force.

Is it possible that MOND might be standard gravity plus an additional force that is inversely proportional to distance. One that is also a property of matter?

I ask, because viewing this way begs the questions: If two, then why not more, and if more then perhaps a series with a hope of definition.

Possible contenders would be nuclear forces and a force that is independent of distance.

Since MOND is proportional to mass (and really to mass-energy) in the same way as gravity is, any formulation of MOND as a two force system could be restated as a more complicated equation for gravity instead. It is a modification of gravity (except in its inertia modification formulation) rather than an additional force.

There are dark matter particle theories that have either a new force only between dark matter particles, or a new force between dark matter and ordinary matter in addition to gravity, or both.

 

[Vanadium 50]

f the choice is between a simple explanation and a complex one that, on examination says exactly the same thing... I would imagine that most would be convinced to go for simple.

We learned A in school. The data seems to support B at least in one type of system. Deciding that there are two forces, A and [B-A] looks to me to be no simpler than one force B.