Why gravity?

Both MOND and dark matter theories suppose that the force behind the
galactic rotation speed is gravity.

Why?

Because it is the only thing we know of course!

What else could it be?

Well, remembering the writings of Lord Kelvin about the sun's energy
source, it is obvious that the conclusion for him should have been:

An unknown source of energy.

He even said that at the conclusion of his paper.

Now, what would happen if we leave gravity and suppose that at
galactic scales new forces become relevant that we can't even suspect
given our minuscule size.

Look at a fly, walking in a wall upwards: gravity is not as important as
the cohesion forces between its legs and the wall. At the scale of a
fly, forces change, WE can't do that, for us, gravity is MUCH more
relevant given our bigger size.

How much would be that force?

Well that is easy: it is the same force that is calculated by MOND
of course. Only the explanation is different: instead of modifying
gravity, we would assume a NEW force that acts perpendicular to the
galaxy rotation, pushing things around and making the galaxy have
a RIGID constant rotation.

Happily this thing can be experimentally checked: we know where the
center of the galaxy is, and with the earth rotation, sun's rotation
around the galaxy center, etc, we can calculate the direction of
this force at any moment.

This would explain why MOND works, and why MOND doesn't work at
scales bigger than a galaxy...

What is this force?

I have no idea, but I do not care. What I do care is that it could
be detected experimentally. Sensitive systems like a spacecraft
in free fall should "feel" this force that could appear pushing
the spacecraft in a direction tangential to the galaxy rotation.

MOND postulates an acceleration of 1.0e-10 m/sec^2.

Is that too small to measure?

This force would be tangential to the galaxy rotation, i.e. easily
distinguished from other random forces that would be directed at
random

Just an idea. I know, I am not qualified, but just take some
minutes to tell me why this is wrong.

It can't be that this acceleration would perturb the orbits
of the planets or whatever since it is A FACT that this acceleration
exists since the ACTUAL rotation curve of the milky way follows
this pattern!

It is just that this is a NEW force, not related to gravity at all.

On 7/24/13 12:37 AM, jacob navia wrote:

It is just that this is a NEW force, not related to gravity at all.

Force associated with vacuum viscosity is suggested as per

Ref1: arXiv:0806.3165v3 [hep-th] 14 Nov 2008
Hydrodynamics of spacetime and vacuum viscosity

It is suggested that all objects slow according to Stokes' law:

http://en.wikipedia.org/wiki/Stokes'_law

This extremely tenuous force
may alter particle motions
imparting observed galactic rotations.

Richard D Saam

In article ,
jacob navia writes:
Now, what would happen if we leave gravity and suppose that at
galactic scales new forces become relevant that we can't even suspect
given our minuscule size.

Unless you can specify how to calculate this force, you might as well
postulate invisible pink fairies pushing the stars around. Put a
little more seriously: MOND postulates one additional free parameter
(a length scale) and attempts to explain rotation curves and other
data with that (so far with no success). Here you are potentially
postulating some vast number of free parameters: some force not
proportional to mass and with some unspecified distance dependence.
No doubt if you make it complicated enough, you can explain
everything, but you will very likely have to have a different force
law for every galaxy (or at least every class of galaxy). That's a
lot of free parameters.

Sensitive systems like a spacecraft
in free fall should "feel" this force that could appear pushing
the spacecraft in a direction tangential to the galaxy rotation.

It would have to be a centripetal force, not tangential. As for
detection, why would this new force not affect the Earth and Sun or
whatever coordinate system you are using to measure the spacecraft
motion? And all the planets, if you are talking about tidal effects?

You might be amused to know that there was quite an industry of
investigating a putative "fifth force" some decades ago.
Investigation petered out when careful observations could find no
evidence. That doesn't mean a fifth force can't exist, but it would
have to have properties outside the parameter space investigated.

Per a later message, I wasn't aware of any unexplained force
involving Galileo. Galileo used frequent thruster firings for
stabilization, so it wasn't a good platform for measuring unexpected
forces. Are you thinking of the "Pioneer Anomaly?" As I wrote in
another message, recent analysis shows no anomaly.

Direct dark matter detection experiments (at useful detection levels)
are only getting started. We'll see what they show.

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In article , Steve Willner
writes:

MOND postulates one additional free parameter
(a length scale) and attempts to explain rotation curves and other
data with that (so far with no success).

I don't follow you here. Whatever one thinks of the various ideas which
could explain MOND, surely it is clear that the basic idea does give a
remarkable fit to rotation curves.

In article ,
Phillip Helbig---undress to reply writes:
Whatever one thinks of the various ideas which
could explain MOND, surely it is clear that the basic idea does give a
remarkable fit to rotation curves.

This is not a field I follow, but my understanding is that no single
"MOND parameter" or even modest number of parameters fits all the
available data. That is, one can easily fit a single galaxy, but
different parameter values are needed to fit different galaxies.

As I wrote, though, I'm far from well informed on this.

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Help keep our newsgroup healthy; please don't feed the trolls.
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