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Fraser
2008-Apr-08, 02:10 PM
According to Newton's Second Law of Dynamics, objects on the farthest edges of galaxies should have lower velocities than objects near the center. But observations confirm that galaxies rotate with a uniform velocity. Some astronomers believe the orbital behavior of galaxies can be explained more accurately with Modified Newtonian Dynamics (MOND) — a [...]

More... (http://www.universetoday.com/2008/04/08/a-case-of-mond-over-dark-matter/)

DyerWolf
2008-Apr-08, 03:52 PM
Very interesting. This is a topic I've followed for a while.

Thanks for the update.

JESMKS
2008-Apr-08, 04:37 PM
Do they know if all galaxies rotate with the same uniform velocity or does each galaxy have it's own specific velocity?

frankuitaalst
2008-Apr-08, 08:04 PM
Reading the article a question I already had some years ago came again floating on the surface .::(
The two most well forces , gravity and electroforce both are said to be proportional to the inverse of the square distance to the source .
This is sufficiently demonstrated in practice , however I don't know of any "rule" or theory why this should be .
Is there some ground , apart from the experiments , why the exponent is exact 2 and not 3 or 4 or even 2.5 or ...
Anybody here has an idea ?

Hornblower
2008-Apr-09, 10:40 PM
Reading the article a question I already had some years ago came again floating on the surface .::(
The two most well forces , gravity and electroforce both are said to be proportional to the inverse of the square distance to the source .
This is sufficiently demonstrated in practice , however I don't know of any "rule" or theory why this should be .
Is there some ground , apart from the experiments , why the exponent is exact 2 and not 3 or 4 or even 2.5 or ...
Anybody here has an idea ?
I cannot think of any reason a priori why any particular exponent should hold. I remember seeing somewhere during my college days that the strong nuclear force falls off roughly as the inverse 9th power of the separation. (Don't take that as gospel. It has been a long time)

I understand why luminance is proportional to the inverse square of the distance, because the light is spread out over a larger area proportional to distance squared. That is simple geometry. I cannot say whether or not that is a valid analogy for gravitation and electrostatic force. I am in no way up to speed on General Relativity.

Jerry
2008-Apr-09, 11:14 PM
Yes - It is purely a matter of geometry. To help get your arms around it, imagine the center of the force is a cube, and that it is the 'square fields' expanding. The total area of each of the six sides of the cube increases as the square of the distance of the cube from the center. Now make the cube an eight, ten, sixty-four or 1028 sided object. A 1028 sided object would look almost perfectly round, but if you measured the area of each section, it would increase as the square of the radius as you expanded the object.

With gravity, this becomes interesting as the distance from the body generating the force increases. The Newtonian solution is a natural, geometric argument; while MOND stretches gravity...unnaturally?

rtomes
2008-Apr-09, 11:17 PM
According to Newton's Second Law of Dynamics, objects on the farthest edges of galaxies should have lower velocities than objects near the center....
Well that isn't true is it? It is true in Solar System because nearly all the mass is located at the centre. But when the mass is spread about like in a galaxy then the part contributing to the rotation is only (roughly) what is inside that objects orbit (and even that assumes spherical symmetry which does not exist in a galaxy).

About 45 years ago I was looking at the relative frequencies (i.e commonness) of different mass stars and noticed that roughly speaking the frequency was inversely proportional to the mass so that there was an equal total mass in each ratio of mass, e.g. equal masses between 0.125 - 0.25 - 0.5 -1 -2 times Sun's mass. It all stops when you get a bit above the Sun because those big blighters blow themselves to smithereens very quickly.

However I started to think about the smaller stuff than stars that we can't see when it is far away, like planets and moons and comets and dust and so on. It seemed to me that a log-object-mass versus log-frequency plot of all sized objects had a slope of quite close to -1 throughout the observable range. Of course we cannot see most of the range but that is no reason why it wouldn't be there.

Over that time interval I have seen astronomers get surprised now and then when they discovered teeming hordes of brown dwarfs or machos, which to me is totally expected. I would love to see a plot of log-mass versus log-frequency of the various classes of objects that have now been established.

I would add one further observation. Nearer the centre of the galaxy you get more of the big stuff and further out you get more of the small stuff as far as stars go. That makes sense because that whole hierarchy of sizes depends on the collision frequency of various objects withe each other for accumulation to take place, and that obviously happens more rapidly in denser zones.

On this basis, wouldn't it be logical to expect a lot more matter to be still in smaller objects on the outer edges of galaxies? Much of this would be unobservable due to being too small to glow even a little bit. It would just be ordinary old baryonic matter at various stages of accumulation in the process from dust to star.

Sometimes I think that observations can be explained by quite mundane things if you look at them the right way. It may require no new physics at all.

PS: It is worth working out how many orders of magnitude there are between atoms and stars and how many of these actually glow. Then you can estimate the missing mass that is not seen.

frankuitaalst
2008-Apr-10, 03:38 PM
Quote : I cannot think of any reason a priori why any particular exponent should hold. I remember seeing somewhere during my college days that the strong nuclear force falls off roughly as the inverse 9th power of the separation. (Don't take that as gospel. It has been a long time)

I understand why luminance is proportional to the inverse square of the distance, because the light is spread out over a larger area proportional to distance squared. That is simple geometry. I cannot say whether or not that is a valid analogy for gravitation and electrostatic force. I am in no way up to speed on General Relativity. End Quote

Well , I had a similar picture in mind : if one imagines that the total attractive "potential" of a body spreads out as a sphere one gets an attractive field of 4piRē over a sphere , 4piRē being the surface of the sphere . The atractive force becomes then : F = Ct / Rē.
Ct being a constant and "of course " being proportional to the involved massas. This idea is consistent with Newtons Law , however the idea itself is purely intuitional and perhaps mentally derived from the Newtons Law itself .
:cry:

Amber Robot
2008-Apr-10, 06:02 PM
On this basis, wouldn't it be logical to expect a lot more matter to be still in smaller objects on the outer edges of galaxies? Much of this would be unobservable due to being too small to glow even a little bit. It would just be ordinary old baryonic matter at various stages of accumulation in the process from dust to star.

Wasn't this idea the basis of the MACHO survey? If I recall correctly, that survey demonstrated that dark matter could not be fully accounted for by these kinds of objects. Otherwise you'd be having far more microlensing events than were seen.

rtomes
2008-Apr-14, 09:36 AM
Wasn't this idea the basis of the MACHO survey? If I recall correctly, that survey demonstrated that dark matter could not be fully accounted for by these kinds of objects. Otherwise you'd be having far more microlensing events than were seen.
Yes, I think it was the idea. However they only detect a small part of the whole spectrum of sizes don't they? There are about 56 orders of magnitude between an atom and a star. How many of those orders of magnitude have we now observed adequately in extended parts of the universe?