Concerning Dark matter

[SamuraiBlue]

I was going through an article concerning dark matter consisting approx.30% of total mass within our space. I also read that candidates for dark matters are sub-atomic particles that do not interact with other matter so we are not able to study this particle except for it's gravitational effect due to it's mass.
One of the reason why dark matter has postulated in the first place was the to explain orbital velocities of http://en.wikipedia.org/wiki/Galaxiesgalaxies in clusters.

My question is, IF dark matter is a sub-atomic particle then it should not be localized within space due to entropy, then should it not show gravitational effect to our own solar system?

 

[Shaw]

I guess the answer is that it does.

One of the early measurements that brought about the concept of dark matter was that stars within the galactic disk of galaxies were not moving at the speeds one would have expected if the stars were just orbiting the galactic center. The gravitational effect of some dark matter seemed the most likely solution.

Beyond that, what effect were you thinking there should be? And are you saying that particles with mass wouldn't be effected by gravity?

I mean think about it... a subatomic particle that doesn't interact with any other particles could pass through the earth as if it wasn't even there (considering that most of what we consider solid matter is actually empty space).

If we could magically hold one of these particles at a couple meters above the ground and then let it go, it would fall through the earth and eventually pop up on the other side and then fall back again... and could do that for years without hitting anything. By not having any other interactions short of a direct hit on another subatomic particle, most solid matter becomes empty space. And other than it's gravitational effect, the earth would seem like it wasn't even there to one of these particles.



Note: I'm not saying that I subscribe to the current theories of Dark Matter, but if it is cause by such a particle, this is why it would be hard to find.

 

[SamuraiBlue]

I guess the answer is that it does.

One of the early measurements that brought about the concept of dark matter was that stars within the galactic disk of galaxies were not moving at the speeds one would have expected if the stars were just orbiting the galactic center. The gravitational effect of some dark matter seemed the most likely solution.

Beyond that, what effect were you thinking there should be? And are you saying that particles with mass wouldn't be effected by gravity?

I mean think about it... a subatomic particle that doesn't interact with any other particles could pass through the earth as if it wasn't even there (considering that most of what we consider solid matter is actually empty space).

If we could magically hold one of these particles at a couple meters above the ground and then let it go, it would fall through the earth and eventually pop up on the other side and then fall back again... and could do that for years without hitting anything. By not having any other interactions short of a direct hit on another subatomic particle, most solid matter becomes empty space. And other than it's gravitational effect, the earth would seem like it wasn't even there to one of these particles.

I don't think so since according to Newton's law of universal gravitation, planets will pull the particles towards the center, larger the mass more particle it would capture within it's gravitational well resulting to change of mass through passage of time which should result to change of orbit which we have not observed.

 

[Shaw]

How would a planet, star or other large body capture one of these particles? If they don't hit anything, then the best you can hope for is a change in their direction. The hypothetical I gave required the particles to start with zero initial momentum relative to the earth... but without that, the best you could hope for is that they might alter their path a little. It would require something very massive (neutron star scale) to capture one of these particles.

And when we are talking about such small effects with no other forces beyond gravity, initial momentum and the exceptionally rare collision, it would take very large gravitational systems (like galaxies and galactic clusters) to capture these in any measurable amount where their combined gravitational effects would come into play.

But in the end this isn't an argument... you asked, I told, you are free to disregard. The physics is pretty straight forward, so even though I don't subscribe to the theory of these particles, there is nothing wrong with the mechanics (even using Newton's laws) of the theory.