The Hunt For Dark Matter

Originally posted by FabianFnas
Is it uniformely distribuated through the space, even in our vicinity, like within the solar system?
And what would then the average density be? Like one kg / litre? More? Less?

Much much less. I am afraid I cannot give an exact figure, but definitely less than a millionth of a gram per litre. The universe is mostly an empty place.

Originally posted by Metal Brain
The spin affects how close stars orbit black holes.

But does it affect our measurement of the mass of the black hole using he orbit of a star?

This is a variable that needs to be taken into account and you have not.
I am not convinced.

It is very relevant. I have proved what you "work out" can be wrong because the spin variable makes those calculations relative.
Relative to what?

Originally posted by FabianFnas
And what is its half-time within a atom nucleus? Is it stable there? If so, why there and why not when it's free?

(I like this discussion, I learn a lot!)

It depends on the atom nucleus, although in that case it is more accurate to say the nucleus decays to another nucleus.

Generally speaking (and oversimplifying a bit), "stability" is determined by whether there is a lower energy state available to the system and whether such a lower energy state is accessible without violating certain conservation laws (e.g. conservation of momentum).

Originally posted by twhitehead
But does it affect our measurement of the mass of the black hole using he orbit of a star?

[b]This is a variable that needs to be taken into account and you have not.
I am not convinced.

It is very relevant. I have proved what you "work out" can be wrong because the spin variable makes those calculations relative.
Relative to what?[/b]

How can you not be convinced? Are you saying that the frame dragging does not exist? Are you an idiot?

Originally posted by Metal Brain
How can you not be convinced? Are you saying that the frame dragging does not exist? Are you an idiot?

No, I am saying that frame dragging does not significantly affect our ability to calculate the mass of a black hole.

Originally posted by KazetNagorra
Generally speaking (and oversimplifying a bit), "stability" is determined by whether there is a lower energy state available to the system and whether such a lower energy state is accessible without violating certain conservation laws (e.g. conservation of momentum).

Great simple explanation of stability!

Dark Matter is probably not Anti-Matter right? What I was reading is that the boundary between where matter and antimatter met would be detectable because of the energy that would be given off at the boundary where the two meet and would be a constant annihilation of the two.There would be emissions that would be detectable. Something is pushing around the visible mass however hence dark matter

Manny

Originally posted by menace71
Dark Matter is probably not Anti-Matter right? What I was reading is that the boundary between where matter and antimatter met would be detectable because of the energy that would be given off at the boundary where the two meet and would be a constant annihilation of the two.There would be emissions that would be detectable. Something is pushing around the visible mass however hence dark matter

Manny

Dark Matter could not possibly be Antimatter for several reasons. One such reason would be that antimatter is not 'dark' which means, if it made up most of the mass of the universe just like dark matter is supposed to, it should be at least as visible as the matter we see.

There would also be the thorny problem that, if dark matter was antimatter, that would mean we would extremely precariously be surrounded my massive amounts of the dangerous stuff which would beg the question of why we haven't yet been annihilated by it a long time ago!

Originally posted by humy
Dark Matter could not possibly be Antimatter for several reasons. One such reason would be that antimatter is not 'dark' which means, if it made up most of the mass of the universe just like dark matter is supposed to, it should be at least as visible as the matter we see.

There would also be the thorny problem that, if dark matter was antimatter, that would ...[text shortened]... stuff which would beg the question of why we haven't yet been annihilated by it a long time ago!

Your statements make sense if you view anti-matter the same way as matter in that it attracts other matter, but what if it repels?

Originally posted by menace71
Dark Matter is probably not Anti-Matter right?

Dark matter particles, if they exist, probably also have antiparticles, but those are not related to the antiparticles that we can currently observe. An anti-up-quark, for instance, is not dark matter by definition since it interacts strongly through forces other than gravity.

Well we do know antimatter is basically the same as matter but opposite charge right ? Like Electron / Positron so visibly it looks just like ordinary matter but it annihilates when it comes in contact with matter. I guess I answered my own question LOL so we really have no clue on what dark matter is other than we know that there is an unaccounted force acting on the mass that we do see in the universe.

Manny

Originally posted by Metal Brain
Your statements make sense if you view anti-matter the same way as matter in that it attracts other matter, but what if it repels?

If it repels then we would have surely already observed this with the antimatter experiments but we didn't observe this. In addition, even if it does 'repel' matter, antimatter still annihilated matter in the experiments so, evidently, it doesn't 'repel' well enough, at least in all our experiments to date, to stop contact and annihilation thus this still begs the question of why we haven't been annihilated yet (and this is not to mention the several other significant problems with the idea of dark matter being antimatter)

Originally posted by twhitehead
But does it affect our measurement of the mass of the black hole using he orbit of a star?

[b]This is a variable that needs to be taken into account and you have not.
I am not convinced.

It is very relevant. I have proved what you "work out" can be wrong because the spin variable makes those calculations relative.
Relative to what?[/b]

The business of measuring or computing the mass of a star orbiting a black hole close enough to be effected by frame dragging is a valid point, if the drag is enough to effect the velocity of the star's orbit around the BH.

For instance, frame dragging is what delays the orbit of Mercury by such a tiny amount it was almost not measurable but it was one of the proof's of relativity when they first made the measurement. I think they knew about the tiny excess precession measured in 1915:

http://en.wikipedia.org/wiki/Tests_of_general_relativity#Frame-dragging_tests

Another related effect is called the Geodetic effect:

http://en.wikipedia.org/wiki/Geodetic_effect

Such effects are hard to even measure with our technology on masses like our sun but would need to be taken into account near the incredible bending of spacetime near a black hole where such effects would be extremely magnified, I suspect enough to alter the measured orbital velocity of a star unfortunate enough to get within grabbing distance of a black hole.

Space time itself is wound into a small spindle near a black hole so it makes sense such alterations of physics would have to be taken into account when trying to make such measurements as the orbital velocity of a star trapped by a black hole.

Originally posted by sonhouse
Space time itself is wound into a small spindle near a black hole so it makes sense such alterations of physics would have to be taken into account when trying to make such measurements as the orbital velocity of a star trapped by a black hole.

But we do not measure the obit of just one star, but rather a number of stars with various orbits, including highly elliptical orbits. Surely such frame dragging if significant would be detectable and could then be taken into account?
To what extent would it affect our estimate of a supermasive black hole?

http://en.wikipedia.org/wiki/Supermassive_black_hole#Milky_Way_galactic_center_black_hole
Wikipedia says the one in our galaxy was estimated from the orbit of a particular star, but then shows a diagram of the orbits of 6 stars.

Interesting how do scientist calculate the mass of a black hole ? it would have to be in a binary system I would think to even observe its effects on orbiting stars. That has to be a part of how they would calculate the black hole's mass to begin with.

Manny