4 ms·

Assuming dark matter is a WIMP, I think it would be invisible and fall through your hand (and the earth) when you tried to pick it up. Stars require the particl

Assuming dark matter is a WIMP, I think it would be invisible and fall through your hand (and the earth) when you tried to pick it up. Stars require the particles in question to bang around against each other and interact with each other. WIMP particles wouldn't even be able to interact with each other except gravitationally - ie. they wouldn't "collide" or create pressure in an enclosed space. A cloud of dark matter is likely to stay a cloud for a very very long time rather than collapsing in on itself. I think of it like an N-body gravity simulation[1] with very very large N and no collision detection. [1] http://justfound.co/gravity/ http://justfound.co/gravity/

BurningFrog 10y ago
> I think it would be invisible and fall through your hand (and the earth) Why would't it fall to the center of the earth and stay there, since that is the local gravitational center?
- openasocket 10y ago
  No, it would speed past the center until losing moment on the surface of the other side of the planet, and oscillate back and forth. Because there's no "friction" slowing down the dark matter it will continue like this forever, by conservation of energy. Check out http://physics.stackexchange.com/questions/214950/if-dark-matter-only-interacts-with-gravity-why-doesnt-it-all-clump-together-in http://physics.stackexchange.com/questions/214950/if-dark-ma...
  - BurningFrog 10y ago
    Aha, DM particles are in eternal orbits! Thanks!
  - TheDauthi 10y ago
    Wouldn't it -very- slowly lose energy due to gravitational radiation?
    - wyager 10y ago
      If the cloud was inhomogeneous, yes. However, (unless I am mistaken, which is fairly likely,) the more symmetric a bulk collection of particles (such as WIMPs), the less gravitational radiation it would emit as it oscillated. In particular, for every particle falling in from one side of the planet, you have another particle falling in from the other side of the planet, so the mass distribution of the system does not change over time. I don't know how gravetomagnetic effects would come into play.
  - grkvlt 10y ago
    > speed past the center What caused these particles to accelerate up to the speeds they are at in the first place, then? Something must be interacting with them quite significantly if they have non-zero velocity?
    - openasocket 10y ago
      the force of gravity
    - gus_massa 10y ago
      Gravity. Actually, almost the only thing that we know about dark mater is that gravity affect it. (Actually, we only know that there is something that is making a gravitational pull of normal mater, and we call this mysterious thing "dark mater".)

BurningFrog 10y ago

> I think it would be invisible and fall through your hand (and the earth) Why would't it fall to the center of the earth and stay there, since that is the local gravitational center?

openasocket 10y ago
No, it would speed past the center until losing moment on the surface of the other side of the planet, and oscillate back and forth. Because there's no "friction" slowing down the dark matter it will continue like this forever, by conservation of energy. Check out http://physics.stackexchange.com/questions/214950/if-dark-matter-only-interacts-with-gravity-why-doesnt-it-all-clump-together-in http://physics.stackexchange.com/questions/214950/if-dark-ma...
- BurningFrog 10y ago
  Aha, DM particles are in eternal orbits! Thanks!
- TheDauthi 10y ago
  Wouldn't it -very- slowly lose energy due to gravitational radiation?
  - wyager 10y ago
    If the cloud was inhomogeneous, yes. However, (unless I am mistaken, which is fairly likely,) the more symmetric a bulk collection of particles (such as WIMPs), the less gravitational radiation it would emit as it oscillated. In particular, for every particle falling in from one side of the planet, you have another particle falling in from the other side of the planet, so the mass distribution of the system does not change over time. I don't know how gravetomagnetic effects would come into play.
- grkvlt 10y ago
  > speed past the center What caused these particles to accelerate up to the speeds they are at in the first place, then? Something must be interacting with them quite significantly if they have non-zero velocity?
  - openasocket 10y ago
    the force of gravity
  - gus_massa 10y ago
    Gravity. Actually, almost the only thing that we know about dark mater is that gravity affect it. (Actually, we only know that there is something that is making a gravitational pull of normal mater, and we call this mysterious thing "dark mater".)