9 ms·

Why would the clump if they don't interact?

by thisiscorrect 6y ago

rurban 6y ago
Gravity still attracts them. They do have mass
- magicalhippo 6y ago
  But without interacting they have no means to shed the energy they gain when attracted to other matter, so they won't clump like normal matter.
  - Google234 6y ago
    They interact with normal matter and themselves via the force of gravity...
    - magicalhippo 6y ago
      But gravity is (effectively) a conservative force. For the most part a dark matter particle attracted to another particle will just convert all the kinetic energy it gained during the attraction back to potential energy as it whizzes past and away from the other particle. Normal matter particles can radiate away photons when they hit each other, ie friction, leading to a non-conservative interaction. This allows them to shed their energy and clump in a way dark matter can't.
      - rurban 6y ago
        No, gravity is different to all other forces. First it's not really a force, then it's attracting and distant. We don't even have a proper theory for it yet.
        magicalhippo 6y ago
        Sure gravity is different from the other forces. That changes nothing about what I said, because we do have a proper theory for it: General Relativity. General Relativity matches a vast number observations with stunning precision. Of course General Relativity is not the final answer, we know that. But that does not mean we know nothing about gravity, or that we somehow can't use our current theory to make predictions. Dark matter as a concept assumes General Relativity is good enough, and takes it as-is. Without modifying gravity we need some other way to explain the discrepancies like galactic rotation curves, and so for dark matter we add in some new, so-far unseen, particles. We also know they have to interact very weakly with normal matter besides gravity, otherwise we'd would already have noticed them in existing observations and experiments. Based on this we can make predictions. What would happen if we have some matter that only interacts gravitationally buzzing around? Well as mentioned in my previous post such matter can't cool down and clump like normal matter, so instead it would be spread out in diffuse blobs around galaxies. And when we model galaxies with such dark matter halos we find that indeed, that might explain galaxy rotation curves. It also makes other predictions which also seem to match other observations. So overall dark matter looks like a pretty good candidate. The alternative of course is to assume there aren't dark matter particles, but rather that GR has to be replaced or modified in some non-trivial way. There are many people working on that. However they're so far struggling, because it turns out to be very difficult to change or replace GR without failing to match existing observations. So that seems to indicate that GR is a pretty darn good theory of gravity.
        steve76 6y ago
        Gravity tends to present as action at a distance. It's just what happens when you say velocity must be relative and does not work with one object alone. I would say gravity is more of a description of the object, what it's really doing, than what produces gravity. Particles tend to present as little tiny building blocks. Einstein looked at Planck's resolution of Newton's Law of Cooling, and said you have a start, you have an end, you have a particle. So now a particle can be a little bit of charge-up needed to make an ember radiate from red to orange, nothing really inside the flame itself. No more absolute right way to tick-tock a clock. Radiate heat, your clock ticks faster, your dumping heat by making a slow ticking dark clock fast and your fast ticking bright clock slow regardless of convective medium. Uncertainty tends to present as a limitation. It's actually very powerful. Everything has an infinite degree of freedom for the fastest path to thermal equilibrium. Spacetime tends to be elevated, even mystical. I see it as a coarse concept, like how probability strips away all sense of weights or lengths or measures. A real gravity application would be popping things in and out of existence. I think of what we could do with matter waves or chemicals or quantum biology or particle beams in orbit. Encircle the Earth, or the Sun, and turn that into a microstate to a cosmic macrostate. Or what we could do with cosmic variance, inflation and parallax. We know this very bright object is here. Everything else from a long distance away thinks it's somewhere else. --- --- --- They mocked the guy for thinking about small black holes. They really would love me. Part of me thinks everything up there is just where a much lesser amount of material down here decided to dump their heat, and that telescopes are disruptive. Point a telescope the right way, and you get them caught in the cross fire, dissolve them or even create them. They really only become real when things from the Earth land on them. Sounds crazy but reminds me of empiricism.
      - PopePompus 6y ago
        This is why there is still a lot of atomic hydrogen in interstellar space. Thermodynamics suggests the atoms should bind together and form molecular hydrogen, but since a hydrogen molecule has no dipole moment, two hydrogen atoms alone in space cannot easily radiate away the energy released when they would bind. So they don't bind.
      - kazagistar 6y ago
        I used to think this as well. However, all orbiting mass can shed energy via gravity waves, and by interaction that fling some mass out at high kinetic velocity while other mass sinks deeper, which is a primary method of star formation afaik. Basically, radiating each other instead of photons. There is also the possibility of dark matter only forces and fields that don't affect normal matter, but allow dark matter to self interact to some extent.
can16358p 6y ago
I think that meant something like don't interact with our "ordinary matter" in a way that we know.

Why would the clump if they don't interact?

rurban 6y ago

Gravity still attracts them. They do have mass

magicalhippo 6y ago
But without interacting they have no means to shed the energy they gain when attracted to other matter, so they won't clump like normal matter.
- Google234 6y ago
  They interact with normal matter and themselves via the force of gravity...
  - magicalhippo 6y ago
    But gravity is (effectively) a conservative force. For the most part a dark matter particle attracted to another particle will just convert all the kinetic energy it gained during the attraction back to potential energy as it whizzes past and away from the other particle. Normal matter particles can radiate away photons when they hit each other, ie friction, leading to a non-conservative interaction. This allows them to shed their energy and clump in a way dark matter can't.
    - rurban 6y ago
      No, gravity is different to all other forces. First it's not really a force, then it's attracting and distant. We don't even have a proper theory for it yet.
      - magicalhippo 6y ago
        Sure gravity is different from the other forces. That changes nothing about what I said, because we do have a proper theory for it: General Relativity. General Relativity matches a vast number observations with stunning precision. Of course General Relativity is not the final answer, we know that. But that does not mean we know nothing about gravity, or that we somehow can't use our current theory to make predictions. Dark matter as a concept assumes General Relativity is good enough, and takes it as-is. Without modifying gravity we need some other way to explain the discrepancies like galactic rotation curves, and so for dark matter we add in some new, so-far unseen, particles. We also know they have to interact very weakly with normal matter besides gravity, otherwise we'd would already have noticed them in existing observations and experiments. Based on this we can make predictions. What would happen if we have some matter that only interacts gravitationally buzzing around? Well as mentioned in my previous post such matter can't cool down and clump like normal matter, so instead it would be spread out in diffuse blobs around galaxies. And when we model galaxies with such dark matter halos we find that indeed, that might explain galaxy rotation curves. It also makes other predictions which also seem to match other observations. So overall dark matter looks like a pretty good candidate. The alternative of course is to assume there aren't dark matter particles, but rather that GR has to be replaced or modified in some non-trivial way. There are many people working on that. However they're so far struggling, because it turns out to be very difficult to change or replace GR without failing to match existing observations. So that seems to indicate that GR is a pretty darn good theory of gravity.
      - steve76 6y ago
        Gravity tends to present as action at a distance. It's just what happens when you say velocity must be relative and does not work with one object alone. I would say gravity is more of a description of the object, what it's really doing, than what produces gravity. Particles tend to present as little tiny building blocks. Einstein looked at Planck's resolution of Newton's Law of Cooling, and said you have a start, you have an end, you have a particle. So now a particle can be a little bit of charge-up needed to make an ember radiate from red to orange, nothing really inside the flame itself. No more absolute right way to tick-tock a clock. Radiate heat, your clock ticks faster, your dumping heat by making a slow ticking dark clock fast and your fast ticking bright clock slow regardless of convective medium. Uncertainty tends to present as a limitation. It's actually very powerful. Everything has an infinite degree of freedom for the fastest path to thermal equilibrium. Spacetime tends to be elevated, even mystical. I see it as a coarse concept, like how probability strips away all sense of weights or lengths or measures. A real gravity application would be popping things in and out of existence. I think of what we could do with matter waves or chemicals or quantum biology or particle beams in orbit. Encircle the Earth, or the Sun, and turn that into a microstate to a cosmic macrostate. Or what we could do with cosmic variance, inflation and parallax. We know this very bright object is here. Everything else from a long distance away thinks it's somewhere else. --- --- --- They mocked the guy for thinking about small black holes. They really would love me. Part of me thinks everything up there is just where a much lesser amount of material down here decided to dump their heat, and that telescopes are disruptive. Point a telescope the right way, and you get them caught in the cross fire, dissolve them or even create them. They really only become real when things from the Earth land on them. Sounds crazy but reminds me of empiricism.
    - PopePompus 6y ago
      This is why there is still a lot of atomic hydrogen in interstellar space. Thermodynamics suggests the atoms should bind together and form molecular hydrogen, but since a hydrogen molecule has no dipole moment, two hydrogen atoms alone in space cannot easily radiate away the energy released when they would bind. So they don't bind.
    - kazagistar 6y ago
      I used to think this as well. However, all orbiting mass can shed energy via gravity waves, and by interaction that fling some mass out at high kinetic velocity while other mass sinks deeper, which is a primary method of star formation afaik. Basically, radiating each other instead of photons. There is also the possibility of dark matter only forces and fields that don't affect normal matter, but allow dark matter to self interact to some extent.

can16358p 6y ago

I think that meant something like don't interact with our "ordinary matter" in a way that we know.