5 ms·

Pithy answer - dark matter, whatever it might be (WIMPs or otherwise) is most importantly, Weakly Interacting (the WI in WIMP). It appears to clump around gravi

Pithy answer - dark matter, whatever it might be (WIMPs or otherwise) is most importantly, Weakly Interacting (the WI in WIMP). It appears to clump around gravitational structures, acting like normal matter, but in more of a field way, rather than a matter way (think of it like a fluid filling a container, with uneven density - where the density is higher is where you've got matter pulling at it). For dark matter to be causing this, you'd need some exotic form of something that's already exotic - it'd have to be able to interact with an intensity we don't associate with it. It'd also be defying what we understand it to normally do (you'd need something to make it pull into a lump from how we think it behaves, which predisposes a physical structure like a planet). So, is there some massive errant lump of dark matter pulling the solar system around? From what we infer about how dark matter behaves from observation, no. (The fluid thing is not an entirely accurate model of how dark matter works, but it's a decent enough metaphor. However, this gets very complex and I'd rather not spend the rest of the day getting into ever more detail about theoretical physics.)

reitanqild 10y ago
@stuff4ben: mission accomplished I guess? : )
- stuff4ben 10y ago
  Indeed. Thank-you petewailes! And I think I actually understood some of that too.
  - petewailes 10y ago
    My pleasure :)
nonbel 10y ago
>"(you'd need something to make it pull into a lump from how we think it behaves, which predisposes a physical structure like a planet). So, is there some massive errant lump of dark matter pulling the solar system around? From what we infer about how dark matter behaves from observation, no." Apparently people find it plausible that dark matter consists of primordial black holes: "An intriguing alternative view is that dark matter is made of black holes formed during the first second of our universe's existence, known as primordial black holes. Now a scientist at NASA’s Goddard Space Flight Center in Greenbelt, Maryland, suggests that this interpretation aligns with our knowledge of cosmic infrared and X-ray background glows and may explain the unexpectedly high masses of merging black holes detected last year." http://www.nasa.gov/feature/goddard/2016/nasa-scientist-suggests-possible-link-between-primordial-black-holes-and-dark-matter http://www.nasa.gov/feature/goddard/2016/nasa-scientist-sugg... So I don't see how the "clumped/lumped" dark matter explanation can be dismissed, black holes are basically lumps of pure mass right?
- Arnt 10y ago
  Black holes are far from the earth, as proved by the continued existence of the earth.
  - nonbel 10y ago
    My understanding of current mainstream thought is that gravity due to a black hole of a given mass will have the exact same effect as from a planet of the same mass.
    - petewailes 10y ago
      Not quite. The way they'd distort spacetime would be different. A small black hole produces more concentrated distortion with nothing around it (think of a marble made of lead on a rubber sheet - lots of distortion, small object), whereas something large with the same mass (a bowling ball made of something very not-dense) would have a different shaped distortion, despite having the same mass. You also get differences in other things too, based on the other different physical properties and behaviours. Basically, if this exists, it's almost certainly a rogue planet or similar large body, not something exotic. It's not impossible that it's something else, but it'd be very very weird if that were the case.
      - nonbel 10y ago
        >"A small black hole produces more concentrated distortion with nothing around it (think of a marble made of lead on a rubber sheet - lots of distortion, small object), whereas something large with the same mass (a bowling ball made of something very not-dense) would have a different shaped distortion, despite having the same mass." I see what you are saying, but am thinking that the distortions need to coincide at a large distance from the object (ie at low accelerations), otherwise GR couldn't reduce to Newtonian mechanics. However, then I found this interesting discussion which leads me to believe that you in fact cannot really derive Newtonian mechanics from GR: https://www.physicsforums.com/threads/advanced-books-papers-on-derivation-of-newtonian-mechanics-from-gr.89178/ https://www.physicsforums.com/threads/advanced-books-papers-...
ccozan 10y ago
Thank you for the explanation, too!

reitanqild 10y ago

@stuff4ben: mission accomplished I guess? : )

stuff4ben 10y ago
Indeed. Thank-you petewailes! And I think I actually understood some of that too.
- petewailes 10y ago
  My pleasure :)

nonbel 10y ago

>"(you'd need something to make it pull into a lump from how we think it behaves, which predisposes a physical structure like a planet). So, is there some massive errant lump of dark matter pulling the solar system around? From what we infer about how dark matter behaves from observation, no." Apparently people find it plausible that dark matter consists of primordial black holes: "An intriguing alternative view is that dark matter is made of black holes formed during the first second of our universe's existence, known as primordial black holes. Now a scientist at NASA’s Goddard Space Flight Center in Greenbelt, Maryland, suggests that this interpretation aligns with our knowledge of cosmic infrared and X-ray background glows and may explain the unexpectedly high masses of merging black holes detected last year." http://www.nasa.gov/feature/goddard/2016/nasa-scientist-suggests-possible-link-between-primordial-black-holes-and-dark-matter http://www.nasa.gov/feature/goddard/2016/nasa-scientist-sugg... So I don't see how the "clumped/lumped" dark matter explanation can be dismissed, black holes are basically lumps of pure mass right?

Arnt 10y ago
Black holes are far from the earth, as proved by the continued existence of the earth.
- nonbel 10y ago
  My understanding of current mainstream thought is that gravity due to a black hole of a given mass will have the exact same effect as from a planet of the same mass.
  - petewailes 10y ago
    Not quite. The way they'd distort spacetime would be different. A small black hole produces more concentrated distortion with nothing around it (think of a marble made of lead on a rubber sheet - lots of distortion, small object), whereas something large with the same mass (a bowling ball made of something very not-dense) would have a different shaped distortion, despite having the same mass. You also get differences in other things too, based on the other different physical properties and behaviours. Basically, if this exists, it's almost certainly a rogue planet or similar large body, not something exotic. It's not impossible that it's something else, but it'd be very very weird if that were the case.
    - nonbel 10y ago
      >"A small black hole produces more concentrated distortion with nothing around it (think of a marble made of lead on a rubber sheet - lots of distortion, small object), whereas something large with the same mass (a bowling ball made of something very not-dense) would have a different shaped distortion, despite having the same mass." I see what you are saying, but am thinking that the distortions need to coincide at a large distance from the object (ie at low accelerations), otherwise GR couldn't reduce to Newtonian mechanics. However, then I found this interesting discussion which leads me to believe that you in fact cannot really derive Newtonian mechanics from GR: https://www.physicsforums.com/threads/advanced-books-papers-on-derivation-of-newtonian-mechanics-from-gr.89178/ https://www.physicsforums.com/threads/advanced-books-papers-...

ccozan 10y ago

Thank you for the explanation, too!