3 ms·

>"(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 mass

by 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-...
  - Arnt 10y ago
    Where in the (observed) solar system is there space for a black hole, even a small one, considering the continued existence and observed orbits of the planets?
petewailes 10y ago
From what we think atm, you can't get a lump of dark matter, without having something to draw it in. It seems to gather where space-time is already distorted. Although it doesn't seem to need much (some galaxies are "made" mostly of dark matter - i.e., they get most of their gravity from it, rather than from the matter we observe), it does look like dark matter only clumps around spacetime distortions (gravity wells basically), rather than being able to clump on its own. So yes, you could in theory have a black hole which has dark matter around it, which could do this, but it'd be mostly the black hole, not the dark matter doing the pulling. Hope that made some sense!
- nonbel 10y ago
  >"you could in theory have a black hole which has dark matter around it" Well, the link I provided gave me the impression it is plausible that the black holes are dark matter. This sounds like a different idea than what you are rejecting as "possible but unlikely".
  - petewailes 10y ago
    Afraid that's not the case. Black holes are categorically not dark matter.
    - nonbel 10y ago
      Well, ok. But apparently someone at NASA thinks it is plausible. Argument from authority heuristic tells me to listen to them rather than you in this case. Since you are not coming with any links or specific argument against what was said there, that is pretty much all I have to go on.
      - petewailes 10y ago
        Primordial black holes are the candidate here as you mentioned (black holes exist, primordial black holes may exist, but we haven't found any yet). Thanks to LIGO we think we may have the first evidence for them, but there is still a lot of work to do to prove them. Decent article on that: https://www.sciencenews.org/article/ligo-black-holes-dark-matter https://www.sciencenews.org/article/ligo-black-holes-dark-ma... The interesting thing will be if we can prove they are around, which would then give some options for showing if they're dark matter or not (looking for gravity waves in areas where there's large dark matter concentrations would be the most likely candidate for now).

by nonbel 10y ago

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-...
- Arnt 10y ago
  Where in the (observed) solar system is there space for a black hole, even a small one, considering the continued existence and observed orbits of the planets?

petewailes 10y ago

From what we think atm, you can't get a lump of dark matter, without having something to draw it in. It seems to gather where space-time is already distorted. Although it doesn't seem to need much (some galaxies are "made" mostly of dark matter - i.e., they get most of their gravity from it, rather than from the matter we observe), it does look like dark matter only clumps around spacetime distortions (gravity wells basically), rather than being able to clump on its own. So yes, you could in theory have a black hole which has dark matter around it, which could do this, but it'd be mostly the black hole, not the dark matter doing the pulling. Hope that made some sense!

nonbel 10y ago
>"you could in theory have a black hole which has dark matter around it" Well, the link I provided gave me the impression it is plausible that the black holes are dark matter. This sounds like a different idea than what you are rejecting as "possible but unlikely".
- petewailes 10y ago
  Afraid that's not the case. Black holes are categorically not dark matter.
  - nonbel 10y ago
    Well, ok. But apparently someone at NASA thinks it is plausible. Argument from authority heuristic tells me to listen to them rather than you in this case. Since you are not coming with any links or specific argument against what was said there, that is pretty much all I have to go on.
    - petewailes 10y ago
      Primordial black holes are the candidate here as you mentioned (black holes exist, primordial black holes may exist, but we haven't found any yet). Thanks to LIGO we think we may have the first evidence for them, but there is still a lot of work to do to prove them. Decent article on that: https://www.sciencenews.org/article/ligo-black-holes-dark-matter https://www.sciencenews.org/article/ligo-black-holes-dark-ma... The interesting thing will be if we can prove they are around, which would then give some options for showing if they're dark matter or not (looking for gravity waves in areas where there's large dark matter concentrations would be the most likely candidate for now).