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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

by 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?

by 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?