4 ms·

There is no difference between a black hole of a certain mass and a star of the same mass, gravitationally. Stars can only get so massive, but black holes can

by LASR 7y ago

There is no difference between a black hole of a certain mass and a star of the same mass, gravitationally. Stars can only get so massive, but black holes can get much much more massive. But even with supermassive black holes, it doesn’t change the odds of colliding solar systems that much. There is just way too much space in between to matter.

saagarjha 7y ago
The supermassive black holes at the center of the galaxy are thought to merge, however.
raattgift 7y ago
Your comment is accurate enough except for: > no difference ... gravitationally. "practically no difference ... gravitationally, provided one is at a very large distance compared to the outer surface of an astrophysical black hole." would be more accurate. Up close to a real black hole additional shear and tension becomes obvious, although you will generally be in the region that would be close to the surface of a star of comparable mass. Astrophysical black holes are more complicated objects than the theoretical Vaidya-Bonnor-Kerr (VBK) black holes, especially when they are not isolated. VBK black holes absorb radiation and emit Hawking radiation (V part), including self-interacting matter (B part, although that is mostly dealing with classical electromagnetism), and they also rotate (K part). VBK breaks several symmetries: the exterior spacetime is not static, not stationary, and spherical symmetry goes to axial symmetry, including all the incoming and outgoing radiation. Real rotating black holes would break axial symmetry because real matter tends to clump (in particular theoretical VBK exterior matter does not look like accretion discs). Small perturbations of the axial symmetry of VBK black holes tend to cause black holes to spin up as matter falls onto them. There is observational support to suggest that most real black holes are likely spinning extremely rapidly. The Kerr vacuum around a VBK is equipped with regions in which objects are dragged around in the direction of the VBK black hole's rotation. As the original angular momentum of the star may be increased by infalling material, a real black hole likely has a strong region extending well outside of where the surface of a star of comparable mass would be. While the dragging effect falls off quickly with distance outside the outermost surface of the outermost of these regions, if we replaced the sun with a real black hole which rotates at extremely high rates, all eight planets' orbits would precess compared to their orbits around the sun. Given enough observation time, this would be especially noticeable for objects on high-inclination, low-ellipticity orbits, even if they are near the outer edge of the Oort cloud. Reducing the angular momentum dramatically increases the observation time needed to distinguish gravitationally between a (growing!) solar-mass black hole and a (shrinking!) solar-mass star. Beyond the Oort cloud things aren't gravitationally bound to our solar system anyway, so by the time you're a couple hundred thousand astronomical units away, you are on safe territory with practical gravitational indistinguishability. In terms of light and radiation, though, swapping the sun for a solar-mass black hole would be pretty noticeable, for example https://en.wikipedia.org/wiki/Poynting%E2%80%93Robertson_effect https://en.wikipedia.org/wiki/Poynting%E2%80%93Robertson_eff... or https://en.wikipedia.org/wiki/Yarkovsky_effect https://en.wikipedia.org/wiki/Yarkovsky_effect

by LASR 7y ago

saagarjha 7y ago

The supermassive black holes at the center of the galaxy are thought to merge, however.

raattgift 7y ago

Your comment is accurate enough except for: > no difference ... gravitationally. "practically no difference ... gravitationally, provided one is at a very large distance compared to the outer surface of an astrophysical black hole." would be more accurate. Up close to a real black hole additional shear and tension becomes obvious, although you will generally be in the region that would be close to the surface of a star of comparable mass. Astrophysical black holes are more complicated objects than the theoretical Vaidya-Bonnor-Kerr (VBK) black holes, especially when they are not isolated. VBK black holes absorb radiation and emit Hawking radiation (V part), including self-interacting matter (B part, although that is mostly dealing with classical electromagnetism), and they also rotate (K part). VBK breaks several symmetries: the exterior spacetime is not static, not stationary, and spherical symmetry goes to axial symmetry, including all the incoming and outgoing radiation. Real rotating black holes would break axial symmetry because real matter tends to clump (in particular theoretical VBK exterior matter does not look like accretion discs). Small perturbations of the axial symmetry of VBK black holes tend to cause black holes to spin up as matter falls onto them. There is observational support to suggest that most real black holes are likely spinning extremely rapidly. The Kerr vacuum around a VBK is equipped with regions in which objects are dragged around in the direction of the VBK black hole's rotation. As the original angular momentum of the star may be increased by infalling material, a real black hole likely has a strong region extending well outside of where the surface of a star of comparable mass would be. While the dragging effect falls off quickly with distance outside the outermost surface of the outermost of these regions, if we replaced the sun with a real black hole which rotates at extremely high rates, all eight planets' orbits would precess compared to their orbits around the sun. Given enough observation time, this would be especially noticeable for objects on high-inclination, low-ellipticity orbits, even if they are near the outer edge of the Oort cloud. Reducing the angular momentum dramatically increases the observation time needed to distinguish gravitationally between a (growing!) solar-mass black hole and a (shrinking!) solar-mass star. Beyond the Oort cloud things aren't gravitationally bound to our solar system anyway, so by the time you're a couple hundred thousand astronomical units away, you are on safe territory with practical gravitational indistinguishability. In terms of light and radiation, though, swapping the sun for a solar-mass black hole would be pretty noticeable, for example https://en.wikipedia.org/wiki/Poynting%E2%80%93Robertson_effect https://en.wikipedia.org/wiki/Poynting%E2%80%93Robertson_eff... or https://en.wikipedia.org/wiki/Yarkovsky_effect https://en.wikipedia.org/wiki/Yarkovsky_effect