4 ms·

> I'm not saying it would happen often. But things do regularly plummet into the sun The flow of mass through the region near the black hole (and well inside t

> I'm not saying it would happen often. But things do regularly plummet into the sun The flow of mass through the region near the black hole (and well inside the surface of our sun) is minuscule in comparison to that in binary systems where the BH's companion is a Roche lobe filling star. The event horizon of the sun would be about 6km in diameter, assuming that we swap the sun for a black hole with little angular momentum. Within the volume 10x that diameter, one might expect violence in objects on intersecting geodesics; with a diameter 50x that of the event horizon, one can start using linearized GR safely; with a diameter 500x that of the event horizon, post-Newtonian corrrections start to become small. The diameter of the sun is a bit more than 450 times larger than that (or about 232 000 times the diameter of the black hole). While there are lots of objects that are known to pass within 10 solar radiuses of the sun's centre-of-mass, it's hard to be definitive about why the number falls off with decreasing radius since the sun does a good job of disintegrating comets, and non-comets are hard to observe that close to the sun. Collisions with our sun are rare, and most colliders are unlikely to have been on orbits that would take them into the central regions discussed in the paragraphs above. Objects that do find themselves only a thousand or two km from the event horizon will certainly take a post-Newtonian long time to emerge from that region, according to a surviving human observer on Earth. I'm not an expert on accretion discs (most of what I know involves the inner edge) nor astrophysical black holes, but absent a binary companion, the disc will be pretty sparse in general, and in particular maybe a factor of ten beyond the ISCO it'd be so sparse that even if it's super-hot, it's not going to do much to an asteroid or comet. Although it's just a guess (you'd have to solve it numerically, good luck) I think tidal dismantling of a comet or asteroid would not put the debris on to many intersecting geodesics, so you need to have the debris linger around the black hole long enough for a different comet or asteroid on a significantly different orbit to add its debris. The bits of the second would almost certainly collide violently with bits of the first. I think that the timing involved for such small objects is so tight that "it would[n't] happen often" is a bit of an understatement, on human scales. :-) Moreover, the energy densities (assuming comets, say) are small, so even being optimistic about the amount and location of inverse Compton scattering, the gamma flux at 1 AU isn't especially worrying. You're not going to destroy the Earth. (Consider a direct conversion of a pair of comets each of 6 * 10^7 kg into gammas radiated isotropically in one second; now extend the duration and reduce the amount converted, both by a lot... Then for convenience, the solid angle of the Earth from the the collision point would be about 6*10^-9 sr). > every few years I think maybe every few million years is more plausible, frankly. There'd be more to see with a young star, since those tend to get a lot more things crashing onto them. > compact black hole Is there a non-compact black hole? :-) :-) :-) ETA : extremely luminous events proposed to be tidal disruption events (and found at https://tde.space/ https://tde.space/ ) are almost wholly from jets from the "pancake flambé" star being disrupted close a supermassive black hole (whose diameter is much much larger than the disrupted star's). The UV emissions are largely absorbed or compton-scattered by the disrupted star's remains on their way out, and so mostly what's seen by us (or anyone outside a few tens of thousands of Schwarzschild radiuses) is in the IR.

garmaine 8y ago
> Is there a non-compact black hole? :-) :-) :-) Yes. The radius of a black hole increases linearly with mass. Supermassive black holes can actually have really small average densities.
- raattgift 8y ago
  Let me reply to your first and final sentence out of order. > [SMBH]s can actually have really small average densities. No kidding. Forgive me if you know all this next three paragraphs. Christodoulou & Rovelli (2014) (Phys. Rev. D 91, 064046 doi:0.1103/PhysRevD.91.064046) [C&R 2014] show the volume of a spherically symmetrical non-eternal BH grows significantly with time (eqn 1 of ). "The bulk of the volume turns out to be due to a region in the vicinity of a constant value of the radial coordinate. That is: inside the hole there is a long spacelike 3d cylinder with slowly varying radius, which grows longer with time ... For instance, the black hole [Sag Astar] has radius ~ 10^6 km and age ~ 10^9 years. Inside it there is space for ~ 10^34 km^3, enough to fit a million Solar Systems!" (emphasis theirs) [C&R 2014] provoked several other papers including Begntsson & Jakobsson (2015) (Mod. Phys. Lett. A, vol 30 no. 21, doi:10.1142/S0217732315501035) extends this to black holes with significant angular momentum, whose interior volumes are much smaller, with [C&R 2014]'s non-rotating volume being an upper limit. Their results are amplified and some implications thereof considered by Ong (2015) (J. Cosmol. Astropart. Phys 04(2015)003 doi:10.1088/1475-7516/2015/04/003), delightfully titled "Never Judge a Black Hole by Its Area", which examines several other types of BH too. In particular Ong clarifies that the [C&R 2014] results are likely robust for astrophysical near-extremals like Sag. Astar. Of course, these are studies of theoretical BHs (and mainly considered statically), but I'm pretty confident that typical stress-energy around mature astrophysical black holes won't perturb dramatically away from that. If you like, I can quickly check with an expert on astrophysical black holes for the case of a close binary where the mass flow from the donor star is very large, or for very young BHs that still have most of their progenitor star outside the horizon, but with agreement that such a system is far from what we would have when swapping our sun with a solar-mass BH. > Yes. It really depends on whose definition of compact object you take. I think the majority view -- bearing in mind this is an astrophysics classification rather than a relativity one and I am in the latter camp -- is that compact objects are characterized in one or more of three ways: (a) the presence of strong gravity in the vicinity, (b) hierarchical or direct formation from gravitationally collapsing matter, (c) the presence of matter densities much higher than that of planets. A quick survey of observational astrophysical groups (Harvard, MIT-Kavli, Cambridge-Kavli, Toronto, KTH), fails to find one that both [i] unambiguously splits supermassive black holes out from compact objects (Harvard comes close), and [ii] restricts their work to small black holes that would meet (c) in the way you want (in fact, all of the groups I quickly looked at explicitly work on SMBHs as well as NSs, white dwarfs, and stellar BHs, but perhaps my selection is unfairly biased). Astrophysicists and relativists can be surprisingly different beasts, but I think few of the former would insist on inserting "mean" into (c) and then exclude very massive BHs as not compact on that basis, especially given [C&R 2015] and subsequent results for spherically symmetrical non-rotating stellar-mass BHs. It also seems like it would be a pretty strange exclusion since SMBHs -- however you define their mean density -- certainly exhibit relevant-to-astrophysicists general-relativistic phenomena near the horizon, and would still do so even if (for a sufficiently massive BH) the curvature just outside the horizon falls below the curvature we experience here on the surface of Earth. Finally, I object if your definition of average density depends on any definition of internal volume, as that would depend on a choice of spatial section, which in turn depends on a choice of coordinates, and thus cannot be other than an observer-dependent quantity. In general, an observer can be found that makes a complete mess of one's expectation for such a quantity. (ETA: Ong makes exactly this point at the even more delightful discussion of his paper op. cit. at https://www.kth.se/profile/ycong/page/the-interior-volumes-of-black-holes https://www.kth.se/profile/ycong/page/the-interior-volumes-o... )

garmaine 8y ago

> Is there a non-compact black hole? :-) :-) :-) Yes. The radius of a black hole increases linearly with mass. Supermassive black holes can actually have really small average densities.

raattgift 8y ago
Let me reply to your first and final sentence out of order. > [SMBH]s can actually have really small average densities. No kidding. Forgive me if you know all this next three paragraphs. Christodoulou & Rovelli (2014) (Phys. Rev. D 91, 064046 doi:0.1103/PhysRevD.91.064046) [C&R 2014] show the volume of a spherically symmetrical non-eternal BH grows significantly with time (eqn 1 of ). "The bulk of the volume turns out to be due to a region in the vicinity of a constant value of the radial coordinate. That is: inside the hole there is a long spacelike 3d cylinder with slowly varying radius, which grows longer with time ... For instance, the black hole [Sag Astar] has radius ~ 10^6 km and age ~ 10^9 years. Inside it there is space for ~ 10^34 km^3, enough to fit a million Solar Systems!" (emphasis theirs) [C&R 2014] provoked several other papers including Begntsson & Jakobsson (2015) (Mod. Phys. Lett. A, vol 30 no. 21, doi:10.1142/S0217732315501035) extends this to black holes with significant angular momentum, whose interior volumes are much smaller, with [C&R 2014]'s non-rotating volume being an upper limit. Their results are amplified and some implications thereof considered by Ong (2015) (J. Cosmol. Astropart. Phys 04(2015)003 doi:10.1088/1475-7516/2015/04/003), delightfully titled "Never Judge a Black Hole by Its Area", which examines several other types of BH too. In particular Ong clarifies that the [C&R 2014] results are likely robust for astrophysical near-extremals like Sag. Astar. Of course, these are studies of theoretical BHs (and mainly considered statically), but I'm pretty confident that typical stress-energy around mature astrophysical black holes won't perturb dramatically away from that. If you like, I can quickly check with an expert on astrophysical black holes for the case of a close binary where the mass flow from the donor star is very large, or for very young BHs that still have most of their progenitor star outside the horizon, but with agreement that such a system is far from what we would have when swapping our sun with a solar-mass BH. > Yes. It really depends on whose definition of compact object you take. I think the majority view -- bearing in mind this is an astrophysics classification rather than a relativity one and I am in the latter camp -- is that compact objects are characterized in one or more of three ways: (a) the presence of strong gravity in the vicinity, (b) hierarchical or direct formation from gravitationally collapsing matter, (c) the presence of matter densities much higher than that of planets. A quick survey of observational astrophysical groups (Harvard, MIT-Kavli, Cambridge-Kavli, Toronto, KTH), fails to find one that both [i] unambiguously splits supermassive black holes out from compact objects (Harvard comes close), and [ii] restricts their work to small black holes that would meet (c) in the way you want (in fact, all of the groups I quickly looked at explicitly work on SMBHs as well as NSs, white dwarfs, and stellar BHs, but perhaps my selection is unfairly biased). Astrophysicists and relativists can be surprisingly different beasts, but I think few of the former would insist on inserting "mean" into (c) and then exclude very massive BHs as not compact on that basis, especially given [C&R 2015] and subsequent results for spherically symmetrical non-rotating stellar-mass BHs. It also seems like it would be a pretty strange exclusion since SMBHs -- however you define their mean density -- certainly exhibit relevant-to-astrophysicists general-relativistic phenomena near the horizon, and would still do so even if (for a sufficiently massive BH) the curvature just outside the horizon falls below the curvature we experience here on the surface of Earth. Finally, I object if your definition of average density depends on any definition of internal volume, as that would depend on a choice of spatial section, which in turn depends on a choice of coordinates, and thus cannot be other than an observer-dependent quantity. In general, an observer can be found that makes a complete mess of one's expectation for such a quantity. (ETA: Ong makes exactly this point at the even more delightful discussion of his paper op. cit. at https://www.kth.se/profile/ycong/page/the-interior-volumes-of-black-holes https://www.kth.se/profile/ycong/page/the-interior-volumes-o... )