4 ms·

We know something which looks exactly like a singularity though does exist - i.e. whatever black holes are, we can observe matching predictions very well. So i

by XorNot 1y ago

We know something which looks exactly like a singularity though does exist - i.e. whatever black holes are, we can observe matching predictions very well. So if singularities don't exist, then some other weird object must that naively looks like one.

exe34 1y ago
No we don't. We know what a blackhole looks like at the event horizon which is entire kilometers away from the alleged singularity.
hoseja 1y ago
You're not ever actually seeing the singularity. The place of INFINITE density. You're seeing the event horizon/curved spacetime around it, at best. Those can also appear around non-singularities.
- radicalbyte 1y ago
  Unless the observable universe itself is within one.
  - hoseja 1y ago
    Well then it is not a singularity either, is it. Feels a bit like a misunderstanding is happening. "Singularity" is a well-defined mathematical concept, not just a cool synonym for "black hole".
oersted 1y ago
Do they look like singularities? As I understand it, any object that crosses a certain density threshold, to a point where light cannot escape its gravitational pull, is effectively a black-hole (even if the mathematical model for them is more purist, only described by a handful of parameters). I don't think they need to be infinitely dense to explain our observations. You could say that we do observe a singularity, not in the centre of the black-hole but in its event horizon. But technically that's just an infinity in the maths not a physical singularity, in the sense that if you were there it would just seem like normal space.
- chasil 1y ago
  I understand that black holes are described by precisely three parameters: mass, spin, and charge. I am assuming that these are the only way to describe anything within an event horizon.
  - raattgift 1y ago
    The key parameter is mass. The no-hair conjecture says that a Kerr-Newman black hole (KN BH: stationary, eternal) has only position, linear momentum, angular momentum, and electromagnetic charge. 3 components of linear momentum and three of position can be removed by keeping the KN BH at the origin of a system of coordinates. The KN BH doesn't evolve with time so we can remove two related time components as well. This leaves us with 3 components of angular momentum ("spin"), electromagnetic charge (because by definition the KN BH is immersed in an electromagnetic field), and mass. Schwarzschild BHs are a special case of KN BH where spin and electromagnetic charge vanish. But we could add other fields with charges, and make those charges more complicated thanks to interactions among the matter fields. That's not a Kerr-Newman black hole any more, though. In practice the other standard model fields don't really make much difference: the charges will tend to neutralize before gravitation is relevant, and won't build up around the BH itself. KN BHs are in that sense electromagnetically quasi-neutral. Things falling into a KN BH cause a perturbation that decays quickly away, changing the mass, and possibly spin, of the BH held at the coordinate origin. A change in electromagnetic charge will probably "reach out" and capture a charged particle in order to neutralize. Electromagnetic attraction is much stronger than gravitational attraction, while also being long range. However neutralization is not necessarily instant (the closest proton might be several light-minutes away, for example) or completely matched (oops, two protons are electromagnetically nudged into an infalling trajectory in response to the small negative charge, so when the second one arrives there'll be a small positive charge for a bit until a further electron is pulled in...). So quasi-neutral. These parameters are only about the horizon. It says nothing about whether the mass was a bunch of individual protons and electrons or a bunch of neutral hydrogen or a bunch of heavier molecules. (The same "says nothing" also exists classically without reference to particles: start with a Schwarzschild BH and drop in a uniform spherical shell of mass M or two concentric uniform spherical shells of mass M/2 each, and after some "balding" time we cannot tell whether our increased-mass Schwarzschild BH had one or two shells dropped into it). More prosaically, "no hair" is a statement about the stability of black holes in the face of small perturbations. If you throw something (a star, a big gravitational wave) into a black hole and wait a bit, does the resulting configuration still look like a black hole in the sense that the trajectories one grinds out of the Kerr-Newman metric (with adjusted mass, spin and charge parameters) accurately represent the trajectories around the new configuration? There is a lot of literature on black hole stability justifying a "yes" answer.
myrmidon 1y ago
Note that the average mass density within the event horizon of a black hole is not particularly large; for big black holes it is actually smaller than the density of main sequence stars (!!). Naively, I would have expected this to provide a good lower bound for "largest possible mass density", but its actually just lower than neutron star density for pretty much all black holes observed so far. See https://en.wikipedia.org/wiki/Schwarzschild_radius https://en.wikipedia.org/wiki/Schwarzschild_radius (=> "Schwarzschild density").
- raattgift 1y ago
  The mean density of the interior is much much much lower than you think, because the Schwarzschild interior volume is enormously larger than that of a closed Euclidean 3-ball of the Schwarzschild radius. Some details: Christodoulou & Rovelli 2014 showed that the maximal interior volume of a Schwarzschild black hole increases with time <https://arxiv.org/abs/1411.2854 https://arxiv.org/abs/1411.2854>. There is plenty of follow-on work in the literature for other varieties of black hole, and it is pretty generic that a black hole gets REALLY BIG inside as it gets older. I have previously aimed HNers at DiNunno & Matzner 2008 <https://arxiv.org/abs/0801.1734 https://arxiv.org/abs/0801.1734> (this is also good teaching material for thinking about different systems of coordinates on a Schwarzschild BH spacetime). One might get some intuition by thinking about pouring material into a (non-static) black hole. When do you saturate the black hole? Does it ever fill completely up? How does its outer boundary (the horizon) evolve as you pour more and more material in?
k__ 1y ago
As far as I know, we never observed a naked singularity. What we see of black holes isn't the singularity.

by XorNot 1y ago

exe34 1y ago

No we don't. We know what a blackhole looks like at the event horizon which is entire kilometers away from the alleged singularity.

hoseja 1y ago

You're not ever actually seeing the singularity. The place of INFINITE density. You're seeing the event horizon/curved spacetime around it, at best. Those can also appear around non-singularities.

radicalbyte 1y ago
Unless the observable universe itself is within one.
- hoseja 1y ago
  Well then it is not a singularity either, is it. Feels a bit like a misunderstanding is happening. "Singularity" is a well-defined mathematical concept, not just a cool synonym for "black hole".

oersted 1y ago

Do they look like singularities? As I understand it, any object that crosses a certain density threshold, to a point where light cannot escape its gravitational pull, is effectively a black-hole (even if the mathematical model for them is more purist, only described by a handful of parameters). I don't think they need to be infinitely dense to explain our observations. You could say that we do observe a singularity, not in the centre of the black-hole but in its event horizon. But technically that's just an infinity in the maths not a physical singularity, in the sense that if you were there it would just seem like normal space.

chasil 1y ago
I understand that black holes are described by precisely three parameters: mass, spin, and charge. I am assuming that these are the only way to describe anything within an event horizon.
- raattgift 1y ago
  The key parameter is mass. The no-hair conjecture says that a Kerr-Newman black hole (KN BH: stationary, eternal) has only position, linear momentum, angular momentum, and electromagnetic charge. 3 components of linear momentum and three of position can be removed by keeping the KN BH at the origin of a system of coordinates. The KN BH doesn't evolve with time so we can remove two related time components as well. This leaves us with 3 components of angular momentum ("spin"), electromagnetic charge (because by definition the KN BH is immersed in an electromagnetic field), and mass. Schwarzschild BHs are a special case of KN BH where spin and electromagnetic charge vanish. But we could add other fields with charges, and make those charges more complicated thanks to interactions among the matter fields. That's not a Kerr-Newman black hole any more, though. In practice the other standard model fields don't really make much difference: the charges will tend to neutralize before gravitation is relevant, and won't build up around the BH itself. KN BHs are in that sense electromagnetically quasi-neutral. Things falling into a KN BH cause a perturbation that decays quickly away, changing the mass, and possibly spin, of the BH held at the coordinate origin. A change in electromagnetic charge will probably "reach out" and capture a charged particle in order to neutralize. Electromagnetic attraction is much stronger than gravitational attraction, while also being long range. However neutralization is not necessarily instant (the closest proton might be several light-minutes away, for example) or completely matched (oops, two protons are electromagnetically nudged into an infalling trajectory in response to the small negative charge, so when the second one arrives there'll be a small positive charge for a bit until a further electron is pulled in...). So quasi-neutral. These parameters are only about the horizon. It says nothing about whether the mass was a bunch of individual protons and electrons or a bunch of neutral hydrogen or a bunch of heavier molecules. (The same "says nothing" also exists classically without reference to particles: start with a Schwarzschild BH and drop in a uniform spherical shell of mass M or two concentric uniform spherical shells of mass M/2 each, and after some "balding" time we cannot tell whether our increased-mass Schwarzschild BH had one or two shells dropped into it). More prosaically, "no hair" is a statement about the stability of black holes in the face of small perturbations. If you throw something (a star, a big gravitational wave) into a black hole and wait a bit, does the resulting configuration still look like a black hole in the sense that the trajectories one grinds out of the Kerr-Newman metric (with adjusted mass, spin and charge parameters) accurately represent the trajectories around the new configuration? There is a lot of literature on black hole stability justifying a "yes" answer.

myrmidon 1y ago

Note that the average mass density within the event horizon of a black hole is not particularly large; for big black holes it is actually smaller than the density of main sequence stars (!!). Naively, I would have expected this to provide a good lower bound for "largest possible mass density", but its actually just lower than neutron star density for pretty much all black holes observed so far. See https://en.wikipedia.org/wiki/Schwarzschild_radius https://en.wikipedia.org/wiki/Schwarzschild_radius (=> "Schwarzschild density").

raattgift 1y ago
The mean density of the interior is much much much lower than you think, because the Schwarzschild interior volume is enormously larger than that of a closed Euclidean 3-ball of the Schwarzschild radius. Some details: Christodoulou & Rovelli 2014 showed that the maximal interior volume of a Schwarzschild black hole increases with time <https://arxiv.org/abs/1411.2854 https://arxiv.org/abs/1411.2854>. There is plenty of follow-on work in the literature for other varieties of black hole, and it is pretty generic that a black hole gets REALLY BIG inside as it gets older. I have previously aimed HNers at DiNunno & Matzner 2008 <https://arxiv.org/abs/0801.1734 https://arxiv.org/abs/0801.1734> (this is also good teaching material for thinking about different systems of coordinates on a Schwarzschild BH spacetime). One might get some intuition by thinking about pouring material into a (non-static) black hole. When do you saturate the black hole? Does it ever fill completely up? How does its outer boundary (the horizon) evolve as you pour more and more material in?

k__ 1y ago

As far as I know, we never observed a naked singularity. What we see of black holes isn't the singularity.