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I think you’re confusing the lack of knowledge of whether or not singularities really form, with whether or not black holes form. There are very clearly black h

I think you’re confusing the lack of knowledge of whether or not singularities really form, with whether or not black holes form. There are very clearly black holes, but of course we can have no way of knowing if they contain singularities. Quark stars could exist, it not with the masses observed in Sag A*. Quark degeneracy pressure isn’t going to support the mass of millions, or even billions of suns. It might be that something other than a singularity is within the event horizon, but there will still be an event horizon. Given that, we’ll never observe the interior of a black hole and be able to communicate those findings, so it’s purely academic at that point. As for time dilation... no. In no way could that magically overcome collapse.

ars 9y ago
What's a black hole without a singularity? A neutron star? Is there a distinction you are making? To me a black hole and a singularity are the same thing. Are you using a different definition? > Quark degeneracy pressure isn’t going to support the mass of millions, or even billions of suns. Why not? If the star is very large the density (and thus gravity) never get high enough. For example, if you consider the milky way as a single object, then that's an object resisting the mass of billions of suns. The mass is irrelevant, what matters is the density. > As for time dilation... no. In no way could that magically overcome collapse. It can keep mass from ever reaching the surface of the black hole (singularity). At least as far as we are concerned. It takes mass an infinite time to actually get to the surface, so the black hole can never grow.
- podiki 9y ago
  A better definition is a black hole is something with an event horizon. (for your last comment, see my previous response)
  - raattgift 9y ago
    > something with an event horizon. ... where the vast majority of that something's mass and charge are within the event horizon. To keep Birkhoff happy, and to avoid confusion with event horizons that pop up where a black hole clearly isn't, maybe add that in a small region of spacetime mass and charge inside an event horizon is a black hole if after stationarization electromagnetic and gravitational perturbations in the Schwarzschild metric are small compared to those in e.g. the Minkowski or Robertson-Walker metrics. I'm tempted to go the other direction: a physically reasonable arrangement of stress-energy can source a black-hole-like metric and that a black hole metric (i.e., an exact solution for a mathematical black hole, like Kerr-Newman) can usefully approximate such that there is a good match between the geodesics structure around a mathematical black hole and the behaviour of observed matter in the vicinity of a black hole candidate. A practical probe of the null geodesics structure around a BH candidate is surface emissions: we routinely detect them directly and through analyses of radiative efficiency for neutron stars and white dwarfs (whether or not these compact objects are accreting), and a detection of surface emission from a compact BH candidate would rule it out as a source of a BH metric. If all BH candidates have surface emissions, then BH metrics are a poor choice of modelling tool. However candidates which survive this probe and other tests of near-horizon geodesics structure might as well be called black holes, even if we have reasons to hope (or even doubt) that its mass and charge within the apparent horizon is concentrated in the singularity point. You'll note here that I'm taking a "quacks like a duck" view of astrophysical black holes; I'd go even further and want to be agnostic about event horizon vs trapping surface and so forth. Around BH-like objects, arranged by increasing ease of observation, there will be a predictable (set of) ISCO(s) below which free-falling trajectories always decay or plunge inward (or outward if retrograde); there will be very strong gravitational lensing; there will be characteristic outflows from Penrose-like mechanisms; and a characteristic efficiency in conversion of accretion matter into radiation compared to matter accreting onto non-BH objects of similar mass. These all depend on whether the exterior region near the candidate object is like that of an exact BH solution, and when you have all of the above, and no evidence to the contrary, it is pretty safe to assume there is something very similar to an event horizon dividing the near-exterior region from the non-exterior region(s).
- saagarjha 9y ago
  > For example, if you consider the milky way as a single object, then that's an object resisting the mass of billions of suns. The mass is irrelevant, what matters is the density. Uh, no. What's keeping the Milky Way from collapsing is the fact that it's rotating, not its density. Any cloud of matter that is initially non-rotating will eventually collapse due to gravity, no matter what its initial density was.
- raattgift 9y ago
  > What's a black hole without a singularity? A black hole with a lumpy interior. (It does not have to be always lumpy). Consider two black holes of the type you envisage, where the mass and charge of each is entirely at the singularity point of each black hole. Let the black holes merge. How do two singularities become one singularity?

ars 9y ago

What's a black hole without a singularity? A neutron star? Is there a distinction you are making? To me a black hole and a singularity are the same thing. Are you using a different definition? > Quark degeneracy pressure isn’t going to support the mass of millions, or even billions of suns. Why not? If the star is very large the density (and thus gravity) never get high enough. For example, if you consider the milky way as a single object, then that's an object resisting the mass of billions of suns. The mass is irrelevant, what matters is the density. > As for time dilation... no. In no way could that magically overcome collapse. It can keep mass from ever reaching the surface of the black hole (singularity). At least as far as we are concerned. It takes mass an infinite time to actually get to the surface, so the black hole can never grow.

podiki 9y ago
A better definition is a black hole is something with an event horizon. (for your last comment, see my previous response)
- raattgift 9y ago
  > something with an event horizon. ... where the vast majority of that something's mass and charge are within the event horizon. To keep Birkhoff happy, and to avoid confusion with event horizons that pop up where a black hole clearly isn't, maybe add that in a small region of spacetime mass and charge inside an event horizon is a black hole if after stationarization electromagnetic and gravitational perturbations in the Schwarzschild metric are small compared to those in e.g. the Minkowski or Robertson-Walker metrics. I'm tempted to go the other direction: a physically reasonable arrangement of stress-energy can source a black-hole-like metric and that a black hole metric (i.e., an exact solution for a mathematical black hole, like Kerr-Newman) can usefully approximate such that there is a good match between the geodesics structure around a mathematical black hole and the behaviour of observed matter in the vicinity of a black hole candidate. A practical probe of the null geodesics structure around a BH candidate is surface emissions: we routinely detect them directly and through analyses of radiative efficiency for neutron stars and white dwarfs (whether or not these compact objects are accreting), and a detection of surface emission from a compact BH candidate would rule it out as a source of a BH metric. If all BH candidates have surface emissions, then BH metrics are a poor choice of modelling tool. However candidates which survive this probe and other tests of near-horizon geodesics structure might as well be called black holes, even if we have reasons to hope (or even doubt) that its mass and charge within the apparent horizon is concentrated in the singularity point. You'll note here that I'm taking a "quacks like a duck" view of astrophysical black holes; I'd go even further and want to be agnostic about event horizon vs trapping surface and so forth. Around BH-like objects, arranged by increasing ease of observation, there will be a predictable (set of) ISCO(s) below which free-falling trajectories always decay or plunge inward (or outward if retrograde); there will be very strong gravitational lensing; there will be characteristic outflows from Penrose-like mechanisms; and a characteristic efficiency in conversion of accretion matter into radiation compared to matter accreting onto non-BH objects of similar mass. These all depend on whether the exterior region near the candidate object is like that of an exact BH solution, and when you have all of the above, and no evidence to the contrary, it is pretty safe to assume there is something very similar to an event horizon dividing the near-exterior region from the non-exterior region(s).
saagarjha 9y ago
> For example, if you consider the milky way as a single object, then that's an object resisting the mass of billions of suns. The mass is irrelevant, what matters is the density. Uh, no. What's keeping the Milky Way from collapsing is the fact that it's rotating, not its density. Any cloud of matter that is initially non-rotating will eventually collapse due to gravity, no matter what its initial density was.
raattgift 9y ago
> What's a black hole without a singularity? A black hole with a lumpy interior. (It does not have to be always lumpy). Consider two black holes of the type you envisage, where the mass and charge of each is entirely at the singularity point of each black hole. Let the black holes merge. How do two singularities become one singularity?