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I wouldn't think it could matter to an outside observer whether the mass inside an event horizon had converged to a point of infinite density or not, there just

I wouldn't think it could matter to an outside observer whether the mass inside an event horizon had converged to a point of infinite density or not, there just needs to be enough mass collapsed to an area small enough to create an event horizon. If the outside observer were to cross the event horizon to check whether there was a singularity in the middle, they'd approach the same frame of reference, no longer be affected by the time dilation, and find the singularity. /notaphysicist

monocasa 11y ago
Sure, it doesn't matter really to an outside observer, I just threw that in to hit home the time dilation aspect. You seem to be assuming a preexisting singularity. My question is how does that singularity get there if the compaction itself that would form the singularity continually pushes the goal post down the line?
- CookieMon 11y ago
  No I'm not assuming a preexisting singularity, I'm assuming (possibly incorrectly) that you needn't reach infinite density for an event horizon. i.e. a singularity doesn't need to have formed, and you couldn't tell whether one had anyway. (My thinking is that if the mass of the singularity was redistributed evenly into a small sphere in the center of the black hole, the gravitational strength around the black hole wouldn't change despite density not being infinite. However that thought experiment is newtonian thinking, not GR, so perhaps flawed) Edit: I assume "singularities" are a red herring here and you're also thinking that as the mass approaches whatever density is required for an event horizon, time slows to zero for the outside observer so no event horizon happens either, but that would leave mass on and around the smaller-or-proto-event-horizon so you get the situation from the link guard-of-terra posted "He also said that the black-hole-with-an-essentially-undetectable-object-just-outside-its-event-horizon is a very good approximation to a black hole of a slightly larger mass.". So perhaps for outside observers, black holes are just very good approximations of black holes - unless you go in for a closer look, in which case you find they are real black holes.
  - monocasa 11y ago
    No, singularities aren't a red herring here, they are very muchthe core of what I'm asking. I don't really care that much about the event horizon (at least for this specific question : ) ). Imagine two neutron stars colliding and forming a black hole. Now imagine the particle inside one of these that's closest to where the singularity's point would be upon formation. How does that singularity form given that the actual compaction that creates it also continually keeps pushing the goal post farther away due to gravitational time dilation.
    - CookieMon 11y ago
      Is there any time dilation in the same reference frame as where the singularity is forming? (I'm running on rules of thumb rather than a genuine understanding of GR: I would normally answer "no", but "frame of reference" might not be a useful concept where a singularity is forming) It is kinda neat that the only time where a singularity has formed is contained within the place where the singluarity formed. That isn't even yo dawg.
  - CookieMon 11y ago
    > (My thinking is that if the mass of the singularity was redistributed evenly into a small sphere in the center of the black hole, the gravitational strength around the black hole wouldn't change despite density not being infinite. However that thought experiment is newtonian thinking, not GR, so perhaps flawed) Update: It turns out I was describing something like the "Schwarzschild radius", confirming that in GR you don't need a singularity for a black hole - the mass just needs to be compacted into a volume smaller than a sphere of the Schwarzschild radius.
- dragonwriter 11y ago
  > My question is how does that singularity get there if the compaction itself that would form the singularity continually pushes the goal post down the line? Isn't it just in the future of every particle within the event horizon (perhaps arbitrarily far from the particle's perspective, and necessarily infinitely far from any outside observer's perspective.)

monocasa 11y ago

Sure, it doesn't matter really to an outside observer, I just threw that in to hit home the time dilation aspect. You seem to be assuming a preexisting singularity. My question is how does that singularity get there if the compaction itself that would form the singularity continually pushes the goal post down the line?

CookieMon 11y ago
No I'm not assuming a preexisting singularity, I'm assuming (possibly incorrectly) that you needn't reach infinite density for an event horizon. i.e. a singularity doesn't need to have formed, and you couldn't tell whether one had anyway. (My thinking is that if the mass of the singularity was redistributed evenly into a small sphere in the center of the black hole, the gravitational strength around the black hole wouldn't change despite density not being infinite. However that thought experiment is newtonian thinking, not GR, so perhaps flawed) Edit: I assume "singularities" are a red herring here and you're also thinking that as the mass approaches whatever density is required for an event horizon, time slows to zero for the outside observer so no event horizon happens either, but that would leave mass on and around the smaller-or-proto-event-horizon so you get the situation from the link guard-of-terra posted "He also said that the black-hole-with-an-essentially-undetectable-object-just-outside-its-event-horizon is a very good approximation to a black hole of a slightly larger mass.". So perhaps for outside observers, black holes are just very good approximations of black holes - unless you go in for a closer look, in which case you find they are real black holes.
- monocasa 11y ago
  No, singularities aren't a red herring here, they are very muchthe core of what I'm asking. I don't really care that much about the event horizon (at least for this specific question : ) ). Imagine two neutron stars colliding and forming a black hole. Now imagine the particle inside one of these that's closest to where the singularity's point would be upon formation. How does that singularity form given that the actual compaction that creates it also continually keeps pushing the goal post farther away due to gravitational time dilation.
  - CookieMon 11y ago
    Is there any time dilation in the same reference frame as where the singularity is forming? (I'm running on rules of thumb rather than a genuine understanding of GR: I would normally answer "no", but "frame of reference" might not be a useful concept where a singularity is forming) It is kinda neat that the only time where a singularity has formed is contained within the place where the singluarity formed. That isn't even yo dawg.
- CookieMon 11y ago
  > (My thinking is that if the mass of the singularity was redistributed evenly into a small sphere in the center of the black hole, the gravitational strength around the black hole wouldn't change despite density not being infinite. However that thought experiment is newtonian thinking, not GR, so perhaps flawed) Update: It turns out I was describing something like the "Schwarzschild radius", confirming that in GR you don't need a singularity for a black hole - the mass just needs to be compacted into a volume smaller than a sphere of the Schwarzschild radius.
dragonwriter 11y ago
> My question is how does that singularity get there if the compaction itself that would form the singularity continually pushes the goal post down the line? Isn't it just in the future of every particle within the event horizon (perhaps arbitrarily far from the particle's perspective, and necessarily infinitely far from any outside observer's perspective.)