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Btw, what I don't get about black holes: why is everyone so fixed on the event horizon and calls everything below 'a singularity'? From our point of view, of c

Btw, what I don't get about black holes: why is everyone so fixed on the event horizon and calls everything below 'a singularity'? From our point of view, of course, we don't have much use for it. But inside it might be rather interesting. For one thing, everything under the event horizon is unlikely to be homogenous. Captured matter is likely to form a much less object in the center of the hole, with a much smaller radius. Between EH and this 'core' many things can happen. For example, if matter forms N accretion disk spinning around the hole and just above the EH, why should it stop doing the same thing after it goes below? Only because we no longer can see it doesn't mean it stops existing and loses all energy and angular momentum. Or it does?

pdonis 6y ago
> why is everyone so fixed on the event horizon and calls everything below 'a singularity'? They don't. The event horizon is at r = 2M. The singularity is at r = 0. There is an interior region in between, so the singularity is not "everything below" the horizon, and I'm not aware of any reputable source that says it is. > why should it stop doing the same thing after it goes below? It's not that infalling matter will suddenly change its behavior as it crosses the horizon: it won't. It's that infalling matter has very little time once it crosses the horizon, before it hits the singularity. For example, in a 1 solar mass black hole, it's only 10 microseconds from the horizon to the singularity. Since the time is linear in the mass of the hole, that means that for, say, the 3 million solar mass hole at the center of the Milky Way, it's only 30 million microseconds, or 30 seconds, from the horizon to the singularity. And even for the most massive hole we've observed, 66 billion solar masses, it's only 660 billion microseconds, or 660,000 seconds, or less than 8 days, from the horizon to the singularity. On a cosmic scale these times are very, very short. > it doesn't mean it stops existing and loses all energy and angular momentum. Or it does? Any energy and angular momentum of matter falling into the hole gets added to the hole's mass and angular momentum.
- Andrew_nenakhov 6y ago
  > It's that infalling matter has very little time once it crosses the horizon, before it hits the singularity. For example, in a 1 solar mass black hole, it's only 10 microseconds from the horizon to the singularity. Why should it be hitting the singularity at all? Why can't particles have a stable orbit just under the event horizon? Is it because the orbit speed would exceed the speed of light? > The singularity is at r = 0. Also, why singularity at all? Neutron stars compress to a certain density and then stop compacting any further. Of course, gravity in black holes is stronger, but why people think that all matter would compact into a singularity? Can't it be a very dense object where compacting force is compensated by matter density limit? (i don't know the exact term in english, sorry)
  - pdonis 6y ago
    > Why should it be hitting the singularity at all? Because the singularity is in the future of every event inside the horizon. It's a moment of time, not a place in space. It can't be avoided because tomorrow can't be avoided. > Why can't particles have a stable orbit just under the event horizon? Is it because the orbit speed would exceed the speed of light? That's one way of looking at it, yes. > why singularity at all? Because that's what the math of GR predicts. > Neutron stars compress to a certain density and then stop compacting any further. Neutron stars have a maximum mass limit. So do white dwarfs and every other kind of stable compact object. Anything over that maximum mass limit has to collapse into a black hole. > Can't it be a very dense object where compacting force is compensated by matter density limit? Not for objects above the maximum mass limit. See above.
    - Andrew_nenakhov 6y ago
      > Neutron stars have a maximum mass limit. So do white dwarfs and every other kind of stable compact object. Anything over that maximum mass limit has to collapse into a black hole. See, this is the crux. From what I read in GR theory, once the neutron star gets over mass limit, the gravity becomes that big that light can't escape it any longer. OK. Light can't escape. Buy why would a matter suddenly work radically differently under the event horizon? Imagine an asteroid, which has an escape velocity of 1m/s, so it's inhabitants can launch pinballs to outer space with a simple catapult. This asteroid eventually collides with another one, with an escape velocity of 2m/s, so pinballs wouldn't leave the body's gravity field. Nothing really changes on the asteroid, all physics processes work in the same regular way. Why should it be that different with neutron star vs black hole? NS with ~3 solar masses has radius ~10km, and is a giant lump of neutrons packed together. To me it looks like BH should be the very similar lump of neutrons, just so heavy that light can no longer escape. So what? It should still be a spherical lump of neutrons underneath, just denser.

pdonis 6y ago

> why is everyone so fixed on the event horizon and calls everything below 'a singularity'? They don't. The event horizon is at r = 2M. The singularity is at r = 0. There is an interior region in between, so the singularity is not "everything below" the horizon, and I'm not aware of any reputable source that says it is. > why should it stop doing the same thing after it goes below? It's not that infalling matter will suddenly change its behavior as it crosses the horizon: it won't. It's that infalling matter has very little time once it crosses the horizon, before it hits the singularity. For example, in a 1 solar mass black hole, it's only 10 microseconds from the horizon to the singularity. Since the time is linear in the mass of the hole, that means that for, say, the 3 million solar mass hole at the center of the Milky Way, it's only 30 million microseconds, or 30 seconds, from the horizon to the singularity. And even for the most massive hole we've observed, 66 billion solar masses, it's only 660 billion microseconds, or 660,000 seconds, or less than 8 days, from the horizon to the singularity. On a cosmic scale these times are very, very short. > it doesn't mean it stops existing and loses all energy and angular momentum. Or it does? Any energy and angular momentum of matter falling into the hole gets added to the hole's mass and angular momentum.

Andrew_nenakhov 6y ago
> It's that infalling matter has very little time once it crosses the horizon, before it hits the singularity. For example, in a 1 solar mass black hole, it's only 10 microseconds from the horizon to the singularity. Why should it be hitting the singularity at all? Why can't particles have a stable orbit just under the event horizon? Is it because the orbit speed would exceed the speed of light? > The singularity is at r = 0. Also, why singularity at all? Neutron stars compress to a certain density and then stop compacting any further. Of course, gravity in black holes is stronger, but why people think that all matter would compact into a singularity? Can't it be a very dense object where compacting force is compensated by matter density limit? (i don't know the exact term in english, sorry)
- pdonis 6y ago
  > Why should it be hitting the singularity at all? Because the singularity is in the future of every event inside the horizon. It's a moment of time, not a place in space. It can't be avoided because tomorrow can't be avoided. > Why can't particles have a stable orbit just under the event horizon? Is it because the orbit speed would exceed the speed of light? That's one way of looking at it, yes. > why singularity at all? Because that's what the math of GR predicts. > Neutron stars compress to a certain density and then stop compacting any further. Neutron stars have a maximum mass limit. So do white dwarfs and every other kind of stable compact object. Anything over that maximum mass limit has to collapse into a black hole. > Can't it be a very dense object where compacting force is compensated by matter density limit? Not for objects above the maximum mass limit. See above.
  - Andrew_nenakhov 6y ago
    > Neutron stars have a maximum mass limit. So do white dwarfs and every other kind of stable compact object. Anything over that maximum mass limit has to collapse into a black hole. See, this is the crux. From what I read in GR theory, once the neutron star gets over mass limit, the gravity becomes that big that light can't escape it any longer. OK. Light can't escape. Buy why would a matter suddenly work radically differently under the event horizon? Imagine an asteroid, which has an escape velocity of 1m/s, so it's inhabitants can launch pinballs to outer space with a simple catapult. This asteroid eventually collides with another one, with an escape velocity of 2m/s, so pinballs wouldn't leave the body's gravity field. Nothing really changes on the asteroid, all physics processes work in the same regular way. Why should it be that different with neutron star vs black hole? NS with ~3 solar masses has radius ~10km, and is a giant lump of neutrons packed together. To me it looks like BH should be the very similar lump of neutrons, just so heavy that light can no longer escape. So what? It should still be a spherical lump of neutrons underneath, just denser.