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Follow-up question: are black holes generally a question of density and not mass? If I could take a laptop and squeeze it hard enough to overcome various nuclea

by kqr 5y ago

Follow-up question: are black holes generally a question of density and not mass? If I could take a laptop and squeeze it hard enough to overcome various nuclear forces, would I get a black hole with an event horizon the size of a laptop's gravitational field? What would it take to get an event horizon on a human scale (a feet or two across?)

codethief 5y ago
> are black holes generally a question of density and not mass? Correct. Any mass M taking up a spherical volume of radius less than 2GM/c² (the Schwarzschild radius) will necessarily be a black hole. Black holes are thus the objects in the universe with the highest mass density and, coincidentally, the highest entropy density. > What would it take to get an event horizon on a human scale (a feet or two across?) A mass M = Lc²/2G, where L = 1ft for a black hole 2ft across.
jfengel 5y ago
Sort of. It depends on both density and mass. The more mass there is, the less density is required to make a black hole. A solar mass black hole is stupid dense. But a supermassive black hole is less dense than the earth, and can be less dense than water. That's still an insane amount of mass, but it's not really all that dense. A human scale black hole would be even denser than a solar mass black hole. It would require over 200 earth masses, though that's still a tiny fraction of a solar mass.
- kqr 5y ago
  In this comment, are you defining density as mass per volume contained by event horizon? Or do we know how the mass is distributed inside the black hole? Does it even make sense to discuss distribution of mass in a black hole? Would clues about that leak out through dynamics like rotation?
  - jfengel 5y ago
    You only need total mass and total volume. No other details leak out. A non spinning black hole is an absolutely perfect sphere, with no "hair". A spinning black hole is flattened, or maybe even a torus, but is still mathematically perfect. Unless quantum mechanics intervenes in ways nobody has yet figured out.
    - kqr 5y ago
      Total volume of... what? Contained within event horizon? Surely not singularity? (Which I would assume to be a... well, singularity.)
      - jfengel 5y ago
        Yep, event horizon. The singularity occurs at a nominal point at the center (or a ring for a rotating black hole). It has no volume, but all of the mass ends up there, causing divide by zero errors.

by kqr 5y ago

codethief 5y ago

> are black holes generally a question of density and not mass? Correct. Any mass M taking up a spherical volume of radius less than 2GM/c² (the Schwarzschild radius) will necessarily be a black hole. Black holes are thus the objects in the universe with the highest mass density and, coincidentally, the highest entropy density. > What would it take to get an event horizon on a human scale (a feet or two across?) A mass M = Lc²/2G, where L = 1ft for a black hole 2ft across.

jfengel 5y ago

Sort of. It depends on both density and mass. The more mass there is, the less density is required to make a black hole. A solar mass black hole is stupid dense. But a supermassive black hole is less dense than the earth, and can be less dense than water. That's still an insane amount of mass, but it's not really all that dense. A human scale black hole would be even denser than a solar mass black hole. It would require over 200 earth masses, though that's still a tiny fraction of a solar mass.

kqr 5y ago
In this comment, are you defining density as mass per volume contained by event horizon? Or do we know how the mass is distributed inside the black hole? Does it even make sense to discuss distribution of mass in a black hole? Would clues about that leak out through dynamics like rotation?
- jfengel 5y ago
  You only need total mass and total volume. No other details leak out. A non spinning black hole is an absolutely perfect sphere, with no "hair". A spinning black hole is flattened, or maybe even a torus, but is still mathematically perfect. Unless quantum mechanics intervenes in ways nobody has yet figured out.
  - kqr 5y ago
    Total volume of... what? Contained within event horizon? Surely not singularity? (Which I would assume to be a... well, singularity.)
    - jfengel 5y ago
      Yep, event horizon. The singularity occurs at a nominal point at the center (or a ring for a rotating black hole). It has no volume, but all of the mass ends up there, causing divide by zero errors.