Some questions about black holes

I enjoy physics, but I don’t actually know much, so I have a couple of questions:

  • As with any other two bodies, you can have two black holes orbiting around each other. The farther they are from each other, the slower they move, and the closer they are, the faster they move (right?). Would it be possible to have them so close their schwarzschild radii intersect (but their centers are not inside the other’s schwarzschild radius), or would they have to be moving too fast for that?
  • If I’m interpreting what I’m reading correctly, a black hole doesn’t have to be infinitely dense, but can instead just be a very large collection of normally-dense matter. Is this right? And if so,
    • Inside this object, some of the gravitational force would be pulling the other way, and thereby lessening the effect. At the very center, the total force would be zero, and choice in movement would be possible! Does this mean there’s a sort of “inner schwarzschild radius” as well, inside which nothing can escape beyond?
      • Could that possibly model our universe?
    • Is it possible to have non-spherical black holes? If you, say, had a very large toroidal collection of matter, could it be large enough that it formed a black hole in the shape of a torus that nothing could escape from inside, but things could pass through the hole?

One Response to “Some questions about black holes”

  1. mollishka Says:

    First question:
    The simple answer is, yes, it is believed that black hole mergers are possible. The idea is that the black holes lose energy via gravitational waves as they orbit one another, eventually their horizons “cross,” and *something happens.* It’s a fairly easy problem to set up, but no one has solved it yet. They know what happens as the black holes get closer and closer to one another, and what happens once they merge (the “ringdown” phase), but not actually what goes on in the middle.

    Second question:
    The whole crazy thing about a black hole is that there isn’t any “matter” in it. It’s just a singularity, an infinitely tiny point at the center of the hole with some associated mass. So wherever you are, inside or outside of the Schwartschild radius, it just looks like there is a bunch of mass at the point at the center of the black hole. But, you’re right, black holes of different mass have different “densities” when you take their volume to be given by their Schwartzchild radius. For instance, a black hole with the mass of one billion suns would have low enough density to float in a bathtub full of water. Of course, if such a bathtub existed, it would collapse under its own force of gravity and also be a black hole.

    Third question:
    The simple answer is “no.” Gravity doesn’t work that way… all directions are seen as the same, which is why the Earth, the Moon, the sun, all the stars and planets, are basically spherical. You could imagine a black hole that is spinning, say, and so slightly squished, such that the effective radius at one of the poles is less than it is at the equator, but one of the things that Stephen Hawking is famous for is showing that even in this case, the singularity (where all the mass “is,” remember) at the center of the black hole is still just a point, not a little line segment or a torus or anything like that.

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