Comments on: Some questions about black holes http://blog.dolphinling.net/2006/05/some-questions-about-black-holes/ ...whatever goes through my head... Fri, 22 Jan 2010 11:50:20 +0000 hourly 1 http://wordpress.org/?v=3.3.1 By: mollishka http://blog.dolphinling.net/2006/05/some-questions-about-black-holes/comment-page-1/#comment-1027 mollishka Wed, 10 May 2006 23:28:49 +0000 http://blog.dolphinling.net/2006/05/some-questions-about-black-holes/#comment-1027 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. 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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