As an astronomer, I get to ignore the details of the things that we don't understand. There's a lot of work that we can do on scales that we do understand, and there is actually a finite size that I can associate with a super massive black hole.
From Andrea M. Ghez
One of the big mysteries about the black hole at the center of the galaxy is, 'Why don't we see emission from matter falling onto the black hole, or, rather, the black hole eating up its surroundings?'
We have this interesting problem with black holes. What is a black hole? It is a region of space where you have mass that's confined to zero volume, which means that the density is infinitely large, which means we have no way of describing, really, what a black hole is!
The key to proving that there's a black hole is showing that there's a tremendous amount of mass in a very small volume. And you can do that with the motions of stars. The way the star moves around the center of the galaxy is very much like the way the planets orbit the sun.
Our galaxy's pretty ordinary, garden-variety. So if we believe our galaxy has a super massive black hole, that tells us that most, if not all, galaxies host such a black hole at their centers.
There's a large cluster of stars that are orbiting the center of our galaxy. And by measuring the motion of stars, and in particular, their orbits, we can figure out whether or not there's a central black hole.
The key to proving that there's a black hole is showing that there's a tremendous amount of mass in a very small volume. And you can do that with the motions of stars.
The Keck telescope, which is the largest in the world, had opened just before I began my faculty position at UCLA.
Fortunately, most things around the supermassive black hole are just going to go around it. They're going to orbit it. They don't actually get sucked in.
How do you observe something you can't see? This is the basic question of somebody who's interested in finding and studying black holes. Because black holes are objects whose pull of gravity is so intense that nothing can escape it, not even light, so you can't see it directly.
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