And the way we've found black holes, to date, is we find these things that are actually very, very bright. And from how bright they are we know they must be very, very massive. And because we see them change in brightness very quickly we also know they must be very small. So these two things: the fact that they are very massive, because they are so bright, and that they are very small because they vary really quickly, tell us that they must be incredibly tiny and incredibly dense.
Mike Garcia
Astrophysicist
Harvard-Smithsonian Center for Astrophysics