Black Holes

Black holes arise from singularities in Einstein’s equations in general relativity. One consequence of that theory is that when you pack enough matter into a given volume, you get a black hole. For example, if you managed to squeeze all the mass of our sun into a region with a radius of less than a few kilometers, you would get a black hole. The escape velocity needed to leave the black hole would exceed the speed of light. Once inside, in other words, nothing can escape, not even light. That is why they are called black holes. The outer boundary, which marks the point of no return, is called the horizon of a black hole. Supermassive black holes are believed to exist at the center of many galaxies, including ours. Astronomers have found abundant evidence for the existence of black holes–not by directly seeing them, but by studying matter on the verge of falling into the abyss. Moreover the gravitational waves unleashed during their mergers have been recently measured in the LIGO experiment.