2003 Evidence for Black Holes

Black holes are common objects in the universe. Each galaxy contains large num-bers—perhaps millions—of stellar-mass black holes, each the remnant of a massive star. In addition, nearly every galaxy contains a supermassive black hole at its center, with a mass ranging from millions to billions of solar masses. This review discusses the demographics of black holes, the ways in which they interact with their environment, factors that may regulate their formation and growth, and progress toward determining whether these objects really warp spacetime as predicted by the general theory of relativity. Black holes are places where gravity is so strong that nothing that enters them—not even light—can escape. Within a finite re-gion surrounding the center of a black hole, all light rays and physically realizable trajec-tories of particles are directed inward. Space and time are so distorted that there is literall