Black Holes Black Hole, a theoretical concept in the study of gravitation. The gravitational field of a black hole, which is an extremely dense body, is so strong that, if the body is large enough, nothing, including electromagnetic radiation, can escape from its vicinity. The body is surrounded by a spherical boundary, called a horizon, through which light can enter but not escape; it therefore appears totally black. Properties The black-hole concept was developed by the German astronomer Karl Schwarzschild in 1916 on the basis of Albert Einstein's theory of general relativity. The radius of the horizon of a