Space & Time

Black Hole

Black Hole, an extremely dense celestial body that has been theorized to exist in the universe. The gravitational field of a black hole 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.


The black-hole concept was developed by the German astronomer Karl Schwarzschild in 1916 on the basis of physicist Albert Einstein’s general theory of relativity. The radius of the horizon of a Schwarzschild black hole depends only on the mass of the body, being 2.95 km (1.83 mi) times the mass of the body in solar units (the mass of the body divided by the mass of the Sun). If a body is electrically charged or rotating, Schwarzschild’s results are modified. An “ergosphere” forms outside the horizon, within which matter is forced to rotate with the black hole; in principle, energy can be emitted from the ergosphere.

According to general relativity, gravitation severely modifies space and time near a black hole. As the horizon is approached from outside, time slows down relative to that of distant observers, stopping completely on the horizon. Once a body has contracted within its Schwarzschild radius, it would theoretically collapse to a singularity—that is, a dimensionless object of infinite density.