The fact that light is bent by a gravitational field brings up the following thought experiment. Imagine adding mass to a body. As the mass increases, so does the gravitational pull and objects require more energy to reach escape velocity. When the mass is sufficiently high enough that the velocity needed to escape is greater than the speed of light we say that a black hole has been created.

Another way of defining a black hole is that for a given mass, there is a radius where if all the mass is compressed within this radius the curvature of spacetime becomes infinite and the object is surrounded by an event horizon. This radius is called the Schwarzschild radius and varies with the mass of the object (large mass objects have large Schwarzschild radii, small mass objects have small Schwarzschild radii).

Schwarzschild radius is the radius below which the gravitational attraction between the particles of a body must cause it to undergo irreversible gravitational collapse. This phenomenon is thought to be the final fate of the more massive stars.

The gravitational radius (R) of an object of mass M is given by the following formula, in which ‘G’ is the universal gravitational constant and ‘c’ is the speed of light: R = 2GM/c2 . For a mass as small as a human being, the gravitational radius is in the order of 10-23 cm, much smaller than the nucleus of an atom; for a typical star such as the Sun, it is about 3 km (2 miles).

The Schwarzschild radius marks the point where the event horizon forms, below this radius no light escapes. The visual image of a black hole is one of a dark spot in space with no radiation being emitted. Any radiation falling on the black hole is not reflected but rather absorbed, and starlight from behind the black hole is lensed.

A black hole can come in any size. Stellar mass black holes are thought to form from supernova events, and have radii of 5 km. Galactic black hole in the cores of most galaxies, millions of solar masses and the radius of a solar system, are built up over time by cannibalizing stars. Mini black holes formed in the early Universe (due to tremendous pressures) down to masses of asteroids with radii the size of a grain of sand.