An image of the core of the Whirlpool galaxy M51 taken by the Wide Field Planetary Camera onboard the Hubble Space Telescope. It shows an immense ring of dust and gas which is thought to surround and hide a giant black hole, 1 million times the mass of the Sun, in the center of the galaxy. The ring forms an accretion disc of gas, about 100 light years across, falling toward the black hole. The two brighter areas perpendicular to the widest dark lane are two jets of particles accelerated by the black hole.
             Anyone who has ever watched the launch of a rocket is familiar with the concept that escape from a gravitational field requires the expenditure of energy. The stronger the gravitational field, more energy is required to escape from its clutches. If the rocket has insufficient fuel, it will return to Earth and escape is impossible. Thus, it is not hard to imagine a gravitational field strong enough to prevent the escape of any object with a finite amount of energy.
             The gravitational force of an object is governed by a combination of the amount of matter it contains and its volume. The more the matter is confined in progressively smaller volume, the larger the gravitational field at the surface of the object. Since even a light beam has a finite amount of energy, one can imagine a massive object in a sufficiently small volume that would posses a gravitational field strong enough to prevent the escape of that light. The French mathematician Simon Laplace reasoned in 1795 that, if Newton's corpuscular theory of light were correct, there could exist massive object from which light could not escape.
             Indeed, any theory of gravity should contain the notion of such an object. In the case of Einstein's theory of General Relativity, we call such an object a black hole.
             However, in the case of general relativity, the path taken by a light beam defines the geometry of space-time for it represents the "shortest distance between two po...