 The simplest picture of a black hole is that of a system whose gravity is so strong that nothing, not even light, can escape from it. Systems of this type are already possible in the familiar Newtonian theory of gravity. The escape velocity of a body is the velocity at which an object would have to begin to travel to escape the gravitational pull of the body and continue flying out to infinity, without further acceleration. Because the escape velocity is measured from the surface of an object, it becomes higher if a body contracts and becomes more dense. (Under such contraction, the mass of the body remains the same, but its surface gets closer to its center of mass; thus the gravitational force at the surface increases.) If the object were to become sufficiently dense, the escape velocity could therefore exceed the speed of light, and light itself would be unable to escape. 