The Emperor's New Mind: Concerning Computers, Minds, and the Laws of Physics (Oxford Landmark Science)

by Roger Penrose

Perhaps the reader is familiar with the concept of escape velocity. This is the speed which an object must attain in order to escape from some massive body. Suppose that body were the earth; then the escape velocity from it would be approximately 40 000 kilometres per hour, which is about 25 000 miles per hour. A stone which is hurled from the earth’s surface (in any direction away from the ground), with a speed exceeding this value, will escape from the earth completely (assuming that we may ignore the effects of air resistance). If thrown with less than this speed, then it will fall back to the earth. (Thus, it is not true that ‘everything that goes up must come down’; an object returns only if it is thrown with less than the escape velocity!) For Jupiter, the escape velocity is 220000 kilometres per hour, i.e. about 140000 miles per hour; and for the sun it is 2 200 000 kilometres per hour, or about 1400000 miles per hour. Now suppose we imagine that the sun’s mass were concentrated in a sphere of just one quarter of its present radius, then we should obtain an escape velocity which is twice as great as its present value; if the sun were even more concentrated, say in a sphere of one-hundredth of its present radius, then the velocity would be ten times as great. We can imagine that for a sufficiently massive and concentrated body, the escape velocity could exceed even the velocity of light! When this happens, we have a black hole.9

A black hole is when the escape velocity required is fastee than light. If we refer to light as the constant of the universe no object can go faster than light