Black Holes

A sufficiently intense gravitational field can prevent the escape not only of matter, but even of light. Bodies known as black holes produce such gravitational fields.

The maximum intensity of a spherical object's gravitational field is a function both of the amount of matter it contains and of its volume. The more matter is contained in an object and the smaller its volume—in other words, the higher its density—the more intense the gravitational field at its surface will be. If Earth were compacted so that it had the same mass, but half its present radius, the force of gravity at its surface would be four times as great as it is now; if it were compacted further, a density would...