Black Hole

A sufficiently intense gravitational field can prevent the escape not only of matter, but even of light. Such gravitational fields are produced by the bodies known as black holes.

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 the 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 eventually be reached at which its constituent subatomic particles would be unable to support their own weight and would collapse to a state of (theoretically) infinite density, producing a black hole. Black holes can (and some do) contain very large amounts of matter—millions or billions of times the mass of the Sun—but may be formed by even a small amount of matter sufficiently compressed.

The idea of black holes is not new; the French mathematician Pierre Simon Laplace (1749–1847) reasoned in 1795 that if the corpuscular theory of light proposed by English physicist Isaac Newton (1642–1727) were correct, there could exist massive objects from which light could not escape. The theory of general relativity, put forward by German physicist Albert Einstein (1679–1959) in 1915 and today basic to physicists' understanding of the Universe, also predicts the existence of black holes, though on from rather different reasoning. In recent decades, much observational evidence has been gathered to support the existence of black holes; there is no debate among astronomers today about whether black holes exist, only regarding their precise properties.