A black hole, long a trope of popular science fiction, is a theoretical region of space in which, as a result of its mass, the force of gravity has become so immense that nothing can escape it, even nearby light. This is why the black hole appears “black,” i.e. cannot be seen: even light which would reach us from the black hole is pulled back by gravity and cannot reach us. At the centre of a black hole is believed to be a point where gravity is so immense and matter so compressed that time and space cease to exist – a point known as a singularity.
– About Black Holes –
The general idea of the black hole is several centuries old, but it took the advent of Albert Einstein’s theory of relativity to lend these notions theoretical and scientific rigour. A black hole is believed to be created when an extremely large star collapses via supernova. The mass at the centre collapses inward under the extraordinary force of its own gravity. First, as occurs with neutron stars, this force collapses atoms. However, unlike with smaller bodies (white dwarfs and neutron stars), in a black hole there is no other subatomic force capable of matching that of gravity. As a result, the body essentially continues to fall inward forever, with matter pushed closer and closer together.
So far, there are no interstellar phenomena which all qualified astronomers agree has been confirmed to be a black hole, although there are a number of possible black holes. Extremely dense black holes, known as supermassive black holes, are believed to lie within the core of galaxies such as the Milky Way.
– Black Hole Singularities –
In the classical physical sense, of course, such a thing as an infinite collapse cannot happen: a black hole still appears to occupy a discrete unit of space. However, what is now believed, on the basis of twentieth-century physics, is that this inward collapse continues due to the curvature of spacetime. In the centre, this curvature becomes infinite at a single point – or along a single ring, if black holes rotate. Here, all of the mass of the black hole is confined at a point which is infinitely small and infinitely dense.
This point, known as a black hole singularity, contains the actual mass of the black hole. Beyond it lies the event horizon beyond which the black hole’s gravity is so powerful that nothing can escape without exceeding the speed of light – and therefore, since nothing (even light) exceeds the speed of light, nothing at all escapes. Any object which passes the event horizon will be inexorably drawn downward into the singularity, and stretched then torn apart by tidal forces. Its mass then falls into the gravitational singularity, adding to the mass of the black hole.