Immediately on publication of Einstein's paper on general relativity, the German astronomer Karl Schwarzschild found a mathematical solution to the new field equations, which corresponds to the gravitational field of a compact massive body, such as a star or planet, and which is now referred to as Schwarzschild's field. If the mass that serves as the source of the field is fairly diffuse, so that the gravitational field on the surface of the astronomical body is fairly weak, Schwarzschild's field will exhibit physical properties similar to those described by Newton. Gross deviations will be found if the mass is so highly concentrated that the field on the surface is strong. At the time of Schwarzschild's work, in 1916, this appeared to be a theoretical speculation; but with the discovery of pulsars and their interpretation as probable neutron stars composed of matter that has the same density as atomic nuclei (so-called nuclear matter), the possibility exists that strong fields may soon be accessible to astronomical observation.
The most conspicuous feature of the Schwarzschild field is that if the total mass is thought of as concentrated at the very centre, a point called a singularity, then at a finite distance from that centre, the Schwarzschild radius, the geometry of space-time changes drastically from that to which we are accustomed. Particles and even light rays cannot penetrate from inside the Schwarzschild radius to the outside and be detected. Conversely, to an outside observer any objects approaching the Schwarzschild radius appear to take an infinite time to penetrate toward the inside. There cannot be any effective communication between the inside and the outside, and the boundary between them is called an event horizon.
The exterior and the interior of the Schwarzschild radius are not cut off from each other entirely, however. Suppose an observer were to attach himself to a particle that is falling freely straight toward the centre and that this observer is equipped with a clock that reads its own proper time. This observer would penetrate the Schwarzschild radius within a finite proper time; moreover, he would find no abnormalities in his environment as he did so. The reason is that his clock would deviate from one permanently kept outside and at a constant distance from the centre, so grossly that the same event that seen from the outside takes forever occurs within a finite time to the free-falling observer.
Excerpt from the Encyclopedia Britannica without permission.