The Event Horizon
According to general relativity, the path taken by a beam of light is the shortest distance between two points; such a path is called a geodesic. Furthermore, gravity warps space, bending geodesics; the stronger a gravitational field is in a certain region, the more bent the geodesics are in that region. Within a certain radius of a black hole, all geodesics are so warped that a photon of light cannot escape to another part of the Universe; essentially, there are no straight lines connecting any point that is within a certain radius of a black hole (which, in theory, has no dimension) to any point that is farther away. The spherical surface defined by this radius is termed the event horizon of the black hole because events inside the event horizon can have no effect on events outside it. Whatever is inside the event horizon is sealed off forever from the space-time of outside observers.
The event horizon thus imposes a form of censorship on the makeup of a black hole; the only properties of a black hole that can be ascertained from the outside are its mass, net charge, and rate of spin. No internal time-dependent processes can be detected in the external environment, for that would involve sending signals from inside the black hole to the outside—which is impossible, for not even light can escape. This "censorship" is what is responsible for the fewness of a black hole's measurable properties: mass, spin, and charge.
Although there are complications in defining the "size" of a black hole, due to the fact that our everyday concept of "size" assumes Euclidean three-dimensional space and such space does not exist even approximately in the near vicinity of a black hole, one can uniquely specify a black hole's circumference and thus. its radius as the circumference divided by 2PI. This value is known as the Schwarzschild radius (Rs) after German astronomer Karl Schwarzschild (1873–1916), who first defined it as Rs = 2GM/c 2 where G is the gravitational constant, M is the mass of the black hole, and c is the speed of light.
Rs, however, cannot be interpreted as the radius of a Euclidean sphere—that is, as the distance from a spherical surface (the event horizon) to its center (the black hole). As mentioned above, the geometry of space-time in the interior of the black hole is so warped that Euclidean notions of distance no longer apply. Nevertheless, Rs does provide a measure of the space around a black hole of mass M. Rs for an object having the mass of the Sun is about 3 km. Thus, in order to turn the Sun into a black hole, one would have to compress it from a sphere with a radius of 696,000 km to a sphere with a radius of just 3 km. Squeezing any mass into a volume dictated by its Schwarzschild radius presents a serious assembly problem; in fact, the only processes that might lead to the formation of a sizable black hole are the explosive death of a moderately massive star or the formation of a supermassive star by sheer accumulation. Physicists also speculate that extremely small black holes might be created by the collision of subatomic particles at high energies. In fact, they estimate that as many as 100 subatomic-size black holes may be produced in the atmosphere of the Earth every year by cosmic rays. The European Laboratory for Particle Physics (CERN, for Conseil Européen pour la Recherche Nucléaire) hopes to produce such microscopic black holes on demand in its new Large Hadron Collider, due to begin operation in 2006.
Very small black holes are predicted by theory to be short-lived, however, due to a quantum phenomenon termed "evaporation." Only large black holes are long-lived enough to have cosmic effects—to swallow millions of suns' worth of mass, to squeeze sufficient energy from the matter approaching their event horizons to outshine entire galaxies, to organize the orbits of billions of stars into well-defined galaxies, and so forth. Large black holes are thought to form primarily from exploding stars or by direct gravitational accumulation of large quantities of matter. A black hole may be produced by an exploding star (nova) as follows: An older star eventually exhausts the nuclear fuel that enables it to produce energy at its core, thus supporting its own weight (and shining steadily for many millions of years). It then begins a rapid collapse. The crushing pressure of the collapsing matter may be sufficient to form a black hole with the mass of several times that of the Sun. Such black holes would have Schwarzschild radii of several to a few tens of kilometers. Considering the amount of mass filling that space, such objects are truly tiny.