The event horizon denotes the boundary of a black hole. The size of the event horizon surrounding a black hole is called the Schwarzschild radius after the German astronomer Karl Schwarzschild (1873-1916), who studied the properties of geometric space around a singularity when warped according to general relativity theory.
The event horizon is an observational boundary on the cosmic scale because no information generated within the black hole can escape. Light is bent by gravitational fields, and within a black hole the gravity is so strong that light is actually bent back upon itself. Another explanation in accord with the wave-particle duality of light is that--inside the event horizon--the gravitational attraction of the singularity is so strong that the required escape velocity is greater than the speed of light. As a consequence, because no object can exceed the speed of light, not even light itself can escape from the region of space within the event horizon.
Processes occurring at or near the event horizon, however, are observable and offer important insights into the physics associated with black holes. An accretion disk surrounding a black hole forms as matter accelerates toward the event horizon. The accelerating matter heats and emits strong, highly energetic electromagnetic radiation, including x rays and gamma rays, that may be associated with some of the properties exhibited by quasars.
Although light and matter cannot escape a black hole by crossing the event horizon, English physicist Stephen Hawking (1942-) formed an important concept, later named Hawking radiation, that offers a possible explanation to possible matter and energy leakage from black holes. Starting with the premise that quantum theories regarding virtual particles--particle-antiparticle pairs that exist so briefly only their effects (not their masses) can be measured--are valid, Hawking surmised that radiation occurs when a virtual particle crosses the event horizon; its partner particle cannot be annihilated and thus becomes a real particle with mass and energy. With Hawking radiation, mass can thus leave the black hole in the form of new particles created just outside the event horizon.
In 2006, researchers from Harvard University and the Massachusetts Institute of Technology noted that a type of x-ray explosion seen on neutron stars was absent from black holes. The most likely explanation for this is that the gas that accretes on a neutron star's surface--eventually leading to an x-ray explosion--does not accrete on a black hole. Instead, the gas falls inside the event horizon of the black hole and disappears completely.