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The event horizon (also termed the Schwarzschild radius; see the right panel) is the point outside the black hole where the gravitational attraction becomes so strong that the escape velocity (the velocity at which an object would have to go to escape the gravitational field) equals the speed of light. Since according to the relativity theory no object can exceed the speed of light, that means that nothing, not even light, could escape the black hole once it is inside this distance from the center of the black hole. The preceding table lists some escape velocities and the corresponding Schwarzschild radius. For example, the Earth presently has an escape velocity of 11.3 km/s, but if we could supply enough pressure to shrink the Earth to 9 millimeters, it would collapse to a black hole with an escape velocity greater than the speed of light.
A more fundamental way of viewing this is that in a black hole the gravitational field is so intense that it bends space and time around itself so that inside the event horizon there are literally no paths in space and time that lead to the outside of the black hole: No matter what direction you went, you would find that your path led back to the center of the black hole. We shall discuss such paths in space and time further a little later.