Black hole event horizon. Conceptual computer artwork illustrating the event horizon of a black hole. This is the boundary beyond which nothing can escape from a black hole's gravitational field, because the escape velocity becomes greater than the speed of light. The theory of general relativity predicts that a clock falling into a black hole would appear (to a distant observer) to run slow (a time dilation effect), eventually seeming to freeze at the event horizon. The freezing is an illusion: a person travelling with the clock would experience the fall across the event horizon as taking a finite amount of time.