The event horizon is a boundary that marks the limits of a black hole. At the event horizon, the escape velocity is equivalent to the speed of light. As general relativity says that nothing can travel faster than the speed of light, nothing inside the event horizon can ever cross the border and leave ahead of it, including light. Hence, nothing that enters a black hole can get out or can be witnessed from outside the event horizon. Although black holes themselves may not transmit energy, electromagnetic radiation and matter particles may be emitted from just outside the event horizon through Hawking radiation.