You will never see the clock hit the event horizon if you are sitting outside the event horizon watching it fall in. The clock will slow as it reaches the horizon, and it will run slower and slower. However, time does not cease at the event horizon in any sense. It is just that there is no escape from it. The event horizon is a theoretical boundary in space-time geometry that marks the point at which objects can no longer escape the gravity of a massive body. An object passing this point would be destroyed by this force. Although light cannot escape from inside a black hole, anything falling into it could remain alive indefinitely by entering a new state of being: eternal unconsciousness. Black holes are very dense objects made up of many electrons orbiting around a nucleus. They have a mass greater than the value given by Einstein's formula for the amount of matter that can exist inside a spherical shell with a radius equal to the distance between two galaxies. This shows that black holes may contain more than just empty space; they may also contain particles such as electrons. In general relativity, the event horizon is the boundary of an invisible region called the "shadow" of the black hole. All objects with less mass than the black hole's mass will fall inside the event horizon when they cross its orbit.