The event horizon is a critical surface surrounding a black hole that marks the point of no return. Once any matter or radiation crosses this invisible boundary, it is forever trapped—unable to escape due to the immense gravitational pull of the black hole. Even light, which travels at the fastest speed in the universe, cannot escape beyond this limit.
To an outside observer, anything approaching the event horizon appears to slow down and fade, never quite crossing the boundary. However, for the object falling in, crossing the event horizon happens in a finite amount of time, and it continues its journey inward toward the singularity.
The event horizon’s radius is proportional to the mass of the black hole and is known as the Schwarzschild radius for non-rotating black holes. This region is essential in understanding black hole thermodynamics, information paradoxes, and the limits of general relativity. It essentially separates the observable universe from the unknowable interior of a black hole.