What Is a Black Hole Event Horizon?

The event horizon is not a physical surface but a mathematical boundary defined by the Schwarzschild radius. Once any object crosses the event horizon, the gravitational pull is so extreme that all possible paths lead inward toward the singularity — even paths travelling at the speed of light. From the outside, an object falling toward the event horizon appears to slow down and redden (gravitational redshift) until it fades from view. The size of the event horizon depends on the black hole mass: for a stellar-mass black hole of about 10 solar masses, the Schwarzschild radius is roughly 30 km; for the supermassive black hole M87* (6.5 billion solar masses), it spans about 19 billion km.