The Schwarzschild radius is the critical radius at which the escape velocity from a massive object equals the speed of light. For a non-rotating, uncharged black hole, this radius defines the event horizon—the boundary beyond which nothing, not even light, can escape the gravitational pull.
This concept arises from Karl Schwarzschild’s solution to Einstein’s equations of general relativity in 1916. If an object’s mass is compressed within its Schwarzschild radius, it collapses into a black hole. The size of the Schwarzschild radius depends only on the mass of the object—the more massive the object, the larger the radius.
Crossing the event horizon marks the point of no return. For an outside observer, anything falling toward this boundary appears to slow down and freeze at the edge, while for the infalling object, it passes through in finite proper time.
The Schwarzschild radius is central to black hole physics and helps define one of nature’s most extreme environments: a region where space and time behave in fundamentally different ways.