The Schwarzschild radius, or gravitational radius is the radius of a sphere that has certain properties: if all the mass of an object is compressed within this sphere, the escape speed from the surface of the sphere would equal the speed of light. An example of an object smaller than its Schwarzschild radius is a black hole. Once a stellar remnant collapses below this radius, light can no longer escape and the object is no longer visible. It is a characteristic radius associated with every quantity of mass. The "Schwarzschild radius" was named after the German astronomer Karl Schwarzschild who calculated this exact solution for the theory of general relativity in 1916.