Schwarzschild radius is defined classically – it is the radius where the escape velocity equals the speed of light. It is equal to the event horizon only for black holes that are not spinning (so-called Schwarzschild black holes.) In 1916, Karl Schwarzschild was the first person to solve Einstein’s equations of General Relativity for a non-spinning spherical massive object. His solution applies to any slowly or non-rotating spherical object, such as the Earth or the Sun. However, in the limit where the size of the object shrinks so that the escape velocity exceeds the speed of light, we call the object a black hole. The singularity represents the place where Einstein’s equations of General Relativity fail – the equations cannot be solved at this point, because the density is infinite, and the dimensional size has collapsed to a point.