A few months later, while serving in World War I, the German physicist Karl Schwarzschild used Einstein's equations to show that a black hole could exist. The Schwarzschild radius is the size of the event horizon of a non-rotating black hole. This radius was the measurement where the escape velocity was equal to the speed of light. If the radius of a star is smaller, then light is unable to escape, and it would be a dark star, or black hole.