Gravitational Radius

The French Mathematician Pierre Laplace used Newton’s laws to calculate the size of a body that would stop light from escaping. Karl Schwarzschild calculated at what distance from the center of a body the escape velocity would be the speed of light. He used Einstein’s relativity theory. Remember that the force of gravity between two objects gets greater as the objects get close together. At a certain distance from a body, the gravity becomes so great that the escape velocity becomes greater than the Speed of light.
Schwarzschild calculated the relationship between this distance from a body’s center and the mass of the body. This distance is known as the Schwarzschild radius. For bodies such as planets and stars, Schwarzschild’s radius is mush smaller than the body. For example for Earth it is less than one half of an inch, and it is about one and a half miles for the sun.
Schwarzschild’s theory was that a black hole was formed if the gravitational radius of a body was larger than its actual radius. This means a body would have to be squeezed into an extremely tiny space. For example, Earth would have to be squashed to the size of a pea for it to become a Black hole.