The escape velocity is important for black holes because, since it has zero radius, you can get close enough to it that the escape velocity is the maximum speed, the speed of light. The radius at which light cannot escape from the black hole is called the event horizon . We can t see anything that happens interior to the event horizon from the outside. M is the mass of the black hole. Note that anything has an event horizon, and black holes of any size can exist.
Interesting effects occur very near black holes because the gravity is so strong.
To try to understand why these effects happen, it may be helpful to think of gravity in a different way than we have before. Newton s laws say that an object will continue in a straight line until acted upon by a force. An example of a force is the gravitational force. So for example, if the sun were not there, the earth would continue through the universe in a straight line. The sun is there, though, and the gravitational force between the earth and the sun cause the earth s motion to be deflected from its straight path.