## Black Hole

One fascinating consequence of General Relativity is the fact that some objects have such intense gravity that nothing can escape them — not even light! These extraordinary occurrences are called black holes. The extreme warping of spacetime around black holes leads to some bizarre phenomena.

## Escape Velocity

Escape velocity is a measure of the speed necessary for a projectile to escape the gravitational pull of an object. For example, the earth’s escape velocity is roughly 11 kilometers per second or about 7 miles per second; therefore (discounting air resistance) an object traveling at that speed from the surface of the earth would be able to escape the earth’s gravity and never fall back again.

## Event Horizon

If you come too close to a black hole, there comes a point when it is physically impossible to return. The imaginary sphere around a black hole that marks this boundary is called the event horizon. Nothing that crosses the event horizon can ever return, including light; therefore, the event horizon marks the region of space that we perceive as black around a black hole.

## General Relativity

Albert Einstein’s theory of General Relativity (GR) replaced Isaac Newton’s theory of Classical Gravity as the most accurate description of gravitation. Specifically, General Gravity transforms the concept of gravity from a force between objects to a property of spacetime. Black holes are one of the many fascinating consequences of General Relativity. (See also Special Relativity.)

The Schwarzschild radius for an object is, simply put, the radius the object would have to be squeezed into to form a black hole. This can be calculated by determining the radius at which the escape velocity exceeds the speed of light. The formula for the Schwarzschild radius is $r=2GM/c^2$. It is named after physicist Karl Schwarzschild.