We've all seen portrayals of black holes in popular media, but since we know there actually are black holes in space, just what is a black hole?
A black hole is a region of space where the force of gravity is so strong that the escape velocity exceeds the speed of light. Nothing, not even light, can escape its pull.
Every object with mass creates its own gravitational field that exerts an attractive force on surrounding objects. At every point in space, there is an associated escape velocity, which is the speed necessary to escape that object’s gravitational field and never come back. The escape velocity at a particular point depends both on the mass of the attracting object and the distance to its center.
Take Earth, for example. Generally speaking, if you throw a baseball up in the air, Earth’s gravitational field will pull it back down. However, if you had an incredibly powerful arm (or a rocket ship) and you sent the baseball upward with a speed greater than the escape velocity at Earth’s surface (about 25,000 miles per hour), it would never come back.
According to Einstein’s theory of general relativity, massive objects exert gravitational force by warping the surrounding spacetime (the three spatial dimensions and the time dimension combined in a four-dimensional continuum). More massive objects warp spacetime more severely, exerting greater gravitational force, and this warping affects the path not only of other massive objects, such as baseballs, but also of massless photons.
What does this mean for black holes?
In vacuum, light travels at about 186,000 miles per hour. This speed is known as the “universal speed limit,” because information cannot travel faster than this limit. At a black hole’s event horizon, spacetime is so severely warped that the escape velocity exceeds this universal limit. Consequently, nothing, not even light, can resist the black hole’s gravitational pull. Everything that crosses the event horizon is lost to the rest of the universe forever.
Will we ever see a black hole? Though it sounds like the stuff of science fiction, scientists plan to image the dark silhouette of the supermassive black hole at our galaxy's center within the decade — read our February 2012 issue or subscribe to find out more about their progress.