Months after Einstein completed his general theory of relativity, the German astronomer Karl Schwarzschild used the theory to attain a thorough understanding of how space and time warp in the proximity of a perfectly spherical star. His results are now known as Schwarzschild's solution. He made the discovery that if a star's mass is compressed so that it becomes dense enough that its mass divided by its radius exceeds a certain crucial value, nothing can escape the gravity of the star, not even light. It is now known that only stars ten to fifty times the size of our sun can become this dense in their final death stage. If the sun became a black hole, it would have to have a radius of less than two miles. A teaspoonful would weigh about as much as Mt. Everest. At first these stars were referred to as dark stars or frozen stars. John Wheeler later introduced the term black holes, and the name stuck. Objects respond to the existence of black holes like they do any other star, unless they cross the event horizon, in which case nothing can escape its gravitational pull. Using a black hole, you could perform a kind of artificial time travel. Recall that where space is warped by the presence of matter, time is warped as well. Since space near a black hole is very warped, so is time. If you could lower yourself to just one inch (2.54 centimeters) above a black hole with a mass of a thousand solar masses (the mass of the sun) and stay there for a year, ten thousand years will have passed on earth. If you went back to earth you would have, in a way, achieved a kind of time travel.

Do Black Holes Really Exist?

No one has ever actually seen a black hole because they are, well, black. Nonetheless, evidence of their existence has accumulated in the last decade by indirect observation. Dust and gas from ordinary nearby stars pick up speed as they fall toward the event horizon. As they approach the speed of light, the friction from the downward spiral causes the dust and gas to heat up and glow, giving off observable light and x-rays. There is evidence that there is a black hole at the center of our galaxy about 2.5 million times the mass of our sun and other black holes may exist inside quasars with billions of solar masses.