Black holes are one of the more bizarre and intriguing predictions of Einstein’s theory of gravity. Surprisingly, there is now a great deal of observational evidence that black holes do exist, both in binary star systems and at the center of most galaxies, including our own. Although we are gaining more knowledge of black holes, they still remain one of the strangest things anyone has ever heard of, and we may never know what exactly one of these things are and can do. It is impossible to manufacture black holes in a laboratory. The density of matter required is too great.
In order to make a black hole the size of a baseball, you would have to pack all the matter in and on the Earth into a volume the size of a fist. Nature can make black holes, however. Matter naturally collapses unless there is some other force to hold it up. The objects in a room are kept from collapsing by electromagnetic forces. The gas in an active star is held up by thermal pressure. However, once a star uses up its thermonuclear fuel, it starts to collapse, and if there is enough mass to overcome other, microscopic forces, it collapses into a black hole. According to
Einstein’s theory, if we could pack enough matter into a small enough volume, the thing created inside will get so deep that the matter inside can never escape. A circle of no return forms. Any matter that passes the point of no return can no longer escape to the outside world. It necessarily keeps collapsing, moving towards the center. It gets deeper and deeper until finally a hole is literally torn in the fabric of spacetime: the density of matter at the center becomes essentially infinite. Thus, what is meant by “a hole in the fabric of spacetime” is: a tiny region of space where the known laws of hysics break down.
A black hole is a region of space so tightly packed with matter, that nothing, not even light can escape. Hidden at its center is a tear in the fabric of spacetime. Stephen Hawking showed in the mid-seventies that black holes aren’t actually black. They glow in the dark. They emit radiation via microscopic processes that occur just outside the horizon. This means black holes ultimately evaporate. In reality, though, a solar mass black hole will take many times the lifetime of the Universe to evaporate. In some sense, a black hole marks a boundary to spacetime: a horizon beyond which no one can see ithout travelling through it.
This radius of no return is called the event horizon of the black hole. All the bumps and wriggles of the matter from which they were formed are smoothed out as the matter contracts, so that the final shape of the horizon is always perfectly smooth and round. This is where everything gets really weird. To a distant observer, events near the horizon appear to slow down. If you drop a clock into a black hole it appears to tick more and more slowly as it approaches the event horizon. Time actually appears to stop right at the horizon.
The clock’s motion towards the black hole also slows down and to a distant observer it takes literally forever to fall through. If you fell in the event horizon with the clock, you would be sucked into the singularity in no time. As you fall, time and space become jumbled, and you cant control your falling to the center as much as you cant help yourself falling into the future. Black holes are definitely one of the most bizarre things anyone has ever heard of. We will never totally understand everything about them. They make up only a small part of our mysterious universe, though.