Black holea re probably the most researched and best known of the topics dealing with relativity. Their existence has been speculated for over 200 years, but has yet to be proven.

A black hole is a region of space where gravity is so strong that nothing, not even light, can escape. (According to relativity, nothing can travel faster than light, so this makes sense.)

But what would cause this strong of a gravitational force? According astronomer John Michell in 1783, it would have to be the collapse of a massive star. And today we know that these massive stars exist-neutron stars.

A neutron star is the final phase of a star more than two times as large as the sun. The sun would collapse into a white dwarf-a dense, earth-sized ball of gas that can collapse no further due to the outward pressure of electrons spinning at near the speed of light.

But in a large star, the gravitational force is so strong that the electrons are pulled into the atomic nucleus where they combine with protons to form neutrons. A neutron star is small and unchanged-only a few miles in diameter. Then one of two things can happen.

That 'strong force'-the force that holds the atomic nucleus together can abruptly stop the implosion and cause the start to burst into a supernova. But if they star is more than two times the sun's size, nothing can stop its collapse into a small, dense object that traps even light- a black hole.

How does aone picture this in the rubber sheet model? Imagine placing an object so massive on the sheet that it cause its indentation to pinch off. This would cause the rest of the sheet to angle down towards the hole, and smaller objects would roll in towards this huge 'gravitational pull'.

Anatomy of a Black Hole...

Although black holes would seem the upitome of chaos, they have a definite structure, and it is even thought that the more matter and energy they pull in, the more ordered they become. (This distintly goes against Newton's Laws of Physics, but who's going to tell the black hole that?)

The outermost edge of a black hole is the event horizon, or Schwarzschild radius. It's the point at which the star collapsed into a black hole, and its size is therefore proportional to the star's mass.

That's the only definite part of a black hole. It used to be also generally agreed that there is a singularity at the center of a black holes where the laws of physics break down, but recently, Stephen Hawking, the leading expert on black holes, has begun to suspect that this may not be true.

Although black holes have the same structure, they are not all alike. Size varies greatly among black holes, with smaller ones causing more distortion of space-time; space-time would have to wrap itself more tightly around a smaller one to close it off. (Think of tine, extremely dense object being dropped on the rubber sheet.)

The physical difference with the most impact, though, is whether it is a rotating or non-rotating black hole. A still black hole is just as was discribed: event horizon, that singularity. But a rotating black hole has two even horizons, and makes some major differences.

Inside the first event horizon, it is just like a non-rotating black hole (the rotation, by the way, is from the rotation of the star before it collapses): gravity pulling you in faster and faster. But when you hit the second event horizon, it's like the eye of a storm: There is no singularity, and space-time seems to return to normal.

There is a problem, of course-there may be anti-gravity after the second event horizon, so you would be spit right back out. To where? Perhaps an alternate universe.

...And some other important schtuff

As was mentioned earlies, black holes are not known for sure to exist. But there may be one at the center of our galaxy, or in the binary system Cygnus X-1, 6000 light years from Earth.

Besides looking for the obvious area of 'nothingness' in space, there are other clues to the presence of one. In a binary system, one looks for accretion disks, a disk of gas that swirls into the suspected black hole from a neighboring star.

There is also the gravitational red shift. Longer light waves (red colors) have less energy. Photons of light will lose more and more energy as they try to escape the steadily stronger gravity of a black hole. The wavelengths lengthens until we must use radio and infrared telescopes to detect it, and then it either becomes to long to detect or it disappears.

The newest proof of black holes may be quasars-stars that radiate enough energy like an entire galaxy. These could be 'white holes', the opposite end of a black hole.

Black holes may have another way of being detected - Hawking Radiatin. Although nothing escapes from a black hole, Steven Hawking theorized that something must, because black holes have entropy. Anything with entropy must have a temperature, and anything with a temperature must emit radiation. The empty space around a black hole, he says, contains miniscule virtual particles that have positive and negative charges. Normally they combine to annhiliate each other. Sometimes the negative one will fall into the black hole, and the positively charged one will escape.

We have, been, plenty of proof that black holes exist. We just have yet to prove that they do exist. If the do, the all important question would be;

What happens when you fall into a black hole???

Good question. And there is two ways to look at it.

From the point of view of the person falling in, not much would change. You would feel fine, time would run normally, and life would be good. That is, until you began to be stretched apart by the difference in gravity between your head and feet. All parts of you body would be pulled towards the center of gravity, so you would be taffy before you reached the event horizon.

To the person watching you, this would not be the case. Say that you were sending singals back to the observer every second. To you, the singals would be sent out every second, but the closer you got the the event horizon, the longer they would take to reach the observer. The one sent out when you are at the event horizon can (ingnoring the fact that you would be dead) would never reach them. In fact, they would never see you reach the event horizon. Time at the event horizon is so slowed down according to the observers relative time, that you would seem to hang at the brink forever.

Black holes are still an unproven enigma, but their presence could lead to the discovery of an even more elusive theory-wormholes.