Black Holes

One of the most interesting phenomenon in the universe are black holes. For those of you who do not what a black hole is, it is a region of space from which nothing, not even light can escape. The existence of black holes is predicted by the General Theory of Relativity, which concludes that a sufficiently compact mass will cause space-time to form a black hole. Surrounding a black hole is a surface called the event horizon. Anything within the event horizon, including any sort of electromagnetic radiation such as light, can never escape.

Black holes are created when a star becomes so massive that it explodes into a supenova at the end of its life cycle. After a black hole is formed, it can grow by absorbing mass from its surroundings, such as other stars and even other black holes, eventually becoming what is called a super massive black hole. Some super-massive black holes have a mass that is as much as millions of solar masses. Super-massive black holes are often found in the center of galaxies. Our own Milky Way galaxy has one.

The question that arises is if no electromagnetic energy can escape a black hole, how can one be detected? It can be inferred through its interaction with other matter. Astronomers have identified many black holes in binary star systems where one of the stars is a black hole by studying the interaction with their companions.  Also, it turns out that quantum theory predicts that the surface of a black hole will radiate thermal energy.

A black hole has only three physical properties: mass, charge and angular momentum. These properties cause the black hole to be visible outside the hole. For example, a charged black hole repels other like charges.

When an object falls into a black hole, information about its shape or charge is lost to outside observers. Note: I doubt whether there are any inside observers either.

At a distance from a black hole, a particle can move in any direction, the only restriction is the speed of light. Close to a black hole space-time starts to deform. Once inside the event horizon the particle cannot escape. To a distant observer, clocks near a black hole slow down. This is called time dilation. On the other hand, an observer falling into a black hole does not experience these effects as he crosses the event horizon. According to his clock he crosses the event horizon after a finite time although he cannot determine the location of the event horizon from local observations. (There seems to be a science fiction story plot here.)      

Within a black hole there is a region of gravitational singularity, a region where space-time curvature becomes infinite. In a non-rotating black hole this region takes the shape of single point, but for a rotating black hole it is smeared out to form a ring singularity in the plane of rotation.

An observer falling into a non-rotating black hole is carried into the singularity once the observer crosses the event horizon. When they reach it, they are crushed to infinite density. However, in the case of a rotating black hole, it is possible to avoid the singularity. Hence there is the possibility of exiting the black hole into a different space-time. (This has been used to travel long distances in space in many a science-fiction story.) But, who knows where the other end of a black hole will take you. It might be an entire other universe.

There is much more to know about black holes. I have just covered the highlights.