Loosely speaking, a black hole is a region of space that has so much mass concentrated in it that there is no way for a nearby object to escape its gravitational pull. Since our best theory of gravity at the moment is Einstein's general theory of relativity, we have to delve into some results of this theory to understand black holes in detail, but let's start of slow, by thinking about gravity under fairly simple circumstances.

Suppose that you are standing on the surface of a planet. You throw a rock straight up into the air. Assuming you don't throw it too hard, it will rise for a while, but eventually the acceleration due to the planet's gravity will make it start to fall down again. If you threw the rock hard enough, though, you could make it escape the planet's gravity entirely. It would keep on rising forever. The speed with which you need to throw the rock in order that it just barely escapes the planet's gravity is called the "escape velocity." As you would expect, the escape velocity depends on the mass of the planet: if the planet is extremely massive, then its gravity is very strong, and the escape velocity is high. A lighter planet would have a smaller escape velocity. The escape velocity also depends on how far you are from the planet's center: the closer you are, the higher the escape velocity. The Earth's escape velocity is 11.2 kilometers per second (about 25,000 m.p.h.), while the Moon's is only 2.4 kilometers per second (about 5300 m.p.h.).

Now imagine an object with such an enormous concentration of mass in such a small radius that its escape velocity was greater than the velocity of light. Then, since nothing can go faster than light, nothing can escape the object's gravitational field. Even a beam of light would be pulled back by gravity and would be unable to escape.

The idea of a mass concentration so dense that even light would be trapped goes all the way back to Laplace in the 18th century. Almost immediately after Einstein developed general relativity, Karl Schwarzschild discovered a mathematical solution to the equations of the theory that described such an object. It was only much later, with the work of such people as Oppenheimer, Volkoff, and Snyder in the 1930's, that people thought seriously about the possibility that such objects might actually exist in the Universe. (Yes, this is the same Oppenheimer who ran the Manhattan Project.) These researchers showed that when a sufficiently massive star runs out of fuel, it is unable to support itself against its own gravitational pull, and it should collapse into a black hole.

In general relativity, gravity is a manifestation of the curvature of spacetime. Massive objects distort space and time, so that the usual rules of geometry don't apply anymore. Near a black hole, this distortion of space is extremely severe and causes black holes to have some very strange properties. In particular, a black hole has something called an 'event horizon.' This is a spherical surface that marks the boundary of the black hole. You can pass in through the horizon, but you can't get back out. In fact, once you've crossed the horizon, you're doomed to move inexorably closer and closer to the 'singularity' at the center of the black hole.

You can think of the horizon as the place where the escape velocity equals the velocity of light. Outside of the horizon, the escape velocity is less than the speed of light, so if you fire your rockets hard enough, you can give yourself enough energy to get away. But if you find yourself inside the horizon, then no matter how powerful your rockets are, you can't escape.

The horizon has some very strange geometrical properties. To an observer who is sitting still somewhere far away from the black hole, the horizon seems to be a nice, static, unmoving spherical surface. But once you get close to the horizon, you realize that it has a very large velocity. In fact, it is moving

Answered by Sharad Singh , an ibibo Master, at 7:21 PM on September 10, 2008

A black hole is a object formed when a star dies is the most powerful thing u can think in the universe. Nothing not any object or any light can escape from it.

Answered by Anvita , an ibibo Citizen, at 3:27 PM on September 10, 2008

According to computer networking terminology, "Black holes" refer to places in the network where incoming traffic is silently discarded (or "dropped"), without informing the source that the data did not reach its intended recipient. It may be used as a slang term to describe a router that goes down (offline) and whose absence goes undetected by other routers on the network. Packets that are forwarded to that router simply get dropped and never arrive at their destination. Black holes are unable to generate destination unreachable messages, so users are left unaware that data is being lost. When examining the topology of the network, the black holes themselves are invisible, and can only be detected by monitoring the lost traffic; hence the name. The most common form of black hole is simply an IP address that specifies a host machine that isn't running or an address to which no host has been assigned. To read more, refer to the source where I took excerpts from: http://tinyurl.com/nwbhole

Answered by A.R. , an ibibo Master, at 1:58 PM on September 10, 2008

Well Shiv , A black hole is a region of space in which the gravitational field is so powerful that nothing, not even electromagnetic radiation (e.g. visible light), can escape its pull after having fallen past its event horizon. The term derives from the fact that the absorbsion of visible light renders the hole's interior invisible, and indistinguishable from the black space around it.

Despite its interior being invisible, a black hole may reveal its presence through an interaction with matter that lies in orbit outside its event horizon. For example, a black hole may be perceived by tracking the movement of a group of stars that orbit its center. Alternatively, one may observe gas (from a nearby star, for instance) that has been drawn into the black hole. The gas spirals inward, heating up to very high temperatures and emitting large amounts of radiation that can be detected from earthbound and earth-orbiting telescopes.

Such observations have resulted in the general scientific consensus that—barring a breakdown in our understanding of nature—black holes do exist in our universe.

For more details , Please visit the source site :

http://en.wikipedia.org/w iki/Black_hole

Answered by Alok Gupta , an ibibo Guru, at 1:57 PM on September 10, 2008

A black hole is a super dense object that has an intense gravitational pull. There are two parts to a black hole, a singularity and a event horizon. If you were to take a slice of a black hole right through its center

The event horizon is where the force of gravity becomes so strong that even light is pulled into the black hole. Although the event horizon is part of a black hole, it is not a tangible object. If you were to fall into a black hole, it would be impossible for you to know when you hit the event horizon. For a mathematical derivation of the radius of a event horizon see below.

The singularity is not really a tangible object either. According to the General Theory of Relativity the Singularity is a point of infinite space time curvature. This means that the force of gravity has become infinitely strong at the center of a black hole. Everything that falls into a black hole by passing the event horizon, including light, will eventually reach the singularity of a black hole. Before something reaches the singularity it is torn apart by intense gravitational forces. Even the atoms themselves are torn apart by the gravitational forces.

Formation of a Black Hole:
Imagine a star which is much more massive than our sun, and which has a mass, called the critical mass, which is large enough to cause a black hole to form. What keeps this star from collapsing onto itself and becoming a black hole? The answer is that there is an intense pressure caused by nuclear reactions within the sun. When the fuel that feeds the nuclear reactions gets used up the massive star cannot support itself anymore. It then collapses to form a black hole.

It is interesting to note that when a black hole is formed by a collapsing star it is actually impossible to watch the final steps of the formation of the black hole from a stationary external reference frame. An external reference frame is a place where one watches the formation of the black hole from far away, like an astronomer on Earth. In addition, it is impossible to see any object fall into a black hole. This is not to say that everything appears to freeze just before entering a black hole. As an object falls into a black hole it gets increasingly dimmer and dimmer from the point of view of an outside observer. By the time an object gets to the edge of a black hole, it will be completely black. This effect, called a gravitational redshift, is caused by the immense gravity near the outside of a black hole.


Cool things about Black Holes:
First of all, if you get close enough to a black hole you will see the back of your own head! This effect, called an Einstein ring, is caused by the intense gravity around a black hole. When you are near a black hole at certain distances the light that leaves from the back of your head will travel though space that is bent so much by gravity that it will enter your eyes.

Another cool thing about black holes is that they might be able to destroy information. The destruction of information is not allowed by quantum mechanics, so Hawking concludes that the usual rules of quantum mechanics cannot apply for black holes!! John Preskill of Caltech has written a paper about this that should be accessible to almost any reader. The paper is located at http://xxx.lanl.gov/abs/hep-th /9209058 and you will need the freely distributed Adobe Acrobat Reader 3.0 to read the PDF document located under other formats.
Evidence for the existence of Black Holes:
This is an interesting problem. How do you prove the existence of something that cannot be observed by definition? There are actually many methods used to see if black holes really exist in our universe. The first method is to look for objects in our universe that have a lot of mass, but are very small. For example we can prove that there exists a black hole in an astronomical object called M87. This object weighs three billion times more than

Answered by Ashish Yadav , an ibibo Wizard, at 1:47 PM on September 10, 2008

although it is still a mystery but what has been heard and read is that black hole is very powerful and has a capability of absorbing big planet even earth .A black hole is a region of space in which the gravitational field is so powerful that nothing, not even electromagnetic radiation (e.g. visible light), can escape its pull after having fallen past its event horizon. The term derives from the fact that the absorbsion of visible light renders the hole's interior invisible, and indistinguishable from the black space around it.

Despite its interior being invisible, a black hole may reveal its presence through an interaction with matter that lies in orbit outside its event horizon. For example, a black hole may be perceived by tracking the movement of a group of stars that orbit its center. Alternatively, one may observe gas (from a nearby star, for instance) that has been drawn into the black hole. The gas spirals inward, heating up to very high temperatures and emitting large amounts of radiation that can be detected from earthbound and earth-orbiting telescopes. Such observations have resulted in the general scientific consensus that—barring a breakdown in our understanding of nature—black holes do exist in our universe.

The idea of an object with gravity strong enough to prevent light from escaping was proposed in 1783 by the Reverend John Michell, an amateur British astronomer. In 1795, Pierre-Simon Laplace, a French physicist independently came to the same conclusion. Black holes, as currently understood, are described by the general theory of relativity. This theory predicts that when a large enough amount of mass is present in a sufficiently small region of space, all paths through space are warped inwards towards the center of the volume, preventing all matter and radiation within it from escaping.

While general relativity describes a black hole as a region of empty space with a pointlike singularity at the center and an event horizon at the outer edge, the description changes when the effects of quantum mechanics are taken into account. Research on this subject indicates that, rather than holding captured matter forever, black holes may slowly leak a form of thermal energy called Hawking radiation. However, the final, correct description of black holes, requiring a theory of quantum gravity, is unknown.

Answered by ani , an ibibo Advisor, at 1:10 PM on September 10, 2008