TinWiki: Black Hole

A Black Hole is a region of space in which the gravitational field is so powerful that nothing can escape after it has past the event horizon of the Black Hole. The reason it is called a Black hole is that even light is unable to escape it. Black holes can be detected, when a black hole interacts with matter out-side of the event horizon a large amount of radiation emits from it. Black holes are currently understood by Einstein's general theory of relativity, which was formed in 1916. The idea of an object with a gravitational pull strong enough to prevent light escaping from it was first thought of in the 18th century. Einstein's theory predicts that when a large enough amount of mass is present within a sufficiently small region of space, all paths through space are warped inwards towards the center of the volume, forcing all matter and radiation to fall inward. 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. Black holes can have any mass. Since the gravitational force of a body on itself, at the surface of a body of any shape, increases in inverse proportion to its characteristic lengthscale squared (as volume-2/3 ), an object of any shape and mass that is sufficiently compressed will collapse under its own gravity and form a black hole. However, when black holes form naturally, only a few mass ranges are realistic.

There are 4 categories of black holes:

• Supermassive black holes contain millions to billion times the mass of the sun and it is believed that in the center of a black hole is located at the center of most galaxies, that includes our own milky way. Active Galactis Nuclei is thought to be from them.
  
• Intermediate-mass black holes, whose size is measured in thousands of solar masses may exist. these black holes have been proposed to be the power source of Ultra-Luminous X ray Sources.
  
• Stellar-mass black holes have masses ranging from about 1.5 - 3.0 solar masses to 15 solar masses. These black holes are generally created by the collapse of individual stars. 20+ solar mass stars may collapse to form black holes.
  
• Micro black holes, which have masses at which the effects of quantum mechanics are expected to become very important. This is usually assumed to be near the Planck mass. Alternatively, the term micro black hole or mini black hole may refer to any black hole with mass much less than that of a star. Black holes of this type have been proposed to have formed during the Big Bang (primordial black holes), but no such holes have been detected as of 2008.
  

There are two theories on how Micro black holes might be produced. They are as follows.

The Big Bang, which produced pressures far larger than that of a supernova and therefore sufficient to produce primordial black holes without needing the powerful gravity fields of collapsing large stars.

High-energy particle accelerators such a the LHC (Large Hadron Collider), if certain non-standard assumptions are all correct. However, any black holes produced in such a matter could evaporate near instantaneously, this means that there is no danger.

General relativity describes mass as changing the shape of spacetime, and the shape of spacetime as describing how matter moves through space. For objects much less dense than black holes, this results in something similar to Newton's laws of gravity: objects with mass attract each other, but it's possible to define an escape velocity which allows a test object to leave the gravitational field of any large object. For objects as dense as black holes, this stops being the case. The effort required to leave the hole becomes infinite, with no escape velocity definable.










External Links

Information on Black holes from the Hubble Team.
A site about black holes and neutron stars.
A topic on AboveTopSecret.com named "Where do black holes lead ?"