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[[Image:Iu8969hu.jpg|right|thumb|300px|Artist's conception of a binary system consisting of a black hole and a main sequence star.  The black hole is drawing matter from the main sequence star via an [[accretion disk]] around it, and some of this matter forms a gas jet.]]

[[Image:Iu8969hu.jpg|right|thumb|300px|Artist's conception of a binary system consisting of a black hole and a main sequence star.  The black hole is drawing matter from the main sequence star via an [[accretion disk]] around it, and some of this matter forms a gas jet.]]

A '''black hole''' is a theoretical prediction of the [[theory of relativity]].  As with the related theoretical concept of a "[[wormhole]]",<ref>The prediction of the existence of wormholes, and its naming in 1957, predates the prediction and naming (1967) of a black hole.[http://www.nytimes.com/2008/04/14/science/14wheeler.html?pagewanted=print]</ref> it is impossible to prove that a black hole does not exist, and thus it fails the [[falsifiability]] requirement of science.  Nevertheless, the theoretical black hole has become an increasingly popular topic for magazines, the science page of the [[New York Times]], and science fiction.

A '''black hole''' is a theoretical prediction of the [[theory of relativity]], a region of [[spacetime]] into which light and other matter can enter, but out of which nothing can ever emerge.  As with the related theoretical concept of a "[[wormhole]]",<ref>The prediction of the existence of wormholes, and its naming in 1957, predates the prediction and naming (1967) of a black hole.[http://www.nytimes.com/2008/04/14/science/14wheeler.html?pagewanted=print]</ref> it is impossible to prove that a black hole does not exist, and thus it fails the [[falsifiability]] requirement of science.  Nevertheless, black holes have become an increasingly popular topic for magazines, the science page of the [[New York Times]], and science fiction.

 

Support and interest in black holes has grown over time.  [[Albert Einstein]] and [[Arthur Eddington]], the two most prominent scientists associated with the [[theory of relativity]], rejected the suggestion that black holes exist.<ref>http://amazing-space.stsci.edu/resources/explorations/blackholes/lesson/whatisit/history.html</ref>  [[Karl Schwarzschild]], whose name is given to the radius of black holes, developed the theory for them but likewise rejected the possibility that they physically exist.  Yet today black holes are frequently depicted on the covers of science magazines.

==Nature of a Black Hole==

==Nature of a Black Hole==

===General Relativity===

===General Relativity===

As it happened, the question became unavoidable shortly after the publication in 1915 of Einstein's [[general theory of relativity]].  [[Schwartzchild]] solved the Einstein field equations in a way that describes the geometry of spacetime outside a spherically symmetric, uncharged, non-rotating distribution of mass. Well away from the center of this distribution of mass, the Schwarzchild solution closely matches the Newtonian model of a gravitational field; only close to the mass, where the curvature of spacetime is large, do significant differences between the two models appear. But if the diameter of the mass distribution is taken to be arbitrarily small, then the region of spacetime immediately surrounding the mass appears to take on extremely curious properties, properties so curious that many questioned whether they had any physical interpretation at all.

As it happened, the question became unavoidable shortly after the publication in 1915 of Einstein's [[general theory of relativity]].  [[Schwartzchild]] solved the Einstein field equations in a way that describes the geometry of spacetime outside a spherically symmetric, uncharged, non-rotating distribution of mass. Well away from the center of this distribution of mass, the Schwarzchild solution closely matches the Newtonian model of a gravitational field; only close to the mass, where the curvature of spacetime is large, do significant differences between the two models appear. But if the diameter of the mass distribution is taken to be arbitrarily small, then the region of spacetime immediately surrounding the mass appears to take on extremely curious properties, properties so curious that many questioned whether they had any physical interpretation at all.  Therefore, Schwartzchild showed that black holes were possible under the theory of [[general relativity]].

[[File:Cygnusx1.jpg|right|200px|thumb|Jet-powered nebula formed from the accretion disk of the binary star Cygnus-X1]]

[[File:Cygnusx1.jpg|right|200px|thumb|Jet-powered nebula formed from the accretion disk of the binary star Cygnus-X1]]

This showed that black holes were possible under the theory of relativity. [[Albert Einstein]], who did not believe that black holes existed, tried to re-work the whole theory to eliminate the need for these singularities. However, Roger Penrose and Stephen Hawking proved the first of many Singularity Theorems, which states that singularities must form under certain conditions. This demonstrated that, rather than mathematical oddities, singularities are a fairly generic feature of realistic solutions to relativity. According to relativity, any mass with radius less than its Schwarzschild Radius is a black hole.

However, neither Schwartzchild himself nor [[Albert Einstein]], who developed the theory of relativity, believed that black holes actually existed<ref>http://amazing-space.stsci.edu/resources/explorations/blackholes/lesson/whatisit/history.html</ref>.  Einstein even tried to re-work general relativity to render these singularities impossible. However, Roger Penrose and Stephen Hawking proved the first of many Singularity Theorems, which states that singularities must form if certain conditions are present. This demonstrated that, rather than mathematical oddities, singularities are a fairly generic consequence of realistic solutions to relativity:  any mass with radius less than its Schwarzschild Radius is a black hole..  Since then, support for black holes among the scientific community has grown.

==Nature of a Black Hole==

==Nature of a Black Hole==