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'''Black holes''' are theoretical entities popularized by pseudoscience despite their implausibility and lack of ever being directly observed.  Suggested by the controversial [[theory of relativity]] (see [[Counterexamples to Relativity]]), black holes are postulated to be collapsed objects, usually [[stars]], which have become so [[dense]] that within a certain radius their [[escape velocity]] exceeds the [[speed of light]]. Thus, they absorb all matter and energy within that radius.  Light and matter can enter, but nothing can ever escape.

'''Black holes''' are theoretical entities popularized by pseudoscience despite their implausibility and lack of ever being directly observed.  Suggested by the controversial [[theory of relativity]] (see [[Counterexamples to Relativity]]), black holes are postulated to be collapsed objects, usually [[stars]], which have become so [[dense]] that within a certain radius their [[escape velocity]] exceeds the [[speed of light]]. Thus, they absorb all matter and energy within that radius.  Light and matter can enter, but nothing can ever escape.

Black holes are increasingly favored by [[liberal]] publications, such as the science page of the [[New York Times]] and glossy magazines, as well as science fiction writers.  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 no black hole exists anywhere, and thus they fail the [[falsifiability]] requirement of science.  

Black holes are increasingly favored by [[liberal]] publications, such as the science page of the [[New York Times]] and glossy magazines, as well as science fiction writers.  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 no black hole exists anywhere, and thus they fail the [[falsifiability]] requirement of physics.  

==History of the idea==

==History of the idea==

The theoretical model of what we now call a black hole has evolved considerably over the centuries. The [[corpuscular theory of light]] held that light was made up of invisibly small particles, and that these particles moved along ballistic trajectories, like tiny bullets. In this framework, it was believed possible that a distant star could be so massive that light emitted from its surface would be dragged back down again. This theory was first advanced by John Michell, who wrote in 1783, "If the semi-diameter of a sphere of the same density as the Sun in the proportion of five hundred to one, and by supposing light to be attracted by the same force in proportion to its [mass] with other bodies, all light emitted from such a body would be made to return towards it, by its own proper gravity."<ref name="r1">http://www.aps.org/publications/apsnews/200911/physicshistory.cfm</ref>

The theoretical model of what we now call a black hole has evolved considerably over the centuries. The [[corpuscular theory of light]] held that light was made up of invisibly small particles, and that these particles moved along ballistic trajectories, like tiny bullets. In this fraemwork, it was believed possible that a distant star could be so massive that light emitted from its surface would be dragged back down again. This theory was first advanced by John Michell, who wrote in 1783, "If the semi-diameter of a sphere of the same density as the Sun in the proportion of five hundred to one, and by supposing light to be attracted by the same force in proportion to its [mass] with other bodies, all light emitted from such a body would be made to return towards it, by its own proper gravity."<ref name="r1">http://www.aps.org/publications/apsnews/200911/physicshistory.cfm</ref>

Suggesting the same possibility independently, Pierre-Simon Laplace wrote in 1796, "It is therefore possible that the greatest luminous bodies in the universe are on this account invisible."<ref name="r1" />

Suggesting the same possibility independently, Pierre-Simon Laplace wrote in 1796, "It is therefore possible that the greatest luminous bodies in the universe are on this account invisible."<ref name="r1" />

===General Relativity===

===General Relativity===

As it happened, the question became unavoidable shortly after the publication in 1915 of Einstein's [[general theory of relativity]].  [[Schwarzschild]] 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 Schwarzschild 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, Schwarzschild showed that black holes were possible under the theory of [[general relativity]].

As it happened, the question became unaviodable shortly after the publication in 1915 of Einstein's [[general theory of relativity]].  [[Schwarzschild]] 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 Schwarzschild 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, Schwarzschild 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]]

===Inside the Event Horizon===

===Inside the Event Horizon===

What actually exists inside the event horizon of a black hole is a question physics is unable to answer. Some postulate that within the event horizon exists a point of zero (or nearly zero) volume but infinite energy density, a point sometimes referred to as a ''gravitational singularity,'' after the notion of a mathematical singularity (a term equaling infinity) in a field equation. Others suspect that infinite energy density is a physical impossibility, and that a black hole contains actual finitely-dense matter compressed into a [[degenerate matter|degenerate]] form, such as quark-degenerate matter. Since all black holes are surrounded by an event horizon which prevents any information or messages from leaving, all these theories are non-[[falsifiable]]; we can never be sure what the interior structure of a black hole is like.

What actually exists inside the event horizon of a black hole is a question physics is unable to answer. Some postulate that within the event horizon exists a point of zero (or nearly zero) volume but infinite energy density, a point sometimes referred to as a ''gravitational singularity,'' after the notion of a mathematical singularity (a term equaling infinity) in a field equation. Others suspect that infinite energy density is a physical impossibility, and that a black hole contains actual finitely-dense matter compressed into a [[degenerate matter|degenerate]] form, such as quark-degererate matter. Since all black holes are surrounded by an event horizon which prevents any information or messages from leaving, all these theories are non-[[falsifiable]]; we can never be sure what the interior structure of a black hole is like.

===Properties of Black Holes===

===Properties of Black Holes===