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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.]]

'''Black holes''' are theoretical entities popularized by liberal pseudoscience enthusiasts 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 astronomical objects that are so dense that their gravitational escape velocity would exceed the speed of light. Because of this, no light or matter can escape them, and they appear "black"<ref>Actually, because of quantum-mechanical phenomena involving Hawking Radiation, discussed later in the article, this is not true on an extremely microscopic level, but is true for all practical purposes.</ref>.  In addition to being of great interest to scientists, they have caught the imagination of science magazines, science fiction writers, and the general public.  This is probably because of the catchy name and the extremely weird behavior of space and time in their vicinity.  This includes a number of movies:  ''Event Horizon'' (1997), ''The Black Hole'' (TV, 2006), and ''Interstellar'' (2014).  The last of those used very sophisticated mathematical models and computer rendering techniques to make show things accurately.

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.  

Like electrons, protons, and neutrons, black holes have never been directly observed, but, as in the case of subatomic particles, evidence for their existence is overwhelming.  This evidence includes observations of radiation from accretion disks, motion of stars near the center of the galaxy, and "gravitational lensing", having all the properties that relativity predicts, around places where there are no visible objects.  Like positrons, their existence was predicted (in both cases based on relativity) before there was any actual evidence of their existence.

 

Black holes are assumed to come into existence from extremely large stars that collapse into a state of high density when they run out of fusion fuel.  An object becomes a black hole when it lies entirely inside the Schwarzschild radius (see below) determined by its mass.  For most objects, the Schwarzschild radius is very tiny compared with its size (for Earth it is about 1 centimeter), so the object does not lie inside that radius.

 

In some very advanced and esoteric theories of physics, black holes are sometimes associated with "[[wormhole]]s",<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>, another very catchy name and very catchy subject for science fiction stories.

==History of the idea==

==History of the idea==

Contrary to popular myth, a black hole is not a cosmic vacuum cleaner. In other words, a one-solar-mass black hole is no better than any other one-solar-mass object (such as, for example, the Sun) at "sucking in" distant objects. However, a one-solar-mass black hole has the same amount of matter as any other one-solar-mass object but compressed into a much smaller space, making it impossible to move fast enough to leave a black hole once there.  If a spaceship could land on a black hole, it would never be able to take off again.

Contrary to popular myth, a black hole is not a cosmic vacuum cleaner. In other words, a one-solar-mass black hole is no better than any other one-solar-mass object (such as, for example, the Sun) at "sucking in" distant objects. However, a one-solar-mass black hole has the same amount of matter as any other one-solar-mass object but compressed into a much smaller space, making it impossible to move fast enough to leave a black hole once there.  If a spaceship could land on a black hole, it would never be able to take off again.

The term "black hole" is also used as a metaphor for a place that it is hard to get out of, generally containing a high concentration of something unpleasant.  Ex: "The inner city is a black hole of crime and drug use." Note that the Black Hole of Calcutta is ''not'' a reference to the celestial object; the name of the place predates the discovery of black holes in space and the Black Hole of Calcutta was a horrible underground prison in [[Calcutta]], India.

The term "black hole" is also used as a metaphor for a place that it is hard to get out of, generally containing a high concentration of something unpleasant.  Ex: "The inner city is a black hole of crime and drug use." The "Black Hole of Calcutta" long predates the celestial use of the term; it was a horrible underground prison in [[Calcutta]], India.

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