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

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

A '''black hole''' is a formation in space with near-[[infinity|infinite]] density and sufficient gravitational force that escape velocity exceeds the speed of light. Thus, "black" refers to the impossibility of any light ([[photon]]s) escaping, thereby making them completely dark.<ref>They 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 could not lie inside that radius.</ref>  

A '''black hole''' is a theoretical formation in space with near-[[infinity|infinite]] density and sufficient gravitational force that escape velocity exceeds the speed of light. Thus, "black" refers to the impossibility of any light ([[photon]]s) escaping, thereby making them completely dark.<ref>They 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 could not lie inside that radius.</ref> But reports of black holes may simply be a scientific form of [[fake news]].

Black holes are reported by reliable publications, such as the science page of the ''[[New York Times]]'', glossy magazines, and movies including ''Event Horizon'' (1997), ''The Black Hole'' (TV, 2006), and ''Interstellar'' (2014). Black holes fail the [[falsifiability]] requirement of science, because it is impossible to prove that they do not exist anywhere in the universe.  Black holes also have meaning in everyday speech, as in "his office has become a black hole!"

Black holes are increasingly promoted by [[liberal]] publications, such as the science page of the ''[[New York Times]]'', glossy magazines, and movies including ''Event Horizon'' (1997), ''The Black Hole'' (TV, 2006), and ''Interstellar'' (2014). Black holes fail the [[falsifiability]] requirement of science, because it is impossible to prove that they do not exist anywhere in the universe.  Black holes also have meaning in everyday speech, as in "his office has become a black hole!"

Black holes had never been directly observed, and there is no direct evidence for their existence. In early 2019 astronomers released a photo-shopped image of a super-massive black hole located in the galaxy Messier 87 - some 55 million light-years away<ref>https://www.washingtonpost.com/science/2019/04/10/see-black-hole-first-time-images-event-horizon-telescope/</ref>. That alleged black hole has since been informally named "Powehi".<ref>http://time.com/5569262/black-hole-hawaii-powehi/</ref>

Black holes had never been directly observed, and there is no direct evidence for their existence. In early 2019 astronomers released a photo-shopped image of a super-massive black hole located in the galaxy Messier 87 - some 55 million light-years away<ref>https://www.washingtonpost.com/science/2019/04/10/see-black-hole-first-time-images-event-horizon-telescope/</ref>. That alleged black hole has since been informally named "Powehi".<ref>http://time.com/5569262/black-hole-hawaii-powehi/</ref>

Black holes are conceptually associated with "[[wormhole]]s",<ref name="nytimes.com">The prediction of the existence of wormholes, and its naming in 1957, predates the prediction and naming (1967) of a black hole.[https://www.nytimes.com/2008/04/14/science/14wheeler.html?pagewanted=print]</ref>  which never caught on with the public, perhaps due to its unattractive terminology.

Black holes are conceptually associated with "[[wormhole]]s",<ref name="nytimes.com">The prediction of the existence of wormholes, and its naming in 1957, predates the prediction and naming (1967) of a black hole.[https://www.nytimes.com/2008/04/14/science/14wheeler.html?pagewanted=print]</ref>  which never caught on with the public, perhaps due to its unattractive terminology.

==Mathematical description==

==Mathematical description==

A '''black hole''' is any object that lies entirely inside its own Schwarzschild radius, given by

A '''black hole''' is any object that lies entirely inside its own Schwarzschild radius, given by