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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 a density so large that its intense gravitational force requires an impossibly large velocity (greater than the speed of light) in order to escape. Thus "black" refers to the impossibility of any light ([[photon]]s) escaping, rendering the matter 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 formation in space with a density so large that its intense gravitational force requires an impossibly large velocity (greater than the speed of light) in order to escape. The "black" refers to the impossibility of any light ([[photon]]s) escaping, rendering the matter 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> |
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| − | Black holes have become extremely common topics of conversation (as well as scientific discussion, of course) over the last few decades. These discussions are common in such things as the science pages of the ''[[New York Times]]'' and other magazines, and movies including ''Event Horizon'' (1997), ''The Black Hole'' (TV, 2006), and ''Interstellar'' (2014). | + | Black holes have become popular topics of conversation (as well as scientific discussion) over the last few decades. These discussions are common in such things as the science pages of the ''[[New York Times]]'' and other magazines, and movies including ''Event Horizon'' (1997), ''The Black Hole'' (TV, 2006), and ''Interstellar'' (2014). |
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| − | Black holes have taken on an everyday meaning, as a place from which nothing emerges, as in the ''New York Times'' referring to Justice [[David Souter]]'s "chambers as a black hole, from which nothing emerges,"<ref>Linda Greenhouse, ''New York Times'' (May 16, 1991).</ref> After that disparagement by the [[liberal media]], Souter flipped from the [[conservative]] to the [[liberal]] side on [[abortion]] and was never criticized like that again by [[liberal]]s. | + | Black holes have taken on an everyday meaning, as a place from which nothing emerges, as in the ''New York Times'' referring to Justice [[David Souter]]'s "chambers as a black hole, from which nothing emerges,"<ref>Linda Greenhouse, ''New York Times'' (May 16, 1991).</ref> After that disparagement by the [[liberal media]], Souter flipped from the [[conservative]] to the [[liberal]] side on [[abortion]] and was never criticized like that again by the [[liberal media]]. |
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| | Black holes had never been '''directly''' observed.<ref>Some argue that electrons, pions, and gamma rays have never been directly observed either, but their existence is accepted.</ref> In early 2019 astronomers released an 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><ref>https://news.harvard.edu/gazette/story/2019/04/harvard-scientists-lead-team-revealing-black-hole/</ref>. This required gathering data from nine observatories around the world and combining them, using atomic clocks. Like all radio astronomy observations, extensive computer processing was used to produce a visible image. That 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.<ref>Some argue that electrons, pions, and gamma rays have never been directly observed either, but their existence is accepted.</ref> In early 2019 astronomers released an 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><ref>https://news.harvard.edu/gazette/story/2019/04/harvard-scientists-lead-team-revealing-black-hole/</ref>. This required gathering data from nine observatories around the world and combining them, using atomic clocks. Like all radio astronomy observations, extensive computer processing was used to produce a visible image. That black hole has since been informally named "Powehi".<ref>http://time.com/5569262/black-hole-hawaii-powehi/</ref> |
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| | What makes the Schwarzschild radius significant is that the behavior of gravity, under General Relativity, is determined by the Schwarzschild metric, and is (not surprisingly) nearly identical to the behavior given by Newton's law of gravity. The difference only shows up at distances at or near the Schwarzschild radius. If the Earth were compressed to a radius of one centimeter or less, the behavior of gravity would become radically different as one approached that distance. Normal world-lines would cease to be "time-like", and such world-lines would be unable to make progress from inside the Schwarzschild radius to outside of it. This is a rather paradoxical situation when viewed in the way we normally view the way the universe works. In short, no object, and no light, can actually pass from inside the radius to outside. The surface at this radius is called the '''event horizon'''. | | What makes the Schwarzschild radius significant is that the behavior of gravity, under General Relativity, is determined by the Schwarzschild metric, and is (not surprisingly) nearly identical to the behavior given by Newton's law of gravity. The difference only shows up at distances at or near the Schwarzschild radius. If the Earth were compressed to a radius of one centimeter or less, the behavior of gravity would become radically different as one approached that distance. Normal world-lines would cease to be "time-like", and such world-lines would be unable to make progress from inside the Schwarzschild radius to outside of it. This is a rather paradoxical situation when viewed in the way we normally view the way the universe works. In short, no object, and no light, can actually pass from inside the radius to outside. The surface at this radius is called the '''event horizon'''. |
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| − | The concept of the Schwarzschild radius was worked out by Karl Schwarzschild in 1916, as part of his solution (the first solution ever worked out) of Einstein's field equation for General Relativity. At first, neither Einstein nor Schwarzschild believed that something could be so dense that these objects could exist. | + | The concept of the Schwarzschild radius was developed by Karl Schwarzschild in 1916, as part of his solution (the first solution ever worked out) of Einstein's field equation for General Relativity. At first, neither Einstein nor Schwarzschild believed that something could be so dense that these objects could exist. |
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| | ==Pre-history== | | ==Pre-history== |