Changes

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]].

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]].

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>

Evidence for the existence of black holes relies on the observations of radiation from accretion disks, the motion of stars near the center of the galaxy, and "gravitational lensing," around places where there are no visible objects.

Evidence for the existence of black holes relies on the observations of radiation from accretion disks, the motion of stars near the center of the galaxy, and "gravitational lensing," around places where there are no visible objects.

By the mid-20th century, the physics of stellar dynamics and quantum mechanics had advanced to the point that it was possible to conceive of incredibly dense objects.  Stars such as the Sun are actually "inflated" to their current size by the radiation pressure from the fusion reactions going on within them.  When stars have consumed all their fuel, that pressure stops, and the material collapses to a much smaller size.  The normal interatomic spacing, with electrons orbiting the nuclei, no longer applies.  A variety of things can occur, including the electrons combining with the protons to make a "neutron star".  Various phenomena were formulated, such as neutron stars, supernovas, pulsars, and magnetars.  Before long these objects were actually observed.  (Pulsars were observed before the theory had been worked out, and supernovas had been observed since antiquity.)

By the mid-20th century, the physics of stellar dynamics and quantum mechanics had advanced to the point that it was possible to conceive of incredibly dense objects.  Stars such as the Sun are actually "inflated" to their current size by the radiation pressure from the fusion reactions going on within them.  When stars have consumed all their fuel, that pressure stops, and the material collapses to a much smaller size.  The normal interatomic spacing, with electrons orbiting the nuclei, no longer applies.  A variety of things can occur, including the electrons combining with the protons to make a "neutron star".  Various phenomena were formulated, such as neutron stars, supernovas, pulsars, and magnetars.  Before long these objects were actually observed.  (Pulsars were observed before the theory had been worked out, and supernovas had been observed since antiquity.)

Stellar evolution and dynamics are quite complicated.  Once a star has burned up all its Hydrogen, it may, depending on its size, burn Helium, and then heavier elements, all the way up to Iron.  (Iron is the most stable nucleus; fission or fusion of Iron nuclei is not energetically favored.)  As the material becomes more compressed, it is held up by the "electron degeneracy pressure" instead of the normal radiation pressure.  The normal atomic structure, with orbiting electrons, is replaced by a collection of nuclei and unbound electrons, obeying the rules of quantum mechanics, particularly the Pauli exclusion principle.  This pressure generally stops further collapse, leading to a [[white dwarf]].  White dwarfs may continue accreting matter from nearby objects.  If the mass is over the "Chandrasekhar limit" of 1.4 solar masses, the gravitational force is too strong even for the degeneracy pressure, and the star becomes a supernova, neutron star, or black hole.  The theory of this gravitational collapse was worked out in the 1930's.

Stellar evolution and dynamics are quite complicated.  Once a star has burned up all its Hydrogen, it may, depending on its size, burn Helium, and then heavier elements, all the way up to Iron.  (Iron is the most stable nucleus; fission or fusion of Iron nuclei is not energetically favored.)  As the material becomes more compressed, it is held up by the "electron degeneracy pressure" instead of the normal radiation pressure.  The normal atomic structure, with orbiting electrons, is replaced by a collection of nuclei and unbound electrons, obeying the rules of quantum mechanics, particularly the Pauli exclusion principle.  This pressure generally stops further collapse, leading to a [[white dwarf]].  White dwarfs may continue accreting matter from nearby objects.  If the mass is over the "Chandrasekhar limit" of 1.4 solar masses, the gravitational force is too strong even for the degeneracy pressure, and the star becomes a supernova, neutron star, or black hole.  The theory of this gravitational collapse was worked out in the 1930s.

If the mass is greater than the Tolman-Oppenheimer-Volkoff limit of about 1.5 solar masses, the neutron star becomes a black hole.

If the mass is greater than the Tolman-Oppenheimer-Volkoff limit of about 1.5 solar masses, the neutron star becomes a black hole.

The properties of these objects (the "event horizon", and matter and information may only pass through the event horizon in one direction) were worked out in the 1950's.  The exact solutions to Einstein's equation were worked out in the 1960's for rotating black holes (the Kerr solution), electrically charged black holes (the Reissner-Nordström solution), or both (the Kerr-Newman solution).  Various theorems relating to thermodynamics were worked out by Stephen Hawking in the 1970's.

The properties of these objects (the "event horizon", and matter and information may only pass through the event horizon in one direction) were worked out in the 1950s.  The exact solutions to Einstein's equation were worked out in the 1960s for rotating black holes (the Kerr solution), electrically charged black holes (the Reissner-Nordström solution), or both (the Kerr-Newman solution).  Various theorems relating to thermodynamics were worked out by Stephen Hawking in the 1970s.

The term "black hole" was coined by John Wheeler in 1967.  The term caught on very quickly in both the scientific community and the public at large, helped by the catchy nature of the phrase.  Because of the remarkable nature of black holes, the concept quickly became a staple of science fiction stories, "popular" science magazines, television shows, and movies such as ''Event Horizon'' (1997), ''The Black Hole'' (TV, 2006), and ''Interstellar'' (2014).  The latter movie put a lot of effort into making the visual depiction of the black hole scientifically accurate.

The term "black hole" was coined by John Wheeler in 1967.  The term caught on very quickly in both the scientific community and the public at large, helped by the catchy nature of the phrase.  Because of the remarkable nature of black holes, the concept quickly became a staple of science fiction stories, "popular" science magazines, television shows, and movies such as ''Event Horizon'' (1997), ''The Black Hole'' (TV, 2006), and ''Interstellar'' (2014).  The latter movie put a lot of effort into making the visual depiction of the black hole scientifically accurate.

This was all theoretical at the time.  There was no evidence that a black hole actually existed until the early 1970's, when scientists began to suspect that the radio source "Cygnus X1" was an actual black hole, and the 1990's, when scientists began to suspect that there was a black hole near the radio source "Sagittarius A*" at the center of our galaxy.  (The name is pronounced with the word "star".)  Since then evidence has accumulated that enormous black holes lie at the center of many galaxies.

This was all theoretical at the time.  There was no evidence that a black hole actually existed until the early 1970s, when scientists began to suspect that the radio source "Cygnus X1" was an actual black hole, and the 1990s, when scientists began to suspect that there was a black hole near the radio source "Sagittarius A*" at the center of our galaxy.  (The name is pronounced with the word "star".)  Since then evidence has accumulated that enormous black holes lie at the center of many galaxies.

==Methods of Observation==

==Methods of Observation==

Black holes described with classical mechanics only have three intrinsic properties by which one differs from another: mass, electric charge, and angular momentum.  Mass describes the amount of matter inside the event horizon.  Angular momentum refers to whether the black hole is stationary or rotating around an axis. While the singularity of a non-rotating black hole may be an infinitely small point, the singularity of a rotating black hole would be in the shape of an infinitely thin ring.  Quantum mechanics, which postulates that information loss cannot occur in a black hole, suggests properties beyond the three suggested by classical physics.  Using this description, information must also be emitted by black holes.  This radiation, referred to as Hawking Radiation, is thermal radiation that has currently only been described mathematically.

Black holes described with classical mechanics only have three intrinsic properties by which one differs from another: mass, electric charge, and angular momentum.  Mass describes the amount of matter inside the event horizon.  Angular momentum refers to whether the black hole is stationary or rotating around an axis. While the singularity of a non-rotating black hole may be an infinitely small point, the singularity of a rotating black hole would be in the shape of an infinitely thin ring.  Quantum mechanics, which postulates that information loss cannot occur in a black hole, suggests properties beyond the three suggested by classical physics.  Using this description, information must also be emitted by black holes.  This radiation, referred to as Hawking Radiation, is thermal radiation that has currently only been described mathematically.

Although radiation from inside the event horizon has not yet been observed, there is much phenomenon that occurs outside the horizon due to the black hole.  Matter entering a spinning black hole is first swirled around by the black hole’s gravity, causing it to heat up and emit x-rays, which can be used to detect the black hole. In the supermassive black holes at the centers of galaxies, some of the matter does not fall into the black hole. Instead it is blasted into space in twin jets of hot gas perpendicular to the accretion disc, in a phenomenon known as an Active Galactic Nucleus.

Although radiation from inside the event horizon has not yet been observed, there is much phenomenon that occurs outside the horizon due to the black hole.  Matter entering a spinning black hole is first swirled around by the black hole's gravity, causing it to heat up and emit x-rays, which can be used to detect the black hole. In the supermassive black holes at the centers of galaxies, some of the matter does not fall into the black hole. Instead it is blasted into space in twin jets of hot gas perpendicular to the accretion disc, in a phenomenon known as an Active Galactic Nucleus.

==Origins of Black Holes==

==Origins of Black Holes==

Scientists have speculated that if a rotating black hole is large enough, a person could pass through the center of the ring-shaped singularity and possibly enter a wormhole. However, it would have to be a very large hole, for if it were not, the hypothetical astronaut would never survive to reach the event horizon due to tidal forces.

Scientists have speculated that if a rotating black hole is large enough, a person could pass through the center of the ring-shaped singularity and possibly enter a wormhole. However, it would have to be a very large hole, for if it were not, the hypothetical astronaut would never survive to reach the event horizon due to tidal forces.

Matter coming close to the event horizon of a small black hole undergoes a process called spaghettification, a term coined by Stephen Hawking in his book ''A Brief History of Time'' to describe extraordinarily strong tidal forces. Because the mass at the center of the black hole is so dense, the gravitational pull on the near end of an object is much greater than the pull on the object’s far end. This causes the object to be stretched out in a way resembling a piece of spaghetti, and generally torn in two.  

Matter coming close to the event horizon of a small black hole undergoes a process called spaghettification, a term coined by Stephen Hawking in his book ''A Brief History of Time'' to describe extraordinarily strong tidal forces. Because the mass at the center of the black hole is so dense, the gravitational pull on the near end of an object is much greater than the pull on the object's far end. This causes the object to be stretched out in a way resembling a piece of spaghetti, and generally torn in two.  

==White Holes==

==White Holes==