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{{quote|''Consideration of black holes suggests, not only that God does play dice, but that He sometimes confuses us by throwing them where they can't be seen.''|'''[[Stephen Hawking]]''', ''The Nature of Space and Time''}}
{{quote|''Inside the event horizon of a black hole, there ''is'' no way out. There are no directions of space that point away from the singularity. Due to the [[Alien Geometries|Lovecraftian curvature]] of spacetime within the event horizon, all the trajectories that ''would'' carry you away from the black hole now point into the past.''
''In fact, this is the definition of the event horizon. It's the boundary separating points in space where there are trajectories that point away from the black hole from points in space where there ''are'' none.''
''Your magical infinitely-accelerating engine is of no use to you...because you cannot find a direction in which to point it. The singularity is all around you, in every direction you look.''
▲''In fact, this is the definition of the event horizon. It's the boundary separating points in space where there are trajectories that point away from the black hole from points in space where there ''are'' none.''<br />
▲''Your magical infinitely-accelerating engine is of no use to you...because you cannot find a direction in which to point it. The singularity is all around you, in every direction you look.''<br />
''And it is getting closer.''|RobotRollCall, [http://www.reddit.com/r/askscience/comments/f1lgu/what_would_happen_if_the_event_horizons_of_two/ here].}}
A black hole is, quite literally, a [[Swirly Energy Thingy]] (okay, rotation is technically optional, but most natural black holes probably do spin). A point of space so massive that even objects going at the speed of light (for example: ''light itself'') cannot escape its gravity (thus its name). This phenomenon has fascinated scientists and writers of fiction for many, many years.
Black holes are collapsed stars, but not many people know ''why'' the stars have collapsed in such a way to create black holes
Stars convert hydrogen into helium via fusion, which produces enormous amounts of energy. This energy keeps the core from collapsing. However, when the star ages, the hydrogen runs out. Stars of sufficient mass can fuse helium into heavier elements (turning into red giants), but even that can't go on forever; producing elements heavier than iron uses energy instead of producing it. So at some point the star runs out of fuel and collapses.
Lighter stars become a degenerate-matter white dwarf which slowly cools over trillions of years into a black dwarf<ref>Not to be confused with "brown dwarfs", which are substellar bodies that were never massive enough to sustain fusion to begin with.</ref>. Stars with more than 1.4 times the mass of the sun have exceeded the "Chandrasekhar limit" and gravity combines electrons and protons to form neutrons, resulting in a neutron star. Stars whose mass exceeds the Tolman-Oppenheimer-Volkoff limit (about two to three solar masses, and definitely no more than five, but it's still unclear) are so massive that even the neutrons can't resist further collapse; it can be assumed that the star collapses down to the event horizon, and past it to a singularity (a single point, or a ring for a rotating black hole).
Black holes can form from masses smaller than stars if the mass is under enough pressure, producing a "micro black hole", but this may require exotic physical conditions such as the ones existing right after the Big Bang.
Black holes are strange things. Besides the singularity at the center, there is the event horizon, the point of no return, that once you cross it...[[Department of Redundancy Department|you can't return]]. Once inside the event horizon, you literally cannot go back
▲Black holes are strange things. Besides the singularity at the center, there is the event horizon, the point of no return, that once you cross it...[[Department of Redundancy Department|you can't return]]. Once inside the event horizon, you literally cannot go back: spacetime is curved in such a way by the black hole's mass that any path you take leads to the same place: the singularity. Rotating black holes also have ergosphere: a region near event horizon, where space-time spin around black holes faster then light.
In fact, space-time will become quite freaky around the event horizon: the closer you get to the event horizon, the [[Year Inside, Hour Outside|slower time becomes]] (due to relativity, however, you won't notice it). In fact, if an observer outside the event horizon could see you, they would see as you get closer and closer (and get redder, due to gravitation red shift, while everything you see would be bluer), you would go slower and slower until you hit the edge of the event horizon at which point you would ''stop'' (nobody would actually see you hit the event horizon, since you appear to slow down as you get closer, for an outside observer, you would take an infinite amount of time to reach it. You wouldn't actually stop, that's just what they'll see). This prediction, however, assumes zero mass of incoming object and neglect quantum effects, so reality may be more tricky.
Of course, nobody knows what
However, you'd probably be long dead before that anyway as black holes come with some dangers attached due to the extremely intense gravity around them: First, you'll be spaghettified (this ''is'' the scientific term for it); the tidal forces of the black hole are so strong that, if you were going in feet first, your feet would feel a stronger attraction than your head and thus your body would stretch out (incidentally, this occurs in more applicable situations, such as returning space shuttles, as well - the difference is that the attraction difference is so minor that the astronauts do not stretch a measurable amount). The gravity exerted by black holes is so strong that it can even deform atoms. On the upside, the bigger a black hole is, the less drastic this effect becomes on its edge; in fact, for a supermassive black hole, an individual should survive at least past the event horizon. The second big danger is good old radiation, due to gravitational blueshifting. Any radiation hitting you from the outside would be blueshifted (given higher frequencies, and therefore energy, as opposed to redshifting, which decreases the frequency of electromagnetic radiation and therefore their energy) and thus a lot more dangerous, to the point that, [http://jila.colorado.edu/~ajsh/insidebh/realistic.html according to some simulations], it would be the thing that would kill you before you could reach the singularity, assuming a black hole big enough to neglect tidal effects. It's known as [http://discovermagazine.com/2011/jun/26-strange-physics-singular-views-inside-black-holes/article_view?b_start:int=2&-C= ''inflationary instability''] and, according to scientists, [[There Is No Kill Like Overkill|its effects would go very far beyond of just vaporizing your body.]]
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To learn more cool facts about black holes, please read [[Irregular Webcomic|David Morgan-Mar's]] [[The Rant|rant]] [http://www.irregularwebcomic.net/2175.html here]. Or [http://www.reddit.com/r/askscience/comments/f1lgu/what_would_happen_if_the_event_horizons_of_two/ this] science question on Reddit. Seriously, they're awesome.
Another useful note is that black holes are one of predictions derived from Einstein's theory of general [[
This means that other theories of relativity and gravity may or (more probably) may not allow similar effects. So all bets are off the moment a fictional 'verse is described as having [[Faster-Than-Light Travel]] other than the rather weird Alcubierre drive. Other signs that the universe is not compatible with General Relativity Theory (GRT) are mentions of either "gravitons" or "anti-gravitation": in GRT gravity isn't a proper field, but the curvature of space. GRT is not, as it stands, compatible with quantum mechanics, so it will probably eventually be extended through a field theory -- the tradeoff being that a field theory does not only allow, but [http://arxiv.org/abs/gr-qc/0412122 support] the existence of [[Deflector Shields|repulsion forces]], which no one has ever seen.
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A black hole's size -- that is, the radius of its event horizon -- depends on its mass, spin, and charge. The simplest case of an uncharged, non-spinning ("Schwarzchild") black hole has a surprisingly straightforward formula:
{{quote|
And for astrophysics, this is more than sufficient to get a ballpark estimate of the size of any black hole based on the mass it contains. Thus, a black hole with a mass equal to the sun has an event horizon 3 kilometers in radius (6 km in diameter).
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{{reflist}}
[[Category:Black Holes]]
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