[[quoteright:266:http://static.tvtropes.org/pmwiki/pub/images/rogue_black_hole_187.jpg]]
[[caption-width-right:266:[[https://www.youtube.com/watch?v=7e5-0t0pTF0 There are monsters out in the cosmos...]]]]
->''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.''
-->-- '''StephenHawking''', ''The Nature of Space and Time''

->''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 [[AlienGeometries 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.''\\
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''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.''\\
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''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.''\\
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''And it is getting closer.[[note]]Not exactly accurate. An engine that could achieve FTL by definition can reverse time and throw you into the past, that is, away from the hole. Just point yourself in ANY fucking direction and hit accelerate: once you break the light barrier, time reverses polarity and shoots you from the hole. And a hypothetical engine that achieves FTL without time paradoxes does so by changing the curvature of space around you, again foiling the black hole. On the other hand, it's hard to predict in what exact time period of the universe will you arrive when you break free; most likely, you'll be lost in time. Oh, and, by the way, if you have a magical spaceship that just can accelerate to any velocity, you have an ability to kick physics in the nuts and take names, because screw you, Einstein. So why not just rip through the event horizon with your bare hands and march out, whistling Dixie?[[/note]]''
-->-- [=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 SwirlyEnergyThingy (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[[note]]Not to be confused with "brown dwarfs", which are substellar bodies, like large planets, that were never massive enough to sustain fusion to begin with.[[/note]]. 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, and they probably don't last too long.

Black Holes don't actually, technically, emit anything, despite what people say about them emitting radiation. That's the extreme conditions the matter entering the black hole is subject to. If you heat something up, it will give off radiation. And the matter entering a black hole gets very, VERY hot with all the spinning and stretching and friction and gravity it's going through. Nothing leaves after it's past the Event Horizon itself. However because a black hole is just so awesome, the matter wouldn't achieve those conditions in any other way, so it kind of it, except it's not.

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...[[DepartmentOfRedundancyDepartment 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. In three-dimensional space the Black Hole is not a disc. The singularity is an infinitely small point in space that sucks things in from all around, so the event horizon is more like a ball - with the singularity at the center. Rotating black holes also have ergosphere: a region near event horizon, where space-time spins around black holes faster than light.

In fact, space-time will become quite freaky around the event horizon: the closer you get to the event horizon, the [[YearInsideHourOutside 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 appear to ''stop''. You won't ''actually'' stop, that's just what they'll see. This is because the light required to see you is now inside the Black Hole and even that cannot escape - so you as you are inside the Black Hole can't be seen. This prediction, however, assumes zero mass of incoming object and neglects quantum effects, so reality may be more tricky.

Of course, nobody knows what'll happen after that, but there still are some theoretical predictions: You'll actually never even notice crossing it. You would just continue accelerating until you hit the singularity and are compacted into an infinitely small point. Or you could find your molecules randomly rearranged as a small, green space-cat with tentacles for legs.

However, you'd probably be long dead before that anyway as black holes come with some dangers attached due to the infinite gravity they exert: 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, [[ThereIsNoKillLikeOverkill its effects would go very far beyond of just vaporizing your body.]]

So, either way, you'll probably end up dead.

Black holes normally can't be seen (thus their moniker), but there are ways they are detectable: if they are near another [[StarKilling star and siphoning off mass]], they can form accretion disks, which glow hot. There's gravitation lensing, in which black holes are detected by the image distortions of objects behind them (TheOtherWiki has a nice animation for that [[http://en.wikipedia.org/wiki/File:BlackHole_Lensing.gif here]]). And then there's Hawking radiation, which basically is a way for black holes to radiate stuff (by quantum mechanics), and is a whole other can of non-zero entropy worms. One of its more practically relevant attributes is that a black hole loses mass/energy this way - the ''smaller it is, the faster it goes''! In other words, really small ones, like the ones that the Large Hadron Collider might produce, would just evaporate and be gone before you even notice them (although the immense release of energy from the Hawking radiation would be noticeable). A sun-mass black hole, on the other hand, would lose about a milligram of its mass-energy every 3.1 x 10[[superscript:31]] (31 nonillion) years. A [[http://arxiv.org/abs/0908.1803v1 scientific paper]] proposes to use a small artificial black hole's Hawking radiation as a means to convert mundane matter into energy and thrust to power a spaceship.

In short: black holes are really, ''[[MindScrew really]]'' [[EldritchLocation weird]]. It's speculated that there are supermassive black holes at the center of every galaxy and that they were there ''before'' the galaxies formed (rather than just have formed by a variety of small black holes merging into one - yes, they can do that, and the simulations of that are pretty spectacular, but predict that the actual event is downright cataclysmic for anything too close). Think of it like this: In the same way that a solar system is a large central star with many planets and other celestial objects orbiting it, a galaxy may be a supermassive black hole with ''stars'' and their solar systems orbiting around it, albeit on an even grander scale, relatively speaking.

If all that still is not weird enough for your taste, look up Einstein-Rosen bridges (think [[OurWormholesAreDifferent wormholes]], but it's rather useless from a practical point of view due to its instability) or really big, (insanely fast) rotating, charged black holes.

To learn more cool facts about black holes, please read [[IrregularWebcomic David Morgan-Mar's]] [[TheRant 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 {{UsefulNotes/relativity}} - and even in its context certain theorists saw the predictions of black holes in relativity and [[http://arxiv.org/abs/gr-qc/0412058 expressed doubts]] at least about the classical model. One such theorist was, initially, Einstein himself, who rejected the premise of a black hole rather strongly. Black holes just didn't make sense, especially how they muck up the nice wonderful understanding of space and time (we think) we have.

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 FasterThanLightTravel 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 [[DeflectorShields repulsion forces]], which no one has ever seen.

Right now there's no strict proof that such things exist: granted, there ''are'' heavy low-radiating objects ("black hole candidates"), but whether some low-emission star inside an enormous gas and dust cloud is really a black hole or not... Yet, there is one [[http://arxiv.org/abs/0903.1105 article]], that states: Sagittarius A* (a source of radio waves, associated with a supermassive object in the center of the Milky Way) must have an event horizon because, given the amount of superhot infalling matter we've detected around it, its surface luminosity is too low to be explained ''without'' something that traps radiation.

!!How big is a black hole?

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 ("Schwarzschild") black hole has a surprisingly straightforward formula:

->R[[subscript:Schwarzschild]] = 3 km * mass in Solar masses

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).

A black hole with a mass equal to the Earth (0.000003 solar masses) would have an event horizon whose radius was 0.000009 km, or 9 millimeters, ''or the size of your average American 25 quarter.''

A black hole with 4 million solar masses, such as the black hole theorized to be at the center of the Milky Way, would have an event horizon whose radius was 12 million km, about a fifth of the orbital radius of Mercury.

Going even further, the largest known black hole in the nearby Universe - the one located in the center of the giant elliptical galaxy M87 that has an estimated mass of 6.4 billion solar masses- would have a radius of 19.2 billion km, larger than our Solar System.

Further ''still'': The galaxy NGC 4889 contains a supermassive black hole with a mass of at ''21 billion'' solar masses, meaning a radius of ''63 billion'' km, and is at present the largest confirmed black hole in the known universe.

The odd thing about this, when compared with most "normal" spherically-shaped objects in the universe, is that a black hole's ''diameter'' is directly proportional to its mass - double the Schwarzschild radius and you've multiplied the mass by 2. For the average spherical object you and I might be familiar with, such as a ball of metal or water, the volume is proportional to its mass ''cubed'' - double the radius and you've multiplied the mass by 8. This means that the larger and more massive the black hole, the ''lower its average density.''[[note]]When discussing the density of the entire region inside the event horizon, ''not'' the density of the singularity at the center.[[/note]] A black hole with 1 solar mass would have an average density on the order of 10[[superscript:16]] grams per cubic centimeter, about 1.5 quadrillion times the density of solid lead. A black hole with 4 million solar masses, on the other hand, would only have an average density of 0.00028 grams per cubic centimeter, about a quarter the density of air at sea level on the Earth, and the supermassive black hole mentioned above would be even less dense.

!!What isn't a black hole?

A black hole is not:
* A hole: It doesn't ''go'' anywhere. As far as the rest of the universe is concerned, you're right there[[note]]In fact, one of the biggest problems with the definition of a black hole, mathematically, is that it seems to completely disregard the laws of conservation of mass and energy. Stephen Hawking admitted that his math had a seriously big hole in the logic as a result, even though the rest added up. Theorists have posited several possible solutions to the problem.[[/note]]. They don't look like they do in Science Fiction, you can't see them from that three-quarters angle that's popular, either. Assuming there's enough stars behind it, it'd just look like a big black spot ''maybe'' with a little light visible around the outside, depending on what's on the opposite side of you, but most of it would probably be red-shifted out of the visual spectrum. Instruments would be able to see much more exciting views in the form of various other kinds of radiation.
* Black: Oh sure, beyond the Event Horizon is black, but they may well glow brightly from heat generated by whatever it is that they're sucking in, and from accretion.
* Flat: Staying away from the more [[TimeyWimeyBall wibbly-wobbly]] stuff, it's convenient to think of a black hole as a tiny sphere and the event horizon is a shell around it. Once you hit the shell, you're stuck. It's also going to look pretty much exactly the same as you circle it, regardless of the direction you chose. The objects orbiting it do so due to spin.
* Magic: Black holes don't gain any super-magic powers of suction when they become black holes. Their mass exerts the same gravitational force as a star, planet, or any other object of the same mass. If our sun were suddenly turned into a black hole, nothing would happen to us. [[note]] other than the fact that all that energy normally radiated from the sun is suddenly gone[[/note]] Well, nothing would happen to the planet, though we'd most likely dies off from cold and starvation. If we wanted to study a black hole, we could put a probe in orbit around it the same way we put probes around other astronomical bodies. It's not going to instantly spiral to it's doom (at least not any faster than it would around anything else)

Unless you're watching ''extremely'' hard SciFi ([[MohsScaleOfScienceFictionHardness like 5.5 or 6]]), a black hole is probably nothing like you've generally seen in fiction. Black holes rank up there with FTL and TimeTravel as one frequently exploited bits of science.

!!How can you exit a Black Hole?

So, you survived the massive radiation poisoning, and the spaghettification, and you're past the Event Horizon and you haven't died yet. And now you want to go home? Wow, you really dream big!

The Event Horizon is not a thing but a location where your escape velocity is a speed faster than 186,000 miles per second (the speed of light). And that escape velocity itself can vary, depending on the mass of the Black Hole or how deep you are. It's perfectly possible that at some point past the Event Horizon, that the escape velocity is twice the speed of light. Theoretically you could make the Black Hole's escape velocity if you had a magical Faster Than Lightspeed ship, but in keeping with the rule that the faster something moves, the slower it ages, this would result in time rewinding for everything going FTL - the ship and its instruments as well as any people inside it. This would mean you could time-travel back to ''before'' you entered the Black Hole, thus 'escaping' it. There are several quasi-logical outcomes of trying this (amid millions of other possibilities). Of course, since time travel is another of those things that spits in the eye of Physics, these are all wild guesses.

# Upon breaking the speed of light, you disintegrate into your component particles. This is the most likely outcome. Happy dying!
# You reverse yourself in time, but fail to move in space. You think you are outside the Black Hole, and so does your magical FTL ship, but you're not. Eventually you hit the singularity and disintegrate, but you don't even notice. You are dead.
# You successfully reverse only yourself to a point and place in time before entering the Black Hole without changing everything else. You appear outside the Hole, perhaps in your partner's ship. However your memories are reversed, but the universe outside continues. Your interactions with the world return through the events you rewound, regardless of the input your brain should be receiving. You are now more like a broken record than a human being. The world is grateful to your institutionalised person for your research.
# You reverse the history of the universe to before you entered the Black Hole. Everything, including your ship's instruments and your memories of everything that happened before that point, has reversed to that point. Not remembering that you have been into the Black Hole before, you enter it again like an idiot. And then again and again. You have just ''doomed the entire universe'' to exist as a broken record, but nobody ever notices.
# You appear outside the Black Hole with your memories and the recordings of your ship intact. Congratulations! You just rewound the entire history of the universe ''except yourself!'' So you don't go in again. Then a collection of paradoxes gangs up on you asking unpleasant questions like how you can have information about the Black Hole if you didn't go in. Your impossible presence causes the universe to disintegrate, implode, or even switch off like a light-bulb.
# You manage to kill yourself faster because space beyond the event horizon has warped by the gravitational forces to the point the only way direction you can move is further in. At best you can manage to stay in place long enough for the black hole to dissipate on its own and you might live long enough to do so since time is moving extra slow for you. A lot of time will have passed for everyone not breaking light speed barriers beyond event horizons [[note]]see page quote for an alternate possibility[[/note]].
# The desired scenario - you appear outside the Black Hole with data in hand, you don't go in again, and the universe somehow doesn't switch off. Congratulations, not only have you rewound ''the entire universe except yourself,'' defying all laws of physics, but you have defied all possible logic too. You are now God. Or possibly [[Series/DoctorWho The Doctor]].
But this is all a moot point. FTL travel is impossible (at least, based on all the numbers and laws and theories and quantum we currently possess) and so you can't have yourself a magical FTL-speed ship anyway.[[note]]Fortunately, since FTL ships don't exist you're not going to be ''able'' to go anywhere near a black hole anyway.[[/note]] Because if you could, you might as well click your heels together three times and wish your way out.

Have fun arguing with infinity.

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