History UsefulNotes / BlackHoles

Black holes are collapsed stars, but not many people know ''how'' the stars have collapsed in such a way to create black holes. We don't really understand what a singularity (the heart of a black hole) even is. When we try to do the math, [[EldritchLocation key calculations utterly collapse and physics melts down as the spacetime has infinite curvature]]. In other words, its mass has collapsed to ''[[MindScrew paradoxically zero]]'' because it's [[RhymesOnADime infinitely small and infinitely dense--thus its gravity is infinitely intense]]. They could thus be interpreted as a Real-Life case of a GameBreakingBug.

By the way, the event horizon is the TropeNamer for the MoralEventHorizon - just like how there is no return if you cross a black hole's event horizon, there is no return to being good if you cross the MoralEventHorizon.
So, either way, you'll probably end up dead (well, according to our classic understanding of it anyway, [[TemptingFate unless you dare to keep reading]]).

* Holes: They don'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 thermodynamics (mass and energy are conserved via an increase in the black hole's mass and Hawking radiation). 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 ScienceFiction, 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: At least, not in the sense we normally think of. No light can escape from inside the event horizon, but if a black hole has an accretion disk, the matter in the disk glows '''very''' brightly due to the immense heat and other radiation generated by friction and other forces, and thus such a black hole can be one of the brightest objects in the universe. [[http://www.wired.com/wp-content/uploads/2014/10/ut_interstellarOpener_f.png A real black hole might look more like this.]]

A black hole with 4 million solar masses, such as [[http://en.wikipedia.org/wiki/Sagittarius_A* Sagittarius A*,]] the black hole known 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[[note]]Seventeen times larger than the Sun, comparable in size to a giant star as [[https://en.wikipedia.org/wiki/Arcturus Arcturus]][[/note]].

'''''The''''' NegativeSpaceWedgie.

A black hole with 4 million solar masses, such as [[http://en.wikipedia.org/wiki/Sagittarius_A* Sagittarius A*,]] 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.

What form that core takes depends on its mass. For lighter stars, such as the Sun, the core becomes 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 sub-stellar bodies, like large planets, that were never massive enough to sustain fusion to begin with.[[/note]] According to current estimates, no black dwarfs yet exist, as a star cooling to that level would take longer than the universe has existed. The Sun is expected to become a black dwarf in approximately 1 quadrillion years. If the core is more than 1.4 times the mass of the Sun, it will exceed the [[https://en.wikipedia.org/wiki/Chandrasekhar_limit Chandrasekhar limit]] and gravity will combine electrons and protons to form neutrons, resulting in a neutron star. If the core mass exceeds the [[https://en.wikipedia.org/wiki/Tolman–Oppenheimer–Volkoff_limit Tolman-Oppenheimer-Volkoff limit]] (about two to three solar masses, and definitely no more than five, but it's still unclear), even the neutrons can't resist further collapse;[[note]]Neutron stars are prevented from collapsing further by a pressure called neutron degeneracy pressure. This is caused by the Pauli exclusion principle, and the degeneracy pressure is insufficient to prevent collapse over the Tolman-Oppenheimer-Volkoff. However, it is possible that there are other forms of degenerate matter, which may be capable of preventing further collapse until the object's mass reaches a new limit.[[/note]] it can be assumed that the core collapses past its Schwarzchild radius[[note]]the radius of any object which, if crushed beyond that point, the object will form a black hole[[/note]], and becomes a singularity (a single point, or a ring for a rotating black hole).

Black holes are strange things. Besides the singularity at the center[[note]]Assuming it exists, since it's believed it's an artifact caused by breakdown of relativity under those conditions, that would disappear with a--still not existent--theory of quantum gravity[[/note]], there is the event horizon, the point of no 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, just like the Sun is not a big yellow circle. The Sun is a sphere, an event horizon is a smaller sphere, and the singularity is an infinitely tiny ball so small and tightly packed that it has basically turned itself ''inside out'' — so when you are inside the event horizon, you are inside the ball. Rotating black holes also have an ergosphere, a region near the event horizon where space-time spins around the black hole at speeds so great that you'd need to move faster than light just to stay still, let alone move in a direction counter to the black hole's rotation.

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[[note]]For less massive ones, you'll be dead before you even cross the event horizon[[/note]]. 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. The thing is 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 just vaporizing your body]].

Black holes normally can't be seen (thus their moniker), but there are ways they are detectable. If they're [[StarKilling siphoning off matter from a nearby star]], they can form accretion disks, which get '''incredibly''' hot due to friction and other forces and emit light and other radiation at intensities on the order of millions of time the brightness of the sun. There's gravitational lensing, in which black holes are detected by the image distortions of objects behind them (Website/TheOtherWiki has [[http://en.wikipedia.org/wiki/File:BlackHole_Lensing.gif a nice animation]] for that). And then there's [[https://en.wikipedia.org/wiki/Hawking_radiation Hawking radiation]], named after Creator/StephenHawking, who proposed the concept. It's a theoretical way for black holes to lose mass via quantum mechanics, and is a whole other can of non-zero entropy worms. One of its more practically relevant attributes is that the energy of the radiation "emitted" by a black hole is inversely proportional to its mass — ''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 solar-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, which they'd more than make up for by consuming the cosmic microwave background. 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.

To learn more cool facts about black holes, please read [[Webcomic/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 Website/{{Reddit}}. Seriously, they're awesome.

Until February 2016, with the [[https://en.wikipedia.org/wiki/First_observation_of_gravitational_waves first detection of gravitational waves]] with the LIGO instrument and other similar detections in the following years[[note]]As most of the detected signals exactly match the theoretical predictions of a black hole merger[[/note]], there was 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.[[note]]That's part of the reason its name includes an asterisk, actually: We ''think'' there's a black hole there, but weren't completely sure; photographs ''would'' help, but there's currently many stars in the way, explaining why it took longer to image it.[[/note]]

In April 2019, a black hole was actually observed for the first time. The Event Horizon Telescope, a network of 8 interlinked telescopes, was able to photograph a black hole in the center of the distant giant galaxy [[https://en.wikipedia.org/wiki/Messier_87 M87]], 500 million trillion km away (see the image that illustrates this article). This is roughly equivalent, in terms of scale, to reading the date on a coin in New York... from Los Angeles. The black hole looks like... [[ShapedLikeItself a black hole]], illuminated by the glowing ring of superheated gas around its event horizon. The internet immediately [[MoeAnthropomorphism anthropomorphized]] it. Three years later, in May 2022, the team operating the same telescope released a picture of [[https://eventhorizontelescope.org/blog/astronomers-reveal-first-image-black-hole-heart-our-galaxy Sagittarius A*]] (the aforementioned supermassive black hole thought to exist at the heart of the Milky Way).

There is one last part about black holes that is still very controversial, the [[https://en.wikipedia.org/wiki/Black_hole_information_paradox Black Hole Information Paradox]]. That is, what ''happens'' to information when in a black hole. Hawking stated that it's irretrievably lost, which would violate the [[https://en.wikipedia.org/wiki/First_law_of_thermodynamics First Law of Thermodynamics]]. However, thanks to the bizarre nature of black holes, it's possible the law might be broken (thank you, infinity). Other theories include it being hidden in a "pocket universe" or it's released when the black hole eventually evaporates, regardless of its size.

A black hole with 4 million solar masses, such as [[http://en.wikipedia.org/wiki/Sagittarius_A* Sagittarius A*]], 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.

Perhaps ''even further'': The ultra-luminous quasar [[https://en.wikipedia.org/wiki/TON_618 TON-618]], around 10.4 billion light-years distant, is calculated to possibly contain a central black hole massing in at '''''66 billion''''' solar masses[[note]]For comparison purposes, this monster black hole is more massive than ''all the stars of the Milky Way galaxy combined''[[/note]]. Scientists are calling it an "''ultramassive'' black hole", because "supermassive" [[BeyondTheImpossible isn't a strong enough word to describe this behemoth]]. Its radius would be 1,300 AU, or '''195 billion km'''[[note]]If you still do not have enough, some calculations show that in the ''[[TimeAbyss very distant]]'' future, assuming an Universe in eternal expansion, the collapse of superclusters of galaxies could produce black holes with masses of up to ''one hundred trillion'' solar masses. Such monsters would have a radius of around 31 light years, or '''295 trillion kilometers'''[[/note]].

* 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 as 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[[note]]Black holes that rotated very fast are expected to look [[https://www.researchgate.net/figure/General-relativistic-ray-tracing-simulations-of-the-black-hole-For-rotating-black-hole_fig4_336768072 distorted]][[/note]]. The objects orbiting it do so due to spin.

# You just fry yourself with the concentrated beam of blueshifted gamma-rays you made [[https://youtu.be/JQnHTKZBTI4?t=2m35s by screwing with relativistic speeds]], which happens ''before'' you even break ''c''. This is the most likely outcome. [[ShootTheShaggyDog Happy dying!]]

What form that core takes depends on its mass. For lighter stars, such as the Sun, the core becomes 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 sub-stellar bodies, like large planets, that were never massive enough to sustain fusion to begin with.[[/note]] According to current estimates, no black dwarfs yet exist, as a star cooling to that level would take longer than the universe has existed. The Sun is expected to become a black dwarf in approximately 1 quadrillion years. If the core is more than 1.4 times the mass of the Sun, it will exceed the [[https://en.wikipedia.org/wiki/Chandrasekhar_limit Chandrasekhar limit]] and gravity will combine electrons and protons to form neutrons, resulting in a neutron star. If the core mass exceeds the [[https://en.wikipedia.org/wiki/Tolman–Oppenheimer–Volkoff_limit Tolman-Oppenheimer-Volkoff limit]] (about two to three solar masses, and definitely no more than five, but it's still unclear), its mass is such that even the neutrons can't resist further collapse;[[note]]Neutron stars are prevented from collapsing further by a pressure called neutron degeneracy pressure. This is caused by the Pauli exclusion principle, and the degeneracy pressure is insufficient to prevent collapse over the Tolman-Oppenheimer-Volkoff. However, it is possible that there are other forms of degenerate matter, which may be capable of preventing further collapse until the object's mass reaches a new limit.[[/note]] it can be assumed that the core collapses past its Schwarzchild radius[[note]]the radius of any object which, if crushed beyond that point, the object will form a black hole[[/note]], and becomes a singularity (a single point, or a ring for a rotating black hole).

What form that core takes depends on the mass of the star. A lighter star, such as the Sun, becomes 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 sub-stellar bodies, like large planets, that were never massive enough to sustain fusion to begin with.[[/note]] According to current estimates, no black dwarfs yet exist, as a star cooling to that level would take longer than the universe has existed. The Sun is expected to become a black dwarf in approximately 1 quadrillion years. Stars with cores weighing in at more than 1.4 times the mass of the Sun have exceeded the [[https://en.wikipedia.org/wiki/Chandrasekhar_limit Chandrasekhar limit]] and gravity combines electrons and protons to form neutrons, resulting in a neutron star. Stars whose core mass exceeds the [[https://en.wikipedia.org/wiki/Tolman–Oppenheimer–Volkoff_limit 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;[[note]]Neutron stars are prevented from collapsing further by a pressure called neutron degeneracy pressure. This is caused by the Pauli exclusion principle, and the degeneracy pressure is insufficient to prevent collapse over the Tolman-Oppenheimer-Volkoff. However, it is possible that there are other forms of degenerate matter, which may be capable of preventing further collapse until the object's mass reaches a new limit.[[/note]] it can be assumed that the star collapses past its Schwarzchild radius[[note]]The radius of any object which, if crushed beyond that point, the object will form a black hole. For reference, the planet Earth's is about 9 millimetres. A human being could theoretically form a black hole, but you'd have to squeeze them into an area ''ten billion times smaller than the radius of a proton''.[[/note]], 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.[[note]]This may not be true. Due to quantum effects at the horizon, a black hole emits a form of black body radiation and '''''VERY''''' slowly loses mass until it eventually evaporates. However, the energy of the radiation "emitted" by a black hole is inversely proportional to its mass. This means that lighter black holes can quickly evaporate in a burst of radiation; this also means that larger black holes won't be able to evaporate, or even lose mass, for quite some time, due to the cosmic microwave background. This form of radiation is called "Hawking radiation", named after its discoverer (well, "predictor" would be more accurate, no one has ever actually been able to observe this radiation yet — due to obvious reasons) Stephen Hawking. You can read more about this in [[https://en.wikipedia.org/wiki/Hawking_radiation the other wiki]]. However, until irrefutable evidence is found, it's just a hypothesis despite the fact that it's mathematically all working out.[[/note]] That's the extreme conditions the matter ''entering'' the black hole is subject to. If you heat something up, it gives 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 once it's past the event horizon itself. However, because a black hole is so unique in how it operates, the matter wouldn't achieve those conditions in any other way, so it kind of is, except it's not.

Black holes normally can't be seen (thus their moniker), but there are ways they are detectable. If they're [[StarKilling siphoning off matter from a nearby star]], they can form accretion disks, which glow hot. There's gravitational lensing, in which black holes are detected by the image distortions of objects behind them (Website/TheOtherWiki has [[http://en.wikipedia.org/wiki/File:BlackHole_Lensing.gif a nice animation]] for that). 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 solar-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, which they'd more than make up for by consuming the cosmic microwave background. 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.