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    Default Why are Accretions Disks Planar? And Why do they Spin?

    So have recently watched "Nova: Black Hole Apocalypse" and the thought occurred to me, why does a black hole form an accretion disk in a planar form? Why would it not be a sphere? And then why does it spin? If the gasses were distributed randomly with random trajectories around a black hole upon forming, why would they begin to spin? And why do they always seem (at least in depictions) to spin in the same direction?

    Now, I assume whatever causes this spin is also what causes galaxies to form in primary disk shapes, and to spin...

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    Default Re: Why are Accretions Disks Planar? And Why do they Spin?

    If the star was spinning when the black hole formed from it, the black hole will spin as well.

    Going by neutron stars and pulsars - conservation of momentum results in the tiny objects spinning faster than the larger ones. So, even if the star was a slow-spinner - it will tend to produce a fast-spinning pulsar or black hole.
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    Default Re: Why are Accretions Disks Planar? And Why do they Spin?

    https://en.m.wikipedia.org/wiki/Protoplanetary_disk

    I don't have time to do a full write-up, so I'll leave this wikipedia article here. The first paragraphs under the 'formation' heading describe how and why the proto-stellar nebula collapses into a disk, and the accretion disk of a black hole follows the same physics.
    Last edited by DeTess; 2019-07-25 at 04:05 AM.
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    Default Re: Why are Accretions Disks Planar? And Why do they Spin?

    Quote Originally Posted by hamishspence View Post
    Going by neutron stars and pulsars - conservation of momentum results in the tiny objects spinning faster than the larger ones. So, even if the star was a slow-spinner - it will tend to produce a fast-spinning pulsar or black hole.
    The demonstration I always remember is a dancer spinning on the spot with her leg held out straight. If she drew the leg in she spun faster even without making any other changes. Worth noting that some neutron stars spin absurdly fast--I think the fastest we've found is up around 700 revolutions per *second*, which means (even given how small neutron stars are) its surface is moving around 0.25c!

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    Default Re: Why are Accretions Disks Planar? And Why do they Spin?

    All the gas and dust particles start being spread out completely randomly and moving in random directions with random velocities. The only thing they have in common is that they are all being pulled towards the common center of gravity. But as they all get closer and closer together, collisions becomes more frequent and every time you have two partices colliding, their new speed and direction will be closer to their combined average speed and direction. This happens over and over and over, until you eventually have all speeds and directions averaged out and the whole thing starts getting closer to being a disk.

    If all the gas were to start out distributed evenly, then it would indeed collapse into a sphere. But it's never perfectly even, which means there is an average speed and direction between all particles.

    I've been looking if there's any good videos on the topic, and I think this one might be helpful.
    Last edited by Yora; 2019-07-22 at 03:40 AM.
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    Default Re: Why are Accretions Disks Planar? And Why do they Spin?

    Quote Originally Posted by Yora View Post
    All the gas and dust particles start being spread out completely randomly and moving in random directions with random velocities. The only thing they have in common is that they are all being pulled towards the common center of gravity. But as they all get closer and closer together, collisions becomes more frequent and every time you have two partices colliding, their new speed and direction will be closer to their combined average speed and direction. This happens over and over and over, until you eventually have all speeds and directions averaged out and the whole thing starts getting closer to being a disk.

    If all the gas were to start out distributed evenly, then it would indeed collapse into a sphere. But it's never perfectly even, which means there is an average speed and direction between all particles.

    I've been looking if there's any good videos on the topic, and I think this one might be helpful.
    This, plus, there's relativistic frame-dragging... that is, a spinning massive object actually warps spacetime to make it easier for other things to spin in the same direction. With the Earth, the effect is minor. With the sun, it's measurable, with a supermassive black hole, we can watch it happen from halfway across the galaxy.

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    Default Re: Why are Accretions Disks Planar? And Why do they Spin?

    Quote Originally Posted by Rakaydos View Post
    This, plus, there's relativistic frame-dragging... that is, a spinning massive object actually warps spacetime to make it easier for other things to spin in the same direction. With the Earth, the effect is minor. With the sun, it's measurable, with a supermassive black hole, we can watch it happen from halfway across the galaxy.
    Yeah, there's a lot of frame dragging involved.

    Then again, galaxies aren't all spirals. There are a lot of ellipticals, they tend to be big, so the guess is that they come from collisions between spirals. Smaller ones tend to be approximately spherical, so maybe the spirals are the results of collisions and the ellipticals are the results of less spectacular absorbtions? At this point it's all about the guessing, some of it looks highly statistically significant, but in my opinion it's all guessing.

    There are planets with rings, and that's really accretion discs too, IMHO. Saturn's accretion disc doesn't sound so fancy, but it's very pretty, and I think that's what it really is, most of the gas giants have them, to a lesser extent.

    OTOH, there are orbiting SMBHs, where the smaller smashes through the accrestion disc of the larger. I can't see a black hole going through an accretion disc being actually that energetic, it absorbs some plasma... However, some of the accrestion disc follows the smaller black hole, it doesn't have time to form a disc, but when the black holes come close, that stuff hits the accretion disc, and that is energetic.

    https://en.wikipedia.org/wiki/OJ_287

    The smaller supermassive black hole with a mass of "only" 100 million M☉ orbits the larger one with an observed 11- to 12-year orbital period. The maximum brightness is obtained when the minor component moves through the accretion disk of the supermassive component at perinigricon.
    Last edited by halfeye; 2019-07-22 at 02:25 PM.
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    Default Re: Why are Accretions Disks Planar? And Why do they Spin?

    Thanks all. Several good (and consistent) explanations that all make sense. I appreciate the time and effort.

    I would like to understand a bit more about "relativistic frame-dragging". Can anyone point me to a good source to understand that concept in regards to this topic?

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    Default Re: Why are Accretions Disks Planar? And Why do they Spin?

    Here is a simple video on why a three-dimensional cloud of particles will tend to form into a spinning disk, and why it's the expected outcome.

    https://www.youtube.com/watch?v=tmNXKqeUtJM
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    Default Re: Why are Accretions Disks Planar? And Why do they Spin?

    Quote Originally Posted by LordEntrails View Post
    So have recently watched "Nova: Black Hole Apocalypse" and the thought occurred to me, why does a black hole form an accretion disk in a planar form? Why would it not be a sphere? And then why does it spin? If the gasses were distributed randomly with random trajectories around a black hole upon forming, why would they begin to spin? And why do they always seem (at least in depictions) to spin in the same direction?

    Now, I assume whatever causes this spin is also what causes galaxies to form in primary disk shapes, and to spin...
    If you have a sphere of randomly-moving gas and dust, the stuff within will start colliding with other stuff. As it does so, some kinetic energy is transformed into heat and light and whatnot, but momentum is conserved—both linear momentum and angular momentum. Some will have clockwise angular momentum, some will have counterclockwise angular momentum, some will have angular momentum perpendicular to it; however, collisions will cause opposing angular momentum to ultimately be zeroed out (ie, clockwise-spinning and counterclockwise-spinning stuff will hit each other and stop orbiting).
    However, just because the original sphere of gas and dust was all moving randomly doesn't mean it was moving symmetrically. If you sum up the angular momentum of every puff of gas and every speck of dust, you'll come up with a nonzero average angular momentum. The solar system or galaxy or whatever that results from this cloud will be rotating in that direction, and not perpendicular to it; thus, it will be flat.

    This minutephysics video explains it pretty well.
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    Default Re: Why are Accretions Disks Planar? And Why do they Spin?

    Of all of the universe of possible values for the total angular momentum, why should it be exactly zero?
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    Default Re: Why are Accretions Disks Planar? And Why do they Spin?

    Quote Originally Posted by GreatWyrmGold View Post
    If you have a sphere of randomly-moving gas and dust, the stuff within will start colliding with other stuff.
    I don't think that's enough of itself. There is a huge amount of space, and the matter is pretty compact, there is a lot of room for stuff to miss other stuff. The Sun's accretion disk is not totally planar, and it's quite lumpy, we live on one of the lumps and there are several others and lots of smaller ones (asteroids).

    One of the factors with black holes is definitely frame dragging, holes statistically almost have to rotate, and if they do they will drag local spacetime around with them.
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    Default Re: Why are Accretions Disks Planar? And Why do they Spin?

    Quote Originally Posted by halfeye View Post
    I don't think that's enough of itself. There is a huge amount of space, and the matter is pretty compact, there is a lot of room for stuff to miss other stuff. The Sun's accretion disk is not totally planar, and it's quite lumpy, we live on one of the lumps and there are several others and lots of smaller ones (asteroids).
    If you're running by that definition of 'totally planar', then black hole accretion disks most likely aren't totally planar either.
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    Default Re: Why are Accretions Disks Planar? And Why do they Spin?

    Quote Originally Posted by Chronos View Post
    Of all of the universe of possible values for the total angular momentum, why should it be exactly zero?
    It isn't.

    But let me explain a bit more.

    What GreatWyrmGold is saying is that for every particular particle in a giant cloud of space dust, it has its own momentum and so it's own angular momentum around the total center of mass. However, in that same giant cloud of space dust, there is also going to be another particle (or combination of particles) which also have an equal momentum but in the opposite direction around the center of mass. Of course, these particles are not likely to meet each other directly, but as the space dust bangs into other particles and as the entire cloud begins to condense due to gravity, those two equal-but-opposite momentums are going to meet together and cancel each other out.

    Of course, it's more complicated than that, but that's a simplified explanation to how it works.

    That's just considering the angular momentum of one particle, though. If we lived in a perfectly symmetrical universe, then the space dust clouds would have every angular momentum matched with another angular momentum and everything cancel to zero. However, our universe is not perfectly symmetrical (thankfully) and so any random space dust cloud, there is going to be some net angular momentum in some direction. And because of the way 3D space just happens to work (and maths) this means that it will rotate in a flat, 2D plane along a single perpendicular axis.

    It's probably best to watch the video on Youtube to be able to visualize it better, as the video shows clouds of particles falling into itself and eventually becoming a swirling galaxy-like shape, due to that imbalance of angular momentum in some direction..
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    Default Re: Why are Accretions Disks Planar? And Why do they Spin?

    Quote Originally Posted by halfeye View Post
    I don't think that's enough of itself. There is a huge amount of space, and the matter is pretty compact, there is a lot of room for stuff to miss other stuff. The Sun's accretion disk is not totally planar, and it's quite lumpy, we live on one of the lumps and there are several others and lots of smaller ones (asteroids).
    Even the smallest clump is slowly built up by gravity. And the more it grows, the faster it continues to grow. Sure it's slow, but a hundred million years or more is a really long time. There are also recent theories that extra-solar objects similar to ʻOumuamua may have helped start that clumping.

    You don't need frame dragging to explain it. Straight Newtonian mechanics will model a slightly rotating, slightly inconsistent, cloud collapsing into a disk and then planets over astrological time periods. Also of note is that the planets did not form where they are today, they moved quite a bit from their initial positions.
    Last edited by Excession; 2019-08-05 at 10:21 PM.

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    Default Re: Why are Accretions Disks Planar? And Why do they Spin?

    Quote Originally Posted by Excession View Post
    Even the smallest clump is slowly built up by gravity. And the more it grows, the faster it continues to grow. Sure it's slow, but a hundred million years or more is a really long time. There are also recent theories that extra-solar objects similar to ʻOumuamua may have helped start that clumping.

    You don't need frame dragging to explain it. Straight Newtonian mechanics will model a slightly rotating, slightly inconsistent, cloud collapsing into a disk and then planets over astrological time periods. Also of note is that the planets did not form where they are today, they moved quite a bit from their initial positions.
    The solar system is a lot less flat than Saturns rings. The orbit of Pluto is tilted by something like 5 or maybe even ten degrees compared to that of Neptune, and Neptune isn't perfectly aligned with the Earth. I'm not exactly sure where I'm aiming with that, but those are the facts as I understand them.
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    Default Re: Why are Accretions Disks Planar? And Why do they Spin?

    Quote Originally Posted by halfeye View Post
    The solar system is a lot less flat than Saturns rings. The orbit of Pluto is tilted by something like 5 or maybe even ten degrees compared to that of Neptune, and Neptune isn't perfectly aligned with the Earth. I'm not exactly sure where I'm aiming with that, but those are the facts as I understand them.
    Pluto's weird orbit is one of the reasons they chose to downgrade it from a planet, AFAIK. Anyway, not sure how Neptune's orbit being inclined a whole 1.7 degrees to the ecliptic somehow disproves that the Solar System is pretty flat? If there was a planet orbiting with a 30 degree inclination then I'd see your point.

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    Default Re: Why are Accretions Disks Planar? And Why do they Spin?

    Quote Originally Posted by halfeye View Post
    The solar system is a lot less flat than Saturns rings. The orbit of Pluto is tilted by something like 5 or maybe even ten degrees compared to that of Neptune, and Neptune isn't perfectly aligned with the Earth. I'm not exactly sure where I'm aiming with that, but those are the facts as I understand them.
    I guess the main question is, why do we have sometimes flat and mostly even rings and other times the matter clumps together into planets (or stars).

    Well, let's go back to our cloud of gas, which will become a solar system. All those little pieces of matter are colliding with each other and those collisions are not ideal, so every time some energy is converted to heat and radiated away. Also sometimes one particle gets enough energy from a collision that it gets propelled away from the cloud, so whatever stays behind has less kinetic energy then before. This is the reason for gravitational collapse of the whole cloud: evaporation (particles escaping) and radiation of heat.

    For the reasons already mentioned, after long enough time the key factor preventig the whole cloud from collapsing into a single dense object in the middle is the total angular momentum, which is always conserved. Hence we expect the matter to concentrate in a disk perpendicular to the axis of rotation.

    The thing is, the cloud of gas is not uniform - some parts are hotter then others or have different initial density and so on. Because of this, different parts of the cloud will collapse faster or slower and the more they differentiate, the more varied the tempo of collapse becomes. Essentially density is very unstable under gravity. After all, the whole cloud was slightly denser from the surrounding and it started collapsing on its own, hasn't it? The same thing happens within this cloud, so galaxies have stars and solar systems have planets. Some moons are also created like this I think.

    Once everything cools down and we have a disk of matter with a solid blob in the middle (which is not yet dense enough to be a star), then those bigger clumps of matter created due to the uneven density have more or less stable orbits and start catching everything in their paths Katamari style and the more more matter is there to take, the faster it goes. This is why beyond Neptune there are multiple dwarf planets - their orbits are extermely long, matter density was way lower then closer to the middle, so all those objects are very unlikely to ever meet and crash into each other. This is also why not all matter in the solar system is in the disk even further away from the Kuiper belt there is Oort cloud - the density of matter is so small that far away from the center that it did not have enough time to dissipate the excessive energy, since collisions are that rare.

    So, if the density of matter is high enough, planets will form from initial slightly more dense parts of the cloud. Why then we have an asteroid belt between perfectly good planets and why does Saturn has such a spiffy ring (note: all gas giants have those just not as gigantic)? Saturn I had to google up, but in general the reasons are violent. Sometimes those are remnants of collisions with massive objects, sometimes the big bully in the neighbourhood (I am looking at you Jupiter) is disturbing orbits of objects in a wide area, so no planet had a chance to form there. Jupiter also has a habit of throwing rocks at other planets in the solar system.

    With accretion disks it has more to do with how fast the matter around the black hole is spinning and how strong are velocity gradients along the radius of the disk: particles closer to the center move faster then those further away, so any larger objects is ground to a paste. Also the matter in the disk is pretty hot, so it simply cannot form large particles - closer to the center we are talking about a milion Kelvin or higher.
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    Default Re: Why are Accretions Disks Planar? And Why do they Spin?

    Quoth erikun:

    It isn't.

    But let me explain a bit more.
    ...
    I think we're in agreement. My comment was in support of it not being perfectly symmetric. There is one possible value of angular momentum that makes it perfectly symmetric (that being zero), but a really, really large number of possible values that would make it not perfectly symmetric.
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    Default Re: Why are Accretions Disks Planar? And Why do they Spin?

    Quote Originally Posted by Chronos View Post
    I think we're in agreement. My comment was in support of it not being perfectly symmetric. There is one possible value of angular momentum that makes it perfectly symmetric (that being zero), but a really, really large number of possible values that would make it not perfectly symmetric.
    How do you explain Saturn's rings?

    https://en.wikipedia.org/wiki/Saturn#Planetary_rings

    The rings extend from 6,630 to 120,700 kilometers (4,120 to 75,000 mi) outward from Saturn's equator and average approximately 20 meters (66 ft) in thickness.
    Last edited by halfeye; 2019-08-09 at 12:01 PM.
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    Default Re: Why are Accretions Disks Planar? And Why do they Spin?

    Quote Originally Posted by halfeye View Post
    How do you explain Saturn's rings?

    https://en.wikipedia.org/wiki/Saturn#Planetary_rings
    They aren't perfectly symmetric. If they were, their thickness would be in the order of cm.
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    Default Re: Why are Accretions Disks Planar? And Why do they Spin?

    Quote Originally Posted by DeTess View Post
    They aren't perfectly symmetric. If they were, their thickness would be in the order of cm.
    They are within an arcsecond of perfect, compared to whole degrees out for the solar system as a whole.
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    Default Re: Why are Accretions Disks Planar? And Why do they Spin?

    Just because the "average" thickness is 20m doesn't mean the entire ring system is that thick. There are apparently parts of them that are 1km thick, according to Wikipedia, which is definitely an error of more than an arcsecond.

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    Default Re: Why are Accretions Disks Planar? And Why do they Spin?

    Quote Originally Posted by factotum View Post
    Just because the "average" thickness is 20m doesn't mean the entire ring system is that thick. There are apparently parts of them that are 1km thick, according to Wikipedia, which is definitely an error of more than an arcsecond.
    Error? this is a physical system, it's not a mistake if it doesn't fit some metric. Even at a km thick, they are ridiculously close to the "perfect", there is no way they got to that point so quickly just by averaging.
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    Default Re: Why are Accretions Disks Planar? And Why do they Spin?

    Quote Originally Posted by halfeye View Post
    Error? this is a physical system, it's not a mistake if it doesn't fit some metric. Even at a km thick, they are ridiculously close to the "perfect", there is no way they got to that point so quickly just by averaging.
    Only if you assume that Saturn's rings where initially more or less evenly distributed, and that's not necessarily true. If they started roughly disk-shaped, then they would fallen out pretty quickly due to collisions. source:(see page 16 &17)
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    Default Re: Why are Accretions Disks Planar? And Why do they Spin?

    Quote Originally Posted by halfeye View Post
    Error? this is a physical system, it's not a mistake if it doesn't fit some metric. Even at a km thick, they are ridiculously close to the "perfect", there is no way they got to that point so quickly just by averaging.
    Actually, it can. The thing to remember here, is that the rings around the Saturn are much smaller and denser then the primordial cloud of the whole solar system, so the process was faster. Moreover, they were not created from the primordial matter (too much ice, too little rock in them). Most likely they are remnants of a moon or two crushing into saturn and being squeezed out by the tidal forces. If so, the matter in the ring was already in a flat orbit give or take the size of the moon in question, so there was not much energy to dissipate.

    As for why there are larger differences in orbit allignment of planets, it just means that planets formed before the averaging out process could finish. Once the matter is concentrated in the planets, there is no friction that shaped the disk.
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    Default Re: Why are Accretions Disks Planar? And Why do they Spin?

    Quote Originally Posted by Chronos View Post
    I think we're in agreement. My comment was in support of it not being perfectly symmetric. There is one possible value of angular momentum that makes it perfectly symmetric (that being zero), but a really, really large number of possible values that would make it not perfectly symmetric.
    Right after the big bang, when the universe infinitely small and all energy and matter was infinitely densely packed, the universe should have been, and I believe is assumed to have been, perfectly symmetric and uniform.

    But the reason this symmetry was lost is quantum fluctuations. Quantum fluctuations are by their very nature completely unpredictable, and as such have no symmetry among each other. They are really, really small, but once you have these tiny specks of irregularity in the distribution of matter and energy, you get a cascading chain reaction of the electromagnetic and gravitational forces making everything starting to clump.

    Those quantum fluctuations are still visible in the Cosmic Microwave background, causing the chaotic pattern in slghtly warmer and colder regions.
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    Default Re: Why are Accretions Disks Planar? And Why do they Spin?

    What is there to explain about Saturn's rings? If they were perfectly symmetric, they wouldn't exist either, just like an accretion disk.
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