# Why does everything rotate?

## Main Question or Discussion Point

Do we know why everything in the universe rotates? The dust clouds, stars, planets all rotate. Even galaxies rotate. Why is that? Gravity causes the dust particles to accrete into rocks and then asteroids, planets, and stars. But why the rotation.

Similarly, why is our galaxy such a relatively flat spiral rather than more globular? I assume the rotation has a lot to do with that, but that does not explain why the rotation itself is in such a narrow plain to create a relatively flat spiral.

I can guess at the answers but would like a more authoritative answer.

Thanks.

PeterDonis
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Do we know why everything in the universe rotates?
Not everything does, at least not to a significant extent. But it is true that many things do; generally, the reason is that the process of forming those things involves matter clumping together, and the clumping process is never perfectly symmetric. As the matter clumps more tightly, to a smaller and smaller size, the initial asymmetry gets magnified so that the object rotates faster, just as a skater spins faster with arms pulled in than when they are stretched out.

Note also that, averaged over the entire universe, the total rotation is, as best we can tell, zero. There are many rotating objects, but they rotate in many different planes, and when you average all the rotations together, they all cancel out leaving no net rotation overall.

Interesting. I had guessed the rotation had to do with objects in linear expansion straight motion falling into a gravity well. But that would poorly explained individual objects rotating.

Why the relatively flat spiral galaxy. I know that not all of galaxies are flat spirals but one would think that a relatively flat spiral galaxy should be a statistical anomaly.

PeterDonis
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one would think that a relatively flat spiral galaxy should be a statistical anomaly.
Why?

Chronos
Gold Member
Conservartion of angular momentum is a big factor

Chalnoth
Do we know why everything in the universe rotates? The dust clouds, stars, planets all rotate. Even galaxies rotate. Why is that? Gravity causes the dust particles to accrete into rocks and then asteroids, planets, and stars. But why the rotation.
Because when you have a big cloud of dust with all the bits of dust having random motions, the probability that that cloud won't have a net angular momentum is virtually zero.

As the cloud collapses in on itself due to gravity, the net rotation increases in speed (rather like a figure skater pulling their arms in). Because objects like galaxies and stars are vastly more dense than the clouds that they formed from, there's pretty much always a good amount of rotation that results.

Planets are a bit of a different deal, because they don't form from diffuse clouds of dust: they form from a spinning disc of matter. The friction within the disc causes the resulting planet to tend to have the same direction of spin as the disc of matter, which means that if the orbit is clockwise, then the spin of the planet probably will be too.

Rotation could come from the so called "Big Bang". I don't have a link, but believe I have read that suspected black holes have been observed producing jets of material. If a rotating black hole "exploded" in a pinwheel fashion the matter emitted would be rotating and would continue to rotate unless some force reduced that rotation.

russ_watters
Mentor
On a number line stretching from negative to positive infinity, the odds of having exactly zero are, well, roughly zero.

Bandersnatch
Why the relatively flat spiral galaxy. I know that not all of galaxies are flat spirals but one would think that a relatively flat spiral galaxy should be a statistical anomaly.
For a similar reason to why the solar system is flat. When the initial rotation of the collapsing cloud of particles gets amplified due to the conservation of angular momentum, it prevents particles from clumping too close to the axis of rotation. But nothing prevents them from clumping in the plane perpendicular to that axis.
A test particle in a cloud with some initial rotation will get attracted to the highest concentration of mass (the centre), but while the component of gravitational force in the direction of the rotational axis is affected by centrifugal force, eventually stopping further motion in that direction, there is no such limitation in the direction parallel to the axis - the particle falls towards the rotating plane, passes it and falls back in. As it does so, frictional interactions tend to dampen oscillations, promoting a flat spinning disc shape.

A picture is worth a thousand words, but an interactive program is worth even more. See here:
The program visualises some of the above. It does not include any damping interactions apart from some basic collisions, so the oscillations never fully stop. The initial collapse before particles get depleted clearly shows the tendency to form a disc, though.
Try playing with the initial values. You can remove clumping, increase particle number, etc.
As with any n-body simulation, it can get a bit resource intensive. Adding enough particles will grind it to a halt even on the fastest machines.

As long as the formation is from a spinning cloud of matter, a flat disc shape is natural. Elliptical galaxies are thought to be the result of subsequent collisions between spiral galaxies. I suspect given enough time, the ellipticals will flatten again.

Jimster41
Gold Member
What about QM objects? Do they rotate? Do they all rotate? I've been confused about what QM "spin" means w/respect to the common notion of rotation for a long time.

Also confused about where the "random" fluctuations come from that cause the system to depart from spherical symmetry?

Why?
Because there are so many possible variations of globular shapes that settling on something so relatively flat would seem almost "artificial".

I am guessing it is the rotation that makes it happen. Without it, a flat spiral would have to be unlikely. That is why I was wondering about why things in the universe rotate in the first place.

For a similar reason to why the solar system is flat. When the initial rotation of the collapsing cloud of particles gets amplified due to the conservation of angular momentum, it prevents particles from clumping too close to the axis of rotation.
Can you comment about what causes that "initial rotation of the collapsing cloud of particles"?

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Bandersnatch
Can you comment about what causes that "initial rotation of the collapsing cloud of particles"?
Imagine a perfectly uniform, perfectly still, infinite in extent cloud of particles suspended in infinite space. If such uniformity could be set up as an initial condition, and there was no inherent randomness to the motion of the particles, all the forces would cancel out and it'd remain in this static form forever.
However, there is (so far) no reason to think there ever was a perfectly uniform initial state at any time in the past of our universe, and even if there were, there is inherent randomness to the motion of the particles due to their quantum nature on small enough scales.
All you need for your perfectly uniform cloud to fall apart into clumps of matter contracting under their own gravity is the tiniest of nudges. Just as with a needle standing on its tip, as soon as you introduce any deviation whatsoever, no matter how small, the balance is irrevocably broken and forces no longer cancel out. Thus just as the needle eventually falls, regions of.overdensity will form in the cloud.

As the cloud collapses and parts of it will start to orbit other parts, you will always be able to select a limited volume in which matter has some angular momentum in one direction. For example, if your initially static cloud of particles collapses to form two locally overdense 'swirls' of matter, whose total angular momentum remains zero, you can always look at just one of those swirls as the portion of the cloud with a non-zero rotation that will govern its further evolution.

You're a good explainer Bandersnatch.

Thanks.

Chalnoth
What about QM objects? Do they rotate? Do they all rotate? I've been confused about what QM "spin" means w/respect to the common notion of rotation for a long time.

Also confused about where the "random" fluctuations come from that cause the system to depart from spherical symmetry?
The matter that makes up these dust clouds has bumped into one another so many times that the individual motions are approximately random.

PeterDonis
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Rotation could come from the so called "Big Bang".
No, it couldn't; at least, not according to our best current model, which is that the overall angular momentum of the universe is zero. Individual objects or systems can have nonzero angular momentum, but averaged over the entire universe all of those individual angular momenta cancel out.

suspected black holes have been observed producing jets of material
Yes, but this has nothing to do with the Big Bang.

If a rotating black hole "exploded"
Black holes cannot explode. The jets of material are produced by the effects of rotating black holes on the matter surrounding them. See here:

http://en.wikipedia.org/wiki/Astrophysical_jet#Rotating_black_hole_as_energy_source

Jimster41
Gold Member
Imagine a perfectly uniform, perfectly still, infinite in extent cloud of particles suspended in infinite space. If such uniformity could be set up as an initial condition, and there was no inherent randomness to the motion of the particles, all the forces would cancel out and it'd remain in this static form forever.
However, there is (so far) no reason to think there ever was a perfectly uniform initial state at any time in the past of our universe, and even if there were, there is inherent randomness to the motion of the particles due to their quantum nature on small enough scales.
All you need for your perfectly uniform cloud to fall apart into clumps of matter contracting under their own gravity is the tiniest of nudges. Just as with a needle standing on its tip, as soon as you introduce any deviation whatsoever, no matter how small, the balance is irrevocably broken and forces no longer cancel out. Thus just as the needle eventually falls, regions of.overdensity will form in the cloud.
Thanks, that is exactly the image I have been struggling with (the initial motionless cloud). I sort of a associate this image with the "moment of last scattering". Is that wrong?

As the cloud collapses and parts of it will start to orbit other parts, you will always be able to select a limited volume in which matter has some angular momentum in one direction. For example, if your initially static cloud of particles collapses to form two locally overdense 'swirls' of matter, whose total angular momentum remains zero, you can always look at just one of those swirls as the portion of the cloud with a non-zero rotation that will govern its further evolution.
So do the two "locally overdense swirls" then cause particles near them to begin to "fall into orbit" around them. Why don't the nearby particles fall straight toward them and only start rotating once they get there? Frame dragging hasn't got anything to do with it does it? Am I wrong to think that there is GR is involved in the spread of "rotational motion" through the cloud?

Bandersnatch
I sort of a associate this image with the "moment of last scattering" is that wrong?
There were already overdense regions in the pre-recombination plasma. Their imprints on the CMBR can be observed.

So do the two "locally overdense swirls" then cause particles near them to begin to "fall into orbit" around them. Why don't the nearby particles fall straight toward them and only start rotating once they get there? Frame dragging hasn't got anything to do with it does it. Am I wrong to think that there is GR is involved in the spread of "rotational motion" through the cloud?
I don't think you need any GR here. It's just that there are more than two particles starting from rest in an otherwise empty space, which is the only way you'd get a straight line collapse. Add a third gravitating particle anywhere in there, and it's now a three body problem, which already doesn't admit any such nice, clean solutions. And there's of course more than 3 particles...
In other words, a test particle can't fall straight onto the massive body (let's call it A) because there are other massive bodies in a non-symmetric distribution (it's no longer an uniform cloud) pulling on it and imparting it with non-zero tangential velocity w/r to A, that then gets amplified due to the conservation of angular momentum.

Jimster41
Gold Member
I don't think you need any GR here.
So just old Sir Newton.
No curved space-time to see.

PeterDonis
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I don't think you need any GR here.
You do if the alternative is SR; but perhaps you just mean that the Newtonian approxmation for gravity is sufficient?

Bandersnatch
That's what I meant. Isn't it, though?

PeterDonis
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Isn't it, though?
For the general question under discussion, yes, the Newtonian approximation should be sufficient. If we were specifically discussing rotating black holes, then it wouldn't be.

Chalnoth
I don't think you need any GR here. It's just that there are more than two particles starting from rest in an otherwise empty space, which is the only way you'd get a straight line collapse. Add a third gravitating particle anywhere in there, and it's now a three body problem, which already doesn't admit any such nice, clean solutions. And there's of course more than 3 particles...
In other words, a test particle can't fall straight onto the massive body (let's call it A) because there are other massive bodies in a non-symmetric distribution (it's no longer an uniform cloud) pulling on it and imparting it with non-zero tangential velocity w/r to A, that then gets amplified due to the conservation of angular momentum.
Not really. This effect exists even with just two particles. It's down to the fact that their initial motion isn't likely to be exactly towards or away from one another (or at rest). Instead, each particle will have some random velocity, and as they fall towards one another, that random velocity will probably make them miss.

The real reason why you probably won't see them actually orbit is friction: you need friction for particles to settle into tighter orbits around one another, and if all you have are two particles, there's no friction to be had.

Bandersnatch