# Why does a gas cloud spontanously rotate?

1. Oct 9, 2005

### BigMacnFries

I have read numerous books that describe the formation of a solar system from a cloud of gas. They state that as the gas contracts it rotates and as it rotates it flattens out into a disk. I do not understand why the gas molecules start to rotate, how the gas molecules decide which way they are all going to spin and once the cloud is spinning why it flattens out. Can somebody please explain?

2. Oct 9, 2005

### Chronos

A Newtonian physics thing. Momentum is preserved by all bodies in motion. The molecules in a gas cloud move linearly in random directions until a force is applied [like gravity]. Once influenced by gravity, their momentum shifts from linear to angular momentum [i.e., their trajectory becomes curved]. Whether or not they escape is purely depends on their initial momentum and the strength of the force acting upon them. The ones that lack enough energy to escape are doomed to spiral inward.

3. Oct 9, 2005

### SpaceTiger

Staff Emeritus
I think it's fair to say that astronomers aren't sure either, but it probably has something to do with tidal forces (i.e. forces that pull differently on different parts of the cloud) from the galaxy and other nearby objects. Remember that it only takes a tiny amount of rotation when the cloud is large to make it rotate quickly when it's small -- angular momentum is conserved.

Randomly is the short answer. The long answer would involve correlations with angular momentum on larger scales. At the largest scales, however, the sources of angular momentum in collapsed structures are almost certainly random. If this weren't the case, the universe would have a net angular momentum and the cosmological principle would be violated.

This is a tough question to answer from first principles. Basically, it has to do with the fact that a disk is the lowest-energy configuration for a strongly rotating system. If there were no net angular momentum, the lowest-energy configuration would be a sphere. The latter case just follows from symmetry. In the absence of a preferred direction or axis, there's no reason the density should be higher in any direction or along any axis. Thus, a sphere. If there is net angular momentum, however, there is a preferred axis (i.e. that around which its rotating), so one would expect the system to form a shape that had symmetry about that axis. This could be a disk or, perhaps, a spheroid, but it just turns out to be a disk.

In real systems, however, the single pseudovector (angular momentum) is not enough to describe the motions of the system. One should also consider tensor terms and complicated gravitational interactions in order derive the final state. You probably need not worry about this, however.

4. Oct 9, 2005

### Chronos

Exactly! My slip into two dimensional thinking is showing [differential geometry still makes me mad]. Obtaining flatened rotational curves in 3D is difficult. Tidal forces [both gravitational and electromagnetic] nudge outlying particles to align with the flow, as I recall.

5. Oct 10, 2005

### BigMacnFries

Ok this is probably a bit oversimplified but Im gettin the idea that these particles have initial momentum and where there are flutuations in mass these particles curve around or orbit the more massive area, I guess creating a vicious cycle. I would assume that because of initial random motion all the different orbits would be on any plane that intersects the center of the mass and would go clockwise or counter clockwise if looking from the same view.

If what I've said is roughly correct then somehow this system transforms itself into one where nearly all particles rotate around the center of mass in the same direction, as all our planets and sun rotate the same way. Is this correct and if so how?

6. Oct 10, 2005

### SpaceTiger

Staff Emeritus
All the laws of physics say is that the total angular momentum must be conserved, not that it must be conserved for any individual object. The individual gas particles or solid bodies will interact with one another both gravitationally and electromagnetically, so it's a simple matter to exchange angular momentum between particles and distribute it more or less uniformly in the cloud. That the distribution should be approximately uniform (in space) is just a consequence of statistical mechanics.

7. Oct 10, 2005

### Turbot

With respect to what frame must angular momentum be conserved? If we reject action at a distance, as Einsten did, we must contemplate an ether:

If we do not admit to an ether (a preferred local reference frame), then the rotation of a rigid body or rotation of a fluid body can have no have no effect on the rigid body or the fluid body, absent action at a distance mediated by remote objects.

8. Oct 11, 2005

### Chronos

There is also the small matter of turbulence that creeps into the picture for a fluid in motion [which accurately describes a rotating gas cloud]. Modeling turbulence is horrendously difficult. Our math is only sufficient to solve simple cases. As Heisenberg is credited with having said:

"When I meet God, I am going to ask him two questions: Why relativity? And why turbulence? I really believe he will have an answer for the first."

9. Oct 11, 2005

### Garth

Perhaps (?): "I have seen the business that God has given to the children of man to be busy with. He has made everything beautiful in its time. Also, he has put eternity into man's heart, yet so that he cannot find out what God has done from the beginning to the end."Ecclesiastes 3:10-11 (English Standard Version)

Garth

10. Oct 14, 2005

### Phobos

Staff Emeritus
Turbot -
That line of discussion needs to be presented in the IR forum rather than here.
thanks

Last edited: Oct 14, 2005
11. Oct 14, 2005

### Turbot

Darn! I was under the impression that if Einstein expressed an opinion relating to cosmology that it might be appropriate to post it here.

12. Oct 15, 2005

### Phobos

Staff Emeritus
I'm asking that you do not hijack/confuse the issue with disproven ether theory.

13. Nov 22, 2005

### CarlB

As has been mentioned above, the direction is the direction they were rotating in before the whole thing contracted. As the contraction continues, the rotation has to speed up in order to preserve angular momentum.

The reason that any given patch of gas is already slightly rotating is pretty much the same reason that any given patch of gas is already moving in some direction. Past influences left a slight rotation.

For example, if another star flew by a patch of gas, it would tend to leave the gas rotating slightly. This is because the parts of the gas that are closest to the star get attracted more to it.

Carl