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Where did angular momentum come from?

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stuh505
 
Reply Mon 30 Apr, 2007 10:50 am
Our understanding of the formation of the solar system relies on some initial angular momentum of the solar nebula.

First of all, what does this actually mean? It is easier to understand angular momentum for a coherent body. I guess it means that (on average) each particle in the nebula has some initial tangential velocity relative to the center of mass of the nebula. However, this velocity must be extremely low, because it cannot exceed the escape velocity of the system or else the nebula would have never collapsed.

Has anyone calculated what the range of average angular momentums would have resulted in forming a solar system similar to ours?

Ok, so where did this angular momentum come from? My guess is that it comes from the spinning of the milky way galaxy. That the arms of the galaxy cause an uneven distribution of mass on one side of the solar nebula, so that as the galaxy turns, it spins the solar nebula like a string spins a yoyo.

That makes sense so I'll assume it is correct. Next question: why does the galaxy have angular momentum? Well, clearly matter created by the big bang was not perfectly homogeneous. If it was, then the universe would just be a large perfectly evenly distributed soup of one or more (probably just one) kinds of fundamental particles. So that didn't happen. Either you can say that time was nonuniform or you can say that matter was created in a nonuniform way...doesn't really matter.

Ok, so instead of a uniform distribution, we get some random self-similar distribution. This explains why it is possible to have gravitational clustering to increase the heterogeneity of the system as opposed to gravity just pathetically trying to counteract expansion forces. So clusters form, galaxies, and within these clusters, solar systems, and within these clusters, planets, and below that the other forces are able to have effects breaking the fractal nature of things.

Under this model, I can see how a galaxy would develop angular momentum...exactly the same way as a solar nebula develops angular momentum, because with a fractal distribution of surrounding matter that is moving this will continue to accelerate the particles in the galaxy towards different locations.

However, it does seem that there would be some very delicate balance between the size, mass, and momentum of a cluster necessary to cause it to collapse into a galaxy or stellar system.

I feel like running some simulations on this to verify because it is easy to talk about these things and say "sounds like it makes sense" but without a simulation to back it up, it's just a guessing game.
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Type: Discussion • Score: 1 • Views: 1,738 • Replies: 25
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Quincy
 
  1  
Reply Mon 30 Apr, 2007 02:22 pm
Can't say I can contribute. Sorry. Sounds inetersting though, but a bit over my head.
0 Replies
 
rosborne979
 
  1  
Reply Mon 30 Apr, 2007 09:56 pm
Re: Where did angular momentum come from?
stuh505 wrote:
Where did angular momentum come from?


It's virtually impossible for any body, or collection of bodies, not to have angular momentum. To demonstrate this, simply try to throw any object without imparting any rotation to it. You won't be able to do it, even if the rotation is small it'll always be there. The same applies to nebulas and proto solar systems.

You don't need to postulate an external rotational component (such as galactic movement), mere chaos is sufficient.

And once collections of masses begin to collapse due to gravity, conservation of angular momentum makes the collection spin. It's inevitible.

Everything in space spins, unless external forces dampen the rotation (such as tidal locking in the Earth/Moon system, and artificial control such as man-made satelites). Once the locking mechanism is removed, even the barest impact from cosmic dust or micro meteors starts a slow spin again.

The ultimate source is as you suggested; asymetry. Without asymetry the Universe would be nothing more than a homogenous quark fog (and perhaps not even that much structure would exist).

Why we have asymetry is a great and fundamental mystery.
0 Replies
 
stuh505
 
  1  
Reply Tue 1 May, 2007 08:52 am
Re: Where did angular momentum come from?
rosborne979 wrote:
The ultimate source is as you suggested; asymetry. Without asymetry the Universe would be nothing more than a homogenous quark fog (and perhaps not even that much structure would exist).


Yes, if the universe was truly homogeneous, there would only be one type of particle...I'm not sure if it would be the quark. However, if it was the quark, then none of them could be confined in hadrons! In which case...they might not even contribute to observable reality, if there were a hypothetical observer.
0 Replies
 
rosborne979
 
  1  
Reply Tue 1 May, 2007 08:44 pm
Re: Where did angular momentum come from?
stuh505 wrote:
rosborne979 wrote:
The ultimate source is as you suggested; asymetry. Without asymetry the Universe would be nothing more than a homogenous quark fog (and perhaps not even that much structure would exist).


Yes, if the universe was truly homogeneous, there would only be one type of particle...I'm not sure if it would be the quark. However, if it was the quark, then none of them could be confined in hadrons! In which case...they might not even contribute to observable reality, if there were a hypothetical observer.


I was just quoting a physicist who once described the early Universe as a "Quark Fog". I'm not sure he was speaking literally. And he wasn't talking about a symetric mature Universe (it just sounded good) Smile
0 Replies
 
patiodog
 
  1  
Reply Wed 2 May, 2007 06:19 am
I'm WAY out of my depth here, but I think of the metaphor we frequently see of objects with mass distorting space: high-mass object are drawn as a concave dimple in in a flat plane of space. A bit like a bathtub drawn. And when stuff goes down the bathtub drain, it spins.




But, then, if I picture the Milky Way as a big storm, with an eye and rotation and whatnot, it's similarly unsurprising to me that the smaller systems that are thrown off of it would spin, as well -- like the tornadoes we get starting this time of year here in the Midwest.
0 Replies
 
Eorl
 
  1  
Reply Wed 2 May, 2007 06:42 am
Re: Where did angular momentum come from?
stuh505 wrote:


Yes, if the universe was truly homogeneous, there would only be one type of particle.


If the universe was truly homogenous, there would only be one actual particle...the universe... no?

Ros, if it's hard to avoid angular momentum, then it would be rare for a planet with any fluid parts in it's core, NOT to have a magnetic field?
0 Replies
 
J-B
 
  1  
Reply Wed 2 May, 2007 06:52 am
Interesting, I'm following this thread.
0 Replies
 
stuh505
 
  1  
Reply Wed 2 May, 2007 07:57 am
Re: Where did angular momentum come from?
Eorl wrote:
stuh505 wrote:


Yes, if the universe was truly homogeneous, there would only be one type of particle.


If the universe was truly homogenous, there would only be one actual particle...the universe... no?


No, by homogeneous we are referring to the distribution of matter/energy in the universe. The cosmological principle says that the universe is homogeneous and isotropic on a large scale. At any rate, homogeneous does not mean that every local "window" is identical, it means that the set is composed of identical elements.

Quote:
Ros, if it's hard to avoid angular momentum, then it would be rare for a planet with any fluid parts in it's core, NOT to have a magnetic field?


Every planet has a magnetic field, because every moving electric charge has a magnetic field.
0 Replies
 
Eorl
 
  1  
Reply Wed 2 May, 2007 03:53 pm
The moving electric charge has to be moving relative to something else right? The earth's core spins at a different speed than the crust. My question is really, is it actually easier for the core to be spinning at a slightly different speed than for it to be the same?
0 Replies
 
rosborne979
 
  1  
Reply Wed 2 May, 2007 06:19 pm
Re: Where did angular momentum come from?
Eorl wrote:
Ros, if it's hard to avoid angular momentum, then it would be rare for a planet with any fluid parts in it's core, NOT to have a magnetic field?


I suppose it would depend on what the fluid was composed of.

Why do you ask?
0 Replies
 
Eorl
 
  1  
Reply Wed 2 May, 2007 06:35 pm
Because most planets have magnetic fields, and therefore have a crust/core rotational difference as the cause (something I worked out many years ago in my head, that turns out to be the most recent theory - thus my interest).

I was thinking big external influences would be required to cause the rotational difference, but if what you say is true (and I don't doubt it is), then it would be difficult for a planet not to have a crust/core difference. I like that idea a lot.
0 Replies
 
rosborne979
 
  1  
Reply Wed 2 May, 2007 06:57 pm
Eorl wrote:
Because most planets have magnetic fields, and therefore have a crust/core rotational difference as the cause (something I worked out many years ago in my head, that turns out to be the most recent theory - thus my interest).

I was thinking big external influences would be required to cause the rotational difference, but if what you say is true (and I don't doubt it is), then it would be difficult for a planet not to have a crust/core difference. I like that idea a lot.


Most planets probably don't have 'crusts', at least not solid crusts.

I'm really not an expert on stuff like this, but I would guess that most fluid plasma interactions of compressed elements (helium, hydrogen, lithium, nickel, iron etc), are prone to generating electromagnetic fields. So in general, I would guess that almost every planetary body with a fluid interior would generate an electromagnetic field.

Solid bodies like the Moon would probably have a much smaller (or no) field.

Does our Moon have any electromagnetic field? I don't even know. Is the Moon even solid all the way through?
0 Replies
 
Eorl
 
  1  
Reply Wed 2 May, 2007 07:05 pm
University of California-Berkley wrote:
Quote:
If the moon today has no magnetic field, then where did the original magnetic field come from? Dating of Apollo moon rocks hints that during the period 3.6-3.85 billion years ago the moon did have a magnetic field, probably because its core was still liquid and spinning enough to generate a magnetic field comparable to that of the Earth.


http://www.xs4all.nl/~carlkop/lunmeti.html
0 Replies
 
stuh505
 
  1  
Reply Wed 2 May, 2007 08:03 pm
Eorl, all planets and moons and basically all matter has magnetic fields because they all are made of molecules and atoms that have electric charges. all electric charges have an electric field, and all moving electric charges have a magnetic field. In fact the magnetic field can be explained by applying the relativistic Lorentz transformations to the electric field...so they are really the same thing. This is different from the large unified magnetic field that a planet can have due to turbulent flows around the core.

rosborne, all planets are differentiated meaning that the denser elements have gravitated towards the center of the planet. You can assume that ALL planets are differentiated because no planet is 100% the same element, and if it had time to spherize, then it had time to differentiate.
so regardless of whether or not the planet has a solid surface, density is going to be roughly radially symmetric...and most planets do have some iron, its heavy it goes to the core. the moon does. our moon does have a magnetic field (in addition to the random magnetic fields that everything has).

A fluid core is not necessary either. You have to think about all planets behaving in a liquid way, because they do...even the rockiest planets are subject to tidal forces that literally bulge the shape of the planet into a non-spherical shape...the side that is facing the sun is stretched towards the sun due to gravitation (or if its a moon, stretched to its planet). as the planet/moon rotates, the bulge shifts across the surface, heating the planet and taking energy from its orbit. the point is that there are fluid-like actions happening in all planets.
0 Replies
 
rosborne979
 
  1  
Reply Wed 2 May, 2007 09:32 pm
stuh505 wrote:
Eorl, all planets and moons and basically all matter has magnetic fields because they all are made of molecules and atoms that have electric charges.

[URL=http://www.nineplanets.org/luna.html]This Source[/URL] wrote:
The Moon has no global magnetic field. But some of its surface rocks exhibit remanent magnetism indicating that there may have been a global magnetic field early in the Moon's history.


While all matter may carry some small EM field, the primary global fields are a result of fluid motion which is not present in all planets and moons.

Tidal forces will result in some compression and heat, but not always enough to allow for fluid motion sufficient to generate global magnetic fields.
0 Replies
 
rosborne979
 
  1  
Reply Wed 2 May, 2007 09:36 pm
Eorl wrote:
University of California-Berkley wrote:
Quote:
If the moon today has no magnetic field, then where did the original magnetic field come from? Dating of Apollo moon rocks hints that during the period 3.6-3.85 billion years ago the moon did have a magnetic field, probably because its core was still liquid and spinning enough to generate a magnetic field comparable to that of the Earth.


http://www.xs4all.nl/~carlkop/lunmeti.html


Yes, I agree. There was a time before the moon had 'frozen', when internal fluid motion must have produced a magnetic field which affected the rocks. But that time is long past.
0 Replies
 
Eorl
 
  1  
Reply Wed 2 May, 2007 09:58 pm
"fluid motion" is not the whole story.

Quote:
The metallic core of our planet is spinning faster than the rest of it, according to evidence unearthed by Harvard geologists. And this hellishly hot core, almost as big as the moon, apparently is growing in size.

"It's like a planet within a planet," says Adam Dziewonski, Baird Professor of Science.
.....

The highly conductive iron moving in a magnetic field generates electricity, creating the equivalent of a huge generator, or dynamo, at the planet's center. This electricity, in turn, has its own magnetic field which is responsible for compasses pointing north, northern and southern lights, and other effects at the planet's surface.

Magnetic fields in the core reach strengths of 200 or more times greater than at the surface. The intense field at the bottom of the outer core penetrates into the inner core, coupling the two together.

"We believe this coupling provides enough twisting motion, or torque, to power for extra rotation of the inner core," Dziewonski says. The mechanism works somewhat like that of a motor wherein a rapidly changing electromagnetic force causes a rotor to rotate. In this case, the motor rotor is the size of the moon.

This neat explanation leaves one frustrating question: where did the magnetic field come from that originally started the geodynamo? "Once you get things going, the electric current generated by the dynamo can reinforce it," Dziewonski notes. "But we don't know how things got started in the first place."


Which brings us back to what got me interested in angular momentum.
0 Replies
 
rosborne979
 
  1  
Reply Wed 2 May, 2007 10:13 pm
Eorl wrote:
Which brings us back to what got me interested in angular momentum.


As Stuh mentioned earlier, planets tend to be stratified by mass density.

As the planets coalesced, angular momentum alone would tend to start them spinning, and then as the heavier elements moved toward the core, the core would tend to spin faster than the upper layers.

Most of the motion probably starts there.

But there is also convection between layers, and probably pockets of convection rotation which may also generate fields.
0 Replies
 
Eorl
 
  1  
Reply Wed 2 May, 2007 10:15 pm
Yep, that's pretty much what I'm thinkin'
0 Replies
 
 

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