gravity question

General physics and astronomy discussions not directly related to Celestia

gravity question

Post #1by Guest » 26.04.2004, 04:43

Probably a weird question but here goes anyway.

Does gravity bring order to a disordered system?

Or to put it another way, does the presence of gravity make a complicated system simpler?

E.G. A particle cloud in space large enough to form a sun and planets is a pretty complex system, lots of random motion and positions. Hypertheticaly, if this cloud has only random motion within the particles, then is the presence of gravity between the particles themselves enough to cause ordering over a long time period that will cause a solar system to form?

Every time I look at the situation closely it seems to me that it will become simpler and ordered over time even though initialy it appears that the cloud should just expand and stay disorganised.

If it is an ordering presence, then shouldn't it be possible to find a relationship between a given volume and a given mass that will create a solar system of a given size and provide a rough idea of the ratios between the mass of formed bodies and their orbits?


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Post #2by selden » 26.04.2004, 10:44

You'll have to define what you mean by "order".

lazy chaos

Post #3by lazy chaos » 26.04.2004, 20:04

Gravity does tend to clump things together which can be interpreted as some kind of "order". However the solar system is chaotic in its dynamics: any gravitational system involving more than two bodies cannot be solved analytically but only by approximations that get worse and worse through time.

And even so there are bursts of high chaos caused by certain alignments of Jupiter and Saturn - if their orbits enter the same plane then they can start disturbing the asteroid belt big time. Working out the dynamics of these planets' motion, it was calculated that the last time this happened was sixty-five million years ago...

As for ratios of orbits, several people (e.g. Patrick Moore) regard "Bode's Law" as a coincidence, and have shown that no Bode's Law relationship applies to the satellite systems of the giant planets.


Post #4by Guest » 27.04.2004, 02:10

Hi guy's,

I guess by order I'm refering to the final system being simpler than the initial system. E.g. It's a lot harder to predict the movement of a given individual particles as opposed to the same mass acting as say a few large objects. Even though both can be chaotic the object version is less influenced by the chaotic nature and should be more easily predicted.

That sounds pretty obvious but if it's the case then it seems like gravity "tending to clump things together" results in diminished influence of the chaotic nature of the individual particles. Again pretty obvious but where does this lead too? Over greater time do we assume the system tends to have less and less chaotic tendencies?

Logically this seems to be the case.
Looking at it another way, efficiency, the most efficient movement in the case of a particle cloud would be rotation. The most efficient form of rotation is circular, ie follows a sine wave. Anything not a sine wave must consist of harmonics and these would be of a lower level than the fundamental. A perfect example is wobbles in orbits. Since there is always some friction(from where is irrelevant) then the harmonics would disipate faster than the fundamental. Over time this leaves the system in a circular orbit.

The above paragraph is the thought proccess that inspired the initial post.

Looking at the example from lazy chaos where the orbits of Jupiter and Saturn cause asteroids to be disturbed, by disturbed I'm assuming knock a few out of their orbits. The knocked out asteroid is now out of the equation's once it's gone or becomes part of another planet, hopefully not ours! The system is simpler. The effect on the orbits of Jupiter and Saturn would also be ever slightly affected, though in this case it could either add or subtract chaos from their orbits. N.B. In this case I'm assuming chaos is the harmonics of the fundamental (which could be B.S. but would be difficult to distinguish in this case over the life span of these systems).

Just looked up Bode's Law, interesting concept but not quite what I was refering to. My reasoning is there would be many "Bode's Laws" where the relationship is defined by the initial mass and volume of the system. Hence the law that applies to the solar system wouldn't be the same as the one as for the giant planets, it makes no sense to even try to check for validity. The mass is different, the volume is different. I'm wondering if there is a "law" that gives us an expression like Bode's law for differing systems, not one expression that fits all. The other thing to note is that the gas giants and their satellites aren't really a system on their own, they are a system within a system hence may not have simple solution. Also such a law would also be most useful when the orbits are circular or near circular, since if they aren't then the effects of the chaos component would still be high.

Does this make sense or am I barking up an empty tree???

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Post #5by Matt McIrvin » 24.05.2004, 01:23


John Baez doesn't answer the question, but he gives a hint.

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Post #6by julesstoop » 25.05.2004, 22:50

Well, to me it seems any contracting cloud af gass is not an insulated system but part of the universe (as part of a galaxy or whatever): the contraction causes heat, heat causes radiation and this radiation increases the entropy of anything nearby (but not part of our 'system' in which entropy seems to fall). Secondly, any star formed will die in the future. The most extreme example of both effects being a supernova.

So, the answer to the question must be: 'space-time in general.'
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