Naked Science Forum
Non Life Sciences => Physics, Astronomy & Cosmology => Topic started by: thebrain13 on 20/08/2006 06:30:26
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Ive heard explanations for why galaxies, planets, stars, basically everthing weve ever heard of, with any significant size, like a planet or bigger, rotates. But why? there is no modern theory which explains why a body would start rotating, just that if it was rotating that it must of been rotating in the first place , or it was hit with some large object in its past, all at random.
I dont see how this explanation makes sense at all. If the rotational periods of objects were "random" we would expect to find random rotations with all the objects we see. You know we might find planets with huge rotational velocity, and some with hardly any. And I would expect that on average a larger object would rotate slower, given the laws of averages an object the size of jupiter or the sun would rotate slower, you know because given its mass any random event would accelerate it slower, for example if a fast moving large asteroid hit the sun it would do harly anything, if it hit pluto, it would spend it spinning. I would think the large amount of objects with random velocity that would hit the sun from all directions would tend to balance its velocity out, so a large object would rotate slower on average than a small one.
This however, is hardly the case. In fact quite the contrary, the planets in our solar system (which are the only planets I have data for) rotate faster as they get bigger. The correlation is strikingly consistant, all planets bigger than earth rotate faster, all planets smaller, rotate slower.
Ill organize their mass and their rotational velocity. the mass of the planet is the first number and the second number is their rotational velocity. Tell me if you see a correlation 1.jupiter1 2.saturn2 3.neptune3 4.uranus4 5.earth5 6.mars6 7.venus9 8.mercury8 9.pluto7 And even the suns rotational velocity is higher than all the planets[:)]
So as you can see a planets\stars rotational velocity is based on the planets size. And its harldy random. this pattern is followed exactly by all the planets, with the only outliers being mercury and venus, whose orbits are highly influenced by the fact that they are so close to the sun, the tidal forces due to being so close to the sun is overwhelming. hence their revolutional period closely matches their year. Like the earth and the moon.
So your telling me that, all the planets randomly follow this pattern? It boggles my mind why people believe that all the planets randomly rotate so consistantly, how can planets as big as jupiter randomly rotate at thousands of miles per hour? Now consider the fact that our galaxy the milky way also rotates. The milky way has 100 billion stars in it, most of the stars are believed to have many planets around them. We rotate the galaxy at a speed of 550,000 miles per hour.
Your telling me an object as humungous as a galaxy randomly rotates at speeds that high? Are you kidding?
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You have hit on a very fundamental point that is generally not very well explained by the text books and it concerns one of the most fundamental laws in the universe, the conservation of angular momentum. People understand the conservation of energy (you don't get something fo nothing)but forget the importanf bit about angular momentum
The conservation of angular momentum means that as clouds contract they rotate faster, like a spinning skater pulling in her arms.
Consider the later stages of the big bang with a lot of hydrogen and helium batting round at temperatures of a few thousand degrees centigrade just about the time that the cosmic microwave background was formed when the electrons combined with the neucleii its a bit like a big explosion and the gas is all turbulent and swirling around. This turbulence continues through the "dark ages" until the gas expands with the universe and gets cool enough for the first stars to form. These stars are forming from turbulent clouds and as they contract the conservation of the angular momentum means that as the clouds contract they rotate faster and faster.
The formation of planets is an important way for the contracting cloud to shed its angular momentum, If all the angular momentum in the solar system were concentrated in the sun it would be rotating too fast to be stable.
You have a point about the rotations of the planets depending on size but there are other interactions that can change the rotation speeds of planets by coupling the energy into the orbits via satellites and it is thought that the early rotation speed of the earth was much faster than it is now.
As things contract they tend to get faster. some neutron stars (about the size of the earth) rotate at around 1000 RPM! I expect that all black holes will be rotating at pretty well the maximum speed possible, with rotation velocites around the speed of light at the event horizon, unless they are very big.
I can also correct you about the rotation of our galaxy the velocity of the sun in its orbit is around 200 km/second a good deal faster than your 55000 mph which is a hood deal less than the 18 miles/second that the earth is moving in its orbit round the sun, but the galaxy is a big place and it takes around 250million years for the sun to complete its orbit but it is interesing to note that there has been complex life on earth during several orbits of the galaxy by the sun.
Learn, create, test and tell
evolution rules in all things
God says so!
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Originally posted by Soul Surfer
You have hit on a very fundamental point that is generally not very well explained by the text books and it concerns one of the most fundamental laws in the universe, the conservation of angular momentum. People understand the conservation of energy (you don't get something fo nothing)but forget the importanf bit about angular momentum
The conservation of angular momentum means that as clouds contract they rotate faster, like a spinning skater pulling in her arms.
The trouble with this is that it confines itself to an explanation of the conservation of rotational motion – but no motion, whether rotational or linear – is such that it cannot be dissipated.
Yes, when a skater pulling in their arms does conserve angular momentum, but that angular momentum was created from the chemical energy in her muscles, and is dissipated by frictional forces – thus the angular motion is not conserved in isolation of the conservation of energy, but is merely an expression of that energy.
Another case where conservation of rotational energy is more difficult to explain is when looking at the workings of a rack and pinion (although you can argue that for any finite length rack, there must always be a nett zero rotational energy, and the real problem only exists if one hypothesises an infinite length rack).
George
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but that angular momentum was created from the chemical energy
Not really she has transferred angular momentum from the earth to herself by the application of energy from her legs, her arms don't manufacture angular momentum just redistribute it, in way that increases her total kinetic energy - this takes energy but doesn't change her angular momentum. Friction will then transfer the angular momentum to the air.
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Since we are talking about rotation and angular momentum, I would like to bring in a question which is related to this. Does it make sense to talk about a rotating universe? If yes, how would this affect the expansion of the universe ? Has anyone attempted to solve the Einstein field equations for this case ?
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Another someone. There is as near as dammit no friction in space so both kinetic energy and angular momentum are totally conserved for larger bodies virtually the only forces acting are grvitational although electromagnetic forces can play a part in special circumstances.
We discussed the possibility of a total rotation in the universe some months ago it is an interesting thought but the mere fact of the expansion from small beginnigs imply that it must be very small by now even if we assume that it must have been a fast as possible when the universe was small.
I myself am very interested in the process of collapse inside the event horizon of a large rotating black hole and feel that a lot of it it can be handled using our current understanding of physics without need of quantum gravity, but I haven't been able to find results from meny people studying it.
Learn, create, test and tell
evolution rules in all things
God says so!
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I neglected this thread a little, and I shouldnt have, this is good stuff. Anyways I have a swivel chair. If I spin the chair with my arms raised outwards, and then move them closer to my body, my body will spin faster, in accordance with conservation of angular momentum. However I also noticed that if I raise my arms, and then bring them in, without initially spinning my chair, No speed is obtained. There is no momentum to conserve.
That is precisely my point, you cant say everything spins now because of conservation of momentum, because that law can only CONSERVE momentum. I want to know, how did all these objects gain this momentum in the first place
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If we accept that the uni-verse is the only body that exist as it name implies the idea that it can be rotating is meaningless, where do we stand to observe this 'rotation'
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Im not talking about the whole universe rotating, Im talking about almost every object in it is rotating. Stars planets galaxies, all rotating relative to each other.
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The average density of the universe is over the past ten billion years or so is only a few atoms per CC (i think) Any solid bodies like planets have forme from rather tenuous material that has contracted a great deal. This material was turbulent gas and dust that was unlikely to be rotating very much to start with but there is almost certain to have been a little bit of inbalance in the rotation which after contraction by factors of many millions becomes a significant rotation. in fact it seems likely that the formation of planets is part of the process that allows stars to form because most of the angular momentum in the solae system is concentrated in the planets and if all of this were to be dumped into the sun it would become rotationaly unstable.
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This is the accepted explanation for why the planets rotate. (i think) However I just dont understand how this is an accurate model of our universe today.
My understanding of angular momentum goes something like this. If you have two seperate clouds of gas, both equal in mass and their distribution of the mass is equivalent, one isnt thicker or thinner in one area than the other. Then the amount of angular momentum it had to begin with will directly affect its rotational velocity once the cloud condenses. So if one cloud had twice the angular momentum to begin with it would rotate at twice the speed when it condenses. If one cloud had 100 times the angular momentum to begin with it would rotate a hundered times faster as a planet. This is correct right?
So, when you look at it like this you find that the amount of angular momentum these clouds must of had to begin with must of been very precise, to explain the very similar rotation speeds of the planets and stars we know of today. That is the opposite of random. If the planets orbited randomly we might find planets that rotate very fast planets that rotate very slow. These planets are too similar to be random.
And one other thing, if the clouds constituent parts are truely random, then the larger a cloud is, the more likely its average angular momentum per mass, is going to be balanced out, or lower. However the opposite has proven true.
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But we do have a wide range of rotational speeds for planets such as the large Jupiter rotating in nine hours and Venus taking several hundred days.
There is a well defined upper limit due the strength of materials and some stars and planets are seen to approach this.
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well, venus and mars days match their years. Thats because those two, have huge tidal effect from the sun. Outside of those two they are very similar. The range is jupiter, the fastest, has a day of .41 earth days to the slowest mars of 1.03 earth days. To me that is very similar. I can hardly consider that random, the amount of energy per mass is very similar. Remember the theory says violent turbulent gas clouds randomnly rotate. So all we know is that the most turbulent violent gas clouds had about twice as much energy as the least. Thats similar.
And if you continue to insist that they are completely random, I can just bring up galaxy rotational velocity.
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AS I have already pointed out there have been a lot of angular momentum transfers in the formation of our solar system so the masses and orbits of the planets and their rotation speeds are not simply what the bits of the cloud that they formed from possessed to start with. but different stellar systems posses considerably different values of angular momentum in many cases they form multiple star systems and not simply solitary stars with planetary systems.