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Name: Jim
Status: educator
Age: 60s
Location: N/A
Country: N/A
Date: 1999 - 2000

What is rotation that it is found everywhere at all size scales in the universe? It exists in spin in fundamental particles, as rotation of planets about suns, of suns in galaxies, of galaxies in clusters, etc.

The person who asked me this question said they had tried to deal with things like conservation of angular momentum, but that doesn't really say why rotation itself is such a common thing at all scales of physical reality. Can you help?

Two facts imply rotation. (1) we have more than one spacial dimension, that is, it takes more than one number to say where in the world you are. Why there are 3 dimensions, and not 1 or 5 billion is something no one, I think, knows. (2) empty space looks the same in all directions. That is, if you go straight forward 10 miles in empty space, or turn immediately left and go 10 miles in that direction, you see no different, by which I mean any physics experiment your perform comes out the same in both locations. Why this is true is not really questioned, because it's the simplest thing you could imagine, and you'd really need a reason for it to be other than this.

Because of (1) and especially (2), in the absence of any other matter, it costs no energy at all for a body to turn to face another direction. Why? Well, if it DID cost energy, then you could tell what direction you were facing, or going toward, and (2) would not be true anymore.

Because it costs no energy for bodies not near other bodies to rotate, they usually do. Or more precisely, if even once they are set rotating, then there is usually nothing to stop them doing so forever. So we see a lot of rotating things.

A further, very important fact, is that when other bodies ARE near, they usually cause orbital motion, which is another form of rotation. This is true because of the interesting fact that (3) gravitational forces between extended spherically symmetric bodies act as if all the mass of each body were concentrated at its center.

Because of (3), if we see two spherical bodies near one another, we can immediately reconsider this system as ONE extended body consisting of two blobs on the ends of a long, massless, invisible, springy stick. The two bodies may approach or recede from one another, which we just consider the springy stick getting longer or shorter. When we consider the whole stick-with-blobs system, we see that the stick-with-blobs will in general rotate, because it is an isolated object, and there is no energy cost for it to do so. Stepping back and looking at the system as two bodies again, we see that the spin of the stick-with-blobs object is equivalent to rotation of the two bodies about their common center of mass. In the case of two equal-mass bodies, like a binary star, that would be about a point equidistant from each. In the case of a heavy and very light body, like the Sun and the Earth, the point of rotation is barely away from the center of the Sun towards the Earth, so we might as well say the Earth rotates around the Sun.

Spin of atomic particles is completely different.


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