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Why Orbits and Rotation?

  1. Jul 24, 2004 #1
    This question has most likely been asked already: Why do planets, stars, galaxies, and other celestial objects rotate? And why do planets/stars orbit other planets/stars. If an object is to start rotating, it must initially have some non-zero net torque. But who/what is providing this torque? Are there planets, stars, etc. somewhere in space that are not rotating or orbiting another body?
  2. jcsd
  3. Jul 24, 2004 #2


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    The rotation is the result of some imbalance in the initial conditions of most systems. For example, take two objects being drawn toward each other by gravity. As they approach one another two possibilities exist; they could head directly toward one another a perfect collision course, but this possibility is extremely remote. The other possibility is that the collision course is not perfect, and the two objects miss on the first pass. Still drawn to one another by gravity, each will "turn" sharply in the direction of the other, resulting in two objects circling one another.

    It is a nearr certainty that there are objects in interstellar space that cannot rotate around a recognizable Center. These objects could be perceived as sitting still (from their own frame of reference), but it is far more likely that an outside observer will see them as "traveling in a straight line".
    Last edited: Jul 27, 2004
  4. Jul 25, 2004 #3


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    Are these two questions the same?

    (1) Is the universe as a whole rotating?

    (2) Is the sum of the angular momentum vectors for all mass in the universe non-zero?

    If the answer to (1) is "yes," then is the rotation susceptible to discovery?

    Jerry Abbott
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