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Am I moving, or is space moving?

  1. May 3, 2015 #1
    I need some help with this because I'm not sure which is true. Two distant stationary objects in the universe will be moving away from each other despite the fact that they're stationary, this is because space itself "moves" or expands correct?

    So in this case space is moving, not the galaxies?
  2. jcsd
  3. May 3, 2015 #2


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    The galaxies are moving, as you say with respect to each other.

    When you say they are stationary, this means each is stationary with respect to its local environnent, and with respect to the universe as a whole : the universe looks as symmetric from either viewpoint.

    Say we are in galaxy A looking at distant galaxy B. B is surrounded by other nearby galaxies B1 B2 etc... On average, from our perspective, B, B1, B2 share the same motion relative to us.
    Which is the same as saying that from B's perspective, the group B1, B2,... has zero overall motion, or that B is stationary with respect to that group.
    Actually, to better accuracy B will see itself at the center of a symmetrical universe and it is stationary with respect to that.

    The "dots on an expanding balloon" image says it all better : look at the motion of the dots, forgetting the balloon. This is exactly how it works. Each dot moves away from other dots, and each is stationary with respect to the center of the group of surrounding dots.

    Last edited: May 3, 2015
  4. May 3, 2015 #3


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    I think you fail to realize that "moving" is a very tricky subject on cosmological scales. Galaxies far apart are RECEDING from each other but "moving" is not quite correct. It's a hard concept but space isn't moving and galaxies aren't moving, but galaxies are getting farther apart. "Moving" is more of a local phenomenon. Google "metric expansion" and as the rabbit suggested, read the link in my signature.
  5. May 3, 2015 #4
    Sure but with the balloons and dots analogy, the balloon (space) is expanding, the dots are not moving... their location is fixed? Like the galaxies in space are not moving, the space is expanding.
  6. May 3, 2015 #5


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    I take it you have not read the link.
  7. May 3, 2015 #6


    Staff: Mentor

    This is a matter of interpretation and choice of coordinates. "Motion" is not an absolute concept. Relative to the standard coordinates used in cosmology, the galaxies are not moving; the scale factor in the metric is changing with time. Sometimes this is described as "space expanding".

    However, we could also choose coordinates centered on us in which distant galaxies are moving away from us, and space is not changing at all. So the question you are asking doesn't have a unique answer.
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