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Does motion require emptiness

  1. Jul 16, 2009 #1
    Does the fact that things move mean that there exists something called emptiness for which them to move into?
  2. jcsd
  3. Jul 16, 2009 #2


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    "The concept of space is considered to be of fundamental importance to an understanding of the universe although disagreement continues between philosophers over whether it is itself an entity, a relationship between entities, or part of a conceptual framework."
  4. Jul 16, 2009 #3

    Not necessarily. Imagine an empty universe where all our laws of physics still hold. Now imagine filling all of it uniformly with matter. Now imagine hollowing out a sphere the size of a basketball. Now put the basketball inside, and start it spinning (imagine there is no friction... or even if there is, so what?) There is no empty space in the universe (the technicalities of there being void inside an atom can be overcome if necessary or better left ignored) and the basketball is spinning. No laws of physics are violated, so sure, why not.

    Another experiment: fill a bucket with water. Put a small piece of wood inside. Close the bucket so that it is airtight and there is no air left in the bucket. Shake the bucket. The wood will move.
  5. Jul 17, 2009 #4
    This is probably the second oldest question in the world (right after, 'what do women want?')

    by "hollowing out a sphere" do you mean "replace the matter with emptiness"? Once you have "hollowed out" this void, how do you get your basketball through the surrounding matter to enter that void?

    I doubt the OP (or anyone else) will be satisfied with a one paragraph answer to this question. There probably isn't really an answer anyway.
  6. Jul 17, 2009 #5
  7. Jul 17, 2009 #6
    "by "hollowing out a sphere" do you mean "replace the matter with emptiness"? Once you have "hollowed out" this void, how do you get your basketball through the surrounding matter to enter that void?"

    Yes, just carve out a sphere. If you have a problem with the thought experiment, put the ball there first, and then fill the rest of space with matter. You can start the ball spinning first, if you like.

    "How would you spin the ball without applying some sort of pressure."

    Technically, pressure is the force applied in a direction perpendicular to the surface of an object. Clearly, to set it spinning, you need only apply a small amount of torque, and this is applied tangentially for maximum affect. So no pressure would need to be applied.

    And you *asked* whether empty space was needed for motion, not whether or not you needed empty space to start motion. These are different questions. Clearly, motion can be sustained in the absence of empty space. I have already shown that with several examples. Can it be initiated, however?

    This is a more difficult question, the answer to which I am inclined to say is "not really". Of course, the supposition of "no empty space" would require not only non-physical ideas of matter, but also necessitate a universe filled with infinitely much mass (an infinite universe, because otherwise there would be empty space outside, and you'd just have to go to the edge of the universe to get motion fairly easily).

    There is a fairly easy experiment for this, I suppose. Get a container and fill it with water. Put a fish inside. Close the container so that no air is in the container and the container is air tight. Can the fish swim around?

    The only problem with this is, of course, that there is empty space between molecules of water and between atoms inside the molecules. However, water does not compress easily.

    I can only imagine that the outcome of this experiment would be that the fish can swim just fine. And I don't see pressure as being a problem... you can have different pressures in water, although the density of water remains nearly constant.
  8. Jul 17, 2009 #7


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    Not in a real world. This would require infinitely dense objects made up of infinitely small particles, zero friction, zero viscosity. If the particles were tiny cubes with no gaps, then the movment between particles would be restricted to parallel to the surfaces of the cubes. I'm not sure if this would allow a solid to flow through a fluid. Also if the cubes were incompressable, then the speed of sound (or information of movement) is infinite.
  9. Jul 17, 2009 #8

    For the question to make sense, you have to make certain simplifying assumptions. For instance, I already mentioned the problem with matter and the infinite universe.

    Otherwise, zero friction and zero viscosity aren't really required at all. We can imagine a continuum of matter with any density and friction characteristics we want.

    If you have a problem with this assumption, then there is no answer to the question, because there's no way of knowing whether or not motion could be sustained in the absence of the void.

    In the real world, there is void, so it the question is sort of superfluous. That's why we have to imagine slightly different, though still physically sensible, worlds.
  10. Jul 17, 2009 #9
    Do you see that you are requiring an empty place to place your ball, and then go on to conclude you don't need empty space to move an object into? Your thought experiment is inconsistent, or ironic, or something. It doesn't hold up to scrutiny; it seems to ignore the issue it professes to explain.
  11. Jul 17, 2009 #10
    We can move on the earth right? Our air is made of tiny gas particles, so we can move through a medium, tough, it would be tougher than moving through a vacuum.
  12. Jul 17, 2009 #11


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    Is it possible you are missing his point?

    OK, granted you need to move the basketball into the void.

    Now pause.

    Now...that basketball can rotate without any void to move into.

    That is one thing that is special about a sphere - it has perfect rotational symmetry.
  13. Jul 18, 2009 #12

    I'm not ignoring anything, really. I'm only doing a thought experiment, since you have to for this sort of question. Why? Because there is empty space in real life.

    In a theoretical sense, I don't see how motion requires empty space in order to be sustained. It's a hard thing to think about because everything we experience occurs with empty space around it, so while something may not actually require empty space, it may seem like it since that's the only way we've ever seen it.

    Is there a thought experiment by which it is not possible? Name one, and see if the same sorts of objections you're making to my assumptions don't equally apply to your own.
  14. Jul 18, 2009 #13
    Yes I was missing AUM's point. Let me think about this some more before I make further noises.
  15. Jul 18, 2009 #14


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    Paraphrasing AUMAthTutor:

    Put the ball in place first.
    Start it spinning.
    Fill the space around it with matter.
  16. Jul 18, 2009 #15
    Is motion, of either bosons or fermions, possible through a medium of Planck density?
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