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Empty space ?

  1. Oct 22, 2006 #1
    Once again, hello to all !:smile: ,

    I'm back with another quick question ...

    Can you explain or redirect me to an explanation of what the term ' empty space ' means.

    It’ all over in the threads I read, but it’s not quite clear to me as to what it pertains to …

    Thank you,

  2. jcsd
  3. Oct 22, 2006 #2


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    "Empty" space is a theoretical construct that does not exist in our Universe. Even in the absence of matter and radiation, "empty" space is a teeming sea of virtual particles, coming into existence and annihilating in accordance with the Heisenberg Uncertainty Principle. You might think that for "something" to exist, there must be a "nothing" with which to contrast it, but that is not the case.
  4. Oct 22, 2006 #3


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    In general, "empty space" is a term used when the space baing refferenced is devoid of anything that would effect the outcum of the scenario in question.
  5. Oct 23, 2006 #4
    In quatum physics (or quantum field theory), 'empty space' means actually 'vacuum' and means the state of lowest energy (which is non-zero). So, empty space means in fact the sea (or soup) of virtual particles poppping in and out of existance.
  6. Oct 23, 2006 #5
    To quote Steven Hawking: "Empty space isn't empty"
  7. Nov 21, 2006 #6

    Chris Hillman

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    "Empty space" in the context of gtr?

    Since you are posting in the relativity forum, I will guess that you have encountered this phrase in the context of spacetime models, probably solutions of the Einstein field equation. If so, "empty space" refers to some region which contains no matter, but possibly contains some field such as an electromagnetic field (which will not interfere with uncharged test particles, except indirectly via the gravitational effects of the electromagnetic field energy). A "vacuum" region almost always refers to a region which contains no matter and no non-gravitational fields. Some authors use "empty space" as a synonym for "vacuum". An "electrovacuum" region contains only a gravitational and an electromagnetic field.

    Examples of regions in a spacetime model which would NOT be called "empty space" under any circumstances include "balls of perfect fluid". Such fluid balls are often used to construct simple models of isolated stars by "matching across the surface" (where the pressure of the fluid falls to zero) to an "exterior vacuum region". Similarly, collapsing balls of pressureless perfect fluid, or collapsing "dust balls" for short, are often used to construct simple models of gravitational collapse by matching across some (topologically) spherical surface (the pressure is everywhere zero in the dust, so we have a lot of freedom here) to an exterior vacuum region.

    In the context of stellar models or collapse models, you might see the terms "interior solution" (perfect fluid) and "exterior solution" (vacuum, or sometimes, imprecisely, "empty space"). If so, don't confuse these with discussion of the "interior region" versus "exterior region" of a black hole model; these would generally both be regions of "empty space"; their common boundary is the event horizon of the black hole.

    Chris Hillman
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