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If the universe is finite in size, what is at the end of it?

  1. Jan 10, 2013 #1
    If the universe is finite in size, what is at the very edge of it?
     
  2. jcsd
  3. Jan 10, 2013 #2
    Its boundary, which lies at infinity. :wink:
     
  4. Jan 10, 2013 #3

    micromass

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    Finite in size [itex]\neq[/itex] has an edge.

    Look at our earth: it's finite in size but it doesn't have an edge.
     
  5. Jan 10, 2013 #4
    I think OP is referring to the 3D projection of the multidimensional universe (or as we normally see it). Although multidimensional universe might not have an edge, our 3D representative one might have one.

    e.g. circle, which is a projection of a sphere in 2D.
     
  6. Jan 10, 2013 #5

    micromass

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    Projection of the universe on what?? The universe is all that there is :confused:
     
  7. Jan 10, 2013 #6
    Cheese lots and lots of it.
     
  8. Jan 10, 2013 #7
    When an apple is cut in two halves, the apple is all that there is. Yet we get a projection of apple in 2D by the framework defined by the movement of a knife. That framework has no existence, yet the apple is cut.
     
  9. Jan 10, 2013 #8

    micromass

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    Universe [itex]\neq[/itex] apple.
    Making physical statements based on loose analogies like this is very dangerous.
     
  10. Jan 10, 2013 #9
    What do you mean? I live on it. (tee hee I live on the edge)

    Seriously though, I am on the edge of the Earth.

    Isn't this one of those "dangerous" & loose analogies?
     
  11. Jan 10, 2013 #10

    jgens

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    I think micromass is looking at the earth as a sphere rather than a ball.
     
  12. Jan 10, 2013 #11
    Would you explain how it is a loose analogy?

    I think nitsuj meant boundary, when he said "edge". It determines whether you are inside or outside.
     
  13. Jan 10, 2013 #12
    Hmm looks like there is a difference between boundary and edge that I didn't know of.
     
  14. Jan 10, 2013 #13
    Sorry I'm not sure of the difference
     
  15. Jan 10, 2013 #14
    Imagine a two dimensional creature living on a plane. If the plane is finite, then it has a boundary line around it and the creature can't go beyond that boundary because that's the edge of the universe. Now consider a creature living on the surface of a very large sphere. To the creature, it seems that the universe is a plane if the radius is large enough. Such a creature lives in a finite universe, but it has no edge. Now think about how we may be living in a finite 4 dimensional curved universe that has no edge.
     
  16. Jan 10, 2013 #15
    I totally appreciate that popular analogy, I find the transition from 2D to 3D is lost on me though :cry:
     
  17. Jan 10, 2013 #16
    Do you mean to tell me that you can't imagine a 3 dimensional object curved in the 4th dimension? Yeesh.
     
  18. Jan 10, 2013 #17
    The trouble with this model is that we can easily predict the cheese would rot and smell bad. No one has ever detected such a cosmic stench, and any attempt to preserve this model will just be more speculation. However, I read a book by a Dutchman whose grandmother told him when you came to the edge of the universe it was covered over with old newspapers and flour paste. This vastly more sophisticated model certainly explains the absence of "The Cosmic Stench", and it also happens that all the "dark" matter they can't find exactly equals the amount of newspaper and flour it would take to cover the edges of the universe. I'd call that 'the smoking gun that shot the magic bullet' if ever there was one.
     
  19. Jan 10, 2013 #18
    lol, i know right. How has the spoon made it's way to my mouth all these years?
     
  20. Jan 11, 2013 #19
    We might be living in that or something else. If we can never literally experience the higher dimensions (except time) in any way, we will never be sure that there exists more than three. One may argue the possibility, but it remains as mathematical one as no convincing evidence can be gathered.

    Think of an an ideal 2D creature on that sphere (a dot, not an ant, not even a human). Whatever the dot does it will never find the edge of the sphere. However, he might suspect the sphere to be 3D if he is an intelligent creature (his ability only comes from some example in his own 2D world). But having never experienced the 3rd dimension in any way he'll never realize/believe/have evidence of this being true. (Remember he is incapable of imagining anything 3D (cause he's never been a 3D creature himself)).

    He'll say "What other stuff can lie in other dimension out there? I already have all stuffs in my universe uniquely mapped in 2D. How can I be sure that there are stuff out there in other dimension? If there are no stuff in other dimension we should not be calling the universe more than 2D." This is similar to what we experience.
     
  21. Jan 11, 2013 #20
    I was not suggesting that there are more than 3 spacial dimensions. Although I didn't specify what the 4th dimension was, I was thinking of time. More correctly, I was thinking of 4 dimensional space-time.

    This is not correct. The angles on a triangle on a sphere add up to more than 180 degrees. A 2D creature could measure it and so detect the third dimension. In similar fashion, if we measure the angles of a triangle to be more or less than 180 degrees, we too would have concrete evidence of curvature.
    The bending of light near the surface of the sun is an example of just such a measurement. This is evidence that space-time is indeed curved, but does not answer the question of whether the universe is so curved that it closes in on itself like a sphere.
     
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