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Question regarding atoms

  1. Sep 16, 2010 #1
    If you placed an atom in an empty universe that was completely dark and cold, would it eventually radiate away all its energy until it had zero KE?

    If so, what would happen to an atom with zero energy? In this sense, are atoms inherently unstable systems?
     
  2. jcsd
  3. Sep 16, 2010 #2

    DaveC426913

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    KE is a relative concept. A single atom with no motion relative to anything else has no KE.

    However, I think you've touched on something profound. An isolated atom would have no constraints on its position or velocity and thus would cease to have a discrete value for either. The atom would smear out across space, potentially without limit. Interestingly, this is the same thing that happens to atoms cooled to (almost) absolute zero (Bose-Einstein condensate).

    Unstable is another way of saying they're not the permanent solid little particles we're used to thinking of them as.
     
  4. Sep 16, 2010 #3
    I was going to say that I presumed when you said it would cease to have a discrete value, that it was because with no outside observer it would be equally likely to appear at any point in the universe. The bold text, however, indicates to me that this is incorrect. Please explain.
     
  5. Sep 16, 2010 #4

    DaveC426913

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    When an atom approaches absolute zero, its velocity approaches zero. As per HUP, if its velocity is well-defined, its position becomes poorly-defined. Atoms near 0K really do smear out.

    When you say "equally likely to appear at any point in the universe", that is the same thing as saying it is essentially at any point in the universe (i.e. when you go to observe it, it's there). i.e. it has smeared out across the universe.

    They are two sides of the same coin.
     
  6. Sep 16, 2010 #5
    Cool. Thanks.
     
  7. Sep 16, 2010 #6
    Even though this is just a "thought experiment" in reality, eventually the entire universe will cool off will it not?

    Forget about "big crunch" and "big rip" theories - if the universe is expanding and always radiating energy in all directions eventually it has to lose all it's energy... (at least in my mind)

    So it seems that no matter what way you slice it, we're living on borrowed time... what a strange thing for such an elegant universe that it seems to be built into it's structure that it can't last forever...
     
  8. Sep 16, 2010 #7

    DaveC426913

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    Well, it will never lose the heat, its just that
    a] it will be spread out over an ever-increasing volume, so its average temperature will approach - but never reach - zero.
    b] the heat will be evenly distributed.
     
  9. Sep 16, 2010 #8
    If we say that universe is fixed size, this would require some kind of mirror or something at the 'edge' of the Universe?
     
  10. Sep 17, 2010 #9

    DaveC426913

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    No.10chars
     
  11. Sep 17, 2010 #10
    Show me the end of a circular line or edge of the surface of a ball.
     
  12. Sep 17, 2010 #11

    Drakkith

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  13. Sep 17, 2010 #12

    DaveC426913

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    No I can't. That's just the way QM is.
     
  14. Sep 18, 2010 #13
    I cannot agree reading this thread. QM is a theory, nothing more. It is has been tested under the condition that we have a universe with many particles and energy. To follow that the eigenstates of such a single atom allow to be expanded in space in infinity would imply that QM is founded on an interaction in the universe with the interaction of other materia.

    QM is more or less a theory of single particles, interacting if at all in a simple case. It needs linearity from the beginning.
    It is not okay to talk about a universe with one atom. You should not forget that we expect the universe to be more or less 4 dimensional, where we have those dimensions because we have more than one single atom.

    Best regards,
    Jens
     
  15. Sep 18, 2010 #14
    Actually, I thought that I did in my first post.
     
  16. Sep 18, 2010 #15
    You have no sense of fun.
     
  17. Sep 18, 2010 #16
    This is not fair. You don't know me in person and I am not an native english speaker. This forum is about physics and I try to answer in way which is only about the subject. It has nothing to do with me in person.
     
  18. Sep 18, 2010 #17
    The opening post posed a hypothetical question, and you said, "It's not ok to talk about a universe with one atom."

    To me, that's not fun. Clearly, there's not much sense in trying to derive anything useful from it, but so what?
     
  19. Sep 18, 2010 #18
    We cannot say that the energy will become absolute zero....because the atom will lose its energy till the temperature on both sides is equal
     
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