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Are Bose Eistein Condensates Classical Objects

  1. May 15, 2013 #1

    bhobba

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    This came up in a thread discussing if quantum effects occur at the macro level. I always thought it was pretty much standard wisdom that Bose-Einstein condensates show quantum effects at the macro level. I mean for such states atoms loose their individuality and behave like one large quantum system. Am I missing something here?

    Thanks
    Bill
     
  2. jcsd
  3. May 15, 2013 #2


    As far as i can understand, the Bose condensate is observed only indirectly(inferred) as interacting with the outside macro world raises the temperature and destroys coherence. Feel free to correct me if i am wrong but direct observation is needed to justify the claim that quantum effects can be observed on the classical scale.

    It is my understanding that brains are not suited to observe quantumness, though i agree with you that the world is entirely quantum. This is supported by the latest understanding of the HUP and some experiements like the DCQE. I am not claiming that i know that brains destroy coherence and single out outcomes but i am genuinely interested if direct quantum behavior can be experimentally observed(even in principle).
     
  4. May 15, 2013 #3

    JK423

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    Why would you think that a BEC is a classical object? In the sense, that all the atoms of a BEC are delocalized and in the same quantum state, hence coherent. But i think you already knew that, so i suppose you should elaborate a little bit more on what you're trying to say because i don't undestand.. :)
     
  5. May 15, 2013 #4

    bhobba

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    I believe they were actually created in 1995. But how they determined its properties I have zero idea.

    Thanks
    Bill
     
  6. May 15, 2013 #5

    DrDu

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    Do you consider classical electromagnetic fields to be classical objects?
     
  7. May 15, 2013 #6

    stevendaryl

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    A large part of the weirdness of Bose-Einstein condensates have nothing to do with quantum mechanics (in the sense of the uncertainty principle, non-commuting operators, etc.) but just follow from the differences between Bose-Einstein statistics and Maxwell-Boltzmann statistics. This difference is purely due to a difference in how you count the number of microstates associated with a macrostates. If you think of particles as in principle distinguishable, then you get Maxwell-Boltmann, and if you think of them as in principle indistinguishable, then you get Bose-Einstein. I'm not sure what the connection is between quantum mechanics and indistinguishability of particles. They are usually taught together, but that might be just because both come into play at the level of atomic particles, and are unimportant for large objects.
     
  8. May 15, 2013 #7

    bhobba

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    Yes - but Quantum Fields - no.

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    Bill
     
  9. May 15, 2013 #8

    bhobba

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    I don't - I think its QM writ large - the particles have lost their individuality and is simply one large quantum object.

    Thanks
    Bill
     
    Last edited: May 15, 2013
  10. May 15, 2013 #9

    bhobba

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    Wait a minute - I don't think that classical systems have the property when you exchange two particles it is exactly the same - that's the reason for the different counting and why it doesn't follow the classical Gaussian Boltzmann law which is based on the binomial distribution.

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    Bill
     
  11. May 15, 2013 #10

    JK423

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    Just one particle bounded inside a potential well shows full quantum behavior. N indistinguishable particles in the same state in a potential well show similarly full quantum behaviour that doesn't have -only- to do with the statistics. The statistics may give you extra features, but the quantum features of just one particle are also adopted by the N particles.
     
  12. May 15, 2013 #11

    JK423

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    Then who does? :confused:
     
  13. May 15, 2013 #12

    stevendaryl

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    Okay, I think that's what I intended to say. The difference is due to a difference in how you count states. If you want to call that "quantum mechanics", I guess you can, but it seems to me that it's separable from the rest of quantum mechanics.
     
  14. May 15, 2013 #13

    stevendaryl

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    I guess I need to get down to specifics. What specific behavior of Bose-Einstein condensates are we talking about?
     
  15. May 15, 2013 #14

    bhobba

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    It arose in the thread I mentioned in my first post. In that thread it was thought ny some, not by me, all macro objects were classical. I mentioned liquid helium, and some other stuff, and thought it pretty much settled the issue - but some thought it didn't. I thought I would do a post to see what people generally thought with BEC's which is even more obvious to me its not classical. Looks like you are in my camp.

    Thanks
    Bill
     
  16. May 15, 2013 #15

    JK423

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    Well i talk generally, but in your previous discussion did you discuss about a specific behaviour of a BEC that depends only on statistics? From the OP i understand that someone implied that BECs do not show quantum behaviour..? I don't know where that came from..
    If you could elaborate a bit it would be very helpfull.
     
  17. May 15, 2013 #16

    bhobba

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    It arose in a general sense in this thread:
    https://www.physicsforums.com/showthread.php?t=690802&page=3

    Perhaps you can contribute to it. I really don't know what else I can say - to me its totally obvious size has nothing to do with quantum behavior and BEC's are a prime example of that.

    Thanks
    Bill
     
  18. May 15, 2013 #17
    If we consider that the quantum state is primary at origin of universe (not classical), then the question becomes when (and why and how) do quantum entities begin to show classical behavior..correct ? In this view, the BEC states (and superfluid states) would be on the cutting edge of the transition when quantum entities begin to show classical behavior. The question is: why do classical events spontaneously appear from primary quantum entities, not the reverse..correct ?

    So, can someone explain what new attributes are found in classical states that are missing in quantum states....the experimental confirmation of these 'new' attributes not found in quantum states should help us better understand where any entity of interest falls along the continuum from quantum <--> classical...correct ?

    Edit: Perhaps we can start with He-3 and Li-6 isotopes for examples. Let us assume both are primarily quantum entities. It is known that both can show quantum superfluid behavior under certain conditions. So, what new attributes are added to He-3 and Li-6 as quantum entities that also let us call them classical entities ?
     
    Last edited: May 15, 2013
  19. May 15, 2013 #18

    bhobba

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    I think a classical object has certain properties you expect in classical mechanics. One is it has a definite position and momentum. BEC's as a whole have that - at least I think they do. But another property is you can break it down into constituent parts and those parts are distinguishable - BEC's are not like that - if they even have the property of parts is open to question - they behave as a single large quantum object. I am sure people more expert than me in condensed matter physics can elucidate even more weird totally counter intuitive behavior not explainable classically.

    Thanks
    Bill
     
  20. May 15, 2013 #19

    stevendaryl

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    I'm saying that the existence of Bose-Einstein condensates depends only on statistics. I didn't make any claims about behavior.
     
  21. May 15, 2013 #20

    stevendaryl

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    I did a quick Google search to try to find examples of weirdness associated with BECs, and I couldn't find a good page describing them. But an example I remember is in the case of superfluids (not all BECs are superfluids, and not all superfluids are BECs), the liquid flows without friction, can squeeze through tiny cracks, and can flow uphill on its way to a lower elevation. If these properties are explained by quantum tunneling, then I would agree that's a macroscopic quantum effect.
     
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