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BEC question

  1. Nov 18, 2003 #1
    Do I understand correctly that a Bose Einstein Condensate ejects electromagnetic field lines when forming, and continues to be anti-magnetic (repelled from both poles of the magnet?) after forming, and does this mean that a BEC formed in free space and then brought near an electromagnetic planet (like Earth) will experience a probability to move away from the electromagnetic core, that is a "force" in the antigravitic direction? If so, how to calculate this force potential in doing work?
     
  2. jcsd
  3. Nov 19, 2003 #2
    Hi,

    It sounds like you're talking about the Meissner effect, which is a property of superconductors, not BEC's. When brought below the critical temperature, a material becomes superconducting and expels magnetic field lines from its interior. It then becomes a perfect diamagnet, which is what you see when people have the superconducting material levitating above a magnet.
     
  4. Nov 20, 2003 #3
    Thanks, xeguy

    Is a BEC a superconductor? Perhaps there is a good reason to keep the terms separate. BEC is studied by confinement in a magnetic trap, so it appears to me that it is, like superconductor, anti-magnetic (I think I recall the correct term is diamagnetic?).

    Anyway the morning light has reminded me that magnetic traps are very strong compared to the earth's magnetic field. It now seems to me that the levitation effect would be present, but negligible.
     
  5. Nov 20, 2003 #4
    BECs are not superconducting (as far as I know). The repulsion of magnetic fields from a SC is an entirely different effect than using magnets to trap atoms in the condensate.
     
  6. Nov 20, 2003 #5
    BEC superfluids?

    I have read that the BEC is a superfluid, and that the fermionic component (in a 2-phase condensate) is composed of Compton pairs of electrons, which as pairs are all in one quantum state, which if I understand means that the waveform could be reduced to a single solution, as if all electrons present in the cloud behave as if they were in a single electron pair. Any effect on one of them will be instantaneously transmitted to all of them? Or perhaps transmitted in some multiple of the planck quantum of change involved in the entire quantum system.

    The fermionic component of the BEC (if I have understood these terms correctly) is like a bubble or droplet of superfluid which encloses a lattice-like structure which is composed of the bosonic portion of the cloud. The result is a cloud which can be made to rotate, and when rotated, the cloud exhibits quantum behavior evidenced by the formation of a large number of highly symmetric vortices. I will try to add a link to a picture of the surface of such a cloud, which seems to me to have the exact form of an {SU(2)}{?} spin network, which usually only exists at much smaller scales.

    http://jilawww.colorado.edu/bec/hi_res_pic_album_macromedia/pages/Vortexlattice250_jpg.htm

    Wow that was easy. I just copied from the Address line in my browser window. I was then able to click on it in the preview window and go to the link.

    So if you look at the link, you see the surface of a liquid with vortice holes reaching down into it, or you can squint your eyes and imagine the hexagonal tileing of the SU(2) plane, or the "perspective?" view of the 3-d {SU(3)}{?} space, in which the line of the observer provides a Z basis to the XY basis plane of the SU(2) spin network surface.

    I notice that the outer vortices look larger than the interior vortices. This leads me to further speculations, but I would rather wait to go on any further until some response helps me understand if what I have written here reads as physics, or if I am merely parroting buzzes and clicks.

    Richard T. Harbaugh, BES '75 BST '89 SCSU
    (index BEC Superfluid Superconducting? link Pic to jilawww . colorado . edu / bec)
     
  7. Nov 20, 2003 #6

    chroot

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    Re: BEC superfluids?

    Er um, Richard,

    It seems you've inappropriately mixed up a lot of different concepts here between BEC's and superconductors.

    - Warren
     
  8. Nov 20, 2003 #7
    Hi Warren. Thanks for the quick reply. What are you watching me type? :).

    Anyway, I am aware that I have only partial use of many of these terms, but knowing that I don't know is not the same as knowing what I don't know. I ask for your corrections.

    Otherwise I guess I am hearing that BEC should not be expected to be diamagnetic, or to superconduct, or to model a single quantum state?

    Thanks
     
  9. Nov 20, 2003 #8
    A BEC is made up of bosons (as the name suggests) which have completely different properties than the fermions (electrons) in a SC. There are two completely different phenomena at work.
     
  10. Feb 26, 2004 #9
    Hi
    Wanted to add; Fermionic BEC also possible when they form Bosonic pairs.
     
  11. Feb 27, 2004 #10

    ZapperZ

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    OK, let's see if I can add to the confusion here... :)

    BEC is the BIG, GENERAL, and in fact, generic term for anything (even elephants) that can condense into a single, coherent state. But to be able to do that, one of the criteria is that this "thing" must have whole integral spin (bosons).

    But here's the interesting part - this "thing" need not be just one particle. It helps and it is easier if it is, but it doesn't have to be. It can be a composite boson. A Cooper pair (not Compton pair) in a superconductor, is one such composite boson. While the individual consituents of the composite boson (in this case electrons) are themselves not a boson, they together form a larger beast that is a "boson". It is this composite boson that condenses into a BEC state. A similar situation can be seen in the triplet state He3 superfluidity.

    So to form this BEC state, one need not have to be only a point or single-particle boson. I think we need to get the basics clear first before tackling the Meissner effect. :)

    Zz.
     
  12. Feb 27, 2004 #11
    Well I am better at speculation when not encumbered by facts, but here is a question: If a BEC (or other cloud of atoms (eg fermionic pairs or He3, probably not elephants!) acts as if in a single quantum state, then it responds to certain stimuli as if it were a single particle. So if a long tube were made to be full of a BEC or equivalent, and a quantum change were applied to one end of the tube, would the result be an instantaneous change in state at the other end of the tube? Or is the speed of light still in effect?

    Just wondering, thanks for any ideas,

    Richard.
     
  13. Feb 27, 2004 #12
    Very good question, Richard, and one of the few questions I've seen lately requiring a reasonable amount of neural network activity. :wink:

    There are differences even though they share many common quantum characteristics based on Bose-Einstein statistics.

    .

    It appears unlikely to me based on that argument since I believe magnetic traps operate to confine the atoms before the onset of BEC; basically entrapment is due to the intrinsic atomic magnetic moment.
    However, that fact doesn't mutual exclude supercurrent from developing in a BEC, and I suppose detection of it (along with the associated Meissner feild) has never been attempted due to the typical tiny sample size and large external B field, (and possibly the theoretical impossibility).

    Theoretically, I'd be interested to hear if it is possible.

    If superconductivity were simply a matter of Bose Condensate development, then lasers would also superconduct.
    At issue are a number of other differential factors, the presence of charge carriers in the supercurrent being the most prominent, a factor not favorable to a BEC.

    Also a very thought provoking idea that you think BEC's would transfer info instantaneously by virtue of its macroscopic quantum state. Hmmm.

    Creator :smile:

    P.S. I also agree: Elephants don't form condensates.
     
    Last edited: Feb 27, 2004
  14. Feb 29, 2004 #13
    Thanks, Creator, for your reply. It seems you have some specific knowlege of BEC. I have only read with interest the website and some of the papers posted at <http://jilawww.colorado.edu/bec>. I am afraid my interests range too widely to be very specific about anything.

    I think I follow your correction about the magnetic trap. The BEC clouds at Colorado are said to be visible to the unassisted eye, so I guess they must be fairly sizeable. And they are said to be suspended, so I made the unqualified assumption that they were still held in the magnetic trap, but perhaps you are correct, although I don't see how suspension can be maintained by means of intrinsic atomic magnetic moment. I think I can follow how the condensation process would be due to the atomic magnetic moment, but again my ideas have very little detail and no formulation.

    I'm not sure about the lasers. I have some idea that lasers operate by producing coherant light, where all photons are in the same quantum state, but of course photons are not bosons. Is the laser crystal itself somehow a BEC?

    Actually I suspect that the speed of light will not be violated in a macroscopic BEC, but that is only due to my naive belief in C, and not due to any direct knowlege. But if not why not? If the cloud really is in a single quantum state by definition, then it would seem to be possible to transfer information instantaneously, but probably the authors mean only to say that the cloud will settle into a single quantum state when undisturbed. Probably a quantum change at one side of the cloud is propagated through the cloud in real time and is not instantaneous, but wouldn't it be fun to play with it and see?

    Thanks for Being,

    Richard
     
  15. Mar 1, 2004 #14


    Photons ARE bosons. That's why lasers work at all. Laser stands for Light Amplification by Stimulated Emission of Radiation. A medium is stimulated by a supply of energy either through a chemical reaction in a liquid or gas, electrons "wriggling" through magnets, or through the use a ruby with a flash tube. The electrons rise an energy level and as they drop back down, release a photon of a particlar energy. Since a good percentage release similar photons that don't interfere with each other, you can get an amplified, coherent beam of light.



    Just for future semantics, it's a lowercase c. Remember that matter follows the rules just as it should. The making of the BEC calls for a group of atoms to be applied to the same process overall. Remove the hot atoms and trap the cold ones. A BEC is a result. BEC's don't last that long anyhow and I'm guessing that an attempt to modify just a section will collapse it although I'm not sure.
     
  16. Mar 1, 2004 #15

    ZapperZ

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    Although my original intention of including "elephants" as possible bosons and undergoing a BEC was in jest, take note that there is nothing, in principle, that would prevent such a thing from occuring if the total spin of an elephant is an integral number. The reason we don't see this happening of course is because it is almost impossible to maintain coherence between all the atoms and molecules in the elephant, both spatially and temporally. Decoherence is the principle cause on why we do not observe quantum effects at macroscopic scale. However, having said that, we do have an ongoing study on extending the size on when quantum effects can be observed. SQUIDs experiments, for instance, have extended this up to the 100 micrometer scale involving up to 10^9 particles (Cooper pairs).[1,2] In fact, Roger Penrose and his co-authors have proposed an experiment involving mirror and optical cavities that would show Schrodinger Cat-type effects involving up to 10^14 atoms![3]

    So "size" here, in principle, does not automatically rule out any QM effects. So if you could make all the atoms and molecules in an elephant to preserve their coherence and form a boson, there's nothing to say that these elephants could not undergo a BEC! I'd pay money to see that! :)

    Zz.

    [1] J.R. Friedman et al., Nature v.406, p.43 (2002).
    [2] C.H. van der Wall et al., Science v.290, p.773 (2000).
    [3] W. Marshall et al., PRL v.91, p.130401 (2003).
     
  17. Mar 1, 2004 #16
    Thanks for the corrections and information. I have had an interesting morning reviewing spin and bosons. Humbling, but interesting. I remember discussions with my chemistry professor about spin, and that I came away with this same feeling of uncertainty.

    If I have a 3 dimensional geometric object in a two dimensional frame of view, and I rotate the object one full turn about any axis, I would naturally expect to see the same view of the object before and after the rotation. This seems to work with any macroscopic object, I think. If the object has symetry like a football, I could rotate it half a turn and see the "same" view, or at least an indistinguishable view. Then what is spin two? I rotate it one full turn, and it is not the same. I rotate it again a full turn, and it is as it was before rotation. This is mysterious. My poor imagination invokes a demon that hides behind the object and changes it as I turn it. I turn the ball half a spin, the demon makes a mark on the hidden side of the ball, then when I get to one full turn, I see the mark and think the ball is not the same. Then I turn it another half turn, the demon erases the mark, and on the next half turn, at spin 2, the ball looks the same again. Tricky demon.

    Or, it is an object with some higher dimensional features, like a hypercube, so when I rotate one turn in my view, I bring up some hidden dimension in which the object is different.

    But how do we count these rotations of particles? How do we know if we have turned it once, or half, or two?

    What if it is not a simple spherical solid, but a two part system, like the Earth and moon? Then I may rotate the system once, but it does not look the same because the internal components have changed position. I could have to rotate it a very large number of times before it returned to its original configuration. Then I could have any number of spin states, even fractional ones.

    Thanks again for the responses.

    Richard
     
  18. Mar 1, 2004 #17
  19. Mar 4, 2004 #18
    All this is great in theory, my question is how we can use the BEC if at all?

    Thanks.
     
  20. Mar 5, 2004 #19
    OK, but what are principles of superconductivity, I see you have bunch of boson(like) particles, but what makes them be better conductors than ordinary electrons (if there is same number of electrons in conductor). What makes Resistance go to Zero or near zero (I=U/R)?

    Q: if we would have nuclei (with bonded) electrons in metal that too have integer spin (bosons), could cooper pairs pass though nuclei like ghosts (no resistance) ? What about Coulombs forces between charges in BEC and superconductors ?

    About BEC: does it mean that if we have blimp full of He it can be compressed in size of ONE atom???
    How the hell did they observe that super fluid behavior of He when it was in BEC state (all He in size of one atom)? In my text book in chapter abut aggregate states is example of super fluid He (BEC I presume) on low temp, without viscosity, and climbing on the walls of tank (or this isn't BEC).
     
  21. Mar 5, 2004 #20

    ZapperZ

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    To be able to understand that, you need to know what is meant by phase coherence, and especially long-range phase coherence. When the cooper pairs condense into the superconducting, or BEC state, these "things" no longer exist as individuals. ALL the pairs that have condensed are now, essentially, ONE object. They are described by ONE single quantum state, and this state has long-range coherence, basically over the entire superconducting region of the material. As a consequence, the long-range phase coherence means that they lattice vibrations no longer "interfere" or scatter with the condensate. This is the fundamental description of the BCS theory's ground state wavefunction.

    The "ordinary" electrons in metals do not have such description. The simplest form of description that these electrons have is known as the Drude model. This treats electrons as if they are in an ideal gas of free electrons. They SCATTER of each other and the lattice ions elastically (very much like an ideal gas in the Kinetic Theory). You can find a description of this model typically in the 1st two chapters of a standard solid state physics text.

    Read above regarding the "immunity" of the condensate against scattering.

    The presence of Coulombic forces is the reason why the formation of cooper pairs is so astounding. Cooper has shown that two electrons in an electron cloud (or the Fermi sea) with a positive lattice ion background CAN, in fact, form a bound state. (This problem, btw, is a typical homework problem in a solid state physics class when the students get to the superconductivity chapter). But keep in mind that this is a MANY-BODY phenomena, meaning two electrons will never attract and from a bound state all by themselves. The fermionic sea and the lattice ions (phonons) equally play vital roles in this case (at least for conventional superconductors).

    Zz.
     
    Last edited: Mar 6, 2004
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