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Magnetic Monopole?

  1. Aug 23, 2009 #1
    We were having a discussion about magnetic monopoles in an AP Physics course a while back, talking about why they can't happen, etc. One of the other students (there were 7 of us, haha) said, "What if you magnetize a bunch of needles and then stick, say, all of the south poles into a styrofoam ball. Wouldn't this act like a magnetic monopole?"

    Even though this probably isn't possible, we, including the teacher, couldn't find a reasonable solution as to why it wouldn't act like a monopole. Any suggestions/solutions as to why it wouldn't act as a monopole?

    Thanks!
     
  2. jcsd
  3. Aug 23, 2009 #2
    The fields would sum to 0 all over.It would be like you don't have a magnet.
     
  4. Aug 23, 2009 #3
    Yes, I'm pretty sure that the net flux through that system is 0 due to the same number of B. field lines entering as escaping.
     
  5. Aug 23, 2009 #4
    Ahh okay thanks you for elaborationg on why it would be 0, makes
    complete sense now!
     
  6. Sep 4, 2009 #5
    Well, the discovery of magnetic monopoles are in the news. Apparently they exist.
     
  7. Sep 4, 2009 #6

    ZapperZ

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    But not in the way that we originally thought, or what is being described out of the Standard Model. The "spin ice" magnetic monopole is not as direct as one would imagine as being analogous to a source charge.

    Zz.
     
  8. Sep 4, 2009 #7
    You got me, I can't follow their reasoning. Heck I can't even
    follow the reasoning of my checkbook balance because of
    the wife variable.

    My interest is in mobius extensions. I just have a knack for
    twisty turney stuff and topography I do in my head and then
    try to ply them somehow. One thing I found resembles a
    magnetic field and you can rotate and collapse the thing
    into a ball. (Mobius monopole) Don't know if it's useful
    for theorist to play with I've heard it is but like I said
    the maths they show me may as well be Chinese.
     
  9. Sep 5, 2009 #8
  10. Sep 7, 2009 #9

    wolram

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    Magnetic monopoles

    Thought this may be interesting.

    http://www.sciencedaily.com/releases/2009/09/090903163725.htm


    ScienceDaily (Sep. 4, 2009) — Researchers from the Helmholtz-Zentrum Berlin für Materialien und Energie have, in cooperation with colleagues from Dresden, St. Andrews, La Plata and Oxford, for the first time observed magnetic monopoles and how they emerge in a real material.
     
  11. Sep 7, 2009 #10

    ZapperZ

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    It would be beneficial of people do a search first to see if such a topic has already been posted.

    In addition, and since no one has made this link yet, one should also read this:

    http://sciencenow.sciencemag.org/cgi/content/full/2009/904/1

    Again, as with the fractional quantum hall effect, these are condensed matter systems. These effects do not survive if the collective interactions are taken apart. It doesn't mean that they do not have any fundamental importance (I consider condensed matter physics to be extremely fundamental), but this is not the same elementary "particles" that we are looking for within the Standard Model zoo.

    Zz.
     
  12. Sep 8, 2009 #11
    (I typed this in response to another reply, which was closed before I could hit the 'Post Reply' button ;). So maybe I'm repeating stuff here.)

    No, they have not. These are emergent monopoles - collective behavior of the underlying degrees of freedom, which from a macroscopic point of view mimics the behavior of a monopole. Well, not even this is completely true actually.

    Note that the monopole emerges as the end point of a Dirac string. A Dirac string is essentially a tube of flux, made to be as small as possible. You can compare it with a normal magnet with a north and a south pole. If you make the magnet very thin, and very long it's endpoints become essentially pointlike "magnetic" sources. The north and south pole are connected by a small tube of flux, such that the overall divergence of the magnetic field is still zero (one of maxwell's equation).

    But for a true monopole to exist the Dirac string should not be visible! This was also the original argument of Dirac. The string is essentially a mathematical construct which restores the divergence of the magnetic field. Since the tube is made to be infinitely thin, the only way the flux would be detectable is via the Bohm-Aharanov effect. The only way this effect is also ruled out leads to Dirac's quantization of electric and magnetic charge.

    However, the article clearly states:
    So what they measure are actually string-like excitations: the spins align themselve into flux tubes. They haven't measured monopoles: they measured Dirac strings! But monopoles sound fancier ofcourse.
     
  13. Sep 12, 2009 #12
    Yes, thank you. The direct measurements were upon something that looked like Dirac strings, rather than magnetic monopoles directly.

    But it's still very inspiring dispite the criticism by some who insist that the strings terminate at infinity. There is no infinity in a finite chunk of material. If they want free-space monopoles that's another matter. But I'd still like to know when solid state physics can tell us anything about the vacuum.
     
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