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  1.  
    oops. Mea culpa!
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      CommentAuthorAngus
    • CommentTimeAug 15th 2014
     
    Posted By: Andrew PalfreymanViolent agreement, I would say


    OK then. I interpreted your comment as implying Maxwell's equations predict that there should be no magnetic monopoles.
  2.  
    div B = 0 is what it is
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      CommentAuthorAngus
    • CommentTimeAug 15th 2014
     
    No it isn't.
  3.  
    Right then.
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      CommentAuthoralsetalokin
    • CommentTimeAug 15th 2014 edited
     
    Of course there can be no monopoles, no more than there can be a bit of string with one end, or three. Magnetic polarity is more like a direction, than a thing of which you can have a pair and then take one away leaving one behind. Travel along a "field line" and you will always find it to be a complete loop, without beginning or end, its "polarity" defined only by the "direction of flux". Magnetic "poles" are illusions caused by viewing matter in bulk and not being able to ride those field lines down to their ultimate origin: moving charges. Can you have a "left" without a right"? An up without a down? Why then should anyone expect to find a N magnetic pole without a S one? Just because it's a prediction of some versions of the SM? Don't let your models fool you.
    divB=0 doesn't specifically say that there are no monopoles, but it _does_ specifically say that field lines are closed loops. Figuring out from that that there can be no monopoles is as simple as cutting a loop of string.
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      CommentAuthorAngus
    • CommentTimeAug 15th 2014 edited
     
    Indeed it says that field lines are closed loops. But it doesn't say they have to be. After all, it is allowed that there are positive and negative electrical charges and electric field lines may end on them. Dirac worked out that, on the available evidence, magnetic monopoles should be possible, with magnetic field lines ending on them.

    Dirac showed that if any magnetic monopoles exist in the universe, then all electric charge in the universe must be quantized.[11] The electric charge is, in fact, quantized, which is consistent with (but does not prove) the existence of monopoles.
    • CommentAuthorjoshs
    • CommentTimeAug 15th 2014
     
    Posted By: TrimNS

    Is this evidence that we can see the future?

    http://www.newscientist.com/article/dn19712

    I thought we only saw the past?
    Soon the future that we have seen will repeat the past that we have also seen.
  4.  
    So it will have been being the same
    • CommentAuthorjoshs
    • CommentTimeAug 15th 2014
     
    Posted By: Andrew PalfreymanSo it will have been being the same
    Only better.
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      CommentAuthoralsetalokin
    • CommentTimeAug 15th 2014 edited
     
    Posted By: AngusIndeed it says that field lines are closed loops. But it doesn't say they have to be. After all, it is allowed that there are positive and negative electrical charges and electric field lines may end on them. Dirac worked out that, on the available evidence, magnetic monopoles should be possible, with magnetic field lines ending on them.

    Dirac showed that if any magnetic monopoles exist in the universe, then all electric charge in the universe must be quantized.[11] The electric charge is, in fact, quantized, which is consistent with _(but does not prove)_ the existence of monopoles.


    The quoted statement skirts by a mere parenthetical phrase several logical fallacies. If A, then B. Observe B. It is therefore concluded _(but not proven)_ that A. In fact, B provides no evidence either way for or against A. Only the observation of _NOT B_ is probative at all, and that would prove that A is false. However, we definitely have observed B. So we may make no legit conclusions about A at all from that.
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      CommentAuthorAngus
    • CommentTimeAug 15th 2014 edited
     
    I don't think the statement is fallacious. It says
    (a) it can be proven that A implies B
    (b) observe B
    (c) therefore A is not excluded.

    Since A is the existence of monopoles this is a statement that we DON'T know that monopoles do not exist. Our evidence is only that we are unable to find any. On some mornings I might equally well conclude that socks do not exist.


    And that contradicts (I think) your statement

    Posted By: alsetalokinOf course there can be no monopoles, no more than there can be a bit of string with one end, or three.
    • CommentAuthortinker
    • CommentTimeAug 15th 2014
     
    You could have a one ended piece of string - so long as the other end was in another universe.
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      CommentAuthorAngus
    • CommentTimeAug 15th 2014
     
    Posted By: tinkerYou could have a one ended piece of string - so long as the other end was in another universe.


    It's not the situation with fields. All that magnetic monopoles would involve is that magnetic fields emanate from magnetic charges just as electric fields emanate from electric charges. The "lines of force" must terminate on a charge in both cases.
  5.  
    Posted By: AngusI don't think the statement is fallacious. It says
    (a) it can be proven that A implies B
    (b) observe B
    (c) therefore A is not excluded.

    Since A is the existence of monopoles this is a statement that we DON'T know that monopoles do not exist. Our evidence is only that we are unable to find any. On some mornings I might equally well conclude that socks do not exist.


    And that contradicts (I think) your statement

    Posted By: alsetalokinOf course there can be no monopoles, no more than there can be a bit of string with one end, or three.


    As I said, the fallacy is avoided by the parenthetical in the original quote. But it's a strawman avoidance, since the writer wants you to conclude that the existence of monopoles is allowed, from the faulty syllogism. My point is that the "logic" is irrelevant and actually tells us nothing about the existence or nonexistence of monopoles, in spite of the author's wish that we conclude that they are possible, given the existence of quantized electric charge. But really we are not justified in concluding that. They may be possible, they may not, but the existence of quantized electric charge doesn't provide any new information that could allow us to decide. Now, if we discovered that electric charge isn't quantized, that there is no "unit charge" , no minimum charge... then that would indeed prove, by Dirac, that monopoles could not exist.
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      CommentAuthorAngus
    • CommentTimeAug 15th 2014
     
    That's right. I agree.
    And therefore you must agree that it is incorrect to state "monopoles do not exist" except as an opinion or article of faith.
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      CommentAuthoralsetalokin
    • CommentTimeAug 15th 2014 edited
     
    Yes, but faith founded on some pretty damned strong analogies and known facts about the World. Such as that moving charges make magnetic fields that are closed loops around the direction of travel of the charge, and that bulk magnetism is believed to arise from the moving charge represented by electrons "orbiting" in their atoms, also generating closed loops of magnetic flux. In fact there are no known magnetism-generating processes other than these. Therefore I conclude that my string analogy is apt, or even the reverse-obverse, left-right, coming-going descriptions.
  6.  
    But... I just realised that when string is being made, it only has one end, the end coming out of the string-making machine. Inside the machine, mysterious processes occur that meld together components, each of which is not string exactly, into the gestalt we call "string". Where does the string begin, the components end? I dunno.
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      CommentAuthorAngus
    • CommentTimeAug 15th 2014
     
    I have stipulated that nobody has ever seen a monopole. My cavil is with the statement that they don't exist.

    After all, okapi didn't exist until 1901. (Which brings up another interesting question - who has to observe a monopole before it exists? WIll pygmy tribes in the African bush do?)

    Perhaps all string is made in a giant loop and bits are cut off as needed. (Do not examine this statement too closely.)
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      CommentAuthoralsetalokin
    • CommentTimeAug 15th 2014 edited
     
    Cavil away, they don't exist nevertheless. Until someone sees one, just like your okapi. Pygmies, qua pygmy? No, the observer has to have some observing cachet, else what was observed might have been a Canada goose, or even nothing. This does not rule out a valid observation made by a PhD grad student or post-doc who just happened to be a pygmy, though.