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Does the photon has a dipole moment?

  1. Jun 24, 2006 #1
    This might be a fantastically stupid question, but does the photon has a dipole? I'm almost certain it doesn't since a dipole is formed when two charges of opposite sign are very close to one another (at least compared to the distance an observer is from them) and unlike say the neutron which has internal charges, yet neutral overall, the photon doesn't have that internal structure and so has no dipole.

    Am I right in guessing that the neutrino is predicted to have no dipole either, since it's supposed to be a neutral fundamental particle? A bit of Googling seemed to imply current experimental results give very small upper bounds on it's possible dipole strength.

    Are all 'fundamental' (at least to current theories) neutral particles dipole-less (Z boson, graviton etc)?

    Or am I just way off the plot and should get back to reading some (quantum) electrodynamics :redface:
  2. jcsd
  3. Jun 24, 2006 #2
    photon has spin - it's the photon's polarity, there are three orthogonal states of polarity for a photon (the standard base is circular left, circular right and linear).
    this means photons are spin 1 particles.

    i think you might have been confused by the idea of spin in fermions...
    while the photon does have spin, it does not have magnetic dipole, it can have intrinsic angular momentum though.
    Last edited: Jun 24, 2006
  4. Jun 24, 2006 #3
    I know about the photon spin and it's various angular momentum and polarisation properites.

    I was just confused as to wether it had a dipole to it, but that would mean all photons emit photons due to their dipole which would just lead to some kind of horrible exponential growth problem wouldn't it! Should have put some thought to it first!
  5. Jun 26, 2006 #4

    Meir Achuz

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    Photons have neither an electric or magnetic dipole moment, as well as no electric charge. It is not inconceivable that some theory be concocted
    (Anything can be concocted.) that does have them, but there is certainly no exptl evidence for anything like that.
    In QCD, there are 8 gluons which behave like photons for QCD.
    Each gluon does have a QCD charge, so gluons can emit gluons, but there is no horrible consequence.
  6. Jun 26, 2006 #5


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    Well, not horrible, but asymptotic freedom comes out of that.
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