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Electromagnetic lagrangian

  1. Dec 29, 2007 #1
    why is there a factor of -1/4 in front of the two tensors? Is it formulated that way for convenience?
  2. jcsd
  3. Dec 29, 2007 #2
    It's not convenience. The kinetic term for any real scalar field has a factor of 1/2. But, when you work out the explicit form of the EM lagrangian in terms of the photon field, you find that the contraction of the tensors gives four terms; but, each of the four is identical to one of the others. This means that, in terms of the field, there's an extra factor of 2 that needs to be accounted for, turning the 1/2 into a 1/4.
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