Sunday, November 25, 2018

electromagnetism - Expanding electromagnetic field Lagrangian in terms of gauge field



The electromagnetic field tensor is given by Fμν=μAννAμ, and it appears in the Lagrangian as L=14F2μνAμJμ.

Schwartz's QFT textbook says that (chapter 8, page 116) F2μν=2(μAν)22(μAμ)2,
but I do not see how that is correct.


The first step of the expansion gives F2μν=(μAν)2μAννAμνAμμAν+(νAμ)2, and I see that the first and last term add up to produce 2(μAν)2. However, the second term μAννAμ when expanded has a term like 0A11A0, and I don't see how this is present in (μAμ)2=(0A0+1A1+2A2+3A3)2.




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