In The Beginning

(A Tribute to Maxwell)

n the beginning, God said unto his angels: ``Let Us create electromagnetic radiation.''
``Use the vector potential!'' they said.
``Ah,'' God replied, ``but the vector potential is not gauge invariant.''
``Then use the field tensor!'' they tried again.
``But,'' God replied, ``the field tensor is not Lorentz invariant.''
``Well,'' the angels said at last, ``the self-contraction of the field tensor is both gauge and Lorentz invariant!''
And God saw that it was the Lagrangian field density, and that it was good.
Then there was light!

--Kerson Huang, 1980