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