The gauge covariant derivative ( /ˌɡeɪdʒ koʊˌvɛəriənt dɨˈrɪvətɪv/) is like a generalization of the covariant derivative used in general relativity. If a theory has gauge transformations, it means that some physical properties of certain equations are preserved under those transformations. Likewise, the gauge covariant derivative is the ordinary derivative modified in such a way as to make it behave like a true vector operator, so that equations written using the covariant derivative preserve their physical properties under gauge transformations.
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In fluid dynamics, the gauge covariant derivative of a fluid may be defined as
where is a velocity vector field of a fluid.
In gauge theory, which studies a particular class of fields which are of importance in quantum field theory, the minimally-coupled gauge covariant derivative is defined as
where is the electromagnetic vector potential.
If a gauge transformation is given by
and for the gauge potential
then transforms as
and transforms as
and transforms as
so that
and in the QED Lagrangian is therefore gauge invariant, and the gauge covariant derivative is thus named aptly.
On the other hand, the non-covariant derivative would not preserve the Lagrangian's gauge symmetry, since
In quantum chromodynamics, the gauge covariant derivative is [1]
where is the coupling constant, is the gluon gauge field, for eight different gluons , is a four-component Dirac spinor, and where is one of the eight Gell-Mann matrices, .
In general relativity, the gauge covariant derivative is defined as
where is the Christoffel symbol.