= Gauge theory
{wiki}
A <classical field theory> is a gauge theory when its description has a local redundancy represented by <gauge transformations>. A <gauge field> defines the <covariant derivative> that compares matter fields at different points, and its <gauge field strength> measures the curvature of that comparison. <Electromagnetism> is the familiar Abelian example; <Yang-Mills theory> generalizes it to a non-Abelian internal group. Both classical equations and their <quantum field theory> can have this structure. Physically equivalent gauge descriptions must give the same observables, even though the <gauge potential> changes.
Back to article page
