Minimal coupling

Minimal coupling is a concept often used in theoretical physics, particularly in the context of quantum field theory and general relativity. It refers to a way of introducing interaction terms between fields in a manner that preserves the symmetries of the theory while introducing minimal modifications to the existing structure of the equations. In the context of gauge theories, for example, minimal coupling involves replacing ordinary derivatives in the equations of motion with covariant derivatives. This is done to ensure that the theory remains invariant under local gauge transformations.