Limitations of gauge invariance
Although gauge invariance preserves the values of physical observables, a gauge transformation can introduce important alterations of physical interpretations. To understand this, it is first shown that a gauge transformation is not, in general, a unitary transformation. Also, physical interpretations are based on both kinetic and potential energy expressions. While the kinetic energy is a measurable quantity, and hence gauge-invariant, the potential energy is gauge-dependent. Two basic examples are examined; one classical and the other quantum-mechanical. The aim is to show that the use of the Coulomb (or radiation) gauge is always consistent with the way that fields are generated in the laboratory. Upon gauge transformation out of the Coulomb gauge, this connection is lost, and physical interpretations can give rise to misleading inferences.