Mistery of Real Scalar Klein-Gordon Field

The mystery that the real scalar Klein-Gordon field has vanishing current densities is resolved. The scalar field is shown to be a complex field due to the condition of possessing a proper non-relativistic limit. Like the Schr\"odinger field, one component complex Klein-Gordon field corresponds to a boson with one flavor, and therefore there exists no physical real scalar field. As a good example, we present the Schwinger model Hamiltonian which is naturally described by the complex scalar field with one flavor. The non-existence of the real scalar field indicates that the Higgs mechanism should be reconsidered.