In non-relativistic quantum mechanics, the square modulus of the wavefunction ψ gives the probability density function ρ = | ψ |. This is the Copenhagen interpretation, circa 1927. In RQM, while ψ (r, t) is a wavefunction, the probability interpretation is not the same as in non-relativistic QM. Some RWEs do not predict a probability density ρ or probability current j (really meaning probability current density) because they are not positive-definite functions of space and time. The Dirac equation does: