In quantum mechanics, the four- probability current or probability four-current is analogous to the electromagnetic four-current : J = ( ρ ρ c , j ) where ρ is the probability density function corresponding to the time component, and j is the probability current vector. In non-relativistic quantum mechanics, this current is always well defined because the expressions for density and current are positive definite and can admit a probability interpretation. In relativistic quantum mechanics and quantum field theory, it is not always possible to find a current, particularly when interactions are involved.