In quantum mechanics, particles of mass m in the quantum state ψ (r, t) have a probability density defined as ρ ρ = ψ ψ ∗ ∗ ψ ψ = | ψ ψ | 2 . So the probability of finding a particle in a differential volume element d r is d P = | ψ ψ | 2 d 3 r . Then the number of particles passing perpendicularly through unit area of a cross-section per unit time is the probability flux; J = i ℏ ℏ 2 m ( ψ ψ ∇ ∇ ψ ψ ∗ ∗ − − ψ ψ ∗ ∗ ∇ ∇ ψ ψ ) . This is sometimes referred to as the probability current or current density, or probability flux density.