If the motion of the particle is constant, the expression simplifies to Δ Δ τ τ = ( Δ Δ t ) 2 − − ( Δ Δ x ) 2 c 2 − − ( Δ Δ y ) 2 c 2 − − ( Δ Δ z ) 2 c 2 , where Δ means the change in coordinates between the initial and final events. The definition in special relativity generalizes straightforwardly to general relativity as follows below.