The relativistic mass-energy relation : E 2 = ( p c ) 2 + ( m c 2 ) 2 where again E = total energy, p = total 3- momentum of the particle, m = invariant mass, and c = speed of light, can similarly yield the Klein–Gordon equation : E ^ ^ 2 = c 2 p ^ ^ 2 + ( m c 2 ) 2 E ^ ^ 2 Ψ Ψ = c 2 p ^ ^ 2 Ψ Ψ + ( m c 2 ) 2 Ψ Ψ where p ^ ^ is the momentum operator. That is: ∂ ∂ 2 Ψ Ψ ∂ ∂ t 2 = c 2 ∇ ∇ 2 Ψ Ψ − − ( m c 2 ℏ ℏ ) 2 Ψ Ψ