π π 2 2 − − p 2 = − − ( ε ε 2 − − A 2 ) 2 = − − ε ε 2 2 + 2 ε ε w A − − A 2 , M 1 2 = m 1 2 + 2 m w S + S 2 , M 2 2 = m 2 2 + 2 m w S + S 2 . The corresponding Klein–Gordon equations are ( π π 1 2 + M 1 2 ) ψ ψ = 0 , ( π π 2 2 + M 2 2 ) ψ ψ = 0 , and each, due to the constraint P ⋅ ⋅ p ≈ ≈ 0 , is equivalent to H ψ ψ = ( p ⊥ ⊥ 2 + Φ Φ − − b 2 ) ψ ψ = 0.