This expression can be related to the vacuum expectation value of the commutator of the free scalar field operator, G ret ( x , y ) = − − i ⟨ ⟨ 0 | [ Φ Φ ( x ) , Φ Φ ( y ) ] | 0 ⟩ ⟩ Θ Θ ( x 0 − − y 0 ) where [ Φ Φ ( x ) , Φ Φ ( y ) ] := Φ Φ ( x ) Φ Φ ( y ) − − Φ Φ ( y ) Φ Φ ( x ) is the commutator.