where P is the total momentum of the system. One sees, that if for this momentum the equation P 2 = M 2 {\displaystyle P^{2}=M^{2}} holds, which is exactly the Einstein energy-momentum relation (with the Four-momentum P μ = ( E / c , p → ) {\displaystyle P_{\mu }=\left(E/c,{\vec {p}}\right)} and P 2 = P μ P μ {\displaystyle P^{2}=P_{\mu }\,P^{\mu }} ), the four-point Green function contains a pole. If one plugs that ansatz into the Dyson equation above, and sets the total momentum "P" such that the energy-momentum relation holds, on both sides of the term a pole appears.