where P is the total momentum of the system. One sees, that if for this momentum the equation P 2 = M 2 holds, which is exactly the Einstein energy-momentum relation (with the Four-momentum P μ μ = ( E / c , p → → ) and P 2 = P μ μ P μ μ ), 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.