One sees that G F ε ε ( x , y ) = 1 ε ε if ( x − − y ) 2 = 0 , and lim ε ε → → 0 G F ε ε ( x , y ) = 0 if ( x − − y ) 2 ≠ ≠ 0. Hence this means that a single massless particle will always stay on the light cone. It is also shown that the total probability for a photon at any time must be normalised by the reciprocal of the following factor: lim ε ε → → 0 ∫ ∫ | G F ε ε ( 0 , x ) | 2 d x 3 = lim ε ε → → 0 ∫ ∫ ε ε 2 ( x 2 − − t 2 ) 2 + ε ε 4 d x 3 = 2 π π 2 | t | . We see that the parts outside the light cone usually are zero in the limit and only are important in Feynman diagrams.