hello I started to read ‘QFT in a Nutshell’ by A. Zee. In the introduction to the path integral formulation of quantum mechanics there is the story about a particle going through a series of screens with holes drilled through them. Then the number of holes in each screen is increased. This results in the spaces on the screen which are not yet drilled becoming very small. My difficulty is this. The undrilled spaces are then so small that they themselves may act as obstacles thereby providing an amplitude of scattering the particles backwards. Am I complicating things by including this backward scattering together with the amplitude of forward propagation of the paticle? Any help? Thanks.