This experiment which is a variation of the double slit experiment , has actually been conducted. We have (a) the source of electrons, (b) screen 1 ( with the double slit ) and finally (c) screen 2 , where the particles are registered. Near the slits in screen 1 and on the far side from the source of electrons are placed (a) light sources , one for each slit , and (b) photodetectors designed for observing the passage of electrons through the screen with slits (the photodetectors register light scattered by the electrons ) . If the electron passes simultaneously through both slits both the photodetectors are activated simultaneously . But if the electron passes through either one of the slits , only one detector is activated ; in this case we can also tell through which slit the electron passes. So we place an electron source at the source and switch on the light sources used for scattering of the electrons after they pass through the slits . We shall assume that the electrons pass through the slits one by one : the source emits an electron only after the preceding one has reached the detector screen. What does the experiment show ? It always turns out that only one photodetector (either right or left ) is activated and that both photodetectors are never activated simultaneously . It means that the electron passes not through two slits but only through one. Moreover , we can always indicate the slit through which any electron passes. It may again be surmised , in order to explain the interference pattern , that the electron which passes through one slit in some way perceives the neighbouring slit by some secret means , while passing through the slit . It is best not to jump to conclusions but to carry out the experiment over and over again until a sufficiently large number of events of electron detection on screen 2 is obtained and to see how they are distributed. But here we are in for a big , big surprise . On screen 2 we get not the interference pattern that we had expected but an additive pattern ! The point to be made here as far as ‘New Field Theory , is concerned , is that this experiment practically proves , without any doubt the existence of a virtual photon field and that this field is responsible for the interference pattern which exists when no detectors or light sources are present.