Another proof that the future can influence the present

A modified Young double-slit experiment proposed by Wootters and Zurek is considered in which a system P of parallel plates covered with a photographic emulsion has been set up in the region where we would normally expect the central interference fringes. Because under certain conditions P makes it possible to conclude with much more than50% certainty through which of the two slits each particular photon passed, the relevant interference pattern becomes blurred. It is proved that this implies a retroactive effect because the setup of P and the interaction of photons with P appear to influence the momentum transferred to the photons by the edges of the slits at an earlier stage of the experiment.