What was in the apparatus before the click
  When a quantum system is described by a wave function consisting in a couple of wave-packets, each wave packet traveling on a separate path, a commonly asked question is why at a given time only one of the wave packets is able to trigger a click in a detector. In the second half of the last century many scientists considered the possibility that not all these wave-packets are identical. Namely, that there exist full waves and empty waves. The two types of waves were supposed to be identical in all the experiments, e.g. able to produce interference when meeting, however, the full wave was supposed to be able to trigger a detector, while the empty wave was supposed to leave the detector silent. The present text describes an experiment which, for explaining the results, seems to beg the hypothesis of full and empty waves. 
Sofia Wechsler