  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.
