A pair of detectors are positioned beyond the mirror in such a way that the photon's superposition collapses and the photon is observed to have either been on the upper path or the lower path, but not both. If the "upper-path" detector encounters the photon, then the photon "actually" took the upper path and no measurement was made of whether or not there was a bomb on the lower path. If, however, the "lower-path" detector encounters a photon, it can be determined that fifty percent of the time, there is a bomb on the lower path--"without actually encountering it".