Interaction-free measurements

By means of thought-experiments it is shown that, contrary to common opinion, there exist measurement processes that do not affect at all the measured system. These "negative" measurements consist in the ascertainment of the absence of measurement outcomes that are predicted to occur with a specific probability, and that allow - in analogy with "genuine" measurements - to make new statements about the measured system, and lead, hence, to "reduction of the wave function" in the same way as this is the case with normal, "positive", observations that do disturb the measured system. The compelling consequence is, that the usual justification for the uncertainty relation as an unavoidable retroaction of any measurement process on the measured system breaks down, and that the real justification consists in the permanent influence on the particle of all matter in its immediate or more distant neighbourhood, independent of whether this matter is part of a measurement apparatus or not.