Fundamental Limit on 'Interaction Free' Measurements

In "interaction free"measurements, one typically wants to detect the presence of an object without touching it with even a single photon. One often imagines a bomb whose trigger is an extremely sensitive measuring device whose presence we would like to detect without triggering it. We point out that all such measuring devices have a maximum sensitivity set by the uncertainty principle. The so-called "interaction free" measurements can indeed detect such a maximally sensitive bomb without triggering it. However, once it is realized that the sensitivity is finite, it is quite clear that one can probe for the presence of the bomb just by touching it sufficiently softly (i.e., using very long wavelength photons, or small angle scattering).