  We show that no device or gedanken experiment is capable of measuring a
distance less than the Planck length. By "measuring a distance less than the
Planck length" we mean, technically, resolve the eigenvalues of the position
operator to within that accuracy. The only assumptions in our argument are
causality, the uncertainty principle from quantum mechanics and a dynamical
criteria for gravitational collapse from classical general relativity called
the hoop conjecture. The inability of any gedanken experiment to measure a
sub-Planckian distance suggests the existence of a minimal length.
