In the third book of Feymann's Lectures on Physics(section 1-8) he describes how "the Uncertainty Principle protects Quantum Mechanics." The experimental situation is a modified double slit experiment where the two slits are put on rollers in an attempt to detect which slit an electron passes through. The idea is that the change in the direction of momentum of the slits on rollers after collision with an electron will be opposite for the two slits. He argues that if one could measure this change in momentum then uncertainty principle would make it impossible to exactly locate the position of the slits and this will blur out the interference pattern. - Why can't the apparatus be reset each time for each electron? In this way we do not care where the slits are after the electron passes through - only the momentum. Is it that the apparatus can not be reset because this would mean knowing where the slits are and knowing that their momentum is zero at the same time?But then doesn't the mere existence of an interference pattern mean that we know both?