# Heisenberg uncertainty principle

In the double slit experiment, an electron interferes with itself and creates the pattern because it is in a superposition, traveling through both slits. If we place a detector at one slit, the wave function collapses and we lose the interference pattern. If we place the detector over one slit then we know exactly where the electron was as it went through the slit. However, if we know exactly what the position is, shouldn't the uncertainty of momentum be infinite? As uncertainty of position approaches zero, uncertainty of momentum must approach infinity to be equal to the reduced plank constant. Couldn't it even shoot backwards? But instead, we don't see this. It places a clump behind the screen, acting just as we would expect marbles to be.

• Yes, it can move backwards and the path integral formalism takes that into account. If you are a beginner on this and you wonder how to think semi-classically about movement of quanta correctly (even if one shouldn't), then you may want to read Feynman's little book "QED: The strange theory of light and matter". Or look up en.wikipedia.org/wiki/Path_integral_formulation. – CuriousOne Jul 6 '16 at 21:01