Hi, I've done a few terms worth of quantum mechanics, which was good on explaining the mathematics, but not good at explaining it's "derivation". I've been reading the Feynman lectures, and I've learnt a hell of a lot. It does seem though, the more I learn, the less I understand :grumpy: . But it's all good. Anyway; the double slit experiment. Forgive me if this is a simple question, but I'm very confused (in general). Feynman's lectures state that if there is an experiment done which can "in principle" be used to determine which of the two slits an electron passes through, then the interference pattern disappears. He uses photons to illustrate this effect. What I don't understand: Why does the double slit its self not count as observing the electron? For example, when the electron passes through the slit, it could rebound off an atom in the side of the slit, providing information which could be used in principle to discover which path it's taken. My thoughts: Maybe these are electrons which don't contribute to the interference effect? If this is the case, where does the y-momentum of the diffracted electrons come from? Where is the force? (Electrons travelling in x-direction, slits in y-direction) Any help would be appreciated, thanks, Geraint.