a friend of mine gave me this thought experiment which appears to show an inconsistency in QM. I realize there must be some false assumption being made, but I don't see it. This thought experiment takes place in a 1D world completely described by QM (no QFT, QCD, GR, SR, string theory etc). As far as I can tell, it does not deviate from the QM model but manages to violate the uncertainty principle. Heres the experiment: At t=0, an electron exists at x=0 with dx = 0 and dp=inf. You are at x=1 and send a photon towards the origin. At t=t', you detect that the photon has returned. you can immediately calculate that the position of the photon-electron collision happened at x' = ct'/2 and that the electron's velocity at the point of collision was x'/t'. Therefore the error in both the electron's position and momentum at the time of collision is 0. Isn't this a violation of the uncertainty principle? I realize that setting dx=0 at t=0 is not physically possible. However, if dx is sufficiently low, than dx*dp will be less than h-bar/2 at the point of collision.