I am just starting to study quantum mechanics, so I realize I am working with over-simplified examples, but that should work out since I'm hoping for simple answers... Among other things, the claim that this eraser would work has me scratching my head. http://sciam.com/media/inline/DD39218F-E7F2-99DF-39D45DA3DD2602A1_p95.gif They say "the photon need not be detected". I interpret this to imply that the detector could be switched off or removed entirely without changing the result. That being the case I hardly see why there is a detector in the diagram at all. Now, the lens "makes it impossible to ascertain which slit the photon came from" _only at the focus point_. If the detector was placed a mm closer to OR further from the lens, the photons would arrive at distinct points corresponding to the slits. And since the detector can be removed from the situation because the "photon need not be detected", All the lens has done is provide one point in space at which the information is unavailable. That seems to me very different from "erasing" it. If someone could inform me of my specific mistakes in interpretation I would appreciate it, Thanks, X'