Hi, I'm new here, and not *really* a physics student, just a guy trying to wrap his head around science by reading lots of books! Stuck on a well known theory, which in my head does not make sense, and hoping someone can clarify... 1. The problem statement, all variables and given/known data A quantum Eraser - this works by adding a vertical polarized lens over one slit of a "2-slit interference pattern creator" and a horizontal polarized lens over the other slit. When added, the interference pattern vanishes and just has one "clump". Replacing with a 50/50 45o lens to both slits, the interference pattern reappears. The books all tell me this is because you can not measure the effect, and this experience attempts to do that, but by doing so, stops it working. The vertical lens only allows through a vertical particle and so on with the other. 2. Relevant equations Surely all this means is that when the photon attempts to "go through both slits at the same time" it is either in a horizontal or vertical state, and so one of the lenses stops that (as it cant be in both states at once). When the 50/50 lens goes on, if the photon can go through one lens it can also go through the other at the same time. 3. The attempt at a solution This says to me that its not a case of measuring it at all that stops it working, its just you cant put a square peg into a triangular hole. As the entire book hinges on this "cant measure" rule, so far to me it all appears to be based on a misunderstanding... therefore the whole quantum thing is collapsing for me. Please, what key part of this am I getting wrong? Thank you so much.