I just have some narrow questions and got a headache searching through the other threads to see if they have been answered. I just want to understand more specifically what is actually going on with the detectors at the slits. 99% of the explanations and descriptions I read about just talk about a "magic" detector without getting into details. So here are my questions. First, how do you detect whether or not a photon has gone through a slit? Second, and this is the more important question, I would like to know more details on the constraints using these detectors for when the photon is being detected and having it's position "known", and when it isn't being detected. Third, in QED Feynman said (if I am remembering correctly) that you could put these detectors on the slits, but you didn't have to "turn them on". What exactly happens when they are on vs. off? Fourth, and here is the one that I'm really curious on, though it seems dependent on all the others. What constitutes "detection" in a greater sense. For example, if the detectors are turned on, but the set up is arranged so no human could possibly read the results, does this have any bearing whatsoever on the experiment? And related, how do you rule out that the detectors aren't "causing" the change just by being on as opposed to the idea that it is the information being detected that is causing the change? Thanks in advance.