May I ask about this configuration: Let's say we install detectors on the slits so the detection at the screen will be 2 dots, one coming from slit A and one coming from slit B. No interference pattern. We also set up the experiment so that there is not 1 observer but 2. Observer A' is placed so that he can only see half of the screen, the half that gets hit when the particle goes through slit A. If the particle goes through slit A he will see a dot on the part of the screen he can see and he can write it down. If it goes through slit B he will see nothing, he doesn't even know if a particle has been fired at all. Conversely, observer B' is placed so that he can only see the half of the screen which gets hit when the particle goes through slit B. If the particle goes through slit B he will see a dot on the part of the screen he can see and he can write it down. If it goes through slit A he will see nothing, he doesn't even know if a particle has been fired at all. Moreover, none of the observers is aware that there is another observer who can see the half of the screen they can not see. In fact they don't even know they are only seeing half of the screen. All they can tell is that they are requested to write down their letter whenever they see a dot in the screen they can see. So if we run the experiment firing a shower of electrons at once, each observer will write down that they see a bright focused spot on their screen, no lines. If we run the experiment firing electrons one by one, say 1000 electrons, observer A' will write down that he saw 500 dots appearing in the screen, and observer B' will write down that he saw 500 dots appearing in the screen, all of them, focused around one point on each screen, no lines. Now we remove the detectors at the slits. What will they see if we fire a shower of electrons at once. Interference lines in their half of the screen? And what will they see if we fire 1000 electrons one by one? Will they always see a dot on the half of the screen they can see? Or only half of the time (500 each)? And will those dots be concentrated on one point, or will they gradually form interference lines? The reason I'm asking is, most interpretations say that it boils down to whether we generate path information or not. According to each observer, every time they see a dot in their screen there is path information: 'yes, an electron has passed through my slit. I'm sure of that'. They do not know about any other observer watching the experiment. But taken in combination, if both saw always a dot, 1000 times each, even when they think there is path information, there is none, the total combined information just says that they all passed through both slits. And going to more extreme scenarios, what if after the experiment we kill both observers and look at their papers? (not that I want to put that in practice lol). Or, kill only one of the observers and burn down his paper before looking at it, and look only at the other observer's paper?