Why isn't the final screen in the double slit experiment a 'detector' which would collapse the wave function retroactively? I'm sorry if this is a stupid question but I'm someone who is not from the physics world trying to learn about this for the first time (I'm reading Daniel Golding's Quantum Physics for Beginners and also watching Leonard Susskind on youtube; if anyone has any other recommendations that you think would be helpful that would also be great!). From the way that Golding describes the electron/photon gun experiment though it's really not clear to me why the screen on which we get the result (e.g. where, without the detector, we would see an interference pattern, and, with the detector, see a particle pattern) itself is not a detector...
It's my understanding that when we place a detector to detect the photon after the photon has passed through the slits, there is 'delayed reaction' (Wheeler's experiments). However I don't really get why the screen itself where we get the results would not itself be a detector which could also cause the delayed reaction (and thus how we get results without the wave collapse... since the screen itself seems to always be measuring the particles at some point?)
I'm sorry, I have a feeling this is a stupid question but I've been trying to find an answer online for the past hour and I really can't seem to figure it out.
 A: The screen is a detector. The crucial point is that you can't just make a measurement in quantum mechanics. You have to choose what measurement to perform, and some alternatives are "incompatible" (noncommuting). The screen measures the particle in a way that doesn't reveal which slit it went through. You can instead measure the particle in a way that does reveal which slit it went through, but doesn't reveal where it would have made a dot on the screen. In Wheeler's delayed choice experiment, you choose which of these measurements to perform after the particle has passed through the slits.
The measurement doesn't have a retroactive effect – at least, not in any physically detectable way that could be used to send a message to the past or something.
I disagree with Retracted's answer. The issue is not when the measurement happens, but the choice of basis. It's never too late to determine which slit the particle went through, unless you've already done the wrong measurement.
A: The screen is a detector, but instead of detecting the electron at one specific slit, it detects the electron after having been through both slits. The screen where you look at the interference is absolutely an example of a quantum measurement, but it measures after the interference, not before. By the time the electron reaches the screen, the interference pattern has already formed.
