# 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.

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.

• "It's never too late to determine which slit the particle went through, unless you've already done the wrong measurement." Yes ($+1$!), but I would add one minor caveat: the longer you wait, the more difficult it becomes to actually measure that observable in practice. Eventually, it becomes prohibitively impractical. Jan 3, 2021 at 3:49
• @benrg thank you! I'm still slightly confused (but if this is a stupid question feel free to ignore it!). My (amateur) understanding is: if we don't have an extraneous detector right after the slits (and interference occurs) we are basically thinking that the light behaved like a wave and that the 'probability waves' passed through both slits (so as to cause interference with itself). I'm not sure how the screen detector is different from the 'interference causing' detector since I thought they would both be showing the position of the electron/photon? Jan 13, 2021 at 1:02
• "I'm not sure how the screen detector is different from the 'interference causing' detector since I thought they would both be showing the position of the electron/photon?" What do you mean here? what is an interference causing detector vs a screen detector? Jan 17 at 12:09
• @ajiyks, if it helps, you know you can use a bunch of small detectors that are all placed in a line to replace the "screen detector" right? Per electron, only one of those individual detectors will see an electron - and in the end when you collect the data it will show an interference pattern shown as a wave. Jan 17 at 12:10

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.

• Thank you!! I guess then even though a detector which is placed 'after' the particle has passed through the slits can cause "delayed reaction," that there is a limit to this? Is there a defined limit? (Or perhaps it's just one wavelength as in the Wheeler experiment.) Thank you in any case! Sorry, it does seem dumb in retrospect. I just wanted some confirmation. Dec 31, 2020 at 2:55