# A Is this experiment real (double slit)?

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1. Jul 22, 2016

### alikim

I heard about the following experiment and I'd like to know if it really works this way:

Double slit experiment with a detector that gathers information about which slit every particle goes through and stores it in an unobserved black box. The screen is an unobserved photo film that needs to be developed later to reveal where particles hit it.

After the experiment is done and particles have hit the screen wherever they did, the experimental set is disassembled and the black box and the film is taken out.

Before developing the film, there are two choices: either to open the black box and see the info about particles paths in side or destroy the box.

In you look in the box then you always get two-clump pattern developed on the film, if you destroy the box first then the film will always show the interference pattern.

In case this is true, I also would like to know what it means to observe the contents of the box: is it enough to just open it or do you need to read the information?

Also, what happens if I read the info in the box but I have a bad memory and I later forget what I've read?

Thank you,

2. Jul 22, 2016

### StevieTNZ

3. Jul 22, 2016

### Strilanc

You have the details of the experiment wrong, but it's clear that you're talking about a delayed choice eraser experiment.

Here's the main differences between the real experiment and your explanation:

1. You're not deciding whether or not to look, you're deciding how to measure something. You can measure it along the X axis, or along the Z axis.

2. The data doesn't magically change based on the measurement you choose. You use the measurement result to split the data. Your choice just affects what the splits end up looking like. Measure one way, and the splits are two complementary interference patterns. Measure it the other way, they're boring lumps.

3. The which-way information does get copied and stored until later, but it's not on a piece of paper hidden inside of a box. It's in an entangled qubit. This is important to making the "choice of measurement" thing possible.

Overall the experiment is a lot more like choosing whether or not to measure the right thing that reveals the information you want. If you don't measure the right thing, you don't find out anything interesting. If you do measure the right thing, you find out something interesting. The only "weird quantum" part is basically that measuring the wrong thing fundamentally prevents you from later measuring the right thing.

Last edited: Jul 22, 2016
4. Jul 22, 2016

### Staff: Mentor

It's not. Any interaction by the photons with anything macroscopic counts as an observation; this includes the interaction with the detectors so there will be no interference pattern if the detectors are actually detecting. Whether we look at the detection results or not is irrelevant - the pattern on the film is what it is as soon as the photons have landed on it.

The idea that we have to look at the results to make them real dates back to the earliest days of quantum mechanics, and was abandoned with the discovery of quantum decoherence. Unfortunately by then it had made it into the popular imagination, so it's one of those things that "every knows" that just isn't so. A good layman-friendly modern treatment is David Lindley's book "Where does the weirdness go?".