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The famous double slit experiment shows how light behave like a wave when sensors near one slit are on and recording vs not recording. I didn't find a lot of details, but especially given the case that something similar could happen for light traveling around the universe... it raises for me the question:

What if someone/something recorded the particle at the slit or not with a 50% chance of recording. Supposing we wouldn't/couldn't know which it is, it should behave like a wave (my expectation). Couldn't we possibly later open that box and ask/check if it was actually recording or not?

More accurately, would the results change if we could and could not later know if the sensor was actually recording or not?

I know of the delayed double slit (especially the Experimental delayed-choice entanglement swapping) but here I'm not asking regarding time travel but more regarding possible knowledge or lack of it. Because if anything that could observe light would force it to collapse, it seems that it could tell us if something observed that particle, ever (ok no need to put aliens here but it could).

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It's unclear what you are asking in the last paragraph of your question.

Perhaps you would be interested to see that observation or measurement is still a hotly debated topic. For the situation you present, let us consider that all particles can act like waves to some degree.

If you were firing electrons through a double slit and only measuring them an unknown part of the time, you would only have data from the electrons that were measured. When not observing the motion of the electrons, you are not hitting them with the photons required to see anything - they follow their original trajectory.

An analogy that is often used to describe measurement goes like this: You are in a dark room with a rolling chair and a number of basketballs. Your task is to find the chair as it rolls across the room. You have two options:

  • Throw basketballs randomly until you hit the chair.
  • Throw many basketballs at once everywhere around the chair.

Both of these have trade-offs, the first changes the trajectory of the rolling chair, the second uses a large amount of energy and can only apply to situations where you are trying to locate something that is not a basketball (photon).

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