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You know the familiar setup: Strong light source behind a wall with two slits in it, electron passing through. If the photon encounters the electron after it's passed through hole 1, then the electron acts as if that's the only path it took and the interference pattern is ruined; same for if it passes through hole 2.

If you only illuminate one of the slits, then any electrons passing through that slit will be disturbed, of course. So they then behave as if they passed through just one of the slits, and there is no interference pattern.

But if the electron passes through the slit that isn't illuminated, no direct observation is ever made on the electron. Because no flash of light occurred, we can deduce the electron almost certainly went through the non-illuminated slit. How did the electron "know" the other slit was being observed- and thus to act accordingly- if it's behaving as if it never went through that slit? How could that information be communicated to it?

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  • $\begingroup$ Electrons are not passing trough either slit. I would suggest to read Feynman's little book "QED: The Strange Theory of Light and Matter" to understand what electrons are "really doing". All of the difficulties people have with the double slit experiment are based on a false understanding of what "particles" are. Unless you are willing to correct that understanding, you will be turning in an endless circle of logical errors. $\endgroup$ – CuriousOne Jan 11 '16 at 16:53
  • $\begingroup$ @CuriousOne Any chance we might get you to explain what's really going on? $\endgroup$ – DanielSank Jan 11 '16 at 16:54
  • $\begingroup$ Peter, if you illuminate slit #2 and don't see anything, then you're living in a world where the electron must have definitely gone through lit #1. If the electron definitely went though slit #1, then there's can't be any interference between the parts of the electron going through each slit. Now that may sound unsatisfying because I'm implying that what you know actually determines what's physically going on. The thing is, this is the best we understand Nature, as unexpected as it may be. I can write a proper answer to this effect, but please tell us, have you taken a QM course yet? $\endgroup$ – DanielSank Jan 11 '16 at 16:58
  • $\begingroup$ @DanielSank: I did. What is really going in is that the OP labors for naught with a false ontological image for "particle". Feynman takes about a hundred pages in his book trying to set the record straight... I honestly don't know how to condense that to a few sentences. To be honest, I don't think your explanation isn't helping, either. You are merely perpetuating the particle myth by pretending that there are situations in the double slit where a certain path had to be taken. Such a path is never taken, no matter what the outcome. $\endgroup$ – CuriousOne Jan 11 '16 at 16:58
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    $\begingroup$ @PeterZed: If you want to have a consistent interpretation of these experiments then you have to let go of pretty much everything that you may have heard about the movement of particles in quantum mechanics. It's old lore from the first twenty years of the theory when people were still struggling with forcing the single particle image into what looked like a wave theory. Eventually it became clear that the correct description is a pure field theory. Particles emerge as secondary phenomena from that theory quite naturally. Mott noticed this in 1929, but his insight was mostly ignored. $\endgroup$ – CuriousOne Jan 11 '16 at 17:17

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