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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?

  • 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. – CuriousOne Jan 11 '16 at 16:53
  • @CuriousOne Any chance we might get you to explain what's really going on? – DanielSank Jan 11 '16 at 16:54
  • 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? – DanielSank Jan 11 '16 at 16:58
  • @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. – CuriousOne Jan 11 '16 at 16:58
  • @CuriousOne I don't remember it taking several hundred pages for me to form my present understanding of quantum mechanics and measurement. It was more like the right two or three conversations. All the problem solving in the textbooks didn't help. – DanielSank Jan 11 '16 at 17:00
  • @DanielSank: You are luckier and, I mean this absolutely honestly, smarter than most of us. It took me years to get the correct understanding of QM. As for the illuminated double slit... I am sure you are aware that that's not a single particle problem, right? If one wanted to do this right, one would have to discuss the entire wavefuntion of electrons and em field... and then there would be many, many, many more paths in the path integral formulation that one would have to integrate over than in the original problem. One can't talk the complexity of this away without falsifying the physics. – CuriousOne Jan 11 '16 at 17:04
  • @CuriousOne Oh, that's definitely a new one: Electrons not passing through either slit. I've read about the experiment a number of times over the years, but I don't think I've heard mention of that before. Thank you very much for the book recommendation, I've put it on my list. –  Jan 11 '16 at 17:08
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    @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. – CuriousOne Jan 11 '16 at 17:17
  • @CuriousOne "I am sure you are aware that that's not a single particle problem, right?" As happens all too often on this site and in physics generally, we now find ourselves distracted by the complexities of the original double slit experiment: in this case you picked the multi-photon issue. Yet, all of the interesting features of the double slit experiment can be observed in e.g. superconducting qubits where no such problem arises. Therefore, we certainly can avoid the complexity you mention without falsifying the physics. Perhaps what you call my "luck" is working in that field. – DanielSank Jan 11 '16 at 17:32
  • @CuriousOne "Eventually it became clear that the correct description is a pure field theory." Except that you can observe the double-slit phenomenon in a zero dimensional effective solid-state system too, so one does not need field theory at all to get at the question originally asked in this post. – DanielSank Jan 11 '16 at 17:34
  • @DanielSank: You may have noticed that I keep telling people that the double slit experiment is of zero value to physics. It has not been of any historical value, QM came out of Planck's black body radiation field problem and atomic physics. The electron double slit has only been performed in the 1960s as a physical curiosa experiment. As a teaching tool it fails horribly because it teaches people all the wrong lessons about QM. Instead of learning the structure, they get hung up on a false ontology play... what's that good for? – CuriousOne Jan 11 '16 at 17:35
  • @DanielSank: You are still dealing with a quantum field, whether you like it, or not. Solid state physicists do, by the way, use the same quantum field theoretical methods to solve their multi-particle problems. The question here is simply this: how long are we going to teach a false ontology just because it's supposed to be "easier" on the minds of the kids? You might as well ask kindergarten teachers to teach that the Earth is flat. It's not a bad approximation when you are only four feet tall... right? – CuriousOne Jan 11 '16 at 17:39
  • All of your debate just went whoosh. There's a lot I don't know - which I already was well aware of, but thanks for showing me more of it.

    It's interesting that Feynman, who is so celebrated, used the thought experiment as an intro to quantum mechanics without comment or criticism from his peers (that I've heard of).

     –  Jan 11 '16 at 17:48
  • @CuriousOne I understand quantum measurement better than the average bear because I work with a system that's simple but quantum. As you say, there are quantum fields underneath, but by putting the system in a dilution refrigerator the extra degrees of freedom in those fields are frozen and only the low energy collective degrees of freedom matter. In this system we have violated Bell's inequality, done double slit, and whatever other "quantum optics" experiments you like (and more!). It's not a lie for the kiddies. It's a great system that helps you cut through the distractions. – DanielSank Jan 11 '16 at 17:52
  • PeterZed Yeah, I do apologize for this back-and-forth without answering your question. It is a good though because @CuriousOne and I often disagree about how to think about stuff so maybe future hapless readers will get extra perspective. Could you answer my question though: have you taken a QM course? Do you know stuff about state vectors and the basic mathematical apparatus we use to describe them? – DanielSank Jan 11 '16 at 17:54
  • @DanielSank: This has nothing to do with quantum measurements to begin with. It has everything to do with nuances in English: "The wave function describes the probability of measuring AN electron at position x." is logically not equivalent to "The wave function describes the probability of measuring THE electron at position x.". The first sentence is approximately correct (for non-relativistic systems), the second sentence is false for all systems, even for non-relativistic single particle systems. – CuriousOne Jan 11 '16 at 17:57
  • @CuriousOne I disagree. I can set up a system such that the sum of all the excitation numbers over all the modes is one, i.e. $\sum_{\text{mode }m} \hat{n}_m = 1$. If there's nothing around able to supply/remove the mass/energy needed to change that number, then there is really nothing wrong with talking about "the excitation" (where "excitation" might be "electron" or whatever). – DanielSank Jan 11 '16 at 18:52
  • @CuriousOne I agree that because first year QM courses only deal with one-excitation systems, many students get all confused about how to think about this stuff hence my post about second quantization. – DanielSank Jan 11 '16 at 18:54
  • @DanielSank: The problem is that those who like to talk about "the electron" almost inevitably talk about "the path of the electron". See above... and then watch them go down the path of "quantum realism". It's a trivial mistake that still hasn't left the physics building. Just look at all the quantum optics papers that are still trying to find the man behind the quantum mechanical curtain when there isn't even a curtain! What surprises me most is that this problem has been solved so early and the solution has been almost universally forgotten. – CuriousOne Jan 11 '16 at 19:00

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