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Note 1: Don't apprehend my question as 'quantum woo'. I do not mean so and hate anyone who does.
Note 2: I am just a 14 year old physics enthusiast. Don't expect any background.

On to the real question. I recently read about the Quantum two slit experiment and learnt about how the particles go through both slits at the same time (thanks Feynman) unless I actually measure the 'which path' (which slit) information. If i do, the interference pattern disappears.

Lets say i have the experiment setup. I have sensitive sensors which detect which path the particle took. The data goes to a computer which processes the data and shows me the data.

Now lets take the following cases:
1)The computer has an inherent flaw. Sometimes, it shows the wrong which path value. That way, i know the which path data, but its wrong. So the interference pattern shouldn't disappear.

2)The computer is set in a different language. Instead of 'path one' or 'path two', it says 'útvonal egy' or 'útvonal két' (hungarian), which i don't know. So i know the which path data, but can't understand it. So the interference pattern shouldn't disappear.(again)

3)Same as above, but i use Google Translate. So immediately after I'm done using it, should the interference pattern immediately disappear?

In the above three cases were my understandings correct? If so, Quantum bizarrenes has no limit :)

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    $\begingroup$ What the computer displays or not is wholly irrelevant. What matters with which interactions happen, and that doesn't change in any of the three case, and I don't see why you would think it does. $\endgroup$ – ACuriousMind Feb 9 '16 at 15:35
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    $\begingroup$ No, your understanding is not correct. What matters is whether the "which-path" information is available in principle, not whether it is actually accessed by the observer. In order for this information to be available in principle, you need to introduce a detector which interacts with the quantum particle and completely changes the nature of the experiment, so that the interference pattern disappears. $\endgroup$ – Mark Mitchison Feb 9 '16 at 15:35
  • $\begingroup$ Re @ACuriousMind well in trivial literature there are often claims that the state of the system depends on "our knowledge" or stuff like that, which goes horribly wrong here. Re OP: Read feynman again and replace "measurement" in your mind with "interaction". There might be subtleties on how "measurement" actually works on a quantum level, but for you "interaction" should suffice. $\endgroup$ – Bort Feb 9 '16 at 15:44
  • $\begingroup$ Hi Udit. There's a physics chat session starting now. You might want to ask your question there. $\endgroup$ – John Rennie Feb 9 '16 at 16:01
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Your flaw lies in that you think "which path information" means that your consciousness is it that collapses the wavefunction, which in fact was the interpretation Wigner and friends (note the pun) had for quantum mechanics. But this interpretation is regarded as incorrect. In your case we could even perform the experiment and we would note that the interference pattern would already have disappeared in all your 3 cases.

What really matters is when the measurement occurs, not when you read off what you measured. I.e. you send a photon or something to tell you which slit it has gone through. That is what makes the pattern disappear. What you do after is just you messing with the photon. (i.e. google translator etc. doent matter)

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  • $\begingroup$ So wavefunction collapse is entirely physical? The cohort of articles referring to the infamous Copenhagen Interpretation seem to say other wise. Look at the EPR Paradox which couldn't really work if wavefunction collapse was entirely physical. Or could they? $\endgroup$ – Udit Dey Feb 9 '16 at 15:51
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    $\begingroup$ @UditDey Wavefunction collapse is the awkward way of describing what happens to part of a quantum system when one willfully neglects the rest of that system (i.e. the measurement apparatus itself). It's useful for teaching undergrads how to solve textbook problems, but ultimately it's just a way of getting the right answer with slipshod methods. $\endgroup$ – user10851 Feb 10 '16 at 1:20
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Pop-science explanations of the two-slit experiment often confuse "a person knowing" with "an instrument detecting". But it's the latter that matters, not the former.

The interaction with a detector is what makes the interference pattern go away. Whether or not the detector then reports the result to you, and how it does so, has no relevant effect on the particle's path.

The reason interaction with the detector matters is interference only happens when every detail ends up the same. The detector's state is a detail that doesn't end up the same, so it prevents the particle-went-left paths from interfering with the particle-went-right paths.

(If you are able to erase the detector's state somehow, then the paths can interfere again. You can even delay the erasure until later, but you'll need some clever measurements and after-the-fact grouping to recover the hidden interference patterns. Also in these cases the term "detector" starts to be misleading, since we're actually talking about manipulating coherent quantum information.)

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There is often a misunderstanding about what observation means. Observation is not necessarily a human activity, it is an interaction with the experiment. Whether you do understand the language is irrelevant, there has been a physical interaction, and this is what counts.

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