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Is the detector a passive device or is it just a fictional mathematical probe. I think the detector is somehow consuming the energy responsible for the wave nature of the photons, electrons or atoms, but I can't find any information about the detector and how it works.

Any help is appreciated since all videos and articles suspiciously skipping the detector or simplifying it as a 3d cat or fictional cartoon eye. I know about the quantum eraser experiment but before moving to it, I need to know about the detector and how it exactly measures. I'm a software programmer trying to understand how quantum computers work.

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    $\begingroup$ You can use a wall as a detector. $\endgroup$
    – rob
    Mar 23, 2019 at 1:11
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    $\begingroup$ @rob I mean the detector that is supposedly causing the photones to change their mind and act as particles withot wave characteristics and not the surface $\endgroup$
    – Amr Berag
    Mar 23, 2019 at 1:21
  • $\begingroup$ @rob Its somthing scientists said or imply they placed behind the slits to determine which slit a particular photon or atom went through before hitting the surface. Th surface hier is the wall you are talking about. $\endgroup$
    – Amr Berag
    Mar 23, 2019 at 1:59
  • $\begingroup$ @AmrBerag I’m glad to see you’re still interested in this considering you never did get an actual physical answer. In experiments involving photons there’s only one way to detect them. Place matter in the path of the photon and the photon will be absorbed. From this you will know what photons are missing or where they landed. Like spectral lines you can have emission lines or absorption lines. It’s all physical and no waves involved. $\endgroup$ Aug 21, 2020 at 15:40
  • $\begingroup$ @BillAlsept On the whole topic the probabilistic interpretation was convincing to me but the interference part is a twist that opens more questions I may not yet be qualified to ask nor evaluate the answers for. My best understanding now is we are facing a scaling problem when trying to observe the behavior of the tiniest things in the tiniest time frames. I'm not sure how to define physical any more after getting into this and I was reading about the string theory in which they treat those strings as physical stringth with tension and who knows if they will turn out to be attached to pulleys $\endgroup$
    – Amr Berag
    Aug 24, 2020 at 2:05

3 Answers 3

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I misunderstood your question at first. I thought you were asking about the detector downstream of the double slit, where the interference pattern is visible; every practical double-slit experiment includes such a detector. But instead you are asking about a hypothetical detector which could "tag" a particle as having gone through one slit or the other. Most interference experiments do not have such a detector.

The idea of "tagging" a particle as having gone through one slit or the other, and the realization that such tagging would destroy the double-slit interference pattern, was hashed out in a long series of debates between Bohr and Einstein. Most introductory quantum mechanics textbooks will have at least some summary of the history of these discussions, which include many possible "detectors" with varying degrees of fancifulness.

A practical way to tag photons as having gone through one slit or another is to cover both slits with polarizing films. If the light polarizations are parallel, it's not possible to use this technique to tell which slit a given photon came through, and the interference pattern survives. If the light polarizations are perpendicular, the it would be possible in principle to detect whether a given photon went through one slit or the other; in this case the interference pattern is also absent. If the polarizers are at some other angle, it's a good homework problem to predict the intensity of the interference pattern.

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    $\begingroup$ Wouldn't perpendicular ploarizers also explain the lack of an interference pattern because perpendicular waves wouldn't interfere with each other? $\endgroup$
    – JohnFx
    Jun 28, 2020 at 1:49
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    $\begingroup$ @JohnFx I thought that's what I wrote? $\endgroup$
    – rob
    Jun 28, 2020 at 6:05
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I watched a couple videos on the double slit experiment and I had exactly the same question as the original poster (Amr Berag) and stumbled upon this post and was just wondering how come everyone else isnt wondering the same thing.

There are so many videos showing the actual double slit experiment but none show the actual wave function collapse in reality when the particles are "observed".

It turns out that it was merely a thought experiment when it was first proposed and its not super trivial to put an actual detector, but in 1987 an experiment was performed and subsequent experiments were performed but none shown on video.

This link explains that a bit

Please look at the which way section here

I'm just surprised that they don't mention this in any of the videos and how come no one else asks for proof for this. When I saw the original video I was just waiting till the end to see them put a "detector" and see the interference pattern disappear, but nope.

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    $\begingroup$ It seems nuts that this is taken as a given when it was just a thought experiment. I read the link but I don't see how it demonstrates the thought experiment practically... $\endgroup$
    – Claudiu
    Dec 3, 2021 at 23:13
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It's anything that gives you information about where the particle passed by. The problem in measurement in QM is that to measure anything you need to interact with the "thing" you want to measure. If you want to measure temperature in a drop of water with a large thermometer, the heat of the thermometer will affect the drop temperature. If you want to measure the distance to the moon you may shoot a laser (knowing c=the speed of light in vacuum) and wait till it return. But if the smallest thing you have is rock, you can throw it and do the same process knowing the rock speed (ignoring gravity and air resistance), but if the smallest thing you have to measure it is another moon, you will affect the position of the "original" moon and so affecting the whole state of the thing you want to measure. Well, the quantum world is so small that to measure things you have to destroy the original state or perturb it. The device in the double slit is just to block or interact with the particle passing through that slit.

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    $\begingroup$ You just rephrased my question bro. Are you telling me that the detector in the double slit experiment is not passive enough to detect without afecting the outcome? Then why all these credited sientists jumping to the assumption that photones are conscious instead of saying that the very act of trying to detect is affecting these photones and particles in a way that they lose their wave characteristics by withdrawing their enrgy for example. Or do you mean the detector is nothing but another surface moving toward the slits but then it will be too close and prevent the interference! $\endgroup$
    – Amr Berag
    Mar 23, 2019 at 1:47
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    $\begingroup$ I din't rephrase your question. There's no single question mark on my reply. The math is a model to represent a behavior that is very consistent. It is not passive enough $\endgroup$
    – Gndk
    Mar 23, 2019 at 1:59
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    $\begingroup$ Do you mean there is no detector? $\endgroup$
    – Amr Berag
    Mar 23, 2019 at 2:04
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    $\begingroup$ The behavior Im asking about is that these things behave like waves when it is unknown which path they take but behave like sand partjcles or bullets when sientists attempt to detect their pass. I would like to know what device they used to detect the path and how it works. This device is referred to as "The Detector" that is causing the mind puzzling behavior implying that such device exists and is good enough to make scientists and univesity professors believe that atoms not only conscious but can communicate and tell their friends that some curious humans are trying to spy on them. $\endgroup$
    – Amr Berag
    Mar 23, 2019 at 2:37
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    $\begingroup$ @Esther You can start here if you are coming from that cartoon robotic eye detector video youtu.be/yotBpxXiivA $\endgroup$
    – Amr Berag
    Dec 5, 2020 at 13:25

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