24
$\begingroup$

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 are 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.

$\endgroup$
11
  • 4
    $\begingroup$ You can use a wall as a detector. $\endgroup$
    – rob
    Commented Mar 23, 2019 at 1:11
  • 5
    $\begingroup$ @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 $\endgroup$
    – USER249
    Commented 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. $\endgroup$
    – USER249
    Commented 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. $\endgroup$ Commented Aug 21, 2020 at 15:40
  • 1
    $\begingroup$ @USER249 did you ever get a good answer or explanation to this? I feel as though it would be impossible to build such a detector withour perturbing the whole experiment in the first place. I'm not a physicist though so I don't know what I don't know. $\endgroup$
    – rollsch
    Commented Aug 7, 2023 at 0:11

3 Answers 3

12
$\begingroup$

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, 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.

$\endgroup$
6
  • 5
    $\begingroup$ Wouldn't perpendicular ploarizers also explain the lack of an interference pattern because perpendicular waves wouldn't interfere with each other? $\endgroup$
    – JohnFx
    Commented Jun 28, 2020 at 1:49
  • 1
    $\begingroup$ @JohnFx I thought that's what I wrote? $\endgroup$
    – rob
    Commented Jun 28, 2020 at 6:05
  • $\begingroup$ When they go through the polarizers, are they in a superposition of both polarities, or just in one of either? $\endgroup$
    – Juan Perez
    Commented May 20, 2023 at 13:50
  • $\begingroup$ @JuanPerez I'm not sure that question has a simple answer like you want it to. It might be possible to use the ambiguity to make a variation of the "quantum eraser" experiment, but I don't think I can explain that in a comment. $\endgroup$
    – rob
    Commented May 20, 2023 at 16:20
  • $\begingroup$ I think that polarizers at the slits are a bad example because even classical electrodynamics predicts the disappearance of the interference pattern in that case, so it can't be said to provide evidence for quantum mechanics. $\endgroup$
    – benrg
    Commented May 20, 2023 at 19:22
11
$\begingroup$

I watched a couple of 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 isn't 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 it's 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 of 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.

$\endgroup$
8
  • 3
    $\begingroup$ 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... $\endgroup$
    – Cloudyman
    Commented Dec 3, 2021 at 23:13
  • $\begingroup$ The reason why no serious physicists does these experiments is trivial: one can't learn anything from them. We know what "a detector" does to a system: it either removes energy from it or it adds energy to it. The consequences of that are trivial to calculate, both in quantum mechanics and in classical physics. So what's the point of wasting time and money on experiments that we can predicts at the introductory textbook level? In practice science is about what we don't know. It's not an endless repetition of stuff that we do know. We owe that to the taxpayer who funds us. $\endgroup$ Commented May 20, 2023 at 19:49
  • 2
    $\begingroup$ For such a big result, experimental verification seems like a good idea! The failure of the thought experiment to happen in practice can lead to new results in themselves, for example. $\endgroup$
    – apg
    Commented Dec 23, 2023 at 20:42
  • 1
    $\begingroup$ It almost seems like a conspiracy that noone ever filmed this famous experiment, while at the same time we have hundreds of thousands of the double-slit experiment adaptations. Good to see that I'm not the only one confused here. This would be one of the greatest resources for introducing newbies to the uncertainity in the world of quantum physics. The description of the experiment is so mind-boggling that it is one of not very few things that I remember from high school. It would be so live-changing to be able to see it then, even on film. $\endgroup$
    – jannis
    Commented Feb 10 at 11:56
  • 1
    $\begingroup$ And you know @FlatterMann "seeing is believing". When I first learned about the uncertainty and wave-particle dualism it was like "meh, this cannot be true, just some random theoretical stuff they are telling us so that they can later do an exam about it". Having this on film would have great educational consequences. IMO putting such content in the Internet would give the author eternal fame (not mentioning likes/views and, therefore, money). And when it comes to cost Mr Beast puts millions of dollars in his movies, maybe he'd get interested:) Anyway I would surely pay to see it. $\endgroup$
    – jannis
    Commented Feb 10 at 12:03
2
$\begingroup$

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 returns. But if the smallest thing you have is a rock, you can throw it and do the same process knowing the rock's 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.

$\endgroup$
13
  • 3
    $\begingroup$ 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! $\endgroup$
    – USER249
    Commented Mar 23, 2019 at 1:47
  • 1
    $\begingroup$ 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
    Commented Mar 23, 2019 at 1:59
  • 1
    $\begingroup$ Do you mean there is no detector? $\endgroup$
    – USER249
    Commented Mar 23, 2019 at 2:04
  • 1
    $\begingroup$ 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. $\endgroup$
    – USER249
    Commented Mar 23, 2019 at 2:37
  • 1
    $\begingroup$ @Esther You can start here if you are coming from that cartoon robotic eye detector video youtu.be/yotBpxXiivA $\endgroup$
    – USER249
    Commented Dec 5, 2020 at 13:25

Not the answer you're looking for? Browse other questions tagged or ask your own question.