22

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.

Duke William
  • 613
USER249
  • 523
  • 4
    You can use a wall as a detector. – rob Mar 23 '19 at 01:11
  • 3
    @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 – USER249 Mar 23 '19 at 01:21
  • @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. – USER249 Mar 23 '19 at 01:59
  • @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. – Bill Alsept Aug 21 '20 at 15:40
  • @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 – USER249 Aug 24 '20 at 02:05
  • Here is a Veritasium video in which he claims to have a frequency counter combined with a "photomultiplier tube" that counts individual photons? https://youtu.be/GzbKb59my3U?t=145 – slashdottir Aug 18 '22 at 20:29
  • A "detector" is anything that adds or removes energy irreversibly from the system. A black piece of paper will do for the optical double slit. Would you believe that covering up one of two slits leads to the same pattern as a single slit? I would. – FlatterMann May 20 '23 at 18:25
  • @FlatterMann The same pattern with less intensity I guess. But I concluded the experiment as described is hypothetical. It appears the observation of those patterns has more than one explanation other than the probabilistic mathematical conclusions. I also concluded if the mysterious behavior and entanglement is true then faster than light communication is also very possible but asking about faster than light communication possibility is strictly prohibited by the guardians of this forum. – USER249 May 20 '23 at 19:29
  • The explanation of these patterns is exactly the same as it was in 1801, when Young invented the double slit experiment to show that light has wave character. There is no mystery here, other than the question why so many people are looking for a mystery in a 220 year old trivial physics experiment, maybe. That I will never understand. – FlatterMann May 20 '23 at 19:32
  • 1
    @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. – rollsch Aug 07 '23 at 00:11

3 Answers3

11

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.

Duke William
  • 613
rob
  • 89,569
  • 3
    Wouldn't perpendicular ploarizers also explain the lack of an interference pattern because perpendicular waves wouldn't interfere with each other? – JohnFx Jun 28 '20 at 01:49
  • 1
    @JohnFx I thought that's what I wrote? – rob Jun 28 '20 at 06:05
  • When they go through the polarizers, are they in a superposition of both polarities, or just in one of either? – Juan Perez May 20 '23 at 13:50
  • @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. – rob May 20 '23 at 16:20
  • 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. – benrg May 20 '23 at 19:22
  • @benrg Classical electrodynamics predicts the disappearance of these patterns in every case. The only question here is why we don't teach in high school what a "measurement" or a "detection" is? In short, it's an irreversible energy exchange. It is that even in classical physics. The only difference is that in classical physics the consequences for the system under measurement can be ignored and in quantum mechanical settings they can't. – FlatterMann May 20 '23 at 19:46
9

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.

Duke William
  • 613
Caezar 69
  • 91
  • 3
    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... – Cloudyman Dec 03 '21 at 23:13
  • 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. – FlatterMann May 20 '23 at 19:49
  • 1
    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. – apg Dec 23 '23 at 20:42
  • 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. – jannis Feb 10 '24 at 11:56
  • 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.

     – jannis Feb 10 '24 at 12:03
  • @jannis What stops you from performing the experiment at home? A double slit can be easily made from aluminum foil with a pen knife. Shine a light on it in a dark room and you will see the diffraction pattern. Now cover up one of the slits. The diffraction pattern changes to that of a single slit. What did you learn that you didn't know, already? 2-1=1. That's all this teaches. It doesn't tell you anything of value about quantum mechanics. – FlatterMann Mar 04 '24 at 16:06
  • @FlatterMann This post is not about the simple version of the experiment, but the version in which the nature of light changes when observed. – jannis Mar 05 '24 at 18:00
  • @jannis The nature of light is always the same. Light is the excitation of a quantum field. You can detect single quanta of energy and then light behaves like the "clicks" of a Geiger-Mueller counter or you can average over many clicks and then the averages can behave like a classical electromagnetic wave. Do averages of events behave like individual events? No. They don't do that in classical mechanics, either. We simply don't emphasize that when we teach classical mechanics and so many people are surprised when they first discover this phenomenon in quantum mechanics. – FlatterMann Mar 07 '24 at 08:58
2

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.

Duke William
  • 613
Gndk
  • 23
  • 3
    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! – USER249 Mar 23 '19 at 01:47
  • 1
    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 – Gndk Mar 23 '19 at 01:59
  • 1
    Do you mean there is no detector? – USER249 Mar 23 '19 at 02:04
  • It is not passive enough to detect without affecting the outcome. That's why you can't measure (or get to know) both, position and velocity with infinite precision. Those Observables (X and P) are related in a way that measuring one will "destroy" the information of the other observable (same thing for L which I'v heard is what its measure in Q computation). The math is a model to represent a behavior that is very consistent. Which scientist jumped to the assumption that photons are conscious? What do you mean by consciousness? – Gndk Mar 23 '19 at 02:14
  • Here's a related topic with links to videos and papers: https://physics.stackexchange.com/questions/297087/double-slit-detector?rq=1 – Gndk Mar 23 '19 at 02:18
  • 1
    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. – USER249 Mar 23 '19 at 02:37
  • I'm not asking about quantum computing and the collapse of the wave function but i came across this while trying to understand it. – USER249 Mar 23 '19 at 02:42
  • Thank you. What do you think of the last three minutes of this video by Jim alkhalili https://www.youtube.com/watch?v=A9tKncAdlHQ&app=desktop&persist_app=1 – USER249 Mar 23 '19 at 03:29
  • I recommend you to read "The Emperor's New Mind" by Roger Penrose chapter 6, it's really clear about the double slit experiment ;) greetings and good luck! – Gndk Mar 25 '19 at 12:14
  • @AmrBerag : Can you please provide the name of a "scientist or university professor" who believes that atoms are conscious? – WillO Aug 21 '20 at 16:31
  • @WillO Its not easy to find a "scientist or a professor who 'BELIEVES '" but maybe hypothesize or hint or maybe a fancy way of saying "something is missing". Eugene Wigner and Freeman Dyson linked consciousness with quantom mechanics and even Einstein's famous dice quote is hinting to that direction as I understand it – USER249 Aug 24 '20 at 01:25
  • 1
    @Esther You can start here if you are coming from that cartoon robotic eye detector video https://youtu.be/yotBpxXiivA – USER249 Dec 05 '20 at 13:25
  • @AmrBerag Absolutely nothing here is "missing", except for proper education about physics. We don't spend a lot of time on teaching what "measurement" means. Why? Because everybody who has to do them in the lab can figure it out in no time for themselves and it doesn't matter for the remainder of the world. – FlatterMann May 20 '23 at 19:52