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Thread: The Double-Slit Experiment - again

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    The Double-Slit Experiment - again

    From:
    http://en.wikipedia.org/wiki/Double-...nt#cite_ref-20


    when electrons are fired at the target screen in bursts, it is easy to account for the interference pattern that results by assuming that electrons that travel in pairs are interfering with each other because they arrive at the screen at the same time, but when a laboratory apparatus was developed that could reliably fire single electrons at the screen, the emergence of an interference pattern suggested that each electron was interfering with itself; and, therefore, in some sense the electron had to be going through both slits. (my bold)
    Now my question: It should be easy to place two detectors very close behind the slits (too close to allow interference) in a way that each detector “clicks” only when a photon (or electron, etc) travelled through its associated slit.
    What will happen?
    a) both detectors will click simultaneously (the photon travelled through both slits)
    b) only one of them (inpredictably) will click (the photon passed only through one of the slits)
    c) something else will happen (e.g. neither d1 nor d2 will click)
    d) no answer to that silly question
    e) this has been discussed before (please give a link)
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    b) is the answer. If you try and find out which way the electron goes, you lose the interference.

    e) is probably also true

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    b)
    And you won't see an interference pattern.

    By measuring the position of the electron at the slits, you lose the possibility of the interference pattern. Stop looking at the slits, and the interference pattern will appear.

    Grant Hutchison

    Edit: I overlapped with PraedSt.

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    Quote Originally Posted by PraedSt View Post
    b) is the answer. If you try and find out which way the electron goes, you lose the interference.

    e) is probably also true
    I should have said it in my post: Iīm NOT asking whether interference occurs or not. The only question I have is whether itīs possible to experimentally prove that the photon passes one slit, or both, or ...

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    Quote Originally Posted by grant hutchison View Post
    b)
    And you won't see an interference pattern.

    By measuring the position of the electron at the slits, you lose the possibility of the interference pattern. Stop looking at the slits, and the interference pattern will appear.

    Grant Hutchison

    Edit: I overlapped with PraedSt.
    Same comment as to PraedStīs answer: Iīm not asking whether an interference pattern will show up or not (it will not, of course)

  6. #6
    Quote Originally Posted by dhd40 View Post
    Now my question: It should be easy to place two detectors very close behind the slits (too close to allow interference) in a way that each detector “clicks” only when a photon (or electron, etc) travelled through its associated slit.
    If it should be easy, maybe you can propose a design, or cite one, that would satisfy those goals of signaling at the right time and also not interfering.
    0 1 1 0 1 0 0 1 1 0 0 1 0 1 1 0 1 0 0 1 0 1 1 0 0 1 1 0 1 0 0 1 1 0 0 1 0 1 1 0 0 1 1 0 1 0 0 1 0 1 1 0 1 0 0 1 1 0 0 1 0 1 1 0 ...

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    Quote Originally Posted by dhd40 View Post
    The only question I have is whether itīs possible to experimentally prove that the photon passes one slit, or both, or ...
    Yes dhd. You'll prove one or the other, but not both. Does that help?

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    Quote Originally Posted by dhd40 View Post
    The only question I have is whether itīs possible to experimentally prove that the photon passes one slit, or both, or ...
    Yes it is possible, and the test would show that the photon only enters one of the slits.

    At least, that's how I've always understood it.

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    Quote Originally Posted by dhd40 View Post
    I should have said it in my post: Iīm NOT asking whether interference occurs or not. The only question I have is whether itīs possible to experimentally prove that the photon passes one slit, or both, or ...
    You got your answer already.

    If you measure which slit the electron passed through without blocking or impeding either slit, you measure one click for one electron through one slit. As a consequence of what you have done, the interference pattern is lost and the electrons act as single particles, bullet like.

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    Quote Originally Posted by dhd40 View Post
    Same comment as to PraedStīs answer: Iīm not asking whether an interference pattern will show up or not (it will not, of course)
    Well, in that case, you got your answer plus bonus material.

    Grant Hutchison

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    Quote Originally Posted by grant hutchison View Post
    Well, in that case, you got your answer plus bonus material.
    Grant Hutchison
    I thought that's what you always got here at BAUT?

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    Quote Originally Posted by Fazor View Post
    I thought that's what you always got here at BAUT?
    We aim to please it seems..

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    Quote Originally Posted by 01101001 View Post
    If it should be easy, maybe you can propose a design, or cite one, that would satisfy those goals of signaling at the right time and also not interfering.
    Maybe, "easy" is a little bit overdone. On the other hand, a double-slit experiment using a 550 nm - laser would use slit widths of approximately 50 ĩm, and a slit distance of appr. 250 ĩm (donīt ask me where I got these figures from, I found it in my files handwritten). I would be really surprised if it wasnīt possible to move two detectors close enough to eachother so that their distance is <250 ĩm (the detectors themselves could be 1m long, 1 km, or ...)

    BTW, vapour deposition of light-sensitive material could be one option?

    But even if itīs not "easy" to "do" it experimentally, there shouldt be a theoretical QM-answer to my question

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    Quote Originally Posted by PraedSt View Post
    Yes dhd. You'll prove one or the other, but not both. Does that help?
    Not really. Do you say that all options are equally valid?

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    Quote Originally Posted by Fazor View Post
    Yes it is possible, and the test would show that the photon only enters one of the slits.

    At least, that's how I've always understood it.
    Thatīs a clear statement. But itīs not in agreement with Wiki:
    and, therefore, in some sense the electron had to be going through both slits.
    Unless "in some sense" has a different meaning

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    Quote Originally Posted by dhd40 View Post
    But even if itīs not "easy" to "do" it experimentally, there shouldt be a theoretical QM-answer to my question
    There is, and you have gotten it several times now-- the concept of "particle" is applicable here. That concept means that a particle cannot be detected in two places at once, nor can it be detected as having two separate trajectories. Whether or not that means it cannot have two separate trajectories when it is not detected is more a philosophical issue of whether one requires reality to be that which is actually detectable, or that which is merely conceptualizable. Probably your question opens into a whole series of other questions that center on this latter issue.

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    Quote Originally Posted by alainprice View Post
    If you measure which slit the electron passed through without blocking or impeding either slit, you measure one click for one electron through one slit.
    Where do you know this from? Any literature/references about such experiments?

    As a consequence of what you have done, the interference pattern is lost and the electrons act as single particles, bullet like.
    Again, Iīm not questioning this at all. But itīs not my point.

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    Sorry dhd, don't think I've understood you. We've answered your multiple choice in the OP. Is there another question?

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    Quote Originally Posted by Ken G View Post
    There is, and you have gotten it several times now-- the concept of "particle" is applicable here. That concept means that a particle cannot be detected in two places at once, nor can it be detected as having two separate trajectories.
    I fully agree with this. But then it should be so easy (sorry
    01101001) to experimentally prove that the "idea" of a photon (or electron, or fulleren, ..) passing both slits must be wrong. Has this kind of experiment ever been done? Or is the outcome so trivially evident that itīs not worth to give it a try?

    Whether or not that means it cannot have two separate trajectories when it is not detected is more a philosophical issue of whether one requires reality to be that which is actually detectable, or that which is merely conceptualizable.
    True. But honestly, I was mostly interested in knowing whether an experiment as described in my OP has ever been done. Just to see if Wikiīs (and probably othersī) statement

    in some sense the electron had to be going through both slits
    is "real" or "conceptual". To me, an answer to this (your) interesting "real-or-conceptual" aspect is extremely interesting

  20. #20
    Quote Originally Posted by dhd40 View Post
    Thatīs a clear statement. But itīs not in agreement with Wiki:


    Unless "in some sense" has a different meaning
    It indeed does have a different meaning. What the statement expresses is:

    Although you can detect one electron passing through only one slit, if you don't setup a detector then there will be interference patterns which indicate that "in some sense" the electron has passed through both slits and has interfered with itself. This is because the interference pattern has characteristics which are a factor of slit separation. In other words if the electron had not passed through both slits (when you were not detecting) then interference patterns could not have formed (again emphasising "when you are not looking", because when you look/detect, there is no interference).

    I have a followup question if I may:

    *- What would happen if we put detectors (clickers which are not recorded) on each slit and put a timer on the electron firing and leave and lock the room. Come back the next day. The only recording will be of the electron hits and the resulting patterns if any.
    **- Would interference patterns collapse even though there has been no conscious being "observing"?

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    Quote Originally Posted by a1call View Post
    I have a followup question if I may:

    *- What would happen if we put detectors (clickers which are not recorded) on each slit and put a timer on the electron firing and leave and lock the room. Come back the next day. The only recording will be of the electron hits and the resulting patterns if any.
    **- Would interference patterns collapse even though there has been no conscious being "observing"?
    Heh. That's like the: 'does a tree falling in the forest make a noise if there's no-one to hear it?' question.
    Answer's yes to both.

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    Quote Originally Posted by dhd40 View Post
    I fully agree with this. But then it should be so easy (sorry
    01101001) to experimentally prove that the "idea" of a photon (or electron, or fulleren, ..) passing both slits must be wrong. Has this kind of experiment ever been done? Or is the outcome so trivially evident that itīs not worth to give it a try?
    It depends on what you will consider a proof that the photon didn't pass through both slits. It is clear that if you track the trajectory of the photon, it will pass through only one slit. However, if you do that, it will also participate in a different pattern on the wall than if you do not track its path. It is common to interpret that as meaning it only passes through a definite slit if you actually track it. Personally, I never say it passes through "both slits"-- I simply say we know nothing about which slit it passed through, or neither, or both, unless we set up an experiment that can detect it. In other words, a question that is never posed to reality is never answered by reality. Reality is too busy with what is real to be bothered with what is hypothetical.

    Just to see if Wikiīs (and probably othersī) statement is "real" or "conceptual".
    It is purely conceptual, to be sure. Indeed, I believe that is what they meant by "in some sense"-- they meant, in a purely conceptual (even philosophical) sense. It should not be taken too literally, expressly because of the kinds of experiments you are talking about.

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    Quote Originally Posted by a1call View Post
    I have a followup question if I may:

    *- What would happen if we put detectors (clickers which are not recorded) on each slit and put a timer on the electron firing and leave and lock the room. Come back the next day. The only recording will be of the electron hits and the resulting patterns if any.
    You'll get no interference pattern. It doesn't matter if the information is actually recorded, only that the question of which slit was physically posed.
    **- Would interference patterns collapse even though there has been no conscious being "observing"?
    Yes. Of course, a conscious person has to show up the next day, but science is in no position to assess the role of the observer.

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    Quote Originally Posted by PraedSt View Post
    Sorry dhd, don't think I've understood you.
    Donīt worry, thatīs my problem. Itīs sometimes extremely difficult to formulate complex contexts in a non-native language. And I really appreciate the patience of the native-language posters here

    We've answered your multiple choice in the OP.
    Yes, but I was expecting some reasoning/explanations/citations as well

  25. #25
    Quote Originally Posted by PraedSt View Post
    Heh. That's like the: 'does a tree falling in the forest make a noise if there's no-one to hear it?' question.
    Answer's yes to both.
    *- Then, wouldn't that imply that the "wave function collapse" is the result of interaction with the detecting apparatus rather than actual measurement/observance?

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    Quote Originally Posted by a1call View Post
    *- Then, wouldn't that imply that the "wave function collapse" is the result of interaction with the detecting apparatus rather than actual measurement/observance?
    Yes, totally correct. But that's when the observation happens you see; 'observer' doesn't have to mean 'human observer'.

    Edit: Whoops! Pressed post by mistake. Is that what you were asking?

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    Quote Originally Posted by dhd40 View Post
    Yes, but I was expecting some reasoning/explanations/citations as well
    Ah. Ken's is a good explanation. But if you want more, that could be a problem.
    The double slit experiment illustrates the wave-particle duality of light. Sometimes light behaves as if it's a wave, sometimes as if it's a particle. But never both together.
    This is a very old experiment; but till this day, no-one knows how or why light behaves like this, we just know that it does. Frankly, it's a pain in the b***.

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    Quote Originally Posted by a1call View Post
    *- Then, wouldn't that imply that the "wave function collapse" is the result of interaction with the detecting apparatus rather than actual measurement/observance?
    It depends on how we interpret what the "wave function collapse" is. In Bohr's way of looking at it, which is sometimes called the "Copenhagen interpretation" (though that phrase mixes in some extraneous stuff and can get you bogged down in a hurry), the wave function collapse happens as soon as you cross a conceptual divide between the quantum and classical realms. The classicalness of the apparatus "rubs off", if you will, on the quantum wave function, collapsing it.

    However, the problem with this approach is that it does not allow you to treat the classical systems with quantum mechanics, which bothers people who like to imagine quantum mechanics is a fundamental theory. If you include the macroscopic apparatus in the closed system under consideration, then it should not be able to collapse the wavefunction, as it would not obey quantum mechanics to do so. Personally, I just think they are taking quantum mechanics in particular, and science in general, too literally.

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    Quote Originally Posted by PraedSt View Post
    Ah. Ken's is a good explanation. But if you want more, that could be a problem.
    The double slit experiment illustrates the wave-particle duality of light. Sometimes light behaves as if it's a wave, sometimes as if it's a particle. But never both together.
    This is a very old experiment; but till this day, no-one knows how or why light behaves like this, we just know that it does. Frankly, it's a pain in the b***.
    I've made the mistake of saying duality means it can't show both at the same time, but this is wrong.

    Fire single electrons at a screen until the interference pattern develops. Each detection on the screen is a single particle, an electron, but the overall pattern is for a wave.

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    Yes, the way I like to think of the duality is that they are particles that are told where to go by waves. If one takes that picture quite literally, it is called the deBroglie-Bohm interpretation of quantum mechanics. But that requires an extraneous equation that is untestable, and to me unscientific, so I don't view that as something that should be taken literally. Rather, it is a kind of anthropomorphism we use to give us a pedagogical picture of what is happening. Some are dissatisfied with that approach because they want physics to be something more than that, but I'm not really sure why.

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