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Novie questions about the double-slit experiment

  1. Nov 27, 2011 #1

    I am not educated in physics, but I would like to gain some understanding on the double-slit experiment. I am suspecting that not only its results are to me hardly understandable (as far as I know, that is the normal case), but also some of the assumptions on which the experiment itself relies.

    There are many obscure points to me, and probably the following will be a stupid question... but I would appreciate someone clarifying the following:

    If I understood what I have been reading, it is alleged that the 'measurement' by the slits changes the behaviour of the travelling electron from a wave-like one to a particle-one. I am talking about the 'measurement' that attempts to determine which slit the electron went through.

    On the other hand, in the experiment there is a detection device, a fluorescent screen or sort of thing, that shows the wave-like behaviour of the electron by displaying the interference patterns.

    So what I am not understanding is: In which way this 'detection' is different in nature from the 'detection' by the slits, so that forces the electron into a 'particle-like' object, but the other one does not?

    After having formulated the question it sounds still more stupid, but it is a price I am eager to pay for some light on the subject... the resources I see in the Internet insist on quasi-mistycally talking about how 'observing' changes the behaviour of the particle, but I do not get the physical sense of this.

    Thanks in advance.

  2. jcsd
  3. Nov 27, 2011 #2


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    1. A "measurement" at the slit allows us to know which slit the electron passes through.

    2. A "measurment" at the screen tell us nothing about which slit the electron passes through, because the screen is after the slits. All we know is that the electron made it through.

  4. Nov 27, 2011 #3
    Thanks for your answers.

    From what I read it seems to be implied (but I am not sure) for the measurement at the slit to cause the trajectories of the electrons to be particle-like, so that the interference patterns do not appear anymore if the electrons are 'observed' to see which slit they went through. That seems not to be the case with the after-split detector, so - does it mean that the slit-by detector is performing before the electron passing through the slit? Most of the layman-level descriptions of the experiment (and that's the level I can read) do not state anything about this.

    But I might still be understanding wrongly your reply.
  5. Nov 27, 2011 #4


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    There's a slight misunderstanding here.

    If you know which slit the electron (or any quantum particle, for that matter) pass through, then the 2-slit system becomes a system with 2 slits. Instead of the 2-slit interference pattern, you will get pattern coming from two individual slits, i.e. the single-slit diffraction pattern from each of the slits. So it isn't quite true that this is "particle-like".

    I have no idea what you mean by " ... That seems not to be the case with the after-split detector, so - does it mean that the slit-by detector is performing before the electron passing through the slit? "

    What is a "slit-by detector", and what is it performing?

  6. Nov 27, 2011 #5
    Ok, I was missing that.

    Let me call "A" the detector at the slit, which tells which slit the electron passes through. And let "B" be the screen onto which electrons impact.

    My point is that, from what I've been reading, the fact of A detecting the electron causes the electron to 'choose' one of the slits. And a single-split difraction pattern to appear at B. If A is not there, then the 2-split interference pattern occurs. Wether we 'detect' or not determines the result.

    I do not understand that fact... but just assuming it is true -that the mere fact of 'detecting' causes one of the paths to be chosen-, so why is that detection by B does not cause it? I mean, B is also detecting, but not 'causing' one path to be chosen.

    Attempting to find an answer to the last question I asked whether A is detecting 'before' the electron passes the slits and B is detecting 'after' it passed.

    I think I need to read more about all this. I will come back when I get some points a bit clearer.
  7. Nov 27, 2011 #6


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    But the detection at "B" is a different beast then the detection at A!

    Just because you make a measurement, doesn't mean that it is the same as another measurement. You are trying to lump all of these measurements to be the same. I've already told you why they are different. So now, you have to tell me why you think they should be the same, or not any different than the other.

    In QM, the order of operations, i.e. the sequence of events, makes a lot of difference.

  8. Nov 27, 2011 #7
    Well, here and here (from 3:47-4:35) it is said (or at least I underestand it like this) that the act of measuring at A "removed the wave element completely" (I'm quoting literally).

    I wanted to understand in which way exactly the act of measuring affects the behaviour of the electron as a wave or as a particle, and why measureing at A forces the electron to choose a slit.

    So I said to myself, "let's ask why the measurement at A changes such behaviour and the measurement at B does not. That would be a beginning in understanding what in the end these measurements do".

    Now, in the wikipedia article "Doble-slit experiment" I read this:

    And that seems to me to contradict my two other sources above, whilst sounding much less mystically. And it changes my view of the experimet.
  9. Nov 27, 2011 #8


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    You could look at it this way.
    Two detectors placed, one at each slit would give equal values of total count as the electrons arrive from the source but the 'clicks' as each detector went off would have a random pattern to them - they wouldn't be alternate, for example but in bursts through one then through the other etc. etc. This would be because the wave function of each electron would have equal values at each slit. There will also be millions and millions of other electrons that just plough straight into the metal plate, with pretty well the same probability for every point on the plate (assuming a uniform beam was falling on the plate). Those electrons have been eliminated from the experiment. So, even at this stage, you can explain what happens in terms of the electron behaving like a wave - it's just that it can only be detected at one point on the plate.

    When you open the two slits and let some SELECTED electrons fall on a distant screen, the probability of on electron hitting a particular part of that screen is now given by the effect of what is now two waves, emanating from what is now two sources - the slits. This probability distribution follows the same pattern as the interference pattern of the two waves.

    Furthermore, if you take the plate away, there will be a uniform distribution on the screen because ALL the electrons are allowed to get there and you now have a simple wave function for the arriving electrons.

    There is only an apparent paradox when one insists that the electrons hitting the detectors at the slits have to be viewed as 'particles'. If, at that stage, you are prepared to accept that they may be behaving as waves, then what follows is no surprise.
  10. Nov 27, 2011 #9
    I'd like to add to that, but I'm a bit hesitant because I've been learning physics from all over the place and not all sources are accurate.
    But if I'd understood properly, if one would send the particles through the slits one at a time, with no other particles whatsoever to interfere with, the interference pattern still emerges.

    In my opinion that's still quite a big surprise...
  11. Nov 27, 2011 #10


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    I was wondering whether I'd get a reply like this. If you had two separate electron guns, firing, independently into two slits, all you would get would be two piles of electrons - one for each slit. The wave functions of electrons hitting each slit would not include a finite value (probability) at the other slit.
    There has to be a finite probability of an electron passing through each slit for the interference to apply.
    Electrons don't 'interfere with each other' - which is what you are implying. Each electron (wave) only 'interferes with itself' (clearly they are all teenage boys haha)
    Just one electron, with two slits to go through could appear anywhere on the screen. One electron doesn't consist of a pattern and you need to look at many electrons before the pattern emerges. But that one electron is just MORE likely to turn up at or near one of the peaks of the interference pattern.

    Incidentally, for this to work properly and for a coherent interference pattern to emerge, the beam of electrons needs to have a fairly well defined velocity / momentum so that their de Broglie wavelengths (λ=h/mv) are all nearly the same.
  12. Nov 27, 2011 #11


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    Welcome to PhysicsForums, fimaun!

    ZapperZ is steering you straight, but I would like to add a few comments.

    Once you know the particles goes through A, you eliminate the potential contribution from B. So in the restricted context of that statement, the wave element is removed. But quantum language is notoriously difficult to translate into English (or any) words, for exactly situations such as this. So perhaps this will help (although you may not be familiar with this particular type of apparatus). If I place polarizers at slit A and slit B, I can use their relative orientations to either: i) give me knowledge of the slit being traversed (if perpendicular); ii) or no knowledge of the slit being traversed (parallel). In either case, a wave emerges from the slits because we did not obtain position information.

    The point being that if there are interference contributions from both A and B, you see that in the resulting pattern. I wouldn't get hung up on the words wave or particle here except as a simple way to picture in your mind. They usually use those words because people understand that a water wave goes through both slits, and bullets (in the particle analogy) go through one or the other but not both.

    I hope this helps. :smile:
  13. Nov 27, 2011 #12


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    The statement "removed the wave element completely" is not a helpful one. If a TV is turned on and you choose to close your eyes, you will hear it or, if you block your ears, you will see it. By your choice of observation method you have not changed the 'true nature' of the TV or 'removed the sound element' or 'removed the vision element'. Sound and Vision are still there.

    By putting the plate in the way of the original beam of electrons, you have altered the wave function, by having one or two open slits in the plate, you have further altered it. The diffraction pattern due to what is placed in the way can always be looked upon as a diffraction pattern. It's just that some diffraction patterns 'look' a lot like the pattern you would get with a stream of particles and that's how many people choose to treat them.
  14. Nov 28, 2011 #13
    Thank you very much everyone for the answers. I will be answering as I get more understanding. As for now, I am seeing individual facts, but still I cannot see the sense of all this :). I need some time to reflect about this and to read a bit more...
  15. Nov 28, 2011 #14
    Detection at detector B does not determine which slit the particle came from. That's why it does not cause the particle to go through one slit or the other. Only detector A (the one by the slit) does this. It's just the way nature is. If the system were to remain in a superposition, don't expect the world to exist as you see it.
  16. Nov 28, 2011 #15

    I think you would get a better handle on this subject by listening to (audiobook) Manjit Kumar's "Quantum, Einstein, Bohr, and the great debate about the nature of reality" Which is well explained (and read) for the lay person such as yourself.

  17. Nov 28, 2011 #16
    You could also consider reading 'Sneaking a Look at God's Cards' by Giancarlo Ghirardi (the GRW theory creator), or 'Quantum Reality: Theory and Philosophy' by Jonathan Allday - which goes into basic quantum mathematics.
  18. Nov 28, 2011 #17
    With a title like that how can I resist.
  19. Nov 29, 2011 #18


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    Stereotypes. I'm afraid. It could be you or me, I guess.
  20. Nov 29, 2011 #19
    Ok, fair enough about a photon/electron/whatever being 'rather' a particle or 'rather' a wave, or 'basically' a wave. I think I understand that better after your helpful explanations, especially from sophiecentaur and Dr Chinese.

    However, my main problem was about the operation of 'measurement' and how it determines whether there will be an interference pattern or not.

    Be it an illusion or not, I have the impression that I got to understand it quite better thanks to the Wikipedia article on the Wheeler's Delayed Choice Experiment.

    Please correct me if the following is wrong, but this was my conclusion:

    1) The interference pattern caused by several 'paths' or 'possibilities' for a photon/electron/whatever to take or follow appears when the measurement is able to register ALL those paths/possibilites. Put is simply - when it's on the way of each one of the trajectories. Just as the metal screen in the double-slit experiment or the screen in Wheeler's experiment.
    2) On the other hand, the interference pattern disappears (in other words: the 'particle-like' behaviour is caused) when the detection method only can detect one of the paths/possibilities. Just as each one of the detectors by the slits in the double-slit experiment or each one of the telescopes in Wheeler's experiment.

    That might be utterly wrong. Or that might be right but obvious to you and I failed to notice it but now.
  21. Nov 29, 2011 #20
    "Photon passes through double slit unobserved, logically either through one, through the other, or through both."

    or neither any of the above three possibilites.
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