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Simple double slit question

  1. Jan 21, 2009 #1
    I have a very basic question about the double slit experiment. Some websites that I have seen state that the simple act of measuring which slit the particle goes through, is enough to collapse the wave function, and it isn't necessary for anyone to actually observe the results of that measurement. Other websites seem to imply that the wave function doesn't collapse until the results of the measurement are actually observed.

    I was wondering which of these is true. Is the act of measuring alone, sufficient to collapse the wave function, or is observation necessary?

    Are there any websites that give a good basic outline of the two slit experiment, and any subsequent experiments that attempt to better understand what is actually happening?
  2. jcsd
  3. Jan 22, 2009 #2
    I didn't think that this question would be so difficult. I figured that it would take five minutes for someone to answer. Maybe the question wasn't phrased correctly. Let me try again.

    If I set up a double slit experiment, and I have detectors set up to determine which slit the particle passes through, but I don't have any way to actually see the results of that measurement, will the probability wave still collapse? Is it the act of measuring, that collapses the wave, or the act of observing?

    I have a lot of questions about the double slit experiment, but I wanted to clear this up first.

  4. Jan 22, 2009 #3


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    Well, the fact that the detector measured which slit each particle went through ensures that if you look at the screen to see the pattern formed by the particles, you won't see an interference pattern. However, the question of whether the wavefunction "collapsed" on measurement even though you didn't look at the results is trickier, you'll get the same prediction about the pattern on the screen regardless of whether you imagine the wavefunction collapsing on measurement or if you just assume the measuring device became entangled with the particles with no collapse until you look at the screen. In principle you might be able to come up with convoluted scenarios where the measuring device stores the information in a way that keeps it completely isolated from the outside world (avoiding environmental decoherence) and then later the information is erased, in which case you might find interference in the subsets of photons associated with particular measured states for the detector once the which-path information has already been erased, similar to the delayed choice quantum eraser; in this case you would probably have to assume the detector just became entangled with the photon on measurement rather than an actual collapse happening (though I'm certainly not confident that's correct, I'm just speculating based on the analogy with the delayed choice quantum eraser here). But in any practical experiment involving a detector I think you'll get all the same predictions about actual observed experimental results regardless of whether you assume detection involves wavefunction collapse or just entanglement.
  5. Jan 22, 2009 #4
    Okay, that answers the question. In fact it's probably a little more information than I needed. I'm easily confused. But I think that I understand the answer.


    Actually, now that I think about it, maybe I'm only slightly clearer then I was before.

    Just to be sure, this is experimentally proven, right?

    What's to keep the detector from being in both states at once? In which case the particle can still go through both slits. Yup, now I'm sure, I'm still confused.

    Or is it impossible to prove either way, since by looking at the resulting interference pattern, I force the detector to be in one state or the other, which in turn forces the particle to chose one slit or the other. Okay, now I'm confusing myself.

    So now I'm back to the original question. Did the detector collapse the particle's wave function, or did my observing the interference pattern collapse the detector's wave function, which then in turn collapsed the particle's wave function? I'm worse off than I was at the beginning and I still don't know if it's the measuring that collapses the wave function, or the observing.

    This quantum stuff is tough.

    But I think that I've got enough info to form a basis for further thought.

    Not that it will help.
    Last edited: Jan 22, 2009
  6. Jan 22, 2009 #5


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    It's not practically possible to keep a macroscopic detector sufficiently isolated from the environment that it can be treated as being in a giant superposition of states until we examine it (this is the issue of environmental decoherence), but you can have analogous experiments where the photon that goes through the slits is entangled with another "idler" photon such that, if the idler is measured in a certain way, that will tell you which slit its entangled twin went through. In this case, even if you don't actually measure the idlers until after their twins are observed on the screen (i.e. when calculating their behavior you don't assume a 'collapse of the wavefunction' until the photons are observed on the screen), you'll still find that there is no interference pattern in the total collection of photons on the screen.
    Substituting the idler photon for the detector, the combination of the original photon and the idler are in a superposition of states which involve traveling through both slits at once, but it's a different superposition of states than you'd have for a single unentangled photon, and one of the differences is that the probability distribution for the photon to end up at different points on the screen does not show interference in the entangled case (though if the idler is measured at a detector that 'erases' its which-path information, then if you look at the subset of photons on the screen whose corresponding idlers went to a particular which-path erasing detector, then you will see interference in this subset even though there was no interference in the total pattern--again, I recommend reading up on the delayed choice quantum eraser experiment). I remember a few threads where people discussed why entangled photons don't show interference in their total pattern on the screen, like Cthugha's post #40 on this thread or vanesch's posts #49 and 52 on this one. Presumably all this stuff would still be true in the case of an entangled state consisting of all the particles in a detector which has been isolated from its environment and the particle it measured going through one of the slits.
  7. Jan 22, 2009 #6
    Thanks, that's given me a much clearer understanding of the methods involved in detecting the particle. Now I've got just enough information to look smart, to dumb people, and dumb, to smart people. All that I need now is the good sense to know when I'm supposed to be the smart one, and when I'm really the dumb one, like now.
  8. Jan 23, 2009 #7
    After studying the quantum eraser experiment, and the delayed choice quantum eraser, it seems as though there is one basic principle involved, and that is, that so long as the information about which path the photon took is stored somewhere, then the interference pattern will disappear. If that information is permanently destroyed, then the interference pattern is present.

    On a very basic level, am I understanding this correctly?
  9. Jan 23, 2009 #8


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    That's right, the only subtlety is that if the information was once accessible viable an entangled system but the information is later destroyed, interference will only be visible when you look at subsets particles going through the slit which are matched with particular "erased" states of the entangled system (in the delayed choice quantum eraser, you can look at the subset of signal photons whose idlers were detected at which-path erasing detector D1, or the subset whose idlers where detected at which-path erasing detector D2), the total pattern of particles on the screen behind the slits won't show any interference.
  10. Jan 26, 2009 #9
    An important point to make here is that the measurement must effect the electron/photon in some way in order to get any details about which slit it went through. So it's not so mysterious after all.

    It is like throwing a rock at someone to make sure they are really there and then wondering why they either run away from you or at you instead of continuing on their normal path.
  11. Jan 30, 2009 #10
    Is there anyone out there who is trying to reproduce Young's double slit experiment by scanning the interference pattern with a photon detector and then determining the total flux on the pattern to calculate photon spacing?

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