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Double Slit Experiment Questions: Detectors

  1. Mar 24, 2010 #1
    Hi Everyone,

    I'm trying to fill in some gaps in my understanding of the double slit experiment. I think I'm most confused about the detectors. Before I start asking questions, I'd like to spell out what I think I understand with a summary:

    --- Summary of DSE ---


    1. source (G1) to sequentially radiate individual quantum particles of one type (photons, electrons, protons, or whatever...) in a random direction.
    2. a barrier (B1) sufficient to prevent transmission of the particles except for two 'slits' which may be openned or closed (S1, S2).
    3. a movable particle detector (D1) and a planar array of particle detectors (P1).*

    *notes on detectors: 1. It is impossible to make a detector which does not disturb the quantum particle (regardless of whether the detector is 'read' or not). P1 may be a detector plate (such as a photographic plate) or an acctual array of particle detectors.


    Place the barrier B1 between the generator G1 and the planar array of detectors P1 such that particles from G1 can reach both slits S1, S2 and any waves originating from S1 and S2 will create an interference pattern on the plane of P1.


    1. Close S1, S2.
    2. Turn G1 on so that it sends quantum particles (photons, electrons, protons, etc) in random directions one at a time.
    3. Open S1 and wait for pattern to emmerge on detctor array P1.
    4. Close S1, Open S2, Resest P1, and wait for pattern to emmerge on detector array P1.
    5. Keep S2 open, Open S1, Reset P1, and wait for pattern to emmerge on detector array P1.
    7. Place the moveable detector D1 in any position such that it measures (interacts with) the quantum particle differently depending on which slit the particle went through, reset P1 and wait for pattern to emmerge on P1.


    When only S1 is open, a corresponding diffraction pattern emerges on P1.
    When only S2 is open, a corresponding diffraction pattern emerges on P1.
    When both S1 and S2 are open, an interferrence pattern emerges on P1.
    When both S1 and S2 are open, and a detector is placed to disturb the particle differently depending on which slit it goes through (measure which slit it goes through) only diffraction patterns emerge.

    Where a particle appears depends on obstructions in the intervening space and when the particle is interferred with.

    --- End of Summary ---

    Background Questions

    I think the following numberred questions stem from my own lack of understanding of how the detectors work. I imagine there are two categories of detectors: Interference detectors and absorbtion detectors. I can imagine how interference detectors could detect a particle differently depending which slot the particle went through, but how does an absorption detector detect which slot the particle went through, or can it? Is it true to say that the planar detector P1 interacts with, detects, the particles the same way regardless of which slot they came through? In other words, at P1 we know a particle 'hit', but we don't know from which direction? Can we make the planar array P1 out of interference type detectors and know not only where the particle hit but also which direction it came from, or does the uncertainty principle prevent that from being possible?

    Question #1
    I guess whether there is a diffraction or interference pattern with both slits open depends entirely upon whether any detector interacts with the partcle differently depending on which slit it came through. Is this correct?

    Question #2
    If you move the planar detector P1 closer to the barrier B1 does the pattern shift from interferrence to two sepperate diffraction patterns? If so, at what point, and what does the transition from the interference pattern to two seperate diffraction patterns look like?

    Question #3
    If I use only planar detector P1, and P1 comprises an array of particle detectors P2...Pn, and I begin to give the array curvature such that some points on the array P1 are closer to barrier B1 than others, how does that affect the wave function (detection pattern on P1) different from what i would expect from distorting P1? Will I at some point get a detection pattern that suggests some kind of diffraction/interference hybrid?

    Thanks in advance. I hope this was clear to read and you find the questions interesting and enjoyable to answer! Cheers.

    P.S. This is not a homework question!
    Last edited: Mar 24, 2010
  2. jcsd
  3. Mar 26, 2010 #2
    In response to question one, from my understanding, by placing a detector over a slit you are interacting with the particle. Therefore, in a way you are localizing it, thus treating it as a particle and you get results that are consistant with a particle experiment.
    The second question you are asking is a little bit confusing. If you mean you let the particles go through undetected through the slits you will always get an interferrence pattern because you are not changing how the particles/waves are going through the double slit. If you put a detector over one slit and get the diffraction patter, moving the plate (P1) will also change nothing. The distance between the double slit barrier and P1 has no affect on whether it is a diffraction or interferrence pattern.
    And for number three, the answer above should answer this question. The only thing moving the dectector will do is change how many 'fringes' appear on your detector. Also, I think you many be confused on what exactly the wave function is. This just mathmatically describes this wave/particle thing. So once again, if you observe this "quantum mechanical entity" then it becomes a particle and no wave function is need and if you don't observe it then it can be treated as a wave, and then you can use the wave function to describe it's motion. But by changing the distance has no affect on wether or not you should treat this QME as a particle or a wave. It's all in the observing or not observing at the double slit barrier. Hope this helps. It could be wrong but that's what my understanding is.
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