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I now know what slit the photon came out of!

  1. Dec 6, 2009 #1
    Correct me if I'm wrong, but isn't it a fact of quantum life that it's not possible to obtain a diffraction pattern from the double-slit, while at the same time knowing what photon emerged from what slit? And if I am not wrong, wouldn't it be a simple matter to conduct such an experiment based on Compton Scattering of certain X-rays to prove I'm not wrong?

    Consider a partition (opaque barrier) with the famous double slits etched into it. Then consider a three-dimensional medium of which the Compton X-rays fall upon after they come out of one slit or the other. This medium is a block of material (silver halide crystal substrate? Corning's PhotoGray silver halide glass blanks?) that will convert silver halide ---> silver under the irradiation of the X-ray. As the X-ray interacts with the silver halide molecule in the glass blank(?), it converts the halide and leaves behind a black streak of silver that can represent the direction the X-ray photon took. After the bombardment of the silver halide glass blank with the X-rays due to Compton Scattering, we should end up with many black streaks; half of which are lined up and pointing toward one slit, and the other half of which are lined up and pointing toward the other slit.
    The X-rays, now at a somewhat lower frequency due to Compton Scattering, will land on a detector screen where they will form the familiar diffraction pattern.

    Thus we have it: a three-dimentional medium that displays the photon paths in the form of unidimensional black streaks; each streak of which points to the slit each photon came from (contrary to QM); and, directly behind this medium, a screen displaying its diffraction pattern.

    Question: why wouldn't it work?
     
    Last edited: Dec 6, 2009
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  3. Dec 6, 2009 #2

    Matterwave

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    I suspect this wouldn't "work" in the same way that shining a light in a regular 2-slit experiment with electrons wouldn't work. There, the light acts in the same manner as the silver-halide molecules do in your experiment (bathing the electrons in a seal of photons). In general, for the double slit experiment to work you need to evacuate the chamber of all particles that could interact with the things you want to diffract. This is why it's so hard to do the 2-slit experiment with things larger than an electron (although they have been able to get it with something as large as like a carbon bucky ball or something). A large object would tend to interact with the environment, and that would screw up your diffraction pattern.

    I'm not so sure what about bragg diffraction however...
     
  4. Dec 6, 2009 #3

    DrChinese

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    You already know the answer. There will be no diffraction pattern to the extent you know which slit.
     
  5. Dec 6, 2009 #4
    Sounds like a variation of http://en.wikipedia.org/wiki/Afshar_experiment" [Broken].

    If there's a distance (a large gap) between the slits and film, then I'd expect the pattern to exhibit in the first layer of film, but I wouldn't interpret trajectories in the subsequent film-bulk to be conveying any true information about which slit (or direction) each particle originally really came from.

    But if you meant to do it with no gap, then (like all other methods of tagging/decohering/entangling/measuring) no, I'd not expect the interference pattern (in the final film-layer nor a distance afterward).
     
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  6. Dec 6, 2009 #5
    Consider an ordinary x-ray photograph. Wouldn't you agree that the x-ray travels through matter with a very high degree of independence? That the x-ray remain unperturbed in spite of its proximity to the molecular system (eg, skeletal system) it's photographing? How else does the X-ray picture display such astounding resolution, if not for the fact that the x-rays make it to the film without interference or disturbance of any kind from the surrounding molecular system?

    Of course the X-ray image is nothing more than the image of regions where some x-rays made it to the film (resulting in bright areas of the skeletal system), and other x-rays were absorbed by the bone, resulting in a dark region on the film. However, Compton Scattering allows us the luxury of having a single photon perform multiple functions. In the diffraction apparatus I illustrated, those 'multiple functions' would be:
    1. The creation of vector-like black streaks in the silver halide medium for determining which slit the x-ray came from. And,
    2. The landing on the detector screen at a longer wavelength unperturbed, for the illustration of the double-slit interference pattern.
     
    Last edited: Dec 6, 2009
  7. Dec 6, 2009 #6

    Matterwave

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    when your photons compton scatter, it's interacting with the environment. It's wave-function collapses (or something more complicated, w/e interpretation you want) and there will be no diffraction pattern. As far I as I know anyways.

    It's the same as when an electron interacts with a photon in the regular 2-slit experiment. No diffraction pattern occurs.
     
  8. Dec 6, 2009 #7
    The lenses shown below are 4.5mm thick. the glass is effectively a silver halide-doped crown glass sliced from an enormous glass blank that's about 3cm x 6cm x 120 cm.
    If you were to send a 1mm-wide laser beam of x-rays through it (just pretend), you'd end up with a 1mm black spot on the glass surface, with the entire axial region of the spot in the glass completely black:

    http://www.allaboutvision.com/art117/transitions-lenses-b-250x81.jpg [Broken]

    Likewise, if you were to send a single x-ray photon of Compton wavelength through the glass, you'd see a single, almost unidimensional streak. Coming out of the other end of the glass would, of course, be the same x-ray photon, just at a lower frequency. And since the Compton x-ray photon acts in a classical manner with the silver halide glass, it emerges from the glass in the same direction it entered the glass. Because of this classical activity, we should see an interference pattern on our screen.
    Here's the classical equation that describes this classical phonomena:

    compeq8.gif

    It's this streak that will point to the direction that the x-ray photon took as it went through the glass blank.

    More silver halide-doped glass blanks:

    http://img.tradekey.com/images/uploadedimages/offers/2/7/B3620506-20091112113927.jpg [Broken]
     
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  9. Dec 6, 2009 #8
    I've been considering my presentation as little more than a gendankenexperiment. However, due to the seeming classical behavior of Compton Scattering in transparent solids (with each action, there's the equal and opposite reaction; both of which can be determined specifically), I can't see quite clearly why we won't get a diffraction pattern; at least in deduction anyways.
     
  10. Dec 6, 2009 #9

    Matterwave

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    I don't get what your point is. Can you clear this up by saying something along the lines of "I think the X-ray will still diffract because...."?
     
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  11. Dec 6, 2009 #10
    "I think the X-ray will still diffract because compton scattering is a purely classical phenomena that does not involve the superposition of photons, nor is the phonomena bound by the HUP."
     
  12. Dec 6, 2009 #11

    Matterwave

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    Compton scattering may be understood in a classical sense (with relativity of course); however, the interaction between a photon and an electron is certainly NOT classical! Indeed, the full description of the phenomenon should require Q.E.D. It's just that, we can model it using a classical model is all.

    All phenomena are bound by the HUP, it is a fundamental property of our universe. For example, even a ball rolling down a hill is bound by the HUP. It's just that the limits of measurement we can carry out on the ball is many orders of magnitude less accurate than the boundaries of the HUP for that ball.
     
  13. Dec 6, 2009 #12
    Consider the following figure (fig. 1):

    compton.gif

    If you look at the recoil electron and its associated equation, it seems as if uncertainty isn't playing a role here. As you can see from the equation, the scattering angle is easy to obtain, and nowhere can the HUP be calculated from it. It looks purely classical.


    compdat.gif

    Notice the frequency shift. This shift resulted from the photon giving up momentum, and transferring that momentum to the carbon electrons. The greater the momentum transfer, the greater is the presumed angle between carbon and x-ray, as you can see in the illustration.
    Replace the source of carbon in the illustration with the silver halide glass blank as previously discussed, only this time place the double slit right before the glass blank, and directly in front of the x-ray source. Rather than the x-ray's transfer of momentum to the electron in the illustration above (resulting in a lower frequency for that x-ray and a consequent angle of deflection), we now have a transfer of momentum to the silver halide molecule, resulting in:
    1. Silver halide converting to a visible streak of silver metal. And,
    2. A lowering of the frequency of the x-ray, which will then go on to help create a diffraction pattern on a detector screen.
    You have to understand the importance of using the same x-rays Compton used in his experiment. We can't, for example, use UV rays, as UV rays won't exhibit this frequency shift that's so essential for the generation of the diffraction pattern on the detector screen.
     

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    Last edited: Dec 6, 2009
  14. Dec 6, 2009 #13
    Dr. Chinese, matterwave, et. al: Am I making sense, or is this nothing but crackpottery?
     
  15. Dec 6, 2009 #14

    Matterwave

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    I'm not entirely following your argument.

    Perhaps you are talking about X-ray diffraction through a crystalline structure (Bragg diffraction). This is a documented phenomenon, but now instead of slits you have the atomic spacing acting as your "slits". In this case it doesn't matter if you know which of the 2 slits the X-ray traveled through, you get a diffraction pattern because the X-rays are traveling and diffracting off the crystalline structure.

    You can use this to determine the atomic spacing between atoms in a crystal. This is not really a 2-slit experiment though.
     
  16. Dec 6, 2009 #15
    I know about Bragg diffraction. BTW, I added some stuff to my last post while you were typing this post I am now replying to. Specifically, the argument that complimentarity doesn't play a priori role in this gendankenexperiment of mine. You might want to go back and have a look; I'd like your opinion.
     
  17. Dec 6, 2009 #16

    Matterwave

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    I would say, again, that the "certainness" of your model is only an illusion. In the semi-classical model of Compton scattering, the electron is there, and it gets scattered through some definite angle, etc; however, in the real world the electron is NOT just there, waiting for the photon to hit it. It's MUCH more complicated than that.

    The electron, even before the scattering incident, has no definite position/momentum. It certainly does obey the HUP. The interaction between the photon and the electron is not a "collision" in the classical sense, since each "particle" is also a wavefunction which interacts with each other. The theory of this is Quantum Electro Dynamics.

    Perhaps an analogy is simpler to understand.

    In the Bohr model of the atom, it would appear that the electrons go in circles around the proton and that's that. However, this is NOT the correct or complete picture of what's happening.

    You can say all you want "According to the Bohr atom, the electron has a definite orbit!". In real life, the electron does not! So what I'm saying is, your picture of Compton scattering is like the Bohr atom. It's very useful for some applications, but it's not the whole picture.
     
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