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Thread: Classical explanation of double slit polarization experiment

  1. #1 Classical explanation of double slit polarization experiment 
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    I have a disagreement with a Quantum mechanical scientist about a double slit experiment with polarizers, which gives interference or not depending on "which path" knowledge of the photon. That is alright with me, but I can calculate the same results with classical wave formula. He does not agree with me, but also does not seem to be familiar working with classical waves calculations. Is there someone here who is? Who can check if my calculations and conclusions are right?


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  3. #2  
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    Yeah nice. Do it now classicaly with electrons.


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  4. #3  
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    Do you mean it serious? It is about photons, not electrons.
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  5. #4  
    Bullshit Intolerant PhDemon's Avatar
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    But it is not only photons which show interference in a double slit experiment, electrons, buckballs and large (10,000 amu) molecules have also been shown to show interference. QM explains this, if you can't extend your classical wave formula to do the same I'll stick with the QM explanation as it can explain more than your version, and do so accurately.
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  6. #5  
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    Then stick to QM, a perfect choice.
    But for classical I need a serious interrested classical expert
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  7. #6  
    Brassica oleracea Strange's Avatar
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    In the classical case, light passes through both slits. If there are polarisers on both slits then there will only be interference when both slits are polarised with the same orientation.

    Is that what you mean? Or, if not, do you want present your argument?

    Edit: It is more accurate to say that there will be no interference when the polarisations are perpendicular.
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  8. #7  
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    I present the question to a classical scientist. I had a lot of discussions with QM scientist, with always an aversion against anything which sounds like classical, certainly when it looks different then OM, and finish with QM is much better. Well, they are quite right. Keep to that choice and donīt spoil your frame of mind. I have heard all arguments and there is no reason to repeat them here again.
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  9. #8  
    Brassica oleracea Strange's Avatar
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    So you had a disagreement with someone about something but you don't want to say what it was.

    What is the point of this thread, then?
    ei incumbit probatio qui dicit, non qui negat
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  10. #9  
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    So you demonstrated that light waves behave like waves? That is to be expected from waves and the double slit experiment verifies it. Your problem appears to be with the particuate nature of light. The double slit experimant does not address that. You are doing the wrong experiment. I think the experiment you want is one demonstrating the photo electric effect. I searched for a simple experiment that demonstrated this, below is the best I found.
    Elizabeth answered 2 years ago
    I take a light bulb and a camera and take a photo of the bulb from a few metres away. I set up the camera so that the image of the bulb entirely fills the camera's sensor.

    I now make the light bulb dimmer. My camera should still detect light across the entire sensor because, if light is a wave, that light will spread out in the space between the bulb and my camera. All that should happen is that the brightness on my camera image will be lower.

    I now make the light bulb even dimmer. I now don't see light across the entire sensor, but speckles. This now proves that light is a particle!

    If light is a particle, and I make the light bulb so dim that it only emits one particle or photon at a time, then I should see random speckles in my image. This is what we see when we do this type of experiment.

    If light is a wave then, even at very very low intensities, the wave should spread out and fill the entire image equally. This isn't what we observe at low intensities of light!

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  11. #10  
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    Quote Originally Posted by Strange View Post
    What is the point of this thread, then?
    Searching a classical scientist
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  12. #11  
    Brassica oleracea Strange's Avatar
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    Quote Originally Posted by DParlevliet View Post
    Quote Originally Posted by Strange View Post
    What is the point of this thread, then?
    Searching a classical scientist
    Many people here have a background in classical physics (you can't learn non-classical physics without it).

    What question did you want to ask this scientist?
    ei incumbit probatio qui dicit, non qui negat
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  13. #12  
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    Yes, but after they learn QM the classical became child's play and is not allowed any more. For a neutral view one need at least accept that according Copenhagen convention the behaviour of a single photon can partly be described and calculated with classical waves, until a certain limit (and not supposing it is a real wave).

    Anyway, it has now been subject to a wiki edit war, so it is not possible to refer to. I will think if there is another way.

    (still remarcable that on a physics forum nobody is really interested in classical waves... Even on another forum in the "classical physics" part. I did not expect that)
    Last edited by DParlevliet; February 6th, 2014 at 05:14 AM. Reason: made error
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  14. #13  
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    Then let me try. But first: for those who are the opinion that classical waves should not be used besides QM, please don't respond here or make another thead. I have heard all arguments, so no need to repeat them here again.

    Then start with a simplified experiment:

    A single photon source sends photons to a double slit. Before each slit is a quarter wave plate Q1 or Q2 who's optical axis are orthogonal to each other, casuing rotating polarization in opposite directions. The results were:

    1. Without Q1/Q2 there was an interference pattern in Ds
    2. With Q1/Q2 there was no interference.
    3. With Q1/Q2 and a polarising filter POL, adjusted on the Q1 fast axis, there was interference.
    4. With Q1/Q2 and a polarising filter POL, adjusted on the Q2 fast axis, there was interference, but 180š shifted with 3.

    The QM explanation is that Q1/Q2 marks the paths, causing the interference pattern to disappear. When placing POL at a suitable angle, the polarization information is erased, so the interference pattern is restored. No problem for me.

    Now the classical explanation, which is giving the same results: quarter wave plates have orthogonal a "fast" and "slow" axis. The slow axis has 90° phase delay to the fast axis (see figure). In 4 the POL forces the polarization to be parallel to the fast axis of Q2 (see figure). Then the output polarisation does not rotate, but is linear, with Q1 having -90° phase shift (slow axis) with Q2. In 3 the same, but Q1 has +90° phase shift with Q2. Therefore 3 and 4 gives interference patterns which are 180° shifted, as has been measured. In 2 the incoming polarization can be resolve in two polarizations on the F and S axis, which each give the same result as 3 en 4. So 2 is the sum of two interference patterns which are shifted 180°.

    So right until now?
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  15. #14  
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    I don`t understand the question. Single photon is harmonic wave. Therefore you will get same results classicaly and QMechanicaly for double slit experiment. Also may I add there is no such thing as classical physicist and quantum physicist. Thats just media simplification. Every scientist is classical where he can be and is quantum where he has to be.

    Considering your latest post bear in mind that single (real) photon can have either righthanded or lefthanded chirality therefore polarizator should not allow such photon to pass, Here is difference between classical and quantum. Classicaly you can write such light just as superposition of lineary polarized light but in real you cannot do the same with single photon.
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  16. #15  
    Brassica oleracea Strange's Avatar
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    Quote Originally Posted by DParlevliet View Post
    I have a disagreement with a Quantum mechanical scientist about a double slit experiment with polarizers, which gives interference or not depending on "which path" knowledge of the photon. That is alright with me, but I can calculate the same results with classical wave formula. He does not agree with me, but also does not seem to be familiar working with classical waves calculations.
    Maybe he is just uncertain because he is a quantum mechanical scientist. (See what I did there?)
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  17. #16  
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    Quote Originally Posted by Strange View Post
    Maybe he is just uncertain because he is a quantum mechanical scientist. (See what I did there?)
    Perhaps, but he is backed up by another quantum mechanical scientist. Afterwards I must redraw the "not familiar". But still I could make a mistake so now the second step:

    The same as above, but now it is the signal photon from a BBO and the POL is in the idler, forcing also the signal photon to a certain polarization. There is of course a polarization difference between idler and signal, but POL is set so the polarizations are the same as in my first example. Again QM and wave results are the same (of course).

    Does this addition change my earlier explanation in #13? I don't think so.
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  18. #17  
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    Now the full (Walborn) experiment:


    Detector Dp completes the full quantum eraser or, when placed at a longer distance the Ds, a delayed quantum eraser.
    I don't think that will change the classical explanation, because absorbing the idler in Dp will not influence the wave of the signal (anyway, Walborn does not mention that).
    My QM friend mostly objects the consequence: the classical explanation is not influenced by detection in Dp, so does not depend on delayed erasing. This undermines Walborn's QM conclusion. One can say that this prooves that classical is used here outside it boundaries, but predicting the results right weakens this statement.

    So is the classical explantion wrong, or is there an explanation which reconciles both conclusions without just deleting the classical explanation?
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  19. #18  
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    No answers? What does that mean? Do you agree or do you all think that classical waves are now allowed to use here?
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  20. #19  
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    You should never take no answer to mean anything.
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