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A little magic trick with polarized film

  1. Apr 26, 2014 #1
    A little "magic" trick with polarized film

    DO try this at home! I took three pieces of polarized film (which I salvaged from a pair of 3-D glasses). First I put two pieces together (A and B), one rotated at 90 degrees to the other. This effectively blocks out almost all the light. Then, I took a third piece (C), rotated it at 45 degrees, and inserted it BETWEEN the the two pieces, and I could see through all three of them! Here's the weird part - if I insert the third piece (C) IN FRONT OF or BEHIND A and B, no light can get through! Weird or what?

    The third piece un-polarizes the light. Notice even the camera "sees" the distant building and focuses on it instead of the film. In the first shot, the camera cannot "see" through the film, so it focuses on the film itself.

    Polarized Film - erased & unerased.jpg

    The next step was to use these three pieces of polarized film in what's called a "Quantum Eraser" experiment. I've performed this experiment with excellent results. I'll be posting the results (with pictures) soon.
  2. jcsd
  3. Apr 26, 2014 #2
    A great demonstration of quantum weirdness!
  4. Apr 26, 2014 #3


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    Feynman has a great lecture talking about this. Sorry I don't have a link but you can probably find it with Google
  5. Apr 26, 2014 #4


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    Nice experiment fizixfan!

    Don't want to 'wipe out' your enthusiasm, but what looks like "magic" can actually be explained scientifically (as in most cases). The rule for calculating this is called Malus' law and was published in 1809 (so this is a classical phenomenon, which is also valid for single QM photons).

    The rule is cos2(θ) where theta θ is the angle between the light's initial polarization and the axis of the polarizer. Unpolarized light (as sunlight) is a mixture of polarizations at all possible angles, and no matter at what angle the polarizer is set, it will always let 50% of the unpolarized light through.

    Also important to know; the light that do go through a polarizer will be polarized along the axis of the polarizer, i.e. it gets 'twisted' in the same direction.

    Hence, this is what we get in your different setups:

    [A] Unpolarized light --> polarizer 0° = 50% intensity
    [B] Polarized light 0° --> polarizer 90° = cos2(90°) = 0 x 50% = 0% intensity

    [A] Unpolarized light --> polarizer 0° = 50% intensity
    [C] Polarized light 0° --> polarizer 45° = cos2(45°) = 0.5 x 50% = 25% intensity
    [B] Polarized light 45° --> polarizer 90° = cos2(45°) = 0.5 x 25% = 12.5% intensity

    [C] Unpolarized light --> polarizer 45° = 50% intensity
    [A] Polarized light 45° --> polarizer 0° = cos2(45°) = 0.5 x 50% = 25% intensity
    [B] Polarized light 0° --> polarizer 90° = cos2(90°) = 0 x 25% = 0% intensity​

    As you see, it doesn't matter at what angle we set C in the last setup, since there is a complete block between A & B, as they are orthogonal 90° to each other = 0% intensity.

    You can play with this Polarizers Applet to get the numbers directly on your screen.

    Good luck & keep up the good work! :wink:
  6. Apr 26, 2014 #5


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    As far back as 1809. That is interesting. And not necessarily as an explanation for quantum behavior. More interesting.
  7. Apr 27, 2014 #6


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  8. Apr 27, 2014 #7


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    Btw, I remember that we did this experiment with microwaves back in high school using wire gratings as polarisers. It can be explained using ordinary wave mechanics and certainly is not a QM effect.
  9. Apr 27, 2014 #8
    Would classical wave mechanics not just treat the polarisers as filters? Or would this effect be seen with water waves for example?
  10. Apr 27, 2014 #9


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    Nope, it does not work for water waves and sound waves, because they have only one possible polarization (i.e. in the direction in which the wave is travelling).

    Electromagnetic waves can oscillate with more than one orientation, linear polarized light consist of two orthogonal (to the wave's direction of travel) in-phase components.

    Linear polarized electromagnetic wave:
    Red = electric field
    Blue = magnetic field

    I guess one could view this "combined wave feature" as some sort of 'link' to QM superposition, which explain why you get 45° polarized light out of 0° polarized... anyone know?

    Attached Files:

    Last edited by a moderator: Apr 15, 2017
  11. Apr 27, 2014 #10


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    That depends on the medium. In solid media, there are transverse sound waves and these can be polarized.
  12. Apr 27, 2014 #11


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    Ah! Thanks, forgot that.

    Do you know if the superposition principle (of classical waves) could be 'linked' to QM superposition and the Schrödinger equation? Both are linear, right?
  13. Apr 28, 2014 #12
    Feynman's lecture on the Double Slit Experiment can be found here:
    Last edited by a moderator: Sep 25, 2014
  14. Apr 28, 2014 #13


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    I wasn't talking about the double slit experiment, I was talking about his lecture on polarization, which is what's relevant to this thread.
    Last edited by a moderator: Sep 25, 2014
  15. Apr 28, 2014 #14


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  16. Apr 28, 2014 #15


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  17. Apr 28, 2014 #16
    Strictly speaking, you're right. But IMHO, the Double Slit experiment is relevant to this thread, at least to me. It's what started me down this path. The three pieces of polarized film I used in this "magic" trick were also used in a Do-It-Yourself Quantum Eraser experiment in a Scientific American article - just in a different arrangement: http://www.arturekert.org/sandvox/quantum-eraser.pdf [Broken]

    I've performed this experiment myself, and it works! I've got all the pictures, I just have to put it together with explanatory text in a format I can post in another thread.
    Last edited by a moderator: May 6, 2017
  18. Apr 28, 2014 #17


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    That is very cool! :cool:

    But... can I be a "party pooper"...? And question this in the sciam conclusion:

    My 'interpretation' is that this is maybe not 100% correct. Nothing what "the photons did at the wire" changes later on, all information is there all the time, i.e. in a mixture of waves in different phases*, which together cancel out any interference fringes.

    What you really do with the Quantum Eraser is filtering out one or the other phase, to see the interference. The proof for this is obvious in the last step with the "misaligned teeth" – bring the teeth together and interference is gone!

    Anyone disagree? :smile:

    This is a beautiful experiment anyway and I'll love to see your pictures! :thumbs:

    *What happens is that the wavefunction is split at the wire, and each part has the same probability to pass through the V/H polarizer, but they will differ in time/phase.
  19. Apr 29, 2014 #18
    This is the caveat from the SciAm article:

    "We will show you how to set up an experiment that illustrates what is known as quantum erasure. This effect involves one of the oddest features of quantum mechanics--the ability to take actions that change our basic interpretation of what happened in past events.

    "Before we explain what we mean by that and outline the experiment itself, we do have to emphasize one caveat in the interest of truth in advertising. The light patterns that you will see if you conduct the experiment successfully can be accounted for by considering the light to be a classical wave, with no quantum mechanics involved. So in that respect the experiment is a cheat and falls short of fully demonstrating the quantum nature of the effect.

    "Nevertheless, the individual photons that make up the light wave are indeed doing the full quantum dance with all its weirdness intact, although you could only truly prove that by sending the photons through the apparatus and detecting them one at a time. Such a procedure, unfortunately, remains beyond the average home experimenter. Still, by observing the patterns in your experiment and by thinking about what they mean in terms of the individual photons, you can get a firsthand glimpse of the bizarre quantum world."

    So, they are basically claiming that this does demonstrate quantum weirdness. Since I don't have an atom interferometer handy to collapse the interference pattern, the best I could do was follow the instructions in this article, and ponder what the quantum implications are in terms of "measuring" photons and the subsequent collapse, and restoration, of the interference pattern.
  20. Apr 29, 2014 #19


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  21. Apr 29, 2014 #20
    Dr Chinese stated in this thread that "It is a purely classical experiment if the intensity is large."

    I noticed this when I used a green laser pointer (which is much brighter than a red laser pointer of the same make). It appeared to "overwhelm" the which-way information and diffraction patterns still appeared on the wall. With the red laser pointer, the diffraction pattern disappeared when the beam of light passed through the wire with orthogonal polarizers on either side.

    Here are a couple of photos showing what happens with the above setup using a green laser pointer and red laser pointer:
    Last edited: Apr 29, 2014
  22. Apr 29, 2014 #21


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  23. Apr 29, 2014 #22


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    I think you have done a great job that shows 'features' that could be linked to QM, but from DrClaude's link I think it safe to say that in current setup with electromagnetic waves – it can all be explained as a purely classical experiment (due to the high intensity).

    But of course, there are striking similarities between the wavefunction and electromagnetic waves, as they are both wavy in their nature (yet very different in 'substance').

    If we first look at EM waves in from of the light from the laser, this is what happens:
    • The EM waves enter the H/V polarizer and 50% of the light passes through both.

    • The two parts that leave the H/V polarizer will be orthogonal polarized, and from the Fresnel–Arago laws we know that they cannot interfere.

    • We also know that the two H/V parts will spread/propagate in exactly the same way as unpolarized (interference) waves.

    • When we introduce the diagonal "eraser polarizer", 50% of the two parts of the H/V polarized light will pass through, and then both become 45° polarized, i.e. they are now allowed according to the Fresnel–Arago laws to interfere!
    As you see, this is only a classical explanation and no "QM weirdness".

    But if we look at single photons from a "dimed down" laser, this is what happens according[1] to QM:
    • The wavefunction enter the H/V polarizer, and we know that if there's a 50/50 probability for the wavefunction to passes through both – it will pass through both!

    • Afaik[2], there will now be a phase shift between the two H/V parts, which prevents any interference.

    • However, the two H/V parts will spread/propagate in exactly the same way as an unpolarized wavefunction.

    • When we introduce the diagonal "eraser polarizer", there's a 50/50 probability for the two H/V parts to pass through. Sometimes none will pass through, sometimes one and sometimes both. When both parts pass through they will be in the same polarization and phase, hence they will create interference.
    [1]There are of course other interpretations explaining what happens, but I chose the good ol' wave/particle view since this is maybe closest to EM waves.
    [2]Please correct me if I'm wrong!

    Thus, this is a true demonstration of a QM "measurement", subsequent "collapse", and what restoration of the interference pattern means, and the weird part is that the photon 'adjusts' its 'behavior' depending on how we choose to do the measurement.

    I.e. you don't need invoke any retrocausality to explain what's happening in a [delayed choice] quantum eraser, which I believe Wheeler also rejected... i.e. this is my main 'objection' to the sciam conclusion.

    I think this becomes clear in the "Simple Interferometer" beams plitter setup:
    In the upper picture, for every one photon that is injected, there will be one coming out in either of the two different directions, and there's a 50/50 chance for both – hence we look at the photon as a localized 'particle'.

    In the lower picture (with a second beam splitter), the wavy nature of photons becomes dominant, since there is no way to tell which path – hence the delocalized 'wave' takes both paths, and creates destructive or constructive interference, exiting the setup.

    This is what's 'weird' about QM!
    Last edited: Apr 29, 2014
  24. May 1, 2014 #23
    I've posted a new thread https://www.physicsforums.com/showthread.php?p=4734794&posted=1#post4734794 with the pictures I took at each step of the Do-It-Yourself Quantum Eraser experiment, as performed in the Scientific American article from April 14, 2007.

    It's a work in progress, and needs an explanation of what is going on in each photo. I will be editing it soon. But I had to get the pictures up before I lost track of them (they are the best of many hundreds I took). I'm still learning the ropes here, so I hope I'm not breaking the rules by cross-posting or presenting incomplete work.
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