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Help make sense of this polarization explanation by Cal Tech!

  1. Jan 22, 2013 #1
    This vid was produced by Cal Tech. Pay attention to Dr. David Goodstein's explanation of the polarization of light near the end of the video. Does any of it make sense to you, or are Cal Tech Professors showing their fallible sides?

    Last edited by a moderator: Sep 25, 2014
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  3. Jan 22, 2013 #2


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    Would you please quote the sentences you do not agree with?

  4. Jan 23, 2013 #3
    Near the end where Dr. Goodstein says, "Listen guys, you thought you were up & down, but that aint true. Everybody has to be oblique this way or that way... And you thought you were up & down because half of you are this way or that way. And he lets through those that are polarized this way...."

    It's not that I don't disagree with it; it just sounds like nonsense to me, like Dr. Goodstein doesn't really know what he's talking about.
  5. Jan 23, 2013 #4
    Most of quantum mechanics sounds like non-sense. The odd thing is that the math which describes that non-sense makes accurate predictions of experimental results. If you can come up with a theory that makes sense AND makes accurate predictions I'd be willing to bet you'd be getting a Nobel prize.

    What it boils down to is that your brain evolved/or was designed to deal with physics within a certain range. Understanding quantum mechanics (or relativity) would not have helped your ancestor kill a buffalo to eat. Understanding how a spear flies would have. Therefore your brain is equipped to model the flight of a spear but not wave/particle duality.
  6. Jan 23, 2013 #5


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    Not only that, arguing against something simply because it doesn't make sense to you is not a physics argument. "Making sense" is nothing more than an accumulation of knowledge that you've acquired. It also means that something that appears to not makes sense could easily mean that you haven't learned enough about that thing. I can come up with many examples of things that do not make sense in the beginning, but will make perfect sense after I've explained what went on.

  7. Jan 23, 2013 #6
    I noticed your other post on polarization. How comfortable are you with vectors? The thing with this stuff is, as others have said, it doesn't make sense IF sense is defined by what you normally see. However, if you accept that the QM description of light is correct, a few fairly simple rules describe this seemingly silly situation. You're right, it doesn't "make sense" but that's because our idea of "making sense" is wrong.

    so, how much do you know about vectors and the postulates of qm? Have you checked out Leonard Susskind's videos on QM. I haven't actually watched Dr. Goodstein's video, but I think that Susskind's video uses the same example.
  8. Jan 23, 2013 #7
    Hey guys, what troubles me with people (Dr. Goodstein, for example) trying to describe quantum phenomena is that in their description, photons can 'think'. That electrons can 'behave'. that, in the case of the quantum eraser experiment, half-wave plates can 'order' (a photon here or there).

    The most esteemed people who teach and write QM theory also attribute human qualities to quantum phenomena ("Listen guys, you thought you were up & down, but that aint true. Everybody has to be oblique this way or that way..."). And yet, you posters accuse me of being obtuse because I'm not familiar with the subtleties of Maxwell's vectors!

    Is it because I really am obtuse? Or is it because, along with me, you guys don't understand the QM action of the polarizers? I suspect it's the latter...

    As far as our brains evolving intuition to comprehend classical activities such as the path of a spear in-flight, all I have to say is this: no human has travelled at near light-speed, yet we comprehend the beauty and simplicity of SR, don't we? How can this be? How can it be that we never move at a rate of speed high enough to our fellow observers (and never have), yet understand SR? How is it that we can visualize the relativistic properties of black-hole gravity without stepping a foot beyond the feeble gravity of our own planet?!

    To the witless argument that says our brains are wired and bound to the conventions of spears in-flight, all I have to say is this: the only people that have such limited range of thought are people who are averse to physics! The rest of us are on this message board, at the University studying, or otherwise proving that the spear in-flight is aging at a slower rate than the tribesman who threw it!

    Despite all I can say, there is no one here who can adequately explain why polarizers act the way they do. Dr. Goodstein--a Cal Tech Professor--can't adequately do it; I'm hoping that someone here can.
  9. Jan 23, 2013 #8
  10. Jan 23, 2013 #9
    Like what part? Bell's inequality violations? Electrons sent thru Stern/Gerlach magnets?
    What? What is so nonsensical about QM?
  11. Jan 24, 2013 #10
    You asked a question that was phrased like you didn't understand what he was trying to say. I'd be willing to bet that he can explain it better and does, but not in entry level classes which this clearly is.

    The reason that putting the polarizer in the middle allows light to pass is that a polarizer is measuring the light. Light passing through the middle polarizer projects the state onto one of the basis states in the rotated frame. The light exiting that polarizer is now in a state which is polarized along this new axis. The fact that the light before the middle polarizer and the light past the middle polarizer are in different states is the reason that the effect happens.

    I'm going to bed. Let me know if that isn't clear enough.

    I'm not sure what you're laughing at. The fact that I'm right or that it was obvious.

    The first, yes definitely. Stern/Gerlach, a little. Sounds like non-sense and IS non-sense are very different.
  12. Jan 24, 2013 #11


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    Yes, it does not sounds a scientific speech but it has sense:)

    You can think that the polarizer is some "observation", "measurement", that makes the particle occupy one of the eigenstates.

    For the first polarizer, the allowed states were up and down, vertical polarization) or left and right (horizontal polarization). Before reaching the polarizer, the photons were in mixed states, neither up and down nor left and right - arbitrary direction of polarization. Think that the polarizer can observe the polarization of the incoming photon, but observation makes the photon jump into one of the allowed states. The polarizer will "observe" an up-and-down photon and a left-and right photon with equal probability. It lets through only up-and down photons.

    The second polarizer is "oblique" with respect the first one. Its direction of polarization can be 45° with the vertical, for example. The allowed states are also oblique, NW-SE and NE-SW. With respect to those states, the up-and-down photons are in mixed states. When the polarizer "observes" them, (they get into contact with the polarizer) they jump into one of the allowed eigenstates. That is meant by the sentence "Listen guys, you thought you were up & down, but that aint true. Everybody has to be oblique this way or that way" and the polarizer lets through only one kind of photons, those in the eigenstate NW-SE.

    Edit: DrewD beat me, my explanation is basically the same as his.

  13. Jan 24, 2013 #12


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    I'm a bit late coming in here but I have to say, his explanation is absolutely fine - as long as you don't want to hang on to an idea that a photon is anything like a little bullet. In fact, I would say that he put it very well - and at a very appropriate level for the class he was teaching.

    We (you?) have already accepted, in particle 'explanations' of Young's Slits, that they don't behave like peas from a peashooter, with a predictable trajectory - what they are observed to do is based totally on the probability of them turning up in a particular place.
    So where's the problem with allowing them to have a random property of polarisation which governs how they are going to interact with a polarisation sensitive detector?

    There is NO room for conventional thinking when you enter QM. When things appear too hard to understand, it is often down to the 'fallibility' of the student and not always that of the teacher.
    Last edited by a moderator: Sep 25, 2014
  14. Jan 25, 2013 #13
    Let me get this straight. The allowed state of the first polarizer is up-and-down. Those photons that are absorbed by the first polarizer are being 'observed' by the polarizer, thus, not up-and-down photons. What about the photons that passed the polarizer? Are they up-and-down photons? We can never know, because they were never observed by the polarizer (how do we know the property of something if we never get to see its property?). The best thing we can say is that the passed photons (the photons that made it through the first polarizer) are in a superposition of up-and-down and side-to-side states. The presumption is that these passed photons are up-and-down photons (because that's the only allowed state of the first polarizer), but we don't know for sure, since the passed photons were never actually observed.
    Is this line of thinking correct?
    Last edited: Jan 25, 2013
  15. Jan 25, 2013 #14


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    You do know the polarization after the first polarizer: it is up-and down. That is the essence of a polarizer: Only the parallel polarized component of the light beam goes through it.

    All photons were observed by the first polarizer, as the photons got into contact with it. The polarizer is a sheet of plastic, the light beam reaching it has almost no reflection, so goes into the sheet. The sheet consist of molecules, the light interacts with them. You can say "photons" instead of light. Te photons interact with the polarizer. You can call this interaction "measurement" or observation.

    During the interaction, the photons get into the polarization states allowed by the polarizer. It is one of the strange postulates of QM, that measurement results are eigenvalues, as the particles occupy one of the eigenstates when get into contact with the apparatus.

    The photons are not individuals, you can not say about a photon leaving the polarizer wich one was it from the incident photons. The photons inside the polarizer are not the same as the photons in free space. They could have any polarization in free state, as the free space was isotropic. All directions were equivalent. No matter what the polarization was. Now, in the polarizer, they are either up-and down or side-to side. And the polarizer absorbs all the side-to side ones, so all photons emerging at the other side of the polarizer are up-and-down.

  16. Jan 26, 2013 #15


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    I have a feeling that the use of the words Eigenvalues and Eigensates doesn't really solve anything or in any way 'prove' that the photons actually exist in any more real sense than the alternative picture of fields and waves. Both models are equally abstract and, if anything, that particle model of a photon is so far away (as it should be) from the little bullet picture as to make it even less approachable for the 'novice' than the field model. To say that electromagnetic energy 'is' one thing or another when it's on its way from A to Bis clearly a nonsense and that should be made plain to anyone who thinks they've 'got it' by explaining things 'simply' in terms of photons. When you get down to it, the Maths is the same in both cases and yields the same answers it's just that people seem to feel that the partical approach, somehow, doesn't need the hard Maths. That's a Big Mistake.
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