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I Cognitive Dissonance and Quantum Mechanics?

  1. Feb 20, 2017 #1
    For someone at my level, the following article is extremely informative. I would like to post a link to it. But first here are a few quotes from the article...

    Here is his definition of wave-particle duality....
    And here is his statement on Cognitive Dissonance...
    "Quantum Mechanics and the Fourier Transform" by Frank Rioux
  2. jcsd
  3. Feb 20, 2017 #2


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    "Wave Particle Duality" is no longer considered (for about the last 90 years) as a valid description. Quantum objects are not waves and they are not particles. They are quantum objects. If you measure them for particle-like behavior, that's what you will see and if you measure them for wave-like behavior, that's what you'll see. That does not make them waves and it does not make them particles. They are quantum objects. There IS no "cognitive dissonance".
  4. Feb 20, 2017 #3
    Maybe at your level there is no cognitive dissonance. It does not do any good to pretend it is not there.

    You need to read the article. He is not saying that wave particle duality is a fact of nature. He says that experimenters always detect particles but they predict and interpret the outcome by assuming wave like behavior prior to particle detection.
  5. Feb 20, 2017 #4


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    You can spare yourself a ton of grief and almost limitless confusion by learning quantum mechanics from a real textbook instead of random stuff that you pick up off the internet and/or from pop-sci presentations. This is why Physics Forums has the rule requiring that all references be to standard textbooks or peer-reviewed papers in reputable and well-refereed journals, which this article is not.

    It is true that in one class of problems we can get useful (often very useful, which is why the technique is used) results by assuming wave-like behavior prior to making a position measurement; and for historical reasons making a position measurement is often described as "detecting a particle". However, that's not how the modern (in this context, "modern" means "after about 1930") formulation of quantum mechanics works; no matter what Rioux says, you won't find wave-particle duality and the accompanying cognitive dissonance there. You will find many other strange and confusing things.... but not that one.
  6. Feb 20, 2017 #5
    I have a degree in Physics. It is from some time ago, but I remember, vaguely, the elementary quantum mechanics principles. I have much more experience with Fourier Transforms. I know that momentum and position are Fourier transform pairs. I chose to link to that article because I believe it is a good article for someone who is trying to catch up. The way the author explains it helps to bridge some gaps between the way that most of the members here speak about it and what I was used to in the past. He makes some interesting comments that are easier for me to understand.
    Last edited: Feb 20, 2017
  7. Feb 20, 2017 #6


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    You might benefit from seeing modern take on the formalism that dispels much of the mystery:

    Now that's the formalism. There are issues remaining but I would rather discuss it once you have got the gist of the above. No need to delve into its mathematical detail - just an overview is fine for this purpose.

  8. Feb 21, 2017 #7


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    Anything cognitive, including cognitive dissonance, exists only in ones head. Since you have a degree in physics, perhaps it would help you to see how something like "wave-particle duality" appears even in classical physics:
    Since, I hope, you don't have cognitive dissonances about classical physics, it might help you to relief the dissonance in the quantum case too.
  9. Feb 21, 2017 #8


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    My definition of cognitive dissonance is what results from trying to learn QM from multiple sources at once, including popular science and philosophical approaches.

    If you have a degree in physics and can handle the maths, there is no reason for you not to tackle the subject from a modern undergraduate text, such as Griffiths, which is available in a new edition from Cambridge University Press at a reasonable price.

    This will give you a consistent and coherent basis for understanding the subject and for further study.
  10. Feb 21, 2017 #9
    I wish I did not use the term "cognitive dissonance" in the title to this thread because the articles that I linked to are not focused on that, quite the opposite.

    Within the article there are links to several other articles, one of which is titled "Using Optical Transforms to Teach Quantum Mechanics Here is a link to the article


    Here are a few quotes from that peer reviewed article...

    And one more ...

    From the above statement, the mystery of the double slit experiment goes away...the electrons are placed in a well defined state by the slit and the corresponding momentum distribution is recorded on the detection screen.

    That is a pretty simple and clear explanation.

    Here are some images from the article showing the position and momentum distributions side by side.
    The slit(s) in the screen IS/ARE the prior position distributions...i.e. positions where the electron is allowed to be. The momentum distributions are where the electron is likely to be found after. Figure 7 is amazing.

    https://lh3.googleusercontent.com/mKCTRSPt9cgEnojXbzyDR320K99BckxSyEPqKyDWrd6ZKgyxvM0-HFSAQdcTRpBY-FNF7E2RqIupP2TQAve7PVNm9_-LaglYg7DtGVP5RyztCLp5oSUxsiPaY_i-A0SeeOAt1Xzm055Mun_BST9brfWhzfLgQ7ksOEMjvZN84amICtR0uUq6oSufFaAYJsOLfrw7EZuhypsLx6ggromK7J__wrNqhoXzagokyAs3YrBDIp3Xb_PtvgFnNzpzORAgZcqS-gpxfrW1vtFkRVEnGM_klZeFm6jaz0182xvX_P7UfZof7Lo63voANQQuDHYR-N-55dZtDZOI4iPRkbqKasYnbh3Jg2ebnYRN7qqcsoH1BBgRAFqwCvkeUsnNiESQtdKuoeqHoE0dHaeluEvaL918ZOSWN1GTJoey8lQXazhkptlChb90bS0MJrIJ_3sPZtItYPzfk9xd1SG2UjT8_S4-o8qZJZQnq7L8ocpfdsOaodo_NwAzuLxJLx5eFQoNgL-7uyNudkZ9ZWrtaTEKlQ2rwwYWolezqycQxG8PQFUy2DD-buMwHQLFoMppsGpQlN_rD5HSEaqMCY-kWTZloP8XOHsQ2t_Tgy1Euu0LDygyleZ0Th6u=w372-h667-no https://lh3.googleusercontent.com/d71Z0eonD9UhnoBJvjiKeV_COYAJ-H1wmg0YbYkY4T3xGOUqhMQvl7TIFkuUBvxsImESv5H9dYBtNiphVIiDBPWK_3jAwbMl70Dmikz096hNuSCWK4lCrY6AwZlQ0ceNxMVeLwGanfA_kOCVUjLyRPmV8skek3S2Uc1ITSaY_9HDBG4RVHa7XV86_1RWYgFLmc75LqcRQ7BOo4YXK5qjrWSYAQbP8UQx6tmFJ1uVNUKH_xMYbreTVcJsF6yG5oew1nouLesaUu0pq107jZpE6DWLRbqfzJklNbozdxuLqwLa3uhubfXQMMGRPRnX4ZCCRNBzNnBPvOv9rCq4REVc0dfXYua4s-2Y6ivN6uN99x-oz4j4r4qOdXYpGcgdBfdBIcTeaYJqcURxm68dJ2BAeeWr4Iz581g45fyccnknzt-oNiqWJn48fLFBMpPjWCX9vgYESpc4RjNP_Bo7QloBLzE8RAUn521x4JKfoyH-npvLGmGtfNPMg0xZtmvF_QbTMxCFKW7bzmaZiHKrY75P1fuKlu_pzE7RvQZBU1thUOFS9Y10DGU9KSMryv34SRWFmuRetx34t7VfhGGHU0IxkGsK7nwoA8z6thZdrGqW0yB79LQs=w354-h932-no

    Do you think that this is not a good way to look at it?
    Last edited: Feb 21, 2017
  11. Feb 21, 2017 #10


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    Well, your continued insistence on it (as in "Maybe at your level there is no cognitive dissonance. It does not do any good to pretend it is not there.") did support the original use

    That statement describes the spead in the single slit experiment, not the single-photon interference of the double slit experiment. If the two slits did in fact explain the mystery that way, it would not BE a mystery. The "mystery", to the extent that there is one, is why do the results of the double slit experiment depend on whether or not the "which path" information is detected. You are not addressing that.

    Put another way, what you just said in this quote explains the vertical spread in Fig. 1 above but does nothing to explain the horizontal interference pattern.
  12. Feb 21, 2017 #11
    Please look at Figures 1 through 7. They show the momentum distribution for multiple slits, including the double slit. (These are 2-dimensional Fourier Transforms).

    The mystery, and something that they do not address, is why don't we see a diffraction pattern if a detector is placed at the slit. Somehow, the detector at the slit, changes the position distribution. It must destroy superposition, in such a way, that the electron cannot take multiple paths through the slit, it always takes the exact same path through the slit.
  13. Feb 21, 2017 #12


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    All of which support what I just said.
  14. Feb 21, 2017 #13
    How do they support your statement?
  15. Feb 21, 2017 #14


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    They all show a spread which is equivalent to the single slit experiment which jibes with your statement about position localization by each single slit, resulting in a subsequent linerar spread due to the uncertainty in momentum exiting the slit. This does not say anything about the interference pattern, and I am trying to point out that the statement of yours which started the last couple of posts, does not address the interference. Clearly the figures SHOW the interference but what I'm saying is that your comment in post #9 does nothing to address that interference, it only addresses the spread caused by each single slit.
  16. Feb 21, 2017 #15
    Unfortunately, they did not show the position and momentum distributions for a single slit. All of those figures are for multiple slit position distributions. The Fourier Transforms are 2-Dimensional transforms and the area of integration includes the area of all the slits. That is why the momentum distributions show interference.

    We do not have to calculate the Fourier transform to see those distributions. All we have to do is conduct an experiment with the appropriate slit geometry and the recording screen will show the momentum distribution to us. Using his terminology, the screens with slits place the electrons in a well defined position state and the detection screen records the resulting momentum distribution, including interference effects.

    The paper has been invaluable to me in many ways. In particular it has shown me what is meant by "state preparation" and the mathematical meaning of the quantum mechanical principle that the initial wave function is a superposition that places the wave function at all slits simultaneously.

    Last edited: Feb 21, 2017
  17. Feb 21, 2017 #16


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    You are continuing to not address the issue of why there is interference if the "which path" information is not detected but no interference if the "which path" information IS detected. THAT is the "mystery" of the double slit experiment and it seems to me that you are not addressing it. You keep talking about "momentum distribution". OK, explain why the momentum distribution is different based on whether or not the "which path" information is known.
  18. Feb 21, 2017 #17
    I did address that in post #11. First, I really do not know the answer, but I can guess an answer now better than I could guess an answer before, because now I have a somewhat intuitive feel of what is going on. (Better than before and I am sure still not correct). Here is what I said about this in post #11

  19. Feb 21, 2017 #18


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    Ah. I missed that. So we are saying the same thing except that I got the mistaken impression that you thought you now completely understood the results of the two slit experiment. I based that inference on your statement "From the above statement, the mystery of the double slit experiment goes away." which you now are apparently admitting ("I really do not know the answer") is not what you meant.

    @mike1000 on rereading the last few posts I notice that it appears that I was being argumentative just for the sake of being argumentative. I did not mean it that way. I was taken aback by my interpretation of your statements as meaning that your analysis of the momentum (which I believe does address the single-slit spread) explained the results of the 2-slit experiment being different based on whether or not the "which path" information is known.
    Last edited: Feb 21, 2017
  20. Feb 21, 2017 #19
    I did not think you were being argumentative. I appreciate the fact that you read my posts. Thank you.
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