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Are Quantum Mechanics and a rotating fan comparable?

  1. Sep 19, 2015 #1
    Being a non- physicist, it is extremely difficult for me to conceptualize QM. I hope to get some definite help at this forum.
    QM says that the sub-atomic particles exist as both waves and particles. It is not that much difficult to conceptualize.
    QM also says that the sub-atomic particles can exist as waves in more than one place simultaneously (superposition state). It is a bit difficult to conceptualize.
    QM further says that the moment one observes or interferes with their superposition states, they collapse to their physical state. This postulate is extremely difficult for me to conceptualize. The difficulty becomes more compounded when scientists extrapolate this observation of micro-world to macro-world. Some people say that the moon is not moon until we observe it.
    Now I would like to draw your attention to a very simple phenomenon that can be witnessed at our home (my attention was drawn towards it by my sons). Let us observe a fan (preferably at night), hanging from the ceiling and rotating at full speed. Suppose for a moment that it is rotating here for thousands of years and we know nothing about the number, shape or the color of its blades. We do not even know that the fan has blades. So, what do we see there? Just a hazy cloud like structure (though with a definite periphery). We cannot get any information about the blades while it is rotating. Can we compare this hazy cloud with the cloud formed by the rotating electrons around the nucleus? Now switch off the lights so that it is dark in the room. Pick up your camera and just throw a flash of light onto the fan (it flashes for a very short moment). In the flashlight, the blades of the fan become clearly visible in their full shape, as if they are not rotating but are stand still.
    However, here we do know that the blades are still rotating and the flashlight is just a technique to make the blades visible. I think here we cannot say that the flashlight just collapses the rotating blades to a stand-still position.
    Is there some analogy between this scenario and the subatomic scenario? Is the collapse of sub-atomic waves into their physical form as a result of observation is similar to the above phenomenon. I would be obliged to get some feedback. Dayalanand
    Regards.
     
  2. jcsd
  3. Sep 19, 2015 #2

    stevendaryl

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    That's a picturesque analogy, but I don't think it is especially close. In the quantum case, after a system has been observed to be in a definite state, it persists in that state (subject to whatever interactions might cause the state to change). So if you perform two position measurements in succession, you will find that a particle's location doesn't change much between measurements (and similarly for other properties). In contrast, with the fan, if the light flash seems to "freeze" it into position, a second later, it's in a completely different position.
     
  4. Sep 19, 2015 #3
    The aspect I would endorse is putting your strobe light between two counter rotating blades and see perfect correlation on any axis, but if you offset the blades alignment you'll only see a classical shift in correlation.
     
  5. Sep 19, 2015 #4

    Nugatory

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    Well, even the physicists find it difficult to conceptualize QM. Feynman is (credibly) quoted as saying "If you think you understand quantum mechanics, you don't understand quantum mechanics". But with that said, there are a bunch of misconceptions floating around in the popular press, and these don't make it any easier for non-physicists to understand QM:
    That is kinda sorta true, but only because the word "particle" doesn't mean the same thing in QM as it does in normal English usage. You have to forget everything that you thought you knew about particles before you can start making sense of the quantum mechanical notion of particles - and the popular press tends not to tell you that.
    It says no such thing. What it does say is that if you measure the position of a particle in a superposed state, you may find it here,or there, or somewhere else, with different probabilities. That doesn't mean that the particle is in more than one place simultaneously - it's just as reasonable (after you've forgotten everything that you thought you knew about particles) that the particle isn't anywhere unless and until we measure its position.
    Yes, there are some ways of formalizing the theory (called "interpretations") that say that. If you find that difficult to conceptualize, you can always choose a different interpretation.
    No, nobody has ever seriously suggested that. The famous quote (variously attributed to Mermin and Einstein) about whether Bohr seriously believed that the moon was not there when nobody was looking was a rhetorical question - it's obvious to a physicist that the moon is there, so if it's not obvious from our understanding of QM that the moon is there, then something has to be wrong with our understanding of QM. There was, and it took a few decades to work out what it was and why we could be confident about the moon being there even when we weren't looking.
     
  6. Sep 20, 2015 #5
    Dear Stevendary
    Many thanks for your reply full of information.
    Regards
    Dayalanand
     
  7. Sep 20, 2015 #6
    Dear Jerromyjon
    Thanks for the reply. However, it was too much technical for a non-physicist like me to grasp.
    regards
    dayalanand
     
  8. Sep 20, 2015 #7
    Respected Mr Nugatory

    I am extremely grateful to you for giving me important tips about QM. It will encourage me to know more about it.
    Regards
    Dayalanand
     
  9. Sep 25, 2015 #8
    Dear Stevedaryl
    I am OK with your suggestion, except one remaining doubt. In QM, when one observes a rotating 'wave-particle' electron, and it transforms itself into a physical form as a result of observation, how does it 'freeze' to its position? Does it stop rotating around the nucleus? Shall be grateful to get a suggestion.
    regards
    dayalanand
     
  10. Sep 25, 2015 #9

    stevendaryl

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    Well, a difference between the fan blades and an electron is that actually, a fan blade is at a definite location at every moment, but this location changes so rapidly that our eyes interpret it as a spread-out blur. In the case of an electron, QM (or some interpretations of it) says that it doesn't actually have a definite location until it's observed. So the electron goes from being a smeared cloud surrounding the nucleus to being a point-like particle at a particular position when you observe it. After observing it, it spreads out from the position where it was found.
     
  11. Oct 1, 2015 #10
    Dear Stevendary
    Many thanks for continuously educating me. I fully agree that what you say is exactly what QM interprets. I only want to know that is there no possibility in QM to search for the possibility of an alternative interpretation for this 'observation effect' on electron? I only want to say that had we no prior inkling about the existence of the fan blades, we could have given a similar interpretation of the torch effect described in this post. But it is you experts only who have the authority to comment upon it. Hence, I accept your suggestion. Thanks and regards.
    Dayalanand
     
  12. Oct 1, 2015 #11

    Nugatory

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    You are describing what is called a "hidden variable" theory, one that says there is some underlying mechanism that we gave not yet discovered and that will explain the quantum-mechanical results. Bell's theorem (google for it, and take a look at http://www.drchinese.com/Bells_Theorem.htm, maintained by member @DrChinese) has produced the surprising result that quantum mechanics cannot be built on top of any theory that uses classically behaving things like your fan blades.
     
  13. Oct 4, 2015 #12
    Dear Sir
    Many thanks for the suggestion. I am going through the site and trying to learn a little more.
    regards
    Dayalanand
     
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