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Classical versus quantum interference.

  1. Jan 18, 2014 #1
    The classical physics developed by people such as Fraunhoffer and Fresnel seems to work very well
    at predicting the results obtained when observing interference and diffraction patterns. Quantum theory seems to work well also and can be used,for example when photons are sent one at a time.
    What I have been trying to find out is which, if any,of the two approaches (classical or quantum),is best at describing the observed results in a classical (eg not one photon at a time) set up? Are both approaches equally good at predicting all of the fine details that can be observed or does one approach work better than the other?
    Thank you
  2. jcsd
  3. Jan 18, 2014 #2


    Staff: Mentor

    Cant quite follow your concern.

    If its classical, then by definition, you don't need anything quantum.

  4. Jan 18, 2014 #3
    The experiment and the results can be defined as classical in different respects one being that they were observed well beyond the advent of quantum theory. Are you suggesting that there are areas of physics which, by definition, cannot be analysed by means of quantum theory?
    My concern is simple. I want to know which of the two approaches works best.
  5. Jan 18, 2014 #4


    Staff: Mentor

    Of course not.

    Everything is quantum.

    But we know a 'regime' exists that emerges from that quantum world and we call it classical. Its the world we see around us day to day. Its the world classical physics describes. By definition if you want to describe classical phenomena you use classical physics.

  6. Jan 18, 2014 #5
    Are you saying that experiments of the type originally carried out before the advent of quantum theory cannot be addressed by quantum theory? Or are you saying that said experiments cannot be addressed by quantum theory if they are referred to as classical experiments.Whatever you mean you seem to be suggesting that my question is pointless in that "you use classical physics" only. Not at all helpful.

    People like Young, Fresnel, Fraunhoffer and others developed what can be described as classical approaches to the phenomena and people like Feynman, Englert, Greenberg and others developed what can be described as quantum approaches to the phenomena.In the context of the question I asked we can't brush one of the groups away simply by referring to definitions such classical physics and modern physics. I wanted a comparison of both approaches.
    Last edited: Jan 18, 2014
  7. Jan 18, 2014 #6


    Staff: Mentor

    Its the context of the question that I simply do not understand.

    You specifically excluded the situation where there is one photon in the double slit apparatus. With that exclusion its perfectly described classically. QM is not needed or required.

    Your question to me looks like what happens when an immovable object meets an irresistible force. Its a nonsense question because one precludes the other.

    The same here - if you preclude quantum phenomena then QM is not necessary.

    What Feynman etc described were experiments where the particle nature of light was apparent - that is a quantum effect pure and simple and if your setup is sensitive enough to detect that you need QM - if not classical physics is all that's needed.

    Now can I perhaps rephrase it along the lines you perhaps intended it.

    Are you asking if in some circumstances, even though the phenomena does not display quantum effects, a QM explanation may be, for some reason, preferred?

    Personally I do not know of any, but am all ears if anyone can think of some.

  8. Jan 18, 2014 #7

    Classical theory (Huygens, Fresnel, Maxwell) regards light as a continuous wave of time-dependent vectors of the electric and magnetic fields. In quantum theory (Einstein, Schroedinger, Feynman), the light is a flow of tiny massless particles - the photons - each with its own wave-like wave function.

    When applied to the interference of high-intensity light (many photons), both theories predict exactly the same interference picture, so both can be used equally successfully. However, as you correctly pointed out, in the low intensity limit (few photons) the classical theory is helpless, and only quantum theory can explain the interference.

    Apparently, in a successful theory there cannot be two different explanations for the same physical effect. Thus, we should admit that only the quantum explanation of the interference is the correct one. Curiously, this means that quantum effects with photons (Newton rings, diffraction, interference, etc.) were seen experimentally long before the idea of quantum mechanics.

    Last edited: Jan 18, 2014
  9. Jan 18, 2014 #8


    User Avatar
    Gold Member

    In a classical experiment, is it a good approach to work with Newtonian mechanics or in the concept of Special Relativity?
    The answer to this is a single one- IT DEPENDS ON your set up and what you wanna measure.

    The same can be said for other fields (Eg again newtonian/deterministic approach or thermodynamics for gases?)
  10. Jan 19, 2014 #9
    I did not preclude QM. In fact it was quite the reverse and I wanted to know how succesful the QM approach is compared to the classical approach when describing what can be described as classical interference/diffraction patterns.
    Consider the one photon at a time experiment. It's fair to say that the results such an experiment is described by QM only. Now consider a series of experiments being carried out with increasing rates of photon emission so that we see the classical pattern emerging and of increasing intensity variation. If QM can explain the results of low rate photon it is reasonable to assume that the theory can also be applied to any rate of photon emission.Perhaps the theory hasn't been developed that far yet but that's the sort of thing I wanted to know.
  11. Jan 19, 2014 #10
    Thanks meopunk,your middle paragraph seems to answer my question.
  12. Jan 20, 2014 #11
    I think its just a matter of scale.

    At the microscopic scale
    (and we can define that as per experimental data, it revolves around the Plank constant):
    QM works better. QM becomes important/dominant.

    A single photon is at the plank constant scale -- thus QM works better.

    A singe photon is not our typical particle. It does not have wave-particle duality either. It's just a different kind of particle than we see/imagine in the classical world.

    Yes QM could be shown, mathematically/conceptually, to merge into classical as the scale grows bigger.
    Last edited: Jan 20, 2014
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