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On the Wave-Particle Duality property of light

  1. Oct 10, 2014 #1
    I just wanna confirm a statement the -
    Light travels in the form of electromagnetic waves in open space, not particles, but converts to a particle while encountering an obstacle deserting its wave form.
    So is the statement correct or not?
    And does it persists any anomaly or exception while abiding through the scientific law?
     
  2. jcsd
  3. Oct 10, 2014 #2

    Nugatory

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    Not correct, but a better description than "wave-particle duality" or "it's a particle and a wave".
    I just recommended Feynmann's "QED: The strange theory of light and matter" in another of your threads, and that's the right answer here as well.
     
  4. Oct 10, 2014 #3

    phinds

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    To extend Nugatory's answer of "not correct", what you have is a popularization that comes from an understanding ("wave particle duality") was tossed aside in science some 100 years ago (maybe it was 80 or 90, but it was a long time) but is unfortunately carried on even by some physicists even though it is incorrect. What is correct is to say that light/photons are quantum objects. That's it. Quantum objects. If you measure particle-like properties of a quantum object, you will get particle-like results but a quantum object is not a particle. Same thing with wave characteristics.
     
  5. Oct 11, 2014 #4
    Sir,
    Can you elaborate it a further, I think I got the real idea but I need explanation to clear it out.
     
  6. Oct 11, 2014 #5

    phinds

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    I really can't think of anything else to add to what I already said. I could say it in different words, but it would say exactly the same thing.
     
  7. Oct 11, 2014 #6

    bhobba

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  8. Oct 13, 2014 #7
    I can't agree more with this. It seems to happen in every field of research: once a seemingly reasonable interpretation is proposed, it would be self-sustaining and very hard to be replaced by a newer interpretation. Lot of discussion of wave-particle duality had been made before wave mechanism was established in 1926. The "successful" explanation of Compton effect with a pure particle understanding in 1923 or so was regarded as a strong support of the particle picture, and some people thus believed the momentum conservation relation in Compton's explanation could not be supported by a wave interpretation. This belief is still stated today in some physcis webpages and even some textbooks.

    My understading is that light is wave, not particle. Photon (phonon) is the basic unit of interaction between light (lattice) wave and other objects.
     
  9. Oct 13, 2014 #8

    phinds

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    You have two contradictory statements here. First you say that you agree strongly with my statement that light is NOT a wave and then you say that you believe that light IS a wave.

    I say again; Light is NOT a wave. Light is NOT a particle. Light is a quantum object that can exhibit different characteristics depending on the measurement.
     
  10. Oct 13, 2014 #9
    My apology. My remarks are not precise.

    I agree with your remarks "To extend Nugatory's answer of "not correct", what you have is a popularization that comes from an understanding ("wave particle duality") was tossed aside in science some 100 years ago (maybe it was 80 or 90, but it was a long time) but is unfortunately carried on even by some physicists even though it is incorrect."

    I did not read your remaining remarks very carefully. But I cannot agree with that light is neither wave-like nor particle-like. Maxwell equations are wave equations, so light is wave. Schrodinger equation is a wave equation, so matter is wave.
     
  11. Oct 13, 2014 #10

    phinds

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    I did not SAY that light is "not wave-like". Please be precise when you are "quoting" someone.

    What I said, and will say again, is that LIGHT IS NOT A WAVE. It is a quantum object that exhibits wave-like properties and particle-like properties.
     
  12. Oct 13, 2014 #11

    Orodruin

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    Maxwell's equations do not describe the fundamental nature of light, only the classical limit. In the classical limit, the wave-like properties of the quantum theory are more apparent than the particle-like properties. This is similar to describing water as a continuous fluid, this is fine as a macroscopic description but does not capture the fact that water in the end is made up out of water molecules (although I would warn about taking this analogy too far ...)

    The Schrödinger equation describes the evolution of quantum states. Other parts of the description in quantum mechanics involve properties that are particle-like. Matter is made up out of quantum objects that have some wave-like properties and some particle-like properties. This does not mean that an electron is alternately a particle and a wave or both at the same time - an electron is an electron and it has a number of properties that we have found through experimentation and modelling.
     
  13. Oct 13, 2014 #12

    bhobba

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    There is unfortunately a lot of confusion floating around on this.

    But, although its contradictory to the sources that create that confusion, its neither particle nor wave - its quantum stuff.

    Sometimes it behaves LIKE a particle and sometimes LIKE a wave - but its actually neither.

    See the link to the FAQ I posted earlier.

    Another thing that may help in breaking down preconceptions is to see a fully quantum analysis of the double slit experiment:
    http://cds.cern.ch/record/1024152/files/0703126.pdf

    Note it has nothing to do with wave-particle duality - its got more to do with the geometry of the situation and each slit is a position measurement - hence the objects leave the slit with an unknown direction.

    Thanks
    Bill
     
  14. Oct 13, 2014 #13

    bhobba

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    Its called a wave equation, but really it isn't in any usual sense - that dates back to the early days of QM.

    Since Dirac came up with the transformation theory in about 1927 that view has been consigned to the dustbin of history.

    To see it note a wave-function is simply an expansion in terms of position eigenstates. You can expand it in momentum eigenstates if desired - in which case the Schroedinger equation (in its usual form) is useless. You need Schroedinger's equation in momentum space:
    https://www.physicsforums.com/threads/schrodinger-equation-in-momentum-space.507762/

    Neither is better or worse in describing physical phenomena - the choice is dimply mathematical convenience or personal taste.

    BTW that's why Dirac called it the transformation theory - and its what basically goes udner the name of QM today.

    Actually matter is a quantum field - but that is another story.

    Thanks
    Bill
     
    Last edited: Oct 13, 2014
  15. Oct 13, 2014 #14

    e.bar.goum

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    Ok, I've been reading along with this thread, and wondering if it really is another story at all. Surely it's better to dispense with wooly statements like

    And just tell it like it is - our best understanding is that what we call photons are excitations of an underlying photon field.

    As a side note, it's interesting to me that the issue of wave-particle duality isn't resolved in lay discussions of physics, even though it hasn't been an issue for nigh on a century in physics. It just hasn't seemed to percolate out to the public.
     
  16. Oct 16, 2014 #15
    Photons are excitation of an underlying photon field!!!. Which photon field are you talking about????
     
  17. Oct 16, 2014 #16

    bhobba

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    The quantised EM field.

    Interestingly its to a large extent determined by U(1) guage symmetry.

    Thanks
    Bill
     
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