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Particle or wave?

  1. Dec 14, 2009 #1
    Hi All,

    I am not a physicist, so my question/comment may be stupid, but if I dont ask I will surely stay stupid, so I'll ask.

    I understand that particles show both particle-like behaviour (when measured or "observed") and wave-like behaviour (when expanding, not observed). My question: does observation or measurement not simply mean "question it with our existing mindset"? To me, it seems obvious that if we question nature with our particle-mindset (or with what do we find out that it is a particle?) it is not surprising that we get a particle back as an answer.

    In other words, may we have only waves and no wave-particle duality?

    I've also tried to make this point on my recent blog post, where I question http://www.spreadinghappiness.org/2009/12/no-emptiness-stillness-or-eternity-questioning-physical-concepts-in-light-of-typical-human-thinking-mistakes/" [Broken].

    A reply would be very much appreciated!
    Last edited by a moderator: May 4, 2017
  2. jcsd
  3. Dec 14, 2009 #2
  4. Dec 14, 2009 #3
    Hi Sokrates,

    thank you for your answer. I know the photoelectric effect. In this experiment we "question" light by observing the effect it has on the electrons. My question starts at an earlier stage: why are we so sure that we "question" nature with the particles called electrons? How do we know they are particles?
  5. Dec 14, 2009 #4
    Hmm.. I am trying to fully understand your point.

    But the reason is, I think, that we "falsify", "eliminate" other methods of questioning.
    We are forced to believe in that conclusion, after a number of tests.

    The name "particle" or "electron" are our inventions, but the phenomena and their properties are about the character of nature.
  6. Dec 14, 2009 #5


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    This question is part physics, part philosophy. Yes, it is true to a certain extent that what you see is a function of how you look. But that does not mean that the wave picture is "correct" and the particle one is not.

    As Sokrates says, there is behavior that the wave picture cannot explain. For example, you never see fraction of a wave, which you would expect. There are higher order photon effects that really require a particle perspective, see for example this:

    "While the classical, wavelike behavior of light ~interference and diffraction! has been easily
    observed in undergraduate laboratories for many years, explicit observation of the quantum nature of light - i.e., photons - is much more difficult. For example, while well-known phenomena such as the photoelectric effect and Compton scattering strongly suggest the existence of photons, they are not definitive proof of their existence. Here we present an experiment, suitable for an undergraduate laboratory, that unequivocally demonstrates the quantum nature of light. Spontaneously downconverted light is incident on a beamsplitter and the outputs are monitored with single-photon counting detectors. We observe a near absence of coincidence counts between the two detectors—a result inconsistent with a classical wave model of light, but consistent with a quantum description in which individual photons are incident on the beamsplitter. More explicitly, we measured the degree of second-order coherence between the outputs to be g(2)(0)=0.0177+/-0.0026, which violates the classical inequality g(2)(0)>1 by 377 standard deviations."

  7. Dec 14, 2009 #6
    We can leave light out of the diffraction picture, as electrons and buckyballs can exhibit diffraction.
    Other than diffraction, are there any other instances of light acting as a wave? Blackbody radiation, fine structure, Compton scattering, photoelectric effect, and inequality violation experiments all suggest particles.
  8. Dec 14, 2009 #7
    I'm a non-physicist as well so this may be a silly question, however, how would you explain redshift from a particle point of view?
  9. Dec 14, 2009 #8
    Other than diffraction, reflection, dispersion, refraction and interference or phase?...

    The whole electromagnetic theory treats light acting as a wave, I think what's astounding is that light can behave as particle..

    That's why from the times of Newton, people figured out the wave properties of light while it took an Einstein to figure out it can also behave as a particle.
  10. Dec 14, 2009 #9
    "Difficult" is a subjective word. These two particular experiments are only "difficult" to explain by a wave picture in the sense that certain math techniques like integration by parts are "difficult". People tend to forget that the particle explanations also have their difficulties. A small example would be to try and explain, since photons and electrons are supposed to be point particles with zero cross section, how do they collide at all?
  11. Dec 14, 2009 #10
    Integration by parts is difficult?

    No - unfortunately, there's no wave-like explanation for the irreducible quantum of electromagnetic radiation. See Dr. Chinese's post on the subject. A wave doesn't come in lumps.

    People weren't baffled with wave-particle dualities, and mysteries of quantum mechanics because they couldn't handle a specific integral.

    Electrons and photons are not supposed to be "point particles with zero cross section". Who says that? The quantum theory has a beautiful way of handling scattering which avoids philosophical questions like the radius of electron etc..

    You can start by looking at Fermi's Golden Rule, etc...
    Last edited: Dec 14, 2009
  12. Dec 14, 2009 #11
    Well, yes. I thought I was pretty clear about this. It's "difficult" in the same sense that the wave explanation for, say, the Compton effect is "difficult".

    Well, now you're upping the ante. I thought you originally said that the wave-like explanations were merely "difficult".

    I thought people generally said that. I would have thought it was something you yourself would say. About the "point particles" issue at least. As for the zero cross section, that was an extrapolation on my part based on what I would intuitively expect for the cross section of a geometrical point. Maybe there is a good explanation for how a point particle has a non-zero cross section, but it does seem to me like exactly the type of thing that would be "difficult" to explain. (In the sense, of course, that integration by parts is also "difficult".)
  13. Dec 14, 2009 #12
    ? - I don't know how a fundamental property of matter can be tied to a well-defined Mathematical method. What's the relation between integration by parts and wave-particle duality?

    Yes, I am still saying that. One can come up with an overly complicated wave-like model just to explain the particle properties of matter. Bohmian Interpretation attempts to do that, in my opinion. This is why I am using the word "difficult". One needs extra tools and machinery to "fit" the data with a wave picture.

    Particle viewpoint on the other hand SIMPLY explains these experiments. So yes, wave-like explanations become difficult under certain conditions.

    I refrain from making comments about things that cannot be determined by experiment. In the future, if people can find a way to probe the geometry of an electron, if it even makes sense (experimentally) to talk about it, then we could talk about it.

    You brought up the "zero-cross section" argument, so I don't think the particulate view has the burden to explain that. Nobody made a comment about the cross section of a particle.
  14. Dec 16, 2009 #13
    I've answered a question on wave particle duality before, heres what I posted:

    Bohr explained wave particle duality using the idea of complimentarity. Have you ever seen that picture where if you think about it in one way it looks like an old woman and if you think about it another way it looks like a young woman?

    Here is the link if you haven't:


    What Bohr suggested was that the wave picture and particle picture are like the old woman/young woman views. Neither view is really what the picture is of, but together they cover all the human ways of thinking about the picture. Objectively the lines that make up the picture exist independently of our ideas concerning their interpretation. Just as we can't "see" both the young woman and the old woman in the picture at the same time, we can't "see" entities as both waves and particles at the same time. They seem opposing and contradictory, but together they encompass all phenomena. The apparent contradiction is due to our human way of interpreting the phenomena. Bohr's point was that nature doesn't really care what we think of it. Opposites are compliments...its all very Zen. Definitely some Eastern philosophy creeping into science.
  15. Dec 17, 2009 #14
    does anyone know what would happen if one were to replace the screen in the double slit experiment with a photoelectric metal sheet. Would we observe both wave and particle nature in the same experiment?
  16. Dec 17, 2009 #15


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    Not really. As a general rule, complementarity means that you can see the wave, or the particle, but not both at the same time.

    Of course, you can do experiments that APPEAR to give you both. But further analysis shows that is merely an illusion.
  17. Dec 17, 2009 #16
    I recognize my fingers typing this message not as waves but as a body composed of some materials. Am I all right?
  18. Dec 17, 2009 #17
    Here is an answer given by R. Feynman on the exact same question.

    Last edited by a moderator: Sep 25, 2014
  19. Dec 17, 2009 #18
    Here is another answer by the same guy on THAT question:
    Last edited by a moderator: Sep 25, 2014
  20. Dec 17, 2009 #19
    Ok, let me have one more go and then I shut up:

    In the photoelectric effect, we see what happens to the electrons when light shines on the metal. We see the electrons emitting from the surface, and conclude quite reasonably that a wave-model could not explain the details of the phenomenon, but that a particle-model (and the mathematics that come along with it) can.

    Now: how do we know that the emitted electrons are actually particles? If we take this for granted then yes, light seems to also be a particle. ("If you ask nature with the particle-mindset, you get a particle back as an answer" - it always shows wave-like behaviour if we don't look at it, i.e. if we don't ask nature with our existing mindset).

    An integral part of quantum mechanics is to take the observer into the equation - have we done it in this experiment too?
  21. Dec 17, 2009 #20
    For the photo-electric and Compton effects, the wave-wave interactions explain things just as well as the particle model. Maybe better.

    If you assume the electron is a particle, yes, then light also has to be a particle. But if you really treat the electron as a wave, then its interaction with classical light makes good sense. That was Schroedinger's motivation in developing the wave equation.
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