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How does Particle Wave Duality Work?

  1. Mar 18, 2013 #1
    I really don't understand particle wave duality whatsoever. Please explain it to me, as I am very interested.

  2. jcsd
  3. Mar 18, 2013 #2
    What part are you confused on?
  4. Mar 18, 2013 #3
    Have you tried google?
    Also you should read this http://en.wikipedia.org/wiki/Photoelectric_effect

    The photoelectric effect the birth of quantum physics , atleast the thing that started it going "bigtime."

    When you read the wiki article keep this quote from it in mind """"The energy of the emitted electrons does not depend on the intensity of the incoming light, but only on the energy or frequency of the individual photons. It is an interaction between the incident photon and the outermost electron."""

    This proves that light (em radiation) can be both a wave and also it can be quantized in pieces - photons. This basically means that if you have a light source of a certain given frequency and you "shine it" onto a metal plate the plate gives off some electrons because the incoming photons excited them.Now this may seem ok but the most important part of it is that if you now have another lightsource with the same frequency and you add that to the previous one , normally you would think that now the outcoming electrons should be "stronger" or higher energy but they are not.You can have a thousand suns shining on a single plate of metal if all of those suns emit the same frequency then the electrons will also be the same energy.Now this proved that light comes with little packets in it called photons.
    Each photon has a battle with each electron , and only how strong the individual photon is the outcoming electron will be.They are related one to one just like two boxers.

    You may think of this as an army.Normally you don't count each individual soldier you just say army.But in the end of the day the strength of the army as a whole depends on the strength of each soldier.
    the army is the wave , the soldiers are the particles.
    Another analogy could be the modern day CPU of the computer mainboard.Now you can have as many cores as you want or can make but that only increase the capability to do more processes at once or simultaneously but not the overall speed with which those are done.The speed depends on the frequency of the CPU.So if you need or want to do more things at once more cores are better but if you just want to do something twice as fast you need higher speed.

    Now just remember this just like any other analogy is just an analogy.Don't think that someone has actually seen the photon or held it in his arm.Photon is a mathematical artifact that we find handy to call the phenomenon of light shining on a metal plate emitting electrons of certain energy level.
    It's like the wind, you can't see it but it makes an effect on leaves , paper bags and peoples hair.And by that you judge that there is a force acting on those things and you call that force wind.
    Last edited: Mar 18, 2013
  5. Mar 18, 2013 #4
    You would be best to look up the photoelectric effect that crazymechanic linked to and the double slit experiment. The photoelectric effect shows how light, classically a wave, can be observed as a particle in certain circumstances, while the double slit experiment can be used to show how matter, such as electrons, can act as a wave in certain circumstances. Once you understand both of these experiments, it is easier to understand wave particle duality.

    It turns out that the wave function of light and matter describes the probability that a particle will be observed in a certain position. For example, in the double slit experiment, where the wave intensity is higher, there is more probability of a particle being detected. This leads to even electrons forming interference patterns as if they were waves, as on a large scale the probability, described by a wave function, will describe the concentration of particles detected. This means that where we would expect to find areas exposed to high intensity waves according to the wave function, we will find high concentrations of particles.

    The wave function of matter is described by De Broglie's equations, which relate the wavelength of a particle to its momentum and its frequency to its kinetic energy, and both of these are also related to planck's constant. Of course, in some circumstances the wave-like properties of matter have little to no effect on our observations, just as light in some circumstances acts simply as a wave.
  6. Mar 18, 2013 #5
    Consider a car driver. He knows how to predict what the car will do when he operates the controls but doesn't know how hidden machinery of the engine or gears work. That's the current state of this field.

    If you are asking what are the equations that define and model wave particle duality, and how we can use it to build machines, that's well understood. We can drive pretty well. We know when it will be a wave, and when it will be a particle, and how to measure both.

    If you asking how it works, that is currently unanswered - you'd need a mechanic for that, not a driver. Sadly we don't yet have any fully qualified (quantum) mechanics. There is as yet no agreement on the interpretation of quantum mechanics and what wave particle duality actually means fundamentally.
  7. Mar 18, 2013 #6
    I would have to agree with the above statements, which cover just about everything we know about wave-particle duality thus far. I hope this was helpful.
  8. Mar 18, 2013 #7
    This is maybe the most important thing to try and get into your mind from the beginning.

    Thinking by analogy can be helpful in some circumstances, but this is not one of them. It turns out the world of the very tiny is not much like the world of the very large. So trying to understand the tiny by thinking about the large is likely to lead to problems.
  9. Mar 18, 2013 #8
    I just watched a lecture with Richard Feynman about QED. In this, he explains that light is actually particles. Not waves, but particles. The "wave-particle" duality, he explains, is a concept used at a time of confusion. However, they know that light is particles.

    The "wave-nature" of light comes from that fact that the path that a light would take to a given position is probabilistic, and this probabilistic nature of the particles are what gives rise to the wave-like nature.

    The reason we see interference patterns in the double slit experiment, is that when we find the probabilities of a particle being somewhere, and it can happen by 2 different paths, we do not add the probabilities, we add the probability amplitudes. And that is the mystery behind Quantum Mechanics. We do not add probabilities but probability amplitudes.

    To get the probability of an event, we take the square of the amplitude. That means if we have one event that can occur in 2 ways, and the probability amplitude of Way 1 is A, and Way 2 is B, then |A + B|2.

    This results in A2 + B2 + AB* + A*B (because the probability amplitude is a complex number).

    The A2 + B2 is the classical view of probabilities, where we just add the probability. But we can see, that when an event can occur in two different ways, the probability gets two extra funny terms. And these terms are what results in the interference pattern in the double slit experiment.

    However, I am not a physicist, but Richard Feynman explained this in a lecture I watched on Youtube :)
  10. Mar 18, 2013 #9


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    This question has been asked so often, we have a FAQ on it:


    I suggest that if one is new here, to read the physics FAQ subforums. We have two. One can be found in the General Physics forum, the other can be found in the Relativity forum.

  11. Mar 18, 2013 #10
    To all;
    Thank you so much for your help, I now think I understand much better.

    Thanks again,
  12. Mar 18, 2013 #11


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    I would check out this talk by Feynman, he makes it pretty clear:

    Last edited by a moderator: Sep 25, 2014
  13. Mar 18, 2013 #12


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    Yes, but Feynman's lecture was 40 years ago or thereabouts. I think the modern way of looking at it, as discussed in the FAQ referenced above, is that it is inappropriate to talk about light "being a wave" or "being a particle" or "being both". Light is not any of those things, it is a quantum object. If you measure it looking for particle behavior, you get its particle behavior, but that does not mean that light is fundamentally a particle. If you measure it looking for wave behavior, you get its wave behavior, but that does not mean that light is fundamentally a wave. Light is a quantum object.
    Last edited by a moderator: Sep 25, 2014
  14. Mar 18, 2013 #13
    Quite dated, I say. I'll say this once. Particles do not exist. Everything is a quantum field. The particle is what you observe when you view the quantum field in a certain manner.
  15. Mar 18, 2013 #14
    @d3mm Well I would say your right and wrong both.
    saying particles don't exist is like saying breakstone also doesn't exist,everything and all is just asphalt, and yet when you stop and look for a small part of the road you see the tiny little rocks and other little round and all other kinds of things that make it up.

    Well I don't want to get into philosophy here , because everytime someone no matter even how capable or intelligent or experienced get's into this discussion it pretty much always ends with nobody understanding anything in the end and everybody leaving just as they went in.
  16. Mar 18, 2013 #15


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    That's because you are applying classical mechanics to quantum objects. Asphalt is not a quantum object.
  17. Mar 18, 2013 #16
    Yes phinds I do realize the shortcoming of my analogy and that asphalt or anything I see "macro" is not a quantum object.
  18. Mar 18, 2013 #17
    It's not a case of scale that breaks the asphalt analogy, it's the fact that a quantum field is fundamentally different.

    Imagine if the asphalt is all one thing (not separate grains) and is randomly fluctuating and bouncing up and down. If it bounces too hard, a bit breaks off. You see that as a particle as it's been separated, but it's just bouncing asphalt. Once it "falls" back to the ground it recombines.

    Err I think this analogy is stretched too thin and I regret making it now. It's terrible. Maybe you should just check out Quantum Field Theory on Wikipedia :-) Anyway there are no particles in QFT. Everything is a field, or a part of a field.
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