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Wave-particle duality

  1. Jun 24, 2007 #1
    Hi to all

    I have a few confusions regarding wave-particle duality. I hope I will get good answers. You need not answer all the questions ( just those that you feel like answering).

    1. Light acts as particles called photons. Photons have a definite energy based on their frequency/wavelength. But frequency/wavelength are terms used when we assume light to be acting as a wave. How do we imagine a photon to possess any wavelength or frequency? How do we explain Double-Slit Experiment i.e. how does a single photon pass through both the slits? How do we explain constructive and destructive interference if we consider light to be a particle?

    2.(a). An electron is said to possess wave nature also. Then what is the definition of a particle or fermion? What is mass? I mean mass is considered to be a property of matter or particle. If ,then, a particle is actually a wave, then how is mass distributed in this wave? We know that energy can be converted to mass. A wave is a kind of energy. Is a particle then just a highly localized chunk of energy/wave ?

    2.(b). If an electron is left in complete isolation, how will it behave- like a wave or a particle? A dear friend of mine in this forum told me that an electron will start spreading like a wave if left in isolation. However, if we emit photons to see this wave, it will undergo wavefunction collapse and we will see a particle. Can anyone explain to me how this collapse actually happens? And whether isolation is a necessary condition for the electron to "exhibit" wave nature (isolation in the sense that there are no photons roaming around)?

    2.(c). Then ,again, when an electron is made to undergo two-slit experiment, when/how does it decide to convert from its particle character to wave character?

    2.(d). Does an electron in an orbital in an atom exhibit wave nature or particle nature, if : i) it is placed in a room full of light/heat (or in other words, the room is full of photons), ii) it is placed in a room devoid of any heat/light?

    2.(e). During formation of chemical bonds, interference between the two electrons of the participant atoms is taken into consideration. Is this just a probability wave, or are we really taking electrons to be acting as waves? And if the case is the latter one, then what is the reason that the particle nature of electron is completely neglected here?

    Eagerly waiting for your answers....:smile:

    Mr V
    Last edited: Jun 25, 2007
  2. jcsd
  3. Jun 25, 2007 #2
    Maybe my questions are too simple. But if anyone could show a little patience and give a good answer, I'd be grateful.

    Again, you don't need to answer all the questions. Only those that you feel like.

    Mr V
  4. Jun 25, 2007 #3
    I'm confused about these questions too.
  5. Jun 25, 2007 #4
    Is Light A Wave or A Particle?


    You cannot explain diffraction, interference or polarization with the particle model. You need to think of photon as a quanta of energy, that is, with definite energy levels.

    The Double-Slit Experiment can be explained with photons (quanta of energy; not a classical particle). You can find a basic explanation of it using Heisenberg's uncertainty principle by watching the http://ocw.mit.edu/OcwWeb/Physics/8-01Physics-IFall1999/VideoLectures/detail/Video-Segment-Index-for-L-34.htm [Broken] with MIT Professor Walter Lewin (RealPlayer is required).

    According to HUP, an electron does not have a fixed value for for either of the two quantities until you try to measure it. It is hard to understand at first, but remember that quantum mechanics is about probability.


    An electron can be described with quantum mechanics as well. A better explanation is available here:

    https://www.physicsforums.com/showpost.php?p=862093&postcount=2 [Broken]

    That is as far as I can get now.
    Last edited by a moderator: May 2, 2017
  6. Jun 25, 2007 #5
    I have never heard of a "photon" having definite "energy levels". What do you mean by "energy levels"?
    Last edited: Jun 25, 2007
  7. Jun 25, 2007 #6


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    I've never heard of this. As far as I'm aware, the particle model cannot explain diffraction. Care to provide a reference?
    The whole point of "wave-particle duality" is that an object in nature admits both the characteristics of a particle and of a wave. If you read the FAQ (the first link in the above post) it may become clearer as to why there is no problem with this.
  8. Jun 25, 2007 #7


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    This is not true. QM can describe ALL of these observations with one single, consistent formalism.

    I have mentioned the Marcella's paper[1] that clearly derived, using QM, a single-slit, double slit, and multiple slit patterns. Please download that Eur. J. Phys. paper if you have access to it. We don't teach students that because it is a lot more involved and tedious, so we continue to use the wave model when we encounter such situation.

    Furthermore, you also seem to miss Feynman's path integral formalism of QM. The most important aspect of QM is the principle of superposition. When quantum objects have the ability to assume more than one state, then all of these states are in a superposition of states. When quantum objects encounter a multiple slit, then you have a superposition of paths! When you stick to this idea, then you'll realize that it doesn't matter if it is a photon, an electron, a proton, a neutron, a buckyball, etc... if they are coherent quantum particles and they have more than one path to go through, then you'll have a superposition of paths for these particles that resulted in the interference pattern. So this ONE principle explains ALL of these observations for all these particles.


    [1] T.V. Marcella Eur. J. Phys. v.23 p.615 (2002).
    Last edited: Jun 25, 2007
  9. Jun 25, 2007 #8
    I'm not an expert, so take this with a grain of salt, but my understanding is that photons/electrons/etc are neither particles nor waves, but rather some other form of entity, which we do not fully understand, but which has the characteristics of both. The wave model and the particle model are then each an approximation, each approximating some properties well and other properties poorly.

    If I am to borrow the elephant analogy from somewhere else, consider this: One blind man feels the leg of the elephant and says 'it's a thick round column'. Another blind man feels the ear of the elephant and says 'it's a big flat sheet'. In fact it is neither - and both.

    We can then in each situation use the model which fits the situation better, to get an approximate result - often it is still accurate enough. Or, we can use a much more complicated model that takes all aspects into account, and get a more accurate result - but the mathematical complexity of such calculations often makes this approach overkill, and the extra accuracy isn't always worth the complexity. This is how science works in many cases.

    Hope that helps.
  10. Jun 25, 2007 #9
    A matter wave is not the matter itself. It is just a probability wave which gives the probability of the particle being in a small region centered around the point of observation. Now the question arises, how does the electron travel between the origin and the point of detection? What makes its path so unpredictible that we cannot predict where the electron will be detected(we can only calculate probabilities)?
  11. Jun 25, 2007 #10


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    Welcome to quantum physics, where the fundamental entities (photons, electrons, etc.) are neither classical waves nor classical particles, but rather, quantum objects that have characteristics of both. We do not have to "switch" between waves and particles when analyzing them. The mathematics of quantum mechanics and quantum field theory contains both kinds of characteristics, and predicts the results of experiments very successfully. When Feynman and other physicists write about particles, these are the kind of particles that they refer to, not the purely classical kind.

    As far as I know, no one has come up with a "classical-like picture" of the way these things "really are." You can find such pictures, used as analogies to help describe quantum experiments, but they always break down somewhere so you have to take them with a grain of salt.

    Fundamentally, there is (so far) no generally accepted picture, or interpretation of the way things "really are" at the quantum level. There are various competing interpretations of QM, but they are all based on the same mathematics and give the same results for the results of actual experiments. They differ in things that cannot be tested by experiment, even in principle, and they all have non-classical features.
  12. Jun 25, 2007 #11
    It's not the frequency/wavelength of the photon. When talk about the energy of the photon, then it is dependent on the frequency of the corresponding wave.
    For the next part of the question, I don't think a single photo can pass through both the slits. It is the probability wave that passes through both the slits, and the photon has equal probability of passing through any one of the slits.
    Now for the interference- It is the waves that undergo interference and form bright and dark fringes on the screen. Now, as the intensity of the fringe gives the relative probability of a photon absorption, the photons tend to be absobed at the maxima, and tend not to be absorbed at the minima, and the probability of absorption is intermediate for intermediate intensities.

    But I should tell you that I'm still confused regarding this wave-particle duality.
  13. Jun 25, 2007 #12
    I think the biggest problem people have understanding the wave-particle duality is imagining that something is definitely a wave or definitely a particle. To explain diffraction for example we think of photons as waves. They are not waves - neither are they particles. That is the whole point. They are both at the same time but exhibit certain properties which imply particle-like behaviour sometimes, and wave-like behaviour other times. If you take the mathematics of quantum theory as a description of the actual processes involved then you end up with statements that appear to defy logic - mathematically, in the double slit experiment, the electron goes through neither hole, and both holes, and only one hole, and only the other hole, all at the same time. Maybe that's exactly what actually happens, but our brains can't deal with this kind of paradox. Or maybe not - if maths is just a way of getting an answer that matches experimental outcomes then it could happen entirely differently.
  14. Jun 25, 2007 #13
    There is no such thing as a wave or a particle. They are mathematical constructs. Even a body of water doesn't implicitly 'know' it's undergoing wave motion. There shouldn't be any paradoxical confusion because there were never two objects called wave and particle for the quantum to be split between. They are not coherent physical concepts in themselves. A quantum isn't a wave and a particle, or partly wave, partly particle, it's a quantum. You wouldn't need either of the former to fully describe and understand the latter.
  15. Jun 25, 2007 #14
    Thanks for your replies.

    Yeah, sorry I forgot about this principle. So what you mean to say is that QED superpositions both wave model and particle model. The experiment in which the amplitude is favourable to wave nature, it has the highest probability of displaying wave-nature of electron/light, whereas if the amplitude is favourable to particle nature, it has the highest probability of displaying particle nature of electron. Right?
    So it really depends on the type of experiment whether we will observe wave nature or particle nature.

    What does that mean? I mean: suppose there are two slits. The electron has some amplitude to pass through the first slit, and it has some amplitude to pass through the second one. But how can there be an amplitude that it passes through both the slits? Basically, how can a particle be present in two places at once (if we completly ignore the wave model of electron here)? It would be nice of you to explain it a bit more.

    This is what I thought previously. But it is a proven fact that "a matter wave is actually the matter itself".

    That is what I was saying- that though we have all the maths for quantum mechanics, we do not have a concrete theory about why things are the way they are. Even QED just says that a particle is a wave, and a wave is a bunch of particles, but why? How? No answer for that.

    Agreed. But can you explain how that wave is formed with the help of these photons? I mean frequency/wavelength represent the oscillations of the wave, but where are these oscillations taking place in the photon stream? A wave is a continuous oscillating thing, but photons are separate from each other- there are gaps between them.

    But it is shown in the Double Slit experiment that when a single photon is fired, it passes through both slits and interferes with itself. It is easily explained using wave theorem, but not so with the particle model.

    Could you please explain this phenomenon more elaborately. I mean what happens to photons during destructive and constructive interference? What do you mean by "absorption" of photons.

    Mr V
  16. Jun 25, 2007 #15
    I read this in Wikipedia

    There is no quantum world. There is only an abstract physical description. It is wrong to think that the task of physics is to find out how nature is. Physics concerns what we can say about nature

    Wow! What an illuminating statement. Does every physicist think like that- keep on finding mathematical answers without having a clue as to what it practically implies, or how it happens in reality?
  17. Jun 25, 2007 #16
    A particle can actually, physically be in 2 places at once. This has recently been shown in an experiment (I'll post a link to the paper when I can find it).
    However, bear in mind that it's not the electron but the wavefunction for the electron (or rather, part of the wavefunction) which passes through both slits at the same time, just as it is part of the wavefunction which goes through one slit or another and also neither. You get bogged down in paradoxes when you start thinking of a physical object doing this. This is just what the maths tells us happens. No-one has ever seen a wavefunction or can assert that one actually exists but it is the best description we have for now.
    Even if you think of the electron as a wave this doesn't make sense - a physical wave cannot go through both slits and only one and neither at the same time. The wavefunction does, at least mathematically.
  18. Jun 25, 2007 #17
    Really? I am anxiously waiting for the link.

    Oh please don't disappoint me. Is it the particle or the wavefunction? Even if it is a wavefunction, how does there exist an amplitude that the electron (a particle) can pass through both the slits at once?
    Last edited: Jun 25, 2007
  19. Jun 25, 2007 #18


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    I can come in and re-edit that Wikipedia entry to be something else. Now what?

    Please do NOT pay that much emphasis on stuff you read on Wikipedia. I have a hard time already trying to correct the misconception that people get from legitimate sources. I do not need the added burden of correcting information from dubious sources.

    There's a whole slew of experiments that directly tests and measure the phenomenon of superposition. That is what I have been trying to impress upon you, and it is something you seem to have missed. The bonding-antibonding states of H2 molecule is something very familiar in chemistry, and that can only be explained by the superposition of the electron's position between the two H nuclei. Furthermore, there's a whole slew of experiments, namely the Stony Brook and Delft experiments that deal with this. Do a search on here.

    Last edited: Jun 25, 2007
  20. Jun 25, 2007 #19
    Oh please do that, so that it does not mislead anyone else.
  21. Jun 25, 2007 #20
    Yeah, I am just gonna do that. After that I will tell you whether I understood or not.
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