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Are sound waves “pure waves”? Or does sound also have a particle nature according to wave-particle duality?

Stéphane Rollandin
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4 Answers4

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Sound waves in air do not exhibit wave/particle duality. They exhibit no particle-like behavior.

niels nielsen
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In a solid sound waves are just coherent lattice vibrations, and lattice vibrations are quantised like any other oscillator. The result is that in solids we get pseudoparticles called phonons. In this sense sound waves have a wave/particle duality just like matter waves.

I'm not sure how useful the concept of phonons is for liquids because liquids don't have long range order and don't have the well defined vibrational modes that a crystal lattice has. I am pretty sure that phonons are not observed in gases because they don't have even the short range order that liquids enjoy, so as Niels says in his answer sound waves in air have no particle nature.

So whether sound waves have a wave particle duality depends on the medium through which they are propagating.

Chris
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John Rennie
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Sound waves propagate as pressure anomalies in a medium and are the result of many particles interacting with each other. There is no quantum mechanical aspect of sound waves and no particle-like properties in the sense of a wave-particle duality.

EDIT: After some comments and other answers it seems that my answer is at best incomplete when solid media are involved. For example, the comparison of the acoustic phonon modes with Newtonian mechanics does neglect many QM effects. Anyway, I didn't change my original answer, so the comments still make sense.

Metalbeard
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    You might want to consider exactly what a "phonon" is. – dmckee --- ex-moderator kitten Mar 01 '18 at 00:09
  • Phonons are related to particle interactions that lead to e.g. thermal conductivity. Sound propagation does not "care" for the random vibrations of individual particles. Is this distinction not correct? – Metalbeard Mar 01 '18 at 00:32
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    Uhm. Not really. Sound propogation in solids is often described in terms of phonons. Because in a lattice sound is fairly coherent and describing is as "random" is rather a stretch. – dmckee --- ex-moderator kitten Mar 01 '18 at 01:47
  • @dmckee, are phonons defined if the transmissive medium is air? – niels nielsen Mar 01 '18 at 05:18
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    @nielsnielsen At this point I am rapidly getting out of my depth. I suspect that one can define the idea, but that it isn't used in such a context because they won't retain the necessary coherence. – dmckee --- ex-moderator kitten Mar 01 '18 at 06:41
  • @dmckee The acoustical modes of phonons are described in terms of Newtonian mechanics (masses connected with springs, group velocity etc.), so wrt the above question we can say that the resulting sound waves do not have any properties or cause any effects that can only be explained with them being particles..., can't we? – Metalbeard Mar 01 '18 at 06:43
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    "The acoustical modes of phonons are described in terms of Newtonian mechanics" - only if you ignore all the substantial literature that treats phonons on a fully quantum mechanical basis. The experiments might be harder (and maybe fatally so in disordered media) but phonons are formally identical to photons as far as the theory goes. – Emilio Pisanty Mar 01 '18 at 08:14
  • Ok, to conclude this: There IS a duality here (at least for solid media) and my answer is wrong. That expands my horizon quite a bit. – Metalbeard Mar 01 '18 at 08:44
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Are sound waves “pure waves”? Or sound also has a particle nature according to wave-particle duality?

Waves are described by solutions of wave equations, mainly sinusoidal ones.

There are classical waves where the amplitude of the functions is related to a propagation of energy in a medium, as sound and water waves, and electromagnetic waves which do not need a medium to propagate the energy in space. The mathematics allows to build up wavepackets, from waves.

wavepacket

Have a look at this soliton in a tank of water..Wave packets in classical waves, like this soliton in the water, have some characteristics of particles, i.e. are localized and carry momentum and energy.

Now wave particle duality is a quantum mechanical issue, not classical. The waves in quantum mechanics are not energy amplitude waves. The amplitude of the quantum mechanical wave is related to the probability of detection/scattering/decaying... of "particles" as described in quantum mechanics. It is called a duality because there is a probability distribution which is wave like, shows the sinusoidal interferences, as seen clearly in the double slit experiment of single electrons at a time, where both aspects of the electron are visible.

enter image description here

The accumulation of individual electron footprints, a quantum mechanical probability distribution for the experiment "electron scattering on two slits of specific width and distance apart", shows the interference pattern expected from a wave solution. The single points in the detecting screen have a specific (x,y,z) denoting the impact of a particle. This is the wave particle duality.

It is evident that this probability interpretation does not exist in the classical waves, solitons or not.

On the other hand, the wavepacket solutions are the mathematical tool which allows the description of particles in quantum field theory, which uses the probabilistic wave theory, as wavepackets in space and time. In this formulation the point particles of the standard model, as the electron etc, are a theoretical base represented by sinusoidal solutions, and the real interacting particles are wavepackets in space time built up by these elementary particle probability waves.

So your question can be reversed, wave-particle duality can be described by the use of soliton type solutions, the mathematics similar to classical wave equations.

One should always have in mind that it is the probability that is waving in quantum mechanics, the amplitude corresponds to a probability and not to the energy/mass of a particle.

anna v
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