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Reading about dual nature of light, and atomic transitions, it seems to me, maybe wrongly, that the dual nature depends on the way we look at the phenomena.

Suppose a water wave travels and reach another water source. Why can't we talk about a condition like:

$$\Delta E=h\nu$$

where $\nu$ is the frequency of water molecules impacts? Another point of view should be the interference of waves.

Isn't talking about light dualism quite the same? When a photon interacts with an electron, is must have an exact frequency, but also it might be understood as a wave interference (if I'm not mistaken).

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    Search terms to try: phonons, plasmons. – rob May 27 '18 at 19:25
  • Similar, if not duplicate: https://physics.stackexchange.com/q/254088/ – Rococo May 27 '18 at 19:26
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    Possible duplicate of Do gases have phonons? – Emilio Pisanty May 27 '18 at 19:27
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    @santimirandarp : Simply put, phonons are the names for a single quantized unit of a sound-like wave in a liquid, solid, or gas. So they are directly the answer to your question. I'm sure you can search for the term to find much more information about them. – Rococo May 27 '18 at 19:34
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    " I am asking if sound can be defined as a particle." Anyone who answers you with "Yes" or "No" will be both right and wrong. The framework of phonons in the answer and it is sometime useful and reasonable and other times not so much (mostly not in low density gasses). Not every question has a simple answer. – dmckee --- ex-moderator kitten May 27 '18 at 19:45
  • With regards to solids (in particular crystals), phonons are already pretty accepted conceptually and are quite interesting to study because there are cases where they can be considered to scatter with other particles like electrons. With regards to fluids, I remember reading about a study that looked at how acoustic pulses interact with dumb-holes (is to sound what a black-hole is to light) formed by having a vortex with a flow rate comparable to the speed of sound in the fluid. I am not an expert on this but it may be along the lines of what you are looking for. – Jepsilon May 27 '18 at 19:54
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    https://en.wikipedia.org/wiki/Sonic_black_hole Just adding this to link to my previous comment should you be interested – Jepsilon May 27 '18 at 19:59
  • @santimirandarp I'm not sure I completely understand what you are asking, but one thing I can say is that just because some object has a resonance (like a bridge's mechanical resonance) need not mean it has a quantum description nor in particular that $\Delta E=h\nu$ applies to it. – Rococo May 28 '18 at 04:00
  • As dmckee has said, the question of which systems this can be applied to is not necessarily a simple one, which is one reason that if you go through to the linked question there is not a complete consensus there. – Rococo May 28 '18 at 04:02
  • We don't keep revision histories on Stack Exchange: anybody who's interested in older versions of the question can view the history with the publicly-visible link under your question which points to this. For more information, please see this meta post: Let's not have posts look like revision histories –  Sep 30 '18 at 06:20

2 Answers2

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A classical wave emerges in a medium of a huge number of molecules , we cannot identify individual molecules with the frequency of the wave, the molecules are the "coordinates" on which the energy and momentum of the wave motion can be mathematically modeled, by their small motions up or down (transverse wave), left or right (longitudinal wave) as the wave passes. It is like a wave in a stadium, the frequency has a very wide spectrum, depending on how fast the people can respond.

A photon and an electron are point particles, i e. have a definite position $(x,y,z,t)$ in the model that describes their interactions. When a photon interacts with an electron, its wave nature is not in energy, but in probability of interaction. It is the probability of interaction that is modeled as a wave, for the point particles which are the electron and the photon.

The classical wave in water, and does not behave as a particle, i.e. it is not defined by an $(x,y,z,t)$ point isolated in space, which is the definition of a point particle. One could extend the definition by allowing a $Δ(χ)$ ... In this extended case yes, there can be wave packets called solitons in water, which do behave as a particle carrying energy and momentum , in this case a one dimensional one. This is also interesting.

anna v
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  • Thanks for the update. I don't think the first paragraph is correct, or possibly I didnt understand it. A pressure wave can be related with movement of gas particles, isn't it? –  May 28 '18 at 05:53
  • Yes, there is energy transfer through motion, but it does not have the frequency that could identify a wave with the molecule. the molecule just moves back and forth or left and right, the energy moves in space over zillions of molecules in the wave direction. – anna v May 28 '18 at 05:59
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One might think that a particle such as a phonon, unable to form structures with other such particles, might not be considered a particle; but several elementary particles also do not form "structures," existing only as "force mediation" and also never themselves at rest. The term "particle" is not used consistently in different areas of physics, so phonons and solitons may reasonably be called particles, but I prefer to be more clear when I speak, and just call them phonons.

Ronald C Alexander
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