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It is a thought Experiment.

Imagine a box that is lined on the inside with 100% perfect mirrors. Shine some light into that box and seal the box. The light will bounce back and forth between the mirrored walls forever, exerting a force on the walls. So it has pressure.And since it has pressure, changes in pressure can propagate. So by this

Can sound be generated?

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    $\begingroup$ If the mirrors vibrate in and out, this will cause a small modulation in the frequency or color of the light, but that is not sound. $\endgroup$ – Guy Inchbald Mar 24 at 12:01
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Your question mixes together light/matter interactions and light/light interactions. The pressure you refer to is light/matter interactions. Sound propagation in your context would refer to light/light interactions. With that context set, I can see a few ways of answering the question, depending on which conditions are allowed.

One way of interpreting the question is "if the box contains photons, and I vibrate one of the walls, can I observe a consequence of the vibration at the other wall". The answer here is "yes". If you add or remove momentum to photons incoming at a wall by moving the wall, these photons will exert a corresponding reduced or increased pressure on the opposite wall, which could be measured. However, it is the same photons, under low energy conditions in vacuum, that propagate the information. This is in opposition to what we usually mean by "sound", in which, for example, the molecules that vibrate in your ears for you to hear are not the molecules that have been vibrated by a speaker. The momentum of the initially vibrated molecules is transmitted from molecule to molecule until they reach your ear.

A second way of interpreting the question is "if the box contains photons, and I vibrate one of the walls, will I influence photons other than the ones I directly touched with the wall". At low energies, photon/photon interactions in vacuum are practically non-existent. You would need very energetic photons to create any significant interactions. This has been mentioned in the other answers.

A variant of the second way of thinking would be to allow matter in the box. Indeed, one way to increase photon-photon interactions is to add some specific non-linear materials, which couple photons to each other. This is the basis for the field of non-linear optics. In that case, under specific conditions, you could transmit information through photons inside the material that would propagate by photon/photon interactions, but the photons interact with the help of the atoms in the material. A very technical paper that describes these phenomena in your context can be found at https://arxiv.org/pdf/1205.6500.pdf.

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"Pressure" just means force per unit area. Changes in pressure don't propagate. What can propagate through the body of some material substance is changes in its internal pressure--the cumulative forces exerted by the atoms or molecules of the substance on each other.

There is no body of material substance in your thought experiment. The photons of light do not interact with each other. They only exert pressure on the walls. There is no internal pressure in your box of light that can act as a medium for sound waves.

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  • $\begingroup$ but photons can interact i.e en.wikipedia.org/wiki/Two-photon_physics and light in that case acts like a "ULTRA RELATIVISTIC GAS" $\endgroup$ – Annamalai Sriram Mar 24 at 12:10
  • $\begingroup$ @AnnamalaiSriram, Very rarely. Since you'll never be able to perform the experiment in reality (no such perfect mirrors exist), then it's up to you how much light you're going to contain in the box. If it's enough for photon-photon scattering to become a significant effect, then what else might happen in there is beyond my ken. $\endgroup$ – Solomon Slow Mar 24 at 12:16
  • $\begingroup$ How do changes in pressure NOT propagate? In every acoustics text the definition of the acoustic field is a change in pressure. I am confused by the wording perhaps. $\endgroup$ – ggcg Mar 24 at 13:43
  • $\begingroup$ Changes in pressure do propagate. $\endgroup$ – Annamalai Sriram Mar 24 at 13:47
  • $\begingroup$ @ggcg, Changes in the internal pressure of a physical medium can propagate through the medium. I said, "Changes in pressure do not propagate," because "pressure" does not mean the same thing as "internal pressure of a physical medium." The word, "pressure," all by itself denotes a much more general concept. $\endgroup$ – Solomon Slow Mar 24 at 14:25
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There is a difference between creating pressure due to force and creating a disturbance in a medium with bulk properties such as pressure, temperature, etc. You have not really asked a well posed question and based on your comments to existing answers it seems like you just want to pull one magic rabbit after another out of your hat.

Acoustics is not a "pressure" or "change in pressure". It is a disturbance in a medium. A bulk medium has a micro structure and a macro description, i.e. a set of macro variables that describe the state of the system as a while without the need to track every particle, though the statistical properties of those particles contributes to the value of the macro variables. Even phonons, a quanta of lattice vibration, are disturbances in a material whose elements are otherwise bound.

Your original post made no mention of whether the light box was in a vacuum or an ordinary atmosphere and made no attempt to connect the process of photons bouncing around to any material disturbance. So we are all left to guess what you meant.

It is possible for photons to excite acoustic vibrations in a medium. This happens all the time in solid state physics. So, if by acoustics you mean can changes in radiation induced pressure cause changes in the vibration motion of the material in the chamber then yes I'd say it is definitely possible.

But now you are reaching for this gamma-gamma coupling that is the result of some type of higher order process in QED, or an anomalous coupling outside the standard model. If gamma-gamma coupling can create an effective photonic fluid or medium then you would need to derive the bulk properties of such a medium to make any claim that acoustic vibrations within it are possible. Keep in mind that this "coupling" is described as a scattering process via the S matrix, and Feynman diagrams. This does NOT in any way make a photonic medium possible. In order to create a material that would support vibrations one need to start generating bound states, like molecular bonds. The process you mention in your comments does not provide any means to create such a static bound state of bound photons. The gamma particles are still moving fast. How would you describe "acoustics" in that medium?

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A beam of light can be modulated to carry an audio (or video) signal (sometimes in digital format as in fiber optic communication).

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  • $\begingroup$ Yes Mr RW Bird it is also an alternative approach $\endgroup$ – Annamalai Sriram Mar 24 at 15:12
  • $\begingroup$ @AnnamalaiSriram, how does this answer your original question? You seemed to be asking about sound traveling through or being created by radiation pressure and this is nothing more that a comment about encrypting data in amp, freq, or phase modulation. That technique does not make use of anything in your post. $\endgroup$ – ggcg Mar 26 at 14:51
  • $\begingroup$ Sorry To all Answerers I am new to stack and I didn't know that I can only select one best answer as applicable . I thought I can select all which I like $\endgroup$ – Annamalai Sriram Mar 26 at 16:29

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