Do sound waves carry momentum?

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Discussion Overview

The discussion revolves around whether sound waves carry momentum, exploring theoretical and practical implications. Participants examine the nature of sound waves, their interaction with materials, and comparisons to light waves, including concepts from classical and quantum physics.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested
  • Mathematical reasoning

Main Points Raised

  • Some participants propose that sound waves carry momentum due to the movement of vibrating molecules, suggesting that every molecule contributes to the overall momentum.
  • Others argue that sound waves, while they may carry energy, do not carry momentum in the same way light does, questioning the applicability of momentum concepts from light to sound.
  • A participant mentions that sound waves in a crystal lattice can be quantized as "phonons," which have momentum, although they note that the total momentum of phonons typically cancels out.
  • Some contributions highlight that acoustic waves can exert force, as seen in phenomena like shock waves, indicating a practical manifestation of momentum.
  • There is a discussion about the wave-particle duality of sound and light, with some participants asserting that both types of waves exhibit similar properties.
  • One participant expresses skepticism about the completeness of current understanding regarding the properties of light and its medium, referencing historical concepts like the luminiferous aether.

Areas of Agreement / Disagreement

Participants do not reach a consensus on whether sound waves carry momentum. Multiple competing views are presented, with some asserting that they do and others questioning this notion.

Contextual Notes

There are unresolved questions regarding the classical versus quantum mechanical treatment of sound waves and their momentum, as well as the implications of comparing sound to light waves.

granpa
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do sound waves carry momentum? any momentum at all (no matter how small)?

the obvious answer is that they carry energy but not momentum but i am wondering whether they don't theoretically carry some very tiny (usually negligible) amount of momentum in the same way that light, even though it can normally be thought of as being massless, theoretically does have some very tiny amount of mass.
 
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sound waves are vibrating molecules, and everyone of these molecules move and thus have a momentum.

Hey I got an idea, if we can get a formula for momentum for sound, ie vibrating particles, then we can use it the other way round to determine the properties of the "medium" that light eventually travels through, right? ;)
 
Kurret said:
sound waves are vibrating molecules, and everyone of these molecules move and thus have a momentum.

Hey I got an idea, if we can get a formula for momentum for sound, ie vibrating particles, then we can use it the other way round to determine the properties of the "medium" that light eventually travels through, right? ;)

Wrong, light does not propagate in the same manner as sound. The properties of materials which effect light transmission are pretty well understood.
 
Okay so do sound waves have momentum or not? I believe they do...as do light waves.

I know sound waves move a lot like a wave and light as a wave function consisting of particles (photons) thus having a wave-particle duality.
 
Uhm... sound waves in a crystal lattice are quantized and can be seen as "phonons" which definitely have a momentum (equal to hk, if I remember correctly).

However I seem to have some problem in seeing the momentum of an acoustic wave with a classical approach. :/ There are certainly cases where an acoustic wave can "push" an object, for example in the case of a "shock wave".
 
or better yet, bass frequencies...

you can "feel" bass
 
Integral said:
Wrong, light does not propagate in the same manner as sound. The properties of materials which effect light transmission are pretty well understood.
It was more like an idea IF there is a medium that light travels through (not any "material"), but more like the luminiferous aether that people thought existed a while ago. Do we really know everything about the properties of light? I doubt that.
 
Domenicaccio said:
Uhm... sound waves in a crystal lattice are quantized and can be seen as "phonons" which definitely have a momentum (equal to hk, if I remember correctly).

Phonons carry pseudomomentum. Summed up, the momentum of all phonons usually cancels to 0. Otherwise the center of mass of the crystal would have to move.
 
Sound waves have momentum and dispersion, a phase and a group velocity. That's one of the reasons why gongs, when struck with a soft mallet, have long attacks.
 
  • #10
Domenicaccio said:
Uhm... sound waves in a crystal lattice are quantized and can be seen as "phonons" which definitely have a momentum (equal to hk, if I remember correctly).

However I seem to have some problem in seeing the momentum of an acoustic wave with a classical approach. :/ There are certainly cases where an acoustic wave can "push" an object, for example in the case of a "shock wave".


same here.
 
  • #11
NYSportsguy said:
I know sound waves move a lot like a wave and light as a wave function consisting of particles (photons) thus having a wave-particle duality.

sounds pretty dumb to say sound waves move a lot like a wave when sounds waves are in fact waves... why do you think they call them sound WAVES haha... of course this coming from a yankee fan... haha just bustin your balls
 

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