Do Phonons Have Momentum and Rest Mass?

[granpa]

do phonons have (significant) momentum? do they have rest mass?

 

[malawi_glenn]

define "significant".

No photons have no rest mass, they are massless.

 

[granpa]

phonons. not photons.

significant as in not trivial. (light carries a trivial amount of momentum)

 

[malawi_glenn]

ok sorry

The momentum of phonons should be related to their dispersion relation yes? But one should not say that they have momentum, they have wave vector. But I guess that this falls into your definition of "trivial"

http://www.ece.rutgers.edu/~maparker/classes/581-chapters/Ch06-Structure-Phonons/Ch06Sec10XalMomentum.pdf

Their dispersion relation shows that they are massless.

 

[granpa]

thank you. I'll have to digest that for a while. its mostly over my head. but it answers my question.

do phonons and antiphonons exist in equal numbers in ordinary materials?

 

[malawi_glenn]

define antiphonon, you mean a phonon that has wave number -k is the antiphonon to phonon with wave number k?

maybe you just should pick up a textbook on solid state physics, e.g Kitell?

 

[genneth]

Acoustic phonons do not have mass; that's guaranteed by them being the Goldstone bosons of broken symmetries. Compared with photons, they have much lower speed, so for the same energy, they have vastly greater momentum. In semiconductors, electronic transitions via pure photon processes are called "vertical" because the photon momentum is negligible, whilst phonon processes are called "horizontal" because they're energies are almost negligible.