Do Phonons Have Momentum and Rest Mass?

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In summary, phonons do not have rest mass and are considered massless particles. Their dispersion relation shows that they have wave vector momentum rather than traditional momentum. Antiphonons, or phonons with wave number -k, exist in equal numbers in ordinary materials. Phonons are Goldstone bosons of broken symmetries and have much lower speed compared to photons, resulting in significantly higher momentum. In semiconductors, phonon processes are called "horizontal" due to their almost negligible energies.
  • #1
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do phonons have (significant) momentum? do they have rest mass?
 
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  • #2
define "significant".

No photons have no rest mass, they are massless.
 
  • #3
phonons. not photons.

significant as in not trivial. (light carries a trivial amount of momentum)
 
  • #4
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.
 
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  • #5
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?
 
  • #6
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?
 
  • #7
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.
 

What is a Phonon?

A phonon is a quantum of vibrational energy in a crystal lattice. It is the smallest unit of vibrational energy that can exist in a crystal lattice and is considered to be a collective excitation of the atoms or molecules in the lattice.

What is Momentum?

Momentum is a physical quantity that measures the motion of an object. It is defined as the product of an object's mass and its velocity. In other words, it is the amount of force needed to stop or change the direction of an object's motion.

What is Rest Mass?

Rest mass is the mass of an object when it is at rest. It is a fundamental property of matter and is independent of an object's speed or motion. The rest mass of an object is also known as its invariant mass.

How are Phonons related to Momentum?

Phonons are related to momentum through the concept of wave-particle duality. Just as particles can exhibit wave-like behavior, phonons can also have a momentum associated with them. In fact, the momentum of a phonon is directly related to its wavelength and frequency.

Can Phonons have Rest Mass?

Yes, phonons can have rest mass. This is because phonons, being a quantum of vibrational energy, can have a small amount of mass associated with them. However, their rest mass is generally very small, on the order of 10^-36 kg, and is often neglected in calculations.

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