Thread: Phonon particles

I've been doing some reading recently, but I can't quite wrap my head around phonons as particles. I do accept the duality of waves and particles, even if I don't fully understand it, but I can't quite see what properties of a phonon are particle-like. Also, would the same be true for the vibrations of a string or sound waves in the air?

Along the same lines, I've read that if a black hole were approaching Earth, you would be able to hear it, as the gravitational waves would stretch and contract your earbones. How would wave/particle duality apply to those waves?

its photon not phonon.

Originally Posted by shawngoldw

its photon not phonon.

Actually, there is such a thing as a phonon. MagiMaster, which are you asking about?

Cheers

wave-particle duality was mentioned, I figured he meant photon.

Originally Posted by MagiMaster

I've been doing some reading recently, but I can't quite wrap my head around phonons as particles. I do accept the duality of waves and particles, even if I don't fully understand it, but I can't quite see what properties of a phonon are particle-like. Also, would the same be true for the vibrations of a string or sound waves in the air?

Along the same lines, I've read that if a black hole were approaching Earth, you would be able to hear it, as the gravitational waves would stretch and contract your earbones. How would wave/particle duality apply to those waves?

I think it's pretty clear that he's talking about phonons.

ok, my bad

Yeah, he's talking about phonons, which are the acoustic analogs of photons.

Nobody deals much with phonons because, on a practical level, it is so much more convenient to deal with sound as a wave rather than as a quantized particle.

clearly I'm not the right person... but it sounds like one of you can answer his question!

Yeah, I meant phonon. I think Wikipedia gave a definition along the lines of vibrations in a crystal. My question was, not really can you look at them as a particle, but how to do so. What particle-like properties do they (or any macroscopic(?) waves) have?

It is clear you are making the standard beginner's mistake of trying to imagine a phonon as a some sort of classical particle... a marble or tiny ball bearing that is subject to the classical laws of physics. Such an approach will only lead you to grief.

A phonon cannot have particle-like properties in the classical sense. It is a useful concept only when certain properties of matter are analyzed using quantum methods. Then vibrational energy is quantized and the fundamental unit is called a phonon.

This treatment is used only when we get down to the atomic level. For macroscopic waves, e.g. sound or water waves, we stick to wave mechanics, strictly classical. No phonons, please.

Oh ok. So a phonon's particle-like property is the quantization of it's energy, making a phonon the carrier of a quantum of vibrational energy in certain situations?

Yeah, that's about it... a quantum of energy going from one atom to another in a crystal. It can be a useful concept in certain heavy-duty mathematical treatments.

Thanks. That's quite a bit clearer now, although it still leaves some questions about some of the other psuedo-particles. (I guess most of them are used for the same general purpose though.)

Like photons, the "particle-like" nature of phonons extends a little beyond being quanta of energy. They also have momentum and the the interaction between photons and phonons (Brillouin scattering) is subject to the conservation of momentum in much the same way as the collision of particles is. However, attempts to visualise either a photon or a phonon as being like a very small golf ball don't lead to anything useful.