This is a SOUND PARTICLE - Phonon and Quasiparticle Physics Explained by Parth G

We know that light behaves as a wave AND a particle... but can we treat sound in exactly the same way? And what about this exciting new particle known as the "dance particle"?

Hey everyone, in this video I wanted to discuss how physicists like to take complicated physics phenomena and model them as atoms in order to hugely simplify the mathematics used to solve problems. The main example I'll be discussing is the treatment of sound waves as particles known as phonons. Now technically, these particles are QUASIparticles because they're not real, and are just a mathematical reimagination of the sound wave. This is slightly different to light, because light particles (photons) DO actually exist. We have plenty of evidence to show us that the wave model of light is not enough to explain certain things we observe, such as the photoelectric effect, black body radiation (ultraviolet catastrophe) and so on. In fact, light needs both the wave model and the particle model to explain all the phenomena we observe. However a sound quasiparticle (phonon) is not like this - it's a mathematical trick.

The reason we work with quasiparticles at all, becomes very evident when we consider the reality of a sound wave moving through a solid, for example. The sound wave is formed by the oscillation of atoms in the solid. The bonds between atoms allow energy to be transferred from one region in the solid to another, and as soon as the disturbance has passed, the atoms return to their original positions. Now if we want to study how sound waves move through the solid in any detail, then we have to consider the position of each affected atom, as well as the strength of the bonds between atoms. We can treat these bonds like springs, though this is a big simplification. But even then we have to consider the "stiffness" of the springs, which is dependent on the type of atoms forming the bond. And then we may have to think about the disturbances to each atom at every instant in time, if there are multiple sound waves moving through the solid in different directions. Things become complicated very quickly. Instead, if we treat the atoms in the solid as a "background", and the sound wave as a sound particle, then we only need to consider the phonon representing each wave rather than the (likely) tens of thousands of atoms in the solid, and how their positions change over time.

Another example we will discuss is when electrons in a solid can escape from their energy levels, leaving behind a vacancy. This vacancy can be filled with nearby electrons, making it seem as if the vacancy is moving around within the solid. This is treated as its own quasiparticle, known as a "hole", and is much easier to deal with than considering all the electrons filling any nearby vacancy.

More information about the electron hole quasiparticle: https://en.wikipedia.org/wiki/Electro...

Timestamps:
0:00 - The DANCE particle + how physicists work with quasiparticles
0:38 - How we deal with light - waves and particles (photons)
1:06 - Sound waves: oscillations in air (+ other gases liquids and solids)
1:59 - Sound wave in a solid: atomic structure and bonds transmit energy
3:53 - Treating sound waves as particles (phonons) - quasiparticles
5:37 - Why phonons are useful (multiple sound waves and phonon-phonon interactions)
6:15 - Electron hole quasiparticles (vacancy vs electron motion)

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