What Is a Phonon?

Soviet physicist Igor Tamm accepts his portion of 1958 Nobel Prize in Physics
Russian physicist Igor Yevgenyevich Tamm receives his share of the Nobel Prize for Physics. Keystone/Hulton Archive/Getty Images

A phonon is a quantum unit of sound. It is the lowest energy, quantized state of a vibration within a lattice structure, such as those created by the atoms of matter. A phonon is the sound analog to the photon.

The name phonon comes from the behavior of long-wavelength phonons, which give rise to sound at higher energy levels. In quantum physics, the principle of wave-particle duality indicates that behavior which is normally treated as a wave (such as sound) can also be treated as a quantized particle: the phonon.

The name phonon comes from the Greek word "phone," which translates into "sound" or "voice."

Though it is treated as if it were a particle, phonons don't exist as independent particles, but rather as a state of behavior within a system that can be thought of as a particle in its own right. For this reason, sometimes physicists refer to emergent phenomena like phonons as a "quasiparticle" or a "collective excitation."

Applications of Phonons

Within a material, the electrical properties of materials are determined by the arrangement of electrons. The phonons of the material, on the other hand, determine things such as its thermal properties, the speed of sound through the materials, and any other properties that have to do with the way the material interacts with vibrational or kinetic energy.

Though this seems pretty abstract, the fact is that the concept of phonons potentially has applications in detectors such as the Cryogenic Dark Matter Search (CDMS), which attempt to detect even a single phonon worth of vibration within its crystal lattice.

Because the CDMS (both the detectors and the air surrounding the detectors) is cooled to such a low temperature, the particles within the detector do not absorb vibrational energy from the surrounding environment. This reduces the lattice grid of the detector into the lowest energy quantum state, and any disturbance - even that of a single phonon - becomes detectable.

This means that even dark matter, which normally interacts only very rarely with ordinary matter, may end up being detected.

History of the Phonon

The concept of the phonon is attributed to work by the Soviet physicist Igor Tamm, who later went on to receive 1958 Nobel Prize in Physics for his work in discovering the Cherenkov effect and Cherenkov radiation.

A 2000 Russia stamp honoring Igor Tamm credited him with the "Idea of phonons, 1929," although several other online sources indicate different years for when he developed the idea. However, a fascinating series of letters posted online by CERN, between Igor Tamm and Paul Dirac in the 1930's, indicate (p. 33, note 7) that Tamm "introduced the notion of quanta of elastic oscillations later called 'phonons'" in a 1930 letter to Dirac. This adds credibility to the 1929 date from the Russia stamp, as it's likely he would have mulled the concept over before discussing it with his European colleague.

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Jones, Andrew Zimmerman. "What Is a Phonon?" ThoughtCo, Nov. 17, 2017, thoughtco.com/phonon-2699277. Jones, Andrew Zimmerman. (2017, November 17). What Is a Phonon? Retrieved from https://www.thoughtco.com/phonon-2699277 Jones, Andrew Zimmerman. "What Is a Phonon?" ThoughtCo. https://www.thoughtco.com/phonon-2699277 (accessed November 20, 2017).