The Photons and Phonons

The photon is a type of elementary particle. It is the quantum of the electromagnetic field including electromagnetic radiation such as light and radio waves, and the force carrier for the electromagnetic force. Photons are massless, so they always move at the speed of light in vacuum, 299792458 m/s (or about 186,282 mi/s). The photon belongs to the class of bosons. 

Like all elementary particles, photons are currently best explained by quantum mechanics and exhibit wave–particle duality, their behavior featuring properties of both waves and particles. The modern photon concept originated during the first two decades of the 20th century with the work of Albert Einstein, who built upon the research of Max Planck. While trying to explain how matter and electromagnetic radiation could be in thermal equilibrium with one another, Planck proposed that the energy stored within a material object should be regarded as composed of an integer number of discrete, equal-sized parts. To explain the photoelectric effect, Einstein introduced the idea that light itself is made of discrete units of energy. In 1926, Gilbert N. Lewis popularized the term photon for these energy units. Subsequently, many other experiments validated Einstein's approach. 

In the Standard Model of particle physics, photons and other elementary particles are described as a necessary consequence of physical laws having a certain symmetry at every point in spacetime. The intrinsic properties of particles, such as charge, mass, and spin, are determined by this gauge symmetry. The photon concept has led to momentous advances in experimental and theoretical physics, including lasers, Bose–Einstein condensation, quantum field theory, and the probabilistic interpretation of quantum mechanics. It has been applied to photochemistry, high-resolution microscopy, and measurements of molecular distances. Recently, photons have been studied as elements of quantum computers, and for applications in optical imaging and optical communication such as quantum cryptography.

- Phonons

Phonons are quantized vibrational modes that occur in a rigid lattice, such as that of atoms in a solid. The study of phonons is an important part of solid-state physics because phonons play an important role in many physical properties of solids, such as thermal and electrical conductivity. In particular, the property of long-wavelength phonons produces sound in solids—hence the name phonons. In insulating solids, phonons are also the main mechanism by which heat conduction occurs. 

Phonons are the quantum mechanical version of a special type of vibrational motion, known in classical mechanics as a normal mode, in which every part of the crystal lattice oscillates at the same frequency. These normal modes are important because, according to well-known results in classical mechanics, any vibratory motion of a lattice can be viewed as a superposition of normal modes with different frequencies; in this sense, the normal modes are the fundamental vibrations of the lattice. While normal modes are wave-like phenomena in classical mechanics, they acquire certain particle-like properties when a lattice is analyzed using quantum mechanics. They are called phonons. Phonons are bosons with zero spin.

According to quantum mechanics, microscopic vibrations (sound waves) in a solid medium are quantized. This means that vibrational energy can only be exchanged in the form of so-called phonons, whose energy is Planck's constant h times the phonon frequency.

Phonons are important to the physics of infrared optics and solid-state lasers. There are different kinds of vibrational modes, involving very different frequencies and phonon energies:

• Acoustic phonons are associated with long-wavelength vibrations in which neighboring particles oscillate nearly in phase. Their frequencies are relatively low, for example in the gigahertz region.
• Optical phonons are associated with vibrations in which neighboring particles oscillate almost in antiphase. Optical phonons have frequencies in the terahertz region (resulting in much higher phonon energies than acoustic phonons), and in ionic crystals or glasses they can participate in the absorption of infrared light. Note that due to the opposite charge of neighboring ions, this vibration can couple to the electromagnetic field through their electric dipole moment.

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