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In physics, a photon is a quantum of excitation of the quantised electromagnetic field. It is considered one of the elementary particles of the Standard Model.

It is usually given the symbol γ (gamma) although in high energy physics this refers to high energy photons (photons of the immediately lower energy branch for instance are noted X and called X rays).

Photons are often loosely associated to light, to which they relate only for a very narrow frequency window of the spectrum. Even there, light is commonly encountered in quantum states which are not pure photons but superpositions of different numbers of photons, to wit, either coherent superpositions (so-called coherent states) describing coherent light such as the one emitted by an ideal laser, or chaotic superpositions (so-called thermal states) describing light in thermal equilibrium (blackbody radiation). Special devices like micromasers can create pure photon type of light. The associated quantum state is the Fock state denoted |n>, meaning n photons in the electromagnectic field mode understood. If the field is multimode, its quantum state is a tensor product of photon states, e.g.,


with ki the possible momenta of the modes and nki the number of photons in this mode.

Photons can be produced in a variety of ways, including emission from electrons as they change energy states or orbitals. They can also be created by nuclear transitions, particle-antiparticle annihilation or any fluctuations in an electromagnetic field.

In a vacuum, photons move at the speed of light c, defined equal to 299,792,458 m/s (this is a definition and hence does not suffer any experimental uncertainty), or about 3x108 m/s. The dispersion relation is linear and the constant of proportionality is Planck's constant h, yielding the useful relations for kinematic studies, E = h ν (with E the photon energy and ν the frequency of the mode, or photon frequency) and p = h ν / c (p the momentum). Photons are believed to be fundamental particles. Their lifetime is infinite. Their spin is 1 and they are therefore bosons. However since they travel at the speed of light, they have only two spin projections, since the zero projection requires a frame where the photon is still. Such a frame does not exist according to the theory of relativity. They have zero invariant mass but a definite finite energy at the speed of light. Even so, the theory of general relativity states that they are affected by gravity, and this is confirmed by observation.

In a material, they couple to the excitations of the media and behave differently. For instance when they couple to phonons or excitons they give rise to polaritons. Their dispersion departs from the straight line and they acquire an effective mass. Therefore their speed gets lower than the speed of light.

see also

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