# Physics Help

## Photon

From Wikipedia the free encyclopedia, by MultiMedia |

# Photon

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 *k _{i}* the possible momenta of the modes
and

*n*the number of photons in this mode.

_{ki}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
3x10^{8} 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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