Photon

For the Japanese anime video, see Photon (anime).

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.

Symbol

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 range for instance are noted X and called X rays).

Photons are commonly associated with light, which is actually only a limited part of the electromagnetic spectrum. Even there, light is commonly encountered in quantum states which are not pure photons but superpositions of large numbers of photons, to wit, either coherent superpositions (so-called coherent states) describing coherent light such as emitted by an ideal laser, or chaotic superpositions (so-called thermal states) describing light in thermal equilibrium (blackbody radiation). Special devices like masers can create coherent low energy photon radiation.

Quantum state

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.,

$|n_{k_{0}}\rangle \otimes |n_{k_{1}}\rangle \otimes \dots \otimes |n_{k_{n}}\rangle \dots$

with k_i the possible momenta of the modes and n_ki the number of photons in this mode.

Creation

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.

Photons have spin 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. Individual photons are circularly polarized on account of their unity spin.

Photons have zero invariant mass but a definite finite energy at the speed of light. Because they have energy, the theory of general relativity states that they are affected by gravity, and this is confirmed by observation.

In media

In a material, they couple to the excitations of the medium and behave differently. These excitations can often be described as qusi-particles (e.g. phonons and excitons, i.e. as quantized wave- or particle-like entities propagadting though the matter. "Coupling" means here that photons can transform into these excitations (i.e. the photon gets absorbed and medium excited, i.e. a quasi-particle created) and vice versa (the quasi-particle transforms back into the photon, or: the medium relaxes by re-emitting the energy as photon. However, as this transformations are only possibilities they are not bound to happen and what actually propagated through the medium is a polariton, i.e. a quantum-mechanical superposition of the energy quantum being a photon and of it being one of the matter excitations (i.e.a phonon, an exciton or the like).

(According to the rules of quantum mechanics, a measurement (here: just looking what happens to the polariton) breaks this superposition, i.e. the quantum either gets absorbed in the medium an stays there (likely to happen in opaque media) or it reemerges as photon from the surface into space (likely to happen in transparent media))

Matter excitations have a non-linear dispersion relation, i.e. their momentum is not proportional to their energy. Hence, these particles propagate slower than the vacuum speed of light. (The propagations speed is the derivative of the dispersion relation w.r.t. momentum.)

This is the formal reason, why light is slower in media than in vacuum. (The reason for difraction can be deduced from this by Huygen's principle.)

Another way of phrasing it is to say that the photon, by being blent with the matter excitation into a polariton, aquires an [[effective mass], which slows it down.

Molecular absorption

A typical molecule has many different energy levels. When a molecule absorbs a photon, its energy is increased by exactly the amount equal to the energy of the photon. The molecule then enters an excited state.

$M+hv\to M^{*}$

See also

• Particle physics
• Photonics
• Optics
• Spectroscopy
• Photoelectric effect
• List of particles

Links

• James Schombert's "Photon self-identity problems" scan (humor)