- For the Japanese anime video, see
Photon (anime).
In physics, the photon (from Greek φοτος, meaning light) is a quantum of excitation of the quantised electromagnetic field and is one of the elementary particles studied by Quantum Electrodynamics (QED) which is the oldest part of the Standard Model of particle physics. In
layman's terms, photons are the building blocks of electromagnetic radiation: that is, a photon is a "particle" of light, although, according to quantum mechanics, all
particles, including the photon, also have some of the properties of a wave.
Symbol
A photon is usually given the symbol γ (gamma), although in high energy physics
this refers to a high energy photon (a gamma ray; a photon of the immediately
lower energy range is denoted X, an X-ray).
Properties
Photons are commonly associated with visible light, which is actually
only a very limited part of the electromagnetic spectrum. All electromagentic
radiation, from radio waves to gamma rays, is quantised as photons: that is, the smallest
amount of electromagnetic radiation that can exist is one photon whatever its wavelength, frequency, energy or momentum.
Even visible light is commonly encountered in quantum states which are
not "pure" photons but combinations (technically, superpositions) of
large numbers of photons—either coherent superpositions (so-called coherent states, describing coherent light such as emitted by an ideal laser) or mixtures (so-called thermal states), describing light in thermal
equilibrium (black-body radiation). Special devices
like masers can create coherent low energy photon radiation.
In a vacuum, all photons move at the speed of light, c, defined as equal to
299,792,458 m s-1 (the metre is defined as
the distance travelled by light in a vacuum in one 299,792,458th of a second, so the
speed of light does not suffer any experimental uncertainty, unlike the metre or the second), or approximately
3×108 m s-1.
The dispersion
relation of photons (that is, the ratio between their angular
velocity and group velocity, or, equivalently, the ratio between
their momentum and energy) is linear and the constant of proportionality is Planck's constant, h. Briefly considering the wave-like properties of
a photon, it is also worth remembering that the speed of a wave, v, is given by the equation
v = λf, where λ (lambda) is the wavelength and f is the frequency (the symbol ν (nu) is often used instead, but f
is used here to avoid ambiguity with v). This yields two useful relations for kinematic
studies: the energy of a photon, E, is given by the equation E =
hf (or equivalently E = hc / λ) and the momentum of a
photon, p, is given by the equation p = hλ
(or equivalently, p = hf / c).
Photons are fundamental particles. Their lifetime is
infinite, although they can be created and destroyed—see below.
Photons have spin 1 and they are therefore bosons. Photons mediate the electromagnetic field (that is, they are the particles that enable
other particles to interact with each other electromagnetically and with the electromagnetic field), so they are gauge bosons. In general, a boson with spin 1 should be observed in three spin
projections (-1, 0 and 1). The zero projection would require a frame where the photon is at rest, but, since photons travel at
the speed of light, such a frame does not exist according to the theory of relativity, and so photons only have two spin projections. Individual photons are circularly
polarized on account of their unit spin.
Photons have zero invariant mass but a definite finite energy. Because they have energy, the theory of general relativity states that they are affected by gravity, and this is confirmed by observation.
Quantum state
The associated quantum state is the Fock state denoted , meaning n photons in the
electromagnectic field mode understood. If the field is multimode, its quantum state is a tensor product of photon states, for example:
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with ki the possible momenta of the modes and 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. Photons can also be created by nuclear
transitions, particle-antiparticle annihilation or any fluctuations in an electromagnetic field.
In media
In a material, photons couple to the excitations of the medium and behave differently. These excitations can often be described as quasi-particles
(such as phonons and excitons); that is, as
quantized wave- or particle-like entities propagating though the matter.
"Coupling" means here that photons can transform into these excitations (that is, the photon gets absorbed and medium excited,
involving the creation of a quasi-particle) and vice versa (the quasi-particle transforms back into a photon, or the medium
relaxes by re-emitting the energy as a photon). However, as these transformations are only possibilities, they are not bound to
happen and what actually propagates through the medium is a polariton; that is, a
quantum-mechanical superposition of the energy quantum being a photon and
of it being one of the quasi-particle matter excitations.
According to the rules of quantum mechanics, a measurement (here: just
looking what happens to the polariton) breaks this superposition; that is, the quantum either gets absorbed in the medium an
stays there (likely to happen in opaque media) or it re-emerges as photon from the surface into space (likely to happen in
transparent media).
Matter excitations have a non-linear dispersion relation; that is, 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 with respect to
momentum.)
This is the formal reason why light is slower in media (such as glass) than in vacuum. (The reason for diffraction can be deduced from this by Huygens' principle.)
Another way of phrasing it is to say that the photon, by being blended with the matter excitation to form a polariton, aquires
an effective mass, which means that it cannot travel at c, the speed of light in a vacuum.
Molecular absorption
A typical molecule, M, has many different
energy levels. When a molecule absorbs a photon, its energy is increased by
an amount equal to the energy of the photon. The molecule then enters an excited state, M * .
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See also
Links
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