Photon

Photon

The photon (from Greek φῶς Phos gene φωτός. PhotoS " light ") is the elementary excitation ( quantum) of the electromagnetic field. Figuratively speaking, photons are that from which electromagnetic radiation is, hence the name photon or particle of light is sometimes used. In quantum electrodynamics, the photon part as a mediator of the electromagnetic interaction of the gauge bosons.

History of Research

Since ancient times there were various, mutually partially contradictory ideas about the nature of light. By the end of the 19th century competed wave and particle theories with one another (see story in the article light). While many phenomena such as interference and polarization phenomena seemed to prove the wave nature of light, there was also evidence of a particle nature. A historically important experiment in this direction was the observation of the photoelectric effect by Heinrich Hertz and William Hall wax in 1887.

The discovery of the quantization of the electromagnetic radiation went in 1900 from the Planckian radiation law, which describes the thermal radiation of a black body. In order to explain this law in theory, Planck had to assume that the surface of the black body can only exchange discrete amounts of energy proportional to the frequency with the electromagnetic field. Planck himself, however, did not stand still, the electromagnetic radiation to be quantized before, but only the energy exchange.

Albert Einstein put then in 1905 in his publication on the photoelectric effect, the light quantum hypothesis. According to her, light is a stream of " localized in points in space quanta of energy which move without dividing, and absorbed only as a whole and can be generated ." Due to widespread doubts about these views, this work was only in 1920 ( Planck ) and 1921 (Einstein) was awarded the Nobel Prize.

However, the particle nature of electromagnetic radiation was still widely doubted until Arthur Holly Compton was able to prove in the years 1923-25 ​​, that X-rays on single electrons just seems like the firing of individual particles, especially their energy and momentum which corresponded to the light quanta. The discovery of the Compton effect and its interpretation, he received the 1927 Nobel Prize.

The formal quantum theory of light was developed starting with works by Max Born, Pascual Jordan and Werner Heisenberg only since 1925. The currently valid theory of electromagnetic radiation, which also describes the photons, is quantum electrodynamics ( QED). She goes into its beginnings back to the work of Paul Dirac in 1927, in which the interaction is analyzed by quantized electromagnetic radiation with an atom. The QED was developed in the 1940's and 1965 honored with the award of the Nobel Prize for Physics to Richard P. Feynman, Julian Schwinger, and Tomonaga Shinichirō.

The term photon for the elementary excitation of the quantized electromagnetic field was introduced in 1926 by the chemist Gilbert Newton Lewis. He used the term in the context of his proposed model of the interaction of atoms with light. This model, which was not generally recognized, among others, saw falsely before a conservation of the photon number.

In a letter to his friend Michele Besso (1873-1955) Albert Einstein wrote in 1951: " The whole 50 years of conscious brooding have ' not brought me the answer to the question, What are light quanta closer. While today everyone believes Lump, he knew it, but he is mistaken ... "

Icon

For the photon in general the ( gamma) is being used. In high energy physics, this symbol is, however, reserved for the high-energy photons of gamma radiation (gamma - quanta), and in this branch of physics is also relevant X-ray photons received the symbol X (of X - rays and English: X -ray).

Very often, a photon is represented by the energy contained:

Or

Properties

Any electromagnetic radiation, from radio waves to gamma radiation is quantized into photons. That is, the smallest amount of electromagnetic radiation of a certain frequency is a photon. Photons have an infinite natural lifetime, but may be created or destroyed in a variety of physical processes. A photon has no mass. It follows that it is always moving in the vacuum speed of light. Optical media in the group velocity (expressed by the refractive index ) compared with the vacuum velocity of light due to the interaction of photons with matter is reduced, the phase velocity can be considered to be even higher.

Generation and detection

Photons can be generated in many ways, especially through transitions ( " quantum jumps" ) of electrons between different states (eg different atomic or molecular orbitals or energy bands in a solid body ). Photons can also be produced in nuclear transitions, particle-antiparticle annihilation reactions ( annihilation) or by any fluctuations in an electromagnetic field.

For the detection of photons flow, among other photomultipliers, photoconductors or photodiodes can be used. CCD, vidicon, PSDs quadrant diodes or photo - plates and films are used for position-sensitive detection of photons. In the IR range also bolometers are used. Photons in the gamma-ray range can be detected individually by Geiger counter. Photomultipliers and avalanche photodiodes can also be used for single- photon detection in the optical region, Photomultiplier generally have the lower dark count, avalanche photodiodes but still at lower photon energies up to the IR range can be used.

Mass

Photons have no mass. This manifests itself in the Maxwell equations, characterized in that the components of the electric field in the vacuum, the wave equation

. meet This wave equation for each component of the electric field (and also the magnetic flux density) of the special case of the Klein-Gordon equation for massless fields or particles. The phase velocity is the velocity of light.

Further, the shape of the Maxwell equations allows to define the electric and magnetic potentials ( calibration fields). For interaction particles with nonzero mass, there would then be no Coulomb potential, but a Yukawa potential. The potential of an electric charge would be attenuated with an additional exponential damping term.

Had a photon mass, so this would change the behavior of magnetic fields. Such differences could not be detected experimentally so far, resulting in the currently (as of 2013) arising existing limits on the mass of a photon.

Conversely, you can see for now also from the relativistic energy-momentum relation, that massless particle trajectories are light-like:

Spin

Photons are spin-1 particles and thus bosons. It can therefore arbitrarily many photons occupy the same quantum state, which is realized, for example, in a laser.

While as the electron spin is parallel or anti- parallel to a predetermined direction as desired, the photon spin can be oriented to the flight direction parallel or anti- parallel due to lack of bulk. The helicity of the photon is therefore a characteristic size. Nevertheless, a single photon can be linearly polarized by two oppositely circularly polarized states are superimposed.

Photons in the vacuum

Under vacuum, the photons move with the vacuum speed of light. The dispersion relation, i.e. the dependence of the energy of the frequency ( ny) is linear and the constant of proportionality is the Planck's constant,

Numerical values ​​, as typically occur in optical spectra, can be determined as follows:

Example: Red light with 620 nm wavelength has a photon energy of about 2 eV.

The momentum of a photon is thus

Photons in media

In a material photons interact with the surrounding medium, resulting in altered properties. The photon can be absorbed. Yet his energy, of course, does not disappear, but goes into other forms of energy on, for example, elementary excitations ( quasiparticles ) of the medium such as phonons or excitons. It is also possible that it is propagated through a medium. In the particle picture is no uniform medium, but a sequence of scattering processes of the photon to the atoms of the medium. This propagation can be obtained by the introduction of a quasi-particle, the polariton, describe. These elementary excitations in matter do not usually have linear dispersion relation, and their propagation velocity is lower than the vacuum speed of light, in experiments, the speed was reduced to a few meters per second.

Interaction of photons with matter

Photons incident on matter, solve certain energies from different processes. In the following, the energy ranges are given for different processes in which they are relevant:

• Rayleigh scattering
• Below 5 eV excitation of higher -energy states of electrons, no ionization
• 5 eV to 100 keV photoelectric effect,
• 50 keV to 1 MeV Compton effect,
• 1.022 to 6 MeV pair production ( under certain conditions and by direct photon-photon interaction is possible),
• From 2.18 to 16 MeV nuclear photo effect.
• Higher energies: photodisintegration

These effects contribute significantly to the fact that one can detect this radiation and can be certain substances with specific effects demonstrate, by gamma spectroscopy.