Photon

In Physical of the particles, the photon is the mediating Elementary particle of the electromagnetic Interaction. In other words, when two electrically charged particles interact, this interaction is translated from a quantum point of view like a exchange of photons. In the current design of the Light, the electromagnetic Waves, Waves radio with the Gamma rays while passing by the visible Light, all consists of photons.

The concept of photon was developed by Albert Einstein between 1905 and 1917 to explain experimental observations which could not be included/understood within the framework of a traditional undulatory model of the light., It thus showed that parallel to its undulatory properties - Interférence S and Diffraction -, the electromagnetic field presents simultaneously corpuscular properties. The photons are “packages” of energy elementary or quanta of electromagnetic Rayonnement which is exchanged during the absorption or of the emission of light by the matter. Moreover, the energy and the Quantité of movement (Pressure of radiation) of a monochromatic electromagnetic wave are equal to an integer of times those of a photon.

The concept of photon gave place to important advances in experimental and theoretical physics, such as the Laser S, the condensates of Bump-Einstein, the Quantum optic, the Quantum theory of the fields and the probabilistic interpretation of the quantum Mécanique. The photon is a particle of null mass and of Spin equal to 1, it is thus a Boson. One generally uses the symbol γ ( gamma ) to indicate it.

The energy of a visible photon of light is about 2 eV, that is to say ~$10^9$ time less than energy necessary to create a hydrogen atom. Consequently, the usual radiation sources (antenna S, Lampe S, Laser, etc) produce very great quantities of photons, which explains why nature " granulaire" luminous energy is negligible in many physical situations. There exist however processes making it possible to produce photons one by one:

• electronic Transition;
• nuclear transition;
• annihilation of pairs particle - antiparticle.

History

Origin of the " term; photon"

The photons were originally called “quanta of light” ( das Lichtquant ) by Albert Einstein. in which the photons were “incréables and indestructibles”. Although the theory of Lewis was never accepted, being contradicted by several experiments, its new name, photon , was adopted immediately by the scientific community.

In physics, a photon is represented by the symbol $\backslash \; gamma\; \backslash !$, the Greek letter Gamma. The use of this symbol for the photon probably comes from the gamma rays, which were discovered and named in 1900 by Paul Ulrich Villard. In 1914, Rutherford and Edward Andrade showed that these gamma rays were a form of light. In Optical Chemistry and , the photons are usually symbolized by $h\; \backslash \; naked\; \backslash !$, the energy of the photon, where $h\; \backslash !$ is the Constante of Planck and the letter Greek $\backslash \; naked\; \backslash !$ (naked) is the frequency of the photon. On the occasion, the photon can be symbolized by HF , where its frequency is identified by F .

Development of the concept of “quanta of light”

The description of the light followed during the history a curious movement of beam between a corpuscular vision and an undulatory vision. In the majority of the theories until the 18th century, one considers that the light consists of particles. Although undulatory models are proposed by Rene Descartes (1637), Robert Hooke (1665), and Christian Huygens (1678), the particulate models remain dominant, partly because of the influence of Isaac Newton. A change of Paradigme takes place starting from the description of the phenomena of Interférence S and Diffraction of the light by Thomas Young and Augustin Fresnel at the beginning of the 19th century, and towards 1850 the undulatory models become the rule. The prediction by Maxwell in 1865 owing to the fact that the light is an electromagnetic wave, followed experimental confirmation of Hertz in 1888, seem to carry a blow of thanks to the corpuscular theories of the light.

The undulatory theory of Maxwell does not give however an account of all the properties of the light. This theory predicts that the energy of a light wave depends only on the amplitude of the wave, but not of its frequency; however of many experiments indicate that the energy transferred from the light to the atoms depends only on the frequency and not on the amplitude. For example, certain chemical reactions are not possible that in the presence of a light wave of sufficient frequency: in lower part of a frequency threshold, some is the incidental intensity, the light cannot start the reaction. In a similar way, in the photoelectric Effet the electrons are ejected of a metal plate only to the top of a certain frequency, and the energy of the emitted electrons depends on the frequency of the wave, and not of its amplitude. In the same order of idea, the results obtained at the end of and the beginning of the 20th century on the radiation of the black Corps are reproduced theoretically by max Planck in 1900 by supposing that the matter interacting with an electromagnetic wave of frequency $\backslash \; nu$ can receive or emit electromagnetic energy only by packages of well defined value equal to $h\; \backslash \; nu$ - these packages being called quanta . ,

Since the Maxwell's equations authorize any value of the electromagnetic energy, the majority of the physicists think initially that this quantification of exchanged energy is due to still unknown constraints on the matter which absorbs or emits the light. In 1905, Einstein is the first to propose that the quantification of energy is a property of the light itself . In 1909 what was worth the Nobel Prize of 1927 to him.

Objections with the assumption of the quanta of light

During all the beginning of 20th century however, the concept of photon remains discussed, mainly because of the absence of a formalism making it possible to combine the undulatory phenomena with the corpuscular phenomena lately discovered. Thus in 1913, in a letter of introduction in favor of the admission of Einstein to the Academy of Science of Prussia, written Planck:

One should not to too hold rigor of him what, in its speculations, it occasionally could exceed its target, such as for example with its assumption of the quanta of light.

Many effects highlighting quantified nature of the light can in fact being also explained by a semiclassic theory, in which the matter is quantified but the light is regarded as a traditional electromagnetic field. Among the phenomena thus explainable, one can for example quote the existence of a threshold in the photoelectric effect, the relation between the energy of the emitted electron and the frequency of the wave, the regrouping of the photoelectrons in an interferometer Hanbury Brown and Twiss, as well as the Poissonnian statistics of the accounts. Contrary to a spread idea, the photoelectric effect is thus not the absolute proof of the existence of the photon (although certain experiments on the photoelectric effect cannot however be explained by a semiclassic theory:

• energy and the impulse are preserved only on average , but not during the elementary processes such as absorption and the emission of light. That makes it possible to reconcile the discontinuous change of the energy of the atom with the continuous variations of the energy of the light.
• causality is abandoned. For example, the spontaneous emission is simply an emission induced by an electromagnetic field " virtuel".

However, of the more precise experiments of Compton diffusion show than energy and the impulse is preserved extraordinarily well during the elementary processes, and also that the retreat of the electron and the generation of a new photon during the Compton diffusion obey causality except for less 10ps. Consequently, Bohr and his collaborators give to their model " funeral as honourable as possible". On the theoretical face, the quantum electrodynamic invented by P.A.M. Dirac manages to give a complete theory of the radiation - and electrons - explaining the Dualité wave-corpuscle. For this time, and in particular thanks to the invention of the Laser , the experiments have confirmed in an increasingly direct way the existence of the photon and the failure of the semiclassic theories. It in particular became possible to measure the presence of a photon without absorbing it, thus showing in a direct way the quantification of the electromagnetic field, so that the prediction of Einstein is regarded as proven.

Nobel Prize in bond with the concept of photon

The reader interested by the history of the ideas is invited to refer to the texts of the conferences Nobel, alive testimonys of science moving (see also the duality wave-corpuscle in the Nobel files).

Nobel Prize allotted in bond with the concept of photon:

• 1918 : max Planck " in recognition off the services He rendered to the advancement off Physics by his discovery off energy quanta"

• 1921 : Albert Einstein " for his services to Theoretical Physics, and especially for his discovery off the law off the photoelectric effect"
• 1923 : Robert A. Millikan " for his work one the elementary load off electricity and one the photoelectric effect"
• 1927 : Arthur H. Compton " for his discovery off the effect named after him" (divided with Charles Thomson Rees Wilson)
• 1965: Sin-Itiro Tomonaga, Julian Schwinger and Richard P. Feynman " for to their fundamental work in quantum electrodynamics, with deep-ploughing consequences for the physics off elementary particles"
• 2005 : Roy J. Glauber " for his contribution to the quantum theory off optical coherence" (divided with John L. Hall and Theodor W. Hänsch)

Physical properties

See also: restricted Relativity

The photon does not have a electric Charge and does not disintegrate in a spontaneous way in the vacuum. The photon is also without mass: in experiments, it is lower than 7.10^-17 eV/c ². A photon has two possible states of Polarization and is described by three continuous parameters: the components of sound Vecteur of wave, which determine its λ and its direction wavelength of propagation. The photons are emitted starting from several processes, for example when a load is accelerated, when a Atome or a core change of a high Energy level on a lower level, or when a particle and its antiparticle is destroyed. Photons are absorbed by the opposite process, for example in the production of a particle and its antiparticle or in the atomic transitions and nuclear worms from the high energy levels.

As the photon is without mass, it moves in the vacuum at the speed C (the Speed of light in the vacuum) and its energy E and the Quantité of movement p is connected by E = C•p . In comparison, the corresponding equation for particles of invariable mass m would be $E^\; \{2\}\; =\; c^\; \{2\}\; p^\; \{2\}\; +\; m^\; \{2\}\; c^\; \{4\}\; \backslash !$, as shown in restricted relativity.

Monochromatic light of Fréquence ν consists of photons of energy E depending only on ν:

$E\; =\; \backslash \; hbar\; \backslash \; Omega\; =\; H\; \backslash \; naked\; =\; \backslash \; frac\; \{H\; C\}\; \{\backslash \; lambda\}$,

and of or momentum (impulse ) p :

$\backslash \; mathbf\; \{p\}\; =\; \backslash \; hbar\; \backslash \; mathbf\; \{K\}$,

where $\backslash \; hbar\; =\; h/2\; \backslash \; pi\; \backslash !$ (constant of Dirac or reduced Planck's constant); $\backslash \; mathbf\; \{K\}$ is the vector of wave photon, of amplitude $k\; =\; 2\; \backslash \; pi\; \backslash \; lambda\; \backslash !$ and directed according to the direction of propagation of the photon, and $\backslash \; Omega\; =\; 2\; \backslash \; pi\; \backslash \; naked\; \backslash !$ is its angular Fréquence. Let us note that as for the other particles, a photon can be in a state whose energy is not well defined, such as for example in the case of a package of wave. In this case, the state of the photon is decomposable in a superposition of monochromatic waves close wavelengths (via a Transformée of Fourier).

The photon also has a Spin which is independent of its frequency. The amplitude of the spin is $\backslash \; sqrt\; \{2\}\; \backslash \; hbar$ and the component measured in the direction of propagation, called Hélicité, must be $\backslash \; pm\; \backslash \; hbar$. The two possible helicities correspond to the two possible states of circular polarization of the photon (time and anti-clockwise). As in traditional electromagnetism, a linear superposition corresponds to a superposition of two states of opposite helicity.

An important consequence of these formulas is that the annihilation of a particle and its antiparticle cannot be done in the shape of only one photon. Indeed, in the reference frame of the center of mass, the particles entering in collision do not have momentum, whereas only one photon always has a certain momentum. The law of conservation of the momentum thus requires that at least two photons are created, with a null momentum clear. The energy of the two photons can be given by respecting the laws of conservation. The opposite process, the Creation of pairs, is the mechanism dominating by which photons of high energy (as the gamma rays) lose their energy while passing through the matter.

The traditional formulas of the energy and the momentum of electromagnetic radiations can be reexpressed in term of events connected to the photons. For example, the pressure of electromagnetic radiations on an object comes from the transfer of momentum of the photons per unit of time and surface of this object.

General properties

Note: This section is to be amalgamated in the preceding section or to erase

Should be raised here an apparent paradox: if the photon is monochromatic (only one wavelength λ), that should be an infinite sinusoid; one can obtain a package of wave only if there is a spectrum of a certain width (for example of Gaussian type). In fact, like any quantum phenomenon, there is an uncertainty on the impulse p (thus a certain width of spectrum) and on the position X . The photon thus represents only one wavelength (that of the maximum of the spectrum, the sinusoid registered in the envelope), but is in fact decomposable in a superposition of sinusoids close wavelengths (via a Transformée of Fourier).

The photon respects the principle of uncertainty of Heisenberg: if one knows with precision his position (i.e. the package of wave is narrow, δ X is weak), then uncertainty on his momentum p , which results in a dispersion wavelength δλ, is important, because dependant on 1/δ X .

One can connect this with the spreading out of the package of wave: at the time of the interaction, the photon is quite localized (δ X small) thus the dispersion of the momentum is large (δλ large). The moment according to, the dispersion of momentum makes that the photon is less quite localized, it is spread out (δ X is larger); its “form” “being brought closer” to the ideal sinusoid, its spectrum east narrows (δλ is smaller). One can see coarsely δ X like the “diameter” of the photon.

When they move in the matter, the photons seem to move more slowly than in the vacuum, the speed being determined by the value of the Index of refraction of this medium, which itself depends on the frequency or the Wavelength on this light. Actually the photon at always same speed: it more or less is absorbed and re-emitted (a little later) by the atoms of the matter, according to the body which it crosses, which gives the impression - macroscopically - that the light slows down.

According to knowledge of the 21e century, the photons are elementary particles of well defined energy and null Rest mass. According to the theory of the General relativity, the photons, in spite of their null rest mass, are subjected to the gravitation since they have a nonnull energy (equivalence masses energy). This could be confirmed by the observations, most spectacular being the gravitational lenses or mirages. In particular, at the time of a solar eclipse, one could note that the image of stars moved when the Sun passed near this image; this is explained by the fact why the trajectory of the photons is modified by the proximity of the Sun. This observation, made in 1919, is one of the first experimental confirmations of the general theory of relativity.

Models

Ball of light

The first image which one has of the photon is the “ball of light”, the light would be made up of grains which would travel to 299  792  458  m/s (Speed of light).

In this model, a flow of luminous energy given is broken up into balls whose energy depends on the Wavelength λ and is worth H. C/λ . Thus, for light monochromatic (i.e. whose spectrum is summarized with only one wavelength), the flow of energy is composed in many “small” balls if the wavelength is large (side of the red), or little of “large” balls if the wavelength is small (side of blue) - the qualifiers “small” and “large” is not relating to the size of the balls, but to the quantity of energy which they comprise.

If the light is made up several wavelengths, then the flow of energy is composed of balls of various “sizes”.

This vision, simplistic according to the current standards, does not make it possible to correctly explain all the properties of the light.

Package of wave

One can represent with the first access the photons by packages of wave : the electromagnetic wave is not a Sinusoïde of infinite extension, there is an envelope of important amplitude framed by other envelopes definitely less significant.

This model is insufficient. Indeed, in such a configuration, the photon should widen its progression progressively (one speaks about the “spreading out of the package of wave”), energy should be less and less concentrated. However, it is noted that even after an interstellar way several thousands of light-years, the properties of the photons are exactly the same ones.

Duality wave-corpuscle

The photon is a concept to explain the interactions between the electromagnetic radiations and the matter. As for the others elementary particles, it has a Dualité wave-particle. One can speak about photon as a particle only at the time of the interaction. Apart from any interaction, one does not know - and one cannot know - which “form” has this radiation. One can imagine that the photon would be a concentration which would be formed only at the time of the interaction, then would be spread out, and would be reformed at the time of an other interaction. One cannot thus speak about “localization” nor of “trajectory” of the photon.

One can in shows the photon like a quantum particle, i.e. a mathematical object defined by his Fonction of wave which gives the probability of presence. Attention not to confuse this function and the traditional electromagnetic wave.

Thus, the electromagnetic wave, i.e. the value of the Electric field and the Magnetic field according to the place and of the moment ($\backslash \; vec\; \{E\}\; (\backslash \; vec\; \{X\},\; T)$ and $\backslash \; vec\; \{B\}\; (\backslash \; vec\; \{X\},\; T)$), thus has two significances:

• macroscopic: when the flow of energy is sufficiently important, they are the fields electric and magnetic measured by a macroscopic apparatus (for example receiving antenna, a electroscope or a probe of Hall);
• microscopic: it represents the probability of presence of the photons, i.e. the probability that in a given place there is a quantified interaction (i.e. of a given energy hν ).

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