Photon
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In physics, a photon is an elementary particle, the quantum of the electromagnetic interaction and the basic unit of light and all other forms of electromagnetic radiation. It is also the force carrier for the electromagnetic force. The effects of this force are easily observable at both the microscopic and macroscopic level, because the photon has no rest mass; this allows for interactions at long distances. Like all elementary particles, photons are governed by quantum mechanics and will exhibit wave–particle duality — they exhibit properties of both waves and . For example, a single photon may be refracted by a lens or exhibit wave interference with itself, but also act as a particle giving a definite result when quantitative momentum is measured.
The modern concept of the photon was developed gradually by Albert Einstein to explain experimental observations that did not fit the classical wave model of light. In particular, the photon model accounted for the frequency dependence of light's energy, and explained the ability of matter and radiation to be in thermal equilibrium. It also accounted for anomalous observations, including the properties of black body radiation, that other physicists, most notably Max Planck, had sought to explain using semiclassical models, in which light is still described by Maxwell's equations, but the material objects that emit and absorb light are quantized. Although these semiclassical models contributed to the development of quantum mechanics, further experiments validated Einstein's hypothesis that light itself is quantized; the quanta of light are photons.
In the modern Standard Model of particle physics, photons are described as a necessary consequence of physical laws having a certain symmetry at every point in spacetime. The intrinsic properties of photons, such as charge, mass and spin, are determined by the properties of this gauge symmetry. The neutrino theory of light, which attempts to describe the photon as a composite structure, has been unsuccessful so far.
The photon concept has led to momentous advances in experimental and theoretical physics, such as lasers, Bose–Einstein condensation, quantum field theory, and the probabilistic interpretation of quantum mechanics. It has been applied to photochemistry, high-resolution microscopy, and measurements of molecular distances. Recently, photons have been studied as elements of quantum computers and for sophisticated applications in optical communication such as quantum cryptography.
Nomenclature
In 1900, Max Planck was working on black-body radiation and suggested that the energy in electromagnetic waves could only be released in "packets" of energy. In his 1901 article In physics, a photon is usually denoted by the symbol γ (the Greek letter gamma). This symbol for the photon probably derives from gamma rays, which were discovered in 1900 by Paul Villard,Physical properties
--> The photon is massless,The mass of the photon is believed to be exactly zero, based on experiment and theoretical considerations described in the article. Some sources also refer to the relativistic mass concept, which is just the energy scaled to units of mass. For a photon with wavelength λ or energy E, this is h/λc or E/c2. This usage for the term "mass" is no longer common in scientific literature. Further info: What is the mass of a photon? http://math.ucr.edu/home/baez/physics/ParticleAndNuclear/photon_mass.html has no electric charge, Photons are emitted in many natural processes. For example, when a charge is accelerated it emits synchrotron radiation. During a molecular, atomic or nuclear transition to a lower energy level, photons of various energy will be emitted, from infrared light to gamma rays. A photon can also be emitted when a particle and its corresponding antiparticle are annihilated (for example, electron-positron annihilation). In empty space, the photon moves at c (the speed of light) and its energy and momentum are related by pc}}, where p is the magnitude of the momentum vector p. This derives from the following relativistic relation, with 0}}:See section 1.6 in {{Cite book |last = Alonso|first=M. |last2 = Finn|first2=E.J. |title = Fundamental University Physics Volume III: Quantum and Statistical Physics |publisher = Addison-Wesley |year = 1968 |isbn=0-201-00262-0 }} E^{2} = p^{2} c^{2} + m^{2} c^{4}. The energy and momentum of a photon depend only on its frequency (ν) or inversely, its wavelength (λ): E = \hbar\omega = h\nu = \frac{h c}{\lambda} \mathbf{p} = \hbar\mathbf{k}, where k is the wave vector (where the wave number k 2π/λ}}), 2πν}} is the angular frequency, and h/2π}} is the reduced Planck constant.Davison E. Soper, Electromagnetic radiation is made of photons, Institute of Theoretical Science, University of Oregon Since p points in the direction of the photon's propagation, the magnitude of the momentum is p = \hbar k = \frac{h\nu}{c} = \frac{h}{\lambda}. The photon also carries spin angular momentum that does not depend on its frequency.This property was experimentally verified by Raman and Bhagavantam in 1931: {{Cite journal |last = Raman|first = C.V. |last2 = Bhagavantam|first2 = S. |title = Experimental proof of the spin of the photon |url = http://dspace.rri.res.in/bitstream/2289/2123/1/1931%20IJP%20V6%20p353.pdf |format=PDF| authorlink = C. V. Raman |journal = Indian Journal of Physics |volume = 6|pages = 353 |year = 1931 }} The magnitude of its spin is \scriptstyle{\sqrt{2} \hbar} and the component measured along its direction of motion, its helicity, must be ±ħ. These two possible helicities, called right-handed and left-handed, correspond to the two possible circular polarization states of the photon.E.g., section 1.3.3.2 in {{Cite book |last = Burgess|first = C.|url=http://books.google.com/books?id=PLYECqs2geEC&pg=PA27 |last2 = Moore|first2=G. |title=The Standard Model. A Primer |publisher= Cambridge University Press |year=2007 |isbn=0-521-86036-9 }} To illustrate the significance of these formulae, the annihilation of a particle with its antiparticle in free space must result in the creation of at least two photons for the following reason. In the center of mass frame, the colliding antiparticles have no net momentum, whereas a single photon always has momentum (since it is determined, as we have seen, only by the photon's frequency or wavelength—which cannot be zero). Hence, conservation of momentum (or equivalently, translational invariance) requires that at least two photons are created, with zero net momentum. (However it is possible if the system interacts with another particle or field for annihilation to produce one photon, as when a positron annihilates with a bound atomic electron, it is possible for only one photon to be emitted, as the nuclear Coulomb field breaks translational symmetry.) The energy of the two photons, or, equivalently, their frequency, may be determined from conservation of four-momentum. Seen another way, the photon can be considered as its own antiparticle. The reverse process, pair production, is the dominant mechanism by which high-energy photons such as gamma rays lose energy while passing through matter.E.g., section 9.3 in {{Cite book |last=Alonso|first=M. |last2=Finn|first2=E.J. |title=Fundamental University Physics Volume III: Quantum and Statistical Physics |publisher= Addison-Wesley |year=1968 }} That process is the reverse of "annihilation to one photon" allowed in the electric field of an atomic nucleus. The classical formulae for the energy and momentum of electromagnetic radiation can be re-expressed in terms of photon events. For example, the pressure of electromagnetic radiation on an object derives from the transfer of photon momentum per unit time and unit area to that object, since pressure is force per unit area and force is the change in momentum per unit time.E.g., Appendix XXXII in {{Cite book |last=Born |first=M. |authorlink=Max Born |title=Atomic Physics |publisher= Blackie & Son |year=1962 |isbn=0486659844 }}Experimental checks on photon mass
The photon is currently believed to be strictly massless, but this is an experimental question. If the photon is not a strictly massless particle, it would not move at the exact speed of light in vacuum, c. Its speed would be lower and depend on its frequency. Relativity would be unaffected by this; the so-called speed of light, c, would then not be the actual speed at which light moves, but a constant of nature which is the maximum speed that any object could theoretically attain in space-time. Thus, it would still be the speed of space-time ripples ( gravitational waves and gravitons), but it would not be the speed of photons. A massive photon would have other effects as well. Coulomb's law would be modified and the electromagnetic field would have an extra physical degree of freedom. These effects yield more sensitive experimental probes of the photon mass than the frequency dependence of the speed of light. If Coulomb's law is not exactly valid, then that would cause the presence of an electric field inside a hollow conductor when it is subjected to an external electric field. This thus allows one to test Coulomb's law to very high precision. A null result of such an experiment has set a limit of m ≲ 10−14 eV/c2. Sharper upper limits have been obtained in experiments designed to detect effects caused by the Galactic vector potential. Although the galactic vector potential is very large because the galactic magnetic field exists on very long length scales, only the magnetic field is observable if the photon is massless. In case of a massive photon, the mass term \scriptstyle\frac{1}{2} m^2 A_{\mu}A^{\mu} would affect the galactic plasma. The fact that no such effects are seen implies an upper bound on the photon mass of m < . The galactic vector potential can also be probed directly by measuring the torque exerted on a magnetized ring. Such methods were used to obtain the sharper upper limit of 10−18eV/c2 given by the Particle Data Group. Summary Table These sharp limits from the non-observation of the effects caused by the galactic vector potential have been shown to be model dependent. preprint If the photon mass is generated via the Higgs mechanism then the upper limit of m≲10−14 eV/c2 from the test of Coulomb's law is valid.Historical development
In most theories up to the eighteenth century, light was pictured as being made up of particles. Since particle models cannot easily account for the refraction, diffraction and birefringence of light, wave theories of light were proposed by René Descartes (1637), The Maxwell wave theory, however, does not account for all properties of light. The Maxwell theory predicts that the energy of a light wave depends only on its intensity, not on its frequency; nevertheless, several independent types of experiments show that the energy imparted by light to atoms depends only on the light's frequency, not on its intensity. For example, some chemical reactions are provoked only by light of frequency higher than a certain threshold; light of frequency lower than the threshold, no matter how intense, does not initiate the reaction. Similarly, electrons can be ejected from a metal plate by shining light of sufficiently high frequency on it (the photoelectric effect); the energy of the ejected electron is related only to the light's frequency, not to its intensity.Frequency-dependence of luminiscence p. 276f., photoelectric effect section 1.4 in {{Cite book |last=Alonso|first=M. |last2=Finn|first2=E.J. |title=Fundamental University Physics Volume III: Quantum and Statistical Physics |publisher= Addison-Wesley |isbn=0-201-00262-0 |year=1968 }}The phrase "no matter how intense" refers to intensities below approximately 1013 W/cm2 at which point perturbation theory begins to break down. In contrast, in the intense regime, which for visible light is above approximately 1014 W/cm2, the classical wave description correctly predicts the energy acquired by electrons, called ponderomotive energy. (See also: Boreham et al. (1996). " Photon density and the correspondence principle of electromagnetic interaction".) By comparison, sunlight is only about 0.1 W/cm2. At the same time, investigations of blackbody radiation carried out over four decades (1860–1900) by various researchers Since the Maxwell theory of light allows for all possible energies of electromagnetic radiation, most physicists assumed initially that the energy quantization resulted from some unknown constraint on the matter that absorbs or emits the radiation. In 1905, Einstein was the first to propose that energy quantization was a property of electromagnetic radiation itself. Although he accepted the validity of Maxwell's theory, Einstein pointed out that many anomalous experiments could be explained if the energy of a Maxwellian light wave were localized into point-like quanta that move independently of one another, even if the wave itself is spread continuously over space. In 1909 and 1916,Early objections
Einstein's 1905 predictions were verified experimentally in several ways in the first two decades of the 20th century, as recounted in Robert Millikan's Nobel lecture. Even after Compton's experiment, Bohr, Hendrik Kramers and John Slater made one last attempt to preserve the Maxwellian continuous electromagnetic field model of light, the so-called BKS model.- Energy and momentum are conserved only on the average in interactions between matter and radiation, not in elementary processes such as absorption and emission. This allows one to reconcile the (at the time believed to be) discontinuously changing energy of the atom (jump between energy states) with the continuous release of energy into radiation. (It is now known that this is actually a continuous process, the combined atom-field system evolving in time according to Schroedinger's equation.)
- Causality is abandoned. For example, spontaneous emissions are merely emissions induced by a "virtual" electromagnetic field.
Wave–particle duality and uncertainty principles
Photons, like all quantum objects, exhibit both wave-like and particle-like properties. Their dual wave–particle nature can be difficult to visualize. The photon displays clearly wave-like phenomena such as diffraction and interference on the length scale of its wavelength. For example, a single photon passing through a double-slit experiment lands on the screen exhibiting interference phenomena but only if no measure was made on the actual slit being run across. To account for the particle interpretation that phenomena is called probability distribution but behaves according to the Maxwell's equations. A key element of quantum mechanics is Heisenberg's uncertainty principle, which forbids the simultaneous measurement of the position and momentum of a particle along the same direction. Remarkably, the uncertainty principle for charged, material particles requires the quantization of light into photons, and even the frequency dependence of the photon's energy and momentum. An elegant illustration is Heisenberg's thought experiment for locating an electron with an ideal microscope. \Delta x \sim \frac{\lambda}{\sin \theta} where \theta is the aperture angle of the microscope. Thus, the position uncertainty \Delta x can be made arbitrarily small by reducing the wavelength λ. The momentum of the electron is uncertain, since it received a "kick" \Delta p from the light scattering from it into the microscope. If light were not quantized into photons, the uncertainty \Delta p could be made arbitrarily small by reducing the light's intensity. In that case, since the wavelength and intensity of light can be varied independently, one could simultaneously determine the position and momentum to arbitrarily high accuracy, violating the uncertainty principle. By contrast, Einstein's formula for photon momentum preserves the uncertainty principle; since the photon is scattered anywhere within the aperture, the uncertainty of momentum transferred equals \Delta p \sim p_{\mathrm{photon}} \sin\theta = \frac{h}{\lambda} \sin\theta giving the product \Delta x \Delta p \, \sim \, h, which is Heisenberg's uncertainty principle. Thus, the entire world is quantized; both matter and fields must obey a consistent set of quantum laws, if either one is to be quantized.E.g., p. 10f. in {{Cite book |last=Schiff |first=L.I. |title=Quantum Mechanics |edition=3rd |publisher= McGraw-Hill |year=1968 |id= |isbn=0070552878 }}. The analogous uncertainty principle for photons forbids the simultaneous measurement of the number n of photons (see Fock state and the Second quantization section below) in an electromagnetic wave and the phase \phi of that wave \Delta n \Delta \phi > 1 See coherent state and squeezed coherent state for more details. Both photons and material particles such as electrons create analogous interference patterns when passing through a double-slit experiment. For photons, this corresponds to the interference of a Maxwell light wave whereas, for material particles, this corresponds to the interference of the Schrödinger wave equation. Although this similarity might suggest that Maxwell's equations are simply Schrödinger's equation for photons, most physicists do not agree.Bose–Einstein model of a photon gas
In 1924, Satyendra Nath Bose derived Planck's law of black-body radiation without using any electromagnetism, but rather a modification of coarse-grained counting of phase space. The modern view on this is that photons are, by virtue of their integer spin, bosons (as opposed to fermions with half-integer spin). By the spin-statistics theorem, all bosons obey Bose–Einstein statistics (whereas all fermions obey Fermi-Dirac statistics).Stimulated and spontaneous emission
In 1916, Einstein showed that Planck's radiation law could be derived from a semi-classical, statistical treatment of photons and atoms, which implies a relation between the rates at which atoms emit and absorb photons. The condition follows from the assumption that light is emitted and absorbed by atoms independently, and that the thermal equilibrium is preserved by interaction with atoms. Consider a cavity in thermal equilibrium and filled with electromagnetic radiation and atoms that can emit and absorb that radiation. Thermal equilibrium requires that the energy density \rho(\nu) of photons with frequency \nu (which is proportional to their number density) is, on average, constant in time; hence, the rate at which photons of any particular frequency are emitted must equal the rate of absorbing them. Einstein began by postulating simple proportionality relations for the different reaction rates involved. In his model, the rate R_{ji} for a system to absorb a photon of frequency \nu and transition from a lower energy E_{j} to a higher energy E_{i} is proportional to the number N_{j} of atoms with energy E_{j} and to the energy density \rho(\nu) of ambient photons with that frequency, R_{ji} = N_{j} B_{ji} \rho(\nu) \! where B_{ji} is the rate constant for absorption. For the reverse process, there are two possibilities: spontaneous emission of a photon, and a return to the lower-energy state that is initiated by the interaction with a passing photon. Following Einstein's approach, the corresponding rate R_{ij} for the emission of photons of frequency \nu and transition from a higher energy E_{i} to a lower energy E_{j} is R_{ij} = N_{i} A_{ij} + N_{i} B_{ij} \rho(\nu) \! where A_{ij} is the rate constant for emitting a photon spontaneously, and B_{ij} is the rate constant for emitting it in response to ambient photons ( induced or stimulated emission). In thermodynamic equilibrium, the number of atoms in state i and that of atoms in state j must, on average, be constant; hence, the rates R_{ji} and R_{ij} must be equal. Also, by arguments analogous to the derivation of Boltzmann statistics, the ratio of N_{i} and N_{j} is g_i/g_j\exp{(E_j-E_i)/kT)}, where g_{i,j} are the degeneracy of the state i and that of j, respectively, E_{i,j} their energies, k the Boltzmann constant and T the system's temperature. From this, it is readily derived that g_iB_{ij} = g_jB_{ji} and A_{ij} = \frac{8 \pi h \nu^{3}}{c^{3}} B_{ij}. The A and Bs are collectively known as the Einstein coefficients.Section 1.4 in {{cite book |last=Wilson |first=J. |last2=Hawkes |first2=F.J.B. |title=Lasers: Principles and Applications |publisher= Prentice Hall |location=New York |year=1987 |isbn=0-13-523705-X }}. Einstein could not fully justify his rate equations, but claimed that it should be possible to calculate the coefficients A_{ij}, B_{ji} and B_{ij} once physicists had obtained "mechanics and electrodynamics modified to accommodate the quantum hypothesis".P. 322 in {{cite journal |last = Einstein|first = A.|authorlink = Albert Einstein |year = 1916|title = Strahlungs-emission und -absorption nach der Quantentheorie |journal = Verhandlungen der Deutschen Physikalischen Gesellschaft |volume = 18|pages = 318–323 }} : In fact, in 1926, Paul Dirac derived the B_{ij} rate constants in using a semiclassical approach, Einstein was troubled by the fact that his theory seemed incomplete, since it did not determine the direction of a spontaneously emitted photon. A probabilistic nature of light-particle motion was first considered by Newton in his treatment of birefringence and, more generally, of the splitting of light beams at interfaces into a transmitted beam and a reflected beam. Newton hypothesized that hidden variables in the light particle determined which path it would follow. Similarly, Einstein hoped for a more complete theory that would leave nothing to chance, beginning his separation from quantum mechanics. Ironically, Max Born's probabilistic interpretation of the wave functionSecond quantization
In 1910, Peter Debye derived Planck's law of black-body radiation from a relatively simple assumption. In 1925, Born, Heisenberg and Jordan reinterpreted Debye's concept in a key way. Dirac took this one step further. He treated the interaction between a charge and an electromagnetic field as a small perturbation that induces transitions in the photon states, changing the numbers of photons in the modes, while conserving energy and momentum overall. Dirac was able to derive Einstein's A_{ij} and B_{ij} coefficients from first principles, and showed that the Bose–Einstein statistics of photons is a natural consequence of quantizing the electromagnetic field correctly (Bose's reasoning went in the opposite direction; he derived Planck's law of black body radiation by assuming BE statistics). In Dirac's time, it was not yet known that all bosons, including photons, must obey BE statistics. Dirac's second-order perturbation theory can involve virtual photons, transient intermediate states of the electromagnetic field; the static electric and magnetic interactions are mediated by such virtual photons. In such quantum field theories, the probability amplitude of observable events is calculated by summing over all possible intermediate steps, even ones that are unphysical; hence, virtual photons are not constrained to satisfy E = pc, and may have extra polarization states; depending on the gauge used, virtual photons may have three or four polarization states, instead of the two states of real photons. Although these transient virtual photons can never be observed, they contribute measurably to the probabilities of observable events. Indeed, such second-order and higher-order perturbation calculations can give apparently infinite contributions to the sum. Such unphysical results are corrected for using the technique of renormalization. Other virtual particles may contribute to the summation as well; for example, two photons may interact indirectly through virtual electron- positron pairs.Photon-photon-scattering section 7-3-1, renormalization chapter 8-2 in {{Cite book |last=Itzykson |first=C. |last2=Zuber |first2=J.-B. |title=Quantum Field Theory |publisher= McGraw-Hill |year=1980 |isbn=0-07-032071-3 }} In fact, such photon-photon scattering, as well as electron-photon scattering, is meant to be one of the modes of operations of the planned particle accelerator, the International Linear Collider. In modern physics notation, the quantum state of the electromagnetic field is written as a Fock state, a tensor product of the states for each electromagnetic mode |n_{k_0}\rangle\otimes|n_{k_1}\rangle\otimes\dots\otimes|n_{k_n}\rangle\dots where |n_{k_i}\rangle represents the state in which \, n_{k_i} photons are in the mode k_i. In this notation, the creation of a new photon in mode k_i (e.g., emitted from an atomic transition) is written as |n_{k_i}\rangle \rightarrow |n_{k_i}+1\rangle. This notation merely expresses the concept of Born, Heisenberg and Jordan described above, and does not add any physics.The photon as a gauge boson
The electromagnetic field can be understood as a gauge field, i.e., as a field that results from requiring a gauge symmetry holds independently at every position in spacetime. The quanta of an Abelian gauge field must be massless, uncharged bosons, as long as the symmetry is not broken; hence, the photon is predicted to be massless, and to have zero electric charge and integer spin. The particular form of the electromagnetic interaction specifies that the photon must have spin ±1; thus, its helicity must be \pm \hbar. These two spin components correspond to the classical concepts of right-handed and left-handed circularly polarized light. However, the transient virtual photons of quantum electrodynamics may also adopt unphysical polarization states. In the prevailing Standard Model of physics, the photon is one of four gauge bosons in the electroweak interaction; the other three are denoted W+, W− and Z0 and are responsible for the weak interaction. Unlike the photon, these gauge bosons have mass, owing to a mechanism that breaks their SU(2) gauge symmetry. The unification of the photon with W and Z gauge bosons in the electroweak interaction was accomplished by Sheldon Glashow, Abdus Salam and Steven Weinberg, for which they were awarded the 1979 Nobel Prize in physics. Sheldon Glashow Nobel lecture, delivered 8 December 1979. Abdus Salam Nobel lecture, delivered 8 December 1979. Steven Weinberg Nobel lecture, delivered 8 December 1979. Physicists continue to hypothesize grand unified theories that connect these four gauge bosons with the eight gluon gauge bosons of quantum chromodynamics; however, key predictions of these theories, such as proton decay, have not been observed experimentally.E.g., chapter 14 in {{cite book |last=Hughes |first=I. S. |title=Elementary particles |edition=2nd |publisher= Cambridge University Press |year=1985 |isbn=0-521-26092-2 }}Contributions to the mass of a system
The energy of a system that emits a photon is decreased by the energy E of the photon as measured in the rest frame of the emitting system, which may result in a reduction in mass in the amount {E}/{c^2}. Similarly, the mass of a system that absorbs a photon is increased by a corresponding amount. As an application, the energy balance of nuclear reactions involving photons is commonly written in terms of the masses of the nuclei involved, and terms of the form {E}/{c^2} for the gamma photons (and for other relevant energies, such as the recoil energy of nuclei).E.g., section 10.1 in {{Cite book |last=Dunlap |first=R.A. |title=An Introduction to the Physics of Nuclei and Particles |publisher= Brooks/Cole |year=2004 |isbn=0-534-39294-6 }} This concept is applied in key predictions of quantum electrodynamics (QED, see above). In that theory, the mass of electrons (or, more generally, leptons) is modified by including the mass contributions of virtual photons, in a technique known as renormalization. Such "radiative corrections" contribute to a number of predictions of QED, such as the magnetic dipole moment of leptons, the Lamb shift, and the hyperfine structure of bound lepton pairs, such as muonium and positronium.Radiative correction to electron mass section 7-1-2, anomalous magnetic moments section 7-2-1, Lamb shift section 7-3-2 and hyperfine splitting in positronium section 10-3 in {{Cite book |last=Itzykson |first=C. |last2=Zuber|first2=J.-B. |title=Quantum Field Theory |publisher= McGraw-Hill |year=1980 |isbn=0-07-032071-3 }} Since photons contribute to the stress-energy tensor, they exert a gravitational attraction on other objects, according to the theory of general relativity. Conversely, photons are themselves affected by gravity; their normally straight trajectories may be bent by warped spacetime, as in gravitational lensing, and their frequencies may be lowered by moving to a higher gravitational potential, as in the Pound-Rebka experiment. However, these effects are not specific to photons; exactly the same effects would be predicted for classical electromagnetic waves.E. g. sections 9.1 (gravitational contribution of photons) and 10.5 (influence of gravity on light) in {{Cite book |last=Stephani |first=H.|url=http://books.google.com/books?id=V04_vLQvstcC&pg=PA86 |last2=Stewart |first2=J.|pages=86 ff, 108 ff. |title=General Relativity: An Introduction to the Theory of Gravitational Field |isbn=0-521-37941-5 |publisher= Cambridge University Press |year=1990 }}Photons in matter
(Visible) light that travels through transparent matter does so at a lower speed than c, the speed of light in a vacuum. X-rays, on the other hand, usually have a phase velocity above c, as evidenced by total external reflection. In addition, light can also undergo scattering and absorption. There are circumstances in which heat transfer through a material is mostly radiative, involving emission and absorption of photons within it. An example would be in the core of the sun. Energy can take about a million years to reach the surface. Alternatively, photons may be viewed as always traveling at c, even in matter, but they have their phase shifted (delayed or advanced) upon interaction with atomic scatters: this modifies their wavelength and momentum, but not speed.Ch 4 in {{Cite book |last = Hecht| first=Eugene |title = Optics |publisher= Addison Wesley |year=2001 |isbn=9780805385663 }} A light wave made up of these photons does travel slower than the speed of light. In this view the photons are "bare", and are scattered and phase shifted, while in the view of the preceding paragraph the photons are "dressed" by their interaction with matter, and move without scattering or phase shifting, but at a lower speed. Light of different frequencies may travel through matter at different speeds; this is called dispersion. In some cases, it can result in extremely slow speeds of light in matter. The effects of photon interactions with other quasi-particles may be observed directly in Raman scattering and Brillouin scattering.Polaritons section 10.10.1, Raman and Brillouin scattering section 10.11.3 in {{Cite book |last = Patterson|first=J.D.|pages=569 ff, 580 ff |last2 = Bailey|first2=B.C.|url=http://books.google.com/books?id=uRQg87Mb6DoC&pg=569 |title = Solid-State Physics: Introduction to the Theory |publisher= Springer |year=2007 |isbn=3-540-24115-9 }} Photons can also be absorbed by nuclei, atoms or molecules, provoking transitions between their energy levels. A classic example is the molecular transition of retinal C20H28O, which is responsible for vision, as discovered in 1958 by Nobel laureate biochemist George Wald and co-workers. The absorption provokes a cis-trans isomerization that, in combination with other such transitions, is transduced into nerve impulses. The absorption of photons can even break chemical bonds, as in the photodissociation of chlorine; this is the subject of photochemistry.E.g., section 11-5 C in {{Cite book |last=Pine |first=S.H. |last2=Hendrickson |first2=J.B. |last3=Cram |first3=D.J. |last4=Hammond |first4=G.S. |title=Organic Chemistry |edition=4th |publisher= McGraw-Hill |year=1980 |isbn=0-07-050115-7 }}Nobel lecture given by G. Wald on December 12, 1967, online at nobelprize.org: The Molecular Basis of Visual Excitation. Analogously, gamma rays can in some circumstances dissociate atomic nuclei in a process called photodisintegration.Technological applications
Photons have many applications in technology. These examples are chosen to illustrate applications of photons per se, rather than general optical devices such as lenses, etc. that could operate under a classical theory of light. The laser is an extremely important application and is discussed above under stimulated emission. Individual photons can be detected by several methods. The classic photomultiplier tube exploits the photoelectric effect: a photon landing on a metal plate ejects an electron, initiating an ever-amplifying avalanche of electrons. Charge-coupled device chips use a similar effect in semiconductors: an incident photon generates a charge on a microscopic capacitor that can be detected. Other detectors such as Geiger counters use the ability of photons to ionize gas molecules, causing a detectable change in conductivity.Photomultiplier section 1.1.10, CCDs section 1.1.8, Geiger counters section 1.3.2.1 in {{cite book |first=C.R. |last=Kitchin |title=Astrophysical Techniques |publisher= CRC Press |location=Boca Raton (FL) |year=2008 |isbn=1420082434 }} Planck's energy formula E=h\nu is often used by engineers and chemists in design, both to compute the change in energy resulting from a photon absorption and to predict the frequency of the light emitted for a given energy transition. For example, the emission spectrum of a fluorescent light bulb can be designed using gas molecules with different electronic energy levels and adjusting the typical energy with which an electron hits the gas molecules within the bulb.An example is US Patent Nr. 5212709. Under some conditions, an energy transition can be excited by "two" photons that individually would be insufficient. This allows for higher resolution microscopy, because the sample absorbs energy only in the region where two beams of different colors overlap significantly, which can be made much smaller than the excitation volume of a single beam (see two-photon excitation microscopy). Moreover, these photons cause less damage to the sample, since they are of lower energy. In some cases, two energy transitions can be coupled so that, as one system absorbs a photon, another nearby system "steals" its energy and re-emits a photon of a different frequency. This is the basis of fluorescence resonance energy transfer, a technique that is used in molecular biology to study the interaction of suitable proteins. Several different kinds of hardware random number generator involve the detection of single photons. In one example, for each bit in the random sequence that is to be produced, a photon is sent to a beam-splitter. In such a situation, there are two possible outcomes of equal probability. The actual outcome is used to determine whether the next bit in the sequence is "0" or "1".Recent research
Much research has been devoted to applications of photons in the field of quantum optics. Photons seem well-suited to be elements of an extremely fast quantum computer, and the quantum entanglement of photons is a focus of research. Nonlinear optical processes are another active research area, with topics such as two-photon absorption, self-phase modulation, modulational instability and optical parametric oscillators. However, such processes generally do not require the assumption of photons per se; they may often be modeled by treating atoms as nonlinear oscillators. The nonlinear process of spontaneous parametric down conversion is often used to produce single-photon states. Finally, photons are essential in some aspects of optical communication, especially for quantum cryptography.Introductory-level material on the various sub-fields of quantum optics can be found in {{Cite book |last=Fox |first=M. |title=Quantum Optics: An Introduction |publisher= Oxford University Press |year=2006|url=http://books.google.com/books?id=Q-4dIthPuL4C&printsec=frontcover |isbn=0-19-856673-5 }}See also
- Doppler shift
- Light
- Electromagnetic radiation
- Quantum optics
- Photonics
- Phonons
- Photon polarization
- Ballistic photon
- Photography
- Laser
- Advanced Photon Source at Argonne National Laboratory
- Static forces and virtual-particle exchange
Notes
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