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Flint, occasionally flintstone, is a sedimentary cryptocrystalline form of the mineral quartz, categorized as the variety of chert that occurs in chalk or marly limestone. Historically, flint was widely used to make stone tools and start fires.
Flint occurs chiefly as nodules and masses in sedimentary rocks, such as chalks and limestones. Inside the nodule, flint is usually dark grey or black, green, white, or brown in colour, and has a glassy or waxy appearance. A thin, oxidised layer on the outside of the nodules is usually different in colour, typically white and rough in texture. The nodules can often be found along streams and beaches.
Flint breaks and chips into sharp-edged pieces, making it useful in constructing a variety of cutting tools, such as knife blades and scrapers. The use of flint to make stone tools dates back more than three million years; flint's extreme durability has made it possible to accurately date its use over this time. Flint is one of the primary materials used to define the Stone Age.
During the Stone Age, access to flint was so important for survival that people would travel or trade long distances to obtain the stone. Grime's Graves was an important source of flint traded across Europe. Flint Ridge in Ohio was another important source of flint, and Native Americans extracted the flint from hundreds of quarries along the ridge. This "Ohio Flint" was traded across the eastern United States, and has been found as far west as the Rocky Mountains and south around the Gulf of Mexico.
When struck against steel, flint will produce enough sparks to ignite a fire with the correct tinder, or gunpowder used in weapons, namely the flintlock firing mechanism. Although it has been superseded in these uses by different processes (the percussion cap), or materials (ferrocerium), "flint" has lent its name as generic term for a fire starter.
Origin | Flint | Wikipedia | 394 | 43701 | https://en.wikipedia.org/wiki/Flint | Physical sciences | Sedimentary rocks | Earth science |
The exact mode of formation of flint is not yet clear, but it is thought that it occurs as a result of chemical changes in compressed sedimentary rock formations during the process of diagenesis. One hypothesis is that a gelatinous material fills cavities in the sediment, such as holes bored by crustaceans or molluscs and that this becomes silicified. This hypothesis would certainly explain the complex shapes of flint nodules that are found. The source of dissolved silica in the porous media could be the spicules of silicious sponges (demosponges). Certain types of flint, such as that from the south coast of England and its counterpart on the French side of the Channel, contain trapped fossilised marine flora. Pieces of coral and vegetation have been found preserved inside the flint similar to insects and plant parts within amber. Thin slices of the stone often reveal this effect.
Flint sometimes occurs in large flint fields in Jurassic or Cretaceous beds, for example, in Europe. Puzzling giant flint formations known as paramoudra and flint circles are found around Europe but especially in Norfolk, England, on the beaches at Beeston Bump and West Runton.
The "Ohio flint" is the official gemstone of Ohio state. It is formed from limey debris that was deposited at the bottom of inland Paleozoic seas hundreds of millions of years ago that hardened into limestone and later became infused with silica. The flint from Flint Ridge is found in many hues like red, green, pink, blue, white, and grey, with the colour variations caused by minute impurities of iron compounds.
Flint can be coloured: sandy brown, medium to dark grey, black, reddish brown or an off-white grey.
Uses
Tools or cutting edges
Flint was used in the manufacture of tools during the Stone Age as it splits into thin, sharp splinters called flakes or blades (depending on the shape) when struck by another hard object (such as a hammerstone made of another material). This process is referred to as knapping. | Flint | Wikipedia | 416 | 43701 | https://en.wikipedia.org/wiki/Flint | Physical sciences | Sedimentary rocks | Earth science |
Flint mining is attested since the Paleolithic, but became more common since the Neolithic (Michelsberg culture, Funnelbeaker culture). In Europe, some of the best toolmaking flint has come from Belgium (Obourg, flint mines of Spiennes), the coastal chalks of the English Channel, the Paris Basin, Thy in Jutland (flint mine at Hov), the Sennonian deposits of Rügen, Grimes Graves in England, the Upper Cretaceous chalk formation of Dobruja and the lower Danube (Balkan flint), the Cenomanian chalky marl formation of the Moldavian Plateau (Miorcani flint) and the Jurassic deposits of the Kraków area and Krzemionki in Poland, as well as of the Lägern (silex) in the Jura Mountains of Switzerland.
In 1938, a project of the Ohio Historical Society, under the leadership of H. Holmes Ellis began to study the knapping methods and techniques of Native Americans. Like past studies, this work involved experimenting with actual knapping techniques by creation of stone tools through the use of techniques like direct freehand percussion, freehand pressure and pressure using a rest. Other scholars who have conducted similar experiments and studies include William Henry Holmes, Alonzo W. Pond, Francis H. S. Knowles and Don Crabtree.
To reduce susceptibility to fragmentation, flint/chert may be heat-treated, being slowly brought up to a temperature of for 24 hours, then slowly cooled to room temperature. This makes the material more homogeneous and thus more knappable and produces tools with a cleaner, sharper cutting edge. Heat treating was known to Stone Age artisans.
To ignite fire or gunpowder
When struck against steel, a flint edge produces sparks. The hard flint edge shaves off a particle of the steel that exposes iron, which reacts with oxygen from the atmosphere and can ignite the proper tinder.
Prior to the wide availability of steel, rocks of pyrite (FeS2) would be used along with the flint, in a similar (but more time-consuming) way. These methods remain popular in woodcraft, bushcraft, and amongst people practising traditional fire-starting skills.
Flintlocks | Flint | Wikipedia | 461 | 43701 | https://en.wikipedia.org/wiki/Flint | Physical sciences | Sedimentary rocks | Earth science |
A later, major use of flint and steel was in the flintlock mechanism, used primarily in flintlock firearms, but also used on dedicated fire-starting tools. A piece of flint held in the jaws of a spring-loaded hammer, when released by a trigger, strikes a hinged piece of steel ("frizzen") at an angle, creating a shower of sparks and exposing a charge of priming powder. The sparks ignite the priming powder and that flame, in turn, ignites the main charge, propelling the ball, bullet, or shot through the barrel. While the military use of the flintlock declined after the adoption of the percussion cap from the 1840s onward, flintlock rifles and shotguns remain in use amongst recreational shooters.
Comparison with ferrocerium
Flint and steel used to strike sparks were superseded in the 20th century by ferrocerium (sometimes referred to as "flint", although not true flint, "mischmetal", "hot spark", "metal match", or "fire steel"). This human-made material, when scraped with any hard, sharp edge, produces sparks that are much hotter than obtained with natural flint and steel, allowing use of a wider range of tinders. Because it can produce sparks when wet and can start fires when used correctly, ferrocerium is commonly included in survival kits. Ferrocerium is used in many cigarette lighters, where it is referred to as "a flint".
Fragmentation
Flint's utility as a fire starter is hampered by its property of uneven expansion under heating, causing it to fracture, sometimes violently, during heating. This tendency is enhanced by the impurities found in most samples of flint that may expand to a greater or lesser degree than the surrounding stone, and is similar to the tendency of glass to shatter when exposed to heat, and can become a drawback when flint is used as a building material.
As a building material | Flint | Wikipedia | 397 | 43701 | https://en.wikipedia.org/wiki/Flint | Physical sciences | Sedimentary rocks | Earth science |
Flint, knapped or unknapped, has been used from antiquity (for example at the Late Roman fort of Burgh Castle in Norfolk) up to the present day as a material for building stone walls, using lime mortar, and often combined with other available stone or brick rubble. It was most common in those parts of southern England where no good building stone was available locally, and where brick-making was not widespread until the later Middle Ages. It is especially associated with East Anglia, but also used in chalky areas stretching through Hampshire, Sussex, Surrey and Kent to Somerset.
Flint was used in the construction of many churches, houses, and other buildings, for example, the large stronghold of Framlingham Castle. Many different decorative effects have been achieved by using different types of knapping or arrangement and combinations with stone (flushwork), especially in the 15th and early 16th centuries. Because knapping flints to a relatively flush surface and size is a highly skilled process with a high level of wastage, flint finishes typically indicate high status buildings.
During World War I, in the chalky-soil country of France, the British filled sandbags with flint and used these sandbags as breastworks.
Ceramics
Flint pebbles are used as the media in ball mills to grind glazes and other raw materials for the ceramics industry. The pebbles are hand-selected based on colour; those having a tint of red, indicating high iron content, are discarded. The remaining blue-grey stones have a low content of chromophoric oxides and so are less deleterious to the colour of the ceramic composition after firing. | Flint | Wikipedia | 333 | 43701 | https://en.wikipedia.org/wiki/Flint | Physical sciences | Sedimentary rocks | Earth science |
Until recently calcined flint was also an important raw material in clay-based ceramic bodies produced in the UK. In clay bodies, calcined flint attenuates the shrinkage whilst drying, and modifies the fired thermal expansion. Flint can also be used in glazes as a network former. In preparation for use flint pebbles, frequently sourced from the coasts of South-East England or Western France, were calcined to around . This heating process both removed organic impurities and induced certain physical reactions, including converting some of the quartz to cristobalite. After calcination the flint pebbles were crushed and milled to a fine particle size. However, the use of flint has now been superseded by quartz. Because of the historical use of flint, the word "flint" is used by some potters (especially in the U.S.) to refer generically to siliceous raw materials used in ceramics that are not flint.
Jewelry
Flint bracelets were known in Ancient Egypt, and several examples have been found. | Flint | Wikipedia | 211 | 43701 | https://en.wikipedia.org/wiki/Flint | Physical sciences | Sedimentary rocks | Earth science |
Radiation pressure (also known as light pressure) is mechanical pressure exerted upon a surface due to the exchange of momentum between the object and the electromagnetic field. This includes the momentum of light or electromagnetic radiation of any wavelength that is absorbed, reflected, or otherwise emitted (e.g. black-body radiation) by matter on any scale (from macroscopic objects to dust particles to gas molecules). The associated force is called the radiation pressure force, or sometimes just the force of light.
The forces generated by radiation pressure are generally too small to be noticed under everyday circumstances; however, they are important in some physical processes and technologies. This particularly includes objects in outer space, where it is usually the main force acting on objects besides gravity, and where the net effect of a tiny force may have a large cumulative effect over long periods of time. For example, had the effects of the Sun's radiation pressure on the spacecraft of the Viking program been ignored, the spacecraft would have missed Mars orbit by about . Radiation pressure from starlight is crucial in a number of astrophysical processes as well. The significance of radiation pressure increases rapidly at extremely high temperatures and can sometimes dwarf the usual gas pressure, for instance, in stellar interiors and thermonuclear weapons. Furthermore, large lasers operating in space have been suggested as a means of propelling sail craft in beam-powered propulsion.
Radiation pressure forces are the bedrock of laser technology and the branches of science that rely heavily on lasers and other optical technologies. That includes, but is not limited to, biomicroscopy (where light is used to irradiate and observe microbes, cells, and molecules), quantum optics, and optomechanics (where light is used to probe and control objects like atoms, qubits and macroscopic quantum objects). Direct applications of the radiation pressure force in these fields are, for example, laser cooling (the subject of the 1997 Nobel Prize in Physics), quantum control of macroscopic objects and atoms (2012 Nobel Prize in Physics), interferometry (2017 Nobel Prize in Physics) and optical tweezers (2018 Nobel Prize in Physics). | Radiation pressure | Wikipedia | 436 | 43709 | https://en.wikipedia.org/wiki/Radiation%20pressure | Physical sciences | Classical mechanics | Physics |
Radiation pressure can equally well be accounted for by considering the momentum of a classical electromagnetic field or in terms of the momenta of photons, particles of light. The interaction of electromagnetic waves or photons with matter may involve an exchange of momentum. Due to the law of conservation of momentum, any change in the total momentum of the waves or photons must involve an equal and opposite change in the momentum of the matter it interacted with (Newton's third law of motion), as is illustrated in the accompanying figure for the case of light being perfectly reflected by a surface. This transfer of momentum is the general explanation for what we term radiation pressure.
Discovery
Johannes Kepler put forward the concept of radiation pressure in 1619 to explain the observation that a tail of a comet always points away from the Sun.
The assertion that light, as electromagnetic radiation, has the property of momentum and thus exerts a pressure upon any surface that is exposed to it was published by James Clerk Maxwell in 1862, and proven experimentally by Russian physicist Pyotr Lebedev in 1900 and by Ernest Fox Nichols and Gordon Ferrie Hull in 1901. The pressure is very small, but can be detected by allowing the radiation to fall upon a delicately poised vane of reflective metal in a Nichols radiometer (this should not be confused with the Crookes radiometer, whose characteristic motion is not caused by radiation pressure but by air flow caused by temperature differentials.)
Theory
Radiation pressure can be viewed as a consequence of the conservation of momentum given the momentum attributed to electromagnetic radiation. That momentum can be equally well calculated on the basis of electromagnetic theory or from the combined momenta of a stream of photons, giving identical results as is shown below.
Radiation pressure from momentum of an electromagnetic wave
According to Maxwell's theory of electromagnetism, an electromagnetic wave carries momentum. Momentum will be transferred to any surface it strikes that absorbs or reflects the radiation. | Radiation pressure | Wikipedia | 388 | 43709 | https://en.wikipedia.org/wiki/Radiation%20pressure | Physical sciences | Classical mechanics | Physics |
Consider the momentum transferred to a perfectly absorbing (black) surface. The energy flux (irradiance) of a plane wave is calculated using the Poynting vector , which is the cross product of the electric field vector E and the magnetic field's auxiliary field vector (or magnetizing field) H. The magnitude, denoted by S, divided by the speed of light is the density of the linear momentum per unit area (pressure) of the electromagnetic field. So, dimensionally, the Poynting vector is , which is the speed of light, , times pressure, . That pressure is experienced as radiation pressure on the surface:
where is pressure (usually in pascals), is the incident irradiance (usually in W/m2) and is the speed of light in vacuum. Here, .
If the surface is planar at an angle α to the incident wave, the intensity across the surface will be geometrically reduced by the cosine of that angle and the component of the radiation force against the surface will also be reduced by the cosine of α, resulting in a pressure:
The momentum from the incident wave is in the same direction of that wave. But only the component of that momentum normal to the surface contributes to the pressure on the surface, as given above. The component of that force tangent to the surface is not called pressure.
Radiation pressure from reflection
The above treatment for an incident wave accounts for the radiation pressure experienced by a black (totally absorbing) body. If the wave is specularly reflected, then the recoil due to the reflected wave will further contribute to the radiation pressure. In the case of a perfect reflector, this pressure will be identical to the pressure caused by the incident wave:
thus doubling the net radiation pressure on the surface:
For a partially reflective surface, the second term must be multiplied by the reflectivity (also known as reflection coefficient of intensity), so that the increase is less than double. For a diffusely reflective surface, the details of the reflection and geometry must be taken into account, again resulting in an increased net radiation pressure of less than double.
Radiation pressure by emission
Just as a wave reflected from a body contributes to the net radiation pressure experienced, a body that emits radiation of its own (rather than reflected) obtains a radiation pressure again given by the irradiance of that emission in the direction normal to the surface Ie: | Radiation pressure | Wikipedia | 485 | 43709 | https://en.wikipedia.org/wiki/Radiation%20pressure | Physical sciences | Classical mechanics | Physics |
The emission can be from black-body radiation or any other radiative mechanism. Since all materials emit black-body radiation (unless they are totally reflective or at absolute zero), this source for radiation pressure is ubiquitous but usually tiny. However, because black-body radiation increases rapidly with temperature (as the fourth power of temperature, given by the Stefan–Boltzmann law), radiation pressure due to the temperature of a very hot object (or due to incoming black-body radiation from similarly hot surroundings) can become significant. This is important in stellar interiors.
Radiation pressure in terms of photons
Electromagnetic radiation can be viewed in terms of particles rather than waves; these particles are known as photons. Photons do not have a rest-mass; however, photons are never at rest (they move at the speed of light) and acquire a momentum nonetheless which is given by:
where is momentum, is the Planck constant, is wavelength, and is speed of light in vacuum. And is the energy of a single photon given by:
The radiation pressure again can be seen as the transfer of each photon's momentum to the opaque surface, plus the momentum due to a (possible) recoil photon for a (partially) reflecting surface. Since an incident wave of irradiance over an area has a power of , this implies a flux of photons per second per unit area striking the surface. Combining this with the above expression for the momentum of a single photon, results in the same relationships between irradiance and radiation pressure described above using classical electromagnetics. And again, reflected or otherwise emitted photons will contribute to the net radiation pressure identically.
Compression in a uniform radiation field
In general, the pressure of electromagnetic waves can be obtained from the vanishing of the trace of the electromagnetic stress tensor: since this trace equals 3P − u, we get
where is the radiation energy per unit volume.
This can also be shown in the specific case of the pressure exerted on surfaces of a body in thermal equilibrium with its surroundings, at a temperature : the body will be surrounded by a uniform radiation field described by the Planck black-body radiation law and will experience a compressive pressure due to that impinging radiation, its reflection, and its own black-body emission. From that it can be shown that the resulting pressure is equal to one third of the total radiant energy per unit volume in the surrounding space.
By using Stefan–Boltzmann law, this can be expressed as
where is the Stefan–Boltzmann constant. | Radiation pressure | Wikipedia | 510 | 43709 | https://en.wikipedia.org/wiki/Radiation%20pressure | Physical sciences | Classical mechanics | Physics |
Solar radiation pressure
Solar radiation pressure is due to the Sun's radiation at closer distances, thus especially within the Solar System. While it acts on all objects, its net effect is generally greater on smaller bodies, since they have a larger ratio of surface area to mass. All spacecraft experience such a pressure, except when they are behind the shadow of a larger orbiting body.
Solar radiation pressure on objects near the Earth may be calculated using the Sun's irradiance at 1 AU, known as the solar constant, or GSC, whose value is set at 1361 W/m2 as of 2011.
All stars have a spectral energy distribution that depends on their surface temperature. The distribution is approximately that of black-body radiation. This distribution must be taken into account when calculating the radiation pressure or identifying reflector materials for optimizing a solar sail, for instance.
Momentary or hours long solar pressures can indeed escalate due to release of solar flares and coronal mass ejections, but effects remain essentially immeasureable in relation to Earth's orbit. However these pressures persist over eons, such that cumulatively having produced a measureable movement on the Earth-Moon system's orbit.
Pressures of absorption and reflection
Solar radiation pressure at the Earth's distance from the Sun, may be calculated by dividing the solar constant GSC (above) by the speed of light c. For an absorbing sheet facing the Sun, this is simply:
This result is in pascals, equivalent to N/m2 (newtons per square meter). For a sheet at an angle α to the Sun, the effective area A of a sheet is reduced by a geometrical factor resulting in a force in the direction of the sunlight of:
To find the component of this force normal to the surface, another cosine factor must be applied resulting in a pressure P on the surface of:
Note, however, that in order to account for the net effect of solar radiation on a spacecraft for instance, one would need to consider the total force (in the direction away from the Sun) given by the preceding equation, rather than just the component normal to the surface that we identify as "pressure".
The solar constant is defined for the Sun's radiation at the distance to the Earth, also known as one astronomical unit (au). Consequently, at a distance of R astronomical units (R thus being dimensionless), applying the inverse-square law, we would find: | Radiation pressure | Wikipedia | 500 | 43709 | https://en.wikipedia.org/wiki/Radiation%20pressure | Physical sciences | Classical mechanics | Physics |
Finally, considering not an absorbing but a perfectly reflecting surface, the pressure is doubled due to the reflected wave, resulting in:
Note that unlike the case of an absorbing material, the resulting force on a reflecting body is given exactly by this pressure acting normal to the surface, with the tangential forces from the incident and reflecting waves canceling each other. In practice, materials are neither totally reflecting nor totally absorbing, so the resulting force will be a weighted average of the forces calculated using these formulas.
Radiation pressure perturbations
Solar radiation pressure is a source of orbital perturbations. It significantly affects the orbits and trajectories of small bodies including all spacecraft.
Solar radiation pressure affects bodies throughout much of the Solar System. Small bodies are more affected than large ones because of their lower mass relative to their surface area. Spacecraft are affected along with natural bodies (comets, asteroids, dust grains, gas molecules).
The radiation pressure results in forces and torques on the bodies that can change their translational and rotational motions. Translational changes affect the orbits of the bodies. Rotational rates may increase or decrease. Loosely aggregated bodies may break apart under high rotation rates. Dust grains can either leave the Solar System or spiral into the Sun.
A whole body is typically composed of numerous surfaces that have different orientations on the body. The facets may be flat or curved. They will have different areas. They may have optical properties differing from other aspects.
At any particular time, some facets are exposed to the Sun, and some are in shadow. Each surface exposed to the Sun is reflecting, absorbing, and emitting radiation. Facets in shadow are emitting radiation. The summation of pressures across all of the facets defines the net force and torque on the body. These can be calculated using the equations in the preceding sections.
The Yarkovsky effect affects the translation of a small body. It results from a face leaving solar exposure being at a higher temperature than a face approaching solar exposure. The radiation emitted from the warmer face is more intense than that of the opposite face, resulting in a net force on the body that affects its motion.
The YORP effect is a collection of effects expanding upon the earlier concept of the Yarkovsky effect, but of a similar nature. It affects the spin properties of bodies. | Radiation pressure | Wikipedia | 469 | 43709 | https://en.wikipedia.org/wiki/Radiation%20pressure | Physical sciences | Classical mechanics | Physics |
The Poynting–Robertson effect applies to grain-size particles. From the perspective of a grain of dust circling the Sun, the Sun's radiation appears to be coming from a slightly forward direction (aberration of light). Therefore, the absorption of this radiation leads to a force with a component against the direction of movement. (The angle of aberration is tiny, since the radiation is moving at the speed of light, while the dust grain is moving many orders of magnitude slower than that.) The result is a gradual spiral of dust grains into the Sun. Over long periods of time, this effect cleans out much of the dust in the Solar System.
While rather small in comparison to other forces, the radiation pressure force is inexorable. Over long periods of time, the net effect of the force is substantial. Such feeble pressures can produce marked effects upon minute particles like gas ions and electrons, and are essential in the theory of electron emission from the Sun, of cometary material, and so on.
Because the ratio of surface area to volume (and thus mass) increases with decreasing particle size, dusty (micrometre-size) particles are susceptible to radiation pressure even in the outer Solar System. For example, the evolution of the outer rings of Saturn is significantly influenced by radiation pressure.
As a consequence of light pressure, Einstein in 1909 predicted the existence of "radiation friction", which would oppose the movement of matter. He wrote: "radiation will exert pressure on both sides of the plate. The forces of pressure exerted on the two sides are equal if the plate is at rest. However, if it is in motion, more radiation will be reflected on the surface that is ahead during the motion (front surface) than on the back surface. The backward acting force of pressure exerted on the front surface is thus larger than the force of pressure acting on the back. Hence, as the resultant of the two forces, there remains a force that counteracts the motion of the plate and that increases with the velocity of the plate. We will call this resultant 'radiation friction' in brief."
Solar sails
Solar sailing, an experimental method of spacecraft propulsion, uses radiation pressure from the Sun as a motive force. The idea of interplanetary travel by light was mentioned by Jules Verne in his 1865 novel From the Earth to the Moon. | Radiation pressure | Wikipedia | 482 | 43709 | https://en.wikipedia.org/wiki/Radiation%20pressure | Physical sciences | Classical mechanics | Physics |
A sail reflects about 90% of the incident radiation. The 10% that is absorbed is radiated away from both surfaces, with the proportion emitted from the unlit surface depending on the thermal conductivity of the sail. A sail has curvature, surface irregularities, and other minor factors that affect its performance.
The Japan Aerospace Exploration Agency (JAXA) has successfully unfurled a solar sail in space, which has already succeeded in propelling its payload with the IKAROS project.
Cosmic effects of radiation pressure
Radiation pressure has had a major effect on the development of the cosmos, from the birth of the universe to ongoing formation of stars and shaping of clouds of dust and gasses on a wide range of scales.
Early universe
The photon epoch is a phase when the energy of the universe was dominated by photons, between 10 seconds and 380,000 years after the Big Bang.
Galaxy formation and evolution
The process of galaxy formation and evolution began early in the history of the cosmos. Observations of the early universe strongly suggest that objects grew from bottom-up (i.e., smaller objects merging to form larger ones). As stars are thereby formed and become sources of electromagnetic radiation, radiation pressure from the stars becomes a factor in the dynamics of remaining circumstellar material.
Clouds of dust and gases
The gravitational compression of clouds of dust and gases is strongly influenced by radiation pressure, especially when the condensations lead to star births. The larger young stars forming within the compressed clouds emit intense levels of radiation that shift the clouds, causing either dispersion or condensations in nearby regions, which influences birth rates in those nearby regions.
Clusters of stars
Stars predominantly form in regions of large clouds of dust and gases, giving rise to star clusters. Radiation pressure from the member stars eventually disperses the clouds, which can have a profound effect on the evolution of the cluster.
Many open clusters are inherently unstable, with a small enough mass that the escape velocity of the system is lower than the average velocity of the constituent stars. These clusters will rapidly disperse within a few million years. In many cases, the stripping away of the gas from which the cluster formed by the radiation pressure of the hot young stars reduces the cluster mass enough to allow rapid dispersal.
Star formation | Radiation pressure | Wikipedia | 454 | 43709 | https://en.wikipedia.org/wiki/Radiation%20pressure | Physical sciences | Classical mechanics | Physics |
Star formation is the process by which dense regions within molecular clouds in interstellar space collapse to form stars. As a branch of astronomy, star formation includes the study of the interstellar medium and giant molecular clouds (GMC) as precursors to the star formation process, and the study of protostars and young stellar objects as its immediate products. Star formation theory, as well as accounting for the formation of a single star, must also account for the statistics of binary stars and the initial mass function.
Stellar planetary systems
Planetary systems are generally believed to form as part of the same process that results in star formation. A protoplanetary disk forms by gravitational collapse of a molecular cloud, called a solar nebula, and then evolves into a planetary system by collisions and gravitational capture. Radiation pressure can clear a region in the immediate vicinity of the star. As the formation process continues, radiation pressure continues to play a role in affecting the distribution of matter. In particular, dust and grains can spiral into the star or escape the stellar system under the action of radiation pressure.
Stellar interiors
In stellar interiors the temperatures are very high. Stellar models predict a temperature of 15 MK in the center of the Sun, and at the cores of supergiant stars the temperature may exceed 1 GK. As the radiation pressure scales as the fourth power of the temperature, it becomes important at these high temperatures. In the Sun, radiation pressure is still quite small when compared to the gas pressure. In the heaviest non-degenerate stars, radiation pressure is the dominant pressure component.
Comets
Solar radiation pressure strongly affects comet tails. Solar heating causes gases to be released from the comet nucleus, which also carry away dust grains. Radiation pressure and solar wind then drive the dust and gases away from the Sun's direction. The gases form a generally straight tail, while slower moving dust particles create a broader, curving tail.
Laser applications of radiation pressure
Optical tweezers
Lasers can be used as a source of monochromatic light with wavelength . With a set of lenses, one can focus the laser beam to a point that is in diameter (or ).
The radiation pressure of a P = 30 mW laser with λ = 1064 nm can therefore be computed as follows.
Area:
force:
pressure:
This is used to trap or levitate particles in optical tweezers.
Light–matter interactions | Radiation pressure | Wikipedia | 474 | 43709 | https://en.wikipedia.org/wiki/Radiation%20pressure | Physical sciences | Classical mechanics | Physics |
The reflection of a laser pulse from the surface of an elastic solid can give rise to various types of elastic waves that propagate inside the solid or liquid. In other words, the light can excite and/or amplify motion of, and in, materials. This is the subject of study in the field of optomechanics. The weakest waves are generally those that are generated by the radiation pressure acting during the reflection of the light. Such light-pressure-induced elastic waves have for example observed inside an ultrahigh-reflectivity dielectric mirror. These waves are the most basic fingerprint of a light-solid matter interaction on the macroscopic scale. In the field of cavity optomechanics, light is trapped and resonantly enhanced in optical cavities, for example between mirrors. This serves the purpose of gravely enhancing the power of the light, and the radiation pressure it can exert on objects and materials. Optical control (that is, manipulation of the motion) of a plethora of objects has been realized: from kilometers long beams (such as in the LIGO interferometer) to clouds of atoms, and from micro-engineered trampolines to superfluids.
Opposite to exciting or amplifying motion, light can also damp the motion of objects. Laser cooling is a method of cooling materials very close to absolute zero by converting some of material's motional energy into light. Kinetic energy and thermal energy of the material are synonyms here, because they represent the energy associated with Brownian motion of the material. Atoms traveling towards a laser light source perceive a doppler effect tuned to the absorption frequency of the target element. The radiation pressure on the atom slows movement in a particular direction until the Doppler effect moves out of the frequency range of the element, causing an overall cooling effect.
An other active research area of laser–matter interaction is the radiation pressure acceleration of ions or protons from thin–foil targets. High ion energy beams can be generated for medical applications (for example in ion beam therapy) by the radiation pressure of short laser pulses on ultra-thin foils. | Radiation pressure | Wikipedia | 437 | 43709 | https://en.wikipedia.org/wiki/Radiation%20pressure | Physical sciences | Classical mechanics | Physics |
Silicon dioxide, also known as silica, is an oxide of silicon with the chemical formula , commonly found in nature as quartz. In many parts of the world, silica is the major constituent of sand. Silica is one of the most complex and abundant families of materials, existing as a compound of several minerals and as a synthetic product. Examples include fused quartz, fumed silica, opal, and aerogels. It is used in structural materials, microelectronics, and as components in the food and pharmaceutical industries. All forms are white or colorless, although impure samples can be colored.
Silicon dioxide is a common fundamental constituent of glass.
Structure
In the majority of silicon dioxides, the silicon atom shows tetrahedral coordination, with four oxygen atoms surrounding a central Si atom (see 3-D Unit Cell). Thus, SiO2 forms 3-dimensional network solids in which each silicon atom is covalently bonded in a tetrahedral manner to 4 oxygen atoms. In contrast, CO2 is a linear molecule. The starkly different structures of the dioxides of carbon and silicon are a manifestation of the double bond rule.
Based on the crystal structural differences, silicon dioxide can be divided into two categories: crystalline and non-crystalline (amorphous). In crystalline form, this substance can be found naturally occurring as quartz, tridymite (high-temperature form), cristobalite (high-temperature form), stishovite (high-pressure form), and coesite (high-pressure form). On the other hand, amorphous silica can be found in nature as opal and diatomaceous earth. Quartz glass is a form of intermediate state between these structures.
All of these distinct crystalline forms always have the same local structure around Si and O. In α-quartz the Si–O bond length is 161 pm, whereas in α-tridymite it is in the range 154–171 pm. The Si–O–Si angle also varies between a low value of 140° in α-tridymite, up to 180° in β-tridymite. In α-quartz, the Si–O–Si angle is 144°. | Silicon dioxide | Wikipedia | 454 | 43710 | https://en.wikipedia.org/wiki/Silicon%20dioxide | Physical sciences | Ceramic compounds | Chemistry |
Polymorphism
Alpha quartz is the most stable form of solid SiO2 at room temperature. The high-temperature minerals, cristobalite and tridymite, have both lower densities and indices of refraction than quartz. The transformation from α-quartz to beta-quartz takes place abruptly at 573 °C. Since the transformation is accompanied by a significant change in volume, it can easily induce fracturing of ceramics or rocks passing through this temperature limit. The high-pressure minerals, seifertite, stishovite, and coesite, though, have higher densities and indices of refraction than quartz. Stishovite has a rutile-like structure where silicon is 6-coordinate. The density of stishovite is 4.287 g/cm3, which compares to α-quartz, the densest of the low-pressure forms, which has a density of 2.648 g/cm3. The difference in density can be ascribed to the increase in coordination as the six shortest Si–O bond lengths in stishovite (four Si–O bond lengths of 176 pm and two others of 181 pm) are greater than the Si–O bond length (161 pm) in α-quartz.
The change in the coordination increases the ionicity of the Si–O bond.
Faujasite silica, another polymorph, is obtained by the dealumination of a low-sodium, ultra-stable Y zeolite with combined acid and thermal treatment. The resulting product contains over 99% silica, and has high crystallinity and specific surface area (over 800 m2/g). Faujasite-silica has very high thermal and acid stability. For example, it maintains a high degree of long-range molecular order or crystallinity even after boiling in concentrated hydrochloric acid.
Molten SiO2
Molten silica exhibits several peculiar physical characteristics that are similar to those observed in liquid water: negative temperature expansion, density maximum at temperatures ~5000 °C, and a heat capacity minimum. Its density decreases from 2.08 g/cm3 at 1950 °C to 2.03 g/cm3 at 2200 °C. | Silicon dioxide | Wikipedia | 447 | 43710 | https://en.wikipedia.org/wiki/Silicon%20dioxide | Physical sciences | Ceramic compounds | Chemistry |
Molecular SiO2
The molecular SiO2 has a linear structure like . It has been produced by combining silicon monoxide (SiO) with oxygen in an argon matrix.
The dimeric silicon dioxide, (SiO2)2 has been obtained by reacting O2 with matrix isolated dimeric silicon monoxide, (Si2O2). In dimeric silicon dioxide there are two oxygen atoms bridging between the silicon atoms with an Si–O–Si angle of 94° and bond length of 164.6 pm and the terminal Si–O bond length is 150.2 pm. The Si–O bond length is 148.3 pm, which compares with the length of 161 pm in α-quartz. The bond energy is estimated at 621.7 kJ/mol.
Natural occurrence
Geology
is most commonly encountered in nature as quartz, which comprises more than 10% by mass of the Earth's crust. Quartz is the only polymorph of silica stable at the Earth's surface. Metastable occurrences of the high-pressure forms coesite and stishovite have been found around impact structures and associated with eclogites formed during ultra-high-pressure metamorphism. The high-temperature forms of tridymite and cristobalite are known from silica-rich volcanic rocks. In many parts of the world, silica is the major constituent of sand.
Biology
Even though it is poorly soluble, silica occurs in many plants such as rice. Plant materials with high silica phytolith content appear to be of importance to grazing animals, from chewing insects to ungulates. Silica accelerates tooth wear, and high levels of silica in plants frequently eaten by herbivores may have developed as a defense mechanism against predation.
Silica is also the primary component of rice husk ash, which is used, for example, in filtration and as supplementary cementitious material (SCM) in cement and concrete manufacturing.
Silicification in and by cells has been common in the biological world and it occurs in bacteria, protists, plants, and animals (invertebrates and vertebrates). | Silicon dioxide | Wikipedia | 443 | 43710 | https://en.wikipedia.org/wiki/Silicon%20dioxide | Physical sciences | Ceramic compounds | Chemistry |
Prominent examples include:
Tests or frustules (i.e. shells) of diatoms, Radiolaria, and testate amoebae.
Silica phytoliths in the cells of many plants including Equisetaceae, many grasses, and a wide range of dicotyledons.
The spicules forming the skeleton of many sponges.
Uses
Structural use
About 95% of the commercial use of silicon dioxide (sand) is in the construction industry, e.g. in the production of concrete (Portland cement concrete).
Certain deposits of silica sand, with desirable particle size and shape and desirable clay and other mineral content, were important for sand casting of metallic products. The high melting point of silica enables it to be used in such applications such as iron casting; modern sand casting sometimes uses other minerals for other reasons.
Crystalline silica is used in hydraulic fracturing of formations which contain tight oil and shale gas.
Precursor to glass and silicon
Silica is the primary ingredient in the production of most glass. As other minerals are melted with silica, the principle of freezing point depression lowers the melting point of the mixture and increases fluidity. The glass transition temperature of pure SiO2 is about 1475 K. When molten silicon dioxide SiO2 is rapidly cooled, it does not crystallize, but solidifies as a glass. Because of this, most ceramic glazes have silica as the main ingredient.
The structural geometry of silicon and oxygen in glass is similar to that in quartz and most other crystalline forms of silicon and oxygen, with silicon surrounded by regular tetrahedra of oxygen centres. The difference between the glass and crystalline forms arises from the connectivity of the tetrahedral units: Although there is no long-range periodicity in the glassy network, ordering remains at length scales well beyond the SiO bond length. One example of this ordering is the preference to form rings of 6-tetrahedra.
The majority of optical fibers for telecommunications are also made from silica. It is a primary raw material for many ceramics such as earthenware, stoneware, and porcelain.
Silicon dioxide is used to produce elemental silicon. The process involves carbothermic reduction in an electric arc furnace:
SiO2 + 2 C -> Si + 2 CO | Silicon dioxide | Wikipedia | 470 | 43710 | https://en.wikipedia.org/wiki/Silicon%20dioxide | Physical sciences | Ceramic compounds | Chemistry |
Fumed silica
Fumed silica, also known as pyrogenic silica, is prepared by burning SiCl4 in an oxygen-rich hydrogen flame to produce a "smoke" of SiO2.
SiCl4 + 2 H2 + O2 -> SiO2 + 4 HCl
It can also be produced by vaporizing quartz sand in a 3000 °C electric arc. Both processes result in microscopic droplets of amorphous silica fused into branched, chainlike, three-dimensional secondary particles which then agglomerate into tertiary particles, a white powder with extremely low bulk density (0.03-0.15 g/cm3) and thus high surface area. The particles act as a thixotropic thickening agent, or as an anti-caking agent, and can be treated to make them hydrophilic or hydrophobic for either water or organic liquid applications.
Silica fume is an ultrafine powder collected as a by-product of the silicon and ferrosilicon alloy production. It consists of amorphous (non-crystalline) spherical particles with an average particle diameter of 150 nm, without the branching of the pyrogenic product. The main use is as pozzolanic material for high performance concrete. Fumed silica nanoparticles can be successfully used as an anti-aging agent in asphalt binders.
Food, cosmetic, and pharmaceutical applications
Silica, either colloidal, precipitated, or pyrogenic fumed, is a common additive in food production. It is used primarily as a flow or anti-caking agent in powdered foods such as spices and non-dairy coffee creamer, or powders to be formed into pharmaceutical tablets. It can adsorb water in hygroscopic applications. Colloidal silica is used as a fining agent for wine, beer, and juice, with the E number reference E551.
In cosmetics, silica is useful for its light-diffusing properties and natural absorbency.
Diatomaceous earth, a mined product, has been used in food and cosmetics for centuries. It consists of the silica shells of microscopic diatoms; in a less processed form it was sold as "tooth powder". Manufactured or mined hydrated silica is used as the hard abrasive in toothpaste.
Semiconductors
Silicon dioxide is widely used in the semiconductor technology: | Silicon dioxide | Wikipedia | 494 | 43710 | https://en.wikipedia.org/wiki/Silicon%20dioxide | Physical sciences | Ceramic compounds | Chemistry |
for the primary passivation (directly on the semiconductor surface),
as an original gate dielectric in MOS technology. Today when scaling (dimension of the gate length of the MOS transistor) has progressed below 10 nm, silicon dioxide has been replaced by other dielectric materials like hafnium oxide or similar with higher dielectric constant compared to silicon dioxide,
as a dielectric layer between metal (wiring) layers (sometimes up to 8–10) connecting elements and
as a second passivation layer (for protecting semiconductor elements and the metallization layers) typically today layered with some other dielectrics like silicon nitride.
Because silicon dioxide is a native oxide of silicon it is more widely used compared to other semiconductors like gallium arsenide or indium phosphide.
Silicon dioxide could be grown on a silicon semiconductor surface. Silicon oxide layers could protect silicon surfaces during diffusion processes, and could be used for diffusion masking.
Surface passivation is the process by which a semiconductor surface is rendered inert, and does not change semiconductor properties as a result of interaction with air or other materials in contact with the surface or edge of the crystal. The formation of a thermally grown silicon dioxide layer greatly reduces the concentration of electronic states at the silicon surface. SiO2 films preserve the electrical characteristics of p–n junctions and prevent these electrical characteristics from deteriorating by the gaseous ambient environment. Silicon oxide layers could be used to electrically stabilize silicon surfaces. The surface passivation process is an important method of semiconductor device fabrication that involves coating a silicon wafer with an insulating layer of silicon oxide so that electricity could reliably penetrate to the conducting silicon below. Growing a layer of silicon dioxide on top of a silicon wafer enables it to overcome the surface states that otherwise prevent electricity from reaching the semiconducting layer.
The process of silicon surface passivation by thermal oxidation (silicon dioxide) is critical to the semiconductor industry. It is commonly used to manufacture metal–oxide–semiconductor field-effect transistors (MOSFETs) and silicon integrated circuit chips (with the planar process).
Other
Hydrophobic silica is used as a defoamer component.
In its capacity as a refractory, it is useful in fiber form as a high-temperature thermal protection fabric.
Silica is used in the extraction of DNA and RNA due to its ability to bind to the nucleic acids under the presence of chaotropes. | Silicon dioxide | Wikipedia | 500 | 43710 | https://en.wikipedia.org/wiki/Silicon%20dioxide | Physical sciences | Ceramic compounds | Chemistry |
Silica aerogel was used in the Stardust spacecraft to collect extraterrestrial particles.
Pure silica (silicon dioxide), when cooled as fused quartz into a glass with no true melting point, can be used as a glass fibre for fibreglass.
Production
Silicon dioxide is mostly obtained by mining, including sand mining and purification of quartz.
Quartz is suitable for many purposes, while chemical processing is required to make a purer or otherwise more suitable (e.g. more reactive or fine-grained) product.
Precipitated silica
Precipitated silica or amorphous silica is produced by the acidification of solutions of sodium silicate. The gelatinous precipitate or silica gel, is first washed and then dehydrated to produce colorless microporous silica. The idealized equation involving a trisilicate and sulfuric acid is:
Na2Si3O7 + H2SO4 -> 3 SiO2 + Na2SO4 + H2O
Approximately one billion kilograms/year (1999) of silica were produced in this manner, mainly for use for polymer composites – tires and shoe soles.
On microchips
Thin films of silica grow spontaneously on silicon wafers via thermal oxidation, producing a very shallow layer of about 1 nm or 10 Å of so-called native oxide.
Higher temperatures and alternative environments are used to grow well-controlled layers of silicon dioxide on silicon, for example at temperatures between 600 and 1200 °C, using so-called dry oxidation with O2
Si + O2 -> SiO2
or wet oxidation with H2O.
Si + 2 H2O -> SiO2 + 2 H2
The native oxide layer is beneficial in microelectronics, where it acts as electric insulator with high chemical stability. It can protect the silicon, store charge, block current, and even act as a controlled pathway to limit current flow.
Laboratory or special methods
From organosilicon compounds
Many routes to silicon dioxide start with an organosilicon compound, e.g., HMDSO, TEOS. Synthesis of silica is illustrated below using tetraethyl orthosilicate (TEOS). Simply heating TEOS at 680–730 °C results in the oxide:
Si(OC2H5)4 -> SiO2 + 2 O(C2H5)2
Similarly TEOS combusts around 400 °C: | Silicon dioxide | Wikipedia | 504 | 43710 | https://en.wikipedia.org/wiki/Silicon%20dioxide | Physical sciences | Ceramic compounds | Chemistry |
Si(OC2H5)4 + 12 O2 -> SiO2 + 10 H2O + 8 CO2
TEOS undergoes hydrolysis via the so-called sol-gel process. The course of the reaction and nature of the product are affected by catalysts, but the idealized equation is:
Si(OC2H5)4 + 2 H2O -> SiO2 + 4 HOCH2CH3
Other methods
Being highly stable, silicon dioxide arises from many methods. Conceptually simple, but of little practical value, combustion of silane gives silicon dioxide. This reaction is analogous to the combustion of methane:
SiH4 + 2 O2 -> SiO2 + 2 H2O
However the chemical vapor deposition of silicon dioxide onto crystal surface from silane had been used using nitrogen as a carrier gas at 200–500 °C.
Chemical reactions
Silicon dioxide is a relatively inert material (hence its widespread occurrence as a mineral). Silica is often used as inert containers for chemical reactions. At high temperatures, it is converted to silicon by reduction with carbon.
Fluorine reacts with silicon dioxide to form SiF4 and O2 whereas the other halogen gases (Cl2, Br2, I2) are unreactive.
Most forms of silicon dioxide are attacked ("etched") by hydrofluoric acid (HF) to produce hexafluorosilicic acid:
Stishovite does not react to HF to any significant degree.
HF is used to remove or pattern silicon dioxide in the semiconductor industry.
Silicon dioxide acts as a Lux–Flood acid, being able to react with bases under certain conditions. As it does not contain any hydrogen, non-hydrated silica cannot directly act as a Brønsted–Lowry acid. While silicon dioxide is only poorly soluble in water at low or neutral pH (typically, 2 × 10−4 M for quartz up to 10−3 M for cryptocrystalline chalcedony), strong bases react with glass and easily dissolve it. Therefore, strong bases have to be stored in plastic bottles to avoid jamming the bottle cap, to preserve the integrity of the recipient, and to avoid undesirable contamination by silicate anions.
Silicon dioxide dissolves in hot concentrated alkali or fused hydroxide, as described in this idealized equation:
SiO2 + 2 NaOH -> Na2SiO3 + H2O | Silicon dioxide | Wikipedia | 505 | 43710 | https://en.wikipedia.org/wiki/Silicon%20dioxide | Physical sciences | Ceramic compounds | Chemistry |
Silicon dioxide will neutralise basic metal oxides (e.g. sodium oxide, potassium oxide, lead(II) oxide, zinc oxide, or mixtures of oxides, forming silicates and glasses as the Si-O-Si bonds in silica are broken successively). As an example the reaction of sodium oxide and SiO2 can produce sodium orthosilicate, sodium silicate, and glasses, dependent on the proportions of reactants:
2 Na2O + SiO2 -> Na4SiO4;
Na2O + SiO2 -> Na2SiO3;
Na2O + SiO2 -> glass.
Examples of such glasses have commercial significance, e.g. soda–lime glass, borosilicate glass, lead glass. In these glasses, silica is termed the network former or lattice former. The reaction is also used in blast furnaces to remove sand impurities in the ore by neutralisation with calcium oxide, forming calcium silicate slag.
Silicon dioxide reacts in heated reflux under dinitrogen with ethylene glycol and an alkali metal base to produce highly reactive, pentacoordinate silicates which provide access to a wide variety of new silicon compounds. The silicates are essentially insoluble in all polar solvent except methanol.
Silicon dioxide reacts with elemental silicon at high temperatures to produce SiO:
SiO2 + Si -> 2 SiO
Water solubility
The solubility of silicon dioxide in water strongly depends on its crystalline form and is three to four times higher for amorphous silica than quartz; as a function of temperature, it peaks around . This property is used to grow single crystals of quartz in a hydrothermal process where natural quartz is dissolved in superheated water in a pressure vessel that is cooler at the top. Crystals of 0.5–1 kg can be grown for 1–2 months. These crystals are a source of very pure quartz for use in electronic applications. Above the critical temperature of water and a pressure of or higher, water is a supercritical fluid and solubility is once again higher than at lower temperatures.
Health effects | Silicon dioxide | Wikipedia | 448 | 43710 | https://en.wikipedia.org/wiki/Silicon%20dioxide | Physical sciences | Ceramic compounds | Chemistry |
Silica ingested orally is essentially nontoxic, with an of 5000 mg/kg (5 g/kg). A 2008 study following subjects for 15 years found that higher levels of silica in water appeared to decrease the risk of dementia. An increase of 10 mg/day of silica in drinking water was associated with a reduced risk of dementia of 11%.
Inhaling finely divided crystalline silica dust can lead to silicosis, bronchitis, or lung cancer, as the dust becomes lodged in the lungs and continuously irritates the tissue, reducing lung capacities. When fine silica particles are inhaled in large enough quantities (such as through occupational exposure), it increases the risk of systemic autoimmune diseases such as lupus and rheumatoid arthritis compared to expected rates in the general population.
Occupational hazard
Silica is an occupational hazard for people who do sandblasting or work with powdered crystalline silica products. Amorphous silica, such as fumed silica, may cause irreversible lung damage in some cases but is not associated with the development of silicosis. Children, asthmatics of any age, those with allergies, and the elderly (all of whom have reduced lung capacity) can be affected in less time.
Crystalline silica is an occupational hazard for those working with stone countertops because the process of cutting and installing the countertops creates large amounts of airborne silica. Crystalline silica used in hydraulic fracturing presents a health hazard to workers.
Pathophysiology
In the body, crystalline silica particles do not dissolve over clinically relevant periods. Silica crystals inside the lungs can activate the NLRP3 inflammasome inside macrophages and dendritic cells and thereby result in production of interleukin, a highly pro-inflammatory cytokine in the immune system.
Regulation
Regulations restricting silica exposure 'with respect to the silicosis hazard' specify that they are concerned only with silica, which is both crystalline and dust-forming.
In 2013, the U.S. Occupational Safety and Health Administration reduced the exposure limit to 50 μg/m3 of air. Prior to 2013, it had allowed 100 μg/m3 and in construction workers even 250 μg/m3.
In 2013, OSHA also required the "green completion" of fracked wells to reduce exposure to crystalline silica and restrict the exposure limit. | Silicon dioxide | Wikipedia | 497 | 43710 | https://en.wikipedia.org/wiki/Silicon%20dioxide | Physical sciences | Ceramic compounds | Chemistry |
Crystalline forms
SiO2, more so than almost any material, exists in many crystalline forms. These forms are called polymorphs.
Safety
Inhaling finely divided crystalline silica can lead to severe inflammation of the lung tissue, silicosis, bronchitis, lung cancer, and systemic autoimmune diseases, such as lupus and rheumatoid arthritis. Inhalation of amorphous silicon dioxide, in high doses, leads to non-permanent short-term inflammation, where all effects heal.
Other names
This extended list enumerates synonyms for silicon dioxide; all of these values are from a single source; values in the source were presented capitalized. | Silicon dioxide | Wikipedia | 139 | 43710 | https://en.wikipedia.org/wiki/Silicon%20dioxide | Physical sciences | Ceramic compounds | Chemistry |
The prisoner's dilemma is a game theory thought experiment involving two rational agents, each of whom can either cooperate for mutual benefit or betray their partner ("defect") for individual gain. The dilemma arises from the fact that while defecting is rational for each agent, cooperation yields a higher payoff for each. The puzzle was designed by Merrill Flood and Melvin Dresher in 1950 during their work at the RAND Corporation. They invited economist Armen Alchian and mathematician John Williams to play a hundred rounds of the game, observing that Alchian and Williams often chose to cooperate. When asked about the results, John Nash remarked that rational behavior in the iterated version of the game can differ from that in a single-round version. This insight anticipated a key result in game theory: cooperation can emerge in repeated interactions, even in situations where it is not rational in a one-off interaction.
Albert W. Tucker later named the game the "prisoner's dilemma" by framing the rewards in terms of prison sentences. The prisoner's dilemma models many real-world situations involving strategic behavior. In casual usage, the label "prisoner's dilemma" is applied to any situation in which two entities can gain important benefits by cooperating or suffer by failing to do so, but find it difficult or expensive to coordinate their choices.
Premise
William Poundstone described this "typical contemporary version" of the game in his 1993 book Prisoner's Dilemma:
Two members of a criminal gang are arrested and imprisoned. Each prisoner is in solitary confinement with no means of speaking to or exchanging messages with the other. The police admit they don't have enough evidence to convict the pair on the principal charge. They plan to sentence both to a year in prison on a lesser charge. Simultaneously, the police offer each prisoner a Faustian bargain. If he testifies against his partner, he will go free while the partner will get three years in prison on the main charge. Oh, yes, there is a catch ... If both prisoners testify against each other, both will be sentenced to two years in jail. The prisoners are given a little time to think this over, but in no case may either learn what the other has decided until he has irrevocably made his decision. Each is informed that the other prisoner is being offered the very same deal. Each prisoner is concerned only with his own welfare—with minimizing his own prison sentence. | Prisoner's dilemma | Wikipedia | 492 | 43717 | https://en.wikipedia.org/wiki/Prisoner%27s%20dilemma | Mathematics | Game theory | null |
This leads to three different possible outcomes for prisoners A and B:
If A and B both remain silent, they will each serve one year in prison.
If one testifies against the other but the other doesn’t, the one testifying will be set free while the other serves three years in prison.
If A and B testify against each other, they will each serve two years.
Strategy for the prisoner's dilemma
Two prisoners are separated into individual rooms and cannot communicate with each other. It is assumed that both prisoners understand the nature of the game, have no loyalty to each other, and will have no opportunity for retribution or reward outside of the game. The normal game is shown below:
Regardless of what the other decides, each prisoner gets a higher reward by betraying the other ("defecting"). The reasoning involves analyzing both players' best responses: B will either cooperate or defect. If B cooperates, A should defect, because going free is better than serving 1 year. If B defects, A should also defect, because serving 2 years is better than serving 3. So, either way, A should defect since defecting is A's best response regardless of B's strategy. Parallel reasoning will show that B should defect.
Defection always results in a better payoff than cooperation, so it is a strictly dominant strategy for both players. Mutual defection is the only strong Nash equilibrium in the game. Since the collectively ideal result of mutual cooperation is irrational from a self-interested standpoint, this Nash equilibrium is not Pareto efficient.
Generalized form
The structure of the traditional prisoner's dilemma can be generalized from its original prisoner setting. Suppose that the two players are represented by the colors red and blue and that each player chooses to either "cooperate" or "defect".
If both players cooperate, they both receive the reward for cooperating. If both players defect, they both receive the punishment payoff . If Blue defects while Red cooperates, then Blue receives the temptation payoff , while Red receives the "sucker's" payoff, . Similarly, if Blue cooperates while Red defects, then Blue receives the sucker's payoff , while Red receives the temptation payoff .
This can be expressed in normal form:
and to be a prisoner's dilemma game in the strong sense, the following condition must hold for the payoffs:
The payoff relationship implies that mutual cooperation is superior to mutual defection, while the payoff relationships and imply that defection is the dominant strategy for both agents. | Prisoner's dilemma | Wikipedia | 512 | 43717 | https://en.wikipedia.org/wiki/Prisoner%27s%20dilemma | Mathematics | Game theory | null |
The iterated prisoner's dilemma
If two players play the prisoner's dilemma more than once in succession, remember their opponent's previous actions, and are allowed to change their strategy accordingly, the game is called the iterated prisoner's dilemma.
In addition to the general form above, the iterative version also requires that , to prevent alternating cooperation and defection giving a greater reward than mutual cooperation.
The iterated prisoner's dilemma is fundamental to some theories of human cooperation and trust. Assuming that the game effectively models transactions between two people that require trust, cooperative behavior in populations can be modeled by a multi-player iterated version of the game. In 1975, Grofman and Pool estimated the count of scholarly articles devoted to it at over 2,000. The iterated prisoner's dilemma is also called the "peace-war game".
General strategy
If the iterated prisoner's dilemma is played a finite number of times and both players know this, then the dominant strategy and Nash equilibrium is to defect in all rounds. The proof is inductive: one might as well defect on the last turn, since the opponent will not have a chance to later retaliate. Therefore, both will defect on the last turn. Thus, the player might as well defect on the second-to-last turn, since the opponent will defect on the last no matter what is done, and so on. The same applies if the game length is unknown but has a known upper limit.
For cooperation to emerge between rational players, the number of rounds must be unknown or infinite. In that case, "always defect" may no longer be a dominant strategy. As shown by Robert Aumann in a 1959 paper, rational players repeatedly interacting for indefinitely long games can sustain cooperation. Specifically, a player may be less willing to cooperate if their counterpart did not cooperate many times, which causes disappointment. Conversely, as time elapses, the likelihood of cooperation tends to rise, owing to the establishment of a "tacit agreement" among participating players. In experimental situations, cooperation can occur even when both participants know how many iterations will be played.
According to a 2019 experimental study in the American Economic Review that tested what strategies real-life subjects used in iterated prisoner's dilemma situations with perfect monitoring, the majority of chosen strategies were always to defect, tit-for-tat, and grim trigger. Which strategy the subjects chose depended on the parameters of the game. | Prisoner's dilemma | Wikipedia | 501 | 43717 | https://en.wikipedia.org/wiki/Prisoner%27s%20dilemma | Mathematics | Game theory | null |
Axelrod's tournament and successful strategy conditions
Interest in the iterated prisoner's dilemma was kindled by Robert Axelrod in his 1984 book The Evolution of Cooperation, in which he reports on a tournament that he organized of the N-step prisoner's dilemma (with N fixed) in which participants have to choose their strategy repeatedly and remember their previous encounters. Axelrod invited academic colleagues from around the world to devise computer strategies to compete in an iterated prisoner's dilemma tournament. The programs that were entered varied widely in algorithmic complexity, initial hostility, capacity for forgiveness, and so forth.
Axelrod discovered that when these encounters were repeated over a long period of time with many players, each with different strategies, greedy strategies tended to do very poorly in the long run while more altruistic strategies did better, as judged purely by self-interest. He used this to show a possible mechanism for the evolution of altruistic behavior from mechanisms that are initially purely selfish, by natural selection.
The winning deterministic strategy was tit for tat, developed and entered into the tournament by Anatol Rapoport. It was the simplest of any program entered, containing only four lines of BASIC, and won the contest. The strategy is simply to cooperate on the first iteration of the game; after that, the player does what his or her opponent did on the previous move. Depending on the situation, a slightly better strategy can be "tit for tat with forgiveness": when the opponent defects, on the next move, the player sometimes cooperates anyway, with a small probability (around 1–5%, depending on the lineup of opponents). This allows for occasional recovery from getting trapped in a cycle of defections.
After analyzing the top-scoring strategies, Axelrod stated several conditions necessary for a strategy to succeed: | Prisoner's dilemma | Wikipedia | 373 | 43717 | https://en.wikipedia.org/wiki/Prisoner%27s%20dilemma | Mathematics | Game theory | null |
Nice: The strategy will not be the first to defect (this is sometimes referred to as an "optimistic" algorithm), i.e., it will not "cheat" on its opponent for purely self-interested reasons first. Almost all the top-scoring strategies were nice.
Retaliating: The strategy must sometimes retaliate. An example of a non-retaliating strategy is Always Cooperate, a very bad choice that will frequently be exploited by "nasty" strategies.
Forgiving: Successful strategies must be forgiving. Though players will retaliate, they will cooperate again if the opponent does not continue to defect. This can stop long runs of revenge and counter-revenge, maximizing points.
Non-envious: The strategy must not strive to score more than the opponent.
In contrast to the one-time prisoner's dilemma game, the optimal strategy in the iterated prisoner's dilemma depends upon the strategies of likely opponents, and how they will react to defections and cooperation. For example, if a population consists entirely of players who always defect, except for one who follows the tit-for-tat strategy, that person is at a slight disadvantage because of the loss on the first turn. In such a population, the optimal strategy is to defect every time. More generally, given a population with a certain percentage of always-defectors with the rest being tit-for-tat players, the optimal strategy depends on the percentage and number of iterations played.
Other strategies
Deriving the optimal strategy is generally done in two ways:
Bayesian Nash equilibrium: If the statistical distribution of opposing strategies can be determined an optimal counter-strategy can be derived analytically.
Monte Carlo simulations of populations have been made, where individuals with low scores die off, and those with high scores reproduce (a genetic algorithm for finding an optimal strategy). The mix of algorithms in the final population generally depends on the mix in the initial population. The introduction of mutation (random variation during reproduction) lessens the dependency on the initial population; empirical experiments with such systems tend to produce tit-for-tat players, but no analytic proof exists that this will always occur.
In the strategy called win-stay, lose-switch, faced with a failure to cooperate, the player switches strategy the next turn. In certain circumstances, Pavlov beats all other strategies by giving preferential treatment to co-players using a similar strategy. | Prisoner's dilemma | Wikipedia | 496 | 43717 | https://en.wikipedia.org/wiki/Prisoner%27s%20dilemma | Mathematics | Game theory | null |
Although tit-for-tat is considered the most robust basic strategy, a team from Southampton University in England introduced a more successful strategy at the 20th-anniversary iterated prisoner's dilemma competition. It relied on collusion between programs to achieve the highest number of points for a single program. The university submitted 60 programs to the competition, which were designed to recognize each other through a series of five to ten moves at the start. Once this recognition was made, one program would always cooperate and the other would always defect, assuring the maximum number of points for the defector. If the program realized that it was playing a non-Southampton player, it would continuously defect in an attempt to minimize the competing program's score. As a result, the 2004 Prisoners' Dilemma Tournament results show University of Southampton's strategies in the first three places (and a number of positions towards the bottom), despite having fewer wins and many more losses than the GRIM strategy. The Southampton strategy takes advantage of the fact that multiple entries were allowed in this particular competition and that a team's performance was measured by that of the highest-scoring player (meaning that the use of self-sacrificing players was a form of minmaxing).
Because of this new rule, this competition also has little theoretical significance when analyzing single-agent strategies as compared to Axelrod's seminal tournament. But it provided a basis for analyzing how to achieve cooperative strategies in multi-agent frameworks, especially in the presence of noise.
Long before this new-rules tournament was played, Dawkins, in his book The Selfish Gene, pointed out the possibility of such strategies winning if multiple entries were allowed, but remarked that Axelrod would most likely not have allowed them if they had been submitted. It also relies on circumventing the rule that no communication is allowed between players, which the Southampton programs arguably did with their preprogrammed "ten-move dance" to recognize one another, reinforcing how valuable communication can be in shifting the balance of the game.
Even without implicit collusion between software strategies, tit-for-tat is not always the absolute winner of any given tournament; more precisely, its long-run results over a series of tournaments outperform its rivals, but this does not mean it is the most successful in the short term. The same applies to tit-for-tat with forgiveness and other optimal strategies. | Prisoner's dilemma | Wikipedia | 495 | 43717 | https://en.wikipedia.org/wiki/Prisoner%27s%20dilemma | Mathematics | Game theory | null |
This can also be illustrated using the Darwinian ESS simulation. In such a simulation, tit-for-tat will almost always come to dominate, though nasty strategies will drift in and out of the population because a tit-for-tat population is penetrable by non-retaliating nice strategies, which in turn are easy prey for the nasty strategies. Dawkins showed that here, no static mix of strategies forms a stable equilibrium, and the system will always oscillate between bounds.
Stochastic iterated prisoner's dilemma
In a stochastic iterated prisoner's dilemma game, strategies are specified in terms of "cooperation probabilities". In an encounter between player X and player Y, Xs strategy is specified by a set of probabilities P of cooperating with Y. P is a function of the outcomes of their previous encounters or some subset thereof. If P is a function of only their most recent n encounters, it is called a "memory-n" strategy. A memory-1 strategy is then specified by four cooperation probabilities: , where Pcd is the probability that X will cooperate in the present encounter given that the previous encounter was characterized by X cooperating and Y defecting. If each of the probabilities are either 1 or 0, the strategy is called deterministic. An example of a deterministic strategy is the tit-for-tat strategy written as , in which X responds as Y did in the previous encounter. Another is the win-stay, lose switch strategy written as . It has been shown that for any memory-n strategy there is a corresponding memory-1 strategy that gives the same statistical results, so that only memory-1 strategies need be considered. | Prisoner's dilemma | Wikipedia | 360 | 43717 | https://en.wikipedia.org/wiki/Prisoner%27s%20dilemma | Mathematics | Game theory | null |
If is defined as the above 4-element strategy vector of X and as the 4-element strategy vector of Y (where the indices are from Y's point of view), a transition matrix M may be defined for X whose ij-th entry is the probability that the outcome of a particular encounter between X and Y will be j given that the previous encounter was i, where i and j are one of the four outcome indices: cc, cd, dc, or dd. For example, from Xs point of view, the probability that the outcome of the present encounter is cd given that the previous encounter was cd is equal to . Under these definitions, the iterated prisoner's dilemma qualifies as a stochastic process and M is a stochastic matrix, allowing all of the theory of stochastic processes to be applied.
One result of stochastic theory is that there exists a stationary vector v for the matrix v such that . Without loss of generality, it may be specified that v is normalized so that the sum of its four components is unity. The ij-th entry in will give the probability that the outcome of an encounter between X and Y will be j given that the encounter n steps previous is i. In the limit as n approaches infinity, M will converge to a matrix with fixed values, giving the long-term probabilities of an encounter producing j independent of i. In other words, the rows of will be identical, giving the long-term equilibrium result probabilities of the iterated prisoner's dilemma without the need to explicitly evaluate a large number of interactions. It can be seen that v is a stationary vector for and particularly , so that each row of will be equal to v. Thus, the stationary vector specifies the equilibrium outcome probabilities for X. Defining and as the short-term payoff vectors for the {cc,cd,dc,dd} outcomes (from Xs point of view), the equilibrium payoffs for X and Y can now be specified as and , allowing the two strategies P and Q to be compared for their long-term payoffs.
Zero-determinant strategies | Prisoner's dilemma | Wikipedia | 441 | 43717 | https://en.wikipedia.org/wiki/Prisoner%27s%20dilemma | Mathematics | Game theory | null |
In 2012, William H. Press and Freeman Dyson published a new class of strategies for the stochastic iterated prisoner's dilemma called "zero-determinant" (ZD) strategies. The long term payoffs for encounters between X and Y can be expressed as the determinant of a matrix which is a function of the two strategies and the short term payoff vectors: and , which do not involve the stationary vector v. Since the determinant function is linear in , it follows that (where ). Any strategies for which are by definition a ZD strategy, and the long-term payoffs obey the relation .
Tit-for-tat is a ZD strategy which is "fair", in the sense of not gaining advantage over the other player. But the ZD space also contains strategies that, in the case of two players, can allow one player to unilaterally set the other player's score or alternatively force an evolutionary player to achieve a payoff some percentage lower than his own. The extorted player could defect, but would thereby hurt himself by getting a lower payoff. Thus, extortion solutions turn the iterated prisoner's dilemma into a sort of ultimatum game. Specifically, X is able to choose a strategy for which , unilaterally setting sy to a specific value within a particular range of values, independent of Ys strategy, offering an opportunity for X to "extort" player Y (and vice versa). But if X tries to set sx to a particular value, the range of possibilities is much smaller, consisting only of complete cooperation or complete defection.
An extension of the iterated prisoner's dilemma is an evolutionary stochastic iterated prisoner's dilemma, in which the relative abundance of particular strategies is allowed to change, with more successful strategies relatively increasing. This process may be accomplished by having less successful players imitate the more successful strategies, or by eliminating less successful players from the game, while multiplying the more successful ones. It has been shown that unfair ZD strategies are not evolutionarily stable. The key intuition is that an evolutionarily stable strategy must not only be able to invade another population (which extortionary ZD strategies can do) but must also perform well against other players of the same type (which extortionary ZD players do poorly because they reduce each other's surplus). | Prisoner's dilemma | Wikipedia | 497 | 43717 | https://en.wikipedia.org/wiki/Prisoner%27s%20dilemma | Mathematics | Game theory | null |
Theory and simulations confirm that beyond a critical population size, ZD extortion loses out in evolutionary competition against more cooperative strategies, and as a result, the average payoff in the population increases when the population is larger. In addition, there are some cases in which extortioners may even catalyze cooperation by helping to break out of a face-off between uniform defectors and win–stay, lose–switch agents.
While extortionary ZD strategies are not stable in large populations, another ZD class called "generous" strategies is both stable and robust. When the population is not too small, these strategies can supplant any other ZD strategy and even perform well against a broad array of generic strategies for iterated prisoner's dilemma, including win–stay, lose–switch. This was proven specifically for the donation game by Alexander Stewart and Joshua Plotkin in 2013. Generous strategies will cooperate with other cooperative players, and in the face of defection, the generous player loses more utility than its rival. Generous strategies are the intersection of ZD strategies and so-called "good" strategies, which were defined by Ethan Akin to be those for which the player responds to past mutual cooperation with future cooperation and splits expected payoffs equally if he receives at least the cooperative expected payoff. Among good strategies, the generous (ZD) subset performs well when the population is not too small. If the population is very small, defection strategies tend to dominate. | Prisoner's dilemma | Wikipedia | 302 | 43717 | https://en.wikipedia.org/wiki/Prisoner%27s%20dilemma | Mathematics | Game theory | null |
Continuous iterated prisoner's dilemma
Most work on the iterated prisoner's dilemma has focused on the discrete case, in which players either cooperate or defect, because this model is relatively simple to analyze. However, some researchers have looked at models of the continuous iterated prisoner's dilemma, in which players are able to make a variable contribution to the other player. Le and Boyd found that in such situations, cooperation is much harder to evolve than in the discrete iterated prisoner's dilemma. In a continuous prisoner's dilemma, if a population starts off in a non-cooperative equilibrium, players who are only marginally more cooperative than non-cooperators get little benefit from assorting with one another. By contrast, in a discrete prisoner's dilemma, tit-for-tat cooperators get a big payoff boost from assorting with one another in a non-cooperative equilibrium, relative to non-cooperators. Since nature arguably offers more opportunities for variable cooperation rather than a strict dichotomy of cooperation or defection, the continuous prisoner's dilemma may help explain why real-life examples of tit-for-tat-like cooperation are extremely rare even though tit-for-tat seems robust in theoretical models.
Real-life examples
Many instances of human interaction and natural processes have payoff matrices like the prisoner's dilemma's. It is therefore of interest to the social sciences, such as economics, politics, and sociology, as well as to the biological sciences, such as ethology and evolutionary biology. Many natural processes have been abstracted into models in which living beings are engaged in endless games of prisoner's dilemma.
Environmental studies
In environmental studies, the dilemma is evident in crises such as global climate change. It is argued all countries will benefit from a stable climate, but any single country is often hesitant to curb emissions. The immediate benefit to any one country from maintaining current behavior is perceived to be greater than the purported eventual benefit to that country if all countries' behavior was changed, therefore explaining the impasse concerning climate-change in 2007. | Prisoner's dilemma | Wikipedia | 427 | 43717 | https://en.wikipedia.org/wiki/Prisoner%27s%20dilemma | Mathematics | Game theory | null |
An important difference between climate-change politics and the prisoner's dilemma is uncertainty; the extent and pace at which pollution can change climate is not known. The dilemma faced by governments is therefore different from the prisoner's dilemma in that the payoffs of cooperation are unknown. This difference suggests that states will cooperate much less than in a real iterated prisoner's dilemma, so that the probability of avoiding a possible climate catastrophe is much smaller than that suggested by a game-theoretical analysis of the situation using a real iterated prisoner's dilemma.
Thomas Osang and Arundhati Nandy provide a theoretical explanation with proofs for a regulation-driven win-win situation along the lines of Michael Porter's hypothesis, in which government regulation of competing firms is substantial.
Animals
Cooperative behavior of many animals can be understood as an example of the iterated prisoner's dilemma. Often animals engage in long-term partnerships; for example, guppies inspect predators cooperatively in groups, and they are thought to punish non-cooperative inspectors.
Vampire bats are social animals that engage in reciprocal food exchange. Applying the payoffs from the prisoner's dilemma can help explain this behavior.
Psychology
In addiction research and behavioral economics, George Ainslie points out that addiction can be cast as an intertemporal prisoner's dilemma problem between the present and future selves of the addict. In this case, "defecting" means relapsing, where not relapsing both today and in the future is by far the best outcome. The case where one abstains today but relapses in the future is the worst outcome: in some sense, the discipline and self-sacrifice involved in abstaining today have been "wasted" because the future relapse means that the addict is right back where they started and will have to start over. Relapsing today and tomorrow is a slightly "better" outcome, because while the addict is still addicted, they haven't put the effort in to trying to stop. The final case, where one engages in the addictive behavior today while abstaining tomorrow, has the problem that (as in other prisoner's dilemmas) there is an obvious benefit to defecting "today", but tomorrow one will face the same prisoner's dilemma, and the same obvious benefit will be present then, ultimately leading to an endless string of defections. | Prisoner's dilemma | Wikipedia | 489 | 43717 | https://en.wikipedia.org/wiki/Prisoner%27s%20dilemma | Mathematics | Game theory | null |
In The Science of Trust, John Gottman defines good relationships as those where partners know not to enter into mutual defection behavior, or at least not to get dynamically stuck there in a loop. In cognitive neuroscience, fast brain signaling associated with processing different rounds may indicate choices at the next round. Mutual cooperation outcomes entail brain activity changes predictive of how quickly a person will cooperate in kind at the next opportunity; this activity may be linked to basic homeostatic and motivational processes, possibly increasing the likelihood of short-cutting into mutual cooperation.
Economics
The prisoner's dilemma has been called the E. coli of social psychology, and it has been used widely to research various topics such as oligopolistic competition and collective action to produce a collective good.
Advertising is sometimes cited as a real example of the prisoner's dilemma. When cigarette advertising was legal in the United States, competing cigarette manufacturers had to decide how much money to spend on advertising. The effectiveness of Firm A's advertising was partially determined by the advertising conducted by Firm B. Likewise, the profit derived from advertising for Firm B is affected by the advertising conducted by Firm A. If both Firm A and Firm B chose to advertise during a given period, then the advertisement from each firm negates the other's, receipts remain constant, and expenses increase due to the cost of advertising. Both firms would benefit from a reduction in advertising. However, should Firm B choose not to advertise, Firm A could benefit greatly by advertising. Nevertheless, the optimal amount of advertising by one firm depends on how much advertising the other undertakes. As the best strategy is dependent on what the other firm chooses there is no dominant strategy, which makes it slightly different from a prisoner's dilemma. The outcome is similar, though, in that both firms would be better off were they to advertise less than in the equilibrium.
Sometimes cooperative behaviors do emerge in business situations. For instance, cigarette manufacturers endorsed the making of laws banning cigarette advertising, understanding that this would reduce costs and increase profits across the industry.
Without enforceable agreements, members of a cartel are also involved in a (multi-player) prisoner's dilemma. "Cooperating" typically means agreeing to a price floor, while "defecting" means selling under this minimum level, instantly taking business from other cartel members. Anti-trust authorities want potential cartel members to mutually defect, ensuring the lowest possible prices for consumers. | Prisoner's dilemma | Wikipedia | 496 | 43717 | https://en.wikipedia.org/wiki/Prisoner%27s%20dilemma | Mathematics | Game theory | null |
Sport
Doping in sport has been cited as an example of a prisoner's dilemma. Two competing athletes have the option to use an illegal and/or dangerous drug to boost their performance. If neither athlete takes the drug, then neither gains an advantage. If only one does, then that athlete gains a significant advantage over the competitor, reduced by the legal and/or medical dangers of having taken the drug. But if both athletes take the drug, the benefits cancel out and only the dangers remain, putting them both in a worse position than if neither had doped.
International politics
In international relations theory, the prisoner's dilemma is often used to demonstrate why cooperation fails in situations when cooperation between states is collectively optimal but individually suboptimal. A classic example is the security dilemma, whereby an increase in one state's security (such as increasing its military strength) leads other states to fear for their own security out of fear of offensive action. Consequently, security-increasing measures can lead to tensions, escalation or conflict with one or more other parties, producing an outcome which no party truly desires. The security dilemma is particularly intense in situations when it is hard to distinguish offensive weapons from defensive weapons, and offense has the advantage in any conflict over defense.
The prisoner's dilemma has frequently been used by realist international relations theorists to demonstrate the why all states (regardless of their internal policies or professed ideology) under international anarchy will struggle to cooperate with one another even when all benefit from such cooperation.
Critics of realism argue that iteration and extending the shadow of the future are solutions to the prisoner's dilemma. When actors play the prisoner's dilemma once, they have incentives to defect, but when they expect to play it repeatedly, they have greater incentives to cooperate.
Multiplayer dilemmas
Many real-life dilemmas involve multiple players. Although metaphorical, Garrett Hardin's tragedy of the commons may be viewed as an example of a multi-player generalization of the prisoner's dilemma: each villager makes a choice for personal gain or restraint. The collective reward for unanimous or frequent defection is very low payoffs and the destruction of the commons. | Prisoner's dilemma | Wikipedia | 435 | 43717 | https://en.wikipedia.org/wiki/Prisoner%27s%20dilemma | Mathematics | Game theory | null |
The commons are not always exploited: William Poundstone, in a book about the prisoner's dilemma, describes a situation in New Zealand where newspaper boxes are left unlocked. It is possible for people to take a paper without paying (defecting), but very few do, feeling that if they do not pay then neither will others, destroying the system. Subsequent research by Elinor Ostrom, winner of the 2009 Nobel Memorial Prize in Economic Sciences, hypothesized that the tragedy of the commons is oversimplified, with the negative outcome influenced by outside influences. Without complicating pressures, groups communicate and manage the commons among themselves for their mutual benefit, enforcing social norms to preserve the resource and achieve the maximum good for the group, an example of effecting the best-case outcome for prisoner's dilemma.
Academic settings
The prisoner's dilemma has been used in various academic settings to illustrate the complexities of cooperation and competition. One notable example is the classroom experiment conducted by sociology professor Dan Chambliss at Hamilton College in the 1980s. Starting in 1981, Chambliss proposed that if no student took the final exam, everyone would receive an A, but if even one student took it, those who didn't would receive a zero. In 1988, John Werner, a first-year student, successfully organized his classmates to boycott the exam, demonstrating a practical application of game theory and the prisoner's dilemma concept.
Nearly 25 years later, a similar incident occurred at Johns Hopkins University in 2013. Professor Peter Fröhlich's grading policy scaled final exams according to the highest score, meaning that if everyone received the same score, they would all get an A. Students in Fröhlich's classes organized a boycott of the final exam, ensuring that no one took it. As a result, every student received an A, successfully solving the prisoner's dilemma in a mutually optimal way without iteration. These examples highlight how the prisoner's dilemma can be used to explore cooperative behavior and strategic decision-making in educational contexts.
Related games
Closed-bag exchange
Douglas Hofstadter suggested that people often find problems such as the prisoner's dilemma problem easier to understand when it is illustrated in the form of a simple game, or trade-off. One of several examples he used was "closed bag exchange": | Prisoner's dilemma | Wikipedia | 474 | 43717 | https://en.wikipedia.org/wiki/Prisoner%27s%20dilemma | Mathematics | Game theory | null |
Friend or Foe?
Friend or Foe? is a game show that aired from 2002 to 2003 on the Game Show Network in the US. On the game show, three pairs of people compete. When a pair is eliminated, they play a game similar to the prisoner's dilemma to determine how the winnings are split. If they both cooperate (Friend), they share the winnings 50–50. If one cooperates and the other defects (Foe), the defector gets all the winnings, and the cooperator gets nothing. If both defect, both leave with nothing. Notice that the reward matrix is slightly different from the standard one given above, as the rewards for the "both defect" and the "cooperate while the opponent defects" cases are identical. This makes the "both defect" case a weak equilibrium, compared with being a strict equilibrium in the standard prisoner's dilemma. If a contestant knows that their opponent is going to vote "Foe", then their own choice does not affect their own winnings. In a specific sense, Friend or Foe has a rewards model between prisoner's dilemma and the game of Chicken.
This is the rewards matrix:
This payoff matrix has also been used on the British television programs Trust Me, Shafted, The Bank Job and Golden Balls, and on the American game show Take It All, as well as for the winning couple on the reality shows Bachelor Pad and Love Island. Game data from the Golden Balls series has been analyzed by a team of economists, who found that cooperation was "surprisingly high" for amounts of money that would seem consequential in the real world but were comparatively low in the context of the game.
Iterated snowdrift
Researchers from the University of Lausanne and the University of Edinburgh have suggested that the "Iterated Snowdrift Game" may more closely reflect real-world social situations, although this model is actually a chicken game. In this model, the risk of being exploited through defection is lower, and individuals always gain from taking the cooperative choice. The snowdrift game imagines two drivers who are stuck on opposite sides of a snowdrift, each of whom is given the option of shoveling snow to clear a path or remaining in their car. A player's highest payoff comes from leaving the opponent to clear all the snow by themselves, but the opponent is still nominally rewarded for their work. | Prisoner's dilemma | Wikipedia | 490 | 43717 | https://en.wikipedia.org/wiki/Prisoner%27s%20dilemma | Mathematics | Game theory | null |
This may better reflect real-world scenarios, the researchers giving the example of two scientists collaborating on a report, both of whom would benefit if the other worked harder. "But when your collaborator doesn't do any work, it's probably better for you to do all the work yourself. You'll still end up with a completed project."
Coordination games
In coordination games, players must coordinate their strategies for a good outcome. An example is two cars that abruptly meet in a blizzard; each must choose whether to swerve left or right. If both swerve left, or both right, the cars do not collide. The local left- and right-hand traffic convention helps to co-ordinate their actions.
Symmetrical co-ordination games include Stag hunt and Bach or Stravinsky.
Asymmetric prisoner's dilemmas
A more general set of games is asymmetric. As in the prisoner's dilemma, the best outcome is cooperation, and there are motives for defection. Unlike the symmetric prisoner's dilemma, though, one player has more to lose and/or more to gain than the other. Some such games have been described as a prisoner's dilemma in which one prisoner has an alibi, hence the term "alibi game".
In experiments, players getting unequal payoffs in repeated games may seek to maximize profits, but only under the condition that both players receive equal payoffs; this may lead to a stable equilibrium strategy in which the disadvantaged player defects every X game, while the other always co-operates. Such behavior may depend on the experiment's social norms around fairness.
Software
Several software packages have been created to run simulations and tournaments of the prisoner's dilemma, some of which have their source code available:
The source code for the second tournament run by Robert Axelrod (written by Axelrod and many contributors in Fortran)
Prison, a library written in Java, last updated in 1998
Axelrod-Python, written in Python
Evoplex, a fast agent-based modeling program released in 2018 by Marcos Cardinot | Prisoner's dilemma | Wikipedia | 424 | 43717 | https://en.wikipedia.org/wiki/Prisoner%27s%20dilemma | Mathematics | Game theory | null |
In fiction
Hannu Rajaniemi set the opening scene of his The Quantum Thief trilogy in a "dilemma prison". The main theme of the series has been described as the "inadequacy of a binary universe" and the ultimate antagonist is a character called the All-Defector. The first book in the series was published in 2010, with the two sequels, The Fractal Prince and The Causal Angel, published in 2012 and 2014, respectively.
A game modeled after the iterated prisoner's dilemma is a central focus of the 2012 video game Zero Escape: Virtue's Last Reward and a minor part in its 2016 sequel Zero Escape: Zero Time Dilemma.
In The Mysterious Benedict Society and the Prisoner's Dilemma by Trenton Lee Stewart, the main characters start by playing a version of the game and escaping from the "prison" altogether. Later, they become actual prisoners and escape once again.
In The Adventure Zone: Balance during The Suffering Game subarc, the player characters are twice presented with the prisoner's dilemma during their time in two liches' domain, once cooperating and once defecting.
In the eighth novel from the author James S. A. Corey, Tiamat's Wrath, Winston Duarte explains the prisoner's dilemma to his 14-year-old daughter, Teresa, to train her in strategic thinking.
The 2008 film The Dark Knight includes a scene loosely based on the problem in which the Joker rigs two ferries, one containing prisoners and the other containing civilians, arming both groups with the means to detonate the bomb on each other's ferries, threatening to detonate them both if they hesitate.
In moral philosophy
The prisoner's dilemma is commonly used as a thinking tool in moral philosophy as an illustration of the potential tension between the benefit of the individual and the benefit of the community.
Both the one-shot and the iterated prisoner's dilemma have applications in moral philosophy. Indeed, many of the moral situations, such as genocide, are not easily repeated more than once. Moreover, in many situations, the previous rounds' outcomes are unknown to the players, since they are not necessarily the same (e.g. interaction with a panhandler on the street).
The philosopher David Gauthier uses the prisoner's dilemma to show how morality and rationality can conflict. | Prisoner's dilemma | Wikipedia | 474 | 43717 | https://en.wikipedia.org/wiki/Prisoner%27s%20dilemma | Mathematics | Game theory | null |
Some game theorists have criticized the use of the prisoner's dilemma as a thinking tool in moral philosophy. Kenneth Binmore argued that the prisoner's dilemma does not accurately describe the game played by humanity, which he argues is closer to a coordination game. Brian Skyrms shares this perspective.
Steven Kuhn suggests that these views may be reconciled by considering that moral behavior can modify the payoff matrix of a game, transforming it from a prisoner's dilemma into other games.
Pure and impure prisoner's dilemma
A prisoner's dilemma is considered "impure" if a mixed strategy may give better expected payoffs than a pure strategy. This creates the interesting possibility that the moral action from a utilitarian perspective (i.e., aiming at maximizing the good of an action) may require randomization of one's strategy, such as cooperating with 80% chance and defecting with 20% chance. | Prisoner's dilemma | Wikipedia | 187 | 43717 | https://en.wikipedia.org/wiki/Prisoner%27s%20dilemma | Mathematics | Game theory | null |
Linear programming (LP), also called linear optimization, is a method to achieve the best outcome (such as maximum profit or lowest cost) in a mathematical model whose requirements and objective are represented by linear relationships. Linear programming is a special case of mathematical programming (also known as mathematical optimization).
More formally, linear programming is a technique for the optimization of a linear objective function, subject to linear equality and linear inequality constraints. Its feasible region is a convex polytope, which is a set defined as the intersection of finitely many half spaces, each of which is defined by a linear inequality. Its objective function is a real-valued affine (linear) function defined on this polytope. A linear programming algorithm finds a point in the polytope where this function has the largest (or smallest) value if such a point exists.
Linear programs are problems that can be expressed in standard form as
Here the components of are the variables to be determined, and are given vectors, and is a given matrix. The function whose value is to be maximized ( in this case) is called the objective function. The constraints and specify a convex polytope over which the objective function is to be optimized.
Linear programming can be applied to various fields of study. It is widely used in mathematics and, to a lesser extent, in business, economics, and some engineering problems. There is a close connection between linear programs, eigenequations, John von Neumann's general equilibrium model, and structural equilibrium models (see dual linear program for details).
Industries that use linear programming models include transportation, energy, telecommunications, and manufacturing. It has proven useful in modeling diverse types of problems in planning, routing, scheduling, assignment, and design.
History
The problem of solving a system of linear inequalities dates back at least as far as Fourier, who in 1827 published a method for solving them, and after whom the method of Fourier–Motzkin elimination is named.
In the late 1930s, Soviet mathematician Leonid Kantorovich and American economist Wassily Leontief independently delved into the practical applications of linear programming. Kantorovich focused on manufacturing schedules, while Leontief explored economic applications. Their groundbreaking work was largely overlooked for decades. | Linear programming | Wikipedia | 456 | 43730 | https://en.wikipedia.org/wiki/Linear%20programming | Mathematics | Other | null |
The turning point came during World War II when linear programming emerged as a vital tool. It found extensive use in addressing complex wartime challenges, including transportation logistics, scheduling, and resource allocation. Linear programming proved invaluable in optimizing these processes while considering critical constraints such as costs and resource availability.
Despite its initial obscurity, the wartime successes propelled linear programming into the spotlight. Post-WWII, the method gained widespread recognition and became a cornerstone in various fields, from operations research to economics. The overlooked contributions of Kantorovich and Leontief in the late 1930s eventually became foundational to the broader acceptance and utilization of linear programming in optimizing decision-making processes.
Kantorovich's work was initially neglected in the USSR. About the same time as Kantorovich, the Dutch-American economist T. C. Koopmans formulated classical economic problems as linear programs. Kantorovich and Koopmans later shared the 1975 Nobel Memorial Prize in Economic Sciences. In 1941, Frank Lauren Hitchcock also formulated transportation problems as linear programs and gave a solution very similar to the later simplex method. Hitchcock had died in 1957, and the Nobel Memorial Prize is not awarded posthumously.
From 1946 to 1947 George B. Dantzig independently developed general linear programming formulation to use for planning problems in the US Air Force. In 1947, Dantzig also invented the simplex method that, for the first time efficiently, tackled the linear programming problem in most cases. When Dantzig arranged a meeting with John von Neumann to discuss his simplex method, von Neumann immediately conjectured the theory of duality by realizing that the problem he had been working in game theory was equivalent. Dantzig provided formal proof in an unpublished report "A Theorem on Linear Inequalities" on January 5, 1948. Dantzig's work was made available to public in 1951. In the post-war years, many industries applied it in their daily planning.
Dantzig's original example was to find the best assignment of 70 people to 70 jobs. The computing power required to test all the permutations to select the best assignment is vast; the number of possible configurations exceeds the number of particles in the observable universe. However, it takes only a moment to find the optimum solution by posing the problem as a linear program and applying the simplex algorithm. The theory behind linear programming drastically reduces the number of possible solutions that must be checked. | Linear programming | Wikipedia | 496 | 43730 | https://en.wikipedia.org/wiki/Linear%20programming | Mathematics | Other | null |
The linear programming problem was first shown to be solvable in polynomial time by Leonid Khachiyan in 1979, but a larger theoretical and practical breakthrough in the field came in 1984 when Narendra Karmarkar introduced a new interior-point method for solving linear-programming problems.
Uses
Linear programming is a widely used field of optimization for several reasons. Many practical problems in operations research can be expressed as linear programming problems. Certain special cases of linear programming, such as network flow problems and multicommodity flow problems, are considered important enough to have much research on specialized algorithms. A number of algorithms for other types of optimization problems work by solving linear programming problems as sub-problems. Historically, ideas from linear programming have inspired many of the central concepts of optimization theory, such as duality, decomposition, and the importance of convexity and its generalizations. Likewise, linear programming was heavily used in the early formation of microeconomics, and it is currently utilized in company management, such as planning, production, transportation, and technology. Although the modern management issues are ever-changing, most companies would like to maximize profits and minimize costs with limited resources. Google also uses linear programming to stabilize YouTube videos.
Standard form
Standard form is the usual and most intuitive form of describing a linear programming problem. It consists of the following three parts:
A linear (or affine) function to be maximized
e.g.
Problem constraints of the following form
e.g.
Non-negative variables
e.g.
The problem is usually expressed in matrix form, and then becomes:
Other forms, such as minimization problems, problems with constraints on alternative forms, and problems involving negative variables can always be rewritten into an equivalent problem in standard form.
Example | Linear programming | Wikipedia | 352 | 43730 | https://en.wikipedia.org/wiki/Linear%20programming | Mathematics | Other | null |
Suppose that a farmer has a piece of farm land, say L hectares, to be planted with either wheat or barley or some combination of the two. The farmer has F kilograms of fertilizer and P kilograms of pesticide. Every hectare of wheat requires F1 kilograms of fertilizer and P1 kilograms of pesticide, while every hectare of barley requires F2 kilograms of fertilizer and P2 kilograms of pesticide. Let S1 be the selling price of wheat and S2 be the selling price of barley, per hectare. If we denote the area of land planted with wheat and barley by x1 and x2 respectively, then profit can be maximized by choosing optimal values for x1 and x2. This problem can be expressed with the following linear programming problem in the standard form:
In matrix form this becomes:
maximize
subject to
Augmented form (slack form)
Linear programming problems can be converted into an augmented form in order to apply the common form of the simplex algorithm. This form introduces non-negative slack variables to replace inequalities with equalities in the constraints. The problems can then be written in the following block matrix form:
Maximize :
where are the newly introduced slack variables, are the decision variables, and is the variable to be maximized.
Example
The example above is converted into the following augmented form:
{|
|-
| colspan="2" | Maximize:
| (objective function)
|-
| subject to:
|
| (augmented constraint)
|-
|
|
| (augmented constraint)
|-
|
|
| (augmented constraint)
|-
|
|
|}
where are (non-negative) slack variables, representing in this example the unused area, the amount of unused fertilizer, and the amount of unused pesticide.
In matrix form this becomes:
Maximize :
Duality
Every linear programming problem, referred to as a primal problem, can be converted into a dual problem, which provides an upper bound to the optimal value of the primal problem. In matrix form, we can express the primal problem as:
Maximize cTx subject to Ax ≤ b, x ≥ 0;
with the corresponding symmetric dual problem,
Minimize bTy subject to ATy ≥ c, y ≥ 0.
An alternative primal formulation is:
Maximize cTx subject to Ax ≤ b;
with the corresponding asymmetric dual problem,
Minimize bTy subject to ATy = c, y ≥ 0. | Linear programming | Wikipedia | 487 | 43730 | https://en.wikipedia.org/wiki/Linear%20programming | Mathematics | Other | null |
There are two ideas fundamental to duality theory. One is the fact that (for the symmetric dual) the dual of a dual linear program is the original primal linear program. Additionally, every feasible solution for a linear program gives a bound on the optimal value of the objective function of its dual. The weak duality theorem states that the objective function value of the dual at any feasible solution is always greater than or equal to the objective function value of the primal at any feasible solution. The strong duality theorem states that if the primal has an optimal solution, x*, then the dual also has an optimal solution, y*, and cTx*=bTy*.
A linear program can also be unbounded or infeasible. Duality theory tells us that if the primal is unbounded then the dual is infeasible by the weak duality theorem. Likewise, if the dual is unbounded, then the primal must be infeasible. However, it is possible for both the dual and the primal to be infeasible. See dual linear program for details and several more examples.
Variations
Covering/packing dualities
A covering LP is a linear program of the form:
Minimize: bTy,
subject to: ATy ≥ c, y ≥ 0,
such that the matrix A and the vectors b and c are non-negative.
The dual of a covering LP is a packing LP, a linear program of the form:
Maximize: cTx,
subject to: Ax ≤ b, x ≥ 0,
such that the matrix A and the vectors b and c are non-negative.
Examples
Covering and packing LPs commonly arise as a linear programming relaxation of a combinatorial problem and are important in the study of approximation algorithms. For example, the LP relaxations of the set packing problem, the independent set problem, and the matching problem are packing LPs. The LP relaxations of the set cover problem, the vertex cover problem, and the dominating set problem are also covering LPs.
Finding a fractional coloring of a graph is another example of a covering LP. In this case, there is one constraint for each vertex of the graph and one variable for each independent set of the graph.
Complementary slackness
It is possible to obtain an optimal solution to the dual when only an optimal solution to the primal is known using the complementary slackness theorem. The theorem states: | Linear programming | Wikipedia | 486 | 43730 | https://en.wikipedia.org/wiki/Linear%20programming | Mathematics | Other | null |
Suppose that x = (x1, x2, ... , xn) is primal feasible and that y = (y1, y2, ... , ym) is dual feasible. Let (w1, w2, ..., wm) denote the corresponding primal slack variables, and let (z1, z2, ... , zn) denote the corresponding dual slack variables. Then x and y are optimal for their respective problems if and only if
xj zj = 0, for j = 1, 2, ... , n, and
wi yi = 0, for i = 1, 2, ... , m.
So if the i-th slack variable of the primal is not zero, then the i-th variable of the dual is equal to zero. Likewise, if the j-th slack variable of the dual is not zero, then the j-th variable of the primal is equal to zero.
This necessary condition for optimality conveys a fairly simple economic principle. In standard form (when maximizing), if there is slack in a constrained primal resource (i.e., there are "leftovers"), then additional quantities of that resource must have no value. Likewise, if there is slack in the dual (shadow) price non-negativity constraint requirement, i.e., the price is not zero, then there must be scarce supplies (no "leftovers").
Theory
Existence of optimal solutions
Geometrically, the linear constraints define the feasible region, which is a convex polytope. A linear function is a convex function, which implies that every local minimum is a global minimum; similarly, a linear function is a concave function, which implies that every local maximum is a global maximum.
An optimal solution need not exist, for two reasons. First, if the constraints are inconsistent, then no feasible solution exists: For instance, the constraints x ≥ 2 and x ≤ 1 cannot be satisfied jointly; in this case, we say that the LP is infeasible. Second, when the polytope is unbounded in the direction of the gradient of the objective function (where the gradient of the objective function is the vector of the coefficients of the objective function), then no optimal value is attained because it is always possible to do better than any finite value of the objective function. | Linear programming | Wikipedia | 489 | 43730 | https://en.wikipedia.org/wiki/Linear%20programming | Mathematics | Other | null |
Optimal vertices (and rays) of polyhedra
Otherwise, if a feasible solution exists and if the constraint set is bounded, then the optimum value is always attained on the boundary of the constraint set, by the maximum principle for convex functions (alternatively, by the minimum principle for concave functions) since linear functions are both convex and concave. However, some problems have distinct optimal solutions; for example, the problem of finding a feasible solution to a system of linear inequalities is a linear programming problem in which the objective function is the zero function (i.e., the constant function taking the value zero everywhere). For this feasibility problem with the zero-function for its objective-function, if there are two distinct solutions, then every convex combination of the solutions is a solution.
The vertices of the polytope are also called basic feasible solutions. The reason for this choice of name is as follows. Let d denote the number of variables. Then the fundamental theorem of linear inequalities implies (for feasible problems) that for every vertex x* of the LP feasible region, there exists a set of d (or fewer) inequality constraints from the LP such that, when we treat those d constraints as equalities, the unique solution is x*. Thereby we can study these vertices by means of looking at certain subsets of the set of all constraints (a discrete set), rather than the continuum of LP solutions. This principle underlies the simplex algorithm for solving linear programs.
Algorithms
Basis exchange algorithms
Simplex algorithm of Dantzig
The simplex algorithm, developed by George Dantzig in 1947, solves LP problems by constructing a feasible solution at a vertex of the polytope and then walking along a path on the edges of the polytope to vertices with non-decreasing values of the objective function until an optimum is reached for sure. In many practical problems, "stalling" occurs: many pivots are made with no increase in the objective function. In rare practical problems, the usual versions of the simplex algorithm may actually "cycle". To avoid cycles, researchers developed new pivoting rules.
In practice, the simplex algorithm is quite efficient and can be guaranteed to find the global optimum if certain precautions against cycling are taken. The simplex algorithm has been proved to solve "random" problems efficiently, i.e. in a cubic number of steps, which is similar to its behavior on practical problems. | Linear programming | Wikipedia | 500 | 43730 | https://en.wikipedia.org/wiki/Linear%20programming | Mathematics | Other | null |
However, the simplex algorithm has poor worst-case behavior: Klee and Minty constructed a family of linear programming problems for which the simplex method takes a number of steps exponential in the problem size. In fact, for some time it was not known whether the linear programming problem was solvable in polynomial time, i.e. of complexity class P.
Criss-cross algorithm
Like the simplex algorithm of Dantzig, the criss-cross algorithm is a basis-exchange algorithm that pivots between bases. However, the criss-cross algorithm need not maintain feasibility, but can pivot rather from a feasible basis to an infeasible basis. The criss-cross algorithm does not have polynomial time-complexity for linear programming. Both algorithms visit all 2D corners of a (perturbed) cube in dimension D, the Klee–Minty cube, in the worst case.
Interior point
In contrast to the simplex algorithm, which finds an optimal solution by traversing the edges between vertices on a polyhedral set, interior-point methods move through the interior of the feasible region.
Ellipsoid algorithm, following Khachiyan
This is the first worst-case polynomial-time algorithm ever found for linear programming. To solve a problem which has n variables and can be encoded in L input bits, this algorithm runs in time. Leonid Khachiyan solved this long-standing complexity issue in 1979 with the introduction of the ellipsoid method. The convergence analysis has (real-number) predecessors, notably the iterative methods developed by Naum Z. Shor and the approximation algorithms by Arkadi Nemirovski and D. Yudin.
Projective algorithm of Karmarkar
Khachiyan's algorithm was of landmark importance for establishing the polynomial-time solvability of linear programs. The algorithm was not a computational break-through, as the simplex method is more efficient for all but specially constructed families of linear programs.
However, Khachiyan's algorithm inspired new lines of research in linear programming. In 1984, N. Karmarkar proposed a projective method for linear programming. Karmarkar's algorithm improved on Khachiyan's worst-case polynomial bound (giving ). Karmarkar claimed that his algorithm was much faster in practical LP than the simplex method, a claim that created great interest in interior-point methods. Since Karmarkar's discovery, many interior-point methods have been proposed and analyzed. | Linear programming | Wikipedia | 508 | 43730 | https://en.wikipedia.org/wiki/Linear%20programming | Mathematics | Other | null |
Vaidya's 87 algorithm
In 1987, Vaidya proposed an algorithm that runs in time.
Vaidya's 89 algorithm
In 1989, Vaidya developed an algorithm that runs in time. Formally speaking, the algorithm takes arithmetic operations in the worst case, where is the number of constraints, is the number of variables, and is the number of bits.
Input sparsity time algorithms
In 2015, Lee and Sidford showed that linear programming can be solved in time, where denotes the soft O notation, and represents the number of non-zero elements, and it remains taking in the worst case.
Current matrix multiplication time algorithm
In 2019, Cohen, Lee and Song improved the running time to time, is the exponent of matrix multiplication and is the dual exponent of matrix multiplication. is (roughly) defined to be the largest number such that one can multiply an matrix by a matrix in time. In a followup work by Lee, Song and Zhang, they reproduce the same result via a different method. These two algorithms remain when and . The result due to Jiang, Song, Weinstein and Zhang improved to .
Comparison of interior-point methods and simplex algorithms
The current opinion is that the efficiencies of good implementations of simplex-based methods and interior point methods are similar for routine applications of linear programming. However, for specific types of LP problems, it may be that one type of solver is better than another (sometimes much better), and that the structure of the solutions generated by interior point methods versus simplex-based methods are significantly different with the support set of active variables being typically smaller for the latter one.
Open problems and recent work
There are several open problems in the theory of linear programming, the solution of which would represent fundamental breakthroughs in mathematics and potentially major advances in our ability to solve large-scale linear programs.
Does LP admit a strongly polynomial-time algorithm?
Does LP admit a strongly polynomial-time algorithm to find a strictly complementary solution?
Does LP admit a polynomial-time algorithm in the real number (unit cost) model of computation? | Linear programming | Wikipedia | 424 | 43730 | https://en.wikipedia.org/wiki/Linear%20programming | Mathematics | Other | null |
This closely related set of problems has been cited by Stephen Smale as among the 18 greatest unsolved problems of the 21st century. In Smale's words, the third version of the problem "is the main unsolved problem of linear programming theory." While algorithms exist to solve linear programming in weakly polynomial time, such as the ellipsoid methods and interior-point techniques, no algorithms have yet been found that allow strongly polynomial-time performance in the number of constraints and the number of variables. The development of such algorithms would be of great theoretical interest, and perhaps allow practical gains in solving large LPs as well.
Although the Hirsch conjecture was recently disproved for higher dimensions, it still leaves the following questions open.
Are there pivot rules which lead to polynomial-time simplex variants?
Do all polytopal graphs have polynomially bounded diameter?
These questions relate to the performance analysis and development of simplex-like methods. The immense efficiency of the simplex algorithm in practice despite its exponential-time theoretical performance hints that there may be variations of simplex that run in polynomial or even strongly polynomial time. It would be of great practical and theoretical significance to know whether any such variants exist, particularly as an approach to deciding if LP can be solved in strongly polynomial time.
The simplex algorithm and its variants fall in the family of edge-following algorithms, so named because they solve linear programming problems by moving from vertex to vertex along edges of a polytope. This means that their theoretical performance is limited by the maximum number of edges between any two vertices on the LP polytope. As a result, we are interested in knowing the maximum graph-theoretical diameter of polytopal graphs. It has been proved that all polytopes have subexponential diameter. The recent disproof of the Hirsch conjecture is the first step to prove whether any polytope has superpolynomial diameter. If any such polytopes exist, then no edge-following variant can run in polynomial time. Questions about polytope diameter are of independent mathematical interest. | Linear programming | Wikipedia | 424 | 43730 | https://en.wikipedia.org/wiki/Linear%20programming | Mathematics | Other | null |
Simplex pivot methods preserve primal (or dual) feasibility. On the other hand, criss-cross pivot methods do not preserve (primal or dual) feasibilitythey may visit primal feasible, dual feasible or primal-and-dual infeasible bases in any order. Pivot methods of this type have been studied since the 1970s. Essentially, these methods attempt to find the shortest pivot path on the arrangement polytope under the linear programming problem. In contrast to polytopal graphs, graphs of arrangement polytopes are known to have small diameter, allowing the possibility of strongly polynomial-time criss-cross pivot algorithm without resolving questions about the diameter of general polytopes.
Integer unknowns
If all of the unknown variables are required to be integers, then the problem is called an integer programming (IP) or integer linear programming (ILP) problem. In contrast to linear programming, which can be solved efficiently in the worst case, integer programming problems are in many practical situations (those with bounded variables) NP-hard. 0–1 integer programming or binary integer programming (BIP) is the special case of integer programming where variables are required to be 0 or 1 (rather than arbitrary integers). This problem is also classified as NP-hard, and in fact the decision version was one of Karp's 21 NP-complete problems.
If only some of the unknown variables are required to be integers, then the problem is called a mixed integer (linear) programming (MIP or MILP) problem. These are generally also NP-hard because they are even more general than ILP programs.
There are however some important subclasses of IP and MIP problems that are efficiently solvable, most notably problems where the constraint matrix is totally unimodular and the right-hand sides of the constraints are integers or – more general – where the system has the total dual integrality (TDI) property.
Advanced algorithms for solving integer linear programs include:
cutting-plane method
Branch and bound
Branch and cut
Branch and price
if the problem has some extra structure, it may be possible to apply delayed column generation.
Such integer-programming algorithms are discussed by Padberg and in Beasley.
Integral linear programs | Linear programming | Wikipedia | 452 | 43730 | https://en.wikipedia.org/wiki/Linear%20programming | Mathematics | Other | null |
A linear program in real variables is said to be integral if it has at least one optimal solution which is integral, i.e., made of only integer values. Likewise, a polyhedron is said to be integral if for all bounded feasible objective functions c, the linear program has an optimum with integer coordinates. As observed by Edmonds and Giles in 1977, one can equivalently say that the polyhedron is integral if for every bounded feasible integral objective function c, the optimal value of the linear program is an integer.
Integral linear programs are of central importance in the polyhedral aspect of combinatorial optimization since they provide an alternate characterization of a problem. Specifically, for any problem, the convex hull of the solutions is an integral polyhedron; if this polyhedron has a nice/compact description, then we can efficiently find the optimal feasible solution under any linear objective. Conversely, if we can prove that a linear programming relaxation is integral, then it is the desired description of the convex hull of feasible (integral) solutions.
Terminology is not consistent throughout the literature, so one should be careful to distinguish the following two concepts,
in an integer linear program, described in the previous section, variables are forcibly constrained to be integers, and this problem is NP-hard in general,
in an integral linear program, described in this section, variables are not constrained to be integers but rather one has proven somehow that the continuous problem always has an integral optimal value (assuming c is integral), and this optimal value may be found efficiently since all polynomial-size linear programs can be solved in polynomial time.
One common way of proving that a polyhedron is integral is to show that it is totally unimodular. There are other general methods including the integer decomposition property and total dual integrality. Other specific well-known integral LPs include the matching polytope, lattice polyhedra, submodular flow polyhedra, and the intersection of two generalized polymatroids/g-polymatroids – e.g. see Schrijver 2003.
Solvers and scripting (programming) languages
Permissive licenses:
Copyleft (reciprocal) licenses:
MINTO (Mixed Integer Optimizer, an integer programming solver which uses branch and bound algorithm) has publicly available source code but is not open source.
Proprietary licenses: | Linear programming | Wikipedia | 471 | 43730 | https://en.wikipedia.org/wiki/Linear%20programming | Mathematics | Other | null |
In telecommunications and computer networking, a network packet is a formatted unit of data carried by a packet-switched network. A packet consists of control information and user data; the latter is also known as the payload. Control information provides data for delivering the payload (e.g., source and destination network addresses, error detection codes, or sequencing information). Typically, control information is found in packet headers and trailers.
In packet switching, the bandwidth of the transmission medium is shared between multiple communication sessions, in contrast to circuit switching, in which circuits are preallocated for the duration of one session and data is typically transmitted as a continuous bit stream.
Terminology
In the seven-layer OSI model of computer networking, packet strictly refers to a protocol data unit at layer 3, the network layer. A data unit at layer 2, the data link layer, is a frame. In layer 4, the transport layer, the data units are segments and datagrams. Thus, in the example of TCP/IP communication over Ethernet, a TCP segment is carried in one or more IP packets, which are each carried in one or more Ethernet frames.
Architecture
The basis of the packet concept is the postal letter: the header is like the envelope, the payload is the entire content inside the envelope, and the footer would be your signature at the bottom.
Network design can achieve two major results by using packets: error detection and multiple host addressing.
Framing
Communications protocols use various conventions for distinguishing the elements of a packet and for formatting the user data. For example, in Point-to-Point Protocol, the packet is formatted in 8-bit bytes, and special characters are used to delimit elements. Other protocols, like Ethernet, establish the start of the header and data elements by their location relative to the start of the packet. Some protocols format the information at a bit level instead of a byte level.
Contents
A packet may contain any of the following components:
Addresses
The routing of network packets requires two network addresses, the source address of the sending host, and the destination address of the receiving host.
Error detection and correction
Error detection and correction is performed at various layers in the protocol stack. Network packets may contain a checksum, parity bits or cyclic redundancy checks to detect errors that occur during transmission. | Network packet | Wikipedia | 470 | 43734 | https://en.wikipedia.org/wiki/Network%20packet | Technology | Networks | null |
At the transmitter, the calculation is performed before the packet is sent. When received at the destination, the checksum is recalculated, and compared with the one in the packet. If discrepancies are found, the packet may be corrected or discarded. Any packet loss due to these discards is dealt with by the network protocol.
In some cases, modifications of the network packet may be necessary while routing, in which cases checksums are recalculated.
Hop limit
Under fault conditions, packets can end up traversing a closed circuit. If nothing was done, eventually the number of packets circulating would build up until the network was congested to the point of failure. Time to live is a field that is decreased by one each time a packet goes through a network hop. If the field reaches zero, routing has failed, and the packet is discarded.
Ethernet packets have no time-to-live field and so are subject to broadcast storms in the presence of a switching loop.
Length
There may be a field to identify the overall packet length. However, in some types of networks, the length is implied by the duration of the transmission.
Protocol identifier
It is often desirable to carry multiple communication protocols on a network. A protocol identifier field specifies a packet's protocol and allows the protocol stack to process many types of packets.
Priority
Some networks implement quality of service which can prioritize some types of packets above others. This field indicates which packet queue should be used; a high-priority queue is emptied more quickly than lower-priority queues at points in the network where congestion is occurring.
Payload
In general, the payload is the data that is carried on behalf of an application. It is usually of variable length, up to a maximum that is set by the network protocol and sometimes the equipment on the route. When necessary, some networks can break a larger packet into smaller packets.
Examples
Internet protocol
IP packets are composed of a header and payload. The header consists of fixed and optional fields. The payload appears immediately after the header. An IP packet has no trailer. However, an IP packet is often carried as the payload inside an Ethernet frame, which has its own header and trailer.
Per the end-to-end principle, IP networks do not provide guarantees of delivery, non-duplication, or in-order delivery of packets. However, it is common practice to layer a reliable transport protocol such as Transmission Control Protocol on top of the packet service to provide such protection. | Network packet | Wikipedia | 505 | 43734 | https://en.wikipedia.org/wiki/Network%20packet | Technology | Networks | null |
NASA Deep Space Network
The Consultative Committee for Space Data Systems (CCSDS) packet telemetry standard defines the protocol used for the transmission of spacecraft instrument data over the deep-space channel. Under this standard, an image or other data sent from a spacecraft instrument is transmitted using one or more packets.
MPEG packetized stream
Packetized elementary stream (PES) is a specification associated with the MPEG-2 standard that allows an elementary stream to be divided into packets. The elementary stream is packetized by encapsulating sequential data bytes from the elementary stream between PES packet headers.
A typical method of transmitting elementary stream data from a video or audio encoder is to first create PES packets from the elementary stream data and then to encapsulate these PES packets inside an MPEG transport stream (TS) packets or an MPEG program stream (PS). The TS packets can then be transmitted using broadcasting techniques, such as those used in an ATSC and DVB.
NICAM
In order to provide mono compatibility, the NICAM signal is transmitted on a subcarrier alongside the sound carrier. This means that the FM or AM regular mono sound carrier is left alone for reception by monaural receivers. The NICAM packet (except for the header) is scrambled with a nine-bit pseudo-random bit-generator before transmission. Making the NICAM bitstream look more like white noise is important because this reduces signal patterning on adjacent TV channels. | Network packet | Wikipedia | 299 | 43734 | https://en.wikipedia.org/wiki/Network%20packet | Technology | Networks | null |
Clostridium botulinum is a gram-positive, rod-shaped, anaerobic, spore-forming, motile bacterium with the ability to produce botulinum toxin, which is a neurotoxin.
C. botulinum is a diverse group of pathogenic bacteria. Initially, they were grouped together by their ability to produce botulinum toxin and are now known as four distinct groups, C. botulinum groups I–IV. Along with some strains of Clostridium butyricum and Clostridium baratii, these bacteria all produce the toxin.
Botulinum toxin can cause botulism, a severe flaccid paralytic disease in humans and other animals, and is the most potent toxin known to science, natural or synthetic, with a lethal dose of 1.3–2.1 ng/kg in humans.
C. botulinum is commonly associated with bulging canned food; bulging, misshapen cans can be due to an internal increase in pressure caused by gas produced by bacteria.
C. botulinum is responsible for foodborne botulism (ingestion of preformed toxin), infant botulism (intestinal infection with toxin-forming C. botulinum), and wound botulism (infection of a wound with C. botulinum). C. botulinum produces heat-resistant endospores that are commonly found in soil and are able to survive under adverse conditions.
Microbiology
C. botulinum is a Gram-positive, rod-shaped, spore-forming bacterium. It is an obligate anaerobe, the organism survives in an environment that lacks oxygen. However, C. botulinum tolerates traces of oxygen due to the enzyme superoxide dismutase, which is an important antioxidant defense in nearly all cells exposed to oxygen. C. botulinum is able to produce the neurotoxin only during sporulation, which can happen only in an anaerobic environment. | Clostridium botulinum | Wikipedia | 418 | 43922 | https://en.wikipedia.org/wiki/Clostridium%20botulinum | Biology and health sciences | Gram-positive bacteria | Plants |
C. botulinum is divided into four distinct phenotypic groups (I-IV) and is also classified into seven serotypes (A–G) based on the antigenicity of the botulinum toxin produced. On the level visible to DNA sequences, the phenotypic grouping matches the results of whole-genome and rRNA analyses, and setotype grouping approximates the result of analyses focused specifically on the toxin sequence. The two phylogenetic trees do not match because of the ability of the toxin gene cluster to be horizontally transferred.
Serotypes
Botulinum neurotoxin (BoNT) production is the unifying feature of the species. Seven serotypes of toxins have been identified that are allocated a letter (A–G), several of which can cause disease in humans. They are resistant to degradation by enzymes found in the gastrointestinal tract. This allows for ingested toxins to be absorbed from the intestines into the bloodstream. Toxins can be further differentiated into subtypes on the bases of smaller variations.
However, all types of botulinum toxin are rapidly destroyed by heating to 100 °C for 15 minutes (900 seconds). 80 °C for 30 minutes also destroys BoNT.
Most strains produce one type of BoNT, but strains producing multiple toxins have been described. C. botulinum producing B and F toxin types have been isolated from human botulism cases in New Mexico and California. The toxin type has been designated Bf as the type B toxin was found in excess to the type F. Similarly, strains producing Ab and Af toxins have been reported.
Evidence indicates the neurotoxin genes have been the subject of horizontal gene transfer, possibly from a viral (bacteriophage) source. This theory is supported by the presence of integration sites flanking the toxin in some strains of C. botulinum. However, these integrations sites are degraded (except for the C and D types), indicating that the C. botulinum acquired the toxin genes quite far in the evolutionary past. Nevertheless, further transfers still happen via the plasmids and other mobile elements the genes are located on. | Clostridium botulinum | Wikipedia | 443 | 43922 | https://en.wikipedia.org/wiki/Clostridium%20botulinum | Biology and health sciences | Gram-positive bacteria | Plants |
Toxin types in disease
Only botulinum toxin types A, B, E, F and H (FA) cause disease in humans. Types A, B, and E are associated with food-borne illness, while type E is specifically associated with fish products. Type C produces limber-neck in birds and type D causes botulism in other mammals. No disease is associated with type G. The "gold standard" for determining toxin type is a mouse bioassay, but the genes for types A, B, E, and F can now be readily differentiated using quantitative PCR. Type "H" is in fact a recombinant toxin from types A and F. It can be neutralized by type A antitoxin and no longer is considered a distinct type.
A few strains from organisms genetically identified as other Clostridium species have caused human botulism: C. butyricum has produced type E toxin and C. baratii had produced type F toxin. The ability of C. botulinum to naturally transfer neurotoxin genes to other clostridia is concerning, especially in the food industry, where preservation systems are designed to destroy or inhibit only C. botulinum but not other Clostridium species.
Metabolism
Many C. botulinum genes play a role in the breakdown of essential carbohydrates and the metabolism of sugars. Chitin is the preferred source of carbon and nitrogen for C. botulinum. Hall A strain of C. botulinum has an active chitinolytic system to aid in the breakdown of chitin. Type A and B of C. botulinum production of BoNT is affected by nitrogen and carbon nutrition. There is evidence that these processes are also under catabolite repression.
Groups
Physiological differences and genome sequencing at 16S rRNA level support the subdivision of the C. botulinum species into groups I-IV. Some authors have briefly used groups V and VI, corresponding to toxin-producing C. baratii and C. butyricum. What used to be group IV is now C. argentinense.
Although group II cannot degrade native protein such as casein, coagulated egg white, and cooked meat particles, it is able to degrade gelatin.
Human botulism is predominantly caused by group I or II C. botulinum. Group III organisms mainly cause diseases in non-human animals. | Clostridium botulinum | Wikipedia | 496 | 43922 | https://en.wikipedia.org/wiki/Clostridium%20botulinum | Biology and health sciences | Gram-positive bacteria | Plants |
Laboratory isolation
In the laboratory, C. botulinum is usually isolated in tryptose sulfite cycloserine (TSC) growth medium in an anaerobic environment with less than 2% oxygen. This can be achieved by several commercial kits that use a chemical reaction to replace O2 with CO2. C. botulinum (groups I through III) is a lipase-positive microorganism that grows between pH of 4.8 and 7.0 and cannot use lactose as a primary carbon source, characteristics important for biochemical identification.
Transmission and sporulation
The exact mechanism behind sporulation of C. botulinum is not known. Different strains of C. botulinum can be divided into three different groups, group I, II, and III, based on environmental conditions like heat resistance, temperature, and biome. Within each group, different strains will use different strategies to adapt to their environment to survive. Unlike other clostridial species, C. botulinum spores will sporulate as it enters the stationary phase. C. botulinum relies on quorum-sensing to initiate the sporulation process. C. botulinum spores are not found in human feces unless the individual has contracted botulism, but C. botulinum cannot spread from person to person.
Motility structures
The most common motility structure for C. botulinum is a flagellum. Though this structure is not found in all strains of C. botulinum, most produce peritrichous flagella. When comparing the different strains, there is also differences in the length of the flagella and how many are present on the cell.
Growth conditions and prevention | Clostridium botulinum | Wikipedia | 349 | 43922 | https://en.wikipedia.org/wiki/Clostridium%20botulinum | Biology and health sciences | Gram-positive bacteria | Plants |
C. botulinum is a soil bacterium. The spores can survive in most environments and are very hard to kill. They can survive the temperature of boiling water at sea level, thus many foods are canned with a pressurized boil that achieves even higher temperatures, sufficient to kill the spores. This bacteria is widely distributed in nature and can be assumed to be present on all food surfaces. Its optimum growth temperature is within the mesophilic range. In spore form, it is a heat resistant pathogen that can survive in low acid foods and grow to produce toxins. The toxin attacks the nervous system and will kill an adult at a dose of around 75 ng. Botulinum toxin can be destroyed by holding food at 100 °C for 10 minutes; however, because of its potency, this is not recommended by the USA's FDA as a means of control.
Botulism poisoning can occur due to preserved or home-canned, low-acid food that was not processed using correct preservation times and/or pressure. Growth of the bacterium can be prevented by high acidity, high ratio of dissolved sugar, high levels of oxygen, very low levels of moisture, or storage at temperatures below 3 °C (38 °F) for type A. For example, in a low-acid, canned vegetable such as green beans that are not heated enough to kill the spores (i.e., a pressurized environment) may provide an oxygen-free medium for the spores to grow and produce the toxin. However, pickles are sufficiently acidic to prevent growth; even if the spores are present, they pose no danger to the consumer.
Honey, corn syrup, and other sweeteners may contain spores, but the spores cannot grow in a highly concentrated sugar solution; however, when a sweetener is diluted in the low-oxygen, low-acid digestive system of an infant, the spores can grow and produce toxin. As soon as infants begin eating solid food, the digestive juices become too acidic for the bacterium to grow. | Clostridium botulinum | Wikipedia | 412 | 43922 | https://en.wikipedia.org/wiki/Clostridium%20botulinum | Biology and health sciences | Gram-positive bacteria | Plants |
The control of food-borne botulism caused by C. botulinum is based almost entirely on thermal destruction (heating) of the spores or inhibiting spore germination into bacteria and allowing cells to grow and produce toxins in foods. Conditions conducive of growth are dependent on various environmental factors.
Growth of C. botulinum is a risk in low acid foods as defined by having a pH above 4.6 although growth is significantly retarded for pH below 4.9.
Taxonomic history
C. botulinum was first recognized and isolated in 1895 by Emile van Ermengem from home-cured ham implicated in a botulism outbreak. The isolate was originally named Bacillus botulinus, after the Latin word for sausage, botulus. ("Sausage poisoning" was a common problem in 18th- and 19th-century Germany, and was most likely caused by botulism.) However, isolates from subsequent outbreaks were always found to be anaerobic spore formers, so Ida A. Bengtson proposed that both be placed into the genus Clostridium, as the genus Bacillus was restricted to aerobic spore-forming rods.
Since 1959, all species producing the botulinum neurotoxins (types A–G) have been designated C. botulinum. Substantial phenotypic and genotypic evidence exists to demonstrate heterogeneity within the species, with at least four clearly-defined "groups" (see ) straddling other species, implying that they each deserve to be a genospecies.
The situation as of 2018 is as follows:
C. botulinum type G (= group IV) strains are since 1988 their own species, C. argentinense.
Group I C. botulinum strains that do not produce a botulin toxin are referred to as C. sporogenes. Both names are conserved names since 1999. Group I also contains C. combesii.
All other botulinum toxin-producing bacteria, not otherwise classified as C. baratii or C. butyricum, is called C. botulinum. This group still contains three genogroups. | Clostridium botulinum | Wikipedia | 453 | 43922 | https://en.wikipedia.org/wiki/Clostridium%20botulinum | Biology and health sciences | Gram-positive bacteria | Plants |
Smith et al. (2018) argues that group I should be called C. parabotulinum and group III be called C. novyi sensu lato, leaving only group II in C. botulinum. This argument is not accepted by the LPSN and would cause an unjustified change of the type strain under the Prokaryotic Code. (The current type strain ATCC 25763 falls into group I.) Dobritsa et al. (2018) argues, without formal descriptions, that group II can potentially be made into two new species.
The complete genome of C. botulinum ATCC 3502 has been sequenced at Wellcome Trust Sanger Institute in 2007. This strain encodes a type "A" toxin.
Diagnosis
Physicians may consider the diagnosis of botulism based on a patient's clinical presentation, which classically includes an acute onset of bilateral cranial neuropathies and symmetric descending weakness. Other key features of botulism include an absence of fever, symmetric neurologic deficits, normal or slow heart rate and normal blood pressure, and no sensory deficits except for blurred vision. A careful history and physical examination is paramount to diagnose the type of botulism, as well as to rule out other conditions with similar findings, such as Guillain–Barré syndrome, stroke, and myasthenia gravis. Depending on the type of botulism considered, different tests for diagnosis may be indicated.
Foodborne botulism: serum analysis for toxins by bioassay in mice should be done, as the demonstration of the toxins is diagnostic.
Wound botulism: isolation of C. botulinum from the wound site should be attempted, as growth of the bacteria is diagnostic.
Adult enteric and infant botulism: isolation and growth of C. botulinum from stool samples is diagnostic. Infant botulism is a diagnosis which is often missed in the emergency room.
Other tests that may be helpful in ruling out other conditions are:
Electromyography (EMG) or antibody studies may help with the exclusion of myasthenia gravis and Lambert–Eaton myasthenic syndrome (LEMS).
Collection of cerebrospinal fluid (CSF) protein and blood assist with the exclusion of Guillan-Barre syndrome and stroke.
Detailed physical examination of the patient for any rash or tick presence helps with the exclusion of any tick transmitted tick paralysis.
Pathology | Clostridium botulinum | Wikipedia | 506 | 43922 | https://en.wikipedia.org/wiki/Clostridium%20botulinum | Biology and health sciences | Gram-positive bacteria | Plants |
Foodborne botulism
Signs and symptoms of foodborne botulism typically begin between 18 and 36 hours after the toxin gets into your body, but can range from a few hours to several days, depending on the amount of toxin ingested. Symptoms include:
Double vision
Blurred vision
Ptosis
Nausea, vomiting, and abdominal cramps
Slurred speech
Trouble breathing
Difficulty in swallowing
Dry mouth
Muscle weakness
Constipation
Reduced or absent deep tendon reactions, such as in the knee
Wound botulism
Most people who develop wound botulism inject drugs several times a day, so determining a timeline of when onset symptoms first occurred and when the toxin entered the body can be difficult. It is more common in people who inject black tar heroin. Wound botulism signs and symptoms include:
Difficulty swallowing or speaking
Facial weakness on both sides of the face
Blurred or double vision
Ptosis
Trouble breathing
Paralysis
Infant botulism
If infant botulism is related to food, such as honey, problems generally begin within 18 to 36 hours after the toxin enters the baby's body. Signs and symptoms include:
Constipation (often the first sign)
Floppy movements due to muscle weakness and trouble controlling the head
Weak cry
Irritability
Drooling
Ptosis
Tiredness
Difficulty sucking or feeding
Paralysis
Beneficial effects of botulinum toxin
Purified botulinum toxin is diluted by a physician for treatment of:
Congenital pelvic tilt
Spasmodic dysphasia (the inability of the muscles of the larynx)
Achalasia (esophageal stricture)
Strabismus (crossed eyes)
Paralysis of the facial muscles
Failure of the cervix
Blinking frequently
Anti-cancer drug delivery
Adult intestinal toxemia
A very rare form of botulism that occurs by the same route as infant botulism but is among adults. Occurs rarely and sporadically. Signs and symptoms include:
Abdominal pain
Blurred vision
Diarrhea
Dysarthria
Imbalance
Weakness in arms and hand area
Treatment
In the case of a diagnosis or suspicion of botulism, patients should be hospitalized immediately, even if the diagnosis and/or tests are pending. Additionally if botulism is suspected, patients should be treated immediately with antitoxin therapy in order to reduce mortality. Immediate intubation is also highly recommended, as respiratory failure is the primary cause of death from botulism. | Clostridium botulinum | Wikipedia | 483 | 43922 | https://en.wikipedia.org/wiki/Clostridium%20botulinum | Biology and health sciences | Gram-positive bacteria | Plants |
In North America, an equine-derived heptavalent botulinum antitoxin is used to treat all serotypes of non-infant naturally occurring botulism. For infants less than one year of age, botulism immune globulin is used to treat type A or type B.
Outcomes vary between one and three months, but with prompt interventions, mortality from botulism ranges from less than 5 percent to 8 percent.
Vaccination
There used to be a formalin-treated toxoid vaccine against botulism (serotypes A-E), but it was discontinued in 2011 due to declining potency in the toxoid stock. It was originally intended for people at risk of exposure. A few new vaccines are under development.
Use and detection
C. botulinum is used to prepare the medicaments Botox, Dysport, Xeomin, and Neurobloc used to selectively paralyze muscles to temporarily relieve muscle function. It has other "off-label" medical purposes, such as treating severe facial pain, such as that caused by trigeminal neuralgia.
Botulinum toxin produced by C. botulinum is often believed to be a potential bioweapon as it is so potent that it takes about 75 nanograms to kill a person ( of 1 ng/kg, assuming an average person weighs ~75 kg); 1 kilogram of it would be enough to kill the entire human population.
A "mouse protection" or "mouse bioassay" test determines the type of C. botulinum toxin present using monoclonal antibodies. An enzyme-linked immunosorbent assay (ELISA) with digoxigenin-labeled antibodies can also be used to detect the toxin, and quantitative PCR can detect the toxin genes in the organism.
C. botulinum in different geographical locations
A number of quantitative surveys for C. botulinum spores in the environment have suggested a prevalence of specific toxin types in given geographic areas, which remain unexplained. | Clostridium botulinum | Wikipedia | 424 | 43922 | https://en.wikipedia.org/wiki/Clostridium%20botulinum | Biology and health sciences | Gram-positive bacteria | Plants |
Parasitism is a close relationship between species, where one organism, the parasite, lives on or inside another organism, the host, causing it some harm, and is adapted structurally to this way of life. The entomologist E. O. Wilson characterised parasites as "predators that eat prey in units of less than one". Parasites include single-celled protozoans such as the agents of malaria, sleeping sickness, and amoebic dysentery; animals such as hookworms, lice, mosquitoes, and vampire bats; fungi such as honey fungus and the agents of ringworm; and plants such as mistletoe, dodder, and the broomrapes.
There are six major parasitic strategies of exploitation of animal hosts, namely parasitic castration, directly transmitted parasitism (by contact), trophicallytransmitted parasitism (by being eaten), vector-transmitted parasitism, parasitoidism, and micropredation. One major axis of classification concerns invasiveness: an endoparasite lives inside the host's body; an ectoparasite lives outside, on the host's surface.
Like predation, parasitism is a type of consumer–resource interaction, but unlike predators, parasites, with the exception of parasitoids, are much smaller than their hosts, do not kill them, and often live in or on their hosts for an extended period. Parasites of animals are highly specialised, each parasite species living on one given animal species, and reproduce at a faster rate than their hosts. Classic examples include interactions between vertebrate hosts and tapeworms, flukes, and those between the malaria-causing Plasmodium species, and fleas.
Parasites reduce host fitness by general or specialised pathology, that ranges from parasitic castration to modification of host behaviour. Parasites increase their own fitness by exploiting hosts for resources necessary for their survival, in particular by feeding on them and by using intermediate (secondary) hosts to assist in their transmission from one definitive (primary) host to another. Although parasitism is often unambiguous, it is part of a spectrum of interactions between species, grading via parasitoidism into predation, through evolution into mutualism, and in some fungi, shading into being saprophytic. | Parasitism | Wikipedia | 475 | 43937 | https://en.wikipedia.org/wiki/Parasitism | Biology and health sciences | Ecology | null |
Human knowledge of parasites such as roundworms and tapeworms dates back to ancient Egypt, Greece, and Rome. In early modern times, Antonie van Leeuwenhoek observed Giardia lamblia with his microscope in 1681, while Francesco Redi described internal and external parasites including sheep liver fluke and ticks. Modern parasitology developed in the 19th century. In human culture, parasitism has negative connotations. These were exploited to satirical effect in Jonathan Swift's 1733 poem "On Poetry: A Rhapsody", comparing poets to hyperparasitical "vermin". In fiction, Bram Stoker's 1897 Gothic horror novel Dracula and its many later adaptations featured a blood-drinking parasite. Ridley Scott's 1979 film Alien was one of many works of science fiction to feature a parasitic alien species.
Etymology
First used in English in 1539, the word parasite comes from the Medieval French , from the Latinised form , . The related term parasitism appears in English from 1611.
Evolutionary strategies
Basic concepts
Parasitism is a kind of symbiosis, a close and persistent long-term biological interaction between a parasite and its host. Unlike saprotrophs, parasites feed on living hosts, though some parasitic fungi, for instance, may continue to feed on hosts they have killed. Unlike commensalism and mutualism, the parasitic relationship harms the host, either feeding on it or, as in the case of intestinal parasites, consuming some of its food. Because parasites interact with other species, they can readily act as vectors of pathogens, causing disease. Predation is by definition not a symbiosis, as the interaction is brief, but the entomologist E. O. Wilson has characterised parasites as "predators that eat prey in units of less than one". | Parasitism | Wikipedia | 373 | 43937 | https://en.wikipedia.org/wiki/Parasitism | Biology and health sciences | Ecology | null |
Within that scope are many possible strategies. Taxonomists classify parasites in a variety of overlapping schemes, based on their interactions with their hosts and on their life cycles, which can be complex. An obligate parasite depends completely on the host to complete its life cycle, while a facultative parasite does not. Parasite life cycles involving only one host are called "direct"; those with a definitive host (where the parasite reproduces sexually) and at least one intermediate host are called "indirect". An endoparasite lives inside the host's body; an ectoparasite lives outside, on the host's surface. Mesoparasites—like some copepods, for example—enter an opening in the host's body and remain partly embedded there. Some parasites can be generalists, feeding on a wide range of hosts, but many parasites, and the majority of protozoans and helminths that parasitise animals, are specialists and extremely host-specific. An early basic, functional division of parasites distinguished microparasites and macroparasites. These each had a mathematical model assigned in order to analyse the population movements of the host–parasite groupings. The microorganisms and viruses that can reproduce and complete their life cycle within the host are known as microparasites. Macroparasites are the multicellular organisms that reproduce and complete their life cycle outside of the host or on the host's body.
Much of the thinking on types of parasitism has focused on terrestrial animal parasites of animals, such as helminths. Those in other environments and with other hosts often have analogous strategies. For example, the snubnosed eel is probably a facultative endoparasite (i.e., it is semiparasitic) that opportunistically burrows into and eats sick and dying fish. Plant-eating insects such as scale insects, aphids, and caterpillars closely resemble ectoparasites, attacking much larger plants; they serve as vectors of bacteria, fungi and viruses which cause plant diseases. As female scale insects cannot move, they are obligate parasites, permanently attached to their hosts. | Parasitism | Wikipedia | 452 | 43937 | https://en.wikipedia.org/wiki/Parasitism | Biology and health sciences | Ecology | null |
The sensory inputs that a parasite employs to identify and approach a potential host are known as "host cues". Such cues can include, for example, vibration, exhaled carbon dioxide, skin odours, visual and heat signatures, and moisture. Parasitic plants can use, for example, light, host physiochemistry, and volatiles to recognize potential hosts.
Major strategies
There are six major parasitic strategies, namely parasitic castration; directly transmitted parasitism; trophically-transmitted parasitism; vector-transmitted parasitism; parasitoidism; and micropredation. These apply to parasites whose hosts are plants as well as animals. These strategies represent adaptive peaks; intermediate strategies are possible, but organisms in many different groups have consistently converged on these six, which are evolutionarily stable.
A perspective on the evolutionary options can be gained by considering four key questions: the effect on the fitness of a parasite's hosts; the number of hosts they have per life stage; whether the host is prevented from reproducing; and whether the effect depends on intensity (number of parasites per host). From this analysis, the major evolutionary strategies of parasitism emerge, alongside predation.
Parasitic castrators
Parasitic castrators partly or completely destroy their host's ability to reproduce, diverting the energy that would have gone into reproduction into host and parasite growth, sometimes causing gigantism in the host. The host's other systems remain intact, allowing it to survive and to sustain the parasite. Parasitic crustaceans such as those in the specialised barnacle genus Sacculina specifically cause damage to the gonads of their many species of host crabs. In the case of Sacculina, the testes of over two-thirds of their crab hosts degenerate sufficiently for these male crabs to develop female secondary sex characteristics such as broader abdomens, smaller claws and egg-grasping appendages. Various species of helminth castrate their hosts (such as insects and snails). This may happen directly, whether mechanically by feeding on their gonads, or by secreting a chemical that destroys reproductive cells; or indirectly, whether by secreting a hormone or by diverting nutrients. For example, the trematode Zoogonus lasius, whose sporocysts lack mouths, castrates the intertidal marine snail Tritia obsoleta chemically, developing in its gonad and killing its reproductive cells.
Directly transmitted | Parasitism | Wikipedia | 502 | 43937 | https://en.wikipedia.org/wiki/Parasitism | Biology and health sciences | Ecology | null |
Directly transmitted parasites, not requiring a vector to reach their hosts, include such parasites of terrestrial vertebrates as lice and mites; marine parasites such as copepods and cyamid amphipods; monogeneans; and many species of nematodes, fungi, protozoans, bacteria, and viruses. Whether endoparasites or ectoparasites, each has a single host-species. Within that species, most individuals are free or almost free of parasites, while a minority carry a large number of parasites; this is known as an aggregated distribution.
Trophically transmitted
Trophically-transmitted parasites are transmitted by being eaten by a host. They include trematodes (all except schistosomes), cestodes, acanthocephalans, pentastomids, many roundworms, and many protozoa such as Toxoplasma. They have complex life cycles involving hosts of two or more species. In their juvenile stages they infect and often encyst in the intermediate host. When the intermediate-host animal is eaten by a predator, the definitive host, the parasite survives the digestion process and matures into an adult; some live as intestinal parasites. Many trophically transmitted parasites modify the behaviour of their intermediate hosts, increasing their chances of being eaten by a predator. As with directly transmitted parasites, the distribution of trophically transmitted parasites among host individuals is aggregated. Coinfection by multiple parasites is common. Autoinfection, where (by exception) the whole of the parasite's life cycle takes place in a single primary host, can sometimes occur in helminths such as Strongyloides stercoralis.
Vector-transmitted | Parasitism | Wikipedia | 360 | 43937 | https://en.wikipedia.org/wiki/Parasitism | Biology and health sciences | Ecology | null |
Vector-transmitted parasites rely on a third party, an intermediate host, where the parasite does not reproduce sexually, to carry them from one definitive host to another. These parasites are microorganisms, namely protozoa, bacteria, or viruses, often intracellular pathogens (disease-causers). Their vectors are mostly hematophagic arthropods such as fleas, lice, ticks, and mosquitoes. For example, the deer tick Ixodes scapularis acts as a vector for diseases including Lyme disease, babesiosis, and anaplasmosis. Protozoan endoparasites, such as the malarial parasites in the genus Plasmodium and sleeping-sickness parasites in the genus Trypanosoma, have infective stages in the host's blood which are transported to new hosts by biting insects.
Parasitoids
Parasitoids are insects which sooner or later kill their hosts, placing their relationship close to predation. Most parasitoids are parasitoid wasps or other hymenopterans; others include dipterans such as phorid flies. They can be divided into two groups, idiobionts and koinobionts, differing in their treatment of their hosts.
Idiobiont parasitoids sting their often-large prey on capture, either killing them outright or paralysing them immediately. The immobilised prey is then carried to a nest, sometimes alongside other prey if it is not large enough to support a parasitoid throughout its development. An egg is laid on top of the prey and the nest is then sealed. The parasitoid develops rapidly through its larval and pupal stages, feeding on the provisions left for it.
Koinobiont parasitoids, which include flies as well as wasps, lay their eggs inside young hosts, usually larvae. These are allowed to go on growing, so the host and parasitoid develop together for an extended period, ending when the parasitoids emerge as adults, leaving the prey dead, eaten from inside. Some koinobionts regulate their host's development, for example preventing it from pupating or making it moult whenever the parasitoid is ready to moult. They may do this by producing hormones that mimic the host's moulting hormones (ecdysteroids), or by regulating the host's endocrine system.
Micropredators | Parasitism | Wikipedia | 499 | 43937 | https://en.wikipedia.org/wiki/Parasitism | Biology and health sciences | Ecology | null |
A micropredator attacks more than one host, reducing each host's fitness by at least a small amount, and is only in contact with any one host intermittently. This behavior makes micropredators suitable as vectors, as they can pass smaller parasites from one host to another. Most micropredators are hematophagic, feeding on blood. They include annelids such as leeches, crustaceans such as branchiurans and gnathiid isopods, various dipterans such as mosquitoes and tsetse flies, other arthropods such as fleas and ticks, vertebrates such as lampreys, and mammals such as vampire bats.
Transmission strategies
Parasites use a variety of methods to infect animal hosts, including physical contact, the fecal–oral route, free-living infectious stages, and vectors, suiting their differing hosts, life cycles, and ecological contexts. Examples to illustrate some of the many possible combinations are given in the table.
Variations
Among the many variations on parasitic strategies are hyperparasitism, social parasitism, brood parasitism, kleptoparasitism, sexual parasitism, and adelphoparasitism.
Hyperparasitism
Hyperparasites feed on another parasite, as exemplified by protozoa living in helminth parasites, or facultative or obligate parasitoids whose hosts are either conventional parasites or parasitoids. Levels of parasitism beyond secondary also occur, especially among facultative parasitoids. In oak gall systems, there can be up to four levels of parasitism.
Hyperparasites can control their hosts' populations, and are used for this purpose in agriculture and to some extent in medicine. The controlling effects can be seen in the way that the CHV1 virus helps to control the damage that chestnut blight, Cryphonectria parasitica, does to American chestnut trees, and in the way that bacteriophages can limit bacterial infections. It is likely, though little researched, that most pathogenic microparasites have hyperparasites which may prove widely useful in both agriculture and medicine.
Social parasitism | Parasitism | Wikipedia | 453 | 43937 | https://en.wikipedia.org/wiki/Parasitism | Biology and health sciences | Ecology | null |
Social parasites take advantage of interspecific interactions between members of eusocial animals such as ants, termites, and bumblebees. Examples include the large blue butterfly, Phengaris arion, its larvae employing ant mimicry to parasitise certain ants, Bombus bohemicus, a bumblebee which invades the hives of other bees and takes over reproduction while their young are raised by host workers, and Melipona scutellaris, a eusocial bee whose virgin queens escape killer workers and invade another colony without a queen. An extreme example of interspecific social parasitism is found in the ant Tetramorium inquilinum, an obligate parasite which lives exclusively on the backs of other Tetramorium ants. A mechanism for the evolution of social parasitism was first proposed by Carlo Emery in 1909. Now known as "Emery's rule", it states that social parasites tend to be closely related to their hosts, often being in the same genus.
Intraspecific social parasitism occurs in parasitic nursing, where some individual young take milk from unrelated females. In wedge-capped capuchins, higher ranking females sometimes take milk from low ranking females without any reciprocation.
Brood parasitism | Parasitism | Wikipedia | 263 | 43937 | https://en.wikipedia.org/wiki/Parasitism | Biology and health sciences | Ecology | null |
In brood parasitism, the hosts suffer increased parental investment and energy expenditure to feed parasitic young, which are commonly larger than host young. The growth rate of host nestlings is slowed, reducing the host's fitness. Brood parasites include birds in different families such as cowbirds, whydahs, cuckoos, and black-headed ducks. These do not build nests of their own, but leave their eggs in nests of other species. In the family Cuculidae, over 40% of cuckoo species are obligate brood parasites, while others are either facultative brood parasites or provide parental care. The eggs of some brood parasites mimic those of their hosts, while some cowbird eggs have tough shells, making them hard for the hosts to kill by piercing, both mechanisms implying selection by the hosts against parasitic eggs. The adult female European cuckoo further mimics a predator, the European sparrowhawk, giving her time to lay her eggs in the host's nest unobserved. Host species often combat parasitic egg mimicry through egg polymorphism, having two or more egg phenotypes within a single population of a species. Multiple phenotypes in host eggs decrease the probability of a parasitic species accurately "matching" their eggs to host eggs.
Kleptoparasitism
In kleptoparasitism (from Greek κλέπτης (kleptēs), "thief"), parasites steal food gathered by the host. The parasitism is often on close relatives, whether within the same species or between species in the same genus or family. For instance, the many lineages of cuckoo bees lay their eggs in the nest cells of other bees in the same family. Kleptoparasitism is uncommon generally but conspicuous in birds; some such as skuas are specialised in pirating food from other seabirds, relentlessly chasing them down until they disgorge their catch.
Sexual parasitism
A unique approach is seen in some species of anglerfish, such as Ceratias holboelli, where the males are reduced to tiny sexual parasites, wholly dependent on females of their own species for survival, permanently attached below the female's body, and unable to fend for themselves. The female nourishes the male and protects him from predators, while the male gives nothing back except the sperm that the female needs to produce the next generation.
Adelphoparasitism | Parasitism | Wikipedia | 500 | 43937 | https://en.wikipedia.org/wiki/Parasitism | Biology and health sciences | Ecology | null |
Adelphoparasitism, (from Greek ἀδελφός (adelphós), brother), also known as sibling-parasitism, occurs where the host species is closely related to the parasite, often in the same family or genus. In the citrus blackfly parasitoid, Encarsia perplexa, unmated females may lay haploid eggs in the fully developed larvae of their own species, producing male offspring, while the marine worm Bonellia viridis has a similar reproductive strategy, although the larvae are planktonic.
Illustrations
Examples of the major variant strategies are illustrated.
Taxonomic range
Parasitism has an extremely wide taxonomic range, including animals, plants, fungi, protozoans, bacteria, and viruses.
Animals
Parasitism is widespread in the animal kingdom, and has evolved independently from free-living forms hundreds of times. Many types of helminth including flukes and cestodes have complete life cycles involving two or more hosts. By far the largest group is the parasitoid wasps in the Hymenoptera. The phyla and classes with the largest numbers of parasitic species are listed in the table. Numbers are conservative minimum estimates. The columns for Endo- and Ecto-parasitism refer to the definitive host, as documented in the Vertebrate and Invertebrate columns.
Plants | Parasitism | Wikipedia | 281 | 43937 | https://en.wikipedia.org/wiki/Parasitism | Biology and health sciences | Ecology | null |
A hemiparasite or partial parasite such as mistletoe derives some of its nutrients from another living plant, whereas a holoparasite such as Cuscuta derives all of its nutrients from another plant. Parasitic plants make up about one per cent of angiosperms and are in almost every biome in the world. All these plants have modified roots, haustoria, which penetrate the host plants, connecting them to the conductive system—either the xylem, the phloem, or both. This provides them with the ability to extract water and nutrients from the host. A parasitic plant is classified depending on where it latches onto the host, either the stem or the root, and the amount of nutrients it requires. Since holoparasites have no chlorophyll and therefore cannot make food for themselves by photosynthesis, they are always obligate parasites, deriving all their food from their hosts. Some parasitic plants can locate their host plants by detecting chemicals in the air or soil given off by host shoots or roots, respectively. About 4,500 species of parasitic plant in approximately 20 families of flowering plants are known.
Species within the Orobanchaceae (broomrapes) are among the most economically destructive of all plants. Species of Striga (witchweeds) are estimated to cost billions of dollars a year in crop yield loss, infesting over 50 million hectares of cultivated land within Sub-Saharan Africa alone. Striga infects both grasses and grains, including corn, rice, and sorghum, which are among the world's most important food crops. Orobanche also threatens a wide range of other important crops, including peas, chickpeas, tomatoes, carrots, and varieties of cabbage. Yield loss from Orobanche can be total; despite extensive research, no method of control has been entirely successful.
Many plants and fungi exchange carbon and nutrients in mutualistic mycorrhizal relationships. Some 400 species of myco-heterotrophic plants, mostly in the tropics, however effectively cheat by taking carbon from a fungus rather than exchanging it for minerals. They have much reduced roots, as they do not need to absorb water from the soil; their stems are slender with few vascular bundles, and their leaves are reduced to small scales, as they do not photosynthesize. Their seeds are small and numerous, so they appear to rely on being infected by a suitable fungus soon after germinating. | Parasitism | Wikipedia | 512 | 43937 | https://en.wikipedia.org/wiki/Parasitism | Biology and health sciences | Ecology | null |
Fungi
Parasitic fungi derive some or all of their nutritional requirements from plants, other fungi, or animals.
Plant pathogenic fungi are classified into three categories depending on their mode of nutrition: biotrophs, hemibiotrophs and necrotrophs. Biotrophic fungi derive nutrients from living plant cells, and during the course of infection they colonise their plant host in such a way as to keep it alive for a maximally long time. One well-known example of a biotrophic pathogen is Ustilago maydis, causative agent of the corn smut disease. Necrotrophic pathogens on the other hand, kill host cells and feed saprophytically, an example being the root-colonising honey fungi in the genus Armillaria. Hemibiotrophic pathogens begin their colonising their hosts as biotrophs, and subsequently killing off host cells and feeding as necrotrophs, a phenomenon termed the biotrophy-necrotrophy switch.
Pathogenic fungi are well-known causative agents of diseases on animals as well as humans. Fungal infections (mycosis) are estimated to kill 1.6 million people each year. One example of a potent fungal animal pathogen are Microsporidia - obligate intracellular parasitic fungi that largely affect insects, but may also affect vertebrates including humans, causing the intestinal infection microsporidiosis.
Protozoa
Protozoa such as Plasmodium, Trypanosoma, and Entamoeba are endoparasitic. They cause serious diseases in vertebrates including humans—in these examples, malaria, sleeping sickness, and amoebic dysentery—and have complex life cycles.
Bacteria | Parasitism | Wikipedia | 354 | 43937 | https://en.wikipedia.org/wiki/Parasitism | Biology and health sciences | Ecology | null |
Many bacteria are parasitic, though they are more generally thought of as pathogens causing disease. Parasitic bacteria are extremely diverse, and infect their hosts by a variety of routes. To give a few examples, Bacillus anthracis, the cause of anthrax, is spread by contact with infected domestic animals; its spores, which can survive for years outside the body, can enter a host through an abrasion or may be inhaled. Borrelia, the cause of Lyme disease and relapsing fever, is transmitted by vectors, ticks of the genus Ixodes, from the diseases' reservoirs in animals such as deer. Campylobacter jejuni, a cause of gastroenteritis, is spread by the fecal–oral route from animals, or by eating insufficiently cooked poultry, or by contaminated water. Haemophilus influenzae, an agent of bacterial meningitis and respiratory tract infections such as influenza and bronchitis, is transmitted by droplet contact. Treponema pallidum, the cause of syphilis, is spread by sexual activity.
Viruses
Viruses are obligate intracellular parasites, characterised by extremely limited biological function, to the point where, while they are evidently able to infect all other organisms from bacteria and archaea to animals, plants and fungi, it is unclear whether they can themselves be described as living. They can be either RNA or DNA viruses consisting of a single or double strand of genetic material (RNA or DNA, respectively), covered in a protein coat and sometimes a lipid envelope. They thus lack all the usual machinery of the cell such as enzymes, relying entirely on the host cell's ability to replicate DNA and synthesise proteins. Most viruses are bacteriophages, infecting bacteria.
Evolutionary ecology | Parasitism | Wikipedia | 369 | 43937 | https://en.wikipedia.org/wiki/Parasitism | Biology and health sciences | Ecology | null |
Parasitism is a major aspect of evolutionary ecology; for example, almost all free-living animals are host to at least one species of parasite. Vertebrates, the best-studied group, are hosts to between 75,000 and 300,000 species of helminths and an uncounted number of parasitic microorganisms. On average, a mammal species hosts four species of nematode, two of trematodes, and two of cestodes. Humans have 342 species of helminth parasites, and 70 species of protozoan parasites. Some three-quarters of the links in food webs include a parasite, important in regulating host numbers. Perhaps 40 per cent of described species are parasitic.
Fossil record
Parasitism is hard to demonstrate from the fossil record, but holes in the mandibles of several specimens of Tyrannosaurus may have been caused by Trichomonas-like parasites. Saurophthirus, the Early Cretaceous flea, parasitized pterosaurs. Eggs that belonged to nematode worms and probably protozoan cysts were found in the Late Triassic coprolite of phytosaur. This rare find in Thailand reveals more about the ecology of prehistoric parasites.
Coevolution
As hosts and parasites evolve together, their relationships often change. When a parasite is in a sole relationship with a host, selection drives the relationship to become more benign, even mutualistic, as the parasite can reproduce for longer if its host lives longer. But where parasites are competing, selection favours the parasite that reproduces fastest, leading to increased virulence. There are thus varied possibilities in host–parasite coevolution.
Evolutionary epidemiology analyses how parasites spread and evolve, whereas Darwinian medicine applies similar evolutionary thinking to non-parasitic diseases like cancer and autoimmune conditions.
Long-term partnerships favouring mutualism | Parasitism | Wikipedia | 386 | 43937 | https://en.wikipedia.org/wiki/Parasitism | Biology and health sciences | Ecology | null |
Long-term partnerships can lead to a relatively stable relationship tending to commensalism or mutualism, as, all else being equal, it is in the evolutionary interest of the parasite that its host thrives. A parasite may evolve to become less harmful for its host or a host may evolve to cope with the unavoidable presence of a parasite—to the point that the parasite's absence causes the host harm. For example, although animals parasitised by worms are often clearly harmed, such infections may also reduce the prevalence and effects of autoimmune disorders in animal hosts, including humans. In a more extreme example, some nematode worms cannot reproduce, or even survive, without infection by Wolbachia bacteria.
Lynn Margulis and others have argued, following Peter Kropotkin's 1902 Mutual Aid: A Factor of Evolution, that natural selection drives relationships from parasitism to mutualism when resources are limited. This process may have been involved in the symbiogenesis which formed the eukaryotes from an intracellular relationship between archaea and bacteria, though the sequence of events remains largely undefined.
Competition favouring virulence
Competition between parasites can be expected to favour faster reproducing and therefore more virulent parasites, by natural selection.
Among competing parasitic insect-killing bacteria of the genera Photorhabdus and Xenorhabdus, virulence depended on the relative potency of the antimicrobial toxins (bacteriocins) produced by the two strains involved. When only one bacterium could kill the other, the other strain was excluded by the competition. But when caterpillars were infected with bacteria both of which had toxins able to kill the other strain, neither strain was excluded, and their virulence was less than when the insect was infected by a single strain.
Cospeciation
A parasite sometimes undergoes cospeciation with its host, resulting in the pattern described in Fahrenholz's rule, that the phylogenies of the host and parasite come to mirror each other. | Parasitism | Wikipedia | 428 | 43937 | https://en.wikipedia.org/wiki/Parasitism | Biology and health sciences | Ecology | null |
An example is between the simian foamy virus (SFV) and its primate hosts. The phylogenies of SFV polymerase and the mitochondrial cytochrome c oxidase subunit II from African and Asian primates were found to be closely congruent in branching order and divergence times, implying that the simian foamy viruses cospeciated with Old World primates for at least 30 million years.
The presumption of a shared evolutionary history between parasites and hosts can help elucidate how host taxa are related. For instance, there has been a dispute about whether flamingos are more closely related to storks or ducks. The fact that flamingos share parasites with ducks and geese was initially taken as evidence that these groups were more closely related to each other than either is to storks. However, evolutionary events such as the duplication, or the extinction of parasite species (without similar events on the host phylogeny) often erode similarities between host and parasite phylogenies. In the case of flamingos, they have similar lice to those of grebes. Flamingos and grebes do have a common ancestor, implying cospeciation of birds and lice in these groups. Flamingo lice then switched hosts to ducks, creating the situation which had confused biologists.
Parasites infect sympatric hosts (those within their same geographical area) more effectively, as has been shown with digenetic trematodes infecting lake snails. This is in line with the Red Queen hypothesis, which states that interactions between species lead to constant natural selection for coadaptation. Parasites track the locally common hosts' phenotypes, so the parasites are less infective to allopatric hosts, those from different geographical regions.
Modifying host behaviour | Parasitism | Wikipedia | 372 | 43937 | https://en.wikipedia.org/wiki/Parasitism | Biology and health sciences | Ecology | null |
Some parasites modify host behaviour in order to increase their transmission between hosts, often in relation to predator and prey (parasite increased trophic transmission). For example, in the California coastal salt marsh, the fluke Euhaplorchis californiensis reduces the ability of its killifish host to avoid predators. This parasite matures in egrets, which are more likely to feed on infected killifish than on uninfected fish. Another example is the protozoan Toxoplasma gondii, a parasite that matures in cats but can be carried by many other mammals. Uninfected rats avoid cat odors, but rats infected with T. gondii are drawn to this scent, which may increase transmission to feline hosts. The malaria parasite modifies the skin odour of its human hosts, increasing their attractiveness to mosquitoes and hence improving the chance for the parasite to be transmitted. The spider Cyclosa argenteoalba often have parasitoid wasp larvae attached to them which alter their web-building behavior. Instead of producing their normal sticky spiral shaped webs, they made simplified webs when the parasites were attached. This manipulated behavior lasted longer and was more prominent the longer the parasites were left on the spiders.
Trait loss
Parasites can exploit their hosts to carry out a number of functions that they would otherwise have to carry out for themselves. Parasites which lose those functions then have a selective advantage, as they can divert resources to reproduction. Many insect ectoparasites including bedbugs, batbugs, lice and fleas have lost their ability to fly, relying instead on their hosts for transport. Trait loss more generally is widespread among parasites. An extreme example is the myxosporean Henneguya zschokkei, an ectoparasite of fish and the only animal known to have lost the ability to respire aerobically: its cells lack mitochondria.
Host defences
Hosts have evolved a variety of defensive measures against their parasites, including physical barriers like the skin of vertebrates, the immune system of mammals, insects actively removing parasites, and defensive chemicals in plants. | Parasitism | Wikipedia | 445 | 43937 | https://en.wikipedia.org/wiki/Parasitism | Biology and health sciences | Ecology | null |
The evolutionary biologist W. D. Hamilton suggested that sexual reproduction could have evolved to help to defeat multiple parasites by enabling genetic recombination, the shuffling of genes to create varied combinations. Hamilton showed by mathematical modelling that sexual reproduction would be evolutionarily stable in different situations, and that the theory's predictions matched the actual ecology of sexual reproduction. However, there may be a trade-off between immunocompetence and breeding male vertebrate hosts' secondary sex characteristics, such as the plumage of peacocks and the manes of lions. This is because the male hormone testosterone encourages the growth of secondary sex characteristics, favouring such males in sexual selection, at the price of reducing their immune defences.
Vertebrates
The physical barrier of the tough and often dry and waterproof skin of reptiles, birds and mammals keeps invading microorganisms from entering the body. Human skin also secretes sebum, which is toxic to most microorganisms. On the other hand, larger parasites such as trematodes detect chemicals produced by the skin to locate their hosts when they enter the water. Vertebrate saliva and tears contain lysozyme, an enzyme that breaks down the cell walls of invading bacteria. Should the organism pass the mouth, the stomach with its hydrochloric acid, toxic to most microorganisms, is the next line of defence. Some intestinal parasites have a thick, tough outer coating which is digested slowly or not at all, allowing the parasite to pass through the stomach alive, at which point they enter the intestine and begin the next stage of their life. Once inside the body, parasites must overcome the immune system's serum proteins and pattern recognition receptors, intracellular and cellular, that trigger the adaptive immune system's lymphocytes such as T cells and antibody-producing B cells. These have receptors that recognise parasites.
Insects | Parasitism | Wikipedia | 386 | 43937 | https://en.wikipedia.org/wiki/Parasitism | Biology and health sciences | Ecology | null |
Insects often adapt their nests to reduce parasitism. For example, one of the key reasons why the wasp Polistes canadensis nests across multiple combs, rather than building a single comb like much of the rest of its genus, is to avoid infestation by tineid moths. The tineid moth lays its eggs within the wasps' nests and then these eggs hatch into larvae that can burrow from cell to cell and prey on wasp pupae. Adult wasps attempt to remove and kill moth eggs and larvae by chewing down the edges of cells, coating the cells with an oral secretion that gives the nest a dark brownish appearance.
Plants
Plants respond to parasite attack with a series of chemical defences, such as polyphenol oxidase, under the control of the jasmonic acid-insensitive (JA) and salicylic acid (SA) signalling pathways. The different biochemical pathways are activated by different attacks, and the two pathways can interact positively or negatively. In general, plants can either initiate a specific or a non-specific response. Specific responses involve recognition of a parasite by the plant's cellular receptors, leading to a strong but localised response: defensive chemicals are produced around the area where the parasite was detected, blocking its spread, and avoiding wasting defensive production where it is not needed. Non-specific defensive responses are systemic, meaning that the responses are not confined to an area of the plant, but spread throughout the plant, making them costly in energy. These are effective against a wide range of parasites. When damaged, such as by lepidopteran caterpillars, leaves of plants including maize and cotton release increased amounts of volatile chemicals such as terpenes that signal they are being attacked; one effect of this is to attract parasitoid wasps, which in turn attack the caterpillars.
Biology and conservation
Ecology and parasitology | Parasitism | Wikipedia | 386 | 43937 | https://en.wikipedia.org/wiki/Parasitism | Biology and health sciences | Ecology | null |
Parasitism and parasite evolution were until the twenty-first century studied by parasitologists, in a science dominated by medicine, rather than by ecologists or evolutionary biologists. Even though parasite-host interactions were plainly ecological and important in evolution, the history of parasitology caused what the evolutionary ecologist Robert Poulin called a "takeover of parasitism by parasitologists", leading ecologists to ignore the area. This was in his opinion "unfortunate", as parasites are "omnipresent agents of natural selection" and significant forces in evolution and ecology. In his view, the long-standing split between the sciences limited the exchange of ideas, with separate conferences and separate journals. The technical languages of ecology and parasitology sometimes involved different meanings for the same words. There were philosophical differences, too: Poulin notes that, influenced by medicine, "many parasitologists accepted that evolution led to a decrease in parasite virulence, whereas modern evolutionary theory would have predicted a greater range of outcomes".
Their complex relationships make parasites difficult to place in food webs: a trematode with multiple hosts for its various life cycle stages would occupy many positions in a food web simultaneously, and would set up loops of energy flow, confusing the analysis. Further, since nearly every animal has (multiple) parasites, parasites would occupy the top levels of every food web.
Parasites can play a role in the proliferation of non-native species. For example, invasive green crabs are minimally affected by native trematodes on the Eastern Atlantic coast. This helps them outcompete native crabs such as the Atlantic Rock and Jonah crabs.
Ecological parasitology can be important to attempts at control, like during the campaign for eradicating the Guinea worm. Even though the parasite was eradicated in all but four countries, the worm began using frogs as an intermediary host before infecting dogs, making control more difficult than it would have been if the relationships had been better understood.
Rationale for conservation | Parasitism | Wikipedia | 408 | 43937 | https://en.wikipedia.org/wiki/Parasitism | Biology and health sciences | Ecology | null |
Although parasites are widely considered to be harmful, the eradication of all parasites would not be beneficial. Parasites account for at least half of life's diversity; they perform important ecological roles; and without parasites, organisms might tend to asexual reproduction, diminishing the diversity of traits brought about by sexual reproduction. Parasites provide an opportunity for the transfer of genetic material between species, facilitating evolutionary change. Many parasites require multiple hosts of different species to complete their life cycles and rely on predator-prey or other stable ecological interactions to get from one host to another. The presence of parasites thus indicates that an ecosystem is healthy.
An ectoparasite, the California condor louse, Colpocephalum californici, became a well-known conservation issue. A large and costly captive breeding program was run in the United States to rescue the California condor. It was host to a louse, which lived only on it. Any lice found were "deliberately killed" during the program, to keep the condors in the best possible health. The result was that one species, the condor, was saved and returned to the wild, while another species, the parasite, became extinct.
Although parasites are often omitted in depictions of food webs, they usually occupy the top position. Parasites can function like keystone species, reducing the dominance of superior competitors and allowing competing species to co-exist.
Quantitative ecology
A single parasite species usually has an aggregated distribution across host animals, which means that most hosts carry few parasites, while a few hosts carry the vast majority of parasite individuals. This poses considerable problems for students of parasite ecology, as it renders parametric statistics as commonly used by biologists invalid. Log-transformation of data before the application of parametric test, or the use of non-parametric statistics is recommended by several authors, but this can give rise to further problems, so quantitative parasitology is based on more advanced biostatistical methods.
History
Ancient
Human parasites including roundworms, the Guinea worm, threadworms and tapeworms are mentioned in Egyptian papyrus records from 3000 BC onwards; the Ebers Papyrus describes hookworm. In ancient Greece, parasites including the bladder worm are described in the Hippocratic Corpus, while the comic playwright Aristophanes called tapeworms "hailstones". The Roman physicians Celsus and Galen documented the roundworms Ascaris lumbricoides and Enterobius vermicularis.
Medieval | Parasitism | Wikipedia | 501 | 43937 | https://en.wikipedia.org/wiki/Parasitism | Biology and health sciences | Ecology | null |
In his Canon of Medicine, completed in 1025, the Persian physician Avicenna recorded human and animal parasites including roundworms, threadworms, the Guinea worm and tapeworms.
In his 1397 book Traité de l'état, science et pratique de l'art de la Bergerie (Account of the state, science and practice of the art of shepherding), wrote the first description of a trematode endoparasite, the sheep liver fluke Fasciola hepatica.
Early modern
In the early modern period, Francesco Redi's 1668 book Esperienze Intorno alla Generazione degl'Insetti (Experiences of the Generation of Insects), explicitly described ecto- and endoparasites, illustrating ticks, the larvae of nasal flies of deer, and sheep liver fluke. Redi noted that parasites develop from eggs, contradicting the theory of spontaneous generation. In his 1684 book Osservazioni intorno agli animali viventi che si trovano negli animali viventi (Observations on Living Animals found in Living Animals), Redi described and illustrated over 100 parasites including the large roundworm in humans that causes ascariasis. Redi was the first to name the cysts of Echinococcus granulosus seen in dogs and sheep as parasitic; a century later, in 1760, Peter Simon Pallas correctly suggested that these were the larvae of tapeworms.
In 1681, Antonie van Leeuwenhoek observed and illustrated the protozoan parasite Giardia lamblia, and linked it to "his own loose stools". This was the first protozoan parasite of humans to be seen under a microscope. A few years later, in 1687, the Italian biologists Giovanni Cosimo Bonomo and Diacinto Cestoni described scabies as caused by the parasitic mite Sarcoptes scabiei, marking it as the first disease of humans with a known microscopic causative agent.
Parasitology | Parasitism | Wikipedia | 427 | 43937 | https://en.wikipedia.org/wiki/Parasitism | Biology and health sciences | Ecology | null |
Modern parasitology developed in the 19th century with accurate observations and experiments by many researchers and clinicians; the term was first used in 1870. In 1828, James Annersley described amoebiasis, protozoal infections of the intestines and the liver, though the pathogen, Entamoeba histolytica, was not discovered until 1873 by Friedrich Lösch. James Paget discovered the intestinal nematode Trichinella spiralis in humans in 1835. James McConnell described the human liver fluke, Clonorchis sinensis, in 1875. Algernon Thomas and Rudolf Leuckart independently made the first discovery of the life cycle of a trematode, the sheep liver fluke, by experiment in 1881–1883. In 1877 Patrick Manson discovered the life cycle of the filarial worms that cause elephantiasis transmitted by mosquitoes. Manson further predicted that the malaria parasite, Plasmodium, had a mosquito vector, and persuaded Ronald Ross to investigate. Ross confirmed that the prediction was correct in 1897–1898. At the same time, Giovanni Battista Grassi and others described the malaria parasite's life cycle stages in Anopheles mosquitoes. Ross was controversially awarded the 1902 Nobel prize for his work, while Grassi was not. In 1903, David Bruce identified the protozoan parasite and the tsetse fly vector of African trypanosomiasis.
Vaccine
Given the importance of malaria, with some 220 million people infected annually, many attempts have been made to interrupt its transmission. Various methods of malaria prophylaxis have been tried including the use of antimalarial drugs to kill off the parasites in the blood, the eradication of its mosquito vectors with organochlorine and other insecticides, and the development of a malaria vaccine. All of these have proven problematic, with drug resistance, insecticide resistance among mosquitoes, and repeated failure of vaccines as the parasite mutates. The first and as of 2015 the only licensed vaccine for any parasitic disease of humans is RTS,S for Plasmodium falciparum malaria.
Biological control
Several groups of parasites, including microbial pathogens and parasitoidal wasps have been used as biological control agents in agriculture and horticulture.
Resistance | Parasitism | Wikipedia | 463 | 43937 | https://en.wikipedia.org/wiki/Parasitism | Biology and health sciences | Ecology | null |
Poulin observes that the widespread prophylactic use of anthelmintic drugs in domestic sheep and cattle constitutes a worldwide uncontrolled experiment in the life-history evolution of their parasites. The outcomes depend on whether the drugs decrease the chance of a helminth larva reaching adulthood. If so, natural selection can be expected to favour the production of eggs at an earlier age. If on the other hand the drugs mainly affects adult parasitic worms, selection could cause delayed maturity and increased virulence. Such changes appear to be underway: the nematode Teladorsagia circumcincta is changing its adult size and reproductive rate in response to drugs.
Cultural significance
Classical times
In the classical era, the concept of the parasite was not strictly pejorative: the parasitus was an accepted role in Roman society, in which a person could live off the hospitality of others, in return for "flattery, simple services, and a willingness to endure humiliation".
Society
Parasitism has a derogatory sense in popular usage. According to the immunologist John Playfair,
The satirical cleric Jonathan Swift alludes to hyperparasitism in his 1733 poem "On Poetry: A Rhapsody", comparing poets to "vermin" who "teaze and pinch their foes":
A 2022 study examined the naming of some 3000 parasite species discovered in the previous two decades. Of those named after scientists, over 80% were named for men, whereas about a third of authors of papers on parasites were women. The study found that the percentage of parasite species named for relatives or friends of the author has risen sharply in the same period.
Fiction
In Bram Stoker's 1897 Gothic horror novel Dracula, and its many film adaptations, the eponymous Count Dracula is a blood-drinking parasite (a vampire). The critic Laura Otis argues that as a "thief, seducer, creator, and mimic, Dracula is the ultimate parasite. The whole point of vampirism is sucking other people's blood—living at other people's expense."
Disgusting and terrifying parasitic alien species are widespread in science fiction, as for instance in Ridley Scott's 1979 film Alien. In one scene, a Xenomorph bursts out of the chest of a dead man, with blood squirting out under high pressure assisted by explosive squibs. Animal organs were used to reinforce the shock effect. The scene was filmed in a single take, and the startled reaction of the actors was genuine. | Parasitism | Wikipedia | 508 | 43937 | https://en.wikipedia.org/wiki/Parasitism | Biology and health sciences | Ecology | null |
The entomopathogenic fungus Cordyceps is represented culturally as a deadly threat to the human race. The video game series The Last of Us (2013–present) and its television adaptation present Cordyceps as a parasite of humans, causing a zombie apocalypse. Its human hosts initially become violent "infected" beings, before turning into blind zombie "clickers", complete with fruiting bodies growing out from their faces. | Parasitism | Wikipedia | 86 | 43937 | https://en.wikipedia.org/wiki/Parasitism | Biology and health sciences | Ecology | null |
Massage is the rubbing or kneading of the body's soft tissues. Massage techniques are commonly applied with hands, fingers, elbows, knees, forearms, feet, or a device. The purpose of massage is generally for the treatment of body stress or pain. In English-speaking European countries, a person professionally trained to give massages is traditionally known as a masseur (male) or masseuse (female). In the United States, these individuals are often referred to as "massage therapists". In some provinces of Canada, they are called "registered massage therapists."
In professional settings, clients are treated while lying on a massage table, sitting in a massage chair, or lying on a mat on the floor. There are many different modalities in the massage industry, including (but not limited to): deep tissue, manual lymphatic drainage, medical, sports, structural integration, Swedish, Thai and trigger point.
Etymology
The word comes from the French 'friction of kneading', which, in turn, comes either from the Arabic word massa meaning 'to touch, feel', the Portuguese 'knead', from the Latin meaning 'mass, dough', or the Greek verb () 'to handle, touch, to work with the hands, to knead dough'.
The ancient Greek word for massage was and the Latin was .
History
Ancient times
Archaeological evidence of massage has been found in many ancient civilizations including China, India, Japan, Egypt, Rome, Greece, and Mesopotamia.
2330 BC: The Tomb of Akmanthor (also known as "The Tomb of the Physician") in Saqqara, Egypt, depicts two men having work done on their feet and hands, possibly depicting a massage.
2000 BC: The word muššu'u ("massage") is written for the first time, and its use is described, in some Sumerian and Akkadian texts found at the beginning of the 21st century in ancient Mesopotamia. | Massage | Wikipedia | 408 | 43945 | https://en.wikipedia.org/wiki/Massage | Biology and health sciences | Treatments | Health |
722–481 BC: Huangdi Neijing is composed during the Chinese Spring and Autumn period. The Nei-jing is a compilation of medical knowledge known up to that date, and is the foundation of traditional Chinese medicine. Massage is referred to in 30 different chapters of the Nei Jing. It specifies the use of different massage techniques and how they should be used in the treatment of specific ailments, and injuries. Also known as "The Yellow Emperor's Inner Canon," the text refers to previous medical knowledge from the time of the Yellow Emperor (), misleading some into believing the text itself was written during the time of the Yellow Emperor (which would predate written history).
762 BC: In the Iliad and the Odyssey, massage with oils and aromatic substances is mentioned as a means to relax the tired limbs of warriors and as a way to help the treatment of wounds.
700 BC: Bian Que, the earliest known Chinese physician, uses massage in medical practice.
500 BC: Jīvaka Komarabhācca was an Indian physician who according to the Pāli Buddhist Canon was Shakyamuni Buddha's physician. Jivaka is sometimes credited with founding and developing a style of massage that led to the type of massage practiced in current-day Thailand. Though this claim is disputed.
493 BC: A possible biblical reference documents daily "treatments" with oil of myrrh as a part of the beauty regimen of the wives of Xerxes (Esther, 2:12).
460 BC: Hippocrates wrote "The physician must be experienced in many things, but assuredly in rubbing."
300 BC: Charaka Samhita, sometimes dated to 800 BCE, is one of the oldest of the three ancient treatises of Ayurvedic medicine, including massage. Sanskrit records indicate that massage had been practiced in India long before the beginning of recorded history.
AD 1st or 2nd: Galen mentioned Diogas (Διόγας) who was an iatralipta (ἰατραλείπτης) (rubber and anointer/physiotherapist).
AD 581: China establishes a department of massage therapy within the Office of Imperial Physicians. | Massage | Wikipedia | 459 | 43945 | https://en.wikipedia.org/wiki/Massage | Biology and health sciences | Treatments | Health |
Middle Ages
Many of Galen's manuscripts, for instance, were collected and translated by Hunayn ibn Ishaq in the 9th century. Later in the 11th-century copies were translated into Latin and again in the 15th and 16th centuries, when they helped enlighten European scholars as to the achievements of the Ancient Greeks. This renewal of the Galenic tradition during the Renaissance played a very important part in the rise of modern science.
One of the greatest Persian medics was Avicenna, also known as Ibn Sina, who lived from 980 AD to 1037 AD. His works included a comprehensive collection and systematization of the fragmentary and unorganized Greco-Roman medical literature that had been translated Arabic by that time, augmented by notes from his own experiences. One of his books, Al-Qānūn fī aṭ-Ṭibb (The Canon of Medicine) has been called the most famous single book in the history of medicine in both East and West. Avicenna excelled in the logical assessment of conditions and comparison of symptoms and took special note of analgesics and their proper use as well as other methods of relieving pain, including massage.
AD 1150: Evidence of massage abortion, involving the application of pressure to the pregnant abdomen, can be found in one of the bas reliefs decorating the temple of Angkor Wat in Cambodia. It depicts a demon performing such an abortion upon a woman who has been sent to the underworld. This is the oldest known visual representation of abortion.
In Southeast Asia, massage traditions and techniques have already been entrenched in the people's diverse cultures for centuries before trade contact with Europe in the 16th century. In the Philippines, a distinct massage and healing tradition called hilot developed, while in Thailand, the tradition of massage that developed was called nuad thai. Nuad thai was declared in 2019 as a UNESCO intangible cultural heritage.
18th and 19th centuries
AD 1776: Jean Joseph Marie Amiot and Pierre-Martial Cibot, French missionaries in China translate summaries of Huangdi Neijing, including a list of medical plants, exercises, and elaborate massage techniques, into the French language, thereby introducing Europe to the highly developed Chinese system of medicine, medical-gymnastics, and medical-massage. | Massage | Wikipedia | 459 | 43945 | https://en.wikipedia.org/wiki/Massage | Biology and health sciences | Treatments | Health |
AD 1776: Pehr Henrik Ling, a Swedish physical therapist and teacher of medical-gymnastics, is born. Ling has often been erroneously credited for having invented "Classic Massage", also known as "Swedish Massage", and has been called the "Father of Massage".
AD 1779: Frenchman Pierre-Martial Cibot publishes "Notice du Cong-fou des Bonzes Tao-see", also known as "The Cong-Fou of the Tao-Tse", a French language summary of medical techniques used by Taoist priests. According to English historian of China Joseph Needham, Cibot's work "was intended to present the physicists and physicians of Europe with a sketch of a system of medical gymnastics which they might like to adopt—or if they found it at fault they might be stimulated to invent something better. This work has long been regarded as of cardinal importance in the history of physiotherapy because it almost certainly influenced the Swedish founder of the modern phase of the art, Pehr Hendrik Ling. Cibot had studied at least one Chinese book but also got much from a Christian neophyte who had become expert in the subject before his conversion."
AD 1813: The Royal Gymnastic Central Institute for the training of gymnastic instructors was opened in Stockholm, Sweden, with Pehr Henrik Ling appointed as principal. Ling developed what he called the "Swedish Movement Cure". Ling died in 1839, having previously named his pupils as the repositories of his teaching. Ling and his assistants left a little proper written account of their methods.
AD 1868: Dutch massage practitioner Johan Georg Mezger applies French terms to name five basic massage techniques, and coins the phrase "Swedish massage system". These techniques are still known by their French names (effleurage (long, gliding strokes), petrissage (lifting and kneading the muscles), friction (firm, deep, circular rubbing movements), tapotement (brisk tapping or percussive movements) and vibration (rapidly shaking or vibrating specific muscles)).
Modern times
China | Massage | Wikipedia | 424 | 43945 | https://en.wikipedia.org/wiki/Massage | Biology and health sciences | Treatments | Health |
As of 2005, with the city of Shanghai alone there were an estimated 1,300–2,000 foot massage centers, with more than 3,000 in Shenzhen. It was also estimated that there were nearly 30,000 massage workers in Shanghai and over 40,000 in Shenzhen. The average rate of pay for a worker in the massage industry in China is over 10,000 yuan per month, making them a well-paying job in China's service sector.
United States
Massage started to become popular in the United States in the middle part of the 19th century and was introduced by two New York physicians, George and Charles Taylor, based on Pehr Henrik Ling's techniques developed in Sweden.
During the 1930s and 1940s, massage's influence decreased as a result of medical advancements of the time, while in the 1970s massage's influence grew once again with a notable rise among athletes. Until the 1970s, nurses used massage to reduce pain and aid sleep. Popular books and videos, such as Massage for Relaxation, helped introduce massage to popular culture outside of a health setting. The massage therapy industry is continuously increasing. In 2009, U.S. consumers spent between $4 and $6 billion on visits to massage therapists. In 2015, research estimates that massage therapy was a $12.1 billion industry.
All but five states require massage therapists to be licensed, and licensure requires the applicant to receive training at an accredited school, and to pass a comprehensive exam. Those states that require licensure also typically require continuing education in massage techniques and in ethics.
United Kingdom
The service of massage or "physiological shampooing" was advertised in The Times from as early as 1880. Adverts claimed it as a cure for obesity amongst other chronic ailments.
Sports, business and organizations
Massage developed alongside athletics in both Ancient China and Ancient Greece. Taoist priests developed massage in concert with their Kung Fu gymnastic movements, while Ancient Greek Olympians used a specific type of trainer ("aleiptes") who would rub their muscles with oil. Pehr Ling's introduction to massage also came about directly as a result of his study of gymnastic movements. | Massage | Wikipedia | 440 | 43945 | https://en.wikipedia.org/wiki/Massage | Biology and health sciences | Treatments | Health |
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