Radiation pressure

From Wikipedia, the free encyclopedia

 

Force on a reflector results from reflecting the photon flux

 

Radiation pressure is the pressure exerted upon any 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 which 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 forces generated by radiation pressure are generally too small to be noticed under everyday circumstances; however, they are important in some physical processes. 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 15,000 km (9,300 mi). 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. 

The radiation pressure of sunlight on earth is equivalent to that exerted by about a thousandth of a gram on an area of 1 square metre (measured in units of force: approx. 10 μN/m2).

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 back 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 it is exposed to 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 feeble, 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 impacting gas molecules).

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, which will be transferred to an opaque surface it strikes. 

The energy flux (irradiance) of a plane wave is calculated using the Poynting vector ${\displaystyle \mathbf {S} =\mathbf {E} \times \mathbf {H} }$, whose magnitude we denote by S. 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 S=(power/area)=(rate of doing work/area)=(ΔF/Δt)Δx/area, which is the speed of light, c=Δx/Δt, times pressure, ΔF/area. That pressure is experienced as radiation pressure on the surface:

${\displaystyle P_{\text{incident}}={\frac {\langle S\rangle }{c}}={\frac {I_{f}}{c}}}$

where ${\displaystyle P}$ is pressure (usually in Pascals), ${\displaystyle I_{f}}$ is the incident irradiance (usually in W/m²) and ${\displaystyle c}$ is the speed of light in vacuum. 

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:

${\displaystyle P_{\text{incident}}={\frac {I_{f}}{c}}\cos ^{2}\alpha }$

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:

${\displaystyle P_{\text{emitted}}={\frac {I_{f}}{c}}}$

thus doubling the net radiation pressure on the surface:

${\displaystyle P_{\text{net}}=P_{\text{incident}}+P_{\text{emitted}}=2{\frac {I_{f}}{c}}}$

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 I_e:

${\displaystyle P_{\text{emitted}}={\frac {I_{e}}{c}}}$

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 very tiny. However, because black-body radiation increases rapidly with temperature (according to the fourth power of temperature as 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 very significant. This becomes important in stellar interiors which are at millions of degrees.

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:

${\displaystyle p={\dfrac {h}{\lambda }}={\frac {E_{p}}{c}},}$

where p is momentum, h is Planck's constant, λ is wavelength, and c is speed of light in vacuum. And E_p is the energy of a single photon given by:

${\displaystyle E_{p}=h\nu ={\frac {hc}{\lambda }}}$

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 I_f over an area A has a power of I_fA, this implies a flux of I_f/E_p 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

${\displaystyle P={\frac {u}{3}}}$

where u is the radiation density 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 T: 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

${\displaystyle P_{\text{compress}}={\frac {u}{3}}={\frac {4\sigma }{3c}}T^{4}}$

where ${\displaystyle \sigma }$ is the Stefan–Boltzmann constant.

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 G_SC, 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.

Pressures of absorption and reflection

Solar radiation pressure at the earth's distance from the sun, may be calculated by dividing the solar constant G_SC (above) by the speed of light c. For an absorbing sheet facing the sun, this is simply:

${\displaystyle P={\frac {G_{\text{SC}}}{c}}\approx 4.5\cdot 10^{-6}{\textsf {Pa}}=4.5\mu {\textsf {Pa}}}$

This result is in the S.I. unit 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:

${\displaystyle F={\frac {G_{\text{SC}}}{c}}(A\cos \alpha )}$

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:

${\displaystyle P={\frac {F}{A}}={\frac {G_{\text{SC}}}{c}}\cos ^{2}\alpha }$

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:

${\displaystyle P={\frac {G_{\text{SC}}}{cR^{2}}}\cos ^{2}\alpha }$

Finally, considering not an absorbing but a perfectly reflecting surface, the pressure is doubled due to the reflected wave, resulting in:

${\displaystyle P=2{\frac {G_{\text{SC}}}{cR^{2}}}\cos ^{2}\alpha }$

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 formulae. 

Solar radiation pressure on perfect reflector at normal incidence (α=0)

Distance from sun	Radiation pressure in μPa (μN/m2)
0.20 AU	227
0.39 AU (Mercury)	60.6
0.72 AU (Venus)	17.4
1.00 AU (Earth)	9.08
1.52 AU (Mars)	3.91
3.00 AU (Typical asteroid)	1.01
5.20 AU (Jupiter)	0.34

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 will be exposed to the Sun and some will be in shadow. Each surface exposed to the Sun will be reflecting, absorbing, and emitting radiation. Facets in shadow will be emitting radiation. The summation of pressures across all of the facets will define 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 will be more intense than that of the opposite face, resulting in a net force on the body that will affect 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.

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 From the Earth to the Moon. 

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.

The 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 Pillars of Creation clouds within the Eagle Nebula shaped by radiation pressure and stellar winds.

 

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

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

A protoplanetary disk with a cleared central region (artist's conception).

 

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

Comet Hale–Bopp (C/1995 O1). Radiation pressure and solar wind effects on the dust and gas tails are clearly seen.

 

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 ${\displaystyle \lambda }$. With a set of lenses, one can focus the laser beam to a point that is ${\displaystyle \lambda }$ in diameter (or ${\displaystyle r={\frac {\lambda }{2}}}$). 

The radiation pressure of a 30 mW laser of 1064 nm can therefore be computed as follows:

${\displaystyle A_{rea}=\pi \left({\frac {\lambda }{2}}\right)^{2}\approx 10^{-12}{\text{ m}}^{2}}$

${\displaystyle F_{orce}={\frac {P_{ower}}{c}}={\frac {30{\text{ mW}}}{299792458{\text{ m/s}}}}\approx 10^{-10}{\text{ N}}}$

${\displaystyle P_{ressure}={\frac {F_{orce}}{A_{rea}}}\approx {\frac {10^{-10}{\text{ N}}}{10^{-12}{\text{ m}}^{2}}}=100{\text{ Pascal}}}$

This is used in optical tweezers.

Other examples

Laser cooling is applied to cooling materials very close to absolute zero. 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. 

Large lasers operating in space have been suggested as a means of propelling sail craft in beam-powered propulsion. 

The reflection of a laser pulse from the surface of an elastic solid gives rise to various types of elastic waves that propagate inside the solid. The weakest waves are generally those that are generated by the radiation pressure acting during the reflection of the light. Recently, such light-pressure-induced elastic waves were observed inside an ultrahigh-reflectivity dielectric mirror. These waves are the most basic fingerprint of a light-solid matter interaction on the macroscopic scale.

Slash-and-burn

From Wikipedia, the free encyclopedia

Satellite photograph illustrating slash-and-burn forest clearing along the Xingu River in the state of Mato Grosso, Brazil.

 

Slash-and-burn agriculture, also called fire-fallow cultivation, is a farming method that involves the cutting and burning of plants in a forest or woodland to create a field called a swidden. The method begins by cutting down the trees and woody plants in an area. The downed vegetation, or "slash", is then left to dry, usually right before the rainiest part of the year. Then, the biomass is burned, resulting in a nutrient-rich layer of ash which makes the soil fertile, as well as temporarily eliminating weed and pest species. After about three to five years, the plot's productivity decreases due to depletion of nutrients along with weed and pest invasion, causing the farmers to abandon the field and move over to a new area. The time it takes for a swidden to recover depends on the location and can be as little as five years to more than twenty years, after which the plot can be slashed and burned again, repeating the cycle. In India, the practice is known as jhum or jhoom.

Slash-and-burn can be part of shifting cultivation, an agricultural system in which farmers routinely move from one cultivable area to another. It may also be part of transhumance, the moving of livestock between seasons. A rough estimate is that 200 million to 500 million people worldwide use slash-and-burn. In 2004, it was estimated that in Brazil alone, 500,000 small farmers each cleared an average of one hectare (2.47105 acres) of forest per year. The technique is not scalable or sustainable for large human populations. Methods such as Inga alley cropping and slash-and-char have been proposed as alternatives which would cause less environmental degradation.

A similar term is assarting, which is the clearing of forests, usually (but not always) for the purpose of agriculture. Assarting does not include burning.

History

Historically, slash-and-burn cultivation has been practiced throughout much of the world, in grasslands as well as woodlands. 

During the Neolithic Revolution, which included agricultural advancements, groups of hunter-gatherers domesticated various plants and animals, permitting them to settle down and practice agriculture, which provides more nutrition per hectare than hunting and gathering. This happened in the river valleys of Egypt and Mesopotamia. Due to this decrease in food from hunting, as human populations increased, agriculture became more important. Some groups could easily plant their crops in open fields along river valleys, but others had forests blocking their farming land.

In this context, humans used slash-and-burn agriculture to clear more land to make it suitable for plants and animals. Thus, since Neolithic times, slash-and-burn techniques have been widely used for converting forests into crop fields and pasture. Fire was used before the Neolithic as well, and by hunter-gatherers up to present times. Clearings created by the fire were made for many reasons, such as to draw game animals and to promote certain kinds of edible plants such as berries. 

Slash-and-burn fields are typically used and owned by a family until the soil is exhausted. At this point the ownership rights are abandoned, the family clears a new field, and trees and shrubs are permitted to grow on the former field. After a few decades, another family or clan may then use the land and claim usufructuary rights. In such a system there is typically no market in farmland, so land is not bought or sold on the open market and land rights are traditional. In slash-and-burn agriculture, forests are typically cut months before a dry season. The "slash" is permitted to dry and then burned in the following dry season. The resulting ash fertilizes the soil and the burned field is then planted at the beginning of the next rainy season with crops such as upland rice, maize, cassava, or other staples. Most of this work is typically done by hand, using such basic tools such as machetes, axes, hoes, and makeshift shovels. Old American civilizations, like the Inca, Maya and Aztecs, also used this agricultural technique. Sometimes, the flames spread and caused forest fires which would lead to loss of life (both wild animals and human beings).

Large families or clans wandering in the lush woodlands long continued to be the most common form of society through human prehistory. Axes to fell trees and sickles for harvesting grain were the only tools people might bring with them. All other tools were made from materials they found at the site, such as fire stakes of birch, long rods (Vanko), and harrows made of spruce tops. The extended family conquered the lush virgin forest, burned and cultivated their carefully selected swidden plots, sowed one or more crops, and then proceeded on to forests that had been noted in their wanderings. In the temperate zone, the forest regenerated in the course of a lifetime. So swidden was repeated several times in the same area over the years. But in the tropics the forest floor gradually depleted. It was not only in the moors, as in Northern Europe, but also in the steppe, Savannah, prairie, pampas and barren desert in tropical areas where shifting cultivation is the oldest type of farming. Cultivation is similar to slash-and-burn. (Clark 1952 91–107).

Historical references

Painting by Eero Järnefelt of forest-burning

 

Southern European Mediterranean climates have favored evergreen and deciduous forests. With slash-and-burn agriculture, this type of forest was less able to regenerate than those north of the Alps. Although in northern Europe one crop was usually harvested before grass was allowed to grow, in southern Europe it was more common to exhaust the soil by farming it for several years. 

Classical authors mentioned large forests, with Homer writing about "wooded Samothrace," Zakynthos, Sicily, and other woodlands. These authors indicated that the Mediterranean area once had more forest; much had already been lost, and the remainder was primarily in the mountains.

Although parts of Europe aside from the north remained wooded, by the Roman Iron and early Viking Ages, forests were drastically reduced and settlements regularly moved. The reasons for this pattern of mobility, the transition to stable settlements from the late Viking period on, or the transition from shifting cultivation to stationary farming are unknown. From this period, plows are found in graves. Early agricultural peoples preferred good forests on hillsides with good drainage, and traces of cattle enclosures are evident there. 

In Italy, shifting cultivation was a thing of the past by the birth of Christ. Tacitus describes it as a strange cultivation method, practiced by the Germans. In 98 AD, he wrote about the Germans that their fields were proportional to the participating cultivators but their crops were shared according to status. Distribution was simple, because of wide availability; they changed fields annually, with much to spare because they were producing grain rather than other crops. A W Liljenstrand wrote 1857 in his doctoral dissertation, "About Changing of Soil" (p. 5 ff.), that Tacitus discusses shifting cultivation: "arva per annos mutant". This is the practice of shifting cultivation.

During the Migration Period in Europe, after the Roman Empire and before the Viking Age, the peoples of Central Europe moved to new forests after exhausting old parcels. Forests were quickly exhausted; the practice had ended in the Mediterranean, where forests were less resilient than the sturdier coniferous forests of Central Europe. Deforestation had been partially caused by burning to create pasture. Reduced timber delivery led to higher prices and more stone construction in the Roman Empire (Stewart 1956, p. 123). Although forests gradually decreased in northern Europe, they have survived in the Nordic countries.

Tribes in pre-Roman Italy (including the Etruscans, Umbrians, Ligurians, Sabines, Latins, Campanians, Apulians, Saliscans, and Sabellians) apparently lived in temporary locations. They cultivated small patches of land, kept sheep and cattle, traded with foreign merchants, and occasionally fought. These Italic groups developed identities as settlers and warriors around 900 BC. They built forts in the mountains which are studied today, as are the ruins of a large Samnite temple and theater at Pietrabbondante. 

Many Italic peoples saw benefits in allying with Rome. When the Romans built the Via Amerina in 241 BC, the Falisci settled in cities on the plains and aided the Romans in road construction; the Roman Senate gradually acquired representatives from Faliscan and Etruscan families, and the Italic tribes became settled farmers.

Classical writers described peoples who practiced shifting cultivation, which characterized the Migration Period in Europe. The exploitation of forests demanded displacement as areas were deforested. Julius Caesar wrote about the Suebi in Commentarii de Bello Gallico 4.1, "They have no private and secluded fields ("privati ac separati agri apud eos nihil est") ... They cannot stay more than one year in a place for cultivation’s sake" ("neque longius anno remanere uno in loco colendi causa licet"). The Suebi lived between the Rhine and the Elbe. About the Germani, Caesar wrote: "No one has a particular field or area for himself, for the magistrates and chiefs give year by year to the people and the clans, who have gathered together, as much land and in such places as seem good to them and then make them move on after a year" ("Neque quisquam agri modum certum aut fines habet proprios, sed magistratus ac principes in annos singulos gentibus cognationibusque hominum, qui tum una coierunt, a quantum et quo loco visum est agri attribuunt atque anno post alio transire cogunt" [Book 6.22]). 

Strabo (63 BC—c. 20 AD) also writes about the Suebi in his Geography (VII, 1, 3): "Common to all the people in this area is that they can easily change residence because of their sordid way of life; they do not cultivate fields or collect property, but live in temporary huts. They get their nourishment from their livestock for the most part, and like nomads, pack all their goods in wagons and go on to wherever they want". Horace writes in 17 BC (Carmen Saeculare, 3, 24, 9ff.) about the people of Macedonia: "The proud Getae also live happily, growing free food and cereal for themselves on land they do not want to cultivate for more than a year" ("Vivunt et rigidi Getae, / immetata quibus iugera liberas / fruges et Cererem ferunt, / nec cultura placet longior annua"). 

Locations of Norwegian tribes described by Jordanes in his Getica

 

Jordanes, of Gothic descent, became a monk in Italy. In his mid-sixth-century AD Getica (De origine actibusque Getarum; The Origin and Deeds of the Goths) he described the large island of Scandza, on which the Goths originated. According to Jordanes, of the tribes living there, some are Adogit from within 40 days of the midnight sun. After the Adogit were the Screrefennae and Suehans, who also lived in the north. The Screrefennae did not raise crops, instead hunting and collecting bird eggs. The Suehans, a semi-nomadic tribe with good horses (comparable to the Thuringii), hunted furs to sell; grain could not be grown so far north. In about 550 AD, Procopius also described a primitive hunting people he called "Skrithifinoi": "Both men and women engaged incessantly just in hunting the rich forests and mountains, which gave them an endless supply of game and wild animals."

Slash-and-burn in Småland, Sweden (1904)

 

The use of fire in northeastern Sweden changed as agriculture evolved. Although the Sami people did not burn land (since burning killed the lichen required by their reindeer), later farmers frequently used slash-and-burn techniques. The 19th-century Swedish timber industry moved north, clearing the land of trees but leaving waste behind as a fire risk; during the 1870s, fires were frequent. There was a fire in Norrland in 1851, followed by fires in 1868 and 1878; two towns were lost in 1888.

Forest Finns

Huuhta cultivation spread: within the circle in 1500 AD, within the line in 1600, and to the dashed line in 1700.

 

One culture which flourished in pre-agricultural Europe survives: the Forest Finns in Scandinavia. Martin Tvengsberg, a descendant of the Forest Finns, studied them in his capacity as curator of the Hedmark Museum in Norway. The Savo-Karelians had a sophisticated system for cultivating spruce forests. A runic poem about Finland's spruce forests reads, "Gåivu on mehdien valgoinen valhe" ("The birch is the forest’s white lie"). The best spruce forests reportedly contain birch trees, which grow only after a forest has burned once or twice.

Modern Western world

Slash-and-burn may be defined as the large-scale deforestation of forests for agricultural use. Ashes from the trees help farmers by providing nutrients for the soil.

In industrialized regions, including Europe and North America, the practice was abandoned with the introduction of market agriculture and land ownership. Slash-and-burn agriculture was initially practiced by European pioneers in North America such as Daniel Boone and his family, who cleared land in the Appalachian Mountains during the late 18th and early 19th centuries. However, land cleared by slash-and-burn farmers was eventually taken over by systems of land tenure focusing on long-term improvement and discouraging practices associated with slash-and-burn agriculture.

Northern European heritage

In the Telkkämäki Heritage Farm in Finland, some areas are still burned annually.

 

Telkkämäki Nature Reserve in Kaavi, Finland, is an open-air museum which still practices slash-and-burn agriculture. Farm visitors can see how people farmed when slash-and-burn agriculture became the norm in the Northern Savonian region of eastern Finland beginning in the 15th century. Areas of the reserve are burnt each year.

South Asia

Tribal groups in the northeastern Indian states of Tripura, Arunachal Pradesh, Meghalaya, Mizoram and Nagaland and the Bangladeshi districts of Rangamati, Khagrachari, Bandarban and Sylhet refer to slash-and-burn agriculture as jhum or jhoom cultivation. The system involves clearing land, by fire or clear-felling, for economically-important crops such as upland rice, vegetables or fruits. After a few cycles, the land's fertility declines and a new area is chosen. Jhum cultivation is most often practiced on the slopes of thickly-forested hills. Cultivators cut the treetops to allow sunlight to reach the land, burning the trees and grasses for fresh soil. Although it is believed that this helps fertilize the land, it can leave it vulnerable to erosion. Holes are made for the seeds of crops such as sticky rice, maize, eggplant and cucumber are planted. After considering jhum's effects, the government of Mizoram has introduced a policy to end the method in the state. Slash-and-burn is typically a type of subsistence agriculture, not focused on a need to sell crops globally; planting decisions are governed by the needs of the family (or clan) for the coming year.

Ecological implications

Although a solution for overpopulated tropical countries where subsistence agriculture may be the traditional method of sustaining many families, the consequences of slash-and-burn techniques for ecosystems are almost always destructive. This happens particularly as population densities increase, and as a result farming becomes more intensively practiced. This is because as demand for more land increases, the fallow period by necessity declines. The principal vulnerability is the nutrient-poor soil, pervasive in most tropical forests. When biomass is extracted even for one harvest of wood or charcoal, the residual soil value is heavily diminished for further growth of any type of vegetation.

Sometimes there are several cycles of slash-and-burn within a few years' time span. For example, in eastern Madagascar, the following scenario occurs commonly. The first wave might be cutting of all trees for wood use. A few years later, saplings are harvested to make charcoal, and within the next year the plot is burned to create a quick flush of nutrients for grass to feed the family zebu cattle. If adjacent plots are treated in a similar fashion, large-scale erosion will usually ensue, since there are no roots or temporary water storage in nearby canopies to arrest the surface runoff. Thus, any small remaining amounts of nutrients are washed away. The area is an example of desertification, and no further growth of any type may arise for generations.

The ecological ramifications of the above scenario are further magnified, because tropical forests are habitats for extremely biologically diverse ecosystems, typically containing large numbers of endemic and endangered species. Therefore, the role of slash-and-burn is significant in the current Holocene extinction.

Slash-and-char is an alternative that alleviates some of the negative ecological implications of traditional slash-and-burn techniques. However, the endgame - unsustainability - is the same as for slash-and-burn.

Emissions taxes could help reduce the magnitude on which slash-and-burn agriculture is practised.

Health effects of particulates

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Air pollution measurement station in Emden, Germany

Size, shape and solubility matter

The size of the particle is a main determinant of where in the respiratory tract the particle will come to rest when inhaled. Larger particles are generally filtered in the nose and throat via cilia and mucus, but particulate matter smaller than about 10 micrometers, can settle in the bronchi and lungs and cause health problems. The 10-micrometer size does not represent a strict boundary between respirable and non-respirable particles, but has been agreed upon for monitoring of airborne particulate matter by most regulatory agencies. Because of their small size, particles on the order of 10 micrometers or less (coarse particulate matter, PM₁₀) can penetrate the deepest part of the lungs such as the bronchioles or alveoli; when asthmatics are exposed to these conditions it can trigger bronchoconstriction.

Similarly, so called fine particulate matter (PM_2.5), tend to penetrate into the gas exchange regions of the lung (alveolus), and very small particles (ultrafine particulate matter, PM_0.1) may pass through the lungs to affect other organs. Penetration of particles is not wholly dependent on their size; shape and chemical composition also play a part. To avoid this complication, simple nomenclature is used to indicate the different degrees of relative penetration of a PM particle into the cardiovascular system. Inhalable particles penetrate no further than the bronchi as they are filtered out by the cilia. Thoracic particles can penetrate right into terminal bronchioles whereas PM_0.1, which can penetrate to alveoli, the gas exchange area, and hence the circulatory system are termed respirable particles. In analogy, the inhalable dust fraction is the fraction of dust entering nose and mouth which may be deposited anywhere in the respiratory tract. The thoracic fraction is the fraction that enters the thorax and is deposited within the lung's airways. The respirable fraction is what is deposited in the gas exchange regions (alveoli).

The smallest particles, less than 100 nanometers (nanoparticles), may be even more damaging to the cardiovascular system. Nanoparticles can pass through cell membranes and migrate into other organs, including the brain. Particles emitted from modern diesel engines (commonly referred to as Diesel Particulate Matter, or DPM) are typically in the size range of 100 nanometers (0.1 micrometer). These soot particles also carry carcinogens like benzopyrenes adsorbed on their surface. Particulate mass is not a proper measure of the health hazard, because one particle of 10 µm diameter has approximately the same mass as 1 million particles of 100 nm diameter, but is much less hazardous, as it is unlikely to enter the alveoli. Legislative limits for engine emissions based on mass are therefore not protective. Proposals for new regulations exist in some countries, with suggestions to limit the particle surface area or the particle count (numerical quantity) instead.

The site and extent of absorption of inhaled gases and vapors are determined by their solubility in water. Absorption is also dependent upon air flow rates and the partial pressure of the gases in the inspired air. The fate of a specific contaminant is dependent upon the form in which it exists (aerosol or particulate). Inhalation also depends upon the breathing rate of the subject.

Another complexity not entirely documented is how the shape of PM can affect health, except for the needle-like shape of asbestos which can lodge itself in the lungs. Geometrically angular shapes have more surface area than rounder shapes, which in turn affects the binding capacity of the particle to other, possibly more dangerous substances.

Health problems

Air quality information on PM10 displayed in Katowice, Poland

 

The effects of inhaling particulate matter that has been widely studied in humans and animals include asthma, lung cancer, respiratory diseases, cardiovascular disease, premature delivery, birth defects, low birth weight, and premature death. 

Inhalation of PM_2.5 – PM₁₀ is associated with elevated risk of adverse pregnancy outcomes, such as low birth weight. Maternal PM_2.5 exposure during pregnancy is also associated with high blood pressure in children. Exposure to PM_2.5 has been associated with greater reductions in birth weight than exposure to PM₁₀. PM exposure can cause inflammation, oxidative stress, endocrine disruption, and impaired oxygen transport access to the placenta, all of which are mechanisms for heightening the risk of low birth weight. Overall epidemiologic and toxicological evidence suggests that a causal relationship exists between long-term exposures to PM_2.5 and developmental outcomes (i.e. low birth weight). However, studies investigating the significance of trimester-specific exposure have proven to be inconclusive, and results of international studies have been inconsistent in drawing associations of prenatal particulate matter exposure and low birth weight.  As perinatal outcomes have been associated with lifelong health and exposure to particulate matter is widespread, this issue is of critical public health importance and additional research will be essential to inform public policy on the matter. 

Increased levels of fine particles in the air as a result of anthropogenic particulate air pollution "is consistently and independently related to the most serious effects, including lung cancer and other cardiopulmonary mortality." A large number of deaths and other health problems associated with particulate pollution was first demonstrated in the early 1970s and has been reproduced many times since. PM pollution is estimated to cause 22,000–52,000 deaths per year in the United States (from 2000) contributed to ~370,000 premature deaths in Europe during 2005. and 3.22 million deaths globally in 2010 per the global burden of disease collaboration.

A 2002 study indicated that PM_2.5 leads to high plaque deposits in arteries, causing vascular inflammation and atherosclerosis – a hardening of the arteries that reduces elasticity, which can lead to heart attacks and other cardiovascular problems. A 2014 meta analysis reported that long term exposure to particulate matter is linked to coronary events. The study included 11 cohorts participating in the European Study of Cohorts for Air Pollution Effects (ESCAPE) with 100,166 participants, followed for an average of 11.5 years. An increase in estimated annual exposure to PM 2.5 of just 5 µg/m³ was linked with a 13% increased risk of heart attacks. In 2017 a study revealed that PM not only affects human cells and tissues, but also impacts bacteria which cause disease in humans. This study concluded that biofilm formation, antibiotic tolerance, and colonisation of both Staphylococcus aureus and Streptococcus pneumoniae was altered by Black Carbon exposure.

The World Health Organization (WHO) estimated in 2005 that "... fine particulate air pollution (PM(2.5)), causes about 3% of mortality from cardiopulmonary disease, about 5% of mortality from cancer of the trachea, bronchus, and lung, and about 1% of mortality from acute respiratory infections in children under 5 years, worldwide.". A 2011 study concluded that traffic exhaust is the single most serious preventable cause of heart attack in the general public, the cause of 7.4% of all attacks.

The largest US study on acute health effects of coarse particle pollution between 2.5 and 10 micrometers in diameter. was published 2008 and found an association with hospital admissions for cardiovascular diseases but no evidence of an association with the number of hospital admissions for respiratory diseases. After taking into account fine particle levels (PM_2.5 and less), the association with coarse particles remained but was no longer statistically significant, which means the effect is due to the subsection of fine particles.

Particulate matter studies in Bangkok Thailand from 2008 indicated a 1.9% increased risk of dying from cardiovascular disease, and 1.0% risk of all disease for every 10 micrograms per cubic meter. Levels averaged 65 in 1996, 68 in 2002, and 52 in 2004. Decreasing levels may be attributed to conversions of diesel to natural gas combustion as well as improved regulations.

The Mongolian government agency recorded a 45% increase in the rate of respiratory illness in the past five years (reported in September 2014). Bronchial asthma, chronic obstructive pulmonary disease and interstitial pneumonia were the most common ailments treated by area hospitals. Levels of premature death, chronic bronchitis, and cardiovascular disease are increasing at a rapid rate.

A study In 2000 conducted in the U.S. explored how fine particulate matter may be more harmful than coarse particulate matter. The study was based on six different cities. They found that deaths and hospital visits that were caused by particulate matter in the air were primarily due fine particulate matter.

Effects on vegetation

Particulate matter can clog stomatal openings of plants and interfere with photosynthesis functions. In this manner, high particulate matter concentrations in the atmosphere can lead to growth stunting or mortality in some plant species.