Polarizer

From Wikipedia, the free encyclopedia
https://en.wikipedia.org/wiki/Polarizer

A polarizing filter cuts down the reflections (top) and made it possible to see the photographer through the glass at roughly Brewster's angle although reflections off the back window of the car are not cut because they are less-strongly polarized, according to the Fresnel equations.

A polarizer or polariser is an optical filter that lets light waves of a specific polarization pass through while blocking light waves of other polarizations. It can filter a beam of light of undefined or mixed polarization into a beam of well-defined polarization, that is polarized light. The common types of polarizers are linear polarizers and circular polarizers. Polarizers are used in many optical techniques and instruments, and polarizing filters find applications in photography and LCD technology. Polarizers can also be made for other types of electromagnetic waves besides visible light, such as radio waves, microwaves, and X-rays.

Linear polarizers

Linear polarizers can be divided into two general categories: absorptive polarizers, where the unwanted polarization states are absorbed by the device, and beam-splitting polarizers, where the unpolarized beam is split into two beams with opposite polarization states. Polarizers which maintain the same axes of polarization with varying angles of incidence are often called Cartesian polarizers, since the polarization vectors can be described with simple Cartesian coordinates (for example, horizontal vs. vertical) independent from the orientation of the polarizer surface. When the two polarization states are relative to the direction of a surface (usually found with Fresnel reflection), they are usually termed s and p. This distinction between Cartesian and s–p polarization can be negligible in many cases, but it becomes significant for achieving high contrast and with wide angular spreads of the incident light.

Absorptive polarizers

Certain crystals, due to the effects described by crystal optics, show dichroism, preferential absorption of light which is polarized in particular directions. They can therefore be used as linear polarizers. The best known crystal of this type is tourmaline. However, this crystal is seldom used as a polarizer, since the dichroic effect is strongly wavelength dependent and the crystal appears coloured. Herapathite is also dichroic, and is not strongly coloured, but is difficult to grow in large crystals.

A Polaroid polarizing filter functions similarly on an atomic scale to the wire-grid polarizer. It was originally made of microscopic herapathite crystals. Its current H-sheet form is made from polyvinyl alcohol (PVA) plastic with an iodine doping. Stretching of the sheet during manufacture causes the PVA chains to align in one particular direction. Valence electrons from the iodine dopant are able to move linearly along the polymer chains, but not transverse to them. So incident light polarized parallel to the chains is absorbed by the sheet; light polarized perpendicularly to the chains is transmitted. The durability and practicality of Polaroid makes it the most common type of polarizer in use, for example for sunglasses, photographic filters, and liquid crystal displays. It is also much cheaper than other types of polarizer.

A modern type of absorptive polarizer is made of elongated silver nano-particles embedded in thin (≤0.5 mm) glass plates. These polarizers are more durable, and can polarize light much better than plastic Polaroid film, achieving polarization ratios as high as 100,000:1 and absorption of correctly polarized light as low as 1.5%. Such glass polarizers perform best for short-wavelength infrared light, and are widely used in optical fiber communications.

Beam-splitting polarizers

Beam-splitting polarizers split the incident beam into two beams of differing linear polarization. For an ideal polarizing beamsplitter these would be fully polarized, with orthogonal polarizations. For many common beam-splitting polarizers, however, only one of the two output beams is fully polarized. The other contains a mixture of polarization states.

Unlike absorptive polarizers, beam splitting polarizers do not need to absorb and dissipate the energy of the rejected polarization state, and so they are more suitable for use with high intensity beams such as laser light. True polarizing beamsplitters are also useful where the two polarization components are to be analyzed or used simultaneously.

Polarization by Fresnel reflection

A stack of plates at Brewster's angle to a beam reflects off a fraction of the s-polarized light at each surface, leaving a p-polarized beam. Full polarization at Brewster's angle requires many more plates than shown. The arrows indicate the direction of the electrical field, not the magnetic field, which is perpendicular to the electric field

When light reflects (by Fresnel reflection) at an angle from an interface between two transparent materials, the reflectivity is different for light polarized in the plane of incidence and light polarized perpendicular to it. Light polarized in the plane is said to be p-polarized, while that polarized perpendicular to it is s-polarized. At a special angle known as Brewster's angle, no p-polarized light is reflected from the surface, thus all reflected light must be s-polarized, with an electric field perpendicular to the plane of incidence.

A simple linear polarizer can be made by tilting a stack of glass plates at Brewster's angle to the beam. Some of the s-polarized light is reflected from each surface of each plate. For a stack of plates, each reflection depletes the incident beam of s-polarized light, leaving a greater fraction of p-polarized light in the transmitted beam at each stage. For visible light in air and typical glass, Brewster's angle is about 57°, and about 16% of the s-polarized light present in the beam is reflected for each air-to-glass or glass-to-air transition. It takes many plates to achieve even mediocre polarization of the transmitted beam with this approach. For a stack of 10 plates (20 reflections), about 3% (= (1 − 0.16)²⁰) of the s-polarized light is transmitted. The reflected beam, while fully polarized, is spread out and may not be very useful.

A more useful polarized beam can be obtained by tilting the pile of plates at a steeper angle to the incident beam. Counterintuitively, using incident angles greater than Brewster's angle yields a higher degree of polarization of the transmitted beam, at the expense of decreased overall transmission. For angles of incidence steeper than 80° the polarization of the transmitted beam can approach 100% with as few as four plates, although the transmitted intensity is very low in this case. Adding more plates and reducing the angle allows a better compromise between transmission and polarization to be achieved.

A wire-grid polarizer converts an unpolarized beam into one with a single linear polarization. Coloured arrows depict the electric field vector. The diagonally polarized waves also contribute to the transmitted polarization. Their vertical components are transmitted (shown), while the horizontal components are absorbed and reflected (not shown).

Because their polarization vectors depend on incidence angle, polarizers based on Fresnel reflection inherently tend to produce s–p polarization rather than Cartesian polarization, which limits their use in some applications.

Birefringent polarizers

Further information: Prism § Polarizing_prisms

Other linear polarizers exploit the birefringent properties of crystals such as quartz and calcite. In these crystals, a beam of unpolarized light incident on their surface is split by refraction into two rays. Snell's law holds for both of these rays, the ordinary or o-ray, and the extraordinary or e-ray, with each ray experiencing a different index of refraction (this is called double refraction). In general the two rays will be in different polarization states, though not in linear polarization states except for certain propagation directions relative to the crystal axis.

A Nicol prism

A Nicol prism was an early type of birefringent polarizer, that consists of a crystal of calcite which has been split and rejoined with Canada balsam. The crystal is cut such that the o- and e-rays are in orthogonal linear polarization states. Total internal reflection of the o-ray occurs at the balsam interface, since it experiences a larger refractive index in calcite than in the balsam, and the ray is deflected to the side of the crystal. The e-ray, which sees a smaller refractive index in the calcite, is transmitted through the interface without deflection. Nicol prisms produce a very high purity of polarized light, and were extensively used in microscopy, though in modern use they have been mostly replaced with alternatives such as the Glan–Thompson prism, Glan–Foucault prism, and Glan–Taylor prism. These prisms are not true polarizing beamsplitters since only the transmitted beam is fully polarized.

A Wollaston prism is another birefringent polarizer consisting of two triangular calcite prisms with orthogonal crystal axes that are cemented together. At the internal interface, an unpolarized beam splits into two linearly polarized rays which leave the prism at a divergence angle of 15°–45°. The Rochon and Sénarmont prisms are similar, but use different optical axis orientations in the two prisms. The Sénarmont prism is air spaced, unlike the Wollaston and Rochon prisms. These prisms truly split the beam into two fully polarized beams with perpendicular polarizations. The Nomarski prism is a variant of the Wollaston prism, which is widely used in differential interference contrast microscopy.

Thin film polarizers

Thin-film linear polarizers (also known as TFPN) are glass substrates on which a special optical coating is applied. Either Brewster's angle reflections or interference effects in the film cause them to act as beam-splitting polarizers. The substrate for the film can either be a plate, which is inserted into the beam at a particular angle, or a wedge of glass that is cemented to a second wedge to form a cube with the film cutting diagonally across the center (one form of this is the very common MacNeille cube). Thin-film polarizers generally do not perform as well as Glan-type polarizers, but they are inexpensive and provide two beams that are about equally well polarized. The cube-type polarizers generally perform better than the plate polarizers. The former are easily confused with Glan-type birefringent polarizers.

Wire-grid polarizers

One of the simplest linear polarizers is the wire-grid polarizer (WGP), which consists of many fine parallel metallic wires placed in a plane. WGPs mostly reflect the non-transmitted polarization and can thus be used as polarizing beam splitters. The parasitic absorption is relatively high compared to most of the dielectric polarizers though much lower than in absorptive polarizers.

Electromagnetic waves that have a component of their electric fields aligned parallel to the wires will induce the movement of electrons along the length of the wires. Since the electrons are free to move in this direction, the polarizer behaves in a similar manner to the surface of a metal when reflecting light, and the wave is reflected backwards along the incident beam (minus a small amount of energy lost to Joule heating of the wire).

For waves with electric fields perpendicular to the wires, the electrons cannot move very far across the width of each wire. Therefore, little energy is reflected and the incident wave is able to pass through the grid. In this case the grid behaves like a dielectric material.

Overall, this causes the transmitted wave to be linearly polarized with an electric field completely perpendicular to the wires. The hypothesis that the waves "slip through" the gaps between the wires is incorrect.

For practical purposes, the separation between wires must be less than the wavelength of the incident radiation. In addition, the width of each wire should be small compared to the spacing between wires. Therefore, it is relatively easy to construct wire-grid polarizers for microwaves, far-infrared, and mid-infrared radiation. For far-infrared optics, the polarizer can be even made as free standing mesh, entirely without transmissive optics. In addition, advanced lithographic techniques can also build very tight pitch metallic grids (typ. 50‒100 nm), allowing for the polarization of visible or infrared light to a useful degree. Since the degree of polarization depends little on wavelength and angle of incidence, they are used for broad-band applications such as projection.

Analytical solutions using rigorous coupled-wave analysis for wire grid polarizers have shown that for electric field components perpendicular to the wires, the medium behaves like a dielectric, and for electric field components parallel to the wires, the medium behaves like a metal (reflective).

Malus's law and other properties

Malus' Law where θ₁ − θ₀ = θ_i.

 

Malus' Law demonstration. No light can pass through a pair of crossed polarizing filters, but when a third filter is inserted between them with its axis not parallel to either one, some light can pass.

Malus's law (/məˈluːs/), which is named after Étienne-Louis Malus, says that when a perfect polarizer is placed in a polarized beam of light, the irradiance, I, of the light that passes through is given by

${\displaystyle I=I_{0}\cos ^{2}\theta _{i},}$

where I₀ is the initial intensity and θ_i is the angle between the light's initial polarization direction and the axis of the polarizer.

A beam of unpolarized light can be thought of as containing a uniform mixture of linear polarizations at all possible angles. Since the average value of ${\displaystyle \cos ^{2}\theta }$ is 1/2, the transmission coefficient becomes

${\displaystyle {\frac {I}{I_{0}}}={\frac {1}{2}}.}$

In practice, some light is lost in the polarizer and the actual transmission will be somewhat lower than this, around 38% for Polaroid-type polarizers but considerably higher (>49.9%) for some birefringent prism types.

If two polarizers are placed one after another (the second polarizer is generally called an analyzer), the mutual angle between their polarizing axes gives the value of θ in Malus's law. If the two axes are orthogonal, the polarizers are crossed and in theory no light is transmitted, though again practically speaking no polarizer is perfect and the transmission is not exactly zero (for example, crossed Polaroid sheets appear slightly blue in colour because their extinction ratio is better in the red). If a transparent object is placed between the crossed polarizers, any polarization effects present in the sample (such as birefringence) will be shown as an increase in transmission. This effect is used in polarimetry to measure the optical activity of a sample.

Real polarizers are also not perfect blockers of the polarization orthogonal to their polarization axis; the ratio of the transmission of the unwanted component to the wanted component is called the extinction ratio, and varies from around 1:500 for Polaroid to about 1:10⁶ for Glan–Taylor prism polarizers.

In X-ray the Malus's law (relativistic form):

${\displaystyle I=I_{0}{\frac {f}{f}}_{0}\left[1+{\frac {\lambda (f_{0}-f)}{2c}}\right]\cos ^{2}\theta _{i}}$

where ${\displaystyle f_{0}}$ – frequency of the polarized radiation falling on the polarizer, ${\displaystyle f}$ – frequency of the radiation passes through polarizer, ${\displaystyle \lambda }$ – Compton wavelength of electron, ${\displaystyle c}$ – speed of light in vacuum.

Circular polarizers

Circular polarizers (CPL or circular polarizing filters) can be used to create circularly polarized light or alternatively to selectively absorb or pass clockwise and counter-clockwise circularly polarized light. They are used as polarizing filters in photography to reduce oblique reflections from non-metallic surfaces, and are the lenses of the 3D glasses worn for viewing some stereoscopic movies (notably, the RealD 3D variety), where the polarization of light is used to differentiate which image should be seen by the left and right eye.

Creating circularly polarized light

Circular polarizer creating left-handed circularly polarized light. It is considered left-handed as viewed from the receiver and right-handed as viewed from the source.

There are several ways to create circularly polarized light, the cheapest and most common involves placing a quarter-wave plate after a linear polarizer and directing unpolarized light through the linear polarizer. The linearly polarized light leaving the linear polarizer is transformed into circularly polarized light by the quarter wave plate. The transmission axis of the linear polarizer needs to be half way (45°) between the fast and slow axes of the quarter-wave plate.

In the arrangement above, the transmission axis of the linear polarizer is at a positive 45° angle relative to the right horizontal and is represented with an orange line. The quarter-wave plate has a horizontal slow axis and a vertical fast axis and they are also represented using orange lines. In this instance the unpolarized light entering the linear polarizer is displayed as a single wave whose amplitude and angle of linear polarization are suddenly changing.

When one attempts to pass unpolarized light through the linear polarizer, only light that has its electric field at the positive 45° angle leaves the linear polarizer and enters the quarter-wave plate. In the illustration, the three wavelengths of unpolarized light represented would be transformed into the three wavelengths of linearly polarized light on the other side of the linear polarizer.

Linearly polarized light, represented using components, entering a quarter-wave plate. The blue and green curves are projections of the red line on the vertical and horizontal planes respectively.

In the illustration toward the right is the electric field of the linearly polarized light just before it enters the quarter-wave plate. The red line and associated field vectors represent how the magnitude and direction of the electric field varies along the direction of travel. For this plane electromagnetic wave, each vector represents the magnitude and direction of the electric field for an entire plane that is perpendicular to the direction of travel. (Refer to these two images in the plane wave article to better appreciate this.)

Light and all other electromagnetic waves have a magnetic field which is in phase with, and perpendicular to, the electric field being displayed in these illustrations.

To understand the effect the quarter-wave plate has on the linearly polarized light it is useful to think of the light as being divided into two components which are at right angles (orthogonal) to each other. Towards this end, the blue and green lines are projections of the red line onto the vertical and horizontal planes respectively and represent how the electric field changes in the direction of those two planes. The two components have the same amplitude and are in phase.

Because the quarter-wave plate is made of a birefringent material, when in the wave plate, the light travels at different speeds depending on the direction of its electric field. This means that the horizontal component which is along the slow axis of the wave plate will travel at a slower speed than the component that is directed along the vertical fast axis. Initially the two components are in phase, but as the two components travel through the wave plate the horizontal component of the light drifts farther behind that of the vertical. By adjusting the thickness of the wave plate one can control how much the horizontal component is delayed relative to vertical component before the light leaves the wave plate and they begin again to travel at the same speed. When the light leaves the quarter-wave plate the rightward horizontal component will be exactly one quarter of a wavelength behind the vertical component making the light left-hand circularly polarized when viewed from the receiver.

The top image is left-handed/counter-clockwise circularly polarized, as viewed from the receiver. The bottom image is that of linearly polarized light. The blue and green curves are projections of the red lines on the vertical and horizontal planes respectively.

At the top of the illustration toward the right is the circularly polarized light after it leaves the wave plate. Directly below it, for comparison purposes, is the linearly polarized light that entered the quarter-wave plate. In the upper image, because this is a plane wave, each vector leading from the axis to the helix represents the magnitude and direction of the electric field for an entire plane that is perpendicular to the direction of travel. All the electric field vectors have the same magnitude indicating that the strength of the electric field does not change. The direction of the electric field however steadily rotates.

The blue and green lines are projections of the helix onto the vertical and horizontal planes respectively and represent how the electric field changes in the direction of those two planes. Notice how the rightward horizontal component is now one quarter of a wavelength behind the vertical component. It is this quarter of a wavelength phase shift that results in the rotational nature of the electric field. It is significant to note that when the magnitude of one component is at a maximum the magnitude of the other component is always zero. This is the reason that there are helix vectors which exactly correspond to the maxima of the two components.

Animation of left-handed/counter-clockwise circularly polarized light. (Left-handed as viewed from the receiver.)

In the instance just cited, using the handedness convention used in many optics textbooks, the light is considered left-handed/counter-clockwise circularly polarized. Referring to the accompanying animation, it is considered left-handed because if one points one's left thumb against the direction of travel, ones fingers curl in the direction the electric field rotates as the wave passes a given point in space. The helix also forms a left-handed helix in space. Similarly this light is considered counter-clockwise circularly polarized because if a stationary observer faces against the direction of travel, the person will observe its electric field rotate in the counter-clockwise direction as the wave passes a given point in space.

To create right-handed, clockwise circularly polarized light one simply rotates the axis of the quarter-wave plate 90° relative to the linear polarizer. This reverses the fast and slow axes of the wave plate relative to the transmission axis of the linear polarizer reversing which component leads and which component lags.

In trying to appreciate how the quarter-wave plate transforms the linearly polarized light, it is important to realize that the two components discussed are not entities in and of themselves but are merely mental constructs one uses to help appreciate what is happening. In the case of linearly and circularly polarized light, at each point in space, there is always a single electric field with a distinct vector direction, the quarter-wave plate merely has the effect of transforming this single electric field.

Absorbing and passing circularly polarized light

Circular polarizers can also be used to selectively absorb or pass right-handed or left-handed circularly polarized light. It is this feature which is utilized by the 3D glasses in stereoscopic cinemas such as RealD Cinema. A given polarizer which creates one of the two polarizations of light will pass that same polarization of light when that light is sent through it in the other direction. In contrast it will block light of the opposite polarization.

Circular polarizer passing left-handed, counter-clockwise circularly polarized light. (Left-handed as viewed from the receiver.)

The illustration above is identical to the previous similar one with the exception that the left-handed circularly polarized light is now approaching the polarizer from the opposite direction and linearly polarized light is exiting the polarizer toward the right.

First note that a quarter-wave plate always transforms circularly polarized light into linearly polarized light. It is only the resulting angle of polarization of the linearly polarized light that is determined by the orientation of the fast and slow axes of the quarter-wave plate and the handedness of the circularly polarized light. In the illustration, the left-handed circularly polarized light entering the polarizer is transformed into linearly polarized light which has its direction of polarization along the transmission axis of the linear polarizer and it therefore passes. In contrast right-handed circularly polarized light would have been transformed into linearly polarized light that had its direction of polarization along the absorbing axis of the linear polarizer, which is at right angles to the transmission axis, and it would have therefore been blocked.

Left-handed/Counter-Clockwise circularly polarized light displayed above linearly polarized light. The blue and green curves are projections of the helix on the vertical and horizontal planes respectively.

To understand this process, refer to the illustration on the right. It is absolutely identical to the earlier illustration even though the circularly polarized light at the top is now considered to be approaching the polarizer from the left. One can observe from the illustration that the leftward horizontal (as observed looking along the direction of travel) component is leading the vertical component and that when the horizontal component is retarded by one quarter of a wavelength it will be transformed into the linearly polarized light illustrated at the bottom and it will pass through the linear polarizer.

There is a relatively straightforward way to appreciate why a polarizer which creates a given handedness of circularly polarized light also passes that same handedness of polarized light. First, given the dual usefulness of this image, begin by imagining the circularly polarized light displayed at the top as still leaving the quarter-wave plate and traveling toward the left. Observe that had the horizontal component of the linearly polarized light been retarded by a quarter of wavelength twice, which would amount to a full half wavelength, the result would have been linearly polarized light that was at a right angle to the light that entered. If such orthogonally polarized light were rotated on the horizontal plane and directed back through the linear polarizer section of the circular polarizer it would clearly pass through given its orientation. Now imagine the circularly polarized light which has already passed through the quarter-wave plate once, turned around and directed back toward the circular polarizer again. Let the circularly polarized light illustrated at the top now represent that light. Such light is going to travel through the quarter-wave plate a second time before reaching the linear polarizer and in the process, its horizontal component is going to be retarded a second time by one quarter of a wavelength. Whether that horizontal component is retarded by one quarter of a wavelength in two distinct steps or retarded a full half wavelength all at once, the orientation of the resulting linearly polarized light will be such that it passes through the linear polarizer.

Had it been right-handed, clockwise circularly polarized light approaching the circular polarizer from the left, its horizontal component would have also been retarded, however the resulting linearly polarized light would have been polarized along the absorbing axis of the linear polarizer and it would not have passed.

To create a circular polarizer that instead passes right-handed polarized light and absorbs left-handed light, one again rotates the wave plate and linear polarizer 90° relative to each another. It is easy to appreciate that by reversing the positions of the transmitting and absorbing axes of the linear polarizer relative to the quarter-wave plate, one changes which handedness of polarized light gets transmitted and which gets absorbed.

Homogeneous circular polarizer

Homogeneous circular polarizer passing left-handed, counter-clockwise circularly polarized light. (Left-handed as viewed from the receiver.)

A homogeneous circular polarizer passes one handedness of circular polarization unaltered and blocks the other handedness. This is similar to the way that a linear polarizer would fully pass one angle of linearly polarized light unaltered, but would fully block any linearly polarized light that was orthogonal to it.

A homogeneous circular polarizer can be created by sandwiching a linear polarizer between two quarter-wave plates. Specifically we take the circular polarizer described previously, which transforms circularly polarized light into linear polarized light, and add to it a second quarter-wave plate rotated 90° relative to the first one.

Generally speaking, and not making direct reference to the above illustration, when either of the two polarizations of circularly polarized light enters the first quarter-wave plate, one of a pair of orthogonal components is retarded by one quarter of a wavelength relative to the other. This creates one of two linear polarizations depending on the handedness the circularly polarized light. The linear polarizer sandwiched between the quarter wave plates is oriented so that it will pass one linear polarization and block the other. The second quarter-wave plate then takes the linearly polarized light that passes and retards the orthogonal component that was not retarded by the previous quarter-wave plate. This brings the two components back into their initial phase relationship, reestablishing the selected circular polarization.

Note that it does not matter in which direction one passes the circularly polarized light.

Circular and linear polarizing filters for photography

Main article: Polarizing filter (photography)

Linear polarizing filters were the first types to be used in photography and can still be used for non-reflex and older single-lens reflex cameras (SLRs). However, cameras with through-the-lens metering (TTL) and autofocusing systems – that is, all modern SLR and DSLR – rely on optical elements that pass linearly polarized light. If light entering the camera is already linearly polarized, it can upset the exposure or autofocus systems. Circular polarizing filters cut out linearly polarized light and so can be used to darken skies, improve saturation and remove reflections, but the circular polarized light it passes does not impair through-the-lens systems.

Contemporary history

From Wikipedia, the free encyclopedia

Contemporary history, in English-language historiography, is a subset of modern history that describes the historical period from approximately 1945 to the present. Contemporary history is either a subset of the late modern period, or it is one of the three major subsets of modern history, alongside the early modern period and the late modern period. In the social sciences, contemporary history is also continuous with, and related to, the rise of postmodernity.

Contemporary history is politically dominated by the Cold War (1947–1991) between the United States and the Soviet Union whose effects were felt across the world. The confrontation, which was mainly fought through proxy wars and through intervention in the internal politics of smaller nations, ultimately ended with the Revolutions of 1989 and the dissolution of the Soviet Union in 1991. The latter stages and aftermath of the Cold War enabled the democratisation of much of Europe, Africa, and Latin America. In the Middle East, the period after 1945 was dominated by conflict involving the new state of Israel and the rise of petroleum politics, as well as the growth of Islamism after the 1980s. The first supranational organizations of government, such as the United Nations and European Union, emerged during the period after 1945, while the European colonial empires in Africa and Asia collapsed, gone by 1975.

Countercultures rose and the sexual revolution transformed social relations in western countries between the 1960s and 1980s, as seen in the Protests of 1968. Living standards rose sharply across the developed world because of the post-war economic boom. Japan and West Germany both emerged as exceptionally strong economies. The culture of the United States, especially consumerism, spread widely. By the 1960s, many western countries had begun deindustrializing; in their place, globalization led to the emergence of new financial or industrial centers in Asia, starting with Japan, which was followed by the Four Asian Tigers of Hong Kong, Singapore, South Korea and Taiwan, and later China after its economic reform, which exported its consumer and technological goods around the world.

Science made new advances after 1945: spaceflight, nuclear technology, laser and semiconductor technology were developed alongside molecular biology and genetics, particle physics, and the Standard Model of quantum field theory. Meanwhile, the first computers were created, followed by the Internet, beginning the Information Age.

Political history

1945–1991

Main articles: Post-war and Cold War

See also: Modernity, 1940s, 1950s, 1960s, 1970s, and 1980s

 

The split of Europe during the Cold War

In 1945, the Allies of World War II had defeated all significant opposition to them. They established the United Nations to govern international relations and disputes. A looming question was how to handle the defeated Axis nations and the shattered nations that the Axis had conquered. Following the Yalta Conference, territory was divided into zones for which Allied country would have responsibility and manage rebuilding. While these zones were theoretically temporary (such as the eventual fate of occupied Austria, which was released to independence as a neutral country), growing tensions between the Western Bloc, led by the United States, with the Eastern Bloc, led by the Soviet Union, meant that many calcified into place. Countries in Soviet zones of Eastern Europe had communist regimes installed as satellite states. The Berlin Blockade of 1948 led to a Western Airlift to preserve West Berlin and signified a cooling of East-West relations. Germany split into two countries in 1949, liberal-democratic West Germany and communist East Germany. The conflict as a whole would become known as the Cold War. The Western Bloc formed NATO in 1949 while the Eastern Bloc formed the Warsaw Pact in 1955. Direct combat between the new Great Powers was generally avoided, although proxy wars fought in other countries by factions equipped by one side against the other side's faction occurred. An arms race to develop and build nuclear weapons happened as policymakers wanted to ensure their side had more if it came to a war.

In East Asia, Chiang Kai-shek's Republic of China was overthrown in the Chinese Communist Revolution from 1945–1949. His government retreated to Taiwan, but both the nationalist KMT government and the new communist mainland government under Mao Zedong continued to claim authority over all of China. Korea was divided similarly to Germany, with the Soviet Union occupying the North and the United States occupying the South (future North Korea and South Korea). Unlike Germany, the conflict there turned hot, as the Korean War erupted from 1950–1953. Korea was not reunified under either government, however, due to strong support from both the US and China for their favored side; it became a frozen conflict instead. Japan was given a new constitution foreswearing aggressive war in 1947, and the American occupation ended in 1952, although a treaty of mutual aid with the US was soon signed. The United States also granted the Philippines their independence in 1946 while keeping close relations.

The Middle East became a hotbed of instability. The new Jewish state of Israel declared its independence, recognized by both the United States and the Soviet Union, after which followed the 1948 Arab-Israeli War. Egypt's weak and ineffective king Farouk was overthrown in the Egyptian revolution of 1952, and replaced by General Nasser; the 1953 Iranian coup d'état saw the American-friendly shah Mohammad Reza Pahlavi remove the democratic constraints on his government and take power directly; and Iraq's monarchy was overthrown in 1958.

Decolonization of the British Empire in Africa.

Decolonization was the most important development across Southeast Asia and Africa from 1946–1975, as the old British, French, Dutch, and Portuguese colonial empires unspooled. Many new states were given their independence, but soon found themselves having to choose between allying with the Western Bloc, Eastern Bloc, or attempting to stay neutral as a member of the Non-Aligned Movement. British India was granted independence in 1947 without an outright war of independence being required. It was partitioned into Hindu-majority India and Muslim-majority Pakistan (West Pakistan and East Pakistan, future Pakistan and Bangladesh); Indo-Pakistani wars were fought in 1947, 1965, and 1971. Sukarno took control of an independent Indonesia in 1950, as attempts to reinstate Dutch rule in 1945–1949 had largely failed, and took an independent-to-Eastern leaning stance. He would later be overthrown by Suharto in 1968, who took a pro-Western stance. The Federation of Malaya was granted independence in 1957, with the concurrent fighting of the Malayan Emergency against communist forces from 1948–1960. The French unsuccessfully fought the First Indochina War in an attempt to hold on to French Indochina; at the 1954 Geneva Conference, the new states of Cambodia, Laos, the Democratic Republic of Vietnam, and the eventual Republic of Vietnam were created. The division of Indochina eventually led to the Vietnam War in the 1960s and 70s (as well as the Laotian Civil War and Cambodian Civil War), which ended in communist North Vietnam unifying the country in 1975 and a stinging defeat for the United States.

In Africa, France fought the grinding Algerian War from 1954–1962 that saw the end of French Algeria and the rise of a new independent Algeria. The British and French both slowly released their vast holdings, leading to the creation of states such as First Nigerian Republic in 1963. Portugal, on the other hand, fiercely held onto their Empire, leading to the Portuguese Colonial War from 1961–1974 in Angola, Guinea-Bissau, and Mozambique until the Estado Novo government fell. Meanwhile, apartheid-era South Africa remained fiercely anti-communist, but withdrew from the British Commonwealth in 1961, and supported various pro-colonial factions across Africa that had lost support from their "home" governments in Europe. Many of the newly independent African governments struggled with the balance between being too weak and overthrown by ambitious coup-plotters, and too strong and becoming dictatorships.

Latin America saw gradual economic growth but also instability in many countries, as the threat of coups and military regimes (juntas) were a major threat. The most famous was the Cuban Revolution that overthrew Batista's American-friendly government for Fidel Castro's Soviet-aligned government. This led to the Cuban Missile Crisis in 1963, generally considered one of the incidents most dangerously close to turning the Cold War into a direct military conflict. The 1968 Peruvian coup d'état and also installed a Soviet-friendly government. Despite this, the region ultimately leaned toward the US in this period, with the CIA supporting American-friendly factions in the 1954 Guatemalan coup d'état, the 1964 Brazilian coup d'état, the 1973 Chilean coup d'état, and others. Nicaragua suffered the most, with the Nicaraguan Revolution seeing major military aid from both great powers to their favored factions that extended a civil war in the country for decades. Mexico escaped this unrest, although functioned largely as a one-party state dominated by the PRI. Argentina had a succession of idiosyncratic governments that courted both the US and USSR, but generally mismanaged the economy.

The Middle East saw events that presaged later conflicts in the 70s and 80s. After the end of the United Arab Republic (UAR), Syria's government was overthrown in the 1966 Syrian coup d'état and replaced with the Neo-Baathist Party, eventually leading to the leadership of the Assad family. Israel and its neighbors fought the Six-Day War in 1967 and the Yom Kippur War of 1973. Under Anwar Sadat and later Hosni Mubarak, Egypt switched from Nasserism to favoring the Western Bloc, and signed a peace treaty with Israel. Lebanon, once among the most prosperous and cultural centers of the region, collapsed into the decade-long Lebanese Civil War from 1975–1990. Iran's unpopular pro-American government was overthrown in the 1979 Iranian Revolution and eventually replaced by a new Islamic Republic headed by Ayatollah Khomeini. Iran and Baathist Iraq under Saddam Hussein soon went to war in the Iran-Iraq War from 1980–1988, which ended inconclusively.

The 1980s saw a general retreat for the communist bloc. The Soviet War in Afghanistan (1979–1989) is often called the "Soviet Union's Vietnam War" in comparison to the American defeat, being an expensive and ultimately unsuccessful war and occupation. More importantly, the intervening decades had seen that Eastern Europe was unable to compete economically with Western Europe, which undermined the promise of communist abundance compared to capitalist poverty. The Western capitalist economies had proven wealthier and better, which made matching the Soviet defense budget to the American one strain limited resources. The Pan-European Picnic in 1989 then set in motion a peaceful chain reaction with the subsequent fall of the Berlin Wall. The Revolutions of 1989 saw many countries of Eastern Europe throw off their communist governments, and the USSR declined to invade to re-establish them. East and West Germany were reunified. Client state status for many states ended, as there was no conflict left to fund. The Malta Summit on 3 December 1989, the failure of the August Coup by Soviet hardliners, and the formal Dissolution of the Soviet Union on 26 December 1991 sealed the end of the Cold War.

1991–2001

See also: 1990s, Postmodernity, and Post–Cold War era

The end of the Cold War left the United States the world's sole superpower. Communism seemed discredited; while China remained an officially communist state, Deng Xiaoping's economic reforms and socialism with Chinese characteristics allowed for the growth of a capitalist private sector in China. In Russia, President Boris Yeltsin pursued a policy of privatization, spinning off former government agencies into private corporations, attempting to handle budget problems inherited from the USSR. The end of Soviet foreign aid caused a variety of changes in countries previously part of the Eastern Bloc; many officially became democratic republics, though some were more accurately described as authoritarian/oligarchic republics and one-party states. Many Western commentators treated the development optimistically; it was thought the world was steadily progressing toward free, liberal democracies. While some Americans had anticipated a "peace dividend" from budget cuts to the Defense Department, these cuts were not as large as some had hoped. International coalitions continued to have a role; the Gulf War saw a large international coalition undo Baathist Iraq's annexation of Kuwait, but other "police" style actions were less successful. Somalia and Afghanistan descended into long, bloody civil wars for almost the entirety of the decade (Somali Civil War, Afghan Civil War (1992–1996), Afghan Civil War (1996–2001)). Russia fought a brutal war in Chechnya that failed to suppress the insurgency there from 1994–1996; war would resume during the Second Chechen War in 1999–2000 that saw a resumption of Russian control after Russia successfully convinced enough rebels to join their cause with promises of autonomy. The Dissolution of Yugoslavia also led to a series of Yugoslav Wars; NATO eventually intervened in the Kosovo War. In the Middle East, the Israeli–Palestinian peace process offered the prospect of a long-term peace deal to many; the Oslo Accords signed in 1993 seemed to offer a "roadmap" to resolving the conflict. Despite these high hopes, they would be largely dashed in 2000–2001 after a breakdown of negotiations and the Second Intifada.

2001–present

See also: 2000s, 2010s, and 2020s (decade)

War on Terror, Afghanistan War, and Iraq War

Main articles: Modern terrorism, War on Terror, War in Afghanistan (2001–present), and Iraq War

 

The World Trade Center on fire and the Statue of Liberty.

The September 11 attacks were a series of coordinated suicide attacks by Al-Qaeda upon the United States on 11 September 2001. On that morning, 19 Al-Qaeda terrorists hijacked four commercial passenger jet airliners. The hijackers intentionally crashed two of the airliners into the Twin Towers of the World Trade Center in New York City, killing everyone on board and many others working in the buildings. Both buildings collapsed within two hours, destroying nearby buildings and damaging others. The hijackers crashed a third airliner into the Pentagon in Arlington, Virginia, just outside Washington, D.C. The fourth plane crashed into a field near Shanksville in rural Somerset County, Pennsylvania, after some of its passengers and flight crew attempted to retake control of the plane, which the hijackers had redirected toward Washington, D.C.

In response, the United States under President George W. Bush enacted the Patriot Act. Many other countries also strengthened their anti-terrorism legislation and expanded law enforcement powers. Major terrorist events after the September 11 attacks include the Moscow Theatre Siege, the 2003 Istanbul bombings, the Madrid train bombings, the Beslan school hostage crisis, the 2005 London bombings, the October 2005 New Delhi bombings, and the 2008 Mumbai Hotel Siege, generally from Islamic terrorism.

U.S. Army troops in Kunar province

The United States responded to the 11 September 2001 attacks by launching a "Global War on Terrorism", invading the Islamic Emirate of Afghanistan to depose the Taliban, who had harbored al-Qaeda terrorists. The War in Afghanistan began in late 2001 and was launched by the UN-authorized ISAF, with the United States and United Kingdom providing most of the troops. The Bush administration policy and the Bush Doctrine stated forces would not distinguish between terrorist organizations and nations or governments that harbor them. Operation Enduring Freedom (OEF) was the United States combat operation involving some coalition partners and operating primarily in the eastern and southern parts of the country along the Pakistan border; the ISAF established by the UN Security Council was in charge of securing the capital of Kabul and its surrounding areas. NATO assumed control of ISAF in 2003.

Despite initial coalition successes, the Taliban were never entirely defeated, and continued to hold territory in mountainous regions as well as threaten the new government, the Islamic Republic of Afghanistan, whose grasp on power outside the major cities was shaky at best. The war was also less successful in restricting al-Qaeda than anticipated.

The Second Gulf War began in March 2003 with the invasion of Iraq by a multinational force. The invasion of Iraq led to an occupation and the eventual capture of Saddam Hussein, who was later executed by the Iraqi Government. Despite government assumptions that the war in Iraq would be over with the fall of Hussein, it continued and intensified. Sectarian groups both fought each other and the occupying coalition forces via asymmetric warfare during the Iraqi insurgency, as Iraq was starkly divided between Sunni, Shia, and Kurdish groups that now competed with each other for power. Al-Qaeda operations in Iraq continued as well. In late 2008, the U.S. and Iraqi governments approved a Status of Forces Agreement effective through to the end of 2011.

The Obama administration re-focused US involvement in the conflict on the withdrawal of its troops from Iraq and a surge of troops and government support in Afghanistan. In May 2011, the bin Laden raid occurred after bin Laden was tracked to his compound in Abbottabad, Pakistan.

In 2011, the United States declared a formal end to the Iraq War. In February 2020, President Donald Trump agreed with the Taliban to withdraw all American troops from Afghanistan over the next year. The incoming Biden administration delayed the withdrawal by a few months, but still largely kept to the deal; the coalition-supported Afghan government soon collapsed, and the Taliban took undisputed control of the country in August 2021 after success in the 2021 Taliban offensive.

Arab Spring and Syria

The Arab Spring began in earnest in 2010 with anti-government protests in the Muslim world, but quickly escalated to full-scale military conflicts in countries like Syria, Libya, and Yemen and also gave the opportunity for the emergence of various militant groups including the Islamic State (IS). The IS was able to take advantage of social media platforms including Twitter to recruit foreign fighters from around the world and seized significant portions of territory in Iraq, Syria, Afghanistan, and the Sinai Peninsula of Egypt from 2013 and ongoing. On the other hand, some violent militant organizations were able to negotiate peace with governments including the Moro Islamic Liberation Front in the Philippines in 2014. The presence of IS and the stalemate in the Syrian Civil War created a migration of refugees to Europe and also galvanized and encouraged high-profile terrorism attacks and armed conflicts around the world, such as the November 2015 Paris attacks and the Battle of Marawi in the Philippines in 2017. In 2014, the United States decided to intervene against the Islamic State in Iraq, with most IS fighters being driven out by the end of 2018. Russia and Iran also jointly launched a campaign against IS and in support of Syrian President Bashar al-Assad. As of 2022, Assad has largely regained authority in the southern half of the country, while the northern reaches are controlled by a mixture of Arab Sunni rebels, Kurds, and Turkey.

Russia

Vladimir Putin, Yeltsin's successor, was very popular in Russia after his victory in the Second Chechen War. He portrayed himself as a corruption fighter initially, checking Russian oligarchs who had acquired vast wealth during Russia's liberalization period. With a combination of genuine popularity and legal rollbacks, Russia gradually moved toward being a one-party state, a democracy but one where Putin's party always won. Russia has since intervened in a variety of military conflicts in its neighboring countries including the 2008 Russo-Georgian War; the 2014 2014 military intervention in Ukraine; a 2015 intervention in the Syrian Civil War; and the expansion of the Russo-Ukrainian War to a full-fledged 2022 invasion where Russia declared their intent to depose Ukraine's government and install a compliant, Russia-friendly new government. Russia's government has often cited the enlargement of NATO as a major complaint.

Economic history

The end of World War II in 1945 saw an increase in international trade and an interconnected system of treaties and agreements to ease its flow. In particular, the United States and the United States dollar took a pivotal role in the world economy, displacing the United Kingdom. The era is sometimes called "Pax Americana" for the relative liberal peace in the Western world, resulting from the preponderance of power enjoyed by the United States of America, as a comparison to the Pax Romana established at the height of the Roman Empire. New York's financial sector ("Wall Street") was the center of the financial world from 1945–1970 in a dominant way unlikely to be seen again. Unlike the aftermath of World War I, America strongly aided in the rebuilding of Europe, including aid to the defeated Axis nations, rather than punishment. The Marshall Plan sent billions of dollars of aid to Western Europe to ensure its stability and ward off a potential economic downturn. The 1944 Bretton Woods Conference established the Bretton Woods system which governed these rules from 1945–1971, as well as the World Bank and the International Monetary Fund (IMF). Western Europe also established the European Economic Community in 1957 to ease customs and aid international trade. In general, vast quality of life improvements affected most every corner of the globe during this period, in both the Western and Eastern spheres. France called them Les Trente Glorieuses ("The Glorious Thirty [Years]"). Despite being largely destroyed in the war, West Germany soon bounced back to being an economic powerhouse by the 1950s with the wirtschaftswunder. Surprisingly, Japan followed Germany, achieving incredible economic growth and becoming the second largest economy in the world in 1968, a phenomenon called the Japanese economic miracle. Many explanations are proffered for the enviable results of these years: relative peace (at least outside the "Third World"); a reduction in average family size; technological improvements; and others.

The 1970s saw economic headwinds. Notably, the price of oil started to go up in the 1970s, as the easiest and most accessible wells had already been pumped dry in the preceding century, and oil is a non-renewable resource. Attention was drawn to the abundant oil in the Middle East, where countries in OPEC controlled substantial untapped oil reserves. Political tensions over the Yom Kippur War and the Iranian Revolution led to the 1973 oil crisis and 1979 oil crisis. The Soviet Union called it the "Era of Stagnation". The 1970s and 80s also saw the rise of the Four Asian Tigers, as South Korea, Taiwan, Singapore, and Hong Kong emulated the Japanese route to prosperity with varying degree of success. In China, the leftist Gang of Four were overthrown in 1976, and Deng Xiaoping pursued a policy of tentatively opening the Chinese economy to capitalist innovations throughout the 1980s, which would be continued by his successors in the 1990s. China's economy, tiny in 1976, would see tremendous growth, and eventually take the spot as second largest economy from Japan in 2010. Among Western economies, the collapse of the Bretton Woods system was replaced by a more flexible era of floating exchange rates. The Group of Seven (G7) first met in 1975 and become one of the main international forums that regulated international trade among industrialized nations.

In the beginning of the 2000s, there was a global rise in prices in commodities and housing, marking an end to the commodities recession of 1980–2000. The US mortgage-backed securities, which had risks that were hard to assess, were marketed around the world and a broad based credit boom fed a global speculative bubble in real estate and equities. The financial situation was also affected by a sharp increase in oil and food prices. The collapse of the American housing bubble caused the values of securities tied to real estate pricing to plummet thereafter, damaging financial institutions. The late-2000s recession, a severe economic recession which began in the United States in 2007, was sparked by the outbreak of the financial crisis of 2007–2010. The modern financial crisis was linked to earlier lending practices by financial institutions and the trend of securitization of American real estate mortgages.

The Great Recession spread to much of the industrialized world, and has caused a pronounced deceleration of economic activity. The global recession occurred in an economic environment characterized by various imbalances. This global recession has resulted in a sharp drop in international trade, rising unemployment and slumping commodity prices. The recession renewed interest in Keynesian economic ideas on how to combat recessionary conditions. However, various industrial countries continued to undertake austerity policies to cut deficits, reduced spending, as opposed to following Keynesian theories.

Countries by real GDP growth rate in 2014. (Countries in brown were in recession.)

From late 2009 European sovereign-debt crisis, fears of a sovereign debt crisis developed among investors concerning rising government debt levels across the globe together with a wave of downgrading of government debt of certain European states. Concerns intensified early 2010 and thereafter making it difficult or impossible for sovereigns to re-finance their debts. On 9 May 2010, Europe's Finance Ministers approved a rescue package worth €750 billion aimed at ensuring financial stability across Europe. The European Financial Stability Facility (EFSF) was a special purpose vehicle financed by members of the eurozone to combat the European sovereign debt crisis. In October 2011 eurozone leaders agreed on another package of measures designed to prevent the collapse of member economies. The three most affected countries, Greece, Ireland and Portugal, collectively account for six percent of eurozone's gross domestic product (GDP). In 2012, Eurozone finance ministers reached an agreement on a second €130-billion Greek bailout. In 2013, the European Union agreed to a €10 billion economic bailout for Cyprus.

Contemporary science and technology

Computing and the Internet

Main articles: Information Age and History of the Internet

 

A Visualization of the various routes through a portion of the Internet. Partial map of the Internet based in 2005.

The Information Age or Information Era, also commonly known as the Age of the Computer, is an idea that the current age will be characterized by the ability of individuals to transfer information freely, and to have instant access to knowledge that would have been difficult or impossible to find previously. The idea is heavily linked to the concept of a Digital Age or Digital Revolution, and carries the ramifications of a shift from traditional industry that the Industrial Revolution brought through industrialization, to an economy based around the manipulation of information. The period is generally said to have begun in the latter half of the 20th century, though the particular date varies. The term began its use around the late 1980s and early 1990s, and has been used up to the present with the availability of the Internet.

During the late 1990s, both Internet directories and search engines were popular—Yahoo! and Altavista (both founded 1995) were the respective industry leaders. By late 2001, the directory model had begun to give way to search engines, tracking the rise of Google (founded 1998), which had developed new approaches to relevancy ranking. Directory features, while still commonly available, became after-thoughts to search engines. Database size, which had been a significant marketing feature through the early 2000s (decade), was similarly displaced by emphasis on relevancy ranking, the methods by which search engines attempt to sort the best results first.

"Web 2.0" is characterized as facilitating communication, information sharing, interoperability, User-centered design and collaboration on the World Wide Web. It has led to the development and evolution of web-based communities, hosted services, and web applications. Examples include social-networking sites, video-sharing sites, wikis, blogs, mashups and folksonomies. Social networking emerged in the early 21st century as a popular social communication, largely replacing much of the function of email, message boards and instant messaging services. Twitter, Facebook, and YouTube are all major examples of social websites that gained widespread popularity. The information distribution continued into the early 21st century with mobile interaction and Internet access growing massively in the early 21st century. By the 2010s, a majority of people in the developed world had Internet access and a majority of people worldwide had a mobile phone. Marking the rise of mobile computing, worldwide sales of personal computers fall 14% during the first quarter of 2013. The Semantic Web (dubbed, "Web 3.0") begins the inclusion of semantic content in web pages, converting the current web dominated by unstructured and semi-structured documents into a "web of data".

With the rise of information technology, computer security, and information security in general, is a concern for computers and networks. Concerns include information and services which are protected from unintended or unauthorized access, change or destruction. This has also raised questions of Internet privacy and personal privacy globally.

Space exploration

Main article: Space exploration

The Space Race was one of the rivalries of the Cold War, with both the United States space program (NASA) and the Soviet space program launching satellites, probes, and planning missions. While the Soviets put the first human into space with Yuri Gagarin, the Americans soon caught up, and the US was the first to launch a successful moon landing mission with Apollo 11.

In the 1970s and 80s, the US took a new approach with the Space Shuttle program, hoping to reduce the cost of launches by creating a re-usable Space Shuttle. The first fully functional Space Shuttle orbiter was Columbia (designated OV-102), launched into low Earth orbit in April 1981. In 1996, Shuttle mission STS-75 conducted research in space with the electrodynamic tether generator and other tether configurations. The program suffered from two incidents that destroyed a shuttle: the Challenger disaster and the Columbia disaster). The program ultimately had 135 missions. The retirement of NASA's Space Shuttle fleet took place from March to July 2011.

The end of the Cold War saw a new era of international cooperation with the International Space Station (ISS). Commercial spaceflight also became possible as governments loosened what had previously been their firm control over satellites, opening new possibilities, but also new risks such as light pollution from satellites. The Commercial Orbital Transportation Services (COTS) program began in 2006.

There are various spaceports, including spaceports of human spaceflight and other launch systems (space logistics). Private spaceflight is flight beyond the Kármán line that is conducted and paid for by an entity other than a government agency. Commercialization of space is the use of equipment sent into or through outer space to provide goods or services of commercial value, either by a corporation or state. Space trade plans and predictions began in the 1960s. Spacecraft propulsion is any method used to accelerate spacecraft and artificial satellites.

NASA announced in 2011 that its Mars Reconnaissance Orbiter captured photographic evidence of possible liquid water on Mars during warm seasons. On 6 August 2012, the Mars Science Laboratory Curiosity, the most elaborate Martian exploration vehicle to date, landed on Mars. After the WMAP observations of the cosmic microwave background, information was released in 2011 of the work done by the Planck Surveyor, estimating the age of the Universe to 13.8 billion years old (a 100 million years older than previously thought). Another technological advancement came in 2012 with European physicists statistically demonstrating the existence of the Higgs boson.

Forecasting the future

Main articles: Third millennium, 21st century, 2020s, and 2022

See also: Future and Timeline of the near future

The world is in the third millennium. The 21st century is the century of the Christian Era or Common Era in accordance with the Gregorian calendar. It began on 1 January 2001 and will end 31 December 2100. The 2020s, or The 20s, decade runs from 1 January 2020, to 31 December 2029.

The third millennium is the third period of one thousand years. As this millennium is in progress, only its first decade, the 2000s (decade), can be the subject of the conventional historian's attention. The remaining part of the 21st century and longer-term trends are researched by futures studies, an approach that uses various models and several methods (such as "forecasting" and "backcasting"). Ever since the invention of history, people have searched for "lessons" that might be drawn from its study, on the principle that to understand the past is potentially to control the future. A famous quote by George Santayana has it that "Those who cannot remember the past are condemned to repeat it." Arnold J. Toynbee, in his monumental Study of History, sought regularities in the rise and fall of civilizations. In a more popular vein, Will and Ariel Durant devoted a 1968 book, The Lessons of History, to a discussion of "events and comments that might illuminate present affairs, future possibilities... and the conduct of states." Discussions of history's lessons often tend to an excessive focus on historic detail or, conversely, on sweeping historiographic generalizations.

Future studies takes as one of its important attributes (epistemological starting points) the ongoing effort to analyze alternative futures. This effort includes collecting quantitative and qualitative data about the possibility, probability, and desirability of change. The plurality of the term "futures" in futurology denotes the rich variety of alternative futures, including the subset of preferable futures (normative futures), that can be studied.

Practitioners of the discipline previously concentrated on extrapolating present technological, economic or social trends, or on attempting to predict future trends, but more recently they have started to examine social systems and uncertainties and to build scenarios, question the worldviews behind such scenarios via the causal layered analysis method (and others) create preferred visions of the future, and use backcasting to derive alternative implementation strategies. Apart from extrapolation and scenarios, many dozens of methods and techniques are used in futures research.

Socio-technological trends

At the end of the 20th century, the world was at a major crossroads. Throughout the century, more technological advances had been made than in all of preceding history. Computers, the Internet, and other modern technology radically altered daily lives. Increased globalization, specifically Americanization, had occurred. While not necessarily a threat, it has sparked anti-Western and anti-American sentiment in parts of the developing world, especially the Middle East. The English language has become a leading global language, with people who did not speak it becoming increasingly disadvantaged.

A trend connecting economic and political events in North America, Asia, and the Middle East is the rapidly increasing demand for fossil fuels, which, along with fewer new petroleum finds, greater extraction costs (see peak oil), and political turmoil, saw the price of gas and oil soar ~500% between 2000 and 2005. In some places, especially in Europe, gas could be $5 a gallon, depending on the currency. Less influential, but omnipresent, is the debate on Turkey's participation in the European Union. New urbanism and urban revival continue to be forces in urban planning in the United States. However, evidence shows that growth of American suburbs still outpaces urban growth.

Emerging technologies

Main article: Emerging technologies

Various emerging technologies, the recent developments and convergences in various fields of technology, hold possible future impacts. Emerging technologies cover various cutting-edge developments in the emergence and convergence of technology, including transportation, information technology, biotechnology, robotics and applied mechanics, and material science. Their status and possible effects involve controversy over the degree of social impact or the viability of the technologies. Though, these represent new and significant developments within a field; converging technologies represent previously distinct fields which are in some way moving towards stronger inter-connection and similar goals.

Challenges and problems

Climate change

Main articles: Climate change and Effects of global warming

Climate change and global warming reflects the notion of the modern climate. The changes of climate over the past century, have been attributed to various factors which have resulted in a global warming. This warming is the increase in the average temperature of the Earth's near-surface air and oceans since the mid-20th century and its projected continuation. Some effects on both the natural environment and human life are, at least in part, already being attributed to global warming. A 2001 report by the Intergovernmental Panel on Climate Change suggests that glacier retreat, ice shelf disruption such as that of the Larsen Ice Shelf, sea level rise, changes in rainfall patterns, and increased intensity and frequency of extreme weather events are attributable in part to global warming. Other expected effects include water scarcity in some regions and increased precipitation in others, changes in mountain snowpack, and adverse health effects from warmer temperatures.

It usually is impossible to connect specific weather events to human impact on the world. Instead, such impact is expected to cause changes in the overall distribution and intensity of weather events, such as changes to the frequency and intensity of heavy precipitation. Broader effects are expected to include glacial retreat, Arctic shrinkage, and worldwide sea level rise. Other effects may include changes in crop yields, addition of new trade routes, species extinctions, and changes in the range of disease vectors. Until 2009, the Arctic Northwest Passage pack ice prevented regular marine shipping throughout most of the year in this area, but climate change has reduced the pack ice, and this Arctic shrinkage made the waterways more navigable.

Pandemics

Main articles: COVID-19 pandemic, Impact of the COVID-19 pandemic, and Timeline of the COVID-19 pandemic

In 2020, an outbreak of the COVID-19 disease, first documented in late 2019 in Wuhan, China, spread to other countries becoming a global pandemic, which caused a major socio-economic disruption all over the world. Many countries ordered mandatory lockdowns on movement and closures of non-essential businesses. The threat of the disease caused the COVID-19 recession, although the distribution of vaccines has since eased the economic impact in many countries.

More generally, COVID-19 has been held up as an example of a global catastrophic risk unique to the modern era's ease of travel. New diseases can spread far faster and further in the contemporary era than any previous era of human history; pandemic prevention is one resulting field to ensure that if this happens with a sufficiently deadly virus, humanity can take measures to stop its spread.

Charts

Timeline

Main article: Timelines of modern history § Contemporary period