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Bolometer

Bolometer

] A bolometer is a device for measuring incident electromagnetic radiation. It was invented in the 19th century by the american astronomer Samuel Pierpont Langley It consists of an "absorber", which is connected to a heat sink (area of constant temperature) through an insulating link. The result is that any radiation absorbed by the absorber raises its temperature above that of the heat sink—the higher the power absorbed, the higher the temperature will be. A thermometer of some kind, attached to the absorber, is used to measure the temperature, from which the absorbed power can be calculated. In some designs the thermometer is also the absorber; in others the absorber and thermometer are separate; this is known as "composite design". While bolometers can be used to measure radiation of any frequency, for most wavelength ranges there are other methods of detection that are more sensitive. However, for sub-millimetre wavelengths (from around 200 µm to 1 mm wavelength), the bolometer is the most sensitive type of detector for any measurement over more than a very narrow wavelength range. Bolometers are therefore used for astronomy at these wavelengths. However, to achieve the best sensitivity, they must be cooled down to a fraction of a degree above absolute zero (typically from 50 millikelvins to 300 mK); this makes their operation technically somewhat challenging. The term bolometer is also used in high-energy physics (particle physics) to designate an unconventional particle detector. They use the same principle described above. The bolometers are sensitive not only to light but to every form of energy. They can be used to search for unknown forms of mass or energy (like dark matter) as well as normal particles and radiation. They are very slow and they have a high dead time. They lack completely of any sort of discimination. On the other hand, they are extremely efficient in energy resolution and in sensitivity. They can be used to test very high radio-purity. They are also known as thermal detectors. Their use as particle detectors is still at the developmental stage. Their usage as particle detectors was advice from the beginning of 20th century but the first regular use, even if in a pioneering way, was only in the 1980s because of the difficulty associated with having a system at cryogenic temperature.

External links


- [http://earthobservatory.nasa.gov/Library/Giants/Langley/langley_2.html NASA on the history of the Bolometer] Category:Radiometry Category:Measuring instruments Category:Particle detectors

Electromagnetic radiation

Electromagnetic radiation is a propagating wave in space with electric and magnetic components. These components oscillate at right angles to each other and to the direction of propagation. The term electromagnetic radiation is also used as a synonym for electromagnetic waves in general, even if they are not radiating or travelling in free space. This sense includes, for example, light travelling through an optical fiber, or electrical energy travelling within a coaxial cable. Electromagnetic (EM) radiation carries energy and momentum which may be imparted when it interacts with matter.

Physics

Theory

Electromagnetic waves of much lower frequency than visible light were predicted by Maxwell's equations and subsequently discovered by Heinrich Hertz. Maxwell derived a wave form of the electric and magnetic equations which made explicit the wave nature of the electric and magnetic fields. These equations displayed the symmetry of the fields. According to the theory, a time-varying electric field generates a magnetic field and vice versa. Thus, an oscillating electric field creates an oscillating magnetic field, which in turn creates an oscillating electric field, and so on. By this means an EM wave is produced which propagates through space.

Properties

Electric and magnetic fields exhibit the property of superposition. This means that the field due to a particular particle or time-varying electric or magnetic field adds to the fields due to other causes. (As magnetic and electric fields are vector fields, this is the vector addition of all the individual electric and magnetic field vectors.) As a result, EM radiation is influenced by various phenomena such as refraction and diffraction. For example, a travelling EM wave incident on a particular arrangement of atoms induces oscillation in the atoms and thus causes them to emit their own EM waves (called wavelets). These emissions interfere with the impinging wave and alter its form. In refraction, a wave moving from one medium to another of a different density changes its speed and direction when it enters the new medium. The ratio of the refractive indices of the media determines the extent of refraction. Refraction is the mechanism by which light disperses into a spectrum when it is shone through a prism. The physics of electromagnetic radiation is electrodynamics, a subfield of electromagnetism. EM radiation exhibits both wave properties and particle properties at the same time (see wave-particle duality). These characteristics are mutually exclusive and appear separately in different circumstances: the wave characteristics appear when EM radation is measured over relatively larger timescales and over larger distances, and the particle characteristics are evident when measuring smaller distances and timescales. EM radiation's behaviours as a wave and as a stream of particles have been confirmed by a large number of experiments.

Wave model

An important aspect of the wave nature of light is frequency. The frequency of a wave is its rate of oscillation and is measured in hertz, the SI unit of frequency, equal to one oscillation per second. Light usually comprises a spectrum of frequencies which sum to form the resultant wave. In addition, frequency affects properties like refraction, in which different frequencies undergo a different level of refraction. A wave has troughs and crests. The wavelength is the distance from crest to crest. Waves in the electromagnetic spectrum vary in size from very long radio waves the size of buildings, to very short gamma-rays smaller than the size of the nucleus of an atom. Frequency has an inverse relationship to the concept of wavelength. When waves travel from one medium to another, their frequency remains exactly the same - only their wavelength and/or speed changes. Waves can also be described by their radiant energy. Interference is the superposition of two or more waves resulting in a new wave pattern. The way that these coincide causes different types of interference.

Particle model

In the particle model of EM radiation, EM radiation is quantized as particles called photons. Quantisation of light represents the discrete packets of energy which constitute the radiation. The frequency of the radiation determines the magnitude of the energy of the particles. Moreover, these particles are emitted and absorbed by charged particles, so photons act as transporters of energy. A photon absorbed by an atom excites an electron and elevates it to a higher energy level. If the energy is great enough, the electron is liberated from the atom in a process called ionization. Conversely, an electron which descends to a lower energy level in an atom emits a photon of light equal to the energy difference. The energy levels of electrons in atoms are discrete. Therefore, each element has its own characteristic frequencies. Together these effects explain the absorption spectra of light. The dark bands in the spectrum are due to the atoms in the intervening medium which absorb different frequencies of the light. The composition of the medium through which the light travels determines the nature of the absorption spectrum. For instance, in a distant star, dark bands in the light it emits are due to the atoms in the atmosphere of the star. These bands correspond to the allowed energy levels in the atoms. A similar phenomenon occurs for emission. As the electrons descend to lower energy levels, a spectrum which represents the jumps between the energy levels of the electrons is exhibited. This is manifested in the emission spectrum of nebulae.

Speed of propagation

Any electric charge which accelerates, or any changing magnetic field, produces electromagnetic radiation. Electromagnetic information about the charge travels at the speed of light. Accurate treatment thus incorporates a concept known as retarded time (as opposed to advanced time, which is unphysical in light of causality), which adds to the expressions for the electrodynamic electric field and magnetic field. These extra terms are responsible for electromagnetic radiation. When any wire (or other conducting object such as an antenna) conducts alternating current, electromagnetic radiation is propagated at the same frequency as the electric current. Depending on the circumstances, it may behave as a wave or as particles. As a wave, it is characterized by a velocity (the speed of light), wavelength, and frequency. When considered as particles, they are known as photons, and each has an energy related to the frequency of the wave given by Planck's relation E = hν, where E is the energy of the photon, h = 6.626 × 10-34 J·s is Planck's constant, and ν is the frequency of the wave. One rule is always obeyed regardless of the circumstances. EM radiation in a vacuum always travels at the speed of light, relative to the observer, regardless of the observer's velocity. (This observation led to Albert Einstein's development of the theory of special relativity.)

Electromagnetic spectrum

Generally, EM radiation is classified by wavelength into electrical energy, radio, microwave, infrared, the visible region we perceive as light, ultraviolet, X-rays and gamma rays. The behavior of EM radiation depends on its wavelength. Higher frequencies have shorter wavelengths, and lower frequencies have longer wavelengths. When EM radiation interacts with single atoms and molecules, its behavior depends on the amount of energy per quantum it carries. Spectroscopy can detect a much wider region of the EM spectrum than the visible range of 400 nm to 700 nm. A common laboratory spectroscope can detect wavelengths from 2 nm to 2500 nm. More in-depth information about the physical properties of objects, gases, or even stars can be obtained from this type of device. It is widely used in astrophysics. For example, many hydrogen atoms emit radio waves which have a wavelength of 21.12 cm.

Light

EM radiation with a wavelength between 400 nm and 700 nm is detected by the human eye and perceived as visible light. If radiation having a frequency in the visible region of the EM spectrum shines on an object, say, a bowl of fruit, this results in our visual perception of identifying information from the scene. Our brain's visual system processes the multitude of reflected frequencies into different shades and hues, and through this not-entirely-understood "psychophysical phenomenon," most humans perceive a bowl of fruit. In the vast majority of cases, however, the information carried by light is not directly apprehensible by human senses. Natural sources produce EM radiation across the spectrum; so, too, can human technology manipulate a broad range of wavelengths. Optical fiber transmits light which, although not suitable for direct viewing, can carry data. Those data can be translated into sound or an image. The coded form of such data is similar to that used with radio waves.

Radio waves

Radio waves carry information by varying amplitude and by varying frequency within a frequency band. When EM radiation impinges upon a conductor, it couples to the conductor, travels along it, and induces an electric current on the surface of that conductor by exciting the electrons of the conducting material. This effect (the skin effect) is used in antennas. EM radiation may also cause certain molecules to absorb energy and thus to heat up; this is exploited in microwave ovens.

Derivation

Electromagnetic waves as a general phenomenon were predicted by the classical laws of electricity and magnetism, known as Maxwell's equations. If you inspect Maxwell's equations without sources (charges or currents) then you will find that, along with the possibility of nothing happening, the theory will also admit nontrivial solutions of changing electric and magnetic fields. (For symbol definitions see magnetic field.) :\nabla \cdot \mathbf = 0 :\nabla \times \mathbf = -\frac \mathbf :\nabla \cdot \mathbf = 0 :\nabla \times \mathbf = \mu_0 \epsilon_0 \frac \mathbf \mathbf=\mathbf=\mathbf is a solution, but there might be other solutions as well. Let us employ a useful identity from vector calculus. :\nabla \times \left( \nabla \times \mathbf \right) = \nabla \left( \nabla \cdot \mathbf \right) - \nabla^2 \mathbf Where \mathbf can be any vector function. Taking the curl of the curl equations and applying the identity, we get the following. :\nabla^2 \mathbf = \mu_0 \epsilon_0 \frac \mathbf :\nabla^2 \mathbf = \mu_0 \epsilon_0 \frac \mathbf These types of equations are identified as linear wave equations with wave speed \frac. Amazingly, this speed happens to be exactly the speed of light! Maxwell's equations have unified the permittivity of free space \epsilon_0, the permeability of free space \mu_0, and the speed of light itself: c = \frac. Before this derivation it was not known that there was such a strong relationship between light and electricity and magnetism. But these are only two equations and we started with four, so there is still more information pertaining to these waves hidden within Maxwell's equations. Let's consider a generic vector wave for the electric field. :\mathbf = \mathbf_0 f\left( \hat \cdot \mathbf - c t \right) Here \mathbf_0 is the constant amplitude, f is any second differentiable function, \hat is a unit vector in the direction of propagation, and \hat is a position vector. We observe that f\left( \hat \cdot \mathbf - c t \right) is a generic solution to the wave equation. In other words :\nabla^2 f\left( \hat \cdot \mathbf - c t \right) = \frac \frac f\left( \hat \cdot \mathbf - c t \right), for a generic wave traveling in the \hat direction. The proof of this is trivial. This form will satisfy the wave equation, but will it satisfy all of Maxwell's equations, and with what corresponding magnetic field? :\nabla \cdot \mathbf = \hat \cdot \mathbf_0 f'\left( \hat \cdot \mathbf - c t \right) = 0 :\mathbf \cdot \hat = 0 The first of Maxell's equations implies that electric field is orthogonal to the direction the wave propagates. :\nabla \times \mathbf = \hat \times \mathbf_0 f'\left( \hat \cdot \mathbf - c t \right) = -\frac \mathbf :\mathbf = \frac \hat \times \mathbf The second of Maxwell's equations yields the magnetic field. The remaining equations will be satisfied by this choice of \mathbf,\mathbf. Not only are the electric and magnetic field waves traveling at the speed of light, but they have a special restricted orientation and proportional magnitudes, \mathbf_0 = c \mathbf_0. The electric field, magnetic field, and direction of wave propagation are all orthogonal and the wave propagates in the same direction as \mathbf \times \mathbf. Visualizing yourself as an electromagnetic wave traveling forward, the electric field might be oscillating up and down, while the magnetic field oscillates right and left; but you can rotate this picture around with the electric field oscillating right and left and the magnetic field oscillating down and up. This is a different solution that is traveling in the same direction. This arbitrariness in the orientation, with respect to propagation direction, is known as polarization.

See also


- Electromagnetic wave equation
- Electromagnetic spectrum
- Electromagnetic radiation hazards
- Radiant energy
- Light
- Electromagnetic pulse
- Control of electromagnetic radiation
- Klystron

References


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External links

; General
- [http://www.sengpielaudio.com/calculator-wavelength.htm Conversion of frequency to wavelength and back - electromagnetic, radio and sound waves]
- [http://www.scienceofspectroscopy.info The Science of Spectroscopy - a learning tool for spectroscopy] ; Patents
- Greenleaf Whittier Pickard - - Intelligence intercommunication by magnetic wave component ko:전자기파 ja:電磁波

Samuel Pierpont Langley

Samuel Pierpont Langley (August 22 1834 in Roxbury, Massachusetts near Boston, – February 27 1906, Aiken, South Carolina) was an American astronomer, physicist, inventor of the bolometer and pioneer of aviation. He was the third Secretary of the Smithsonian Institution and the founder of the Smithsonian Astrophysical Observatory. In 1886, Langley received the Henry Draper Medal from the National Academy of Sciences for his contributions to solar physics. His publication in 1890 of infrared observations at the Allegheny Observatory in Pittsburgh together with Frank Washington Very was used by Svante Arrhenius to make the first calculations on the greenhouse effect.

Aviation work

Langley attempted to make the first working piloted heavier-than-air aircraft. His models flew but his two attempts at piloted flight, though less ambitious than the Wright brothers' flights, were not successful. Langley began experimenting with rubber powered models and gliders. (According to one book, he was not able to reproduce Alphonse Pénaud's time aloft with rubber power but persisted anyway.) He built a rotating arm (with function similar to the Wright brothers' wind tunnel) for testing. He obtained a War Department grant of $50,000 to develop a piloted airplane and proceeded to larger models with steam and gasoline power. These flew free for considerable distances, demonstrating stability and sufficient lift. They had elaborate wire braced structures. He hired a successful glider pilot to work with him, offered financial support to the Wright brothers (not accepted), and, most important hired Charles M. Manly as engineer and test pilot. While the full scale vehicle was being designed and built, the internal combustion engine development was contracted out to an engine manufacturer. When the contractor failed to produce an engine to the power and weight specifications, Manly finished the design. This engine had far more power per weight than did the Wright brothers' engine that powered the first airplane. The engine, though mostly not the direct technical work of Langley, was probably the project's main contribution to aviation. [http://aerostories.free.fr/precurseurs/langley/mot_lang.JPG] His piloted machine had wire braced tandem wings (one behind the other). It had pitch and yaw control but no roll control, depending on stability, like the models, for maintaining its roll angle. In contrast to the Wright brothers' approach of designing a light and agile airplane that could be flown against a strong wind, Langley avoided fatal accidents by practicing over water, the Potomac River. This required a catapult for launching. The craft had no landing gear, the plan being to crash into the water, after demonstrating flight. They gave up the project after two crashes on take-off on October 7 and December 8, 1903. Manly was recovered unhurt from the river. 1903 Langley's aircraft was modified and flown by Glenn Curtiss, in 1914, as part of his attempt to fight the Wright brothers' patent, but the court upheld the patent. Although, in 1897 and 1898, radio controlled boats had been demonstrated to the military and to the public by Nikola Tesla, the state of radio was very primitive. Though he did experiments with rotating structures and had the help of a successful hang glider pilot, he appears to have had no effective way of addressing the Wright Brothers' central problem of controlling an airplane, too big to be controlled by the weight of the pilot's body. So if the "Airdrome" had taken off and flown stably, as the models did, Manly would have been in considerable danger and the Wright Brothers' credit would be little reduced. To his credit, Langley had to write reports and proposals during this project, while the Wright brothers were spending their own money. A number of things related to aviation have been named in Langley's honor, including:
- Langley medal
- NASA Langley X-43A Hyper-X
- NASA Langley Research Center (NASA LaRC), Hampton, Virginia
- Langley field
- Langley Air Force Base
- Langley Memorial Aeronautical Laboratory
- Langley unit of solar radiation
- USS Langley (CV-1)
- USS Langley (CVL-27)

At the Smithsonian

Langley served as Secretary of the Smithsonian Institution from 1887-1906 . He was preceded by Spencer Fullerton Baird and succeeded by Charles Doolittle Walcott.

References


- A Heritage of Wings, An Illustrated History of Naval Aviation, by Richard C. Knott, Naval Institute Press, Annapolis, Maryland, 1997
- Aviation, The Pioneer Years, edited by Ben Mackworth-Praed, Studio Editions, Ltd., London, 1990

External links


- http://www.flyingmachines.org/lang.html
- http://www.centennialofflight.gov/index2.cfm Langley, Samuel Langley, Samuel Langley, Samuel Langley, Samuel Langley, Samuel Langley, Samuel ja:サミュエル・ラングレー

Wavelength

:For the album by Van Morrison, see Wavelength (album). The wavelength is the distance between repeating units of a wave pattern. It is commonly designated by the Greek letter lambda (λ). In a sine wave, the wavelength is the distance between the midpoints of the wave: Image:Wavelength.png The x axis represents distance, and I would be some varying quantity at a given point in time as a function of x, for instance air pressure for a sound wave or strength of the electric or magnetic field for light. Wavelength λ has an inverse relationship to frequency f, the number of peaks to pass a point in a given time. The wavelength is equal to the speed of the wave type divided by the frequency of the wave. When dealing with electromagnetic radiation in a vacuum, this speed is the speed of light c, for signals (waves) in air, this is the speed of sound in air. The relationship is given by: : \lambda = \frac where: :λ = wavelength of a sound wave or electromagnetic wave :c = speed of light in vacuum = 299,792.458 km/s ~ 300,000 km/s = 300,000,000 m/s or :c = speed of sound in air = 343 m/s at 20 °C (68 °F) :f = frequency of the wave in 1/s = Hz For radio waves this relationship is approximated with the formula: wavelength λ (in metres) = 300 / frequency (in megahertz).
For sound waves this relationship is approximated with the formula: wavelength λ (in metres) = 333 / frequency (in hertz). When light waves (and other electromagnetic waves) enter a medium, their wavelength is reduced by a factor equal to the refractive index n of the medium but the frequency of the wave is unchanged. The wavelength of the wave in the medium, λ' is given by: : \lambda^\prime = \frac where: :λ0 is the vacuum wavelength of the wave Wavelengths of electromagnetic radiation, no matter what medium they are travelling through, are usually quoted in terms of the vacuum wavelength, although this is not always explicitly stated. Louis de Broglie discovered that all particles with momentum have a wavelength associated with their quantum mechanical wavefunction, called the de Broglie wavelength.

See also


- amplitude
- periodic function

External link


- [http://www.sengpielaudio.com/calculator-wavelength.htm Conversion: Wavelength to frequency and vice versa - The calculator] Category:Length Category:Wave mechanics ko:파장 ja:波長 th:ความยาวคลื่น

Astronomy

:This article is about the science branch. For information about the magazine, see Astronomy (magazine). Astronomy (magazine) as they circled the Moon in 1969. Located near the center of the far side of Earth's Moon, its diameter is about 58 miles (93 km).]] Astronomy (Greek: αστρονομία = άστρον + νόμος, astronomia = astron + nomos, literally, "law of the stars") is the science of celestial objects and phenomena that originate outside the Earth's atmosphere, such as stars, planets, comets, galaxies, and the cosmic background radiation. It is concerned with the formation and development of the universe, the evolution and physical and chemical properties of celestial objects and the calculation of their motions. Astronomical observations are not only relevant for astronomy as such, but provide essential information for the verification of fundamental theories in physics, such as general relativity theory. Complementary to observational astronomy, theoretical astrophysics seeks to explain astronomical phenomena. Astronomy is one of the oldest sciences, with a scientific methodology existing at the time of Ancient Greece and advanced observation techniques possibly much earlier (see archaeoastronomy). Historically, amateurs have contributed to many important astronomical discoveries, and astronomy is one of the few sciences where amateurs can still play an active role, especially in the discovery and observation of transient phenomena. Astronomy is not to be confused with astrology, which assumes that people's destiny and human affairs in general are correlated to the apparent positions of astronomical objects in the sky -- although the two fields share a common origin, they are quite different; astronomers embrace the scientific method, while astrologers do not. In other words, there is no proof that the stars predict our future, but there is proof that our planet is 93 million miles from the sun.

Divisions

In ancient Greece and other early civilizations, astronomy consisted largely of astrometry, measuring positions of stars and planets in the sky. Later, the work of Kepler and Newton, whose work led to the development of celestial mechanics, mathematically predicting the motions of celestial bodies interacting under gravity, and solar system objects in particular. Much of the effort in these two areas, once done largely by hand, is highly automated nowadays, to the extent that they are rarely considered as independent disciplines anymore. Motions and positions of objects are now more easily determined, and modern astronomy is more concerned with observing and understanding the actual physical nature of celestial objects. Since the twentieth century, the field of professional astronomy has split into observational astronomy and theoretical astrophysics. Although most astronomers incorporate elements of both into their research, because of the different skills involved, most professional astronomers tend to specialize in one or the other. Observational astronomy is concerned mostly with acquiring data, which involves building and maintaining instruments and processing the results; this branch is at times referred to as "astrometry" or simply as "astronomy". Theoretical astrophysics is concerned mainly with ascertaining the observational implications of different models, and involves working with computer or analytic models. The fields of study can also be categorized in other ways. Categorization by the region of space under study (for example, Galactic astronomy, Planetary Sciences); by subject, such as star formation or cosmology; or by the method used for obtaining information.

By subject or problem addressed

theoretical astrophysics. Photographed by Mars Global Surveyor, the long dark streak is formed by a moving swirling column of Martian atmosphere (with similarities to a terrestrial tornado). The dust devil itself (the black spot) is climbing the crater wall. The streaks on the right are sand dunes on the crater floor.]]
- Astrometry: the study of the position of objects in the sky and their changes of position. Defines the system of coordinates used and the kinematics of objects in our galaxy.
- Astrophysics: the study of physics of the universe, including the physical properties (luminosity, density, temperature, chemical composition) of astronomical objects.
- Cosmology: the study of the origin of the universe and its evolution. The study of cosmology is theoretical astrophysics at its largest scale.
- Galaxy formation and evolution: the study of the formation of the galaxies, and their evolution.
- Galactic astronomy: the study of the structure and components of our galaxy and of other galaxies.
- Extragalactic astronomy: the study of objects (mainly galaxies) outside our galaxy.
- Stellar astronomy: the study of the stars.
- Stellar evolution: the study of the evolution of stars from their formation to their end as a stellar remnant.
- Star formation: the study of the condition and processes that led to the formation of stars in the interior of gas clouds, and the process of formation itself.
- Planetary Sciences: the study of the planets of the Solar System.
- Astrobiology: the study of the advent and evolution of biological systems in the Universe. Other disciplines that may be considered part of astronomy:
- Archaeoastronomy
- Astrochemistry
- Astrosociobiology
- Astrophilosophy See the list of astronomical topics for a more exhaustive list of astronomy-related pages.

Ways of obtaining information

list of astronomical topics :Main article: Observational astronomy. In astronomy, information is mainly received from the detection and analysis of light and other forms of electromagnetic radiation. Other cosmic rays are also observed, and several experiments are designed to detect gravitational waves in the near future. A traditional division of astronomy is given by the region of the electromagnetic spectrum observed:
- Optical astronomy is the part of astronomy that uses optical components (mirrors, lenses, CCD detectors and photographic films) to observe light from near infrared to near ultraviolet wavelengths. Visible light astronomy (using wavelengths that can be detected with the eyes, about 400 - 700 nm) falls in the middle of this range. The most common tool is the telescope, with electronic imagers and spectrographs.
- Infrared astronomy deals with the detection and analysis of infrared radiation (wavelengths longer than red light). The most common tool is the telescope but using a detector which is sensitive to the infrared. Space telescopes are also used to avoid atmospheric thermal emission, atmospheric opacity, and the effects of astronomical seeing at infrared and other wavelengths.
- Radio astronomy detects radiation of millimetre to dekametre wavelength. The receivers are similar to those used in radio broadcast transmission but much more sensitive. See also Radio telescopes.
- High-energy astronomy includes X-ray astronomy, gamma-ray astronomy, and extreme UV (ultraviolet) astronomy, as well as studies of neutrinos and cosmic rays. Optical and radio astronomy can be performed with ground-based observatories, because the atmosphere is transparent at the wavelengths being detected. Infrared light is heavily absorbed by water vapor, so infrared observatories have to be located in high, dry places or in space. The atmosphere is opaque at the wavelengths of X-ray astronomy, gamma-ray astronomy, UV astronomy and (except for a few wavelength "windows") Far infrared astronomy, so observations must be carried out mostly from balloons or space observatories. Powerful gamma rays can, however be detected by the large air showers they produce, and the study of cosmic rays can also be regarded as a branch of astronomy.

History of astronomy

cosmic ray :Main article: History of astronomy. In early times, astronomy only comprised the observation and predictions of the motions of the naked-eye objects. Aristotle said that the Earth was the center of the Universe and everything rotated around it in orbits that were perfect circles. Aristotle had to be right because people thought that Earth had to be in the center with everything rotating around it because the wind would not scatter leaves, and birds would only fly in one direction. For a long time, people thought that Aristotle was right, but it is probable that Aristotle accidentally did more to hinder our knowledge than help it. The Rigveda refers to the 27 constellations associated with the motions of the sun and also the 12 zodiacal divisions of the sky. The ancient Greeks made important contributions to astronomy, among them the definition of the magnitude system. The Bible contains a number of statements on the position of the earth in the universe and the nature of the stars and planets, most of which are poetic rather than literal; see Biblical cosmology. In 500 AD, Aryabhata presented a mathematical system that described the earth as spinning on its axis and considered the motions of the planets with respect to the sun. Observational astronomy was mostly stagnant in medieval Europe, but flourished in the Iranian world and other parts of Islamic realm. The late 9th century Persian astronomer al-Farghani wrote extensively on the motion of celestial bodies. His work was translated into Latin in the 12th century. In the late 10th century, a huge observatory was built near Tehran, Persia (now Iran), by the Persian astronomer al-Khujandi, who observed a series of meridian transits of the Sun, which allowed him to calculate the obliquity of the ecliptic. Also in Persia, Omar Khayyám performed a reformation of the calendar that was more accurate than the Julian and came close to the Gregorian. Abraham Zacuto was responsible in the 15th century for the adaptations of astronomical theory for the practical needs of Portuguese caravel expeditions. During the Renaissance, Copernicus proposed a heliocentric model of the Solar System. His work was defended, expanded upon, and corrected by Galileo Galilei and Johannes Kepler. Galileo added the innovation of using telescopes to enhance his observations. Kepler was the first to devise a system that described correctly the details of the motion of the planets with the Sun at the center. However, Kepler did not succeed in formulating a theory behind the laws he wrote down. It was left to Newton's invention of celestial dynamics and his law of gravitation to finally explain the motions of the planets. Newton also developed the reflecting telescope. Stars were found to be faraway objects. With the advent of spectroscopy it was proved that they were similar to our own sun, but with a wide range of temperatures, masses, and sizes. The existence of our galaxy, the Milky Way, as a separate group of stars was only proven in the 20th century, along with the existence of "external" galaxies, and soon after, the expansion of the universe, seen in the recession of most galaxies from us. Modern astronomy has also discovered many exotic objects such as quasars, pulsars, blazars and radio galaxies, and has used these observations to develop physical theories which describe some of these objects in terms of equally exotic objects such as black holes and neutron stars. Physical cosmology made huge advances during the 20th century, with the model of the Big Bang heavily supported by the evidence provided by astronomy and physics, such as the cosmic microwave background radiation, Hubble's Law, and cosmological abundances of elements.

Timelines in astronomy

cosmological abundances of elements
- Artificial satellites and space probes
- Astronomical maps, catalogs, and surveys
- Big Bang
- Black hole physics
- Cosmic microwave background astronomy
- Cosmology
- Galaxies, clusters of galaxies, and large scale structure
- Interstellar medium and intergalactic medium
- Natural satellites
- Other background radiation fields
- Solar astronomy
- Solar system astronomy
- Stellar astronomy
- Telescopes, observatories, and observing technology
- White dwarfs, neutron stars, and supernovae

See also


- List of astronomical topics
- Astronomers and Astrophysicists
- Astronomical cycles
- Astronomical naming conventions
- Astronomical object
- Astronomical observatories
- Astronomy organizations
- Astronomical symbols
- Space science
- Celestial navigation

Astronomy tools


- Binoculars
- Telescope
- Computers
- Calculator
- Observatory
- Space observatory
- Maksutov telescope

External Links


- [http://www.space.com/ Space.com]
- [http://www.Astronomy.com/ Astronomy.com]
- [http://www.AbsoluteAstronomy.com/ AbsoluteAstronomy.com]
- [http://www.badastronomy.com/ Bad Astronomy]
- [http://www.nasa.gov/ Nasa]
- [http://www.run4space.com Run4Space Forum]
- [http://antwrp.gsfc.nasa.gov/apod/astropix.html/ Astronomy Picture of the Day] ko:천문학 ms:Astronomi ja:天文学 simple:Astronomy th:ดาราศาสตร์

Kelvin

The kelvin (symbol: K) is the SI unit of temperature, and is one of the seven SI base units. It is defined as the fraction 1/273.16 of the thermodynamic temperature of the triple point of water. A temperature given in kelvins, without further qualification, is measured with respect to absolute zero, where molecular motion stops. It is also common to give a temperature relative to the reference temperature of 273.15 K, approximately the melting point of water under ordinary conditions; this convention is the Celsius temperature scale. The kelvin is named after the British physicist and engineer William Thomson, 1st Baron Kelvin; his barony was in turn named after the River Kelvin, which runs through the grounds of the University of Glasgow.

SI multiples

Typographical conventions

The word kelvin as an SI unit is correctly written with a lowercase k (unless at the beginning of a sentence), and is never preceded by the words degree or degrees, or the symbol °, unlike degrees Fahrenheit, or degrees Celsius. This is because the latter are adjectives, whereas kelvin is a noun. It takes the normal plural form by adding an s in English: kelvins. When the kelvin was introduced in 1954 (10th General Conference on Weights and Measures (CGPM), Resolution 3, CR 79), it was the "degree Kelvin", and written °K; the "degree" was dropped in 1967 (13th CGPM, Resolution 3, CR 104). Note that the symbol for the kelvin unit is always a capital K and never italicised. There is a space between the number and the K, as with all other SI units. Unicode includes the "kelvin sign" at U+212A (in your browser it looks like K). However, the "kelvin sign" is canonically decomposed into U+004B, thereby seen as a (preexisting) encoding mistake, and it is better to use U+004B (K) directly.

Conversion factors

Kelvins and Celsius

The Celsius temperature scale is now defined in terms of the kelvin, with 0 °C corresponding to 273.15 kelvins.
- kelvins to degrees Celsius
- : \mathrm = \mathrm - 273.15

Temperature and energy

In a thermodynamic system, the energy of the particles of a perfect gas is proportional to the absolute temperature, where the constant of proportionality is the Boltzmann constant. As a result, it is possible to determine the average kinetic energy \overline of the gas particles at the temperature T or to calculate the temperature of the gas from the average kinetic energy of the particles: : \overline = \frac \cdot k_B \cdot \mathrm

See also


- ITS-90 International Temperature Scale

External link


- [http://www1.bipm.org/en/si/si_brochure/chapter2/2-1/2-1-1/kelvin.html BIPM brochure on the kelvin] Category:SI base units Category:Units of temperature ko:켈빈 ja:ケルビン simple:Kelvin th:เคลวิน

Dark matter

In cosmology, dark matter refers to hypothetical matter particles, of unknown composition, that do not emit or reflect enough electromagnetic radiation to be detected directly, but whose presence can be inferred from gravitational effects on visible matter such as stars and galaxies. The dark matter hypothesis aims to explain several anomalous astronomical observations, such as anomalies in the rotational speed of galaxies (the galaxy rotation problem). Estimates of the amount of matter present in galaxies, based on gravitational effects, consistently suggest that there is far more matter than is directly observable. The existence of dark matter would also resolve a number of inconsistencies in the Big Bang theory, and is crucial for structure formation. If dark matter does exist, it vastly outmasses the "visible" part of the universe [http://map.gsfc.nasa.gov/m_mm/mr_limits.html]. Only about 4% of the total mass in the universe (as inferred from gravitational effects) can be seen directly. About 23% is thought to be composed of dark matter. The remaining 73% is thought to consist of dark energy, an even stranger component, distributed diffusely in space, that probably cannot be thought of as ordinary particles. Determining the nature of this missing mass is one of the most important problems in modern cosmology and particle physics. The first to hypothesize dark matter was Fritz Zwicky, of the California Institute of Technology (Caltech) in 1933. He applied the virial theorem to the Coma cluster of galaxies and obtained evidence of unseen mass.

Evidence for dark matter

In 1913, Norwegian explorer and physicist Kristian Birkeland may have been the first to predict that space is not only a plasma, but also contains "dark matter". He wrote: "It seems to be a natural consequence of our points of view to assume that the whole of space is filled with electrons and flying electric ions of all kinds. We have assumed that each stellar system in evolutions throws off electric corpuscles into space. It does not seem unreasonable therefore to think that the greater part of the material masses in the universe is found, not in the solar systems or nebulae, but in "empty" space. (Ref. See notes) Dark matter was first hypothesized to exist by the Swiss astrophysicist Fritz Zwicky. In 1933 Zwicky estimated the total amount of mass in a cluster of galaxies, the Coma Cluster, based on the motions of the galaxies near the edge of the cluster. When he compared this mass estimate to one based on the number of galaxies and total brightness of the cluster, he found that there was about 400 times more mass than expected. The gravity of the visible galaxies in the cluster would be far too small for such fast orbits, so something extra was required. This is known as the "missing mass problem". Based on these conclusions, Zwicky inferred that there must be some other form of matter existent in the cluster which we have not detected, which provides enough of the mass and gravity to hold the cluster together. At present, the density of ordinary baryons and radiation in the universe is estimated to be about one hydrogen atom per cubic meter of space. However, dark matter and dark energy are together said to account for 96% of all matter in the universe. This means that only about 4% of all matter can be directly observed. Things like gas clouds, small lumps of iron, and brown dwarfs are ordinary baryonic matter and are all accounted for in the 4% that is observed [http://arxiv.org/abs/astro-ph/0007444] [http://arxiv.org/abs/astro-ph/0002058]. The term dark matter specifically refers to stuff other than what makes up ordinary matter (baryons) that we are familiar with. Since it cannot be directly detected via optical means, many aspects of dark matter remain speculative. The DAMA/NaI experiment has claimed to directly detect dark matter passing through the Earth, though most scientists remain sceptical since negative results of other experiments are (almost) incompatible with the DAMA results if dark matter consists of neutralinos.

Galactic rotation

Much of the evidence for dark matter comes from the study of the motions of galaxies. Many of these appear to be fairly uniform, so by the virial theorem the total kinetic energy should be half the total gravitational binding energy of the galaxies. Experimentally, however, it is found to be much greater: in particular, stars far from the center of galaxies have much higher velocities than predicted by the virial theorem. Galactic rotation curves, which illustrate the velocity of rotation versus the distance from the galactic center, cannot be explained by only the visible matter. Assuming that the visible material makes up only a small part of the cluster is the most straightforward way of accounting for this. Galaxies show signs of being composed largely of a roughly spherical halo of dark matter with the visible matter concentrated in a disc at the center. Low surface brightness dwarf galaxies are important sources of information for studying dark matter, as they have an uncommonly low ratio of visible matter to dark matter, and have few bright stars at the center which impair observations of the rotation curve of outlying stars. Recently, astronomers from Cardiff University claim to have discovered a galaxy made almost entirely of dark matter, 50 million light years away in the Virgo Cluster, which was named VIRGOHI21 (Wikinews, [http://www.newscientist.com/article.ns?id=dn7056 New Scientist]). Unusually, VIRGOHI21 does not appear to contain any visible stars: it was seen with radio frequency observations of hydrogen. Based on rotation profiles, the scientists estimate that this object contains approximately 1000 times as much dark matter as hydrogen and has a total mass of about 1/10th of that of the Milky Way Galaxy we live in. For comparison, the Milky Way is believed to have roughly 10 times as much dark matter as ordinary matter. Models of the Big Bang and structure formation have suggested that such dark galaxies should be very common in the universe, but none have previously been detected. If the existence of this dark galaxy is confirmed, it provides strong evidence for the theory of galaxy formation and poses problems for alternative explanations of dark matter. Dark matter is believed to affect galaxy clusters as well. The galaxy cluster Abell 2029 is composed of thousands of galaxies enveloped in a cloud of hot gas, and an amount of dark matter equivalent to more than a hundred trillion Suns. At the center of this cluster is an enormous, elliptically shaped galaxy that is thought to have been formed from the mergers of many smaller galaxies. More info is available here: [http://chandra.harvard.edu/photo/2003/abell2029/ http://chandra.harvard.edu/photo/2003/abell2029/].

Structure formation

A significant amount of non-baryonic, cold matter is necessary to explain the large-scale structure of the universe. Observations suggest that structure formation in the universe proceeds hierarchically, with the smallest structures, such as stars, forming first, and followed by galaxies and then clusters of galaxies. In the universe, it is thought that the first structures that form are quasars, which are supermassive black holes. This, bottom up model of structure formation requires something like cold dark matter to succeed. Ordinary baryonic matter had too high a temperature, and too much pressure left over from the big bang to collapse and form smaller structures, such as stars, via the Jeans instability. Large computer simulations of billions of dark matter particles have been used to confirm that the cold dark matter model of structure formation is consistent with the structures observed in the universe through galaxy surveys, such as the Sloan Digital Sky Survey and 2dF Galaxy Redshift Survey, as well as observations of the Lyman-alpha forest. These studies have been crucial in constructing the Lambda-CDM model which measures the cosmological parameters, including the fraction of the universe made up of baryons and dark matter. Another important tool for future dark matter observations is gravitational lensing, in particular a technique called weak lensing that allows astrophysicists to characterize the distribution of dark matter by statistical means.

Composition

Data from galaxy rotation curves indicate that nearly 90% of the mass of a galaxy cannot be seen. It can only be detected by its gravitational effect. Several categories of dark matter have been postulated.
- Hot dark matter
- Warm dark matter
- Cold dark matter
- Baryonic dark matter Hot dark matter consists of particles that travel with relativistic velocities. One kind of hot dark matter is known, the neutrino. Neutrinos have a very small mass, do not interact via either the electromagnetic or the strong nuclear force and so are incredibly difficult to detect. This is what makes them appealing as dark matter. However, bounds on neutrinos indicate that ordinary neutrinos make only a small contribution to the density of dark matter. Hot dark matter cannot explain how individual galaxies formed from the Big Bang. The microwave background radiation as measured by the COBE and WMAP satellites, while incredibly smooth, indicates that matter has clumped on very small scales. Fast moving particles, however, cannot clump together on such small scales and, in fact, suppress the clumping of other matter. Hot dark matter, while it certainly exists in our universe in the form of neutrinos, is therefore only part of the story. To explain structure in the universe it is necessary to invoke cold (non-relativistic) dark matter. Large masses, like galaxy-sized black holes can be ruled out on the basis of gravitational lensing data. Possibilities involving normal baryonic matter include brown dwarfs or perhaps small, dense chunks of heavy elements; such objects are known as massive compact halo objects, or "MACHOs". However, studies of big bang nucleosynthesis have convinced most scientists that baryonic matter such as MACHOs cannot be more than a small fraction of the total dark matter. At present, the most common view is that most dark matter is made of one or more elementary particles other than the usual electrons, protons, neutrons, and ordinary neutrinos. Currently, the most commonly considered particles are axions, sterile neutrinos, SIMPs (Strongly Interacting Massive Particles), and WIMPs (Weakly Interacting Massive Particles) (which include neutralinos). None of these are part of the standard model of particle physics. Instead, particles in this last category are frequently suggested by theorists proposing supersymmetric extensions of the standard model of particle physics. In such theories, the WIMP involved is usually the neutralino. Another candidate is so-called sterile neutrinos. Sterile neutrinos can be added to the standard model to explain the small neutrino mass. These sterile neutrinos are expected to be heavier than the ordinary neutrinos, and are a candidate for dark matter.

Alternative explanations

An alternative to dark matter is to suppose that the inconsistencies are due to an incomplete understanding of gravitation. One task could be given through the need of conciling gravitation with quantum mechanics and to explain mass and its creation (Higgs) within gravitation, as in some scalar-tensor theories, which couple scalar fields like the Higgs one to the curvature given through the Riemann tensor or its traces. In many of such theories, the scalar field equals the inflaton field, which is needed in some theories for explaining the inflation of the universe after the Big Bang, as the dominating factor of the quintessence or Dark Energy. To explain the observations, the gravitational force has to become stronger than the Newtonian approximation at great distances or in weak fields. For instance, this can be done by assuming a negative value of the cosmological constant (the value of which is believed to be positive based on recent observations) or by assuming Modified Newtonian Dynamics (MOND), which corrects Newton's laws at small acceleration. However, constructing a relativistic MOND theory has been troublesome, and it is not clear how the theory can be reconciled with gravitational lensing measurements of the deflection of light around galaxies. The leading relativistic MOND theory, proposed by Milgrom's colleague Professor Bekenstein in 2004 is called "TeVeS" for Tensor-Vector-Scalar and solves many of the problems of earlier attempts. Another approach, proposed by Finzi (1963) and again by Sanders (1984), is to replace the gravitational potential energy with the expression :U=\frac where B and ρ are adjustable parameters. However, such approaches run into difficulties explaining the different behavior of different galaxies and clusters, whereas one can easily describe such differences by assuming different quantities of dark matter. For a deeper discussion of this subject, see Modified Newtonian dynamics. Another proposed explanation of the mystery is Nonsymmetric Gravitational Theory. Two other theories which propose modifications to general relativity have recently been proposed. M. Reuter and H. Weyer have proposed that Newton's constant grows at large scales due to quantum effects [http://arxiv.org/abs/hep-th/0410117]. Another proposal by Cooperstock and Tieu suggested that the galaxy rotation problem could be explained with the results of general relativity, amplified by non-linear effects so that the behavior of the galaxy as a whole becomes non-Newtonian [http://arxiv.org/abs/astro-ph/0507619]. A problem in this model was found when it was shown that this model gives rise to a "thin, singular disk" of 2-dimensional matter in the galactic plane [http://arxiv.org/abs/astro-ph/0508377/]. In a [http://arxiv.org/abs/astro-ph/0510750 recent article] it is shown that Cooperstock's and Tieu's model implies that the thin disk must be made out of "exotic matter, either cosmic strings or struts with negative energy density". Cooperstock and Tieu have since [http://arxiv.org/abs/astro-ph/0512048 responded to the potential flaw in their model.] Their amended model has no singularity at the plane of symmetry, yet still it can explain galactic rotation without assuming dark matter exists.

Dark matter in popular culture

Mentions of dark matter occur in some video games and other works of fiction. In such cases, it is usually attributed extraordinary physical or magical properties. Such descriptions are often inconsistent with the properties of dark matter proposed in physics and cosmology.

See also


- Dark energy star

References


- Polar Magnetic Phenomena and Terrella Experiments, in The Norwegian Aurora Polaris Expedition 1902-1903 (publ. 1913, p.720 on 'dark matter')

External links


- [http://astron.berkeley.edu/~mwhite/darkmatter/hdm.html Hot dark matter]
- [http://lpsc.in2p3.fr/tep/fred/dm.html Dark matter Portal]
- [http://arxiv.org/abs/hep-ph/0404175 G. Bertone, D. Hooper, and J. Silk, "Particle Dark Matter: Evidence, Candidates and Constraints"]
- [http://www.livingreviews.org/lrr-2002-4 Timothy J. Sumner, "Experimental Searches for Dark Matter"]
- [http://www.economist.com/science/displaystory.cfm?story_id=3556105 The Economist: Young solar systems are like cosmic snooker games, and the universe is flat]
- [http://www.wired.com/news/space/0,2697,66487,00.html Scientists Find Missing Matter (Wired.com Feb 3rd 2005) ]
- [http://news.bbc.co.uk/1/hi/wales/south_east/4288633.stm Astronomers find star-less galaxy (BBC News Feb 23rd 2005) ]
- [http://www.physorg.com/news6850.html Elliptical galaxies have dark matter halo as well]
- [http://www.isracast.com/tech_news/061005_tech.htm Dark matter History and More on Elliptical galaxies and the Mystery of dark matter]
- [http://www.ebicom.net/~rsf1/missmass.htm Cosmology's Missing Mass Problems]
- [http://xxx.lanl.gov/find/grp_physics/1/abs:+AND+Dark+Matter/0/1/0/past,2005/0/1 recent papers on dark matter on arXiv.org] Category:Celestial mechanics Category:Cosmology
- Category:Large-scale structure of the cosmos ko:암흑물질 ja:暗黒物質

Radiation

Radiation can refer to one of the following:
- Alpha radiation
- Beta radiation
- Gamma radiation
- Delta radiation
- Epsilon radiation
- Neutron radiation
- Cherenkov radiation, radiation by a particle moving through an insulating medium faster than the speed of light in that medium.
- Electromagnetic radiation, a stream of photons of a variety of different energies.
- Ionizing radiation, a stream of particles with sufficient energy to cause ionization.
- Gravitational radiation, a predicted consequence of general relativity.
- Non-ionizing radiation, electromagnetic radiation that does not carry enough energy to ionize living material.
- Particle radiation, any kind of radiation in which the individual elements behave like particles.
- Synchrotron radiation, the emission of radiation by a charged particle undergoing acceleration.
- Thermal radiation, the process by which a hot object emits electromagnetic radiation.
- Radiant energy, radiation emitted by a source into the surrounding environment.
- Adaptive radiation, in evolutionary biology, a process by which one species becomes many in order to adapt to specific ecological niches. In fiction, radiation can also refer to:
- Theta radiation and Omicron radiation, which are found in Star Trek

See also


- Radioactive ko:방사선 ja:放射線 simple:Radiation

Dead time

In gaseous ionization detectors the dead time is the time after each event detected by the detector in which the detector is not able to reveal another event if it happens. In this time the detector is as if it were dead or frozen or paralysed. Even if it is really live, it is not able to count. This may be due both to real events and spurious events. In some cases the detector is not even sensitive to another event (for example in a spark chamber until the potential between the plates recovers above an high enough value); in other cases, the detector is sensitive but is not able to give out information or the information is not trustworthy. An easy example in every day life is given by the screen of Cathode ray tube television set. When a beam arrives on the screen and lights the pixel, then it remains on for a fraction of second even after the beam is stopped. This effect and the persistence effect in the retina of human eyes make it possible to view a complete image though it is composed of points of light that are illuminated at different moments. From the total time a detector is running, the dead time must be subtracted to obtain the live time of the experiment. Category:Nuclear physics

20th century

The 20th century lasted from 1901 to 2000 in the Gregorian calendar. Common usage sometimes regards it as lasting from 1900 to 1999, but this is incorrect since counting of calendar years begins with the year 1. The 20th century is also sometimes known as the nineteen hundreds (1900s). Decades are almost always considered as starting with the "0" year and named accordingly ("1960s", etc.). However, a number of arguments have been used to justify the common usage. One was advanced, erroneously, by Stephen Jay Gould. He claimed that the first decade had only nine years, thus contradicting the definition of decade equaled 10 years. Another argument is that the astronomical year numbering system for years does have a year zero, the year normally known as 1 BC. In 2000 the International Organization for Standardization clarified ISO 8601 to use the astronomical year numbering system, which could be interpreted as retrospectively endorsing all the people who had celebrated the new century a few months earlier. The term is also used to describe various periods that overlap with the calendar definition, most notably the Short twentieth century, which claims that the 20th Century spanned from 1914 to 1989, rendering the pre-WWI 1900s into the 19th Century and putting the 1990s at the beginning of the 21st Century. Indeed, the part of the 20th Century before World War I is quite identical to the late 1800s culturally and technologically and the 1990s decade pointed in many ways (such as the rise of the Internet) to the 21st Century and is seen by some as not being truly a part of the 20th Century.

Overview

The twentieth century saw a remarkable shift in the way that vast numbers of people lived, as a result of technological, medical, social, ideological, and political innovations. Terms like ideology, world war, genocide, and nuclear war entered common usage and became an influence on the lives of everyday people. War reached an unprecedented scale and level of sophistication; in the Second World War (1939-1945) alone, approximately 57 million people died, mainly due to massive improvements in weaponry. The trends of mechanization of goods and services and networks of global communication, which were begun in the 19th century, continued at an ever-increasing pace in the 20th. In spite of the terror and chaos, the 20th century saw many attempts at world peace. As the 35th President of the United States John F. Kennedy said: :What kind of peace do we seek? I am talking about a genuine peace, the kind of peace that makes life on earth worth living. Not merely peace in our time, but peace in all time. Our problems are man-made, therefore they can be solved by man. For in the final analysis, our most basic common link is that we all inhabit this small planet, we all breathe the same air, we all cherish our children's future, and we are all mortal. Virtually every aspect of life in virtually every human society changed in some fundamental way or another during the twentieth century and for the first time, any individual could influence the course of history no matter their background. Arguably, the 20th century re-shaped the face of the planet in more ways than any previous century.
- Death rates
- Infant mortality
- Infectious disease
- Life expectancy
- Maternal death rates
- Battles Scientific discoveries such as relativity and quantum physics radically changed the worldview of scientists, causing them to realize that the universe was much more complex than they had previously believed, and dashing the hopes at the end of the preceding century that the last few details of knowledge were about to be filled in. For a more coherent overview of the historical events of the century, see The 20th century in review. The 20th century has sometimes been called, both within and outside the United States, the American Century, though this is a controversial term.

Important developments, events and achievements

Science and technology


- The assembly line and mass production of motor vehicles and other goods allowed manufacturers to produce more and cheaper products. This allowed the automobile to become the most important means of transportation.
- The invention of heavier-than-air flying machines and the jet engine allowed for the world to become "smaller". Space flight increased knowledge of the rest of the universe and allowed for global real-time communications via geosynchronous satellites.
- Mass media technologies such as film, radio, and television allow the communication of political messages and entertainment with unprecedented impact
- Mass availability of the telephone and later, the computer, especially through the Internet, provides people with new opportunities for near-instantaneous communication
- Applied electronics, notably in its miniaturized form as integrated circuits, made possible the above mentioned rise of mass media, telecommunications, ubiquitous computing, and all kinds of "intelligent" appliances; as well as many advances in natural sciences such as physics, by the use of exponentially growing calculation power (see supercomputer).
- The development of Nitrogen fertilizer, pesticides and herbicides resulted in significantly higher agricultural yield.
- Advances in fundamental physics through the theory of relativity and quantum mechanics led to the development of nuclear weapons (known informally as "the Bomb" and dropped on the industrial town of Hiroshima and the historic one of Nagasaki), the nuclear reactor, and the laser. Fusion power was studied extensively but remained an experimental technology at the end of the century.
- Inventions such as the washing machine and air conditioning led to an increase in both the quantity and quality of leisure time for the middle class in Western societies.
- Most influential inventions in the 20th century: antibiotics, oral contraceptives, new plastics, transistors, Internet
- More...

Wars and politics


- Democratic nations began to extend voting privileges to all adults.
- Rising nationalism and increasing national awareness were among the causes of World War I, the first of two wars to involve all the major world powers including Germany, France, Italy, Japan, the United States and the British Commonwealth. World War I led to the creation of many new countries, especially in Eastern Europe. Ironically, it was said by many to be the 'War to end all Wars'.
- The economic and political aftermath of World War I led to the rise of Fascism and Nazism in Europe, and shortly to World War II. This war also involved Asia and the Pacific, in the form of Japanese aggression against China and the United States. While the First World War mainly cost lives among soldiers, civilians suffered greatly in the Second -- from the bombing of cities on both sides, and in the unprecedented German genocide of the Jews and others, known as the Holocaust.
- During World War I, in Russia the Bolshevik putsch led to the Russian Revolution of 1917. After the Soviet Union's involvement in World War II, Communism became a major force in global politics, spreading all over the world: notably, to Eastern Europe, China, Indochina and Cuba. This led to the Cold War and proxy wars with the western world, including wars in Korea (1950-53) and Vietnam (1957 - 75).
- The "fall of Communism" in the late 1980s freed Eastern and Central Europe from Soviet supremacy. It also led to the dissolution of the Soviet Union and Yugoslavia into successor states, many rife with ethnic nationalism, and left the United States as the world's superpower.
- Through the League of Nations and, after World War II, the United Nations, international cooperation increased. Other efforts included the formation of the European Union, leading to a common currency in much of Western Europe, the euro around the turn of the millennium.
- The end of colonialism led to the independence of many African and Asian countries. During the Cold War, many of these aligned with the USA, the USSR, or China for defense.
- The creation of Israel, a Jewish state in a mostly Arab region of the world, fueled many conflicts in the region, which were also influenced by the vast oil fields in many of the Arab countries.
- The term Southeast Asia coined.

Culture and entertainment


- Movies, music and the media had a major influence on fashion and trends in all aspects of life. As many movies and music originate from the United States, American culture spread rapidly over the world.
- After gaining political rights in the United States and much of Europe in the first part of the century, and with the advent of new birth control techniques women became more independent throughout the century.
- Rock and Roll and Jazz styles of music are developed in the United States, and quickly become the dominant forms of popular music in America, and later, the world. The Beatles, a 1960s British Rock and Roll band, becomes one of the most successful acts of all time, and is credited, in their experimental later albums, with permanently changing what was thought possible in popular music.
- Modern art developed new styles such as expressionism, cubism, and surrealism.
- The automobile provided vastly increased transportation capabilities for the average member of Western societies in the early to mid-century, spreading even further later on. City design throughout most of the West became focused on transport via car. The car became a leading symbol of modern society, with styles of car suited to and symbolic of particular lifestyles.
- Sports became an important part of society, becoming an activity not only for the privileged. Watching sports, later also on television, became a popular activity.

Disease and medicine


- Although the availability and quality of medicine continued to improve, epidemic diseases continued to spread, aided by modern transportation. An influenza pandemic, the Spanish Flu, killed 25 million between 1918 and 1919, while AIDS is yet uncured and treatments remain too expensive for wide use in developing countries.
- Advances in medicine, such as the invention of antibiotics, decreased the number of people dying from diseases. Contraceptive drugs and organ transplantation were developed. The discovery of DNA molecules and the advent of molecular biology allowed for cloning and genetic engineering.

Natural resources and the environment


- The widespread use of petroleum in industry -- both as a chemical precursor to plastics and as a fuel for the automobile and airplane -- led to the vital geopolitical importance of petroleum resources. The Middle East, home to many of the world's oil deposits, became a center of geopolitical and military tension throughout the latter half of the century. (For example, oil was a factor in Japan's decision to go to war against the United States in 1941, and the oil cartel, OPEC, used an oil embargo of sorts in the wake of the Yom Kippur War in the 1970s).
- A vast increase in fossil fuel consumption leads to depletion of natural resources, while air pollution has led to the develoment of an ozone hole and, many believe, global warming and both local and global climate change. The problem is increased by world-wide deforestation, also causing a loss of biodiversity. The problem of a depletion of natural resources is decreased by advances in drilling technology which led to a net increase in the amount of fossil fuel that is readily obtainable at the end of the century, as compared with the amount considered obtainable at the beginning of the century.

Significant people

World leaders


- Africa
  - Gnassingbe Eyadema, Togo
  - Félix Houphouët-Boigny, Côte d'Ivoire
  - Kenneth Kaunda, Zambia
  - Jomo Kenyatta, Kenya
  - Idi Amin, Uganda
  - Nelson Mandela, South Africa
  - Robert Mugabe, Zimbabwe
  - Gamal Abdal Nasser, Egypt
  - Kwame Nkrumah, Ghana
  - Julius Nyerere, Tanzania
  - Habib Bourguiba, Tunisia
  - Muammar al-Qaddafi, Libya
  - Haile Selassie, Ethiopia
  - Léopold Sédar Senghor, Senegal
  - Ahmed Sékou Touré, Guinea
- Americas
  - Juan Perón, Argentina
  - Eva Perón, Argentina
  - Getúlio Vargas, Brazil
  - Luis Carlos Prestes, Brazil
  - Juscelino Kubitschek, Brazil
  - Wilfrid Laurier, Canada
  - William Lyon Mackenzie King, Canada
  - Pierre Trudeau, Canada
  - Salvador Allende, Chile
  - Augusto Pinochet, Chile
  - Fidel Castro, Cuba
  - Ernesto 'Che' Guevara, Argentina/Cuba
  - Emiliano Zápata, Mexico
  - Pancho Villa, Mexico
  - Lázaro Cárdenas del Río, Mexico
  - Augusto César Sandino, Nicaragua
  - Fernando Belaúnde Terry, Peru
  - Alberto Kenya Fujimori, Peru
  - Theodore Roosevelt, USA
  - Woodrow Wilson,USA
  - Franklin D. Roosevelt, USA
  - Harry S Truman, USA
  - Dwight Eisenhower, USA
  - John F. Kennedy, USA
  - Lyndon B. Johnson, USA
  - Richard Nixon, USA
  - Ronald Reagan, USA
  - Bill Clinton, USA
  - George H. W. Bush, USA
  - José Batlle y Ordóñez, Uruguay
  - Romulo Betancourt, Venezuela
- Asia
  - Mahatma Gandhi, India
  - Lee Kuan Yew, Singapore
  - Ferdinand Marcos, the Philippines
  - Corazon Aquino, the Philippines
  - Mao Zedong, People's Republic of China
  - Deng Xiaoping, People's Republic of China
  - Pol Pot, Cambodia
  - Muhammad Ali Jinnah, Pakistan
  - Indira Gandhi, India
  - Mahathir Mohamad, Malaysia
  - Jawaharlal Nehru, India
  - Emperor Hirohito, Japan
  - Ho Chi Minh, Vietnam
  - Sun Yat-sen, Republic of China
  - Chiang Kai-shek, Republic of China
  - Achmad Sukarno, Indonesia
  - Suharto, Indonesia
- Australia and Oceania
  - Edmund Barton, Australia
  - Sir Robert Menzies, Australia
  - Peter Fraser, New Zealand
  - Michael Joseph Savage, New Zealand
  - David Lange, New Zealand
- Europe
  - Franz Joseph of Austria, Austria-Hungary
  - Václav Havel, Czech Republic
  - Franjo Tuđman, Croatia
  - Archbishop Makarios III,