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Metre

Metre

:This article is about the unit of length. For other uses of metre or meter, see meter (disambiguation). The metre (Commonwealth English) or meter (American English) (symbol: m) is the SI base unit of length. It is defined as the length of the path travelled by light in absolute vacuum during a time interval of 1/299,792,458 of a second. Adding SI prefixes to metre creates multiples and submultiples; for example kilometre (1000 metres; kilo- = 1000) and millimetre (one thousandth of a metre; milli- = 1 / 1 000).

Conversions

1 metre is equivalent to:
- exactly 1/0.9144 yards (approximately 1.0936 yards)
- exactly 1/0.3048 feet (approximately 3.2808 feet)
- exactly 10000/254 inches (approximately 39.370 inches)

History

The word metre is from the Greek metron (μετρον), "a measure" via the French mètre. Its first recorded usage in English is from 1797. In the 18th century, there were two favoured approaches to the definition of the standard unit of length. One suggested defining the metre as the length of a pendulum with a half-period of one second. The other suggested defining the metre as one ten-millionth of the length of the earth's meridian along a quadrant (one-fourth the polar circumference of the earth). In 1791, the French Academy of Sciences selected the meridional definition over the pendular definition because of the slight variation of the force of gravity over the surface of the earth, which affects the period of a pendulum. In 1793, France adopted the metre, with this definition, as its official unit of length. Although it was later determined that the first prototype metre bar was short by a fifth of a millimetre due to miscalculation of the flattening of the earth, this length became the standard. So, the circumference of the Earth through the poles is approximately forty million metres. Earth in a vacuum.]] In the 1870s and in light of modern precision, a series of international conferences were held to devise new metric standards. The Metre Convention (Convention du Mètre) of 1875 mandated the establishment of a permanent International Bureau of Weights and Measures (BIPM: Bureau International des Poids et Mesures) to be located in Sèvres, France. This new organisation would preserve the new prototype metre and kilogram when constructed, distribute national metric prototypes, and would maintain comparisons between them and non-metric measurement standards. This organisation created a new prototype bar in 1889 at the first General Conference on Weights and Measures (CGPM: Conférence Générale des Poids et Mesures), establishing the International Prototype Metre as the distance between two lines on a standard bar of an alloy of ninety percent platinum and ten percent iridium, measured at the melting point of ice. In 1893, the standard metre was first measured with an interferometer by Albert A. Michelson, the inventor of the device and an advocate of using some particular wavelength of light as a standard of distance. By 1925, interferometry was in regular use at the BIPM. However, the International Prototype Metre remained the standard until 1960, when the eleventh CGPM defined the metre in the new SI system as equal to 1,650,763.73 wavelengths of the orange-red emission line in the electromagnetic spectrum of the krypton-86 atom in a vacuum. The original international prototype of the metre is still kept at the BIPM under the conditions specified in 1889. To further reduce uncertainty, the seventeenth CGPM of 1983 replaced the definition of the metre with its current definition, thus fixing the length of the metre in terms of time and the speed of light: :The metre is the length of the path travelled by light in vacuum during a time interval of 1/299 792 458 of a second. Note that this definition exactly fixes the speed of light in a vacuum at 299,792,458 metres per second. Definitions based on the physical properties of light are more precise and reproducible because the properties of light are considered to be universally constant.

Timeline of definition

- 1790 May 8 — The French National Assembly decides that the length of the new metre would be equal to the length of a pendulum with a half-period of one second.
- 1791 March 30 — The French National Assembly accepts the proposal by the French Academy of Sciences that the new definition for the metre be equal to one ten-millionth of the length of the earth's meridian along a quadrant (one-fourth the polar circumference of the earth).
- 1795 — Provisional metre bar constructed of brass.
- 1799 December 10 — The French National Assembly specifies that the platinum metre bar, constructed on 23 June 1799 and deposited in the National Archives, as the final standard.
- 1889 September 28 — The first CGPM defines the length as the distance between two lines on a standard bar of an alloy of platinum with ten percent iridium, measured at the melting point of ice.
- 1927 October 6 — The seventh CGPM adjusts the definition of the length to be the distance, at 0 °C, between the axes of the two central lines marked on the prototype bar of platinum-iridium, this bar being subject to one standard atmosphere of pressure and supported on two cylinders of at least one centimetre diameter, symmetrically placed in the same horizontal plane at a distance of 571 millimetres from each other.
- 1960 October 20 — The eleventh CGPM defines the length to be equal to 1,650,763.73 wavelengths in vacuum of the radiation corresponding to the transition between the 2p¹⁰ and 5d⁵ quantum levels of the krypton-86 atom.
- 1983 October 21 — The seventeenth CGPM defines the length to be distance travelled by light in vacuum during a time interval of 1/299 792 458 of a second.

External links

- [http://www.unitconversion.org/unit_converter/length.html?unit=meter&value=1 Length Converter: convert metre to other units, such as yard, mile, and so on]
- [http://physics.nist.gov/cuu/Units/meter.html History of the metre at the U.S. National Institute of Standards and Technology (NIST)]
- [http://www.mel.nist.gov/div821/museum/timeline.htm Timeline of history of the metre at the NIST]
- [http://www1.bipm.org/en/scientific/length/ Bureau International des Poids et Measures - Lengths] Category:SI base units Category:Units of length ko:미터 ms:Meter ja:メートル simple:Metre th:เมตร

Meter (disambiguation)

Meter or metre can mean:
- Metre, a unit of measurement
- Meter (poetry), the regular linguistic sound patterns of verse
- Metre (music), the regular rhythmic patterns of music
- The Meters, a funk band
- Meter, or measuring instrument, including electric meter, gas meter, parking meter, postage meter
- Meter maid, or parking attendant, a person who reads a meter In Commonwealth and Canadian English the unit of length and the poetical or musical concepts are spelt metre, but a measuring device is spelt meter. In American English all of the above are spelt meter. See also American and British English differences. ja:メーター

American English

American English (AmE) is the dialect of the English language used mostly in the United States of America. Crystal (1997) estimates that approximately two thirds of native speakers of English live in the United States. American English is also sometimes called United States English or U.S. English.

History

English was inherited from British colonization. The first wave of English-speaking immigrants was settled in North America in the 17th century. In that century, there were also speakers in North America of the Dutch, French, German, myriad Native American, Spanish, Swedish, Scots, Welsh, Irish, Scottish Gaelic and Finnish languages.

Phonology

In many ways, compared to British English, American English is conservative in its phonology. The conservatism of American English is largely the result of the fact that it represents a mixture of various dialects from the British Isles. Dialect in North America is most distinctive on the East Coast of the continent; this is largely because these areas were in contact with England, and imitated prestigious varieties of British English at a time when those varieties were undergoing changes. The interior of the country was settled by people who were no longer closely connected to England, as they had no access to the ocean during a time when journeys to Britain were always by sea. As such the inland speech is much more homogeneous than the East Coast speech, and did not imitate the changes in speech from England. East Coast-influenced non-rhotic pronunciations may be found among blacks throughout the country.]]Most North American speech is rhotic, as English was in most places in the 17th century. Rhoticity was further supported by Hiberno-English, Scottish English, and West Country English. In most varieties of North American English, the sound corresponding to the letter "R" is a retroflex semivowel rather than a trill or a tap. The loss of syllable-final r in North America is confined mostly to the accents of eastern New England, New York City and surrounding areas, South Philadelphia, and the coastal portions of the South. Dropping of syllable-final r sometimes happens in natively rhotic dialects if r is located in unaccented syllables or words and the next syllable or word begins in a consonant. In England, lost 'r' was often changed into (schwa), giving rise to a new class of falling diphthongs. Furthermore, the 'er' sound of (stressed) fur or (unstressed) butter, which is represented in IPA as stressed or unstressed is realized in American English as a monophthongal r-colored vowel. This does not happen in the non-rhotic varieties of North American speech. Some other British English changes in which most North American dialects do not participate:
- The shift of to (the so-called "broad A") before alone or preceded by . This is the difference between the British Received Pronunciation and American pronunciation of bath and dance. In the United States, only linguistically conservative eastern-New-England speakers took up this innovation.
- The shift of intervocalic to glottal stop , as in for bottle. This change is not universal for British English (and in fact is not considered to be part of Received Pronunciation), but it does not occur in most North American dialects. Newfoundland English and the dialect of New Britain, Connecticut are notable exceptions. On the other hand, North American English has undergone some sound changes not found in Britain, at least not in standard varieties. Many of these are instances of phonemic differentiation and include
- The merger of and , making father and bother rhyme. This change is nearly universal in North American English, occurring almost everywhere except for parts of eastern New England, like the Boston accent.
- The replacement of the lot vowel with the strut vowel in what, was, of, from, everybody, nobody, somebody, anybody, because, and in some dialects want.
- The merger of and . This is the so-called cot-caught merger, where cot and caught are homophones. This change has occurred in eastern New England, in Pittsburgh and surrounding areas, and from the Great Plains westward.
- Vowel merger before intervocalic . Which (if any) vowels are affected varies between dialects.
- The merger of and after palatals in some words, so that cure, pure, mature and sure rhyme with fir in some speech registers for some speakers.
- Dropping of after alveolar consonants so that new, duke, Tuesday, suit, resume, lute are pronounced , , , , , .
- Æ-tensing in environments that vary widely from accent to accent. In some accents, particularly those from Philadelphia to New York City, and can even contrast sometimes, as in Yes, I can vs. tin can .
- Laxing of , and to , and before , causing pronunciations like , and for pair, peer and pure.
- The flapping of intervocalic and to alveolar tap before reduced vowels. The words ladder and latter are mostly or entirely homophonous, possibly distinguished only by the length of preceding vowel. For some speakers, the merger is incomplete and 't' before a reduced vowel is sometimes not tapped following or when it represents underlying 't'; thus greater and grader, and unbitten and unbidden are distinguished. Even among those words where and are flapped, words that would otherwise be homophonous are, for some speakers, distinguished if the flapping is immediately preceded by the diphthongs or ; these speakers tend to pronounce writer with and rider with . This is called Canadian raising; it is general in Canadian English, and occurs in some northerly versions of American English as well (often just applying to the diphthong , but not to ).
- Both intervocalic and may be realized as or , making winter and winner homophones. This does not occur when the second syllable is stressed, as in entail.
- The pin-pen merger, by which is raised to before nasal consonants, making pairs like pen/pin homophonous. This merger originated in Southern American English but is now widespread in the Midwest and West as well. Some mergers found in most varieties of both American and British English include:
- The horse-hoarse merger of the vowels and before 'r', making pairs like horse/hoarse, corps/core, for/four, morning/mourning etc. homophones.
- The wine-whine merger making pairs like wine/whine, wet/whet, Wales/whales, wear/where etc. homophones. Many older varieties of southern and western American English still keep these distinct, but the merger appears to be spreading.

Differences in British English and American English

Main article: American and British English differences American English has both spelling and grammatical differences from British English (or Commonwealth English), some of which were made as part of an attempt to rationalize the English spelling used by British English at the time. Unlike many 20th century language reforms (for example, Turkey's alphabet shift, Norway's spelling reform) the American spelling changes were not driven by government, but by textbook writers and dictionary makers. The first American dictionary was written by Noah Webster in 1828. At the time America was a relatively new country and Webster's particular contribution was to show that the region spoke a different dialect from Britain, and so he wrote a dictionary with many spellings differing from the standard. Many of these changes were initiated unilaterally by Webster. Webster also argued for many "simplifications" to the idiomatic spelling of the period. Somewhat ironically, many, although not all, of his simplifications fell into common usage alongside the original versions, resulting in a situation even more confused than before. Many words are shortened and differ from other versions of English. Spellings such as center are used instead of centre in other versions of English. Conversely, American English sometimes favors words that are morphologically more complex, whereas British English uses clipped forms, such as AmE transportation and BrE transport or where the British form is a back-formation, such as AmE burglarize and BrE burgle (from burglar).

English words that arose in the U.S.

A number of words that arose in the United States have become common, to varying degrees, in English as it is spoken internationally. Although its origin is disputed, the most famous word is probably OK, which is sometimes used in other languages as well. Other American introductions include "belittle," "gerrymander" (from Elbridge Gerry), "blizzard", "teenager", and many more.

English words obsolete outside the U.S.

A number of words that originated in the English of the British Isles are still in everyday use in North America, but are no longer used in most varieties of British English. The most conspicuous of these words are fall, the season; to quit, as in "to cease an activity" (as opposed to "to leave a location" as still used in most other Anglophone countries); and gotten as a past participle of get. Americans are more likely than Britons to name a stream whose breadth or volume is judged insufficient for it to be a river or a creek. The word diaper goes back at least to Shakespeare, and usage was maintained in the U.S. and Canada, but was replaced in the British Isles with nappy. Some of these words are still used in various dialects of the British Isles, but not in formal standard British English. Many of these older words have cognates in Lowland Scots. The subjunctive mood is livelier in North American English than it is in British English; it appears in some areas as a spoken usage, and is considered obligatory in more formal contexts in American English. British English has a strong tendency to replace subjunctives with auxiliary verb constructions.

Regional differences

Main article: American English regional differences Spoken American English is not homogeneous throughout the country, and various regional and ethnic variants exist. These differences affect both pronunciation and the lexicon, and can make one accent a little difficult for speakers of another accent to understand. General American is the name given to any American accent that is relatively free of noticeable regional influences. It enjoys high prestige among Americans, but is not a standard accent in the way that Received Pronunciation is in England.

References

External links

- [http://www.pbs.org/speak/ Do You Speak American]: PBS special
- [http://cfprod01.imt.uwm.edu/Dept/FLL/linguistics/dialect/ Dialect Survey] of the United States, by Bert Vaux et al., Harvard University. The answers to various questions about pronunciation, word use etc. can be seen in relationship to the regions where they are predominant.
- [http://www.ling.upenn.edu/phono_atlas/home.html Phonological Atlas of North America] at the University of Pennsylvania
- [http://students.csci.unt.edu/~kun Guide to Regional English Pronunciation] includes working versions of the Telsur Project maps from the Phonologial Atlas site
- [http://www.peak.org/~jeremy/dictionary/ The American•British British•American Dictionary]
- [http://classweb.gmu.edu/accent/ Speech Accent Archive]
- [http://www.world-english.org/ World English Organization]
- [http://www.esuus.org English Speaking Union of the United States]
- [http://canadianenglish1.narod.ru American Canadian British English Lexical Differences In One Table]
- [http://australianenglish1.narod.ru Australian American British English Lexical Differences In One Table And More]
- [http://www.englisch-hilfen.de/en/words_list/british_american.htm British, American, Australian English - Lists and Online Exercises]
- [http://www.globalenglishsalon.com/ Listen to spoken American English (midwest

Length

:This article is about the concept and measurement of distance. For usage in cricket, see line and length. In general English usage, length (symbols: l, L) is but one particular instance of distance – an object's length is how long the object is – but in the physical sciences and engineering, the word length is in some contexts used synonymously with "distance". Height is vertical distance; width (or breadth) is a lateral distance; an object's width is less than its length. No one speaks of "the length from here to Alpha Centauri", but rather of "the distance from here to Alpha Centauri," but when one speaks of distance more abstractly, one says "A kilometre or a mile, is a unit of length" or "...of distance", and the two statements are synonymous. Likewise, a mountain might be a mile in height. Length is the metric of one dimension of space. The metric of space itself is volume, or (length)³. Length is commonly considered to be one of the fundamental units, meaning that it cannot be defined in terms of other dimensions. However, a set of units can be constructed where units of length can be derived from fundamental physical constants - see Planck units and Planck length. Colloquially length sometimes refers to duration, especially when used in context of music.

Units of length(SI)

The SI unit of Length is the metre (U.S. spelling: meter), from which can be derived:from the regular basis of the foundation of the whole world
- centimetre
- kilometre

Other units of length

The Imperial and US customary units of length

- inch
- foot
- yard
- mile

Units are used in astronomy

- Astronomical unit
- Light year
- Parsec

External links

- [http://www.unitconversion.org/unit_converter/length.html Length Converter: convert between units of length, such as meter, yard, mile, and so on]
- [http://www.unitconversion.org/unit_converter/length-v.html Length Conversion table: convert selected unit to all other units of length]
- [http://calc.skyrocket.de/en/ Online Unit Converter - Conversion of many different units]
- Category:Norm ko:길이 ja:長さ

Light

Light is electromagnetic radiation with a wavelength that is visible to the eye (visible light) or, in a technical or scientific context, electromagnetic radiation of any wavelength. The three basic dimensions of light (i.e., all electromagnetic radiation) are:
- Intensity (or brilliance or amplitude), which is related to the human perception of brightness of the light,
- Frequency (or wavelength), perceived by humans as the color of the light, and
- Polarization (or angle of vibration), which is not perceptible by humans under ordinary circumstances. Due to wave-particle duality, light simultaneously exhibits properties of both waves and particles. The precise nature of light is one of the key questions of modern physics.

Visible electromagnetic radiation

Visible light is the portion of the electromagnetic spectrum between the frequencies of 380 THz (3.8×10¹⁴ hertz) and 750 THz (7.5×10¹⁴ hertz). The speed (

c

), frequency (

f

\nu

), and wavelength (

\lambda

) of a wave obey the relation: :

c = f~\lambda \,\!

Because the speed of light in a vacuum is fixed, visible light can also be characterised by its wavelength of between 400 nanometres (abbreviated 'nm') and 800 nm (in a vacuum). Light entering the eye is absorbed by light-sensitive pigments within the rod cells and cone cells in the retina, triggering a cascade of events that creates electrical nerve impulses that travel through the optic nerve to the brain, producing vision.

Speed of light

Although some people speak of the "velocity of light", the word velocity should be reserved for vector quantities, that is, those with both magnitude and direction. The speed of light is a scalar quantity, having only magnitude and no direction, and therefore speed is the correct term. The speed of light has been measured many times, by many physicists. The best early measurement is Ole Rømer's (a Danish physicist), in 1676. By observing the motions of Jupiter and one of its moons, Io, with a telescope, and noting discrepancies in the apparent period of Io's orbit, Rømer calculated a speed of 227,000 kilometres per second (approximately 141,050 miles per second). The first successful measurement of the speed of light using an earthbound apparatus was carried out by Hippolyte Fizeau in 1849. Fizeau directed a beam of light at a mirror several thousand metres away, and placed a rotating cog wheel in the path of the beam from the source to the mirror and back again. At a certain rate of rotation, the beam could pass through one gap in the wheel on the way out and the next gap on the way back. Knowing the distance to the mirror, the number of teeth on the wheel, and the rate of rotation, Fizeau measured the speed of light as 313,000 kilometres per second. Léon Foucault used rotating mirrors to obtain a value of 298,000 km/s (about 185,000 miles/s) in 1862. Albert A. Michelson conducted experiments on the speed of light from 1877 until his death in 1931. He refined Foucault's results in 1926 using improved rotating mirrors to measure the time it took light to make a round trip from Mt. Wilson to Mt. San Antonio in California. The precise measurements yielded a speed of 186,285 mile/s (299,796 km/s [1,079,265,600 km/h]). In daily use, the figures are rounded off to 300,000 km/s and 186,000 miles/s.

Refraction

All light propagates at a finite speed. Even moving observers always measure the same value of c, the speed of light in vacuum, as c = 299,792,458 metres per second (186,282.397 miles per second). When light passes through a transparent substance, such as air, water or glass, its speed is reduced, and it undergoes refraction. The reduction of the speed of light in a denser material can be indicated by the refractive index, n, which is defined as: :

n = \frac \;\!

Thus, n=1 in a vacuum and n>1 in matter. When a beam of light enters a medium from vacuum or another medium, it keeps the same frequency and changes its wavelength. If the incident beam is not orthogonal to the edge between the media, the direction of the beam will change. Refraction of light by lenses is used to focus light in magnifying glasses, spectacles and contact lenses, microscopes and refracting telescopes.

Optics

The study of light and the interaction of light and matter is termed optics. The observation and study of optical phenomena such as rainbows offers many clues as to the nature of light as well as much enjoyment.

Color and wavelengths

The different wavelengths are detected by the human eye and then interpreted by the brain as colors, ranging from red at the longest wavelengths of about 700 nm. (lowest frequencies) to violet at the shortest wavelengths of about 400 nm. (highest frequencies). The intervening frequencies are seen as orange, yellow, green, cyan, blue, and, conventionally, indigo.
indigo
The wavelengths of the electromagnetic spectrum immediately outside the range that the human eye is able to perceive are called ultraviolet (UV) at the short wavelength (high frequency) end and infrared (IR) at the long wavelength (low frequency) end. Some animals, such as bees, can see UV radiation while others, such as pit viper snakes, can see infrared light. UV radiation is not normally directly perceived by humans except in a very delayed fashion, as overexposure of the skin to UV light can cause sunburn, or skin cancer, and underexposure can cause vitamin D deficiency. However, because UV is a higher frequency radiation than visible light, it very easily can cause materials to fluoresce visible light. Cameras that can detect IR and convert it to light are called, depending on their application, night-vision cameras or infrared cameras. These are different from image intensifier cameras, which only amplify available visible light. When intense radiation (of any frequency) is absorbed in the skin, it causes heating which can be felt. Since hot objects are strong sources of infrared radiation, IR radiation is commonly associated with this sensation. Any intense radiation that can be absorbed in the skin will have the same effect, however.

Measurement of light

The following quantities and units are used to measure the quantity or "brightness" of light. Light can also be characterised by:
- amplitude,
- color, wavelength, or frequency, and
- polarization (or angle of vibration).

Light sources

polarization There are many sources of light. The most common light sources are thermal: a body at a given temperature emits a characteristic spectrum of black body radiation. Examples include sunlight (the radiation emitted by the chromosphere of the Sun at around 6,000 K peaks in the visible region of the electromagnetic spectrum), incandescent light bulbs (which emit only around 10% of their energy as visible light and the remainder as infrared), and glowing solid particles in flames. The peak of the blackbody spectrum is in the infrared for relatively cool objects like human beings. As the temperature increases, the peak shifts to shorter wavelengths, producing first a red glow, then a white one, and finally a blue color as the peak moves out of the visible part of the spectrum and into the ultraviolet. These colors can be seen when metal is heated to "red hot" or "white hot". The blue color is most commonly seen in a gas flame or a welder's torch. Atoms emit and absorb light at characteristic energies. This produces "emission lines" in the spectrum of each atom. Emission can be spontaneous, as in light-emitting diodes, gas discharge lamps (such as neon lamps and neon signs, mercury-vapor lamps, etc.), and flames (light from the hot gas itself—so, for example, sodium in a gas flame emits characteristic yellow light). Emission can also be be stimulated, as in a laser or a microwave maser. Acceleration of a free charged particle, such as an electron, can produce visible radiation: cyclotron radiation, synchrotron radiation, and bremsstrahlung radiation are all examples of this. Particles moving through a medium faster than the speed of light in that medium can produce visible Cherenkov radiation. Certain chemicals produce visible radiation by chemoluminescence. In living things, this process is called bioluminescence. For example, fireflies produce light by this means, and boats moving through water can disturb plankton which produce a glowing wake. Certain substances produce light when they are illuminated by more energetic radiation, a process known as fluorescence. This is used in fluorescent lights. Some substances emit light slowly after excitation by more energetic radiation. This is known as phosphorescence. Phosphorescent materials can also be excited by bombarding them with subatomic particles. Cathodoluminescence is one example of this. This mechanism is used in cathode ray tube televisions. Certain other mechanisms can produce light:
- scintillation
- scintillator
- electroluminescence
- sonoluminescence
- triboluminescence
- radioactive decay
- particle-antiparticle annihilation

Theories about light

Early Greek ideas

In 55 BC Lucretius, continuing the ideas of earlier atomists, wrote that light and heat from the Sun were composed of minute particles. Ptolemy also wrote about the refraction of light.

10th century optical theory

The scientist Abu Ali al-Hasan ibn al-Haytham (965-c.1040), also known as Alhazen, developed a broad theory that explained vision, using geometry and anatomy, which stated that each point on an illuminated area or object radiates light rays in every direction, but that only one ray from each point, which strikes the eye perpendicularly, can be seen. The other rays strike at different angles and are not seen. He used the example of the pinhole camera, which produces an inverted image, to support his argument. Alhazen held light rays to be streams of minute particles that travelled at a finite speed. He improved Ptolemy's theory of the refraction of light. Alhazen's work did not become known in Europe until the late 16th century.

The 'plenum'

René Descartes (1596-1650) held that light was a disturbance of the plenum, the continuous substance of which the universe was composed. In 1637 he published a theory of the refraction of light which wrongly assumed that light travelled faster in a denser medium, by analogy with the behaviour of sound waves. Descartes' theory is often regarded as the forerunner of the wave theory of light.

Particle theory

Pierre Gassendi (1592-1655), an atomist, proposed a particle theory of light which was published posthumously in the 1660s. Isaac Newton studied Gassendi's work at an early age, and preferred his view to Descartes' theory of the plenum. He stated in his Hypothesis of Light of 1675 that light was composed of corpuscles (particles of matter) which were emitted in all directions from a source. One of Newton's arguments against the wave nature of light was that waves were known to bend around obstacles, while light travelled only in straight lines. He did, however, explain the phenomenon of the diffraction of light (which had been observed by Francesco Grimaldi) by allowing that a light particle could create a localised wave in the aether. Newton's theory could be used to predict the reflection of light, but could only explain refraction by incorrectly assuming that light accelerated upon entering a denser medium because the gravitational pull was greater. Newton published the final version of his theory in his Opticks of 1704. His reputation helped the particle theory of light to dominate physics during the 18th century.

Wave theory

In the 1660s, Robert Hooke published a wave theory of light. Christian Huygens worked out his own wave theory of light in 1678, and published it in his Treatise on light in 1690. He proposed that light was emitted in all directions as a series of waves in a medium called the aether. As waves are not affected by gravity, it was assumed that they slowed down upon entering a denser medium. The wave theory predicted that light waves could interfere with each other like sound waves (as noted in the 18th century by Thomas Young), and that light could be polarized. Young showed by means of a diffraction experiment that light behaved as waves. He also proposed that different colors were caused by different wavelengths of light, and explained color vision in terms of three-colored receptors in the eye. Another supporter of the wave theory was Euler. He argued in Nova theoria lucis et colorum (1746) that diffraction could more easily be explained by a wave theory. Later, Fresnel independently worked out his own wave theory of light, and presented it to the Académie des Sciences in 1817. Simeon Denis Poisson added to Fresnel's mathematical work to produce a convincing argument in favour of the wave theory, helping to overturn Newton's corpuscular theory. The weakness of the wave theory was that light waves, like sound waves, would need a medium for transmission. A hypothetical substance called the luminiferous aether was proposed, but its existence was cast into strong doubt by the Michelson-Morley experiment. Newton's corpuscular theory implied that light would travel faster in a denser medium, while the wave theory of Huygens and others implied the opposite. At that time, the speed of light could not be measured accurately enough to decide which theory was correct. The first to make a sufficiently accurate measurement was Léon Foucault, in 1850. His result supported the wave theory, and the classical particle theory was finally abandoned.

Electromagnetic theory

In 1845, Faraday discovered that the angle of polarisation of a beam of light as it passed through a polarising material could be altered by a magnetic field, an effect now known as Faraday rotation. This was the first evidence that light was related to electromagnetism. Faraday proposed in 1847 that light was a high-frequency electromagnetic vibration, which could propagate even in the absence of a medium such as the aether. Faraday's work inspired James Clerk Maxwell to study electromagnetic radiation and light. Maxwell discovered that self-propagating electromagnetic waves would travel through space at a constant speed, which happened to be equal to the previously measured speed of light. From this, Maxwell concluded that light was a form of electromagnetic radiation: he first stated this result in 1862 in On Physical Lines of Force. In 1873, he published A Treatise on Electricity and Magnetism, which contained a full mathematical description of the behaviour of electric and magnetic fields, still known as Maxwell's equations. The technology of radio transmission was, and still is, based on this theory. The constant speed of light predicted by Maxwell's equations contradicted the mechanical laws of motion that had been unchallenged since the time of Galileo, which stated that all speeds were relative to the speed of the observer. A solution to this contradiction would later be found by Albert Einstein.

Particle theory revisited

The wave theory was accepted until the late 19th century, when Einstein described the photoelectric effect, by which light striking a surface caused electrons to change their momentum, which indicated a particle-like nature of light. This clearly contradicted the wave theory, and for years physicists tried in vain to resolve this contradiction.

Quantum theory

In 1900, Max Planck described quantum theory, in which light is considered to be as a particle that could exist in discrete amounts of energy only. These packets were called quanta, and the particle of light was given the name photon, to correspond with other particles being described around this time, such as the electron and proton. A photon has an energy, E, proportional to its frequency, f, by :

E_f = hf = \frac \,\!

where h is Planck's constant,

\lambda

is the wavelength and c is the speed of light. As it originally stood, this theory did not explain the simultaneous wave-like nature of light, though Planck would later work on theories that did. The Nobel Committee awarded Planck the Physics Prize in 1918 for his part in the founding of quantum theory.

Wave-particle duality

The modern theory that explains the nature of light is wave-particle duality, described by Albert Einstein in the early 1900s, based on his work on the photoelectric effect and Planck's results. Einstein determined that the energy of a photon is proportional to its frequency. More generally, the theory states that everything has both a particle nature and a wave nature, and various experiments can be done to bring out one or the other. The particle nature is more easily discerned if an object has a large mass, so it took until an experiment by Louis de Broglie in 1924 to realise that electrons also exhibited wave-particle duality. Einstein received the Nobel Prize in 1921 for his work with the wave-particle duality on photons, and de Broglie followed in 1929 for his extension to other particles.

A light wave

1929 that oscillate perpendicular to each other and to the direction of motion (a transverse wave).]] The electric and magnetic fields are perpendicular to the direction of travel and to each other. This picture depicts a very special case, linearly polarized light. See Polarization for a description of the general case and an explanation of linear polarization. While these relations of the electric and magnetic fields are always true, the subtle difference in the general case is that the direction and amplitude of the magnetic (or electric) field can vary, in one place, with time, or, in one instant, can vary along the direction of propagation.

Vacuum

For other uses, see vacuum cleaner and Vacuum (musical group). The root of the word vacuum is the Latin word vacuum (pl. vacua) which means a space devoid of matter. In physics, a vacuum is the absence of matter in a volume of space.

Vacuum ranges

Vacuum ranges are defined as follows:

Perfect vacuum

A perfect vacuum is an ideal state that cannot practically be obtained in a laboratory, nor even in outer space, where there are a few hydrogen atoms per cubic centimeter at 10⁻¹⁴ pascal or 10⁻¹⁶ torr. In modern day usage vacuum is considered to exist in an enclosed space or chamber, when the pressure of gaseous environment is lower than atmospheric pressure (760 Torr or 101 kPa), or has been reduced as much as necessary to prevent the influence of some gas on a process being carried out in that space.

Partial vacuum

Physicists use the term partial vacuum to describe real-life non-ideal vacuum. A complete characterization of the physical state would require further parameters, such as temperature. The antithesis of a vacuum, which is also an ideal unachievable state, is called a plenum. In engineering, a vacuum is any region where the gas pressure is less than atmospheric pressure. Engineers measure the degree of vacuum in units of pressure. The SI unit of pressure is the pascal (abbreviation Pa), but vacuum is usually measured in millimeters of mercury (mmHg) or torr, with 1 mmHg or 1 torr equaling 133.3223684 pascals. It is often also measured using the barometric scale, or as a percentage of atmospheric pressure in bars or atms. For commercial purposes, vacuum is often measured in inches of mercury (inHg). This means that the pressure in vacuum, when specified in inches of mercury, is equal to the specified inches of mercury subtracted from 29.92. Thus a vacuum of 26 inHg is equivalent to a pressure of (29.92 - 26) or 3.92 inHg. Here, 29.92 inHg means perfect vacuum.

Degrees of vacuum

- Atmospheric pressure = variable, but standardised at 101.325 kPa (760 Torr) or 760 mm of mercury
- Vacuum cleaner = approximately 80 kPa (600 Torr)
- Mechanical vacuum pump = approximately 100 Pa to 100 μPa (1 Torr to 10⁻⁶ Torr)
- Near earth outer space = approximately 100 μPa (10⁻⁶ Torr)
- Cryopumped MBE chamber = 100 nPa to 1 nPa (10⁻⁹ Torr to 10⁻¹¹ Torr)
- Pressure on the Moon = approximately 1 nPa (10⁻¹¹ Torr)
- Interstellar space = approximately 1 fPa (10⁻¹⁷ Torr)
- [http://www-ssg.sr.unh.edu/ism/what1.html Source for interstellar vacuum] As gas pressure decreases, the mean free path (MFP) of the gas molecules increases. When the MFP is greater than the chamber, pump, spacecraft, or other objects present, the continuum assumptions of fluid mechanics do not apply. This vacuum state is called high vacuum, and the study of fluid flows in this regime is called particle gas dynamics. In interplanetary and interstellar space, isotropic gas pressure is insignificant when compared to solar pressure, solar wind, and dynamic pressure. Astrophysicists prefer to use density to describe these environments, in units of particles per cubic metre.

Creating a vacuum

The easiest way to create an artificial vacuum is to expand the volume of a container. For example, your muscles expand your lungs to create a partial vacuum inside them, and air rushes in to fill the vacuum. By repeatedly closing off a compartment of the vacuum and exhausting it, it is possible to pump air out of a chamber of fixed size in a manner analogous to pumping a milkshake out of a glass. This is the principle behind most mechanical vacuum pumps. Inside the pump, a mechanism expands a small sealed cavity to create a deep vacuum. Because of the pressure differential, some air from the chamber is pushed into the pump's small cavity. The pump's cavity is then sealed from the chamber, opened to the atmosphere, and squeezed back to a minute size. A mechanical vacuum pump moves the same volume of gas with each cycle, but as the chamber's pressure drops, this volume contains less and less mass. So although the pumping speed remains constant when measured in litres/second, it drops exponentially when measured in kilograms/second. Meanwhile, the leakage rates, evaporation rates, and sublimation rates produce a constant mass flow into the system. When the pump's mass flow drops to the same level as the mass flows into the chamber, the system asymptotically approaches a constant pressure called the base pressure. Evaporation and sublimation into a vacuum is called outgassing, and the most common source is water absorbed by materials in the chamber. Outgassing can be reduced by desiccation prior to vacuum pumping. The base pressure of a rubber- and plastic-sealed piston pump system is typically 1 to 50 kPa, while a scroll pump might reach 10 Pa and a rotary vane oil pump with a clean and empty metallic chamber can easily achieve 0.1 Pa. If the dominant mass flow into the vacuum system is chamber leakage or outgassing of materials under vacuum, then the vacuum can be improved simply by installing bigger pumps. However, there is a point where backstream leakage through the pump and outgassing of the pump oils become the dominant mass flows into the chamber. In this situation, the vacuum will approach the pump's ultimate pressure - the best vacuum that this type of pump can achieve under ideal conditions. Adding more pumps in parallel or bigger pumps of the same type can still improve the pump-down speed, but they will not reduce the base pressure below ultimate. Better pumping technologies must be used to go beyond this barrier.

High vacuum

Fortunately, once the pressure has dropped below 1 kPa or so, another vacuum pumping technique becomes possible. Matter flows differently at different pressures based on the laws of fluid dynamics. At atmospheric pressure and mild vacuums, molecules interact with each other and push on their neighboring molecules in what is known as viscous flow. When the distance between the molecules increases, the molecules interact with the walls of the chamber more often than the other molecules, and molecular pumping becomes more effective than compression pumping. This regime is generally called high vacuum. One such method to create a high vacuum to ultra high vacuum is by the use of cryopumps. Cryopumping incorporates the use of introducing cryogenics and a vacuum system. On a larger scale, the principles are the same as in a Cryomodule Molecular pumps sweep out a larger area than mechanical pumps, and do so more frequently, making them capable of much higher pumping speeds as measured in volume per time. They do this at the expense of the seal between the vacuum and their exhaust. Since there is no seal, a small pressure at the exhaust can easily force flow backstream through the pump; this is called stall. In high vacuum, however, pressure gradients have little effect on fluid flows, and molecular pumps can attain their full potential. The two main types of molecular pumps are the diffusion pump and the turbomolecular pump. Both types of pumps blow out gas molecules that diffuse into the pump. Diffusion pumps blow out molecules with jets of oil, while turbomolecular pumps use high speed fans. Both of these pumps will stall and fail to pump if exhausted directly to atmospheric pressure, so they must be exhausted to a lower grade vacuum created by a mechanical pump. As with mechanical pumps, the base pressure will be reached when leakage, outgassing, and backstreaming equal the pump speed, but now minimizing leakage and outgassing to a level comparable to backstreaming becomes much more difficult. High vacuum systems generally require metal chambers with metal O-ring seals such as Klein flanges or ISO flanges. The system must be clean and free of organic matter to minimize outgassing. All materials, solid or liquid, have a small vapour pressure, and their outgassing becomes important when the vacuum pressure falls below this vapour pressure. As a result, many materials that work well in low vacuums, such as epoxy, will become a problematic source of outgassing when attempting to achieve high vacuums. With these standard precautions, vacuums of 1 mPa are easily achieved with off-the-shelf molecular pumps. With careful design and operation, 1μPa is possible.

Ultra-high vacuum

:Main article: Ultra high vacuum Even higher vacuums are possible, but they generally require custom-built equipment, strict operational procedures, and a fair amount of trial-and-error. Yet more specialized pumps become useful: # Converting the molecules of gas to their solid phase by freezing them, called cryopumping or cryotrapping # Converting them to solids by electrically combining them with other materials, called ion pumping Ultra-high vacuum systems are usually made of stainless steel with metal-gasketed conflat flanges. The system is usually baked, preferably under vacuum, to temporarily raise the vapour pressure of all outgassing materials in the system and boil them off. If necessary, this outgassing of the system can also be performed at room temperature, but this takes much more time. Once the bulk of the outgassing materials are boiled off and evacuated, the system may be cooled to lower vapour pressures and minimize residual outgassing during actual operation. Some systems are cooled well below room temperature by liquid nitrogen to shut down residual outgassing and simultaneously cryopump the system. In ultra-high vacuum systems, some very odd leakage paths and outgassing sources must be considered. The water absorption of aluminium and palladium becomes an unacceptable source of outgassing, and even the absorptivity of hard metals such as stainless steel or titanium must be considered. Some oils and greases will boil off in extreme vacuums. The porosity of the metallic chamber walls may have to be considered, and the grain direction of the metallic flanges should be parallel to the flange face. The impact of molecular size must be considered. Smaller molecules can leak in more easily and are more easily absorbed by certain materials, and molecular pumps are less effective at pumping gases with lower molecular weights. Your system may be able to evacuate nitrogen, (the main component of air,) to the desired vacuum, but your chamber could still be full of residual atmospheric hydrogen and helium. Vessels lined with a highly gas-permeable material such as palladium (which is a high-capacity hydrogen sponge) create special outgassing problems. The lowest pressures currently achievable in laboratory are about 10^-13 Pa.

Vacuum in space

Pa Much of outer space has the density and pressure of an almost perfect vacuum. It is cold and has no friction. The properties of the vacuum remain largely unknown. A perfect vacuum is an ideal state that cannot practically be obtained in a laboratory, nor even in outer space, where there are a few hydrogen atoms per cubic centimeter at 10−14 pascal or 10−16 torr. All of the observable universe is also filled with large numbers of photons, the so-called cosmic background radiation, and quite likely a correspondingly large number of neutrinos. The current temperature is about 3 K, being merely 3 degrees above the absolute zero of temperature. Neither these photons nor the neutrinos produce a significant interaction with matter, so stars, planets and spacecraft move freely in this near perfect vacuum of interstellar space. Stars, planets and moons keep their atmosphere by gravitational attraction, so atmospheres have no firm boundary. The density of gas decreases with distance from the object. In Low Earth Orbit (about 300 km altitude) the atmospheric density is still sufficient to produce significant drag on satellites. Most Earth satellites operate in this region, and they need to fire their engines every few days to maintain orbit. The atmosphere in Low Earth Orbit is increasingly being polluted with man-made debris. Studies have discovered that some satellites retrieved from orbit are coated with a very thin layer of urine and fecal matter evidently released from Russian and US space missions. [http://see.msfc.nasa.gov/sparkman/Section_Docs/article_1.htm] Beyond planetary atmospheres, the pressure from photons and other particles from the sun become significant. Spacecraft can be buffeted by solar winds, but planets are too massive to be affected. The idea of using this wind with a solar sail has been proposed for interplanetary travel. The deep vacuum of space could make it an attractive environment for certain processes, for instance those that require ultraclean surfaces. 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. (See "Polar Magnetic Phenomena and Terrella Experiments", in The Norwegian Aurora Polaris Expedition 1902-1903 (publ. 1913, p.720)

The quantum-mechanical vacuum

Even an ideal vacuum, thought of as the complete absence of anything, will not in practice remain empty. One reason is that the walls of a vacuum chamber emit light in the form of black-body radiation: visible light if they are at a temperature of thousands of degrees, infrared light if they are cooler. If this soup of photons is in thermodynamic equilibrium with the walls, it can be said to have a particular temperature, as well as a pressure. More fundamentally, quantum mechanics predicts that vacuum energy can never be exactly zero. The lowest possible energy state is called the zero-point energy and consists of a seething mass of virtual particles that have brief existence. This is called vacuum fluctuation. While most agree that this represents a significant part of particle physics, it is a concept that would benefit from a deeper understanding than currently available. Vacuum fluctuations may also be related to the so-called cosmological constant in the theory of gravitation, if indeed this entity were to be observed in nature on a macroscopic scale. The best support for vacuum fluctuations is the Casimir effect. In quantum field theory and string theory, the term "vacuum" is used to represent the ground state in the Hilbert space, that is, the state with the lowest possible energy. In free (non-interacting) quantum field theories, this state is analogous to the ground state of a quantum harmonic oscillator. If the theory is obtained by quantization of a classical theory, each stationary point of the energy in the configuration space gives rise to a single vacuum. String theory is believed to be analogous to quantum field theory but one with a huge number of vacua - with the so-called anthropic landscape.

Historical interpretation

Historically, there has been much dispute over whether such a thing as a vacuum can exist. Ancient Greek philosophers did not like to admit the existence of a vacuum, asking themselves "how can 'nothing' be something?". Plato found the idea of a vacuum inconceivable. He believed that all physical things were instantiations of an abstract Platonic ideal, and could not imagine an "ideal" form of a vacuum. Similarly, Aristotle considered the creation of a vacuum impossible—nothing could not be something. Later Greek philosophers thought that a vacuum could exist outside the cosmos, but not inside it. In the Middle Ages, the idea of a vacuum was thought to be immoral or even heretical. The absence of anything implied the absence of God, and hearkened back to the void prior to the story of creation in the book of Genesis. Medieval thought experiments into the idea of a vacuum considered whether a vacuum was present, if only for an instant, between two flat plates when they were rapidly separated. There was much discussion of whether the air moved in quickly enough as the plates were separated, or, following William Burley whether a 'celestial agent' prevented the vacuum arising—that is, whether nature abhorred a vacuum. This speculation became irrelevant after the Paris condemnations of Bishop Tempier, which required there to be no restrictions on the powers of God, which led to the conclusion that God could create a vacuum if he so wished. Following work by Galileo, Evangelista Torricelli argued in 1643 that there was a vacuum at the top of a mercury barometer. Some people believe that although Torricelli produced the first vacuum, it was Blaise Pascal who recognized it for what it was. Robert Boyle later conducted experiments on the effects of a vacuum. For example, a canary exposed to vacuum would rupture open due to the lack of pressure. In 1654, Otto von Guericke conducted his famous Magdeburg hemispheres experiment, showing that teams of horses could not separate two hemispheres from which the air had been evacuated. Concurrently, theories of the nature of light had proposed the idea of a aethereal medium which would be the medium to convey waves of light (Newton relied on this idea to explain refraction and radiated heat). This evolved into the luminiferous aether idea of the 19th century, but it was known to have significant shortcomings. In 1887 the Michelson-Morley experiment, using an interferometer to attempt to detect the change in the speed of light caused by the Earth moving with respect to the aether, was a famous null result, showing that there really was no static, pervasive medium throughout space and through which the Earth moved as though through a wind. (Of course, if the aether were the medium in which light waves traveled and electromagnetic and gravitational fields manifest, then it would be exceedingly difficult to distinguish the characteristics of such medium from those of the field or fields one was in. It would no more be possible to show that the Earth moved in relation to such an aether than it would be to illustrate that it moved in relation to its own electromagnetic and gravitational fields.)

External links

- [http://www.avs.org/ American Vacuum Society]
- [http://scitation.aip.org/jvsta/ Journal of Vacuum Science and Technology A]
- [http://scitation.aip.org/jvstb/ Journal of Vacuum Science and Technology B]
- [http://imagine.gsfc.nasa.gov/docs/ask_astro/answers/970603.html Discussion of the effects on humans of exposure to hard vacuums].
- [http://www.arXiv.org/abs/hep-th/0012062 Vacuum Energy in High Energy Physics]
- [http://vacuumscientists.com/ Scientist of vacuum]
- http://www.mcallister.com/vacuum.html (Short History of Vacuum Terminology and Technology) Category:Industrial processes ja:真空

Time

Attempting to understand Time has long been a prime occupation for philosophers, scientists and artists. There are widely divergent views about its meaning, hence it is difficult to provide an uncontroversial and clear definition of time. The Oxford English Dictionary defines it as "the indefinite continued progress of existence and events in the past, present, and future, regarded as a whole". Another standard dictionary definition is "a non-spatial linear continuum wherein events occur in an apparently irreversible order." This article looks at some of the main philosophical and scientific issues relating to time. The measurement of time has also occupied scientists and technologists, and was a prime motivation in astronomy. Time is also a matter of significant social importance, having economic value ("time is money") as well as personal value due to an awareness of the limited time in each day and in our lives. Units of time have been agreed upon to quantify the duration of events and the intervals between them. Regularly recurring events and objects with apparently periodic motion have long served as standards for units of time - such as the apparent motion of the sun across the sky, the phases of the moon, the swing of a pendulum.

Philosophy of time

Main article: Philosophy of space and time; Ontology In ancient thought, Zeno's paradoxes challenged the conception of infinite divisibility, and eventually led to the development of calculus. Parmenides (of whom Zeno was a follower) believed that time, motion, and change were illusions, basing this on a rather interesting argument. More recently, McTaggart held a similar belief. Newton believed time and space form a container for events, which is as real as the objects it contains. In contrast, Leibniz believed that time and space are a conceptual apparatus describing the interrelations between events. Leibniz and others thought of time as a fundamental part of an abstract conceptual framework, together with space and number, within which we sequence events, quantify their duration, and compare the motions of objects. In this view, time does not refer to any kind of entity that "flows", that objects "move through", or that is a "container" for events. The bucket argument proved problematic for Leibniz, and his account fell into disfavour, at least amongst scientists, until the development of Mach's principle. Modern physics views the curvature of spacetime around an object as much a feature of that object as are its mass and volume. Immanuel Kant, in the Critique of Pure Reason, described time as an a priori notion that allows us (together with other a priori notions such as space) to comprehend sense experience. With Kant, neither space nor time are conceived as substances, but rather both are elements of a systematic framework necessarily structuring the experiences of any rational agent. Spatial measurements are used to quantify how far apart objects are, and temporal measurements are used to quantify how far apart events occur. Nietzsche, inspired by the concept of eternal return in his book Thus Spoke Zarathustra, argued that time possesses a circular characteristic. Postulating an infinite past, "all things" must have come to pass therein; the same for an infinite future. In Existentialism, time is considered fundamental to the question of being, in particular by the philosopher Martin Heidegger.

Contemporary theses in the philosophy of time

In contempoary philosophy there has been a very active debate over the nature of time, especially in light of the big changes in physics since the 1920s. Contributors include Ned Markosian, Ted Sider, Quentin Smith, and L. Nathan Oaklander. Two major theses have been developed, along with some hybrids. There is no real consensus among philosophers about which, if any, is correct. The two major theories can be summed up as follows: 1. A-theory of time: Presentism: Oaklander writes: "[A] version of the pure A-theory, known as "", purports to avoid… the problem of change... According to presentism, only the present exists. Thus, it is not the case that, say, O is green and [then] O is red [if, for example, O is a tomato]." (Oaklander, L. Nathan. In Smith, Quentin, and Oaklander, L. Nathan. 1995. Time, Change, and Freedom. New York: Routledge. 2004, 27.) 2. B-theory of time: Eternalism: the following passage from L. Nathan Oaklander sums this up

…[T]ime [involves] events strung out along a series united to one another by the relations of earlier than, later and simultaneity… The events in the temporal series are fixed in that they never change their position relative to each other… It has become customary to call the entire series of events spread out along the time-line from earlier to later, the “B-series.” When viewed solely in terms of the B-series, time is thought of as static or unchanging for there is nothing about temporal relations between events that changes... Time not only has a static aspect, it also has a transitory aspect. In addition to conceiving of time in terms of events standing in temporal relations, we also conceive of time and the events in time as moving or passing from the far future to the near future, from the hear future to the present, and then from present they recede into the more and more distant past… When events are ordered in terms of the notions of past, present, or future they form what is called an “A-series.” It should be noted, of course, that the A- and B-series are not really “two” different series of events, but the same series ordered in two different ways. (Oaklander 2004,Page 69)

Time in physics

never change Main article: Time in physics Time is currently one of the few fundamental quantities (quantities which cannot be defined via other quantities because there is nothing more fundamental known at present). Thus, similar to definition of other fundamental quantities (like space and mass), time is defined via measurement. Currently, the standard time interval (called conventional second, or simply second) is defined as 9 192 631 770 oscillations of a hyperfine transition in the ¹³³Cs atom. Prior to Albert Einstein's relativistic physics, time and space had been treated as distinct dimensions; Einstein linked time and space into spacetime. Einstein showed that people traveling at different speeds will measure different times for events and different distances between objects, though these differences are minute unless one is traveling at a speed close to that of light. Many subatomic particles exist for only a fixed fraction of a second in a lab relatively at rest, but some that travel close to the speed of light can be measured to travel further and survive longer than expected. According to the special theory of relativity, in the high-speed particle's frame of reference, it exists for the same amount of time as usual, and the distance it travels in that time is what would be expected for that velocity. Relative to a frame of reference at rest, time seems to "slow down" for the particle. Relative to the high-speed particle, distances seems to shorten. Even in Newtonian terms time may be considered the fourth dimension of motion; but Einstein showed how both temporal and spatial dimensions can be altered (or "warped") by high-speed motion. Einstein (The Meaning Of Relativity - 1968): "Two events taking place at the points A and B of a system K are simultaneous if they appear at the same instant when observed from the middle point, M, of the interval AB. Time is then defined as the ensemble of the indications of similar clocks, at rest relatively to K, which register the same simultaneously."

Measurement

Present day standards

The standard unit for time is the SI second, from which larger units are defined like the minute, hour, and day. Because they do not use the decimal system, and because of the occasional need for a leap-second, the minute, hour, and day are "non-SI" units, but are officially accepted for use with the International System. There are no fixed ratios between seconds (or days) on the one hand and months and years on the other hand -- months and years having significant variations in length. Despite its great social importance, the week is not mentioned even as a "non-SI" unit. ([http://www1.bipm.org/utils/en/pdf/si-brochure.pdf See external pdf file: The International System of Units].) The measurement of time is so critical to the functioning of our modern societies that it is coordinated at an international level. The basis for scientific time is a continuous count of seconds based on atomic clocks around the world, known as International Atomic Time (TAI). This is the yardstick for other time scales including Coordinated Universal Time (UTC) which is the basis for civil time. The 60 base used for seconds, minutes and hours is all the remains of the ancient Phoenician counting base, using 60 as the equivalent of 10, or 100 in modern times. A 60 base is known as sexagesimal.

Chronology

Another form of time measurement consists of studying the past. Events in the past can be ordered in a sequence (creating a chronology), and be put into chronological groups (periodization). One of the most important systems of periodization is Geologic time, which is a system of periodizing the events that shaped the Earth and its life. Chronology, periodization, and interpretation of the past are together known as the study of history.

Psychology

Different people may judge identical lengths of time quite differently. Time can "fly"; that is, a long period of time can seem to go by very quickly. Likewise, time can seem to "drag," as in when one performs a boring task. The psychologist Jean Piaget called this form of time perception "lived time". Time appears to go fast when sleeping, or, to put it differently, time seems not to have passed while asleep. Time also appears to pass more quickly as one gets older. For example, a day for a child seems to last longer than a day for an adult. One possible reason for this is that with increasing age, each segment of time is an increasingly smaller percentage of the person's total experience. Altered states of consciousness are sometimes characterised by a different estimation of time. Some psychoactive substances--such as entheogens--may also dramatically alter a person's temporal judgement. In explaining his theory of relativity, Albert Einstein is often quoted as saying that although sitting next to a pretty girl for an hour feels like a minute, placing one's hand on a hot stove for a minute feels like an hour. This is intended to introduce the listener to the concept of the interval between two events being perceived differently by different observers.

Use of time

The use of time is an important issue in understanding human behaviour, education, and travel behaviour. The question concerns how time is allocated across a number of activities (such as time spent at home, at work, shopping, etc.). Time use changes with technology, as the television or the Internet created new opportunities to use time in different ways. However, some aspects of time use are relatively stable over long periods of time, such as the amount of time spent traveling to work, which despite major changes in transport, has been observed to be about 20-30 minutes one-way for a large number of cities over a long period of time. This has led to the disputed time budget hypothesis. Time management is the organization of tasks or events by first estimating how much time a task will take to be completed, when it must be completed, and then adjusting events that would interfere with its completion so that completion is reached in the appropriate amount of time. Calendars and day planners are common examples of time management tools. Arlie Russell Hochschild and Norbert Elias have written on the use of time from a sociological perspective.

References

- Oxford English Dictionary - [http://www.askoxford.com/concise_oed/time?view=uk]

External links

Perception of time

- [http://plato.stanford.edu/entries/time-experience/ The Experience and Perception of Time]
- [http://cogprints.ecs.soton.ac.uk/archive/00003125/ Subjective Perception of Time and a Progressive Present Moment: The Neurobiological Key to Unlocking Consciousness]
- [http://www.primitivism.com/time.htm Time and Its Discontents]
- [http://www.ericdigests.org/2003-5/time.htm Time and Learning]
- [http://mixingmemory.blogspot.com/2004/12/by-request-time-perception-i.html Time Perception I] and [http://mixingmemory.blogspot.com/2004/12/time-perception-ii-cognitive-factors.html II]
- [http://theorderoftime.org/ The Order of Time: Platform for an Alternative Time Consciousness]
- [http://www.chabad.org/article.asp?AID=74335 What is Time?] An elucidation of the Lubavitcher Rebbe's comments on the topic.

Physics

- [http://physics.nist.gov/GenInt/Time/world.html A walk through Time]
- [http://pages.britishlibrary.net/lobster/tmx Time Travel and Multi-Dimensionality]
- [http://arxiv.org/abs/physics/0310055 Time and classical and quantum mechanics: Indeterminacy vs. discontinuity]
- [http://www.sankey.ws/time.html Time as a universal consequence of quanta]

Timekeeping

- [http://tycho.usno.navy.mil/systime.html Different systems of measuring time]
- [http://physics.nist.gov/cuu/Units/outside.html non-SI units]
- [http://www1.bipm.org/en/scientific/tai/time_server.html UTC/TAI Timeserver]
- [http://tycho.usno.navy.mil/leapsec.html Leapsecond]
- [http://www.intuitor.com/hex/hexclock.html Hex Time]
- [http://www.florencetime.net Florencetime.net]
- [http://news.bbc.co.uk/2/hi/science/nature/3486160.stm BBC article on shortest time ever measured]
- [http://www.awi-net.org American Watchmakers-Clockmakers Institute]
- [http://www.timeanddate.com/worldclock/ The World Clock - Time Zones]

Miscellaneous

- [http://www.boost.org/doc/html/date_time.html Boost Date-Time Library -- Powerful C++ Library for date-time manipulation]
- [http://www.cyclesresearchinstitute.org/ Cycles Research Institute]
- [http://www.timeticker.com/ TimeTicker and the time tickers...]
- [http://www.welt-zeit-uhr.de/worldtime.php World Time and Zones]
- [http://www.timetools.co.uk Time Servers] NTP Time Servers provide accurate timing for computers and computer networks.

Second

:This article is about the unit of time. For other uses, see second (disambiguation). The second (symbol: s) is the SI base unit of time.

Definition

The second is the duration of 9 192 631 770 periods of the radiation corresponding to the transition between the two hyperfine levels of the ground state of the caesium-133 atom.

Origin

Originally, the second was known as a "second minute", meaning the second minute (i.e. small) division of an hour. The first division was known as a "prime minute" and is equivalent to the minute we know today.

Conversions

- 60 seconds = 1 minute
- 3 600 seconds = 1 hour
- 86.4 kiloseconds (86 400 seconds) = 1 day (in the SI sense)

Explanation

The factor of 60 may have been influenced by the Babylonians who used factors of 60 in their counting system. The hour had previously been defined by the Egyptians in terms of the rotation of the Earth as 1/24 of a mean solar day. This m

Metre

Conversions

History

Timeline of definition

See also

External links

Meter (disambiguation)

American English

History

Phonology

Differences in British English and American English

English words that arose in the U.S.

English words obsolete outside the U.S.

Regional differences

See also

Further reading

References

External links

Length

Units of length(SI)

Other units of length

The Imperial and US customary units of length

Units are used in astronomy

See also

External links

Light

Visible electromagnetic radiation

Speed of light

Refraction

Optics

Color and wavelengths

Measurement of light

Light sources

Theories about light

Early Greek ideas

10th century optical theory

The 'plenum'

Particle theory

Wave theory

Electromagnetic theory

Particle theory revisited

Quantum theory

Wave-particle duality

A light wave

See also

Vacuum

Vacuum ranges

Perfect vacuum

Partial vacuum

Degrees of vacuum

Creating a vacuum

High vacuum

Ultra-high vacuum

Vacuum in space

The quantum-mechanical vacuum

Historical interpretation

See also

External links

Time

Philosophy of time

Contemporary theses in the philosophy of time

Time in physics

Measurement

Present day standards

Chronology

Psychology

Use of time

See also

General units of time

Special units of time

Time measurement and horology

Theory and study of time

References

External links

Perception of time

Physics

Timekeeping

Miscellaneous

Further reading

Second