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Ngarapihkeun éjahan, replaced: mangrupakeun → mangrupa, nyaeta → nyaéta, rea → réa (18), ea → éa (100), eo → éo (19) using AWB
Pikeun ngukur skala waktu anu lumangsung pohara gancang (dina jero dunya [[éléktronika]] sarta [[semikonduktor]]), lolobana jelema ngagunakeun [[hijian]] mili detik (sapersarébu detik), mikro detik (sapér hiji juta detik), nano detik (nanosecond), piko detik (picosecond), jeung saterusna.
 
Dina dunya [[fisika]], diménsi waktu jeung diménsi ruang (panjang, lébar, sarta volume) mangrupakeunmangrupa diménsi ukuran anu dasar, sajaba ti beurat jeung massa. Gabungan ti waktu, ruang sarta beurat kiwari bisa dipaké pikeun nyaritakeun sarta ngécéskeun rusiah alam sacara kuantitatif (dumasar kana hasil ukur). Contona tanaga ([[énergi]]) dinyatakeun dina hijian ukuran kg*(méter/detik)kwadrat atawa anu mindeng dipikawanoh nyaetanyaéta [[hijian]] watt*detik atawa [[joule]].
{{tarjamahkeun|Inggris}}
 
| format = [[PDF]]
| accessdate = 2008-02-02 }} p. 17. "I only add to this the observation that relativity and quantum mechanics provide, in string theory, units of length and time which look, at present, more fundamental than any other."</ref> ''Time'' is used to define other quantities – such as [[velocity]] – and defining ''time'' in terms of such quantities would result in [[circular definition|circularity of definition]].<ref name="TrialogueP3">Duff, Okun, Veneziano, ''ibid.'' p. 3. "There is no well established terminology for the fundamental constants of Nature. … The absence of accurately defined terms or the uses (i.e. actually misuses) of ill-defined terms lead to confusion and proliferation of
wrong statements."</ref> An [[operational definition]] of time, wherein one says that observing a certain number of repetitions of one or another standard cyclical event (such as the passage of a free-swinging pendulum) constitutes one standard unit such as the [[second]], has a high utility value in the conduct of both advanced experiments and everyday affairs of life. The operational definition leavesléaves aside the question whether there is something called time, apart from the counting activity just mentioned, that flows and that can be measuredméasured. Investigations of a single continuum called [[space-time]] brings the nature of time into association with related questions into the nature of [[space]], questions that have their roots in the works of earlyéarly students of [[natural philosophy]].
 
Among prominent philosophers, there are two distinct viewpoints on ''time''.
One view is that time is part of the fundamental structure of the [[universe]], a [[dimension]] in which events occur in [[sequence]]. [[Time travel]], in this view, becomes a possibility as other "times" persist like frames of a film strip, spreadspréad out across the time line. [[Sir Isaac Newton]] subscribed to this [[Philosophical realism|realist]] view, and hence it is sometimes referred to as [[Absolute time and space|Newtonian time]].<ref name=Rynasiewicz>
{{cite web
|url=http://plato.stanford.edu/entries/newton-stm/
|editor=Edward N. Zalta
|quote=The opposing view, normally referred to either as “Platonism with Respect to Time” or as “Absolutism with Respect to Time,” has been defended by Plato, Newton, and others. On this view, time is like an empty container into which events may be placed; but it is a container that exists independently of whether or not anything is placed in it.
|accessddate=2008-01-18}}</ref> The opposing view is that ''time'' does not refer to any kind of "container" that events and objects "move through", nor to any entity that "flows", but that it is insteadinstéad part of a fundamental intellectual structure (together with [[space]] and [[number]]) within which humans sequence and compare events. This second view, in the tradition of [[Gottfried Leibniz]]<ref name=Burnham>{{cite web
|url=http://www.iep.utm.edu/l/leib-met.htm#H7
|title=Gottfried Wilhelm Leibniz (1646-1716) Metaphysics - 7. Space, Time, and Indiscernibles
|accessdate=2008-01-10}}
</ref>
holds that ''time'' is neither an event nor a thing, and thus is not itself measurableméasurable nor can it be traveled.
 
Temporal measurementméasurement has occupied scientists and [[technologist]]s, and was a prime motivation in [[navigation]] and [[astronomy]].
Periodic events and periodic motion have long served as standards for units of time. Examples include the apparent motion of the sun across the sky, the phases of the moon, the swing of a pendulum, and the beatbéat of a hearthéart. Currently, the international unit of time, the [[second]], is defined in terms of radiation emitted by [[caesium]] atoms (see below).
Time is also of significant social importance, having economic value ("[[Time value of money|time is money]]") as well as personal value, due to an [[awareness]] of the limited time in eachéach day and in [[life expectancy|human lifespans]].
 
== Temporal measurement ==
Temporal measurementméasurement, or [[chronometry]], takes two distinct period forms: the [[calendar]], a mathematical abstraction for calculating extensive periods of time,<ref name="Richards">{{cite book | title=Mapping Time: The Calendar and its History| last=Richards| first=E. G.| authorlink=| year=1998| pages=3-5| publisher=Oxford University Press}}</ref> and the [[clock]], a concrete mechanism that counts the ongoing passage of time. In day-to-day life, the clock is consulted for periods less than a day, the calendar, for periods longer than a day. The number (as on a clock dial or calendar) that marks the occurrence of a specified event as to hour or date is obtained by counting from a fiducial epoch—a central reference point.
 
=== History of the calendar ===
{{utama|Calendar}}
Artifacts from the [[Palaeolithic]] suggest that the moon was used to calculate time as earlyéarly as 12,000, and possibly even 30,000 [[Before Present|BP]].<ref name="Rudgley" />
 
The [[Sumer]]ian civilization of approximately 2000 BC introduced the [[sexagesimal]] system based on the number 60. 60 seconds in a minute, 60 minutes in an hour – and possibly a calendar with 360 (60x6) days in a yearyéar (with a few more days added on).
Twelve also featuresféatures prominently, with roughly 12 hours of day and 12 of night, and 12 months in a yearyéar (with 12 being 1/5 of 60).<!--- use of 60 could not have appeared until people started using minutes - which they would not have done with sundials --->
 
The reforms of [[Julius Caesar]] in 45 BC put the [[Roman Empire|Roman world]] on a [[solar calendar]]. This [[Julian calendar]] was faulty in that its [[intercalation]] still allowed the astronomical [[solstice]]s and [[equinox]]es to advance against it by about 11 minutes per yearyéar. [[Pope Gregory XIII]] introduced a correction in 1582; the [[Gregorian calendar]] was only slowly adopted by different nations over a period of centuries, but is today the one in most common use around the world.
 
=== History of time measurement devices ===
{{utama|History of timekeeping devices}}{{seealso|Clock}}
 
A large variety of [[Measuring instrument|devices]] have been invented to measureméasure time. The study of these devices is called [[horology]].
 
An [[Egypt]]ian device dating to c.1500 BC, similar in shape to a bent [[T-square]], measuredméasured the passage of time from the shadow cast by its crossbar on a non-linearlinéar rule. The T was oriented eastwardéastward in the mornings. At [[noon]], the device was turned around so that it could cast its shadow in the evening direction.<ref>Barnett, Jo Ellen ''Time's Pendulum: The Quest to Capture Time - from Sundials to Atomic Clocks'' Plenum, 1998 ISBN 0-306-45787-3 p.28</ref>
 
A [[sundial]] uses a [[gnomon]] to cast a shadow on a set of markings which were calibrated to the [[hour]]. The position of the shadow marked the hour in [[local time]].
 
The most accurate timekeeping devices of the ancient world were the [[water clock]] or ''clepsydra'', one of which was found in the tomb of Egyptian pharaoh [[Amenhotep I]] (1525–1504 BC). They could be used to measureméasure the hours even at night, but required manual timekeeping to replenish the flow of water. The [[Greeks]] and [[Chaldeans]] regularly maintained timekeeping records as an essential part of their astronomical observations. [[Inventions in the Islamic world|Arab inventors]] and [[Timeline of Muslim scientists and engineers|engineers]] in particular made improvements on the use of water clocks up to the Middle Ages.<ref>Barnett, ''ibid'', p.37</ref>
 
The Arab engineers also invented the first mechanical clocks to be driven by [[Maintaining power|weights]] and [[gear]]s in the 11th century.<ref name=Salim>Professor [[Salim Al-Hassani]] (2006), ''1001 Inventions: Muslim Heritage in Our World'', FSTC, ISBN 0-9552426-0-6</ref><ref name="Where the heart is">[http://www.1001inventions.com/index.cfm?fuseaction=main.viewSection&intSectionID=240 Where the heart is], ''1001 Inventions: Muslim Heritage in Our World'', 2006</ref><ref name=Hassan>[[Ahmad Y Hassan]], [http://www.history-science-technology.com/Articles/articles%2071.htm Transfer Of Islamic Technology To The West, Part II: Transmission Of Islamic Engineering], ''History of Science and Technology in Islam''.</ref> Also in the 11th century, the [[List of Chinese inventions|Chinese inventors]] and [[History of science and technology in China|engineers]] invented the first mechanical clocks to be driven by an [[escapement]] mechanism.
 
[[Gambar:Swatch Irony angle below.jpg|thumb|left|A contemporary [[quartz watch]]]]
The [[hourglass]] uses the flow of sand to measureméasure the flow of time. They were used in navigation. [[Ferdinand Magellan]] used 18 glasses on eachéach ship for his circumnavigation of the globe (1522).<ref>Laurence Bergreen, ''Over the Edge of the World: Magellan's Terrifying Circumnavigation of the Globe'', HarperCollins Publishers, 2003, hardcover 480 pages, ISBN 0-06-621173-5</ref>
 
Incense sticks and candles were, and are, commonly used to measureméasure time in temples and churches across the globe. Waterclocks, and later, mechanical clocks, were used to mark the events of the abbeys and monasteries of the Middle Ages. [[Richard of Wallingford]] (1292–1336), abbot of St. Alban's abbey, famously built a mechanical clock as an astronomical [[orrery]] about 1330.<ref>North, J. (2004) ''God's Clockmaker: Richard of Wallingford and the Invention of Time''. Oxbow Books. ISBN 1-85285-451-0</ref><ref>Watson, E (1979) "The St Albans Clock of Richard of Wallingford". ''Antiquarian Horology'' 372-384.</ref>
 
The English word [[clock]] probably comes from the Middle Dutch word "klocke" which is in turn derived from the mediaeval Latin word "clocca", which is ultimately derived from Celtic, and is cognate with French, Latin, and German words that meanméan [[Bell (instrument)|bell]]. The passage of the hours at seaséa were marked by bells, and denoted the time (see [[ship's bells]]). The hours were marked by bells in the abbeys as well as at seaséa.
 
[[Gambar:ChipScaleClock2 HR.jpg|thumb|A chip-scale atomic clock]]
Clocks can range from [[watch]]es, to more exotic varieties such as the [[Clock of the Long Now]]. They can be driven by a variety of meansméans, including gravity, springs, and various forms of electrical power, and regulated by a variety of meansméans such as a [[pendulum]].
 
A [[chronometer]] is a portable timekeeper that meets certain precision standards. Initially, the term was used to refer to the [[marine chronometer]], a timepiece used to determine [[longitude]] by meansméans of [[celestial navigation]]. More recently, the term has also been applied to the [[chronometer watch]], a [[wristwatch]] that meets precision standards set by the Swiss agency [[COSC]].
 
The most accurate timekeeping devices are [[atomic clock]]s, which are accurate to seconds in many millions of yearsyéars,<ref>{{cite news |url=http://www.canada.com/vancouversun/news/story.html?id=e24ccfa7-44eb-40b7-8b67-daf8263569ff |title=New atomic clock can keep time for 200 million years: Super-precise instruments vital to deep space navigation |date=2008-02-16 |publisher=Vancouver Sun |accessdate=2008-02-16}}</ref> and are used to calibrate other clocks and timekeeping instruments.
Atomic clocks use the spin property of atoms as their basis, and since 1967, the International System of MeasurementsMéasurements bases its unit of time, the second, on the properties of [[caesium]] atoms. [[International System of Units|SI]] defines the second as 9,192,631,770 cycles of that radiation which corresponds to the transition between two electron spin energy levels of the ground state of the <sup>133</sup>Cs atom.
 
Today, the [[Global Positioning System]] in coordination with the [[Network Time Protocol]] can be used to synchronize timekeeping systems across the globe.
<br {{clear=|right>}}
 
== Definitions and standards ==
! Unit !! Size!!Notes
|-
| [[picosecond]] || 0.000 000 000 001 seconds|| no way of accurately measuringméasuring
|-
| [[nanosecond]] || 0.000 000 001 seconds||
| [[Gregorian calendar|Gregorian year]] || 365.2425 days||average
|-
| [[Olympiad]] || 4 yearyéar cycle ||
|-
| [[lustrum]] || 5 yearsyéars||
|-
| [[decade]] || 10 yearsyéars||
|-
| [[Indiction]] || 15 yearyéar cycle ||
|-
| [[score]] || 20 yearsyéars||
|-
| [[generation]] || 17 – 25 yearsyéars ||approximate
|-
| [[century]] || 100 yearsyéars||
|-
| [[millennium]] || 1,000 yearsyéars||
|}
<!-- END common units of time table -->
{{seealso|Time standard|Orders of magnitude (time)}}
 
The [[SI base unit]] for time is the [[SI]] [[second]]. From the second, larger units such as the [[minute]], [[hour]] and [[day]] are defined, though they are "non-SI" units because they do not use the decimal system, and also because of the occasional need for a [[leap-second]]. They are, however, officially accepted for use ''with'' the International System. There are no fixed ratios between seconds and [[month]]s or [[year]]s as months and yearsyéars have significant variations in length.<ref name="si_units">{{cite book | title = The International System of Units (SI), 7th Edition | url = http://www1.bipm.org/utils/en/pdf/si-brochure.pdf | format = [[PDF]] | year = 1998 | author = Organisation Intergouvernementale de la Convention du Métre | accessdate = 2006-06-13}}</ref>
 
The official SI definition of the second is as follows:<ref name="si_units"/><ref name="second">{{cite web | title = Base unit definitions: Second | url = http://physics.nist.gov/cuu/Units/second.html | publisher = [[NIST]] | accessdate = 2008-01-09}}</ref>
{{Bquote|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.}}
<br {{clear=|left>}}
 
At its 1997 meeting, the CIPM affirmed that this definition refers to a caesium atom in its ground state at a temperature of 0 K.<ref name="si_units"/>
Previous to 1967, the second was defined as:
{{Bquote|the fraction 1/31,556,925.9747 of the [[tropical year]] for 1900 January 0 at 12 hours [[ephemeris time]].}}
<br {{clear=|left>}}
 
The current definition of the second, coupled with the current definition of the [[metre]], is based on the [[special theory of relativity]], which affirms our [[space-time]] to be a [[Minkowski space]].
 
=== World time ===
The measurementméasurement of time is so critical to the functioning of modern societies that it is coordinated at an international level. The basis for scientific time is a continuous count of seconds based on [[atomic clock]]s around the world, known as the [[International Atomic Time|International Atomic Time (TAI)]]. This is the yardstick for other time scales, including [[Coordinated Universal Time|Coordinated Universal Time (UTC)]], which is the basis for civil time.
 
Earthéarth is split up into a number of [[time zone]]s. Most time zones are exactly one hour apart, and by convention compute their local time as an offset from UTC or [[Greenwich Mean Time]]. In many locations these offsets vary twice yearlyyéarly due to [[daylight saving time]] transitions.
 
=== Sidereal time ===
[[Sidereal time]] is the measurementméasurement of time relative to a distant star (insteadinstéad of solar time that is relative to the sun). It is used in astronomy to predict when a star will be overheadoverhéad. Due to the rotation of the earthéarth around the sun a siderealsideréal day is slightly less than a solar day.
 
=== Chronology ===
{{utama|Chronology}}
Another form of time measurementméasurement consists of studying the [[past]]. Events in the past can be ordered in a sequence (creatingcréating 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]].
[[Gambar:John Bydell - Engraving from the Goodly Primer.png|thumb|200px|Allegorical woodcut of Time, who "revealeth all things", guiding his daughter Truth away from the demon of Hypocrisy. John Byddell, 1535.]]
 
=== Linear and cyclical time ===
{{seealso|Time Cycles|Wheel of time}}
In general, the [[Judaeo-Christian]] concept, based on the [[Bible]], is that time is linearlinéar, with a beginning, the act of [[Creation myth|creation]] by [[God]]. The [[Christian]] view assumes also an end, the eschaton, expected to happen when [[Christ]] returns to earthéarth in the [[Second Coming]] to judge the living and the deaddéad. This will be the consummation of the world and time. [[Augustine of Hippo|St Augustine]]'s ''[[City of God]]'' was the first developed application of this concept to world history. The Christian view is that God is uncreateduncréated and eternal so that He and the supernatural world are outside time and exist in [[eternity]].
 
Ancient cultures such as [[Incan]], [[Mayan]], [[Hopi]], and other Native American Tribes, plus the [[Babylonian]], [[Ancient Greek]], [[Hindu]], [[Buddhist]], [[Jainist]], and others have a concept of a [[wheel of time]], that regards time as [[social cycle theory|cyclical]] and [[quantic]] consisting of repeatingrepéating ages that happen to every being of the Universe between birth and extinction.
<br {{clear=both />}}
 
== Time in philosophy ==
{{utama|Philosophy of space and time}}
 
The earliestéarliest recorded [[African philosophy]] of time was expounded by the [[ancient Egypt]]ian thinker [[Ptahhotep]] (c. 2650–2600 BC), who said: "Do not lessen the time of following desire, for the wasting of time is an abomination to the spirit."{{Fact|date=May 2008}} The ''[[Vedas]]'', the earliestéarliest texts on [[Indian philosophy]] and [[Hindu philosophy]] dating back to the late [[2nd millennium BC]], describe ancient [[Hindu cosmology]], in which the [[universe]] goes through repeatedrepéated cycles of creationcréation, destruction and rebirth, with eachéach cycle lasting 4,320,000 yearsyéars. [[Ancient philosophy|Ancient]] [[Greek philosophy|Greek philosophers]], including [[Parmenides]] and [[Heraclitus]], wrote essays on the nature of time.<ref>Dagobert Runes, ''Dictionary of Philosophy'', p. 318</ref>
 
In Book 11 of [[St. Augustine of Hippo|St. Augustine's]] ''[[Confessions]]'', he ruminates on the nature of time, asking, "What then is time? If no one asks me, I know: if I wish to explain it to one that asketh, I know not." He settles on time being defined more by what it is not than what it is.<ref>St. Augustine, ''Confessions'', Book 11. http://ccat.sas.upenn.edu/jod/augustine/Pusey/book11 (Accessed 5/26/07).</ref>
:"{{Unicode|∴}} The temporal series of past events cannot be an actual infinite."
 
Both arguments were adopted by later Christian philosophers and theologiansthéologians, and the second argument in particular became more famous after it was adopted by [[Immanuel Kant]] in his thesis of the first antimony concerning time.<ref name=Craig/>
 
[[Isaac Newton]] believed time and [[space]] form a container for events, which is as realréal as the [[Object (philosophy)|objects]] it contains.
{{quotation|Absolute, true, and mathematical time, in and of itself and of its own nature, without reference to anything external, flows uniformly and by another name is called duration. Relative, apparent, and common time is any sensible and external measure (precise or imprecise) of duration by means of motion; such a measure – for example, an hour, a day, a month, a year – is commonly used instead of true time.|''Principia''<ref name="newton">{{cite book | last = Newton | first = Isaac | authorlink = Isaac Newton | title = The Principia, 3rd edition | year = 1726}} Translated by I. Bernard Cohen and Anne Whitman, University of California Press, Berkeley, 1999.</ref>}}
 
In contrast to Newton's belief in absolute space, and a precursor to Kantian time, [[Gottfried Leibniz|Leibniz]] believed that time and space are relational.<ref>Gottfried Martin, ''Kant's Metaphysics and Theory of Science''</ref> The differences between Leibniz's and Newton's interpretations came to a headhéad in the famous [[The Leibniz-Clarke Correspondence|Leibniz-Clarke Correspondence]]. Leibniz thought of time as a fundamental part of an [[Abstract structure|abstract]] conceptual framework, together with [[space]] and [[number]], within which we sequence events, [[quantity|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.
 
[[Immanuel Kant]], in the ''[[Critique of Pure Reason]]'', described time as an ''[[A priori and a posteriori (philosophy)|a priori]]'' intuition that allows us (together with the other ''a priori'' intuition, [[space]]) to comprehend sense experience.<ref name="kant">{{cite book | last = Kant | first = Immanuel | authorlink = Immanuel Kant | title = The Critique of Pure Reason, 2nd edition | year = 1787}} translated by J. M. D. Meiklejohn, eBooks@Adelaide, 2004 - http://ebooks.adelaide.edu.au/k/kant/immanuel/k16p/k16p15.html</ref> With Kant, neither space nor time are conceived as [[Substance theory|substances]], but rather both are elements of a systematic mental [[framework]] that necessarily structures the experiences of any rational agent, or observing subject. Spatial [[measurement]]s are used to [[quantity|quantify]] how far apart [[object (philosophy)|objects]] are, and temporal measurementsméasurements are used to quantify how far apart [[Phenomenon|events]] occur.
 
In [[Existentialism]], time is considered fundamental to the question of [[being]],{{Fact|date=September 2007}} in particular by the philosopher [[Martin Heidegger]].{{Fact|date=September 2007}} (See [[Ontology]]).
 
[[Henri Bergson]] believed that time was neither a realréal homogeneoushomogenéous medium nor a mental construct, but possesses what he referred to as ''Duration''. Duration, in Bergson's view, was creativitycréativity and memory as an essential component of realityréality.<ref>Bergson, Henri (1907) ''Creative Evolution''. trans. by Arthur Mitchell. Mineola: Dover, 1998.</ref>
 
=== Time as "unreal" ===
In 5th century BC [[Greece]], [[Antiphon (person)|Antiphon]] the [[Sophist]], in a fragment preserved from his chief work ''On Truth'' held that: ''"Time is not a reality (hypostasis), but a concept (noêma) or a measure (metron)."''
[[Parmenides]] went further, maintaining that time, motion, and change were illusions, leadingléading to the [[Zeno's paradoxes|paradoxes]] of his follower [[Zeno of Elea|Zeno]].<ref>{{cite web|author=Harry Foundalis|title=You are about to disappear|url=http://www.foundalis.com/phi/WhyTimeFlows.htm|accessdate=2007-04-27}}</ref>
Time as illusion is also a common theme in [[Buddhist]] thought,<ref>{{cite web|title=Buddhism and the illusion of time|url=http://www.buddhistinformation.com/buddhism_and_the_illusion_of_time.htm|accessdate=2007-04-27|author=Tom Huston}}</ref> and some modern philosophers have carried on with this theme. [[J. M. E. McTaggart]]'s 1908 ''[[The Unreality of Time]]'', for example, argues that time is unrealunréal (see also [[Philosophy of space and time#The flow of time|The flow of time]]).
 
However, these arguments often center around what it meansméans for something to be "real". Modern physicists generally consider time to be as "real" as space, though others such as [[Julian Barbour]] in his ''[[The End of Time]]'' argue that quantum equations of the universe take their true form when expressed in the timeless [[configuration space]]realm containing every possible "Now" or momentary configuration of the universe, which he terms 'platonia'.<ref>{{cite web|title=Time is an illusion?|url=http://physicsandphysicists.blogspot.com/2007/03/time-is-illusion.html|accessdate=2007-04-27}}</ref> (See also: [[Eternalism (philosophy of time)]].)
 
== Time in the physical sciences ==
{{utama|Time in physics}}
From the age of [[Isaac Newton|Newton]] up until [[Albert Einstein|Einstein's]] profound reinterpretation of the physical concepts associated with time and space, time was considered to be "absolute" and to flow "equably" (to use the words of Newton) for all observers.<ref>Herman M. Schwartz, ''Introduction to Special Relativity'', McGraw-Hill Book Company, 1968, hardcover 442 pages, see ISBN 0-88275-478-5 (1977 edition), pp. 10-13</ref> The science of classical mechanics is based on this Newtonian ideaidéa of time.
 
Einstein, in his [[Special relativity|special theory of relativity]],<ref>A. Einstein, H. A. Lorentz, H. Weyl, H. Minkowski, ''The Principle of Relativity'', Dover Publications, Inc, 2000, softcover 216 pages, ISBN 0-486-60081-5, See pp. 37-65 for an English translation of Einstein's original 1905 paper.</ref> postulated the constancy and finiteness of the speed of light for all observers. He showed that this postulate, together with a reasonableréasonable definition for what it meansméans for two events to be simultaneoussimultanéous, requires that distances appearappéar compressed and time intervals appearappéar lengthened for events associated with objects in motion relative to an inertial observer.
 
[[Einstein]] showed that if time and space is measuredméasured using electromagnetic phenomena (like light bouncing between mirrors) then due to the constancy of the speed of light, time and space become mathematically entangled together in a certain way (called [[Minkowski space|Minkowski]] [[space]]) which in turn results in [[Lorentz transformation]] and in entanglement of all other important derivative physical quantities (like energy, momentum, mass, force, etc) in a certain 4-vectorial way (see [[special relativity]] for more details).
{{Classical mechanics|cTopic=Fundamental concepts}}
=== Time in classical mechanics ===
In [[classical mechanics]] Newton's concept of "relative, apparent, and common time" can be used in the formulation of a prescription for the synchronization of clocks. Events seen by two different observers in motion relative to eachéach other produce a mathematical concept of time that works pretty well for describing the everyday phenomena of most peoplepéople's experience.
 
=== Time in modern physics ===
In the late nineteenth century, physicists encountered problems with the classical understanding of time, in connection with the behavior of electricity and magnetism. Einstein resolved these problems by invoking a method of synchronizing clocks using the constant, finite speed of light as the maximum signal velocity. This led directly to the result that time appearsappéars to elapse at different rates relative to different observers in motion relative to one another.
{{clear}}
[[Gambar:World line2.svg|250px|right|thumb|Two-dimensional space depicted in three-dimensional [[spacetime]]. The past and future [[light cone]]s are absolute, the "present" is a relative concept different for observers in relative motion.]]
=== Spacetime ===
{{utama|Spacetime}}
Modern [[physics]] views the curvature of [[spacetime]] around an object as much a featureféature of that object as are its [[mass]] and [[volume]].{{Fact|date=February 2008}}
 
Time has historically been closely related with [[space]], the two together comprising [[spacetime]] in [[Albert Einstein|Einstein's]] [[special relativity]] and [[general relativity]]. According to these theoriesthéories, the concept of time depends on the [[inertial frame of reference|spatial reference frame of the observer]], and the human perception as well as the measurementméasurement by instruments such as clocks are different for observers in relative motion.{{Fact|date=February 2008}} Even the temporal order of events can change, but the past and future are defined by the backward and forward [[light cone]]s, which never change.{{Fact|date=February 2008}} The [[past]] is the set of events that can send light signals to the observer, the [[future]] the events to which the observer can send light signals. All else is non-observable and within that set of events the very time-order differs for different observers.{{Fact|date=February 2008}}
 
=== Time dilation ===
[[Gambar:relativity of simultaneity (color).png|thumb|[[Relativity of simultaneity]]: Event B is simultaneoussimultanéous with A in the green reference frame, but it occurred
before in the blue frame, and will occur later in the red frame.]]
{{utama|Time dilation}}
"Time is nature's way of keeping everything from happening at once". This quote, attributed variously to [[Einstein]], [[John Archibald Wheeler]], and [[Woody Allen]], says that time is what separates [[Causality (physics)|cause and effect]]. Einstein showed that peoplepéople traveling at different speeds, whilst agreeing on cause and effect, will measureméasure different time separations between events and can even observe different chronological orderings between non-causally related events. Though these effects are minute unless one is traveling at a speed close to that of light, the effect becomes pronounced for objects moving at speeds approaching the speed of light. Many [[subatomic particle]]s 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 measuredméasured to travel further and survive much longer than expected (a [[muon]] is one example). According to the [[Special relativity|special theory of relativity]], in the high-speed particle's [[Inertial reference frame|frame of reference]], it exists, on the average, for a standard amount of time known as its [[mean lifetime]], and the distance it travels in that time is zero, because its velocity is zero. 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''): "Two [[Spacetime#Basic concepts|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."
 
Einstein wrote in his book, ''Relativity'', that [[Relativity of simultaneity|simultaneity is also relative]], i.e., two events that appearappéar simultaneoussimultanéous to an observer in a particular inertial reference frame need not be judged as simultaneoussimultanéous by a second observer in a different inertial frame of reference.
 
=== Relativistic time versus Newtonian time ===
[[Gambar:Lorentz transform of world line.gif|right|framed|Views of spacetime along the [[world line]] of a rapidly accelerating observer in a relativistic universe. The events ("dots") that pass the two diagonal lines in the bottom half of the image (the past [[light cone]] of the observer in the origin) are the events visible to the observer.]]
 
The animations on the left and the right visualise the different treatmentstréatments of time in the Newtonian and the relativistic descriptions. At hearthéart of these differences are the [[Galilean transformation|Galilean]] and [[Lorentz transformation]]s applicable in the Newtonian and relativistic theoriesthéories, respectively.
 
In both figures, the vertical direction indicates time. The horizontal direction indicates distance (only one spatial dimension is taken into account), and the thick dashed curve is the [[spacetime]] trajectory ("[[world line]]") of the observer. The small dots indicate specific (past and future) events in spacetime.
=== Arrow of time ===
{{utama|Arrow of time}}
Time appearsappéars to have a direction – the past lies behind, fixed and incommutable, while the future lies aheadahéad and is not necessarily fixed. Yet the majority of the laws of physics don't provide this [[arrow of time]]. The exceptions include the [[Second law of thermodynamics]], which states that [[entropy]] must increaseincréase over time (see [[Entropy (arrow of time)|Entropy]]); the [[Physical cosmology|cosmological]] arrow of time, which points away from the [[Big Bang]], and the radiative arrow of time, caused by [[light]] only traveling forwards in time. In [[particle physics]], there is also the weakwéak arrow of time, from [[CPT symmetry]], and also [[measurement]] in [[quantum mechanics]] (see [[Measurement in quantum mechanics]]).
 
=== Quantised time ===
{{seealso|Chronon}}
Time quantization is a hypothetical concept. In the modern established physical theoriesthéories (the [[Standard Model]] of Particles and Interactions and [[General Relativity]]) time is not quantized.
 
[[Planck time]] (~ [[1 E-44 s|5.4 × 10<sup>−44</sup>]] seconds) is the unit of time in the system of [[natural units]] known as [[Planck units]]. Current established physical theoriesthéories are believed to fail at this time scale, and many physicists expect that the Planck time might be the smallest unit of time that could ever be measuredméasured, even in principle. Tentative physical theoriesthéories that describe this time scale exist; see for instance [[loop quantum gravity]].
 
== Time and the Big Bang ==
[[Stephen Hawking]] in particular has addressed a connection between time and the [[Big Bang]]. He has sometimes stated that we may as well assume that time began with the Big Bang because trying to answer any question about what happened ''before'' the Big Bang is trying to answer a question that is meaninglessméaningless ''as those events would have been part of a different time frame and different universe outside of the scope of the Big Bang theory''.<ref name=BOT-lecture>{{cite web
|url=http://www.hawking.org.uk/lectures/bot.html
|title=The Beginning of Time
</ref> has criticized some expositions that Hawking has given stating that time didn't exist before the big bang.
 
Hawking, in ''[[A Brief History of Time]]'' and elsewhere, along with several other modern physicists, has stated his position more clearlycléarly and less controversially: that even if time did not begin with the Big Bang and there were another time frame before the Big Bang, no information from events then would be accessible to us, and nothing that happened then would have any effect upon the present time-frame.<ref name=BOT-lecture-B>{{cite web
|url=http://www.hawking.org.uk/lectures/bot.html
|title=The Beginning of Time
[[Gambar:CMB Timeline75.jpg|right|300px|thumb|A graphical representation of the expansion of the universe with the inflationary epoch represented as the dramatic expansion of the [[metric tensor|metric]] seen on the left. Image from [[WMAP]] press release, 2006.]]
 
While the Big Bang model is well established in cosmology, it is likely to be refined in the future. Little is known about the earliestéarliest moments of the universe's history. The [[Penrose-Hawking singularity theorems]] require the existence of a singularity at the beginning of cosmic time. However, these theoremsthéorems assume that [[general relativity]] is correct, but general relativity must breakbréak down before the universe reachesréaches the [[Planck temperature]], and a correct treatmenttréatment of [[quantum gravity]] may avoid the singularity.<ref>{{cite book | author=Hawking, Stephen; and Ellis, G. F. R. | title = The Large Scale Structure of Space-Time | location= Cambridge | publisher=Cambridge University Press | year=1973 |id = ISBN 0-521-09906-4}}</ref>
 
There may also be parts of the universe well beyond what can be observed in principle. If inflation occurred this is likely, for exponential expansion would push large regions of space beyond our observable horizon.
 
Some proposals, eachéach of which entails untested hypotheses, are:
* models including the [[Hartle-Hawking state|Hartle-Hawking boundary condition]] in which the whole of space-time is finite; the Big Bang does represent the limit of time, but without the need for a singularity.<ref>{{cite journal | author=[[James Hartle|J. Hartle]] and [[Stephen Hawking|S. W. Hawking]] | title=Wave function of the universe | journal=Phys. Rev. D | volume=28 | pages=2960 | year=1983 | doi=10.1103/PhysRevD.28.2960}}</ref>
* [[brane cosmology]] models<ref>{{cite journal | author=Langlois, David | title=Brane cosmology: an introduction | year=2002 | id={{arxiv|archive=hep-th|id=0209261}} }}</ref> in which inflation is due to the movement of branes in [[string theory]]; the pre-big bang model; the [[ekpyrotic]] model, in which the Big Bang is the result of a collision between branes; and the [[cyclic model]], a variant of the ekpyrotic model in which collisions occur periodically.<ref>{{cite journal | last=Linde | first=Andre | year=2002 | title=Inflationary Theory versus Ekpyrotic/Cyclic Scenario | id={{arxiv|archive=hep-th|id=0205259}} }}</ref><ref name="rebirth">{{cite news | url=http://www.space.com/scienceastronomy/060508_mm_cyclic_universe.html | title=Recycled Universe: Theory Could Solve Cosmic Mystery | publisher=[[Space.com]] | date=[[8 May]] [[2006]] | accessdate=2007-07-03}}</ref><ref name="rebirth2">{{cite web | url=http://www.science.psu.edu/alert/Bojowald6-2007.htm | title=What Happened Before the Big Bang? | accessdate=2007-07-03}}</ref>
* [[chaotic inflation]], in which inflation events start here and there in a random quantum-gravity foam, eachéach leadingléading to a ''bubble universe'' expanding from its own big bang.<ref>{{cite journal | author = A. Linde |title = Eternal chaotic inflation | journal = Mod. Phys. Lett. |volume = A1 |year =1986 | pages=81}}<br />{{cite journal | author = A. Linde |title = Eternally existing self-reproducing chaotic inflationary universe | journal = Phys. Lett. |volume = B175 |year =1986|pages=395–400}}</ref>
 
Proposals in the last two categories see the Big Bang as an event in a much larger and older universe, or [[multiverse]], and not the literal beginning.
{{seealso|Time travel in fiction|Grandfather paradox}}
 
Time travel is the concept of moving backwards and/or forwards to different points in time, in a manner analogous to moving through [[space]] and different than the "normal" flow of time to an earthboundéarthbound observer. Although time travel has been a [[plot device]] in [[fiction]] since the 19th century, and one-way travel into the future is arguably possible given the phenomenon of [[time dilation]] in the [[theory of relativity]], it is currently unknown whether the [[laws of physics]] would allow time travel to the past. Any technological device, whether fictional or hypothetical, that is used to achieve time travel is known as a [[time machine]].
A central problem with time travel to the past is the violation of [[causality]]; should an effect precede its cause, it would give rise to the possibility of [[temporal paradox]]. Some interpretations of time travel resolve this by accepting the possibility of travel between [[Multiverse (science)|parallel realities]] or [[universe]]s.
 
TheoryThéory would point toward there having to be a physical [[dimension]] in which one could travel to, where the [[present]] (i.e. the point that which you are leavingléaving) would be present at a point fixed in either the past or future. Seeing as this theorythéory would be dependent upon the theorythéory of a [[multiverse]], it is uncertain how or if it would be possible to just prove the possibility of time travel.
 
== Perception of time ==
Even in the presence of timepieces, different individuals may judge an identical length of time to be passing at different rates.{{Fact|date=February 2008}} Commonly, this is referred to as time seeming to "fly" (a period of time seeming to pass faster than possible) or time seeming to "drag" (a period of time seeming to pass slower than possible). The psychologist [[Jean Piaget]] called this form of time perception "lived time."{{Fact|date=February 2008}}
 
This common experience was used to familiarize the general public to the ideasidéas presented by [[Einstein]]'s theorythéory of relativity in a 1930 cartoon by [[Sidney "George" Strube]]:<ref name="Priestley">{{cite book | last = Priestley | first = J. B. | authorlink = J. B. Priestley | title = Man and Time | publisher = Crescent Books | location = New York | year = 1964 | pages = 96 | doi = | isbn = }}</ref><ref name="Sunrise">{{cite web | last = Sunrise | first = | title = Unified Field Theory: A new interpretation | work = Chapter 2 - The Development of the Unified Field Theory, pg. 31 | publisher = Sunrise Information Services | year = 2008 | url = http://www.sunrisepage.com/uft/history.pdf| format = | doi = | accessdate = }}</ref>
 
{{quotation|'''Man:''' Well, it's like this,—supposing I were to sit next to a pretty girl for half an hour it would seem like half a minute,—<br />'''Einstein:''' Braffo! You the idea haf! [''[[sic]]'']<br />'''Man:''' But if I were to sit on a hot stove for two seconds then it would seem like two hours.}}
 
A form of temporal illusion verifiable by experiment is the [[kappa effect]],<ref name="Wada">Wada Y, Masuda T, Noguchi K, 2005, "Temporal illusion called 'kappa effect' in event perception" Perception 34 ECVP Abstract Supplement</ref> whereby time intervals between visual events are perceived as relatively longer or shorter depending on the relative spatial positions of the events. In other words: the perception of temporal intervals appearsappéars to be directly affected, in these cases, by the perception of spatial intervals.
 
Time also appearsappéars to pass more quickly as one gets older.{{Fact|date=July 2008}} [[Stephen Hawking]] suggests that the perception of time is a ratio: ''Unit of Time : Time Lived''.{{Fact|date=February 2008}} For example, one hour to a six-month-old person would be approximately "1:4032", while one hour to a 40-yearyéar-old would be "1:349,440". Therefore an hour appearsappéars much longer to a young child than to an aged adult, even though the measureméasure of time is the same.
 
=== Time in altered states of consciousness ===
Altered states of consciousness are sometimes characterized by a different estimation of time. Some psychoactive substances – such as [[entheogen]]s – may also dramatically alter a person's temporal judgement. When viewed under the influence of such substances as [[LSD]], [[psychedelic mushrooms]] and [[peyote]], a clock may appearappéar to be a strange reference point and a useless tool for measuringméasuring the passage of events as it does not correlate with the user's experience. At higher doses, time may appearappéar to slow down, stop, speed up, go backwards and even seem out of sequence. A typical thought might be "I can't believe it's only 8 o'clock, but then again, what does 8 o'clock mean?" As the boundaries for experiencing time are removed, so is its relevance. Many users claim this unbounded timelessness feels like a glimpse into spiritual infinity. To imagine that one exists somewhere "outside" of time is one of the hallmark experiences of a psychedelic voyage.{{Fact|date=February 2008}} [[cannabis (drug)|Marijuana]], a milder psychedelic, may also distort the perception of time to a lesser degree.<ref>{{cite web |url=http://www.erowid.org/plants/cannabis/cannabis_effects.shtml |title=Cannabis Effects |accessdate=2008-02-15 |work=Erowid |quote=Time sense altered: cars seem like they are moving too fast, time dilation and compression are common at higher doses.}}</ref>
 
The practice of [[meditation]], central to all Buddhist traditions, takes as its goal the reflection of the mind back upon itself, thus altering the subjective experience of time; the so called, 'entering the now', or 'the moment'.{{Fact|date=February 2008}}
== Use of time ==
{{see also|Time management|Time discipline}}
In [[sociology]] and [[anthropology]], [[time discipline]] is the general name given to [[society|social]] and [[economics|economic]] rules, conventions, customs, and expectations governing the measurementméasurement of time, the social currency and awareness of time measurementsméasurements, and peoplepéople's expectations concerning the observance of these customs by others.
 
The use of time is an important issue in understanding [[human behaviour]], [[education]], and [[travel behaviour]]. [[Time use research]] is a developing field of study. 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]] createdcréated 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 reachedréached 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.
== Further reading ==
* {{cite book | authorlink = Julian Barbour | last = Barbour | first = Julian | title = The End of Time: The Next Revolution in Physics | Publisher = Oxford University Press | year = 1999 | id = ISBN 0-19-514592-5 |}}
* {{cite book | last = Das | first = Tushar Kanti | title = The Time Dimension: An Interdisciplinary Guide | year = 1990 | location = New York | publisher = Praeger | id=ISBN 0-275-92681-8 }}- ResearchReséarch bibliography
* {{cite book | authorlink = Paul Davies | last = Davies | first = Paul | title=About Time: Einstein's Unfinished Revolution | year = 1996|id=ISBN 0-684-81822-1}}
* {{cite book | authorlink = Richard Feynman | last = Feynman | first = Richard | title=The Character of Physical Law | year = 1994|origyear=1965|location=Cambridge (Mass)|publisher=The MIT Press|id=ISBN 0-262-56003-8|pages=108-126|url=http://mitpress.mit.edu/catalog/item/default.asp?ttype=2&tid=5277}}
 
=== Philosophy ===
;'''Easternéastern Philosophy'''
* [http://www.literati-tradition.com/time.html The Conceptual Scheme of Chinese Philosophical Thinking - Time]
* [http://nariphaltan.virtualave.net/time.pdf An article on Time and Universal Consciousness]