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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 each day and in [[life expectancy|human lifespans]].
== Temporal measurement ==
Temporal measurement, 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 ===
Artifacts from the [[Palaeolithic]] suggest that the moon was used to calculate time as early as 12,000, and possibly even 30,000 [[Before Present|BP]].<ref name="Rudgley" />
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 year. [[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 ===
[[ImageGambar:Sundial Taganrog.jpg|thumb|right|Horizontal [[sundial]] in [[Taganrog]] (1833)]]
{{main|History of timekeeping devices}}{{seealso|Clock}}
A large variety of [[Measuring instrument|devicedevices]]s have been invented to measure 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]], measured the passage of time from the shadow cast by its crossbar on a non-linear rule. The T was oriented eastward 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>
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 measure 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 09552426060-9552426-0-6</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.
[[ImageGambar:Swatch Irony angle below.jpg|thumb|left|A contemporary [[quartz watch]]]]
The [[hourglass]] uses the flow of sand to measure the flow of time. They were used in navigation. [[Ferdinand Magellan]] used 18 glasses on each 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>
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 mean [[Bell (instrument)|bell]]. The passage of the hours at sea 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 sea.
[[ImageGambar: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 means, including gravity, springs, and various forms of electrical power, and regulated by a variety of means such as a [[pendulum]].
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 measurement 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.
Earth 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 yearly due to [[daylight saving time]] transitions.
=== Sidereal time ===
[[Sidereal time]] is the measurement of time relative to a distant star (instead of solar time that is relative to the sun). It is used in astronomy to predict when a star will be overhead. Due to the rotation of the earth around the sun a sidereal day is slightly less than a solar day.
=== 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]].
[[ImageGambar: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.]]
== Time in religion and mythology ==
{{see|:Category:Time and fate deities}}
In the [[Old Testament]] book [[Ecclesiastes]], traditionally ascribed to [[Solomon]] (970–928 BC), time (as the Hebrew word עדן, זמן ''`iddan(time) zĕman(season)'' is often translated) was traditionally regarded as a medium for the passage of [[predestination|predestined]] events. (Another word, זמן ''zman'', was current as meaning ''time fit for an event'', and is used as the modern [[Hebrew language|Hebrew]] equivalent to the English word "time".)
[[ImageGambar:HinduMeasurements.svg|thumb|left|110px|[[Hindu units of measurement|Hindu units of time]] shown [[logarithmic scale|logarithmically]] ]]
=== Linear and cyclical time ===
{{seealso|Time Cycles|Wheel of time}}
In general, the [[Judaeo-Christian]] concept, based on the [[Bible]], is that time is linear, 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 earth in the [[Second Coming]] to judge the living and the dead. 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 uncreated and eternal so that He and the supernatural world are outside time and exist in [[eternity]].
<br clear=both />
== Time in philosophy ==
{{main|Philosophy of space and time}}
Both arguments were adopted by later Christian philosophers and theologians, 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 real as the [[Object (philosophy)|objectobjects]]s 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 head 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|substancesubstances]]s, 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)|objectobjects]]s are, and temporal measurements are used to quantify how far apart [[Phenomenon|eventevents]]s 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 real homogeneous medium nor a mental construct, but possesses what he referred to as ''Duration''. Duration, in Bergson's view, was creativity and memory as an essential component of reality.<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, leading 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>
However, these arguments often center around what it means 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 ==
{{main|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 08827547850-88275-478-5 (1977 edition), pp. 10-13</ref> The science of classical mechanics is based on this Newtonian idea 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 04866008150-486-60081-5, 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 reasonable definition for what it means for two events to be simultaneous, requires that distances appear compressed and time intervals appear lengthened for events associated with objects in motion relative to an inertial observer.
[[Einstein]] showed that if time and space is measured 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 each other produce a mathematical concept of time that works pretty well for describing the everyday phenomena of most people'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 appears to elapse at different rates relative to different observers in motion relative to one another.
[[ImageGambar: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 ===
Modern [[physics]] views the curvature of [[spacetime]] around an object as much a feature 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 theories, 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 measurement 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 ===
[[ImageGambar:relativity of simultaneity (color).png|thumb|[[Relativity of simultaneity]]: Event B is simultaneous with A in the green reference frame, but it occurred
before in the blue frame, and will occur later in the red frame.]]
{{main|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 people traveling at different speeds, whilst agreeing on cause and effect, will measure 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 measured 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|eventevents]]s 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 appear simultaneous to an observer in a particular inertial reference frame need not be judged as simultaneous by a second observer in a different inertial frame of reference.
=== Relativistic time versus Newtonian time ===
[[ImageGambar:Galilean transform of world line.gif|left|framed|Views of spacetime along the [[world line]] of a rapidly accelerating observer in a Newtonian universe. The events ("dots") that pass the horizontal line are the events current to the observer.]]
[[ImageGambar: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 treatments of time in the Newtonian and the relativistic descriptions. At heart of these differences are the [[Galilean transformation|Galilean]] and [[Lorentz transformation]]s applicable in the Newtonian and relativistic theories, respectively.
However, in the relativistic description the ''observability of events'' is absolute: the movements of the observer influences whether an event passes the light cone of the observer. Notice that with the change from a Newtonian to a relativistic description, the concept of ''absolute time'' is no longer applicable: events move up-and-down in the figure depending on the acceleration of the observer.
=== Arrow of time ===
{{main|Arrow of time}}
Time appears to have a direction – the past lies behind, fixed and incommutable, while the future lies ahead 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 increase 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 weak arrow of time, from [[CPT symmetry]], and also [[measurement]] in [[quantum mechanics]] (see [[Measurement in quantum mechanics]]).
=== Quantised time ===
Time quantization is a hypothetical concept. In the modern established physical theories (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 theories 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 measured, even in principle. Tentative physical theories 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 meaningless ''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
|first=Mortimer J., Ph.D.
|quote=Where Einstein had said that what is not measurable by physicists is of no interest to them, Hawking flatly asserts that what is not measurable by physicists does not exist -- has no reality whatsoever.<br />With respect to time, that amounts to the denial of psychological time which is not measurable by physicists, and also to everlasting time -- time before the Big Bang -- which physics cannot measure. Hawking does not know that both Aquinas and Kant had shown that we cannot rationally establish that time is either finite or infinite.
}} {{cite encyclopedia
Scientists have come to some agreement on descriptions of events that happened 10<sup>−35</sup> seconds after the Big Bang, but generally agree that descriptions about what happened before one [[Planck time]] (5 × 10<sup>−44</sup> seconds) after the Big Bang will likely remain pure speculation.
=== Speculative physics beyond the Big Bang ===
<!-- article is about TIME, not about the Big Bang. This section needs to go to [[Big Bang]] article -->
[[ImageGambar: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 earliest 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 theorems assume that [[general relativity]] is correct, but general relativity must break down before the universe reaches the [[Planck temperature]], and a correct treatment 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>
* 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, each leading 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&ndash;400395–400}}</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.
== Time travel ==
{{main|Time travel}}
{{seealso|Time travel in fiction|Grandfather paradox}}
Theory 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 leaving) would be present at a point fixed in either the past or future. Seeing as this theory would be dependent upon the theory of a [[multiverse]], it is uncertain how or if it would be possible to just prove the possibility of time travel.
== Perception of time ==
=== Time in psychology ===
{{see also|Mental chronometry|Sense 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 ideas presented by [[Einstein]]'s theory 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 appears to be directly affected, in these cases, by the perception of spatial intervals.
Time also appears 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-year-old would be "1:349,440". Therefore an hour appears much longer to a young child than to an aged adult, even though the measure 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 appear to be a strange reference point and a useless tool for measuring the passage of events as it does not correlate with the user's experience. At higher doses, time may appear 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}}
=== Culture ===
Culture is another variable contributing to the perception of time. Anthropologist [[Benjamin Lee Whorf]] reported after studying the [[Hopi]] cultures that: "… the Hopi language is seen to contain no words, grammatical forms, construction or expressions or that refer directly to what we call “time”, or to past, present, or future…"<ref>Carroll, John B. (ed.)(1956). ''Language Thought and Reality. Selected Writings of Benjamin Lee Whorf''. MIT Press, Boston, Massachusetts. ISBN 02627300650-262-73006-5 9780262730068</ref> Whorf's assertion has been challenged and modified. Pinker debunks Whorf's claims about time in the Hopi language, pointing out that the anthropologist Malotki (1983) has found that the Hopi do have a concept of time very similar to that of other cultures; they have units of time, and a sophisticated calendar.<ref>{{Citation | last = Parr-Davies | first = Neil | author-link = | title = The Sapir-Whorf Hypothesis: A Critique | publisher = [[Aberystwyth University]] | date = April 2001 | year = 2001 | url = http://www.aber.ac.uk/media/Students/njp0001.html | accessdate = [[2008-02-02]] }}</ref>
== 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 measurement of time, the social currency and awareness of time measurements, and people's expectations concerning the observance of these customs by others.
[[Arlie Russell Hochschild]] and [[Norbert Elias]] have written on the use of time from a sociological perspective.
== Tempo ogé ==
[[ImageGambar:Le Temps.JPG|thumb|Time's mortal aspect is personified in this bronze statue by [[Charles van der Stappen]]]]
:''See the Time [[#Navigation templates|navigation templates]] below for an exhaustive list of related articles.''
=== Books ===
* ''[[A Brief History of Time]]''
* ''[[About Time (book)|About Time]]''
* ''[[An Experiment with Time]]''
=== Organizations ===
''Leading scholarly organizations for researchers on the history and technology of time and timekeeping''
* [[Antiquarian Horological Society]] - AHS (United Kingdom)
=== Miscellaneous arts and sciences ===
* [[Anachronistic]]
* [[Change]]
=== Miscellaneous units of time ===
* [[Fiscal year]]
* [[Half-life]]
== Notes and references ==
{{Refimprove|date=July 2008}}
== 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 02759268180-275-92681-8 }}- Research 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}}
* {{cite book | last = Whitrow | first = Gerald J. | title = The Natural Philosophy of Time | publisher = Clarendon Press (Oxford) | year = 1980}}
* {{cite book | last = Whitrow | first = Gerald J. | title = Time in History. The evolution of our general awareness of time and temporal perspective | publisher = Oxford University Press | year = 1988 | id = ISBN 0-19-285211-6}}
* {{cite book | last = Rovelli | first = Carlo | title = What is time? What is space? | publisher = Di Renzo Editore |location=Rome | year = 2006|id=ISBN 888323146588-8323-146-5|url=http://www.direnzo.it/main.phtml?Language=en&Doc=0001&ISBN=8883231465}}
== Tumbu ka luar ==
{{external links}}
=== Perception of time ===
* [http://www.primitivism.com/time.htm Time and Its Discontents]
* [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://manchestertiming.co.uk/ Time Perception Research at the University of Manchester]
=== Physics ===
* [http://physics.thinkingpal.com/what-is-time/ Do we actually measure time?]
* [http://physics.nist.gov/GenInt/Time/world.html A walk through Time]
* [http://www.sciam.com/print_version.cfm?articleID=00042F0D-1A0E-1085-94F483414B7F0000 Myth of the Beginning of Time]
=== Philosophy ===
;'''Eastern Philosophy'''
* [http://www.literati-tradition.com/time.html The Conceptual Scheme of Chinese Philosophical Thinking - Time]
=== 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://www.worldtimeengine.com/ World Time for any place on earth]
=== Miscellaneous ===
* [http://wwp.greenwichmeantime.com/ GMT and all other timezones...]
* [http://www.timeticker.com/ TimeTicker and the time tickers...]