View source for Time

{{Short description|Continuous progression from past to future}}
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{{Not to be confused with|Thyme}}
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'''Time''' is the continuous progression of [[existence]] that occurs in an apparently [[irreversible process|irreversible]] succession from the [[past]], through the [[present]], and into the [[future]].<ref>
{{cite dictionary |url=http://oxforddictionaries.com/definition/time |title=Time |dictionary=Oxford Dictionaries |quote=The indefinite continued progress of existence and events in the past, present, and future regarded as a whole |publisher=Oxford University Press |access-date=18 May 2017 |url-status=dead |archive-url=https://web.archive.org/web/20120704084938/http://oxforddictionaries.com/definition/time |archive-date=4 July 2012}}</ref><ref name=DefRefs02/><ref>{{Cite dictionary |url=http://ahdictionary.com/word/search.html?q=time |title=Time |dictionary=The American Heritage Dictionary of the English Language |edition=Fourth |date=2011 |quote=A nonspatial continuum in which events occur in apparently irreversible succession from the past through the present to the future.|url-status=live |archive-url=http://archive.wikiwix.com/cache/20120719114813/http://ahdictionary.com/word/search.html?q=time |archive-date=19 July 2012}}</ref> Time dictates all forms of action, age, and [[causality]], being a component quantity of various measurements used to [[sequence]] events, to compare the duration of events (or the intervals between them), and to quantify [[rates of change]] of quantities in material reality or in the [[qualia|conscious experience]].<ref>[http://www.merriam-webster.com/dictionary/time Merriam-Webster Dictionary] {{webarchive|url=https://web.archive.org/web/20120508031242/http://www.merriam-webster.com/dictionary/time |date=8 May 2012 }} the measured or measurable period during which an action, process, or condition exists or continues: duration; a nonspatial continuum which is measured in terms of events that succeed one another from past through present to future</ref><ref>Compact [[Oxford English Dictionary]] A limited stretch or space of continued existence, as the interval between two successive events or acts, or the period through which an action, condition, or state continues. (1971).</ref><ref name=DefRefs01/><ref name=Poidevin/> Time is often referred to as a fourth [[dimension]], along with [[Three-dimensional space|three spatial dimensions]].<ref>"Newton did for time what the Greek geometers did for space, idealized it into an exactly measurable dimension." ''About Time: Einstein's Unfinished Revolution'', Paul Davies, p. 31, Simon & Schuster, 1996, {{isbn|978-0-684-81822-1}}
</ref>

Time is primarily measured in linear spans or periods, ordered from shortest to longest. Practical, human-scale measurements of time are performed using [[clock]]s and [[calendar]]s, reflecting a 24-hour [[day]] collected into a 365-day [[year]] linked to the astronomical motion of the [[Earth]]. Scientific measurements of time instead vary from [[Planck time]] at the shortest to billions of years at the longest. Measurable time is believed to have effectively begun with the [[Big Bang]] 13.8 billion years ago, encompassed by the [[chronology of the universe]]. Modern physics understands time to be inextricable from [[space]] within the concept of [[spacetime]] described by [[general relativity]].<ref>{{cite book |title=Partial Differential Equations in General Relativity |edition=illustrated |first1=Alan D. |last1=Rendall |publisher=Oxford University Press |year=2008 |isbn=978-0-19-921540-9 |page=9 |url=https://books.google.com/books?id=CPkaAQAAIAAJ}}</ref> Time can therefore be [[Time dilation|dilated]] by velocity and matter to pass faster or slower for an external observer, though this is considered negligible outside of extreme conditions, namely [[relativistic speed]]s or the gravitational pulls of [[black hole]]s.

Throughout history, time has been an important subject of study in religion, philosophy, and science. Temporal measurement has occupied scientists and [[Engineering technologist|technologists]], and has been a prime motivation in navigation and astronomy. 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]] ("''[[carpe diem]]''") and in [[life expectancy|human life spans]].

== Definition ==

The concept of time can be complex. Multiple notions exist, and defining time in a manner applicable to all fields without [[circular definition|circularity]] has consistently eluded scholars.<ref name=Poidevin/><ref name="Carroll2009">{{cite journal|last=Carroll |first=Sean M. |title=From Eternity to Here: The Quest for the Ultimate Theory of Time |title-link=From Eternity to Here: The Quest for the Ultimate Theory of Time |date=2009 |journal=Physics Today |publisher=Dutton |isbn=978-0-525-95133-9 |volume=63 |pages=54–55 |bibcode=2010PhT....63d..54C |doi=10.1063/1.3397046 |issue=4}}</ref><ref>{{Cite web |title=The Feynman Lectures on Physics Vol. I Ch. 5: Time and Distance |url=https://www.feynmanlectures.caltech.edu/I_05.html |access-date=2023-12-15 |website=www.feynmanlectures.caltech.edu}}</ref> Nevertheless, diverse fields such as business, industry, sports, the sciences, and the performing arts all incorporate some notion of time into their respective [[measurement|measuring systems]].<ref name="MLB">{{cite web |last= |first= |date=2011 |title=Official Baseball Rules – 8.03 and 8.04 |url=http://mlb.mlb.com/mlb/downloads/y2011/Official_Baseball_Rules.pdf |url-status=live |archive-url=https://web.archive.org/web/20170701102437/http://mlb.mlb.com/mlb/downloads/y2011/Official_Baseball_Rules.pdf |archive-date=1 July 2017 |access-date=18 May 2017 |publisher=Major League Baseball |format=Free PDF download |quote=Rule 8.03 Such preparatory pitches shall not consume more than one minute of time...Rule 8.04 When the bases are unoccupied, the pitcher shall deliver the ball to the batter within 12 seconds...The 12-second timing starts when the pitcher is in possession of the ball and the batter is in the box, alert to the pitcher. The timing stops when the pitcher releases the ball.}}</ref><ref name=Guinness>{{cite web |title=Guinness Book of Baseball World Records |publisher=Guinness World Records, Ltd. |url=http://www.baseball-almanac.com/recbooks/rb_guin.shtml |quote=The record for the fastest time for circling the bases is 13.3 seconds, set by Evar Swanson at Columbus, Ohio in 1932...The greatest reliably recorded speed at which a baseball has been pitched is 100.9 mph by Lynn Nolan Ryan (California Angels) at Anaheim Stadium in California on 20 August 1974. |access-date=7 July 2012 |url-status=live |archive-url=https://web.archive.org/web/20120606114603/http://www.baseball-almanac.com/recbooks/rb_guin.shtml |archive-date=6 June 2012  }}</ref><ref name=Zeigler>{{cite book |last=Zeigler |first=Kenneth |title=Getting organized at work : 24 lessons to set goals, establish priorities, and manage your time |publisher=McGraw-Hill |date=2008 |url=https://books.google.com/books?id=acPPD6lCCxcC |isbn=978-0-07-159138-6 |access-date=30 July 2019 |archive-date=18 August 2020 |archive-url=https://web.archive.org/web/20200818215121/https://books.google.com/books?id=acPPD6lCCxcC |url-status=live }} 108 pages.</ref> Traditional definitions of time involved the observation of periodic motion such as the apparent motion of the sun across the sky, the phases of the moon, and the passage of a free-swinging pendulum. More modern systems include the [[Global Positioning System]], other satellite systems, [[Coordinated Universal Time]] and [[mean solar time]]. Although these systems differ from one another, with careful measurements they can be synchronized.

In physics, time is a fundamental concept to define other quantities, such as [[velocity]]. To avoid a circular definition,<ref name="TrialogueP3">Duff, Okun, Veneziano, ''ibid''. p. 3. "There is no well established terminology for the fundamental constants of Nature.&nbsp;... 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> [[time in physics]] is [[operational definition|operationally defined]] as "what a [[clock]] reads", specifically a count of repeating events such as the [[SI second]].<ref name=DefRefs01/><ref name=Burnham/><ref>{{cite book |title=Process instruments and controls handbook |edition=3 |first1=Douglas M. |last1=Considine |first2=Glenn D. |last2=Considine |publisher=McGraw-Hill |year=1985 |isbn=978-0-07-012436-3 |pages=18–61 |url=https://books.google.com/books?id=kt1UAAAAMAAJ |bibcode=1985pich.book.....C |access-date=1 November 2016 |archive-date=31 December 2013 |archive-url=https://web.archive.org/web/20131231121535/http://books.google.com/books?id=kt1UAAAAMAAJ |url-status=live }}</ref> Although this aids in practical measurements, it does not address the essence of time. Physicists developed the concept of the [[spacetime]] continuum, where events are assigned four coordinates: three for space and one for time. Events like [[particle collision]]s, [[supernova]]s, or [[rocket launch]]es have coordinates that may vary for different observers, making concepts like "now" and "here" relative. In [[general relativity]], these coordinates do not directly correspond to the causal structure of events. Instead, the [[spacetime interval]] is calculated and classified as either space-like or time-like, depending on whether an observer exists that would say the events are separated by space or by time.<ref>{{cite book |author1=Lilley |first=Sam |url=https://books.google.com/books?id=ZS45AAAAIAAJ |title=Discovering Relativity for Yourself |publisher=Cambridge University Press Archive |year=1981 |isbn=978-0-521-23038-4 |edition=illustrated |page=125 }} [https://books.google.com/books?id=ZS45AAAAIAAJ&pg=PA125 Extract of page 125] {{Webarchive|url=https://web.archive.org/web/20240804095000/https://books.google.com/books?id=ZS45AAAAIAAJ&pg=PA125 |date=4 August 2024 }}.</ref> Since the time required for light to travel a specific distance is the same for all observers—a fact first publicly demonstrated by the [[Michelson–Morley experiment]]—all observers will consistently agree on this definition of time as a [[causal relation]].<ref>{{cite journal |last1=Surya |first1=Sumati |title=The causal set approach to quantum gravity |journal=Living Reviews in Relativity |date=December 2019 |volume=22 |issue=1 |article-number=5 |doi=10.1007/s41114-019-0023-1 |quote=Thus, the causal structure poset (M, ≺) of a future and past distinguishing spacetime is equivalent to its conformal geometry.|doi-access=free |arxiv=1903.11544 |bibcode=2019LRR....22....5S }}</ref>

General relativity does not address the nature of time for extremely small intervals where quantum mechanics holds. In quantum mechanics, time is treated as a universal and absolute parameter, differing from general relativity's notion of independent clocks. The [[problem of time]] consists of reconciling these two theories.<ref>{{cite journal |last1=Macías |first1=Alfredo |last2=Camacho |first2=Abel |title=On the incompatibility between quantum theory and general relativity |journal=Physics Letters B |date=May 2008 |volume=663 |issue=1–2 |pages=99–102 |doi=10.1016/j.physletb.2008.03.052 |quote=In our opinion, it is not possible to reconciliate and integrate into a common scheme the absolute and non-dynamical character of Newtonian time of canonical quantization and path integral approaches with the relativistic and dynamical character of time in general relativity.|doi-access=free |bibcode=2008PhLB..663...99M }}</ref> As of 2025, there is no generally accepted theory of quantum general relativity.<ref>{{cite news |last1=Shavit |first1=Joseph |title=Revolutionary theory finally unites quantum mechanics and Einstein's theory of general relativity |url=https://www.thebrighterside.news/post/revolutionary-theory-finally-unites-quantum-mechanics-and-einsteins-theory-of-general-relativity/ |work=The Brighter Side of News |date=18 July 2024 |language=en |quote=The prevailing consensus has been that Einstein's theory of gravity must be modified to fit within the framework of quantum theory [...] when it comes to merging these two theories into a single, comprehensive framework, the scientific community has hit a roadblock. |archive-date=4 August 2024 |access-date=4 August 2024 |archive-url=https://web.archive.org/web/20240804154452/https://www.thebrighterside.news/post/revolutionary-theory-finally-unites-quantum-mechanics-and-einsteins-theory-of-general-relativity/ |url-status=live }}</ref>

== Measurement ==
[[File:Wooden hourglass 3.jpg|thumb|upright=0.8|The flow of sand in an [[hourglass]] can be used to measure the passage of one [[hour]] of time. It also concretely represents the present as being between the past and the future.]]
	
Methods of temporal measurement, or [[chronometry]], generally take two forms. The first is a [[calendar]], a mathematical tool for organising intervals of time on Earth,<ref name="Richards">{{cite book |title=Mapping Time: The Calendar and its History |url=https://archive.org/details/mappingtimecalen00rich_059 |url-access=limited |last=Richards |first=E. G. |date=1998 |pages=[https://archive.org/details/mappingtimecalen00rich_059/page/n23 3]–5 |publisher=[[Oxford University Press]]|isbn=978-0-19-850413-9 }}</ref> consulted for periods longer than a day. The second is a [[clock]], a physical mechanism that indicates the passage of time, consulted for periods less than a day. The combined measurement marks a specific moment in time from a reference point, or ''[[epoch]]''.

Time is one of the seven fundamental [[physical quantities]] in both the [[International System of Units]] (SI) and [[International System of Quantities]]. The SI base [[unit of time]] is the [[second]], which is defined by measuring the [[electronic transition]] frequency of [[caesium]] atoms.

=== History of the calendar ===
{{Main|Calendar}}

Artifacts from the [[Paleolithic]] suggest that the moon was used to reckon time as early as 6,000 years ago.<ref name="Rudgley">{{cite book |title=The Lost Civilizations of the Stone Age |last=Rudgley |first=Richard |author-link=Richard Rudgley |date=1999 |pages=86–105 |publisher=Simon & Schuster |location=New York}}
</ref>{{Disputed inline|Talk page section|date=January 2025}} [[Lunar calendar]]s were among the first to appear, with years of either 12 or 13 [[lunar month]]s (either 354 or 384 days). Without [[Intercalation (timekeeping)|intercalation]] to add days or months to some years, seasons quickly drift in a calendar based solely on twelve lunar months. [[Lunisolar calendar]]s have a thirteenth month added to some years to make up for the difference between a full year (now known to be about 365.24 days) and a year of just twelve lunar months. The numbers twelve and thirteen came to feature prominently in many cultures, at least partly due to this relationship of months to years.

Other early forms of calendars originated in [[Mesoamerica]], particularly in ancient Mayan civilization, in which they developed the [[Maya calendar]], consisting of multiple interrelated calendars. These calendars were religiously and astronomically based; the [[Haabʼ|Haab']] calendar has 18 months in a year and 20 days in a month, plus five [[epagomenal]] days at the end of the year.<ref>{{cite journal | last1 = Van Stone | first1 = Mark | year = 2011 | title = The Maya Long Count Calendar: An Introduction | journal = Archaeoastronomy | volume = 24 | pages = 8–11 }}</ref> In conjunction, the Maya also used a 260-day sacred calendar called the [[Tzolkʼin|Tzolk'in]].<ref>{{Cite web |last=Davies |first=Diane |title=The Maya Calendar Explained |url=https://www.mayaarchaeologist.co.uk/public-resources/maya-world/maya-calendar-system/ |access-date=2025-02-26 |website=Maya Archaeologist |language=en-GB |archive-date=26 February 2025 |archive-url=https://web.archive.org/web/20250226110722/https://www.mayaarchaeologist.co.uk/public-resources/maya-world/maya-calendar-system/ |url-status=live }}</ref>

The reforms of [[Julius Caesar]] in 45&nbsp;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&nbsp;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 it is now by far the most commonly used calendar around the world.

During the [[French Revolution]], a new clock and calendar were invented as part of the [[Dechristianization of France during the French Revolution|dechristianization of France]] and to create a more rational system in order to replace the Gregorian calendar. The [[French Republican Calendar]]'s days consisted of ten hours of a hundred minutes of a hundred seconds, which marked a deviation from the base 12 ([[duodecimal]]) system used in many other devices by many cultures. The system was abolished in 1806.<ref>{{Cite web |title=French republican calendar {{!}} Revolutionary period, decimal system, reform |url=https://www.britannica.com/science/French-republican-calendar |access-date=2025-02-26 |website=Britannica |language=en |archive-date=8 March 2025 |archive-url=https://web.archive.org/web/20250308214916/https://www.britannica.com/science/French-republican-calendar |url-status=live }}</ref>

=== History of other devices ===
[[File:Canberra (AU), Albert Hall, Sundial -- 2019 -- 1740.jpg|thumb|right|Horizontal [[sundial]] in Canberra]]
[[File:Florence-Duomo-Clock.jpg|thumb|24-hour clock face in Florence]]
{{Main|History of timekeeping devices}}
{{See also|Clock}}

A large variety of [[Measuring instrument|devices]] have been invented to measure time. The study of these devices is called [[horology]].<ref>{{Cite web | url=https://nawcc.org/index.php/main-education | title=Education | access-date=1 July 2018 | archive-date=1 May 2019 | archive-url=https://web.archive.org/web/20190501213902/https://nawcc.org/index.php/main-education | url-status=live }}</ref> 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.

A [[sundial]] is any device that uses the direction of sunlight to cast shadows from a [[gnomon]] onto a set of markings calibrated to indicate the [[local time]], usually to the hour. The idea to separate the day into smaller parts is credited to Egyptians because of their sundials, which operated on a duodecimal system. The importance of the number 12 is due to the number of lunar cycles in a year and the number of stars used to count the passage of night.<ref>{{Cite web |last=Lombardi |first=Michael A. |date=March 5, 2007 |title=Why is a minute divided into 60 seconds, an hour into 60 minutes, yet there are only 24 hours in a day? |url=https://www.scientificamerican.com/article/experts-time-division-days-hours-minutes/ |access-date=2025-02-26 |website=Scientific American |language=en |archive-date=20 September 2023 |archive-url=https://web.archive.org/web/20230920130907/https://www.scientificamerican.com/article/experts-time-division-days-hours-minutes/ |url-status=live }}</ref> [[Obelisk|Obelisks]] made as a gnomon were built as early as {{Circa|3500 BC}}.<ref>{{Cite journal |date=2009-08-12 |title=A Walk Through Time - Early Clocks |url=https://www.nist.gov/pml/time-and-frequency-division/popular-links/walk-through-time/walk-through-time-early-clocks |journal=National Institute of Standards and Technology |language=en |archive-date=2 August 2016 |access-date=26 February 2025 |archive-url=https://web.archive.org/web/20160802075828/http://www.nist.gov/pml/general/time/early.cfm |url-status=live }}</ref> An Egyptian device that dates to {{Circa|1500 BC}}, similar in shape to a bent [[T-square]], also measured the passage of time from the shadow cast by its crossbar on a nonlinear 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>{{cite book |author1=Barnett |first=Jo Ellen |url=https://books.google.com/books?id=-ygDdx73hvcC |title=Time's Pendulum: From Sundials to Atomic Clocks, the Fascinating History of Timekeeping and how Our Discoveries Changed the World |publisher=Harcourt Brace |year=1999 |isbn=978-0-15-600649-1 |edition=reprinted |page=28}}</ref>

Alarm clocks reportedly first appeared in ancient Greece {{Circa|250 BC}} with a water clock made by [[Plato]] that would set off a whistle.<ref>{{Cite book |last=Wagner |first=M. F. |title=The Enigmatic Reality of Time |date=2008 |publisher=Koninklijke Brill |isbn=978-90-474-4360-5 |location=Leiden}}</ref> The hydraulic alarm worked by gradually filling a series of vessels with water. After some time, the water emptied out of a [[siphon]].<ref>{{Cite web |date=2023-11-28 |title=The alarm clock of Plato (5th c. B.C.) - The first awakening device in human history |url=https://kotsanas.com/en/the-alarm-clock-of-plato-5th-c-b-c-the-first-awakening-device-in-human-history/ |access-date=2025-02-25 |website=Museum of the Ancient Greek Technology |language=en-GB}}</ref> Inventor [[Ctesibius]] revised Plato's design; the water clock uses a float as the power drive system and uses a sundial to correct the water flow rate.<ref>{{Cite journal |last1=Hwang |first1=Zheng-Hui |last2=Yan |first2=Hong-Sen |last3=Lin |first3=Tsung-Yi |date=2021-02-19 |title=Historical development of water-powered mechanical clocks |url=https://ms.copernicus.org/articles/12/203/2021/ |journal=Mechanical Sciences |language=English |volume=12 |issue=1 |pages=203–219 |doi=10.5194/ms-12-203-2021 |doi-access=free |bibcode=2021MecSc..12..203H |issn=2191-9151}}</ref>

In medieval philosophical writings, the atom was a unit of time referred to as the smallest possible division of time. The earliest known occurrence in English is in [[Byrhtferth]]'s ''Enchiridion'' (a science text) of 1010–1012,<ref>{{cite encyclopedia |year=2008 |title=Byrhtferth of Ramsey |encyclopedia=[[Encyclopædia Britannica]] |url=http://search.eb.com/eb/article-9438957 |access-date=15 September 2008 |url-status=live |archive-url=https://web.archive.org/web/20200614055906/https://academic.eb.com/?target=%2Flevels |archive-date=14 June 2020}}</ref> where it was defined as 1/564 of a ''momentum'' (1{{frac|1|2}}&nbsp;minutes),<ref>"atom", [[Oxford English Dictionary]], Draft Revision September 2008 (contains relevant citations from Byrhtferth's ''Enchiridion'')</ref> and thus equal to 15/94 of a second. It was used in the ''[[computus]]'', the process of calculating the date of Easter. The most precise timekeeping device of the [[ancient world]] was the [[water clock]], or ''clepsydra'', one of which was found in the tomb of Egyptian pharaoh [[Amenhotep I]]. They could be used to measure the hours even at night but required manual upkeep to replenish the flow of water. The [[Ancient Greece|ancient Greeks]] and the people from [[Chaldea]] (southeastern Mesopotamia) regularly maintained timekeeping records as an essential part of their astronomical observations. Arab inventors and engineers, in particular, made improvements on the use of water clocks up to the Middle Ages.<ref>Barnett, ''ibid'', p. 37.</ref> In the 11th century, [[List of Chinese inventions|Chinese inventors]] and [[History of science and technology in China|engineers]] invented the first mechanical clocks driven by an [[escapement]] mechanism.

[[File:Swatch Irony angle below.jpg|thumb|A contemporary [[quartz watch]], 2007 ]]
Incense sticks and candles were, and are, commonly used to measure time in temples and churches across the globe. Water clocks, and, later, mechanical clocks, were used to mark the events of the abbeys and monasteries of the Middle Ages. The passage of the hours at sea can also be marked by [[Ship's bell|bell]]. The hours were marked by bells in abbeys as well as at sea. [[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,'' pp. 372–384.</ref> The [[hourglass]] uses the flow of sand to measure the flow of time. They were also used in navigation. [[Ferdinand Magellan]] used 18 glasses on each ship for his circumnavigation of the globe (1522).<ref>Bergreen, Laurence. ''Over the Edge of the World: Magellan's Terrifying Circumnavigation of the Globe'' (HarperCollins Publishers, 2003), {{isbn|0-06-621173-5}}{{page needed|date=October 2018}}</ref> The English word [[clock]] probably comes from the Middle Dutch word ''klocke'' which, in turn, derives from the medieval Latin word ''clocca'', which ultimately derives from Celtic and is cognate with French, Latin, and German words that mean [[Bell (instrument)|bell]].

Great advances in accurate time-keeping were made by [[Galileo Galilei]] and especially [[Christiaan Huygens]] with the invention of pendulum-driven clocks along with the invention of the minute hand by Jost Burgi.<ref name="Clocks 2016">"History of Clocks." About.com Inventors. About.com, n.d. Web. 21 February 2016.</ref> There is also a clock that was designed to keep time for 10,000 years called the [[Clock of the Long Now]]. Alarm clock devices were later mechanized. [[Levi Hutchins]]{{'s}} alarm clock has been credited as the first American alarm clock, though it can only ring at 4 a.m. [[Antoine Rédier|Antoine Redier]] was also credited as the first person to patent an adjustable mechanical alarm clock in 1847.<ref>{{Cite web |last=Tikkanen |first=Amy |date=15 January 2009 |title=22 Questions About Time and Timekeeping Answered |url=https://www.britannica.com/list/22-questions-about-time-and-timekeeping-answered |access-date=2025-02-25 |website=Britannica |language=en |archive-date=12 March 2025 |archive-url=https://web.archive.org/web/20250312000400/https://www.britannica.com/list/22-questions-about-time-and-timekeeping-answered |url-status=live }}</ref> Digital forms of alarm clocks became more accessible through digitization and integration with other technologies, such as [[Smartphone|smartphones]].

[[File:ChipScaleClock2 HR.jpg|thumb|Chip-scale [[atomic clock]]s, such as this one unveiled in 2004, are expected to greatly improve [[Global Positioning System|GPS]] location.<ref>{{cite journal |url=https://www.nist.gov/public_affairs/releases/miniclock.cfm |title=NIST Unveils Chip-Scale Atomic Clock |journal=NIST |date=27 August 2004 |access-date=9 June 2011 |url-status=live |archive-url=https://web.archive.org/web/20110522125607/http://www.nist.gov/public_affairs/releases/miniclock.cfm |archive-date=22 May 2011  }}</ref>]]

The most accurate timekeeping devices are [[atomic clock]]s, which are accurate to seconds in many millions of years,<ref>{{cite news |url=http://www.canada.com/vancouversun/news/story.html?id=e24ccfa7-44eb-40b7-8b67-daf8263569ff |archive-url=https://web.archive.org/web/20120211190407/http://www.canada.com/vancouversun/news/story.html?id=e24ccfa7-44eb-40b7-8b67-daf8263569ff |url-status=dead |archive-date=11 February 2012 |title=New atomic clock can keep time for 200 million years: Super-precise instruments vital to deep space navigation |date=16 February 2008 |work=Vancouver Sun |access-date=9 April 2011}}</ref> and are used to calibrate other clocks and timekeeping instruments. Atomic clocks use the frequency of [[electronic transition]]s in certain atoms to measure the second. One of the atoms used is [[caesium]]; most modern atomic clocks probe caesium with microwaves to determine the frequency of these electron vibrations.<ref>{{cite web |url=https://www.nist.gov/public_affairs/releases/n99-22.cf |title=NIST-F1 Cesium Fountain Clock |access-date=24 July 2015 |archive-date=25 March 2020 |archive-url=https://web.archive.org/web/20200325220323/https://www.nist.gov/public_affairs/releases/n99-22.cf |url-status=dead }}</ref> Since 1967, the International System of Measurements 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 the radiation that corresponds to the transition between two electron spin energy levels of the ground state of the <sup>133</sup>Cs atom. A portable timekeeper that meets certain precision standards is called a [[chronometer watch|chronometer]]. Initially, the term was used to refer to the [[marine chronometer]], a timepiece used to determine [[longitude]] by means of [[celestial navigation]], a precision first achieved by [[John Harrison]]. More recently, the term has also been applied to the [[chronometer watch]], a watch that meets precision standards set by the Swiss agency [[COSC]].

In modern times, the [[Global Positioning System]] in coordination with the [[Network Time Protocol]] can be used to synchronize timekeeping systems across the globe. {{As of|May 2010}}, the smallest time interval uncertainty in direct measurements is on the order of 12 [[attosecond]]s (1.2&nbsp;×&nbsp;10<sup>−17</sup> seconds), about 3.7&nbsp;×&nbsp;10<sup>26</sup> [[Planck time]]s.<ref>{{cite web |url=http://www.physorg.com/news192909576.html |title=12 attoseconds is the world record for shortest controllable time |date=12 May 2010 |access-date=19 April 2012 |url-status=live |archive-url=https://web.archive.org/web/20110805173204/http://www.physorg.com/news192909576.html |archive-date=5 August 2011  }}
</ref> The time measured was the delay caused by out-of-sync electron waves' [[Wave interference|interference patterns]].<ref>{{Cite web |last=Andrewes |first=William J. H. |date=February 1, 2006 |title=A Chronicle Of Timekeeping |url=https://www.scientificamerican.com/article/a-chronicle-of-timekeeping-2006-02/ |access-date=2025-02-26 |website=Scientific American |language=en}}</ref>

=== Units ===
{{See also|Time (Orders of magnitude)|Unit of time#List}}

The second (s) is the [[SI]] base unit. A [[minute]] (min) is 60 seconds in length (or, rarely, 59 or 61 seconds when leap seconds are employed), and an [[hour]] is 60 minutes or 3600 seconds in length. A day is usually 24 hours or 86,400 seconds in length; however, the duration of a calendar day can vary due to [[daylight saving time]] and [[leap second]]s.

=== Standards ===
{{main|Time standard}}

A time standard is a specification for measuring time: assigning a number or [[calendar date]] to an [[instant]] (point in time), quantifying the duration of a time interval, and establishing a [[chronology]] (ordering of events). In modern times, several time specifications have been officially recognized as standards, where formerly they were matters of custom and practice. The invention in 1955 of the caesium [[atomic clock]] has led to the replacement of older and purely astronomical time standards such as [[sidereal time]] and [[ephemeris time]], for most practical purposes, by newer time standards based wholly or partly on atomic time using the SI second.

[[International Atomic Time]] (TAI) is the primary international time standard from which other time standards are calculated. [[Universal Time]] (UT1) is mean solar time at 0° longitude, computed from astronomical observations. It varies from TAI because of the irregularities in Earth's rotation. [[Coordinated Universal Time]] (UTC) is an atomic time scale designed to approximate Universal Time. UTC differs from TAI by an integral number of seconds. UTC is kept within 0.9&nbsp;second of UT1 by the introduction of one-second steps to UTC, the [[leap second]]. The [[Global Positioning System]] broadcasts a very precise time signal based on UTC time.

The surface of the Earth is split into a number of [[time zone]]s. Standard time or [[civil time]] in a time zone deviates a fixed, round amount, usually a whole number of hours, from some form of Universal Time, usually UTC. Most time zones are exactly one hour apart, and by convention compute their local time as an offset from UTC. For example, time zones at sea are based on UTC. In many locations (but not at sea) these offsets vary twice yearly due to [[daylight saving time]] transitions.

Some other time standards are used mainly for scientific work. [[Terrestrial Time]] is a theoretical ideal scale realized by TAI. [[Geocentric Coordinate Time]] and [[Barycentric Coordinate Time]] are scales defined as [[coordinate time]]s in the context of the general theory of relativity, with TCG applying to Earth's center and TCB to the solar system's [[Barycenter (astronomy)|barycenter]]. [[Barycentric Dynamical Time]] is an older relativistic scale related to TCB that is still in use.

== Philosophy ==

=== Religion ===
[[File:Jain scale of time.JPG|thumb|right|upright=1.2|Scale of time in [[Jain]] texts shown [[logarithmic scale|logarithmically]] ]]
{{Further|Time and fate deities}}

==== Cyclical views of time ====
{{See also|Wheel of time}}
Many ancient cultures, particularly in the East, had a cyclical view of time. In these traditions, time was often seen as a recurring pattern of ages or cycles, where events and phenomena repeated themselves in a predictable manner. One of the most famous examples of this concept is found in [[Hindu philosophy]], where time is depicted as a wheel called the "[[Kalachakra]]" or "Wheel of Time." According to this belief, the universe undergoes endless cycles of creation, preservation, and destruction.<ref name="newman51">{{cite book |author=Newman |first=John |url=https://books.google.com/books?id=iGDCQNn0C9oC |title=The Wheel of Time: Kalachakra in Context |publisher=Shambhala |year=1991 |isbn=978-1-55939-779-7 |editor=Geshe Lhundub Sopa |pages=51–54, 62–77}}</ref>

Similarly, in other ancient cultures such as those of the Mayans, Aztecs, and Chinese, there were also beliefs in cyclical time, often associated with astronomical observations and calendars.<ref>{{Cite web |date=2012-02-27 |title=Chichén Itzá: Venus Cycle |url=https://www.nationalgeographic.com/travel/article/chichen-itza-venus-cycle |access-date=2024-05-08 |website=Travel |language=en |archive-date=8 May 2024 |archive-url=https://web.archive.org/web/20240508213816/https://www.nationalgeographic.com/travel/article/chichen-itza-venus-cycle |url-status=live }}</ref> These cultures developed complex systems to track time, seasons, and celestial movements, reflecting their understanding of cyclical patterns in nature and the universe.

The cyclical view of time contrasts with the linear concept of time more common in Western thought, where time is seen as progressing in a straight line from past to future without repetition.<ref>{{cite book |title=The Future: A Very Short Introduction |author1=Jennifer M. Gidley |edition= |publisher=Oxford University Press |year=2017 |isbn=978-0-19-105424-2 |page=13 |url=https://books.google.com/books?id=I3NdDgAAQBAJ }} [https://books.google.com/books?id=I3NdDgAAQBAJ&pg=PA13 Extract of page 13] {{Webarchive|url=https://web.archive.org/web/20240614171103/https://books.google.com/books?id=I3NdDgAAQBAJ&pg=PA13 |date=14 June 2024 }}</ref>

==== Time in Abrahamic religions ====
In general, the Islamic and [[Judeo-Christian]] world-view regards time as linear<ref>{{cite book |last1=Rust |first1=Eric Charles |url=https://books.google.com/books?id=AcSw6GDlLVwC |title=Religion, Revelation and Reason |publisher=Mercer University Press |year=1981 |isbn=978-0-86554-058-3 |publication-date=1981 |page=60 |quote=Profane time, as [[Mircea Eliade|Eliade]] points out, is linear. [...] In the Judaeo-Christian religions – Judaism, Christianity, Islam – history is taken seriously, and linear time is accepted. |access-date=20 August 2015 |archive-url=https://web.archive.org/web/20170403232350/https://books.google.com/books?id=AcSw6GDlLVwC |archive-date=3 April 2017 |url-status=live}}</ref> and directional,<ref>{{cite book |editor1-last=Betz |editor1-first=Hans Dieter |title=Religion Past & Present: Encyclopedia of Theology and Religion |url=https://books.google.com/books?id=kjIOAQAAMAAJ |volume=4 |edition=4 |publisher=Brill |publication-date=2008 |page=101 |isbn=978-90-04-14688-4 |quote=[...] God produces a creation with a directional time structure [...]. |year=2008 |access-date=20 August 2015 |archive-date=24 September 2015 |archive-url=https://web.archive.org/web/20150924154435/https://books.google.com/books?id=kjIOAQAAMAAJ |url-status=live }}</ref> beginning with the act of [[Creation myth|creation]] by God. The traditional Christian view sees time ending, teleologically,<ref>{{cite book |last1=Lundin |first1=Roger |last2=Thiselton |first2=Anthony C. |author-link2=Anthony Thiselton |last3=Walhout |first3=Clarence |title=The Promise of Hermeneutics |url=https://books.google.com/books?id=3Y2F6rSq9VMC |publisher=Wm. B. Eerdmans Publishing |publication-date=1999 |page=121 |isbn=978-0-8028-4635-8 |quote=We need to note the close ties between teleology, eschatology, and utopia. In Christian theology, the understanding of the teleology of particular actions is ultimately related to the teleology of history in general, which is the concern of eschatology. |year=1999 |access-date=20 August 2015 |archive-date=19 September 2015 |archive-url=https://web.archive.org/web/20150919095555/https://books.google.com/books?id=3Y2F6rSq9VMC |url-status=live }}</ref> with the [[Christian eschatology|eschatological]] end of the present order of things, the "[[Eschatology|end time]]". Though some Christian theologians (such as [[Augustine of Hippo]] and [[Thomas Aquinas|Aquinas]]<ref>{{Cite journal |last=Staley |first=Kevin M. |date=2006 |title=Omniscience, Time, and Eternity: Is Aquinas Inconsistent? |url=https://www.anselm.edu/sites/default/files/Documents/Institute%20of%20SA%20Studies/4.5.3.24_32Staley.pdf |journal=The Saint Anselm Journal}}</ref>) believe that God is outside of time, seeing all events simultaneously, that time did not exist before God, and that God created time.<ref>{{Cite web |last=Barr |first=Stephen M. |date=February 7, 2020 |title=St. Augustine's Relativistic Theory of Time |url=https://churchlifejournal.nd.edu/articles/augustines-push-against-the-limits-of-time/ |archive-url=https://web.archive.org/web/20240415085520/https://churchlifejournal.nd.edu/articles/augustines-push-against-the-limits-of-time/ |archive-date=15 April 2024 |access-date=2025-02-28 |website=Church Life Journal |language=en-US |url-status=live }}</ref><ref>{{Cite web |last=Selzer |first=Mark Albert |title=Augustine on Time: Human Time, Divine Eternity, and Why the Former is Really the Latter |url=https://www.csueastbay.edu/philosophy/reflections/2009/contents/mark-selz.html |access-date=2025-02-28 |website=California State University, East Bay |language=en |archive-date=15 January 2025 |archive-url=https://web.archive.org/web/20250115122951/https://www.csueastbay.edu/philosophy/reflections/2009/contents/mark-selz.html |url-status=live }}</ref>

In the [[Old Testament]] book [[Ecclesiastes]], traditionally ascribed to [[Solomon]] (970–928 BC), time is depicted as cyclical and beyond human control.<ref>Ecclesiastes 1:4–11</ref> The book wrote that there is an appropriate season or time for every activity.<ref>Ecclesiastes 3:1–8</ref>

==== Time in Greek mythology ====
The Greek language denotes two distinct principles, [[Chronos]] and [[Kairos]]. The former refers to numeric, or chronological, time. The latter, literally "the right or opportune moment", relates specifically to metaphysical or Divine time. In theology, Kairos is qualitative, as opposed to quantitative.<ref>{{cite web |url=https://www.perseus.tufts.edu/hopper/text?doc=Perseus%3Atext%3A1999.04.0057%3Aentry%3Dkairo%2Fs1 |title=(Dictionary Entry) |work=Henry George Liddell, Robert Scott, A Greek-English Lexicon |access-date=13 July 2015 |archive-date=7 May 2022 |archive-url=https://web.archive.org/web/20220507073935/https://www.perseus.tufts.edu/hopper/text?doc=Perseus%3Atext%3A1999.04.0057%3Aentry%3Dkairo%2Fs1 |url-status=live }}</ref>

In Greek mythology, Chronos (ancient Greek: Χρόνος) is identified as the personification of time. His name in Greek means "time" and is alternatively spelled Chronus (Latin spelling) or Khronos. Chronos is usually portrayed as an old, wise man with a long, gray beard, such as "Father Time". Some English words whose etymological root is khronos/chronos include ''chronology'', ''chronometer'', ''chronic'', ''[[anachronism]]'', ''synchronise'', and ''chronicle''.

==== Time in Kabbalah & Rabbinical thought====
Rabbis sometimes saw time like "an accordion that was expanded and collapsed at will."<ref>New Myths and Meanings in Jewish New Moon Rituals
David M. Rosen, Victoria P. Rosen
Ethnology, Vol. 39, No. 3 (Summer, 2000), pp. 263–277
(referencing Yerushalmi 1989)
</ref> According to [[Kabbalah|Kabbalists]], "time" is a [[paradox]]<ref>{{cite book |title=Kabbalah and Modernity: Interpretations, Transformations, Adaptations |first1=Boʿaz |last1=Hus |first2=Marco |last2=Pasi |first3=Kocku von |last3=Stuckrad |author3-link=Kocku von Stuckrad |publisher=BRILL |date=2011 |isbn=978-90-04-18284-4 |url=https://books.google.com/books?id=mIGMsLiol7EC |access-date=27 February 2016 |archive-date=13 May 2016 |archive-url=https://web.archive.org/web/20160513114415/https://books.google.com/books?id=mIGMsLiol7EC |url-status=live }}</ref> and an [[illusion]].<ref>{{cite book |title=Alef, Mem, Tau: Kabbalistic Musings on Time, Truth, and Death |first1=Elliot R. |last1=Wolfson |publisher=University of California Press |date=2006 |isbn=978-0-520-93231-9 |page=111 |url=https://books.google.com/books?id=ozhPY2fcNCcC |access-date=7 May 2020 |archive-date=19 August 2020 |archive-url=https://web.archive.org/web/20200819171639/https://books.google.com/books?id=ozhPY2fcNCcC |url-status=live }} [https://books.google.com/books?id=ozhPY2fcNCcC&pg=PA111 Extract of page 111] {{Webarchive|url=https://web.archive.org/web/20220511191004/https://books.google.com/books?id=ozhPY2fcNCcC&pg=PA111 |date=11 May 2022 }}</ref>

==== Time in Advaita Vedanta ====
According to [[Advaita Vedanta]], time is integral to the phenomenal world, which lacks independent reality. Time and the phenomenal world are products of [[Maya (religion)|''maya'']], influenced by our senses, concepts, and imaginations. The phenomenal world, including time, is seen as impermanent and characterized by plurality, suffering, conflict, and division. Since phenomenal existence is dominated by temporality (''[[Kāla|kala]]''), everything within time is subject to change and decay. Overcoming pain and death requires knowledge that transcends temporal existence and reveals its eternal foundation.<ref>{{Cite journal |last=Puligandla |first=R. |date=1974 |title=Time and History in the Indian Tradition |url=https://www.jstor.org/stable/1398019 |journal=Philosophy East and West |volume=24 |issue=2 |pages=165–170 |doi=10.2307/1398019 |jstor=1398019 |issn=0031-8221 |archive-date=13 June 2024 |access-date=13 June 2024 |archive-url=https://web.archive.org/web/20240613231718/https://www.jstor.org/stable/1398019 |url-status=live |url-access=subscription }}</ref>

=== In Western philosophy ===
{{Main|Philosophy of space and time|Temporal finitism}}
[[File:Le Temps.JPG|thumb|Time's mortal aspect is personified in this bronze statue by [[Charles van der Stappen]].]]

Two contrasting viewpoints on time divide prominent philosophers. One view is that time is part of the fundamental structure of the [[universe]]—a [[dimension]] independent of events, in which events occur in [[sequence]]. [[Isaac Newton]] subscribed to this [[Philosophical realism|realist]] view, and hence it is sometimes referred to as [[Absolute space and time|Newtonian time]].<ref name="Rynasiewicz">
{{cite encyclopedia |title=Newton's Views on Space, Time, and Motion |encyclopedia=[[Stanford Encyclopedia of Philosophy]] |publisher=Stanford University |url=http://plato.stanford.edu/entries/newton-stm/ |access-date=5 February 2012 |last=Rynasiewicz |first=Robert: Johns Hopkins University |date=12 August 2004 |archive-url=https://archive.today/20120716191122/http://plato.stanford.edu/entries/newton-stm/ |archive-date=16 July 2012 |quote=Newton did not regard space and time as genuine substances (as are, paradigmatically, bodies and minds), but rather as real entities with their own manner of existence as necessitated by God's existence&nbsp;... To paraphrase: Absolute, true, and mathematical time, from its own nature, passes equably without relation to anything external, and thus without reference to any change or way of measuring of time (e.g., the hour, day, month, or year). |url-status=live}}
</ref><ref name=Markosian >{{cite encyclopedia |url=http://plato.stanford.edu/entries/time/#3 |last=Markosian |first=Ned |author-link=Ned Markosian |title=Time |encyclopedia=The Stanford Encyclopedia of Philosophy (Winter 2002 Edition) |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. |access-date=23 September 2011 |archive-date=14 September 2006 |archive-url=https://web.archive.org/web/20060914153738/http://plato.stanford.edu//entries/time/#3 |url-status=live }}</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 instead 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 encyclopedia |title=Gottfried Wilhelm Leibniz (1646–1716) Metaphysics&nbsp;– 7. Space, Time, and Indiscernibles |encyclopedia=The Internet Encyclopedia of Philosophy |url=http://www.iep.utm.edu/leib-met/#H7 |access-date=9 April 2011 |last=Burnham |first=Douglas: Staffordshire University |date=2006 |archive-url=https://web.archive.org/web/20110514000548/http://www.iep.utm.edu/leib-met/#H7 |archive-date=14 May 2011 |quote=First of all, Leibniz finds the idea that space and time might be substances or substance-like absurd (see, for example, "Correspondence with Clarke," Leibniz's Fourth Paper, §8ff). In short, an empty space would be a substance with no properties; it will be a substance that even God cannot modify or destroy.... That is, space and time are internal or intrinsic features of the complete concepts of things, not extrinsic.... Leibniz's view has two major implications. First, there is no absolute location in either space or time; location is always the situation of an object or event relative to other objects and events. Second, space and time are not in themselves real (that is, not substances). Space and time are, rather, ideal. Space and time are just metaphysically illegitimate ways of perceiving certain virtual relations between substances. They are phenomena or, strictly speaking, illusions (although they are illusions that are well-founded upon the internal properties of substances).... It is sometimes convenient to think of space and time as something "out there," over and above the entities and their relations to each other, but this convenience must not be confused with reality. Space is nothing but the order of co-existent objects; time nothing but the order of successive events. This is usually called a relational theory of space and time. |url-status=live}}
</ref> and [[Immanuel Kant]],<ref name="Mattey">
{{cite web |last=Mattey |first=G. J. |date=22 January 1997 |title=Critique of Pure Reason, Lecture notes: Philosophy 175 UC Davis |url=http://www-philosophy.ucdavis.edu/mattey/kant/TIMELEC.HTM |url-status=dead |archive-url=https://web.archive.org/web/20050314201600/http://www-philosophy.ucdavis.edu/mattey/kant/TIMELEC.HTM |archive-date=14 March 2005 |access-date=9 April 2011 |quote=What is correct in the Leibnizian view was its anti-metaphysical stance. Space and time do not exist in and of themselves, but in some sense are the product of the way we represent things. The[y] are ideal, though not in the sense in which Leibniz thought they are ideal (figments of the imagination). The ideality of space is its mind-dependence: it is only a condition of sensibility.... Kant concluded ... "absolute space is not an object of outer sensation; it is rather a fundamental concept which first of all makes possible all such outer sensation."...Much of the argumentation pertaining to space is applicable, ''mutatis mutandis'', to time, so I will not rehearse the arguments. As space is the form of outer intuition, so time is the form of inner intuition.... Kant claimed that time is real, it is "the real form of inner intuition."}}
</ref><ref name="McCormick">
{{cite encyclopedia |title=Immanuel Kant (1724–1804) Metaphysics: 4. Kant's Transcendental Idealism |url=http://www.iep.utm.edu/kantmeta/#H4 |encyclopedia=The Internet Encyclopedia of Philosophy |first=Matt |last=McCormick |date=2006 |quote=Time, Kant argues, is also necessary as a form or condition of our intuitions of objects. The idea of time itself cannot be gathered from experience because succession and simultaneity of objects, the phenomena that would indicate the passage of time, would be impossible to represent if we did not already possess the capacity to represent objects in time.... Another way to put the point is to say that the fact that the mind of the knower makes the ''a priori'' contribution does not mean that space and time or the categories are mere figments of the imagination. Kant is an empirical realist about the world we experience; we can know objects as they appear to us. He gives a robust defense of science and the study of the natural world from his argument about the mind's role in making nature. All discursive, rational beings must conceive of the physical world as spatially and temporally unified, he argues. |access-date=9 April 2011 |url-status=live |archive-url=https://web.archive.org/web/20110426002607/http://www.iep.utm.edu/kantmeta/#H4 |archive-date=26 April 2011  }}
</ref> holds that ''time'' is neither an event nor a thing, and thus is not itself measurable nor can it be travelled. Furthermore, it may be that there is a [[Subjectivity|subjective]] component to time, but whether or not time itself is "felt", as a sensation, or is a judgment, is a matter of debate.<ref name="DefRefs02">*{{cite web |url=http://www.yourdictionary.com/time |title=Webster's New World College Dictionary |date=2010 |quote=1.indefinite, unlimited duration in which things are considered as happening in the past, present, or future; every moment there has ever been or ever will be… a system of measuring duration 2.the period between two events or during which something exists, happens, or acts; measured or measurable interval |access-date=9 April 2011 |url-status=live |archive-url=https://web.archive.org/web/20110805101704/http://www.yourdictionary.com/time |archive-date=5 August 2011 }}
*{{cite web |url=http://dictionary.reference.com/browse/time?r=66 |title=The American Heritage Stedman's Medical Dictionary |date=2002 |quote=A duration or relation of events expressed in terms of past, present, and future, and measured in units such as minutes, hours, days, months, or years. |access-date=9 April 2011 |url-status=live |archive-url=https://web.archive.org/web/20120305015803/http://dictionary.reference.com/browse/time?r=66 |archive-date=5 March 2012 }}
*{{cite web |url=http://www.collinslanguage.com/results.aspx?context=3&reversed=False&action=define&homonym=-1&text=time |title=Collins Language.com |publisher=HarperCollins |date=2011 |quote=1. The continuous passage of existence in which events pass from a state of potentiality in the future, through the present, to a state of finality in the past. 2. ''physics'' a quantity measuring duration, usually with reference to a periodic process such as the rotation of the earth or the frequency of electromagnetic radiation emitted from certain atoms. In classical mechanics, time is absolute in the sense that the time of an event is independent of the observer. According to the theory of relativity it depends on the observer's frame of reference. Time is considered as a fourth coordinate required, along with three spatial coordinates, to specify an event. |access-date=18 December 2011 |url-status=dead |archive-url=https://web.archive.org/web/20111002190630/http://www.collinslanguage.com/results.aspx?context=3&reversed=False&action=define&homonym=-1&text=time |archive-date=2 October 2011 }}
*{{cite web |url=http://dictionary.reference.com/browse/time?r=66 |title=The American Heritage Science Dictionary @dictionary.com |date=2002 |quote=1. A continuous, measurable quantity in which events occur in a sequence proceeding from the past through the present to the future. 2a. An interval separating two points of this quantity; a duration. 2b. A system or reference frame in which such intervals are measured or such quantities are calculated. |access-date=9 April 2011 |url-status=live |archive-url=https://web.archive.org/web/20120305015803/http://dictionary.reference.com/browse/time?r=66 |archive-date=5 March 2012 }}
*{{cite web |url=http://scienceworld.wolfram.com/physics/Time.html |title=Eric Weisstein's World of Science |date=2007 |quote=A quantity used to specify the order in which events occurred and measure the amount by which one event preceded or followed another. In special relativity, ct (where c is the speed of light and t is time), plays the role of a fourth dimension. |access-date=9 April 2011 |archive-date=29 November 2017 |archive-url=https://web.archive.org/web/20171129190443/http://scienceworld.wolfram.com/physics/Time.html |url-status=live }}</ref><ref name="DefRefs01">*{{cite web |url=http://www.iep.utm.edu/time/ |title=Internet Encyclopedia of Philosophy |date=2010 |quote=Time is what clocks measure. We use time to place events in sequence one after the other, and we use time to compare how long events last... Among philosophers of physics, the most popular short answer to the question "What is physical time?" is that it is not a substance or object but rather a special system of relations among instantaneous events. This working definition is offered by Adolf Grünbaum who applies the contemporary mathematical theory of continuity to physical processes, and he says time is a linear continuum of instants and is a distinguished one-dimensional sub-space of four-dimensional spacetime. |access-date=9 April 2011 |url-status=live |archive-url=https://web.archive.org/web/20110411140140/http://www.iep.utm.edu/time/ |archive-date=11 April 2011 }}
*{{cite web |url=http://dictionary.reference.com/browse/time?r=66 |title=Dictionary.com Unabridged, based on Random House Dictionary |date=2010 |quote=1. the system of those sequential relations that any event has to any other, as past, present, or future; indefinite and continuous duration regarded as that in which events succeed one another.... 3. (sometimes initial capital letter) a system or method of measuring or reckoning the passage of time: mean time; apparent time; Greenwich Time. 4. a limited period or interval, as between two successive events: a long time.... 14. a particular or definite point in time, as indicated by a clock: What time is it? ... 18. an indefinite, frequently prolonged period or duration in the future: Time will tell if what we have done here today was right. |access-date=9 April 2011 |url-status=live |archive-url=https://web.archive.org/web/20120305015803/http://dictionary.reference.com/browse/time?r=66 |archive-date=5 March 2012 }}
*{{cite book |url=https://books.google.com/books?id=HrHvAAAAMAAJ&q=%22time+is+what+clocks+measure%22 |title=Physics |first1=Donald G. |last1=Ivey |first2=J.N.P. |last2=Hume |volume=1 |page=65 |date=1974 |publisher=Ronald Press |quote=Our operational definition of time is that time is what clocks measure. |access-date=7 May 2020 |archive-date=14 April 2021 |archive-url=https://web.archive.org/web/20210414052527/https://books.google.com/books?id=HrHvAAAAMAAJ&q=%22time+is+what+clocks+measure%22 |url-status=live }}</ref><ref name="Poidevin">{{cite encyclopedia |url=http://plato.stanford.edu/archives/win2004/entries/time-experience |title=The Experience and Perception of Time |last=Le Poidevin |first=Robin |date=Winter 2004 |encyclopedia=The Stanford Encyclopedia of Philosophy |editor=Edward N. Zalta |access-date=9 April 2011 |archive-date=22 October 2013 |archive-url=https://web.archive.org/web/20131022013501/http://plato.stanford.edu/archives/win2004/entries/time-experience/ |url-status=live }}</ref><ref>
{{cite book |last=Carrol|first= Sean |chapter=Chapter One, Section Two, Plume |title=From Eternity to Here: The Quest for the Ultimate Theory of Time |title-link=From Eternity to Here: The Quest for the Ultimate Theory of Time |isbn=978-0-452-29654-1 |quote=As human beings we 'feel' the passage of time.|year=2010 |publisher=Penguin }}
</ref><ref name="lehar">
Lehar, Steve. (2000). [http://cns-alumni.bu.edu/~slehar/webstuff/consc1/consc1.html The Function of Conscious Experience: An Analogical Paradigm of Perception and Behavior] {{webarchive|url=https://web.archive.org/web/20151021061139/http://cns-alumni.bu.edu/~slehar/webstuff/consc1/consc1.html |date=21 October 2015 }}, ''Consciousness and Cognition''.
</ref>

In philosophy, time was questioned throughout the centuries; what time is and if it is real or not. Ancient Greek philosophers asked if time was linear or cyclical and if time was endless or [[wikt:finite|finite]].<ref name="Philosophy of Time">{{Cite web |url=http://www.exactlywhatistime.com/philosophy-of-time/ |title=Philosophy of Time – Exactly What Is Time? |access-date=2019-03-28 |archive-date=28 March 2019 |archive-url=https://web.archive.org/web/20190328174952/http://www.exactlywhatistime.com/philosophy-of-time/ |url-status=live}}</ref> These philosophers had different ways of explaining time; for instance, ancient Indian philosophers had something called the [[Wheel of time|Wheel of Time.]] It is believed that there was repeating ages over the lifespan of the universe.<ref name="Ancient Philosophy">{{Cite web|url=http://www.exactlywhatistime.com/philosophy-of-time/ancient-philosophy/|title=Ancient Philosophy – Exactly What Is Time?|access-date=2019-03-28|archive-date=28 March 2019|archive-url=https://web.archive.org/web/20190328174951/http://www.exactlywhatistime.com/philosophy-of-time/ancient-philosophy/|url-status=live}}</ref> This led to beliefs like cycles of rebirth and [[reincarnation]].<ref name="Ancient Philosophy" /> The Greek philosophers believe that the universe was infinite, and was an illusion to humans.<ref name="Ancient Philosophy" /> [[Plato]] believed that time was made by the Creator at the same instant as the heavens.<ref name="Ancient Philosophy" /> He also says that time is a period of motion of the [[Astronomical objects|heavenly bodies]].<ref name="Ancient Philosophy" /> [[Aristotle]] believed that time correlated to movement, that time did not exist on its own but was relative to motion of objects.<ref name="Ancient Philosophy" /> He also believed that time was related to the motion of [[celestial bodies]]; the reason that humans can tell time was because of [[Orbitial periods|orbital periods]] and therefore there was a duration on time.<ref name="Bunnag-2017">{{Cite journal|last=Bunnag|first=Anawat|date=August 2017|title=The concept of time in philosophy: A comparative study between Theravada Buddhist and Henri Bergson's concept of time from Thai philosophers' perspectives|url=https://linkinghub.elsevier.com/retrieve/pii/S2452315117300140|journal=Kasetsart Journal of Social Sciences|doi=10.1016/j.kjss.2017.07.007|doi-access=free|access-date=11 April 2019|archive-date=2 April 2019|archive-url=https://web.archive.org/web/20190402163654/https://linkinghub.elsevier.com/retrieve/pii/S2452315117300140|url-status=live}}</ref>

The ''[[Vedas]]'', the earliest texts on [[Indian philosophy]] and [[Hindu philosophy]] dating to the late [[2nd millennium BC]], describe ancient [[Hindu cosmology]], in which the [[universe]] goes through repeated cycles of creation, destruction and rebirth, with each cycle lasting 4,320&nbsp;million years.<ref>{{Cite book |last=Layton |first=Robert |url=https://books.google.com/books?id=7TPIDL9RdsoC&pg=PA7 |title=Who Needs the Past?: Indigenous Values and Archaeology |date=1994 |publisher=Psychology Press |isbn=978-0-415-09558-7 |pages=7 |language=en}}</ref> [[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> [[Plato]], in the [[Timaeus (dialogue)|''Timaeus'']], identified time with the period of motion of the heavenly bodies. [[Aristotle]], in Book IV of his [[Physics (Aristotle)|''Physica'']] defined time as 'number of movement in respect of the before and after'.<ref>{{cite web |last1=Hardie |first1=R. P. |last2=Gaye |first2=R. K. |title=Physics by Aristotle |url=http://classics.mit.edu//Aristotle/physics.4.iv.html |url-status=live |archive-url=https://web.archive.org/web/20140626030252/http://classics.mit.edu/Aristotle/physics.4.iv.html |archive-date=26 June 2014 |access-date=4 May 2014 |publisher=Massachusetts Institute of Technology}} "''Time then is a kind of number. (Number, we must note, is used in two senses – both of what is counted or the countable and also of that with which we count. Time obviously is what is counted, not that with which we count: there are different kinds of thing.) [...] It is clear, then, that time is 'number of movement in respect of the before and after', and is continuous since it is an attribute of what is continuous.''"</ref> In Book 11 of his ''[[Confessions (St. Augustine)|Confessions]]'', [[Augustine of Hippo|St. Augustine of Hippo]] 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 begins to define time by what it is not rather than what it is,<ref>
{{cite book |url=http://en.wikisource.org/wiki/Nicene_and_Post-Nicene_Fathers:_Series_I/Volume_I/Confessions/Book_XI/Chapter_14 |author-link=Augustine of Hippo |author=Augustine of Hippo |title=Confessions |access-date=9 April 2011 |url-status=live |archive-url=https://web.archive.org/web/20120119043221/http://en.wikisource.org/wiki/Nicene_and_Post-Nicene_Fathers:_Series_I/Volume_I/Confessions/Book_XI/Chapter_14 |archive-date=19 January 2012  }} Book 11, Chapter 14.
</ref> an approach similar to that taken in other [[negative theology|negative definitions]]. However, Augustine ends up calling time a "distention" of the mind (Confessions 11.26) by which we simultaneously grasp the past in memory, the present by attention, and the future by expectation.

Philosophers in the 17th and 18th century questioned if time was real and absolute, or if it was an intellectual concept that humans use to understand and sequence events.<ref name="Philosophy of Time" /> These questions lead to realism vs anti-realism; the realists believed that time is a fundamental part of the universe, and be perceived by events happening in a sequence, in a dimension.<ref name="Early Modern Philosophy">{{Cite web|url=http://www.exactlywhatistime.com/philosophy-of-time/early-modern-philosophy/|title=Early Modern Philosophy – Exactly What Is Time?|access-date=2019-03-28|archive-date=28 March 2019|archive-url=https://web.archive.org/web/20190328174950/http://www.exactlywhatistime.com/philosophy-of-time/early-modern-philosophy/|url-status=live}}</ref> [[Isaac Newton]] said that we are merely occupying time, he also says that humans can only understand [[relative time]].<ref name="Early Modern Philosophy" /> Isaac Newton believed in absolute space and absolute time; 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 [[Leibniz–Clarke correspondence]]. Relative time is a measurement of objects in motion.<ref name="Early Modern Philosophy" /> The anti-realists believed that time is merely a convenient intellectual concept for humans to understand events.<ref name="Early Modern Philosophy" /> This means that time was useless unless there were objects that it could interact with, this was called [[relational time]].<ref name="Early Modern Philosophy" /> [[René Descartes]], [[John Locke]], and [[David Hume]] said that one's mind needs to acknowledge time, in order to understand what time is.<ref name="Bunnag-2017" /> [[Immanuel Kant]] believed that we can not know what something is unless we experience it first hand.<ref>{{Cite web|url=https://www.iep.utm.edu/kantview/|title=Immanuel Kant|last=Jankowiak|first=Tim|access-date=2019-04-02|archive-date=23 May 2021|archive-url=https://web.archive.org/web/20210523180236/https://iep.utm.edu/kantview/|url-status=live}}</ref>

{{Quote box
 |quote=Time is not an empirical concept. For neither co-existence nor succession would be perceived by us, if the representation of time did not exist as a foundation ''a priori''. Without this presupposition, we could not represent to ourselves that things exist together at one and the same time, or at different times, that is, contemporaneously, or in succession.
 |source=[[Immanuel Kant]], ''[[Critique of Pure Reason]]'' (1781), trans. [[Vasilis Politis]] (London: Dent., 1991), p. 54.
 |align=right
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}}
[[Immanuel Kant]], in the ''[[Critique of Pure Reason]]'', described time as an ''[[A priori and a posteriori|a priori]]'' intuition that allows us (together with the other ''a priori'' intuition, space) to comprehend [[empirical evidence|sense experience]].<ref name="kant">
{{cite book |last=Kant |first=Immanuel |author-link=Immanuel Kant |url=https://ebooks.adelaide.edu.au/k/kant/immanuel/k16p/complete.html |title=The Critique of Pure Reason, 2nd edition |date=1787 |access-date=9 April 2011 |archive-url=https://web.archive.org/web/20110413222609/http://ebooks.adelaide.edu.au/k/kant/immanuel/k16p/ |archive-date=13 April 2011 |url-status=dead}} translated by [[J.M.D. Meiklejohn]], eBooks@Adelaide, 2004
</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. Kant 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. Spatial [[measurement]]s are used to [[quantity|quantify]] the extent of and distances between [[object (philosophy)|objects]], and temporal measurements are used to quantify the durations of and between [[Phenomenon|events]]. Time was designated by Kant as the purest possible [[Schema (Kant)#Time|schema]] of a pure concept or category.

[[Henri Bergson]] believed that time was neither a real homogeneous medium nor a mental construct, but possesses what he referred to as ''[[Duration (philosophy)|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>

According to [[Martin Heidegger]] we do not exist inside time, we ''are'' time. Hence, the relationship to the past is a present awareness of ''having been'', which allows the past to exist in the present. The relationship to the future is the state of anticipating a potential possibility, task, or engagement. It is related to the human propensity for caring and being concerned, which causes "being ahead of oneself" when thinking of a pending occurrence. Therefore, this concern for a potential occurrence also allows the future to exist in the present. The present becomes an experience, which is qualitative instead of quantitative. Heidegger seems to think this is the way that a linear relationship with time, or temporal existence, is broken or transcended.<ref name=Balslev>{{Cite book |last=Balslev |first=Anindita N. |author2=Jitendranath Mohanty |title=Religion and Time |publisher=Brill Academic Publishers |series=Studies in the History of Religions, 54. |date=November 1992 |location=The Netherlands |pages=53–59 |url=https://books.google.com/books?id=y94cKeEVa3sC |isbn=978-90-04-09583-0 |access-date=30 July 2019 |archive-date=20 August 2020 |archive-url=https://web.archive.org/web/20200820024824/https://books.google.com/books?id=y94cKeEVa3sC |url-status=live }}</ref> We are not stuck in sequential time. We are able to remember the past and project into the future; we have a kind of random access to our representation of temporal existence; we can, in our thoughts, step out of (ecstasis) sequential time.<ref>{{cite book |author=Heidegger |first=Martin |title=Being and Time |date=1962 |publisher=Blackwell |isbn=978-0-631-19770-6 |page=425 |chapter=V |access-date=30 July 2019 |chapter-url=https://books.google.com/books?id=S57m5gW0L-MC&pg=PA425 |archive-url=https://web.archive.org/web/20200819212325/https://books.google.com/books?id=S57m5gW0L-MC&pg=PA425 |archive-date=19 August 2020 |url-status=live}}</ref>

Modern era philosophers asked: is time real or unreal, is time happening all at once or a duration, is time tensed or tenseless, and is there a future to be?<ref name="Philosophy of Time"/> There is a theory called the tenseless or [[B-theory of time|B-theory]]; this theory says that any tensed terminology can be replaced with tenseless terminology.<ref name="Modern Philosophy">{{Cite web|url=http://www.exactlywhatistime.com/philosophy-of-time/modern-philosophy/|title=Modern Philosophy – Exactly What Is Time?|access-date=2019-03-28|archive-date=28 March 2019|archive-url=https://web.archive.org/web/20190328175027/http://www.exactlywhatistime.com/philosophy-of-time/modern-philosophy/|url-status=live}}</ref> For example, "we will win the game" can be replaced with "we do win the game", taking out the future tense. On the other hand, there is a theory called the tense or [[A Theory of Time|A-theory]]; this theory says that our language has tense verbs for a reason and that the future can not be determined.<ref name="Modern Philosophy" /> There is also something called [[imaginary time]], this was from [[Stephen Hawking]], who said that space and imaginary time are finite but have no boundaries.<ref name="Modern Philosophy" /> Imaginary time is not real or unreal, it is something that is hard to visualize.<ref name="Modern Philosophy" /> Philosophers can agree that physical time exists outside of the human mind and is objective, and psychological time is mind-dependent and subjective.<ref name="Bunnag-2017"/>

=== Unreality ===
In 5th century BC [[Greece]], [[Antiphon (orator)|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=Foundalis |first=Harry |title=You are about to disappear |url=http://www.foundalis.com/phi/WhyTimeFlows.htm |url-status=live |archive-url=https://web.archive.org/web/20110512072013/http://www.foundalis.com/phi/WhyTimeFlows.htm |archive-date=12 May 2011 |access-date=9 April 2011}}</ref> Time as an illusion is also a common theme in [[Buddhism|Buddhist]] thought.<ref>{{cite web |title=Buddhism and the illusion of time |url=http://www.buddhasvillage.com/teachings/time.htm |first=Tom |last=Huston |access-date=9 April 2011 |url-status=live |archive-url=https://web.archive.org/web/20110708103214/http://www.buddhasvillage.com/teachings/time.htm |archive-date=8 July 2011  }}</ref><ref>{{cite book |url=https://books.google.com/books?id=kfsyfoO1IlYC&pg=RA1-PR19 |last=Garfield |first=Jay L. |title=The fundamental wisdom of the middle way: Nāgārjuna's Mūlamadhyamakakārikā |date=1995 |publisher=Oxford University Press |location=New York |isbn=978-0-19-509336-0 |access-date=19 May 2018 |archive-date=19 August 2020 |archive-url=https://web.archive.org/web/20200819134845/https://books.google.com/books?id=kfsyfoO1IlYC&pg=RA1-PR19 |url-status=live }}</ref>

These arguments often center on what it means for something to be ''unreal''. Modern physicists generally believe that time is as ''real'' as space—though others, such as [[Julian Barbour]], argue quantum equations of the universe take their true form when expressed in the timeless [[Configuration space (physics)|realm]] containing every possible ''now'' or momentary configuration of the universe.<ref>{{Cite web |last=Folger |first=Tim |date=30 November 2000 |title=From Here to Eternity |url=https://www.discovermagazine.com/the-sciences/from-here-to-eternity-02 |access-date=2025-02-26 |website=Discover Magazine |language=en}}</ref> [[J. M. E. McTaggart]]'s 1908 article ''[[The Unreality of Time]]'' argues that, since every event has the characteristic of being both present and not present (i.e., future or past), that time is a self-contradictory idea.

Another modern philosophical theory called [[Philosophical presentism|presentism]] views the past and the future as human-mind interpretations of movement instead of real parts of time (or "dimensions") which coexist with the present. This theory rejects the existence of all direct interaction with the past or the future, holding only the present as tangible. This is one of the philosophical arguments against time travel.<ref>{{Cite journal |last=Sakon |first=Takeshi |date=2020 |title=Presentists Should Not Believe in Time Travel |url=https://www.jstage.jst.go.jp/article/jpssj/53/2/53_191/_pdf |journal=Philosophy of Science |volume=53 |issue=2 |via=Osaka City University}}</ref> This contrasts with [[eternalism (philosophy of time)|eternalism]] (all time: present, past and future, is real) and the [[Growing block universe|growing block theory]] (the present and the past are real, but the future is not).

== Physical definition ==
{{Classical mechanics|cTopic=Fundamental concepts}}
{{Main|Time in physics}}

Until [[Albert Einstein|Einstein's]] reinterpretation of the physical concepts associated with time and space in 1907, time was considered to be the same everywhere in the universe, with all observers measuring the same time interval for any event.<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> Non-relativistic [[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|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 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.

The theory of special relativity finds a convenient formulation in [[Minkowski space]]time, a mathematical structure that combines three dimensions of space with a single dimension of time. In this formalism, distances in space can be measured by how long light takes to travel that distance, e.g., a [[light-year]] is a measure of distance, and a meter is now defined in terms of how far light travels in a certain amount of time. Two [[Event (relativity)|events]] in Minkowski spacetime are separated by an ''[[Spacetime interval|invariant interval]]'', which can be either [[space-like]], [[light-like]], or [[time-like]]. Events that have a time-like separation cannot be simultaneous in any [[frame of reference]], there must be a temporal component (and possibly a spatial one) to their separation. Events that have a space-like separation will be simultaneous in some frame of reference, and there is no frame of reference in which they do not have a spatial separation. Different observers may calculate different distances and different time intervals between two events, but the ''invariant interval'' between the events is independent of the observer and their velocity.

=== Arrow of time ===
{{Main|Arrow of time}}

Unlike space, where an object can travel in the opposite directions (and in 3 dimensions), time appears to have only one dimension and only one direction—the past lies behind, fixed and immutable, while the future lies ahead and is not necessarily fixed. Yet most laws of physics allow any process to proceed both forward and in reverse. There are only a few physical phenomena that violate the reversibility of time. This time directionality is known as the [[arrow of time]]. Acknowledged examples of the arrow of time are:<ref>{{Cite journal |last1=O’Byrne |first1=J. |last2=Kafri |first2=Y. |last3=Tailleur |first3=J. |last4=van Wijland |first4=F. |date=2022-01-25 |title=Time irreversibility in active matter, from micro to macro |url=https://doi.org/10.1038/s42254-021-00406-2 |journal=Nature Reviews Physics |volume=4 |issue=3 |pages=167–183 |doi=10.1038/s42254-021-00406-2 |issn=2522-5820|arxiv=2104.03030 |bibcode=2022NatRP...4..167O }}</ref><ref>{{Citation |last=Maxwell |first=Nicholas |title=Relativity Theory May not Have the Last Word on the Nature of Time: Quantum Theory and Probabilism |date=2017 |work=Space, Time and the Limits of Human Understanding |series=The Frontiers Collection |pages=109–124 |editor-last=Wuppuluri |editor-first=Shyam |url=http://link.springer.com/10.1007/978-3-319-44418-5_9 |access-date=2025-03-01 |place=Cham |publisher=Springer International Publishing |doi=10.1007/978-3-319-44418-5_9 |isbn=978-3-319-44417-8 |editor2-last=Ghirardi |editor2-first=Giancarlo|url-access=subscription }}</ref><ref>{{Cite book |url=https://www.cambridge.org/core/product/identifier/9781139225700/type/book |title=Complexity and the Arrow of Time |date=2013-08-08 |publisher=Cambridge University Press |isbn=978-1-139-22570-0 |editor-last=Lineweaver |editor-first=Charles H. |edition=1 |doi=10.1017/cbo9781139225700 |editor-last2=Davies |editor-first2=Paul C. W. |editor-last3=Ruse |editor-first3=Michael}}</ref>
# Radiative arrow of time, manifested in waves (e.g., light and sound) travelling only expanding (rather than focusing) in time (see [[light cone]]);
# [[Entropy (arrow of time)|Entropic arrow of time]]: according to the [[second law of thermodynamics]] an isolated system evolves toward a larger disorder rather than orders spontaneously;
# Quantum arrow time, which is related to irreversibility of [[measurement in quantum mechanics]] according to the [[Copenhagen interpretation]] of [[quantum mechanics]];
# Weak arrow of time: preference for a certain time direction of [[weak force]] in [[particle physics]] (see [[CP violation|violation of CP symmetry]]);
# [[Physical cosmology|Cosmological]] arrow of time, which follows the accelerated [[expansion of the Universe]] after the [[Big Bang]].

The relationships between these different arrows of time is a hotly debated topic in [[theoretical physics]].<ref>{{Cite book |last1=Coveney |first1=Peter |title=The arrow of time: a voyage through science to solve time's greatest mystery |last2=Highfield |first2=Roger |date=1991 |publisher=Fawcett Columbine |isbn=978-0-449-90630-9 |edition=1st |location=New York}} </ref>

The [[second law of thermodynamics]] states that [[entropy]] must increase over time. [[Brian Greene]] theorizes that, according to the equations, the change in entropy occurs symmetrically whether going forward or backward in time. So entropy tends to increase in either direction, and our current low-entropy universe is a statistical aberration, in a similar manner as tossing a coin often enough that eventually heads will result ten times in a row. However, this theory is not supported empirically in local experiment.<ref>{{cite book |last=Greene |first=Brian |author-link=Brian Greene |title=The Fabric of the Cosmos |title-link=The Fabric of the Cosmos |date=2005 |publisher=Penguin Books Limited |isbn=978-0-14-195995-5 |chapter=Chapter 6: Chance and the Arrow |access-date=16 September 2017 |chapter-url=https://books.google.com/books?id=yZujlUD1oAAC |archive-url=https://web.archive.org/web/20200820035526/https://books.google.com/books?id=yZujlUD1oAAC |archive-date=20 August 2020 |url-status=live}}</ref>

=== Classical mechanics ===
In non-relativistic [[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 sufficiently well for describing the everyday phenomena of most people's experience. In the late nineteenth century, physicists encountered problems with the classical understanding of time, in connection with the behavior of electricity and magnetism. The 1860s [[Maxwell's equations]] described that light always travels at a [[Speed of light|constant speed]] (in a vacuum).<ref>{{Cite journal |date=2013 |title=Deriving 0 and 0 from First Principles and Defining the Fundamental Electromagnetic Equations Set |url=https://ijerd.com/paper/vol7-issue4/G0704032039.pdf |journal=International Journal of Engineering Research and Development |volume=7 |issue=4 |pages=33 |issn=2278-067X}}</ref> However, classical mechanics assumed that motion was measured relative to a fixed reference frame. The [[Michelson–Morley experiment]] contradicted the assumption. Einstein later proposed a method of synchronizing clocks using the constant, finite speed of light as the maximum signal velocity. This led directly to the conclusion that observers in motion relative to one another measure different elapsed times for the same event.

[[File:World line.svg|upright=1.2|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.|left]]

=== Spacetime ===
{{Main|Spacetime}}
Time has historically been closely related with space, the two together merging into 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. For example, if a spaceship carrying a clock flies through space at (very nearly) the speed of light, its crew does not notice a change in the speed of time on board their vessel because everything traveling at the same speed slows down at the same rate (including the clock, the crew's thought processes, and the functions of their bodies). However, to a stationary observer watching the spaceship fly by, the spaceship appears flattened in the direction it is traveling and the clock on board the spaceship appears to move very slowly.

On the other hand, the crew on board the spaceship also perceives the observer as slowed down and flattened along the spaceship's direction of travel, because both are moving at very nearly the speed of light relative to each other. Because the outside universe appears flattened to the spaceship, the crew perceives themselves as quickly traveling between regions of space that (to the stationary observer) are many light years apart. This is reconciled by the fact that the crew's perception of time is different from the stationary observer's; what seems like seconds to the crew might be hundreds of years to the stationary observer. In either case, however, causality remains unchanged: the [[past]] is the set of events that can send light signals to an entity and the [[future]] is the set of events to which an entity can send light signals.<ref>{{cite web |url=https://www.youtube.com/watch?v=ev9zrt__lec |title=Albert Einstein's Theory of Relativity |publisher=YouTube |date=30 November 2011 |access-date=24 September 2013 |url-status=live |archive-url=https://web.archive.org/web/20131017182611/http://www.youtube.com/watch?v=ev9zrt__lec |archive-date=17 October 2013  }}</ref><ref>{{cite web |url=https://www.youtube.com/watch?v=V7vpw4AH8QQ |title=Time Travel: Einstein's big idea (Theory of Relativity) |publisher=YouTube |date=9 January 2007 |access-date=24 September 2013 |url-status=live |archive-url=https://web.archive.org/web/20131017182714/http://www.youtube.com/watch?v=V7vpw4AH8QQ |archive-date=17 October 2013  }}</ref>

{{Clear left}}

=== Dilation ===
{{Main|Time dilation}}
[[File:Relativity of Simultaneity.svg|thumb|[[Relativity of simultaneity]]: Event B is simultaneous with A in the green reference frame, but it occurred before in the blue frame, and occurs later in the red frame.]]

[[Albert Einstein|Einstein]] showed in his thought experiments that people travelling at different speeds, while agreeing on [[Causality (physics)|cause and effect]], measure different time separations between events, and can even observe different chronological orderings between non-causally related events. Though these effects are typically minute in the human experience, the effect becomes much more pronounced for objects moving at speeds approaching the speed of light. [[Subatomic particle]]s exist for a well-known average fraction of a second in a lab relatively at rest, but when travelling close to the speed of light they are measured to travel farther and exist for much longer than when at rest.

According to the [[Special relativity|special theory of relativity]], in the high-speed particle's [[Inertial frame of reference|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 seem to shorten. Einstein showed how both temporal and spatial dimensions can be altered (or "warped") by high-speed motion.

Einstein (''[[The Meaning of Relativity]]''): "Two [[Event (relativity)|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 relative 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.

According to general relativity, time also runs slower in stronger [[Gravitational field|gravitational fields]]; this is [[gravitational time dilation]].<ref>{{Cite journal |last=Barukčića |first=Ilija |date=2011 |title=The Equivalence of Time and Gravitational Field |journal=Physics Procedia |language=en |volume=22 |pages=56–62 |doi=10.1016/j.phpro.2011.11.008|doi-access=free |bibcode=2011PhPro..22...56B }}</ref> The effect of the dilation becomes more noticeable in a mass-dense object. A famous example of time dilation is a thought experiment of a subject approaching the [[event horizon]] of a [[black hole]]. As a consequence of how gravitational fields warp spacetime, the subject will experience gravitational time dilation. From the perspective of the subject itself, they will experience time normally. Meanwhile, an observer from the outside will see the subject move closer to the black hole until the extreme, in which the subject appears 'frozen' in time and eventually fades to nothingness due to the diminishing amount of light returning.<ref>{{Cite web |last=Gohd |first=Chelsea |date=2023-05-03 |title=What Happens When Something Gets 'Too Close' to a Black Hole? |url=https://science.nasa.gov/universe/what-happens-when-something-gets-too-close-to-a-black-hole/ |access-date=2025-03-01 |website=NASA Science |language=en-US}}</ref><ref>{{Cite web |last=Gunn |first=Alastair |date=2025-01-12 |title=The (very strange) reason black holes are secret time machines |url=https://www.sciencefocus.com/space/black-hole-time-machine |access-date=2025-03-01 |website=BBC Science Focus Magazine |language=en}}</ref>

=== Relativistic versus Newtonian ===
[[File:Lorentz transform of world line.gif|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.|thumb]]

The animations visualise the different treatments of time in the Newtonian and the relativistic descriptions. At the heart of these differences are the [[Galilean transformation|Galilean]] and [[Lorentz transformation]]s applicable in the Newtonian and relativistic theories, respectively. In the 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. The slope of the world line (deviation from being vertical) gives the relative velocity to the observer.

In the Newtonian description these changes are such that ''time'' is absolute:<ref>{{Cite book |last1=Knudsen |first1=Jens M. |url=https://books.google.com/books?id=rkP1CAAAQBAJ&pg=PA30 |title=Elements of Newtonian Mechanics |last2=Hjorth |first2=Poul G. |date=2012-12-06 |publisher=Springer Science & Business Media |isbn=978-3-642-97599-8 |page=30 |language=en}}</ref> the movements of the observer do not influence whether an event occurs in the 'now' (i.e., whether an event passes the horizontal line through the observer). However, in the relativistic description the ''observability of events'' is absolute: the movements of the observer do not influence 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.

=== Quantization ===
{{See also|Chronon}}

Time quantization refers to the theory that time has a smallest possible unit. Time quantization is a hypothetical concept. In the modern established physical theories like the [[Standard Model]] of particle physics and [[general relativity]] time is not quantized. [[Planck time]] (~ 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. Though tentative physical theories that attempt to describe phenomena at this scale exist; an example is [[loop quantum gravity]]. Loop quantum gravity suggests that time is quantized; if gravity is quantized, spacetime is also quantized.<ref>{{Cite web |last=Lewton |first=Thomas |date=2023-07-10 |title=The Physicist Who Bets That Gravity Can't Be Quantized |url=https://www.quantamagazine.org/the-physicist-who-bets-that-gravity-cant-be-quantized-20230710/ |access-date=2025-03-02 |website=Quanta Magazine |language=en |archive-date=13 February 2025 |archive-url=https://web.archive.org/web/20250213110413/https://www.quantamagazine.org/the-physicist-who-bets-that-gravity-cant-be-quantized-20230710/ |url-status=live }}</ref>

== Travel ==
{{Main|Time travel}}
{{See also|Time travel in fiction|Wormhole|Twin paradox}}

Time travel is the concept of moving backwards or forwards to different points in time, in a manner analogous to moving through space, and different from the normal "flow" of time to an earthbound observer. In this view, all points in time (including future times) "persist" in some way. Time travel has been a [[plot device]] in fiction since the 19th century. Travelling backwards or forwards in time has never been verified as a process, and doing so presents many theoretical problems and contradictory logic which to date have not been overcome. Any technological device, whether fictional or hypothetical, that is used to achieve time travel is known as a [[Time travel|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 a [[temporal paradox]]. Some interpretations of time travel resolve this by accepting the possibility of travel between [[Many-worlds interpretation|branch points]], [[Multiverse|parallel realities]], or [[universe]]s. The many-worlds interpretation has been used as a way to solve causality paradoxes arising from time travel. Any quantum event creates another branching timeline, and all possible outcomes coexist without any [[wave function collapse]].<ref>{{Cite arXiv |last=Ghosh |first=Arnub |date=11 May 2024 |title=Revolutionizing Quantum Mechanics: The Birth and Evolution of the Many-Worlds Interpretation |class=physics.hist-ph |eprint=2405.06924}}</ref> This interpretation was an alternative but is opposite from the [[Copenhagen interpretation]], which suggests that wave functions do collapse.<ref>{{Cite journal |last=Crease |first=Robert P. |date=2019-09-02 |title=The bizarre logic of the many-worlds theory |url=https://www.nature.com/articles/d41586-019-02602-8 |journal=Nature |language=en |volume=573 |issue=7772 |pages=30–32 |doi=10.1038/d41586-019-02602-8 |bibcode=2019Natur.573...30C |archive-date=3 February 2025 |access-date=28 February 2025 |archive-url=https://web.archive.org/web/20250203133712/https://www.nature.com/articles/d41586-019-02602-8 |url-status=live }}</ref> In science, hypothetical faster-than-light particles are known as [[Tachyon|tachyons]]; the mathematics of Einstein's relativity suggests that they would have an ''imaginary [[Invariant mass|rest mass]]''. Some interpretations suggest that it might move backward in time. General relativity permits the existence of [[Closed timelike curve|closed timelike curves]], which could allow an observer to travel back in time to the same space.<ref>{{Cite journal |last1=Tobar |first1=Germain |last2=Costa |first2=Fabio |date=2020-10-22 |title=Reversible dynamics with closed time-like curves and freedom of choice |url=https://iopscience.iop.org/article/10.1088/1361-6382/aba4bc |journal=Classical and Quantum Gravity |volume=37 |issue=20 |pages=205011 |doi=10.1088/1361-6382/aba4bc |issn=0264-9381|arxiv=2001.02511 |bibcode=2020CQGra..37t5011T }}</ref> Though for the [[Gödel metric]], such an occurrence requires a globally rotating universe, which has been contradicted by observations of the [[Redshift|redshifts]] of distant galaxies and the [[cosmic background radiation]].<ref>{{Cite journal |last=Luminet |first=Jean-Pierre |date=2018-05-31 |title=Closed Timelike Curves and Singularities |url=https://doi.org/10.37282/991819.18.32 |journal=Inference: International Review of Science |volume=4 |issue=1 |doi=10.37282/991819.18.32 |issn=2576-4403|url-access=subscription }}</ref> However, it has been suggested that a slowly rotating universe model may solve the [[Hubble tension]], so it can not yet be ruled out.<ref>{{cite journal
 | title=Can rotation solve the Hubble Puzzle?
 | first1=Balázs Endre | last1=Szigeti | first2=István | last2=Szapudi
 | first3=Imre Ferenc | last3=Barna | first4=Gergely Gábor | last4=Barnaföldi
 | journal=Monthly Notices of the Royal Astronomical Society
 | volume=538 | issue=4 | date=April 2025 | pages=3038–3041
 | doi=10.1093/mnras/staf446 | doi-access=free | arxiv=2503.13525 }}</ref>

Another solution to the problem of causality-based temporal paradoxes is that such paradoxes cannot arise simply because they have not arisen. As illustrated in numerous works of fiction, [[free will]] either ceases to exist in the past or the outcomes of such decisions are predetermined. A famous example is the [[grandfather paradox]], in which a person is supposed to travel back in time to kill their own grandfather. This would not be possible to enact because it is a historical fact that one's grandfather was not killed before his child (one's parent) was conceived. This view does not simply hold that history is an unchangeable constant, but that any change made by a hypothetical future time traveller would already have happened in their past, resulting in the reality that the traveller moves from. The [[Novikov self-consistency principle]] asserts that due to causality constraints, time travel to the past is impossible.

== Perception ==
[[File:William James b1842c.jpg|thumb|upright|Philosopher and psychologist [[William James]]]]
{{Main|Time perception}}

The [[specious present]] refers to the time duration wherein one's [[perception]]s are considered to be in the present. The experienced present is said to be 'specious' in that, unlike the objective present, it is an interval and not a durationless instant. The term ''specious present'' was first introduced by the psychologist E. R. Clay, and later developed by [[William James]].<ref name="Andersen">{{cite journal |author=Andersen |first1=Holly |last2=Grush |first2=Rick |date=2009 |title=A brief history of time-consciousness: historical precursors to James and Husserl |url=http://mind.ucsd.edu/papers/bhtc/Andersen&Grush.pdf |url-status=dead |journal=Journal of the History of Philosophy |volume=47 |issue=2 |pages=277–307 |doi=10.1353/hph.0.0118 |s2cid=16379171 |archive-url=https://web.archive.org/web/20080216100320/http://mind.ucsd.edu/papers/bhtc/Andersen%26Grush.pdf |archive-date=16 February 2008 |access-date=9 April 2011}}
</ref>

=== Biopsychology ===
The brain's judgment of time is known to be a highly distributed system, including at least the [[cerebral cortex]], [[cerebellum]], and [[basal ganglia]] as its components. One particular component, the [[suprachiasmatic nucleus|suprachiasmatic nuclei]], is responsible for the [[circadian rhythm|circadian (or daily) rhythm]], while other cell clusters appear capable of shorter-range ([[ultradian]]) timekeeping. [[Mental chronometry]] is the use of response time in perceptual-motor tasks to infer the content, duration, and temporal sequencing of cognitive operations. Judgments of time can be altered by [[temporal illusion]]s (like 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> age,<ref>{{cite web |last=Adler |first=Robert |title=Look how time flies |url=https://www.newscientist.com/article/mg16422180.900-look-how-time-flies |url-status=live |archive-url=https://web.archive.org/web/20110614233936/http://www.newscientist.com/article/mg16422180.900-look-how-time-flies |archive-date=14 June 2011 |access-date=9 April 2011}}
</ref> [[Psychoactive drug|psychoactive drugs]], and [[hypnosis]].<ref>{{Cite journal |author=Bowers, Kenneth |last2=Brenneman |first2=Heather A. |date=January 1979 |title=Hypnosis and the perception of time |journal=International Journal of Clinical and Experimental Hypnosis |volume=27 |issue=1 |pages=29–41 |doi=10.1080/00207147908407540 |pmid=541126}}
</ref> The sense of time is impaired in some people with neurological diseases such as [[Parkinson's disease]] and [[attention deficit disorder]].

Psychoactive drugs can impair the judgment of time. [[Stimulant]]s can lead both humans and rats to overestimate time intervals,<ref>{{cite journal |last1=Wittmann |first1=M. |last2=Leland |first2=D. S. |last3=Churan |first3=J. |last4=Paulus |first4=M. P. |date=8 October 2007 |title=Impaired time perception and motor timing in stimulant-dependent subjects |journal=Drug Alcohol Depend. |volume=90 |issue=2–3 |pages=183–192 |doi=10.1016/j.drugalcdep.2007.03.005 |pmc=1997301 |pmid=17434690}}
</ref><ref>
{{cite journal |last=Cheng |first=Ruey-Kuang |author2=Macdonald, Christopher J. |author3=Meck, Warren H. |date=2006 |title=Differential effects of cocaine and ketamine on time estimation: Implications for neurobiological models of interval timing |journal=[[Pharmacology Biochemistry and Behavior]] |volume=85 |issue=1 |pages=114–122 |pmid=16920182 |doi=10.1016/j.pbb.2006.07.019 |s2cid=42295255 |url=http://cat.inist.fr/?aModele=afficheN&cpsidt=18303059 |format=online abstract |access-date=9 April 2011 |url-status=live |archive-url=https://web.archive.org/web/20110810071741/http://cat.inist.fr/?aModele=afficheN&cpsidt=18303059 |archive-date=10 August 2011  |url-access=subscription }}
</ref> while [[depressant]]s can have the opposite effect.<ref>{{cite journal |last1=Tinklenberg |first1=Jared R. |author-link=Jared Tinklenberg |last2=Roth |first2=Walton T. |last3=Kopell |first3=Bert S. |date=January 1976 |title=Marijuana and ethanol: Differential effects on time perception, heart rate, and subjective response |journal=Psychopharmacology |volume=49 |issue=3 |pages=275–279 |doi=10.1007/BF00426830 |pmid=826945 |s2cid=25928542}}
</ref> The level of activity in the brain of [[neurotransmitter]]s such as [[dopamine]] and [[norepinephrine]] may be the reason for this.<ref>{{cite journal |last1=Arzy |first1=Shahar |last2=Molnar-Szakacs |first2=Istvan |last3=Blanke |first3=Olaf |date=18 June 2008 |title=Self in Time: Imagined Self-Location Influences Neural Activity Related to Mental Time Travel |journal=The Journal of Neuroscience |volume=28 |issue=25 |pages=6502–6507 |doi=10.1523/JNEUROSCI.5712-07.2008 |pmc=6670885 |pmid=18562621}}
</ref> Such chemicals will either excite or inhibit the firing of [[neuron]]s in the brain, with a greater firing rate allowing the brain to register the occurrence of more events within a given interval (speed up time) and a decreased firing rate reducing the brain's capacity to distinguish events occurring within a given interval (slow down time).<ref name="Carter 186–187">{{cite book |title=The Human Brain Book |first1=Rita |last1=Carter |publisher=Dorling Kindersley Publishing |date=2009 |isbn=978-0-7566-5441-2 |pages=186–187 |url=https://books.google.com/books?id=eCV6cwU3qm0C |access-date=27 February 2016 |archive-date=13 May 2016 |archive-url=https://web.archive.org/web/20160513113657/https://books.google.com/books?id=eCV6cwU3qm0C |url-status=live }}</ref>

Psychologists assert that time seems to go faster with age, but the literature on this age-related perception of time remains controversial.<ref>{{cite book |last1=Gruber |first1=Ronald P. |title=Studies on the structure of time: from physics to psycho(patho)logy |last2=Wagner |first2=Lawrence F. |last3=Block |first3=Richard A. |date=2000 |publisher=Springer |isbn=978-0-306-46439-3 |editor1-last=Buccheri |editor1-first=R. |page=54 |chapter=Subjective Time Versus Proper (Clock) Time |access-date=9 April 2011 |editor2-last=Di Gesù |editor2-first=V. |editor3-last=Saniga |editor3-first=Metod |chapter-url=https://books.google.com/books?id=LMsDqsvcxckC |archive-url=https://web.archive.org/web/20110721230232/http://books.google.com/books?id=LMsDqsvcxckC |archive-date=21 July 2011 |url-status=live}} [https://books.google.com/books?id=LMsDqsvcxckC&pg=PA54 Extract of page 54] {{Webarchive|url=https://web.archive.org/web/20160513024117/https://books.google.com/books?id=LMsDqsvcxckC&pg=PA54|date=13 May 2016}}</ref> Those who support this notion argue that young people, having more excitatory neurotransmitters, are able to cope with faster external events.<ref name="Carter 186–187" /> Some also argued that the perception of time is also influenced by memory and how much one have experienced; for example, as one get older, they will have spent less part of their total life waiting a month.<ref>{{Cite web |last=Wilson |first=Jillian |date=2022-12-16 |title=Time Flies By Faster As We Get Older. Here's Why. |url=https://www.huffpost.com/entry/time-perception-aging_l_63973dc2e4b0169d76d92560 |access-date=2025-02-26 |website=HuffPost |language=en}}</ref> Meanwhile, children's expanding cognitive abilities allow them to understand time in a different way. Two- and three-year-olds' understanding of time is mainly limited to "now and not now". Five- and six-year-olds can grasp the ideas of past, present, and future. Seven- to ten-year-olds can use clocks and calendars.<ref>{{cite magazine |last=Kennedy-Moore |first=Eileen |author-link=Eileen Kennedy-Moore |date=28 March 2014 |title=Time Management for Kids |url=http://www.psychologytoday.com/blog/growing-friendships/201403/time-management-kids |url-status=live |archive-url=https://web.archive.org/web/20220730093159/https://www.psychologytoday.com/us/blog/growing-friendships/201403/time-management-kids |archive-date=30 July 2022 |access-date=26 April 2014 |magazine=Psychology Today}}</ref> [[Socioemotional selectivity theory]] proposed that when people perceive their time as open-ended and nebulous, they focus more on future-oriented goals.<ref>{{Cite journal |last1=Giasson |first1=Hannah L. |last2=Liao |first2=Hsiao-Wen |last3=Carstensen |first3=Laura L. |date=2019 |title=Counting down while time flies: implications of age-related time acceleration for goal pursuit across adulthood |journal=Current Opinion in Psychology |volume=26 |pages=85–89 |doi=10.1016/j.copsyc.2018.07.001 |issn=2352-2518 |pmc=6436994 |pmid=30048830}}</ref>

=== Spatial conceptualization ===
Although time is regarded as an abstract concept, there is increasing evidence that time is [[Conceptual metaphor|conceptualized]] in the mind in terms of space.<ref name="Núñez-2012">{{Cite journal |last1=Núñez |first1=Rafael |last2=Cooperrider |first2=Kensy |last3=Doan |first3=D |last4=Wassmann |first4=Jürg |date=1 July 2012 |title=Contours of time: Topographic construals of past, present, and future in the Yupno valley of Papua New Guinea  |journal=Cognition |volume=124 |issue=1 |pages=25–35 |doi=10.1016/j.cognition.2012.03.007|pmid=22542697 |s2cid=17215084 }}</ref> That is, instead of thinking about time in a general, abstract way, humans think about time in a spatial way and mentally organize it as such. Using space to think about time allows humans to mentally organize temporal events in a specific way. This spatial representation of time is often represented in the mind as a [[mental timeline]] (MTL).<ref name="Bottini-2015">{{Cite journal |last1=Bottini |first1=Roberto |last2=Crepaldi |first2=Davide |last3=Casasanto |first3=Daniel |last4=Crollen |first4=Virgine |last5=Collignon |first5=Olivier |date=1 August 2015 |title=Space and time in the sighted and blind   |journal=Cognition |volume=141 |pages=67–72 |doi=10.1016/j.cognition.2015.04.004|pmid=25935747 |hdl=2078.1/199842 |s2cid=14646964 |hdl-access=free }}</ref> These origins are shaped by many environmental factors.<ref name="Núñez-2012" /> [[Literacy]] appears to play a large role in the different types of MTLs, as reading/[[Writing system|writing direction]] provides an everyday temporal orientation that differs from culture to culture.<ref name="Bottini-2015" /> In Western cultures, the MTL may unfold rightward (with the past on the left and the future on the right) since people mostly read and write from left to right.<ref name="Bottini-2015" /> Western calendars also continue this trend by placing the past on the left with the future progressing toward the right. Conversely, speakers of Arabic, Farsi, Urdu, and Hebrew read from right to left, and their MTLs unfold leftward (past on the right with future on the left); evidence suggests these speakers organize time events in their minds like this as well.<ref name="Bottini-2015" />

There is also evidence that some cultures use an allocentric spatialization, often based on environmental features.<ref name="Núñez-2012" /> A study of the indigenous Yupno people of [[Papua New Guinea]] found that they may use an allocentric MTL, in which time flows uphill; when speaking of the past, individuals gestured downhill, where the river of the valley flowed into the ocean. When speaking of the future, they gestured uphill, toward the source of the river. This was common regardless of which direction the person faced.<ref name="Núñez-2012" /> A similar study of the Pormpuraawans, an [[Aboriginal groupings of Western Australia|aboriginal group]] in Australia, reported that when they were asked to organize photos of a man aging "in order," individuals consistently placed the youngest photos to the east and the oldest photos to the west, regardless of which direction they faced.<ref name="Boroditsky-2010">{{Cite journal |last1=Boroditsky |first1=Lera |author-link=Lera Boroditsky |last2=Gaby |first2=Alice |year=2010 |title=Remembrances of Times East |journal=Psychological Science |volume=21 |issue=11 |pages=1635–1639 |doi=10.1177/0956797610386621 |pmid=20959511 |s2cid=22097776}}</ref> This directly clashed with an American group that consistently organized the photos from left to right. Therefore, this group also appears to have an allocentric MTL, but based on the cardinal directions instead of geographical features.<ref name="Boroditsky-2010" /> The wide array of distinctions in the way different groups think about time leads to the broader question that different groups may also think about other abstract concepts in different ways as well, such as causality and number.<ref name="Núñez-2012" />

== Use ==
{{See also|Time management}}

In sociology and [[anthropology]], time discipline is the general name given to [[society|social]] and 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]]<ref>{{cite book |last=Hochschild |first=Arlie Russell |author-link=Arlie Russell Hochschild |title=The time bind: when work becomes home and home becomes work |title-link=Time bind |publisher=Metropolitan Books |year=1997 |location=New York}} {{isbn|978-0-8050-4471-3}}.</ref><ref>{{cite news |last=Russell Hochschild |first=Arlie |author-link=Arlie Russell Hochschild |url=https://www.nytimes.com/1997/04/20/magazine/there-s-no-place-like-work.html |title=There's no place like work |work=[[The New York Times Magazine]] |date=20 April 1997 |url-status=live |archive-url=https://web.archive.org/web/20170323235804/http://www.nytimes.com/1997/04/20/magazine/there-s-no-place-like-work.html |archive-date=23 March 2017 |access-date=20 February 2017 }}</ref> and [[Norbert Elias]]<ref>{{cite book |last=Elias |first=Norbert | author-link = Norbert Elias |title=Time: an essay |publisher=Blackwell |location=Oxford, UK Cambridge, US |year=1992 |isbn=978-0-631-15798-4}}</ref> have written on the use of time from a sociological perspective.

The use of time is an important issue in understanding [[human behavior]], education, and [[travel behavior]]. [[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 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.

[[Time management]] is the organization of tasks or events by first estimating how much time a task requires and when it must be completed, and adjusting events that would interfere with its completion so it is done in the appropriate amount of time. Calendars and day planners are common examples of time management tools.

== Sequence of events ==
A sequence of events, or series of events, is a [[sequence]] of items, facts, events, actions, changes, or procedural steps, arranged in time order (chronological order), often with [[causality]] relationships among the items.<ref>{{cite web |title=Sequence – Order of Important Events |url=http://www.austinschools.org/curriculum/la/resources/documents/instResources/LA_res_Seq_ORS_Module.pdf |publisher=[[Austin Independent School District]] |date=2009 |url-status=dead |archive-url=https://web.archive.org/web/20110927172713/http://www.austinschools.org/curriculum/la/resources/documents/instResources/LA_res_Seq_ORS_Module.pdf |archive-date=27 September 2011  }}
</ref><ref>
{{cite web |title=Sequence of Events Worksheets |url=http://www.reference.com/motif/Science/sequence-of-events-worksheets |archive-url=https://web.archive.org/web/20101013032311/http://www.reference.com/motif/Science/sequence-of-events-worksheets |url-status=dead |archive-date=13 October 2010 |publisher=[[Reference.com]] }}
</ref><ref>
{{cite web |author1=Compiled by David Luckham |author2=Roy Schulte |name-list-style=amp |title=Event Processing Glossary&nbsp;– Version 2.0 |url=http://www.complexevents.com/2011/08/23/event-processing-glossary-version-2-0/ |publisher=Complex Event Processing |url-status=live |archive-url=https://web.archive.org/web/20111015182053/http://www.complexevents.com/2011/08/23/event-processing-glossary-version-2-0/ |archive-date=15 October 2011  |date=23 August 2011 }}
</ref> Because of [[causality]], cause precedes [[result|effect]], or cause and effect may appear together in a single item, but effect never precedes cause. A sequence of events can be presented in text, [[Table (information)|tables]], [[chart]]s, or timelines. The description of the items or events may include a [[timestamp]]. A sequence of events that includes the time along with place or location information to describe a sequential path may be referred to as a [[world line]].

Uses of a sequence of events include stories,<ref>{{cite web |author=Nordquist |first=Richard |title=narrative |url=http://grammar.about.com/od/mo/g/narrative2term.htm |url-status=live |archive-url=https://web.archive.org/web/20110904221452/http://grammar.about.com/od/mo/g/narrative2term.htm |archive-date=4 September 2011 |publisher=[[About.com]]}}
</ref> historical events ([[chronology]]), directions and steps in procedures,<ref>{{cite web |author=Piasecki |first=David J. |title=Inventory Accuracy Glossary |url=http://www.accuracybook.com/glossary.htm |url-status=live |archive-url=https://web.archive.org/web/20110903094536/http://www.accuracybook.com/glossary.htm |archive-date=3 September 2011 |publisher=AccuracyBook.com (OPS Publishing)}}
</ref> and timetables for scheduling activities. A sequence of events may also be used to help describe [[Process (engineering)|processes]] in science, technology, and medicine. A sequence of events may be focused on past events (e.g., stories, history, chronology), on future events that must be in a predetermined order (e.g., [[plan]]s, [[schedule (project management)|schedules]], procedures, timetables), or focused on the observation of past events with the expectation that the events will occur in the future (e.g., processes, projections). The use of a sequence of events occurs in fields as diverse as machines ([[cam timer]]), documentaries (''[[Seconds From Disaster]]''), law ([[Choice of law#Sequence of events in conflict cases|choice of law]]), finance ([[directional-change intrinsic time]]), [[computer simulation]] ([[discrete event simulation]]), and [[electric power transmission]]<ref>{{cite web |title=Utility Communications Architecture (UCA) glossary |url=http://www.nettedautomation.com/glossary_menue/glossary_uca.html |publisher=NettedAutomation |url-status=live |archive-url=https://web.archive.org/web/20111210131225/http://nettedautomation.com/glossary_menue/glossary_uca.html |archive-date=10 December 2011  }}
</ref> ([[sequence of events recorder]]). A specific example of a sequence of events is the [[timeline of the Fukushima Daiichi nuclear disaster]].

== See also ==
* {{anl|List of UTC timing centers}}
* {{anl|Loschmidt's paradox}}
* {{anl|Time metrology}}

=== Organizations ===
* Antiquarian Horological Society&nbsp;– AHS (United Kingdom)
* [[Chronometrophilia]] (Switzerland)
* [[Deutsche Gesellschaft für Chronometrie]]&nbsp;– DGC (Germany)
* [[National Association of Watch and Clock Collectors]]&nbsp;– NAWCC (United States)

{{col-begin}}
{{col-break|width=25%}}

=== Miscellaneous arts and sciences ===
* [[Date and time representation by country]]
* [[List of cycles]]
* {{anl|Nonlinear narrative}}
* {{anl|Philosophy of physics}}
* {{anl|Rate (mathematics)}}

{{col-break|width=25%}}

=== Miscellaneous units ===
* {{anl|Fiscal year}}
* {{anl|Half-life}}
* {{anl|Hexadecimal time}}
* {{anl|Tithi}}
* [[Unix epoch]]
{{col-end}}

== References ==
{{reflist}}

== Further reading ==
<div style="font-size:90%;">
{{Div col}}
* {{cite book |author-link=Julian Barbour |last=Barbour |first=Julian |title=The End of Time: The Next Revolution in Our Understanding of the Universe |publisher=Oxford University Press |date=1999 |isbn=978-0-19-514592-2}}
* Craig Callendar, ''Introducing Time'', Icon Books, 2010, {{isbn|978-1-84831-120-6}}
* {{cite book |last=Das |first=Tushar Kanti |title=The Time Dimension: An Interdisciplinary Guide |date=1990 |location=New York |publisher=Praeger |isbn=978-0-275-92681-6}} – Research bibliography
* {{cite book |author-link=Paul Davies |last=Davies |first=Paul |title=About Time: Einstein's Unfinished Revolution |date=1996 |isbn=978-0-684-81822-1 |publisher=Simon & Schuster Paperbacks |location=New York}}
* {{cite book |author-link=Richard Feynman |last=Feynman |first=Richard |title=The Character of Physical Law |date=1994 |orig-year=1965 |location=Cambridge (Mass) |publisher=The MIT Press |isbn=978-0-262-56003-0 |pages=[https://archive.org/details/characterofphysi0000feyn_u5j3/page/108 108–126] |url=https://archive.org/details/characterofphysi0000feyn_u5j3/page/108 }}
* {{cite book |author-link=Peter Galison |last=Galison |first=Peter |title=Einstein's Clocks and Poincaré's Maps: Empires of Time |date=1992 |location=New York |publisher=W.W. Norton |isbn=978-0-393-02001-4 |url=https://archive.org/details/einsteinsclocksp00gali }}
* Benjamin Gal-Or, ''Cosmology, Physics and Philosophy'', Springer Verlag, 1981, 1983, 1987, {{isbn|0-387-90581-2|0-387-96526-2}}.
* [[Charlie Gere]], (2005) ''Art, Time and Technology: Histories of the Disappearing Body'', Berg
* {{cite book |last=Highfield |first=Roger |title=Arrow of Time: A Voyage through Science to Solve Time's Greatest Mystery |publisher=Random House |date=1992 |isbn=978-0-449-90723-8 |url=https://archive.org/details/arrowoftimevoy00cove }}
* {{cite book |author-link=David S. Landes |last=Landes |first=David |title=Revolution in Time |publisher=[[Harvard University Press]] |date=2000 |isbn=978-0-674-00282-1|title-link=Revolution in Time }}
* {{cite journal |last= Lebowitz |first=Joel L. |title=Time's arrow and Boltzmann's entropy |journal=Scholarpedia |date=2008 |volume=3 |number=4 |page=3448 |doi=10.4249/scholarpedia.3448 |bibcode=2008SchpJ...3.3448L |doi-access=free }}
* {{cite book |author-link=N. David Mermin |last=Mermin |first=N. David |title=It's About Time: Understanding Einstein's Relativity |date=2005 |publisher=Princeton University Press |isbn=978-0-691-12201-4 |url=https://archive.org/details/itsabouttimeunde0000merm |url-access=registration }}
* {{Cite book |title=Time's Arrows: Scientific Attitudes Toward Time |last=Morris |first=Richard |year=1985 |publisher=Simon and Schuster |location=New York |isbn=978-0-671-61766-0 |url-access=registration |url=https://archive.org/details/timesarrows00rich |ref=none}}
* {{cite book |author-link=Roger Penrose |last=Penrose |first=Roger |title=The Emperor's New Mind: Concerning Computers, Minds, and the Laws of Physics |date=1999 |orig-year=1989 |location=New York |publisher=Oxford University Press |isbn=978-0-19-286198-6 |pages=391–417 |url=http://ukcatalogue.oup.com/product/9780192861986.do |access-date=9 April 2011 |url-status=dead |archive-url=https://web.archive.org/web/20101226155727/http://ukcatalogue.oup.com/product/9780192861986.do |archive-date=26 December 2010  }}
* {{cite book |last=Price |first=Huw |title=Time's Arrow and Archimedes' Point |date=1996 |publisher=Oxford University Press |isbn=978-0-19-511798-1 |url=https://archive.org/details/timesarrowarchim00pric |access-date=9 April 2011 }}
* {{cite book |author-link=Hans Reichenbach |last=Reichenbach |first=Hans |title=The Direction of Time |date=1999 |orig-year=1956 |location=New York |publisher=Dover |isbn=978-0-486-40926-9 |url=http://store.doverpublications.com/0486409260.html}}
* {{cite book |last=Rovelli |first=Carlo |title=What is time? What is space? |publisher=Di Renzo Editore |location=Rome |date=2006 |isbn=978-88-8323-146-9 |url=http://www.direnzo.it/main.phtml?Language=en&Doc=0001&ISBN=8883231465 |archive-url=https://web.archive.org/web/20070127102006/http://www.direnzo.it/main.phtml?Language=en&Doc=0001&ISBN=8883231465 |url-status=dead |archive-date=27 January 2007 }}
* {{cite book |last=Rovelli |first=Carlo |title=The Order of Time |publisher=Riverhead  |location=New York |date=2018 |isbn=978-0735216105 |url=https://www.penguinrandomhouse.com/books/551483/the-order-of-time-by-carlo-rovelli/}}
* [[Bernard Stiegler|Stiegler, Bernard]], ''[[Technics and Time, 1: The Fault of Epimetheus]]''
* Roberto Mangabeira Unger and Lee Smolin, ''[[The Singular Universe and the Reality of Time]]'', Cambridge University Press, 2014, {{isbn|978-1-107-07406-4}}.
* {{cite book |author-link=Gerald James Whitrow |last=Whitrow |first=Gerald J. |title=The Nature of Time |publisher=Holt, Rinehart and Wilson (New York) |date=1973|bibcode=1973nati.book.....W }}
* {{cite book |last=Whitrow |first=Gerald J. |title=The Natural Philosophy of Time |publisher=Clarendon Press (Oxford) |date=1980|bibcode=1980npt..book.....W }}
* {{cite book |last=Whitrow |first=Gerald J. |title=Time in History. The evolution of our general awareness of time and temporal perspective |url=https://archive.org/details/timeinhistoryevo00whit |url-access=registration |publisher=Oxford University Press |date=1988 |isbn=978-0-19-285211-3}}
</div>
{{div col end}}

== External links ==
{{Sister project links|n=no|s=no|v=no}}
* [https://web.archive.org/web/20151016211032/http://tycho.usno.navy.mil/systime.html Different systems of measuring time] (archived 16 October 2015).
* {{In Our Time|Time|p005465z|Time}}.
* {{Merriam-Webster|Time}}
* [http://www.iep.utm.edu/time/ Time] in the ''[[Internet Encyclopedia of Philosophy]]'', by Bradley Dowden.
* {{cite encyclopedia |url=http://plato.stanford.edu/archives/win2004/entries/time-experience |title=The Experience and Perception of Time |last=Le Poidevin |first=Robin |date=Winter 2004 |encyclopedia=The Stanford Encyclopedia of Philosophy |editor=Edward N. Zalta |access-date=9 April 2011}}
*[https://www.psychologytoday.com/intl/blog/out-of-the-darkness/202501/time-expansion-experiences Time Expansion Experiences: Time may just be a creation of our minds] by Steve Taylor Ph.D. January 31, 2025, Psychology Today.

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