Time

For the magazine, seeTime(magazine).For other uses, seeTime (disambiguation).

For musical conceptions of "time", seeTime (music)andTempo.

Not to be confused withThyme.

Timeis the continuedsequenceofexistenceandeventsthat occurs in an apparentlyirreversiblesuccession from thepast,through thepresent,and into thefuture.It is a component quantity of variousmeasurementsused tosequenceevents, to compare the duration of events or the intervals between them, and toquantifyrates of changeofquantitiesinmaterial realityor in theconsciousexperience.Time is often referred to as a fourthdimension,along withthree spatial dimensions.

Time is one of the seven fundamentalphysical quantitiesin both theInternational System of Units(SI) andInternational System of Quantities.The SI baseunit of timeis thesecond,which is defined by measuring theelectronic transitionfrequencyofcaesiumatoms.General relativityis the primary framework for understanding how spacetime works.Through advances in both theoretical and experimental investigations of spacetime, it has been shown that time can be distorted anddilated,particularly at the edges ofblack holes.

Throughout history, time has been an important subject of study in religion, philosophy, and science. Temporal measurement has occupied scientists andtechnologistsand has been a prime motivation innavigationandastronomy.Time is also of significant social importance, having economic value ( "time is money") as well as personal value, due to anawarenessof the limited time in each day and inhuman life spans.

Definition

Defining time in a manner applicable to all fields withoutcircularityhas consistently eluded scholars.Nevertheless, diverse fields such as business, industry, sports, the sciences, and the performing arts all incorporate some notion of time into their respectivemeasuring systems.In physics, time is used to define other quantities, such asvelocity,so defining time in terms of such quantities would result in circularity of definition.

Time in physicsis operationally defined as "what aclockreads ".Thisoperational definitionof time, wherein one says that observing a certain number of repetitions of one or another standard cyclical event constitutes one standard unit, such as the second, is useful in the conduct of both advanced experiments and everyday affairs of life. There are many systems for determining what time it is. Periodic events and periodic motion have long served as standards for units of time. Examples include the apparent motion of the sun across the sky, the phases of the moon, and the passage of a free-swinging pendulum. More modern systems include theGlobal Positioning System,other satellite systems,Coordinated Universal Timeandmean solar time.In general, the numbers obtained from different time systems differ from one another, but with careful measurements they can be synchronized.

The operational definition of time does not address the fundamental nature of time. Investigations into the relationship between space and time led physicists to define thespacetimecontinuum, where every event is assigned four numbers representing its time and position (the event's coordinates). Examples of events are thecollision of two particles,the explosion of asupernova,or the arrival of a rocket ship.General relativityexplains why the observed time of an event may be different for different observers. In general relativity, the question of what time it is now only has meaning relative to a particular observer. Distance and time are intimately related, and the time required for light to travel a specific distance is the same for all observers, as first publicly demonstrated byMichelson and Morley.Events can be separated in many directions in space, but if two events are separated by time, then one event must precede the other, and all observers will agree on this. 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. Reconciling these two theories is known as theproblem of time.As of 2023, there is no generally accepted theory of quantum general relativity.

Measurement

The flow of sand in anhourglasscan be used to measure the passage of time. It also concretely represents the present as being between thepastand thefuture.

Generally speaking, methods of temporal measurement, orchronometry,take two distinct forms: thecalendar,a mathematical tool for organising intervals of time,and theclock,a physical mechanism that counts the passage of time. In day-to-day life, the clock is consulted for periods less than a day, whereas the calendar is consulted for periods longer than a day. Increasingly, personal electronic devices display both calendars and clocks simultaneously. The number (as on a clock dial or calendar) that marks the occurrence of a specified event as to hour or date is obtained by counting from a fiducial epoch – a central reference point.

History of the calendar

Main article:Calendar

Artifacts from thePaleolithicsuggest that the moon was used to reckon time as early as 6,000 years ago.Lunar calendarswere among the first to appear, with years of either 12 or 13lunar months(either 354 or 384 days). Withoutintercalationto add days or months to some years, seasons quickly drift in a calendar based solely on twelve lunar months.Lunisolar calendarshave 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. These calendars were religiously and astronomically based, with 18 months in a year and 20 days in a month, plus fiveepagomenaldays at the end of the year.

The reforms ofJulius Caesarin 45 BC put theRoman worldon asolar calendar.ThisJulian calendarwas faulty in that its intercalation still allowed the astronomicalsolsticesandequinoxesto advance against it by about 11 minutes per year.Pope Gregory XIIIintroduced a correction in 1582; theGregorian calendarwas 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 theFrench Revolution,a new clock and calendar were invented as part of thedechristianization of Franceand to create a more rational system in order to replace the Gregorian calendar. TheFrench 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.

History of other devices

Horizontalsundialin Canberra

24-hour clock face in Florence

Main article:History of timekeeping devices

See also:Clock

A large variety ofdeviceshave been invented to measure time. The study of these devices is calledhorology.

An Egyptian device that dates toc. 1500 BC,similar in shape to a bentT-square,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.

Asundialuses agnomonto cast a shadow on a set of markings calibrated to the hour. The position of the shadow marks the hour inlocal time.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.

The most precise timekeeping device of theancient worldwas thewater clock,orclepsydra,one of which was found in the tomb of Egyptian pharaohAmenhotep I.They could be used to measure the hours even at night but required manual upkeep to replenish the flow of water. Theancient Greeksand the people fromChaldea(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.In the 11th century,Chinese inventorsandengineersinvented the first mechanical clocks driven by anescapementmechanism.

A contemporaryquartz watch,2007

Thehourglassuses the flow of sand to measure the flow of time. They were used in navigation.Ferdinand Magellanused 18 glasses on each ship for his circumnavigation of the globe (1522).

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.Richard of Wallingford(1292–1336), abbot of St. Alban's abbey, famously built a mechanical clock as an astronomicalorreryabout 1330.

Great advances in accurate time-keeping were made byGalileo Galileiand especiallyChristiaan Huygenswith the invention of pendulum-driven clocks along with the invention of the minute hand by Jost Burgi.

The English wordclockprobably comes from the Middle Dutch wordklockewhich, in turn, derives from the medieval Latin wordclocca,which ultimately derives from Celtic and is cognate with French, Latin, and German words that meanbell.The passage of the hours at sea was marked by bells and denoted the time (seeship's bell). The hours were marked by bells in abbeys as well as at sea.

Chip-scaleatomic clocks,such as this one unveiled in 2004, are expected to greatly improveGPSlocation.

Clocks can range from watches to more exotic varieties such as theClock of the Long Now.They can be driven by a variety of means, including gravity, springs, and various forms of electrical power, and regulated by a variety of means such as apendulum.

Alarm clocks first appeared in ancient Greece around 250 BC with a water clock that would set off a whistle. This idea was later mechanized by Levi Hutchins andSeth E. Thomas.

Achronometeris a portable timekeeper that meets certain precision standards. Initially, the term was used to refer to themarine chronometer,a timepiece used to determinelongitudeby means ofcelestial navigation,a precision first achieved byJohn Harrison.More recently, the term has also been applied to thechronometer watch,a watch that meets precision standards set by the Swiss agencyCOSC.

The most accurate timekeeping devices areatomic clocks,which are accurate to seconds in many millions of years,and are used to calibrate other clocks and timekeeping instruments.

Atomic clocks use the frequency ofelectronic transitionsin certain atoms to measure the second. One of the atoms used iscaesium;most modern atomic clocks probe caesium with microwaves to determine the frequency of these electron vibrations.Since 1967, the International System of Measurements bases its unit of time, the second, on the properties ofcaesiumatoms.SIdefines 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 the¹³³Cs atom.

Today, theGlobal Positioning Systemin coordination with theNetwork Time Protocolcan be used to synchronize timekeeping systems across the globe.

In medieval philosophical writings, theatomwas a unit of time referred to as the smallest possible division of time. The earliest known occurrence in English is inByrhtferth'sEnchiridion(a science text) of 1010–1012,where it was defined as 1/564 of amomentum(11⁄2minutes),and thus equal to 15/94 of a second. It was used in thecomputus,the process of calculating the date of Easter.

As of May 2010,the smallest time interval uncertainty in direct measurements is on the order of 12attoseconds(1.2 × 10⁻¹⁷seconds), about 3.7 × 10²⁶Planck times.

Units

See also:Time (Orders of magnitude)andUnit of time § List

The second (s) is theSIbase unit. Aminute(min) is 60 seconds in length (or, rarely, 59 or 61 seconds when leap seconds are employed), and anhouris 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 toDaylight saving timeandLeap seconds.

Time standards

Main article:Time standard

A time standard is a specification for measuring time: assigning a number orcalendar dateto aninstant(point in time), quantifying the duration of a time interval, and establishing achronology(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 caesiumatomic clockhas led to the replacement of older and purely astronomical time standards such assidereal timeandephemeris 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 second of UT1 by the introduction of one-second steps to UTC, the "leap second". TheGlobal Positioning Systembroadcasts a very precise time signal based on UTC time.

The surface of the Earth is split into a number oftime zones.Standard time orcivil timein 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 todaylight saving timetransitions.

Some other time standards are used mainly for scientific work.Terrestrial Timeis a theoretical ideal scale realized by TAI.Geocentric Coordinate TimeandBarycentric Coordinate Timeare scales defined ascoordinate timesin the context of the general theory of relativity.Barycentric Dynamical Timeis an older relativistic scale that is still in use.

Philosophy

Religion

Scale of time inJaintexts shownlogarithmically

Further information:Time and fate deities

Religions which view time as cyclical

See also:CalendarandWheel 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 inHindu 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.

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.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.

Time as Linear for Abrahamic Religions

In general, the Islamic andJudeo-Christianworld-view regards time aslinear anddirectional, beginning with the act ofcreationby God. The traditional Christian view sees time ending, teleologically, with theeschatologicalend of the present order of things, the "end time".

In theOld TestamentbookEcclesiastes,traditionally ascribed toSolomon(970–928 BC), time (as the Hebrew word עידן, זמןiddan (age, as in "Ice age" ) zĕman(time)is often translated) is a medium for the passage ofpredestinedevents.(Another word, زمان "זמן"zamān,meanttime fit for an event,and is used as the modernArabic,Persian,andHebrewequivalent to the English word "time".)

Time in Greek mythology

The Greek language denotes two distinct principles,ChronosandKairos.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.

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 includechronology,chronometer,chronic,anachronism,synchronise,andchronicle.

Time in Kabbalah & Rabbinical thought

Rabbis sometimes saw time like "an accordion that was expanded and collapsed at will." According toKabbalists,"time" is aparadoxand anillusion.

Time in Advaita Vedanta

According toAdvaita Vedanta,time is integral to the phenomenal world, which lacks independent reality. Time and the phenomenal world are products ofmaya,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 (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.

In Western philosophy

Main articles:Philosophy of space and timeandTemporal finitism

Time's mortal aspect is personified in this bronze statue byCharles van der Stappen.

Two contrasting viewpoints on time divide prominent philosophers. One view is that time is part of the fundamental structure of theuniverse– adimensionindependent of events, in which events occur insequence.Isaac Newtonsubscribed to thisrealistview, and hence it is sometimes referred to asNewtonian time.

The opposing view is thattimedoes 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 withspaceand number) within which humans sequence and compare events. This second view, in the tradition ofGottfried LeibnizandImmanuel Kant,holds thattimeis neither an event nor a thing, and thus is not itself measurable nor can it be travelled.

Furthermore, it may be that there is asubjectivecomponent to time, but whether or not time itself is "felt", as a sensation, or is a judgment, is a matter of debate.

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 orfinite.These philosophers had different ways of explaining time; for instance, ancient Indian philosophers had something called theWheel of Time.It is believed that there was repeating ages over the lifespan of the universe.This led to beliefs like cycles of rebirth andreincarnation.The Greek philosophers believe that the universe was infinite, and was an illusion to humans.Platobelieved that time was made by the Creator at the same instant as the heavens.He also says that time is a period of motion of theheavenly bodies.Aristotlebelieved that time correlated to movement, that time did not exist on its own but was relative to motion of objects.He also believed that time was related to the motion ofcelestial bodies;the reason that humans can tell time was because oforbital periodsand therefore there was a duration on time.

TheVedas,the earliest texts onIndian philosophyandHindu philosophydating to the late2nd millennium BC,describe ancientHindu cosmology,in which theuniversegoes through repeated cycles of creation, destruction and rebirth, with each cycle lasting 4,320 million years. AncientGreek philosophers,includingParmenidesandHeraclitus,wrote essays on the nature of time. Plato,in theTimaeus,identified time with the period of motion of the heavenly bodies.Aristotle,in Book IV of hisPhysicadefined time as 'number of movement in respect of the before and after'.

In Book 11 of hisConfessions,St. Augustine of Hipporuminates 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,an approach similar to that taken in othernegative 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.

Isaac Newtonbelieved in absolute space and absolute time; Leibniz believed that time and space are relational. The differences between Leibniz's and Newton's interpretations came to a head in the famousLeibniz–Clarke correspondence.

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.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.Isaac Newtonsaid that we are merely occupying time, he also says that humans can only understandrelative time.Relative time is a measurement of objects in motion.The anti-realists believed that time is merely a convenient intellectual concept for humans to understand events.This means that time was useless unless there were objects that it could interact with, this was calledrelational time.René Descartes,John Locke,andDavid Humesaid that one's mind needs to acknowledge time, in order to understand what time is.Immanuel Kantbelieved that we can not know what something is unless we experience it first hand.

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 foundationa 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.

Immanuel Kant,Critique of Pure Reason(1781), trans.Vasilis Politis(London: Dent., 1991), p. 54.

Immanuel Kant,in theCritique of Pure Reason,described time as ana prioriintuition that allows us (together with the othera prioriintuition, space) to comprehendsense experience. With Kant, neither space nor time are conceived assubstances,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 anabstractconceptual framework, together with space and number, within which we sequence events,quantifytheir duration, and compare the motions of objects. In this view,timedoes not refer to any kind of entity that "flows," that objects "move through," or that is a "container" for events. Spatialmeasurementsare used toquantifythe extent of and distances betweenobjects,and temporal measurements are used to quantify the durations of and betweenevents.Time was designated by Kant as the purest possibleschemaof a pure concept or category.

Henri Bergsonbelieved that time was neither a real homogeneous medium nor a mental construct, but possesses what he referred to asDuration.Duration, in Bergson's view, was creativity and memory as an essential component of reality.

According toMartin Heideggerwe do not exist inside time, wearetime. Hence, the relationship to the past is a present awareness ofhaving 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. 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.

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?There is a theory called the tenseless orB-theory;this theory says that any tensed terminology can be replaced with tenseless terminology.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 orA-theory;this theory says that our language has tense verbs for a reason and that the future can not be determined.There is also something called imaginary time, this was fromStephen Hawking,who said that space and imaginary time are finite but have no boundaries.Imaginary timeis not real or unreal, it is something that is hard to visualize.Philosophers can agree that physical time exists outside of the human mind and is objective, and psychological time is mind-dependent and subjective.

Unreality

In 5th century BCGreece,AntiphontheSophist,in a fragment preserved from his chief workOn Truth,held that: "Time is not a reality (hypostasis), but a concept (noêma) or a measure (metron)."Parmenideswent further, maintaining that time, motion, and change were illusions, leading to theparadoxesof his followerZeno.Time as an illusion is also a common theme inBuddhistthought.

J. M. E. McTaggart's 1908The Unreality of Timeargues 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 (see alsoThe flow of time).

These arguments often center on what it means for something to beunreal.Modern physicists generally believe that time is asrealas space – though others, such asJulian Barbour,argue quantum equations of the universe take their true form when expressed in the timelessrealmcontaining every possiblenowor momentary configuration of the universe.

A modern philosophical theory calledpresentismviews 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. This contrasts witheternalism(all time: present, past and future, is real) and thegrowing block theory(the present and the past are real, but the future is not).

Physical definition

Main article:Time in physics

UntilEinstein'sreinterpretation 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.Non-relativisticclassical mechanicsis based on this Newtonian idea of time.

Einstein, in hisspecial theory of relativity,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 inMinkowski spacetime,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., alight-yearis a measure of distance, and a meter is now defined in terms of how far light travels in a certain amount of time. Twoeventsin Minkowski spacetime are separated by aninvariant interval,which can be eitherspace-like,light-like,ortime-like.Events that have a time-like separation cannot be simultaneous in anyframe 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 theinvariant intervalbetween the events is independent of the observer (and his or her velocity).

Arrow of time

Main article: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 thearrow of time.Acknowledged examples of the arrow of time are:

1. Radiative arrow of time, manifested in waves (e.g. light and sound) travelling only expanding (rather than focusing) in time (seelight cone);
2. Entropic arrow of time:according to thesecond law of thermodynamicsan isolated system evolves toward a larger disorder rather than orders spontaneously;
3. Quantum arrow time, which is related to irreversibility ofmeasurement in quantum mechanicsaccording to theCopenhagen interpretationofquantum mechanics;
4. Weak arrow of time: preference for a certain time direction ofweak forceinparticle physics(seeviolation of CP symmetry);
5. Cosmologicalarrow of time, which follows the acceleratedexpansion of the Universeafter theBig Bang.

The relationship(s) between these different Arrows of Time is a hotly debated topic intheoretical physics.

Classical mechanics

In non-relativisticclassical 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. Einstein resolved these problems by invoking a method of synchronizing clocks using the constant, finite speed of light as the maximum signal velocity. This led directly to the conclusion that observers in motion relative to one another measure different elapsed times for the same event.

Two-dimensional space depicted in three-dimensional spacetime. The past and futurelight conesare absolute, the "present" is a relative concept different for observers in relative motion.

Spacetime

Main article:Spacetime

Time has historically been closely related with space, the two together merging into spacetime inEinstein'sspecial relativityandgeneral relativity.According to these theories, the concept of time depends on thespatial 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: thepastis the set of events that can send light signals to an entity and thefutureis the set of events to which an entity can send light signals.

Dilation

Main article:Time dilation

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.

Einsteinshowed in his thought experiments that people travelling at different speeds, while agreeing oncause 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 particlesexist 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 thespecial theory of relativity,in the high-speed particle'sframe of reference,it exists, on the average, for a standard amount of time known as itsmean 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): "Twoeventstaking 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,thatsimultaneity is also relative,i.e., two events that appear simultaneous to an observer in a particular inertial reference frame need not be judged as simultaneous by a second observer in a different inertial frame of reference.

Relativistic versus Newtonian

Views of spacetime along theworld lineof 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 pastlight coneof the observer in the origin) are the events visible to the observer.

The animations visualise the different treatments of time in the Newtonian and the relativistic descriptions. At the heart of these differences are theGalileanandLorentz transformationsapplicable 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 both pictures the view of spacetime changes when the observer accelerates.

In the Newtonian description these changes are such thattimeis absolute: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 theobservability of eventsis 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 ofabsolute timeis no longer applicable: events move up and down in the figure depending on the acceleration of the observer.

Quantization

See also:Chronon

Time quantization is a hypothetical concept. In the modern established physical theories (theStandard Modelof Particles and Interactions andGeneral Relativity) time is not quantized.

Planck time(~ 5.4 × 10⁻⁴⁴seconds) is the unit of time in the system ofnatural unitsknown asPlanck units.Current established physical theories are believed to fail at this time scale, and many physicists expect that the Planck time might be the smallest unit of time that could ever be measured, even in principle. Tentative physical theories that describe this time scale exist; see for instanceloop quantum gravity.

Thermodynamics

Thesecond law of thermodynamicsstates thatentropymust increase over time (seeEntropy). This can be in either direction –Brian Greenetheorizes 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.

Travel

Main article:Time travel

See also:Time travel in fiction,Wormhole,andTwin 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 aplot devicein 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 contradictive 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 atime machine.

A central problem with time travel to the past is the violation ofcausality;should an effect precede its cause, it would give rise to the possibility of atemporal paradox.Some interpretations of time travel resolve this by accepting the possibility of travel betweenbranch points,parallel realities,oruniverses.

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 willeither ceases to exist in the past or the outcomes of such decisions are predetermined. As such, it would not be possible to enact thegrandfather paradoxbecause 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 his or her past, resulting in the reality that the traveller moves from. More elaboration on this view can be found in theNovikov self-consistency principle.

Perception

Philosopher and psychologistWilliam James

Main article:Time perception

Thespecious presentrefers to the time duration wherein one'sperceptionsare 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 termspecious presentwas first introduced by the psychologist E. R. Clay, and later developed byWilliam James.

Biopsychology

The brain's judgment of time is known to be a highly distributed system, including at least thecerebral cortex,cerebellumandbasal gangliaas its components. One particular component, thesuprachiasmatic nuclei,is responsible for thecircadian (or daily) rhythm,while other cell clusters appear capable of shorter-range (ultradian) timekeeping.

Psychoactive drugs can impair the judgment of time.Stimulantscan lead both humans and rats to overestimate time intervals,whiledepressantscan have the opposite effect.The level of activity in the brain ofneurotransmitterssuch asdopamineandnorepinephrinemay be the reason for this.Such chemicals will either excite or inhibit the firing ofneuronsin 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).

Mental chronometryis the use of response time in perceptual-motor tasks to infer the content, duration, and temporal sequencing of cognitive operations.

Early childhood education

Children's expanding cognitive abilities allow them to understand time more clearly. 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.

Alterations

In addition to psychoactive drugs, judgments of time can be altered bytemporal illusions(like thekappa effect),age,andhypnosis.The sense of time is impaired in some people with neurological diseases such asParkinson's diseaseandattention deficit disorder.

Psychologists assert that time seems to go faster with age, but the literature on this age-related perception of time remains controversial.Those who support this notion argue that young people, having more excitatory neurotransmitters, are able to cope with faster external events.

Spatial conceptualization

Although time is regarded as an abstract concept, there is increasing evidence that time isconceptualizedin the mind in terms of space.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 Time Line (MTL).Using space to think about time allows humans to mentally organize temporal order. These origins are shaped by many environmental factors––for example,literacyappears to play a large role in the different types of MTLs, as reading/writing directionprovides an everyday temporal orientation that differs from culture to culture.In western cultures, the MTL may unfold rightward (with the past on the left and the future on the right) since people read and write from left to right.Western calendars also continue this trend by placing the past on the left with the future progressing toward the right. Conversely, Arabic, Farsi, Urdu andIsraeli-Hebrewspeakers read from right to left, and their MTLs unfold leftward (past on the right with future on the left), and evidence suggests these speakers organize time events in their minds like this as well.

This linguistic evidence that abstract concepts are based in spatial concepts also reveals that the way humans mentally organize time events varies across cultures––that is, a certain specific mental organization system is not universal. So, although Western cultures typically associate past events with the left and future events with the right according to a certain MTL, this kind of horizontal, egocentric MTL is not the spatial organization of all cultures. Although most developed nations use an egocentric spatial system, there is recent evidence that some cultures use an allocentric spatialization, often based on environmental features.

A study of the indigenous Yupno people ofPapua New Guineafocused on the directional gestures used when individuals used time-related words.When speaking of the past (such as "last year" or "past times" ), 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, revealing that the Yupno people may use an allocentric MTL, in which time flows uphill.

A similar study of the Pormpuraawans, anaboriginal groupin Australia, revealed a similar distinction in which when 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.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.

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.

Use

See also:Time management

In sociology andanthropology,time discipline is the general name given tosocialand economic rules, conventions, customs, and expectations governing the measurement of time, thesocial currencyand awareness of time measurements, and people's expectations concerning the observance of these customs by others.Arlie Russell HochschildandNorbert Eliashave written on the use of time from a sociological perspective.

The use of time is an important issue in understandinghuman behavior,education, andtravel behavior.Time-use researchis 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 managementis 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 asequenceof items, facts, events, actions, changes, or procedural steps, arranged in time order (chronological order), often withcausalityrelationships among the items. Because ofcausality,cause precedeseffect,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,tables,charts,or timelines. The description of the items or events may include atimestamp.A sequence of events that includes the time along with place or location information to describe a sequential path may be referred to as aworld line.

Uses of a sequence of events include stories,historical events (chronology), directions and steps in procedures,and timetables for scheduling activities. A sequence of events may also be used to help describeprocessesin 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.,plans,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), finance (directional-change intrinsic time),computer simulation(discrete event simulation), andelectric power transmission(sequence of events recorder). A specific example of a sequence of events is thetimeline of the Fukushima Daiichi nuclear disaster.

See also

• List of UTC timing centers
• Loschmidt's paradox
• Time metrology

Organizations

• Antiquarian Horological Society– AHS (United Kingdom)
• Chronometrophilia(Switzerland)
• Deutsche Gesellschaft für Chronometrie– DGC (Germany)
• National Association of Watch and Clock Collectors– NAWCC (United States)

Miscellaneous arts and sciences

• Date and time representation by country
• List of cycles
• Nonlinear narrative
• Philosophy of physics
• Rate (mathematics)

Miscellaneous units

• Fiscal year
• Half-life
• Hexadecimal time
• Tithi
• Unix epoch

Further reading

• Barbour, Julian(1999).The End of Time: The Next Revolution in Our Understanding of the Universe.Oxford University Press.ISBN978-0-19-514592-2.
• Craig Callendar,Introducing Time,Icon Books, 2010,ISBN978-1-84831-120-6
• Das, Tushar Kanti (1990).The Time Dimension: An Interdisciplinary Guide.New York: Praeger.ISBN978-0-275-92681-6.– Research bibliography
• Davies, Paul(1996).About Time: Einstein's Unfinished Revolution.New York: Simon & Schuster Paperbacks.ISBN978-0-684-81822-1.
• Feynman, Richard(1994) [1965].The Character of Physical Law.Cambridge (Mass): The MIT Press. pp.108–126.ISBN978-0-262-56003-0.
• Galison, Peter(1992).Einstein's Clocks and Poincaré's Maps: Empires of Time.New York: W.W. Norton.ISBN978-0-393-02001-4.
• Benjamin Gal-Or,Cosmology, Physics and Philosophy,Springer Verlag, 1981, 1983, 1987,ISBN0-387-90581-2,0-387-96526-2.
• Charlie Gere,(2005)Art, Time and Technology: Histories of the Disappearing Body,Berg
• Highfield, Roger (1992).Arrow of Time: A Voyage through Science to Solve Time's Greatest Mystery.Random House.ISBN978-0-449-90723-8.
• Landes, David(2000).Revolution in Time.Harvard University Press.ISBN978-0-674-00282-1.
• Lebowitz, Joel L. (2008)."Time's arrow and Boltzmann's entropy".Scholarpedia.3(4): 3448.Bibcode:2008SchpJ...3.3448L.doi:10.4249/scholarpedia.3448.
• Mermin, N. David(2005).It's About Time: Understanding Einstein's Relativity.Princeton University Press.ISBN978-0-691-12201-4.
• Morris, Richard (1985).Time's Arrows: Scientific Attitudes Toward Time.New York: Simon and Schuster.ISBN978-0-671-61766-0.
• Penrose, Roger(1999) [1989].The Emperor's New Mind: Concerning Computers, Minds, and the Laws of Physics.New York: Oxford University Press. pp. 391–417.ISBN978-0-19-286198-6.Archived fromthe originalon 26 December 2010.Retrieved9 April2011.
• Price, Huw (1996).Time's Arrow and Archimedes' Point.Oxford University Press.ISBN978-0-19-511798-1.Retrieved9 April2011.
• Reichenbach, Hans(1999) [1956].The Direction of Time.New York: Dover.ISBN978-0-486-40926-9.
• Rovelli, Carlo (2006).What is time? What is space?.Rome: Di Renzo Editore.ISBN978-88-8323-146-9.Archived fromthe originalon 27 January 2007.
• Rovelli, Carlo (2018).The Order of Time.New York: Riverhead.ISBN978-0735216105.
• 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,ISBN978-1-107-07406-4.
• Whitrow, Gerald J.(1973).The Nature of Time.Holt, Rinehart and Wilson (New York).
• Whitrow, Gerald J. (1980).The Natural Philosophy of Time.Clarendon Press (Oxford).
• Whitrow, Gerald J. (1988).Time in History. The evolution of our general awareness of time and temporal perspective.Oxford University Press.ISBN978-0-19-285211-3.

External links

• Different systems of measuring time(archived 16 October 2015)
• TimeonIn Our Timeat theBBC.
• "Time".Merriam-Webster.com Dictionary.
• Timein theInternet Encyclopedia of Philosophy,by Bradley Dowden.
• Le Poidevin, Robin (Winter 2004)."The Experience and Perception of Time".In Edward N. Zalta (ed.).The Stanford Encyclopedia of Philosophy.Retrieved9 April2011.