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Time is the continued
sequence In mathematics Mathematics (from Greek: ) includes the study of such topics as numbers (arithmetic and number theory), formulas and related structures (algebra), shapes and spaces in which they are contained (geometry), and quantities and t ...

sequence
of
existence Existence is the ability of an entity to interact with physical reality Reality is the sum or aggregate of all that is real or existent within a system, as opposed to that which is only imaginary Imaginary may refer to: * Imaginary (sociolog ...

existence
and
events Event may refer to: Gatherings of people * Ceremony, an event of ritual significance, performed on a special occasion * Convention (meeting), a gathering of individuals engaged in some common interest * Event management, the organization of event ...
that occurs in an apparently irreversible succession from the
past The past is the set of all Spacetime#Basic concepts, events that occurred before a given point in time. The past is contrasted with and defined by the present and the future. The concept of the past is derived from the linear fashion in which h ...

past
, through the
present The present (or here and now) is the time that is associated with the events perception, perceived directly and in the first time, not as a recollection (perceived more than once) or a speculation (predicted, hypothesis, uncertain). It is a peri ...

present
, into the
future The future is the time after the past and present. Its arrival is considered inevitable due to the existence of time and the laws of physics. Due to the apparent nature of reality and the unavoidability of the future, everything that currently ...

future
. It is a component quantity of various
measurement Measurement is the quantification (science), quantification of variable and attribute (research), attributes of an object or event, which can be used to compare with other objects or events. The scope and application of measurement are dependen ...

measurement
s used to
sequence In mathematics Mathematics (from Greek: ) includes the study of such topics as numbers (arithmetic and number theory), formulas and related structures (algebra), shapes and spaces in which they are contained (geometry), and quantities and t ...

sequence
events, to compare the duration of events or the intervals between them, and to quantify of
quantities Quantity or amount is a property that can exist as a multitude Multitude is a term for a group of people who cannot be classed under any other distinct category, except for their shared fact of existence. The term has a history of use reaching ba ...
in material reality or in the
conscious Consciousness, at its simplest, is sentience or awareness of internal and external existence. Despite millennia of analyses, definitions, explanations and debates by philosophers and scientists, consciousness remains puzzling and controversial ...

conscious
experience Experience refers to conscious , an English Paracelsian physician Consciousness, at its simplest, is " sentience or awareness of internal and external existence". Despite millennia of analyses, definitions, explanations and debates by philosoph ...
. Time is often referred to as a fourth
dimension In physics Physics is the that studies , its , its and behavior through , and the related entities of and . "Physical science is that department of knowledge which relates to the order of nature, or, in other words, to the regular s ...

dimension
, along with three spatial dimensions. Time has long been an important subject of study in religion, philosophy, and science, but defining it in a manner applicable to all fields without circularity has consistently eluded scholars. Nevertheless, diverse fields such as business, industry, sports, the sciences, and the
performing arts The performing arts are arts such as music, dance, and drama which are performed for an audience. It is different from visual arts The visual arts are Art#Forms, genres, media, and styles, art forms such as painting, drawing, printmaking, s ...
all incorporate some notion of time into their respective . 108 pages.
Time in physics Time in physics is defined by its measurement ' Measurement is the number, numerical quantification (science), quantification of the variable and attribute (research), attributes of an object or event, which can be used to compare with other object ...
is operationally defined as "what a
clock A clock or a timepiece is a device used to measure and indicate time Time is the continued sequence of existence and event (philosophy), events that occurs in an apparently irreversible process, irreversible succession from the past ...

clock
reads". The physical nature of time is addressed by
general relativity General relativity, also known as the general theory of relativity, is the geometric Geometry (from the grc, γεωμετρία; '' geo-'' "earth", '' -metron'' "measurement") is, with arithmetic, one of the oldest branches of mathema ...
with respect to events in space-time. Examples of events are the collision of two particles, the explosion of a supernova, or the arrival of a rocket ship. Every event can be assigned four numbers representing its time and position (the event's coordinates). However, the numerical values are 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 by Michelson and Morley. General relativity does not address the nature of time for extremely small intervals where quantum mechanics holds. At this time, there is no generally accepted theory of quantum general relativity. Time is one of the seven fundamental
physical quantities A physical quantity is a physical property A physical property is any property Property is a system of rights that gives people legal control of valuable things, and also refers to the valuable things themselves. Depending on the nature of t ...
in both the
International System of Units The International System of Units, known by the international abbreviation SI in all languages and sometimes Pleonasm#Acronyms_and_initialisms, pleonastically as the SI system, is the modern form of the metric system and the world's most wi ...
(SI) and
International System of Quantities The International System of Quantities (ISQ) consists of the quantities Quantity is a property that can exist as a Counting, multitude or Magnitude (mathematics), magnitude, which illustrate discontinuity (mathematics), discontinuity and continu ...
. The SI base
unit of time A unit of time is any particular time Time is the indefinite continued sequence, progress of existence and event (philosophy), events that occur in an apparently irreversible process, irreversible succession from the past, through the presen ...
is the
second The second (symbol: s, also abbreviated: sec) is the base unit of time Time is the continued sequence of existence and event (philosophy), events that occurs in an apparently irreversible process, irreversible succession from the past, th ...
. Time is used to define other quantities – such as
velocity The velocity of an object is the Time derivative, rate of change of its Position (vector), position with respect to a frame of reference, and is a function of time. Velocity is equivalent to a specification of an object's speed and direction ...

velocity
 – so defining time in terms of such quantities would result in circularity of definition.Duff, Okun, Veneziano, ''ibid''. p. 3. "There is no well established terminology for the fundamental constants of Nature. ... The absence of accurately defined terms or the uses (i.e., actually misuses) of ill-defined terms lead to confusion and proliferation of wrong statements." An
operational definition An operational definition specifies concrete, replicable procedures designed to represent a construct. In the words of American psychologist S.S. Stevens (1935), "An operation is the performance which we execute in order to make known a concept." ...
of time, wherein one says that observing a certain number of repetitions of one or another standard cyclical event (such as the passage of a free-swinging pendulum) constitutes one standard unit such as the second, is highly useful in the conduct of both advanced experiments and everyday affairs of life. To describe observations of an event, a location (position in space) and time are typically noted. The operational definition of time does not address what the fundamental nature of it is. It does not address why events can happen forward and backward in space, whereas events only happen in the forward progress of time. Investigations into the relationship between space and time led physicists to define the
spacetime In physics, spacetime is any mathematical model which fuses the three-dimensional space, three dimensions of space and the one dimension of time into a single four-dimensional manifold. Minkowski diagram, Spacetime diagrams can be used to visuali ...
continuum.
General relativity General relativity, also known as the general theory of relativity, is the geometric Geometry (from the grc, γεωμετρία; '' geo-'' "earth", '' -metron'' "measurement") is, with arithmetic, one of the oldest branches of mathema ...
is the primary framework for understanding how spacetime works. Through advances in both theoretical and experimental investigations of space-time, it has been shown that time can be distorted and , particularly at the edges of s. Temporal measurement has occupied scientists and technologists, and was a prime motivation in
navigation Navigation is a field of study that focuses on the process of monitoring and controlling the movement of a craft or vehicle from one place to another.Bowditch, 2003:799. The field of navigation includes four general categories: land navigation, ...

navigation
and
astronomy Astronomy (from el, ἀστρονομία, literally meaning the science that studies the laws of the stars) is a natural science that studies astronomical object, celestial objects and celestial event, phenomena. It uses mathematics, phys ...
. Periodic events and periodic motion have long served as standards for units of time. Examples include the apparent motion of the sun across the sky, the phases of the moon, the swing of a pendulum, and the beat of a heart. Currently, the international unit of time, the second, is defined by measuring the
electronic transition A quantum jump is the abrupt transition of a quantum system (atom An atom is the smallest unit of ordinary matter In classical physics and general chemistry, matter is any substance that has mass and takes up space by having volume. All ...
frequency Frequency is the number of occurrences of a repeating event per unit of time A unit of time is any particular time Time is the indefinite continued sequence, progress of existence and event (philosophy), events that occur in an apparen ...

frequency
of
caesium Caesium (IUPAC spelling) (American and British English spelling differences, also spelled cesium in American English) is a chemical element with the Symbol (chemistry), symbol Cs and atomic number 55. It is a soft, silvery-golden alkali ...

caesium
atoms (see
below Below may refer to: *Earth *Ground (disambiguation) *Soil *Floor *Bottom (disambiguation) *Less than *Temperatures below freezing *Hell or underworld People with the surname *Fred Below (1926–1988), American blues drummer *Fritz von Below (1853 ...
). Time is also of significant social importance, having economic value (" time is money") as well as personal value, due to an
awareness Awareness is the state of being conscious of something. More specifically, it is the ability to directly know and perceive, to feel, or to be cognizant of events. Another definition describes it as a state wherein a subject is aware of some infor ...

awareness
of the limited time in each day and in . There are many systems for determining what time it is, including the
Global Positioning System The Global Positioning System (GPS), originally Navstar GPS, is a satellite-based radionavigation system owned by the United States government The federal government of the United States (U.S. federal government) is the national ...
, other satellite systems,
Coordinated Universal Time Coordinated Universal Time or UTC is the primary time standard A time standard is a specification for measuring time: either the rate at which time passes; or points in time; or both. In modern times, several time specifications have been o ...
and
mean solar time planet like the Earth Earth is the third planet from the Sun and the only astronomical object known to harbor life. About 29% of Earth's surface is land consisting of continent A continent is one of several large landmasses. ...
. In general, the numbers obtained from different time systems differ from one another.


Measurement

Generally speaking, methods of temporal measurement, or
chronometry Chronometry (from Ancient Greek, Greek χρόνος ''chronos'', "time" and μέτρον ''metron'', "measure") is the science of the measurement of time, or timekeeping. And with the measurement, chronometry employs the standardisation of time as ...
, take two distinct forms: the
calendar A calendar is a system of organizing days. This is done by giving names to periods of time, typically days, weeks, months and years. A calendar date, date is the designation of a single, specific day within such a system. A calendar is also ...

calendar
, a mathematical tool for organising intervals of time, and the
clock A clock or a timepiece is a device used to measure and indicate time Time is the continued sequence of existence and event (philosophy), events that occurs in an apparently irreversible process, irreversible succession from the past ...

clock
, 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

Artifacts from the
Paleolithic The Paleolithic or Palaeolithic or Palæolithic (), also called the Old Stone Age (from Greek palaios - old, lithos - stone), is a period in prehistory Prehistory, also known as pre-literary history, is the period of human history ...
suggest that the moon was used to reckon time as early as 6,000 years ago.
Lunar calendar A lunar calendar is a calendar A calendar is a system of organizing days. This is done by giving names to periods of time, typically days, weeks, months and years. A calendar date, date is the designation of a single, specific day with ...
s were among the first to appear, with years of either 12 or 13
lunar month In lunar calendar A lunar calendar is a calendar based on the monthly cycles of the Moon's lunar phase, phases (Lunar month#Synodic month, synodic months, lunations), in contrast to solar calendars, whose annual cycles are based only directly o ...
s (either 354 or 384 days). Without intercalation to add days or months to some years,
season A season is a division of the year based on changes in weather Weather is the state of the atmosphere An atmosphere (from the greek words ἀτμός ''(atmos)'', meaning 'vapour', and σφαῖρα ''(sphaira)'', meaning 'ball' or ...

season
s quickly drift in a calendar based solely on twelve lunar months.
Lunisolar calendar A lunisolar calendar is a calendar A calendar is a system of organizing days. This is done by giving names to periods of time, typically days, weeks, months and years. A calendar date, date is the designation of a single, specific day ...
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. These calendars were religiously and astronomically based, with 18 months in a year and 20 days in a month, plus five
epagomenal The intercalary month or epagomenal days. of the ancient Egyptian calendar, Egyptian, Coptic calendar, Coptic, and Ethiopian calendars are a period of five days in common years and six days in leap years in addition to those calendars' 12 standard ...
days at the end of the year. The reforms of
Julius Caesar Gaius Julius Caesar (; 12 July 100 BC – 15 March 44 BC) was a Roman Roman or Romans most often refers to: *, the capital city of Italy *, Roman civilization from 8th century BC to 5th century AD *, the people of ancient Rome *', shortened ...

Julius Caesar
in 45 BC put the
Roman world The culture of ancient Rome existed throughout the almost 1200-year history of the civilization  A civilization (or civilisation) is a that is characterized by , , a form of government, and systems of communication (such as ). Ci ...

Roman world
on a
solar calendar A solar calendar is a calendar A calendar is a system of organizing days. This is done by giving names to periods of time, typically days, weeks, months and years. A calendar date, date is the designation of a single, specific day with ...
. This
Julian calendar The Julian calendar, proposed by Julius Caesar Gaius Julius Caesar (; 12 July 100 BC – 15 March 44 BC) was a Roman Roman or Romans usually refers to: *Rome, the capital city of Italy *Ancient Rome, Roman civilization from 8th century B ...
was faulty in that its intercalation still allowed the astronomical
solstice A solstice is an event that occurs when the Sun appears to reach its most northerly or southerly excursion relative to the celestial equator on the celestial sphere In astronomy and navigation, the celestial sphere is an abstraction, abstr ...

solstice
s and
equinox An equinox is traditionally defined as the time when the plane In mathematics, a plane is a flatness (mathematics), flat, two-dimensional surface (mathematics), surface that extends infinitely far. A plane is the two-dimensional space, two-di ...

equinox
es to advance against it by about 11 minutes per year.
Pope Gregory XIII Pope Gregory XIII ( la, Gregorius XIII; 7 January 1502 – 10 April 1585), born Ugo Boncompagni, was head of the Catholic Church The Catholic Church, also known as the Roman Catholic Church, is the , with 1.3 billion Catholics . ...

Pope Gregory XIII
introduced a correction in 1582; the
Gregorian calendar The Gregorian calendar is the calendar A calendar is a system of organizing days. This is done by giving names to periods of time, typically days, weeks, months and years. A calendar date, date is the designation of a single, speci ...
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 The French Revolution ( ) was a period of radical political and societal change in France France (), officially the French Republic (french: link=no, République française), is a spanning and in the and the , and s. Its ...

French Revolution
, a new clock and calendar were invented in an attempt to de-Christianize time and create a more rational system in order to replace the Gregorian calendar. The
French Republican Calendar The French Republican calendar (french: calendrier républicain français), also commonly called the French Revolutionary calendar (), was a calendar A calendar is a system of organizing days. This is done by giving names to periods of ...
's days consisted of ten hours of a hundred minutes of a hundred seconds, which marked a deviation from the base 12 (
duodecimal The duodecimal system (also known as base 12, dozenal, or, rarely, uncial) is a positional notation Positional notation (or place-value notation, or positional numeral system) usually denotes the extension to any of the (or ). More generally ...
) system used in many other devices by many cultures. The system was abolished in 1806.


History of other devices

A large variety of have been invented to measure time. The study of these devices is called
horology Horology ("the study of time", related to Latin ''horologium'' from Greek language, Greek , "instrument for telling the hour", from ''hṓra'' "hour; time" and -o- interfix and suffix ''-logy''), . is the study of the measurement of time. Clo ...
. An Egyptian device that dates to c. 1500 BC, similar in shape to a bent
T-square A T-square is a technical drawing Drawing is a form of visual art in which an artist uses instruments to mark paper Paper is a thin sheet material produced by mechanically and/or chemically processing cellulose fibres derived from wood ...

T-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. A
sundial A sundial is a horological device that tells the time of day (in modern usage referred to as civil time In modern usage, civil time refers to statutory time scales designated by civilian authorities, or to local time indicated by clocks. M ...

sundial
uses a
gnomon A gnomon (, from Greek , ''gnōmōn'', literally: "one that knows or examines") is the part of a sundial that casts a shadow. The term is used for a variety of purposes in mathematics and other fields. History A painted stick dating from 2 ...

gnomon
to cast a shadow on a set of markings calibrated to the hour. The position of the shadow marks the hour in
local timeLocal time may refer to: * Local mean time ** Apparent solar time * Local time (mathematics) * Local time in the Lorentz ether theory {{disambiguation ...
. 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 the
ancient world Ancient history is the aggregate of past eventsWordNet Search – 3.0
"History"
from t ...

ancient world
was the
water clock A water clock or clepsydra (Greek#REDIRECT Greek Greek may refer to: Greece Anything of, from, or related to Greece Greece ( el, Ελλάδα, , ), officially the Hellenic Republic, is a country located in Southeast Europe. Its populati ...
, or ''clepsydra'', one of which was found in the tomb of Egyptian pharaoh
Amenhotep I Amenhotep I () ( egy, jmn-ḥtp(w) /jaˌmanuwˈħatpaw/ "Amun Amun (; also ''Amon'', ''Ammon'', ''Amen''; egy, jmn, ''reconstructed'' ; Ancient Greek, Greek ''Ámmōn'', ''Hámmōn'') was a major ancient Egyptian deities, ancient Egy ...

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 Greeks Ancient Greece ( el, Ἑλλάς, Hellás) was a civilization belonging to a period of History of Greece, Greek history from the Greek Dark Ages of the 12th–9th centuries BC to the end of Classical Antiquity, antiquity ( AD 600). This era was ...
and the people from
Chaldea Chaldea () was a small country that existed between the late 10th or early 9th and mid-6th centuries BCE, after which the country and its people were absorbed and assimilated into the indigenous population Babylonia. Semitic language, Semitic-s ...
(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 inventors and
engineers Engineers, as practitioners of engineering Engineering is the use of scientific principles to design and build machines, structures, and other items, including bridges, tunnels, roads, vehicles, and buildings. The discipline of enginee ...
invented the first mechanical clocks driven by an
escapement An escapement is a mechanical linkage in mechanical watches and clocks that gives impulses to the timekeeping element and periodically releases the gear train to move forward, advancing the clock's hands. The impulse action transfers energy to ...

escapement
mechanism. The
hourglass An hourglass (or sandglass, sand timer, sand clock or egg timer) is a device used to measure the passage of time Time is the continued sequence of existence and event (philosophy), events that occurs in an apparently irreversible process, ...

hourglass
uses the flow of sand to measure the flow of time. They were used in navigation.
Ferdinand Magellan Ferdinand Magellan ( or ; pt, Fernão de Magalhães, ; es, link=no, Fernando de Magallanes, ; c. 1480 – 27 April 1521) was a Portuguese explorer Exploration is the act of searching for the purpose of discovery of information In ...

Ferdinand Magellan
used 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. Waterclocks, and later, mechanical clocks, were used to mark the events of the abbeys and monasteries of the Middle Ages.
Richard of Wallingford The first or given name Richard originates, via Old French Old French (, , ; French language, Modern French: ) was the language spoken in Northern France from the 8th century to the 14th century. Rather than a unified Dialect#Dialect or lan ...

Richard of Wallingford
(1292–1336), abbot of St. Alban's abbey, famously built a mechanical clock as an astronomical
orrery An orrery is a mechanical Solar system model, model of the Solar System that illustrates or predicts the relative positions and motions of the planets and natural satellite, moons, usually according to the heliocentric Scientific modelling, model ...
about 1330. Great advances in accurate time-keeping were made by
Galileo Galilei Galileo di Vincenzo Bonaiuti de' Galilei (; 15 February 1564 – 8 January 1642) was an Italian astronomer An astronomer is a scientist in the field of astronomy who focuses their studies on a specific question or field outside the ...

Galileo Galilei
and especially
Christiaan Huygens Christiaan Huygens ( , also , ; la, Hugenius; 14 April 1629 – 8 July 1695), also spelled Huyghens, was a Dutch mathematician, physicist, astronomer and inventor, who is regarded as one of the greatest scientists of all time and a major fig ...

Christiaan Huygens
with the invention of pendulum-driven clocks along with the invention of the minute hand by Jost Burgi."History of Clocks." About.com Inventors. About.com, n.d. Web. 21 February 2016. The English word
clock A clock or a timepiece is a device used to measure and indicate time Time is the continued sequence of existence and event (philosophy), events that occurs in an apparently irreversible process, irreversible succession from the past ...

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 A bell is a struck idiophone, directly struck idiophone percussion instrument. Most bells have the shape of a hollow cup that when struck vibrates in a single strong strike tone, with its sides forming an efficient resonator. The strike may be ...
. The passage of the hours at sea was marked by bells and denoted the time (see
ship's bell Image:Titanic bell.JPG, Bell from RMS ''Titanic'' File:Eight Bells.jpg, ''Eight Bells (painting), Eight Bells'' (1887) by Winslow Homer A ship's bell is a bell on a ship that is used for the indication of time as well as other traditional funct ...
). The hours were marked by bells in abbeys as well as at sea. Clocks can range from watches to more exotic varieties such as the
Clock of the Long Now The Clock of the Long Now, also called the 10,000-year clock, is a mechanical clock under construction that is designed to keep time for 10,000 years. It is being built by the Long Now Foundation. A two-meter prototype is on display at the Scie ...
. They can be driven by a variety of means, including gravity, springs, and various forms of electrical power, and regulated by a variety of means such as a
pendulum A pendulum is a weight suspended from a pivot Pivot may refer to: *Pivot, the point of rotation in a lever A lever ( or ) is a simple machine consisting of a beam or rigid rod pivoted at a fixed hinge, or '':wikt:fulcrum, fulcrum''. A lever ...

pendulum
. 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 and Seth E. Thomas. A chronometer is a portable timekeeper that meets certain precision standards. Initially, the term was used to refer to the
marine chronometer Marine is an adjective meaning of or pertaining to the sea or ocean The ocean (also the sea or the world ocean) is the body of salt water which covers approximately 71% of the surface of the Earth.
, a timepiece used to determine
longitude Longitude (, ) is a geographic coordinate A geographic coordinate system (GCS) is a coordinate system associated with position (geometry), positions on Earth (geographic position). A GCS can give positions: *as Geodetic coordinates, ...

longitude
by means of
celestial navigation Celestial navigation, also known as astronavigation, is the ancient and modern practice of position fixing that enables a navigator to transition through a space without having to rely on estimated calculations, or dead reckoning, to know their p ...

celestial navigation
, a precision firstly achieved by
John Harrison John Harrison ( – 24 March 1776) was a self-educated English carpenter Carpenters in an Indian village Carpentry is a skilled trade and a craft in which the primary work performed is the cutting, shaping and installation of building mate ...

John Harrison
. More recently, the term has also been applied to the
chronometer watch A chronometer (; Literally, a measurer of time) is a specific type of Mechanical watch, mechanical timepiece. In Switzerland, only timepieces certified by the Contrôle Officiel Suisse des Chronomètres (COSC) may use the word '' certified chrono ...
, a watch that meets precision standards set by the Swiss agency
COSC The Contrôle Officiel Suisse des Chronomètres (COSC), the Official Swiss Chronometer Testing Institute, is the institute responsible for certifying the accuracy and precision of Swiss watch Swiss made is a label or marker used to indicate tha ...

COSC
. The most accurate timekeeping devices are
atomic clock An atomic clock is a clock A clock is a device used to measure, verify, keep, and indicate time. The clock is one of the oldest human inventions, meeting the need to measure intervals of time shorter than the natural units: the day, t ...

atomic clock
s, 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 of
electronic transition A quantum jump is the abrupt transition of a quantum system (atom An atom is the smallest unit of ordinary matter In classical physics and general chemistry, matter is any substance that has mass and takes up space by having volume. All ...
s in certain atoms to measure the second. One of the atoms used is
caesium Caesium (IUPAC spelling) (American and British English spelling differences, also spelled cesium in American English) is a chemical element with the Symbol (chemistry), symbol Cs and atomic number 55. It is a soft, silvery-golden alkali ...

caesium
, 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 of
caesium Caesium (IUPAC spelling) (American and British English spelling differences, also spelled cesium in American English) is a chemical element with the Symbol (chemistry), symbol Cs and atomic number 55. It is a soft, silvery-golden alkali ...

caesium
atoms. 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 133Cs atom. Today, the
Global Positioning System The Global Positioning System (GPS), originally Navstar GPS, is a satellite-based radionavigation system owned by the United States government The federal government of the United States (U.S. federal government) is the national ...
in coordination with the
Network Time Protocol The Network Time Protocol (NTP) is a networking protocol for clock synchronization between computer systems over packet-switched, variable-Latency (engineering), latency data networks. In operation since before 1985, NTP is one of the oldest I ...
can be used to synchronize timekeeping systems across the globe. 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 Byrhtferth ( ang, Byrhtferð; ) was a priest and monk who lived at Ramsey Abbey in Huntingdonshire (now part of Cambridgeshire) in England. He had a deep impact on the intellectual life of later Anglo-Saxon England and wrote many computistic, ...
's ''Enchiridion'' (a science text) of 1010–1012, where it was defined as 1/564 of a ''momentum'' (1½ minutes), and thus equal to 15/94 of a second. It was used in the ''
computus As a moveable feast, the date of Easter is determined in each year through a calculation known as ''computus'' (Latin for 'computation'). Easter is celebrated on the first Sunday after the Paschal full moon, which is the first full moon on or af ...
'', the process of calculating the date of Easter. , the smallest time interval uncertainty in direct measurements is on the order of 12
attosecond An attosecond is 1×10−18 of a second The second (symbol: s, abbreviation: sec) is the SI base unit, base unit of time in the International System of Units (SI) (French: Système International d’unités), commonly understood and historical ...
s (1.2 × 10−17 seconds), about 3.7 × 1026
Planck time In particle physics Particle physics (also known as high energy physics) is a branch of that studies the nature of the particles that constitute and . Although the word ' can refer to various types of very small objects (e.g. , gas particl ...
s.


Units

The
second The second (symbol: s, also abbreviated: sec) is the base unit of time Time is the continued sequence of existence and event (philosophy), events that occurs in an apparently irreversible process, irreversible succession from the past, th ...
(s) is the base unit. A
minute The minute is a unit Unit may refer to: Arts and entertainment * UNIT, a fictional military organization in the science fiction television series ''Doctor Who'' * Unit of action, a discrete piece of action (or beat) in a theatrical presentation ...
(min) is 60 seconds in length, and an
hour An hour (symbol A symbol is a mark, sign, or that indicates, signifies, or is understood as representing an , , or . Symbols allow people to go beyond what is n or seen by creating linkages between otherwise very different s and s. All ...
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#REDIRECT Daylight saving time Daylight saving time (DST), also daylight savings time or daylight time (the United States and Canada) and summer time (United Kingdom, European Union, and others), is the practice of advancing clocks (typically by ...

Daylight saving time
and
Leap second A leap second is a one-second adjustment that is occasionally applied to Coordinated Universal Time (UTC), to accommodate the difference between precise time (International Atomic Time (TAI), as measured by atomic clocks) and imprecise solar tim ...

Leap second
s.


Definitions and standards

The Mean
Solar Time Solar time is a calculation of the passage of time Time is the continued of and that occurs in an apparently succession from the , through the , into the . It is a component quantity of various s used to events, to compare the dura ...
system defines the second as 1/86,400 of the mean
solar day A synodic day (or synodic rotation period or solar day) is the rotation period, period for a celestial object to rotate once in relation to the star it is orbiting, and is the basis of solar time. The synodic day is distinguished from the sidereal ...
, which is the year-average of the solar day. The solar day is the time interval between two successive solar noons, i.e., the time interval between two successive passages of the Sun across the local meridian. The local meridian is an imaginary line that runs from celestial north pole to celestial south pole passing directly over the head of the observer. At the local meridian, the Sun reaches its highest point on its daily arc across the sky. In 1874 the British Association for the Advancement of Science introduced the CGS (centimetre/gramme/second system) combining fundamental units of length, mass and time. The second is "elastic", because tidal friction is slowing the earth's rotation rate. For use in calculating ephemerides of celestial motion, therefore, in 1952 astronomers introduced the "ephemeris second", currently defined as The CGS system has been superseded by the ''Système international''. The
SI base unit The SI base units are the standard units of measurement A unit of measurement is a definite magnitude Magnitude may refer to: Mathematics *Euclidean vector, a quantity defined by both its magnitude and its direction *Magnitude (mat ...

SI base unit
for time is the second. The
International System of Quantities The International System of Quantities (ISQ) consists of the quantities Quantity is a property that can exist as a Counting, multitude or Magnitude (mathematics), magnitude, which illustrate discontinuity (mathematics), discontinuity and continu ...
, which incorporates the SI, also defines larger units of time equal to fixed integer multiples of one second (1 s), such as the minute, hour and day. These are not part of the SI, but may be used alongside the SI. Other units of time such as the month and the year are not equal to fixed multiples of 1 s, and instead exhibit significant variations in duration. The official SI definition of the second is as follows: At its 1997 meeting, the CIPM affirmed that this definition refers to a caesium atom in its ground state at a temperature of 0 K. The current definition of the second, coupled with the current definition of the meter, is based on the special theory of relativity, which affirms our spacetime to be a
Minkowski space In mathematical physics, Minkowski space (or Minkowski spacetime) () is a combination of three-dimensional Euclidean space Euclidean space is the fundamental space of classical geometry. Originally it was the three-dimensional space of Euclid ...
. The definition of the second in mean solar time, however, is unchanged.


UTC

While in theory, the concept of a single worldwide universal time-scale may have been conceived of many centuries ago, in practicality the technical ability to create and maintain such a time-scale did not become possible until the mid-19th century. The timescale adopted was Greenwich Mean Time, created in 1847. A few countries have replaced it with Coordinated Universal Time, UTC.


History of development

With the advent of the
industrial revolution The Industrial Revolution was the transition to new manufacturing processes in Great Britain, continental Europe Continental Europe or mainland Europe is the contiguous continent A continent is any of several large landmasse ...
, a greater understanding and agreement on the nature of time itself became increasingly necessary and helpful. In 1847 in Britain,
Greenwich Mean Time Greenwich Mean Time (GMT) is the Local mean time, mean solar time at the Royal Observatory, Greenwich, Royal Observatory in Greenwich, London, counted from midnight. At different times in the past, it has been calculated in different ways, i ...
(GMT) was first created for use by the British railways, the British navy, and the British shipping industry. Using telescopes, GMT was calibrated to the
mean solar time planet like the Earth Earth is the third planet from the Sun and the only astronomical object known to harbor life. About 29% of Earth's surface is land consisting of continent A continent is one of several large landmasses. ...
at the
Royal Observatory, Greenwich The Royal Observatory, Greenwich (ROG; known as the Old Royal Observatory from 1957 to 1998, when the working Royal Greenwich Observatory, RGO, temporarily moved south from Greenwich to Herstmonceux Herstmonceux ( , ) is a village and ci ...

Royal Observatory, Greenwich
in the UK. As international commerce continued to increase throughout Europe, in order to achieve a more efficiently functioning modern society, an agreed-upon, and highly accurate ''international standard'' of time measurement became necessary. In order to find or determine such a time-standard, three steps had to be followed: # An internationally agreed-upon time-standard had to be defined. # This new time-standard then had to be consistently and accurately measured. # The new time-standard then had to be freely shared and distributed around the world. The development of what is now known as UTC time began as a collaboration between 41 nations, officially agreed and signed at the
International Meridian Conference The International Meridian Conference was a conference held in October 1884 in Washington, D.C. ) , image_skyline = , image_caption = Clockwise from top left: the Washington Monument and Lincoln Mem ...
, in Washington D.C. in 1884. At this conference, the local mean solar time at the Royal Observatory, Greenwich in England was chosen to define the "universal day", counted from 0 hours at Greenwich mean midnight. This agreed with the civil Greenwich Mean Time used on the island of Great Britain since 1847. In contrast, astronomical GMT began at mean noon, i.e. astronomical day ''X'' began at noon of civil day ''X''. The purpose of this was to keep one night's observations under one date. The civil system was adopted as of 0 hours (civil) 1 January 1925. Nautical GMT began 24 hours before astronomical GMT, at least until 1805 in the
Royal Navy The Royal Navy (RN) is the United Kingdom's naval warfare Naval warfare is combat Combat ( French for ''fight'') is a purposeful violent conflict meant to physically harm or kill the opposition. Combat may be armed (using weapon A ...
, but persisted much later elsewhere because it was mentioned at the 1884 conference. In 1884, the Greenwich meridian was used for two-thirds of all charts and maps as their
Prime Meridian #REDIRECT Prime meridian#REDIRECT Prime meridian A prime meridian is the meridian (geography), meridian (a line of longitude) in a geographic coordinate system at which longitude is defined to be 0°. Together, a prime meridian and its anti-meri ...

Prime Meridian
. Among the 41 nations represented at the conference, the advanced time-technologies that had already come into use in Britain were fundamental components of the agreed method of arriving at a universal and agreed international time. In 1928 Greenwich Mean Time was rebranded for scientific purposes by the
International Astronomical Union The International Astronomical Union (IAU; french: link=yes, Union astronomique internationale, UAI) is a Non-governmental organization, nongovernmental organisation with the objective of advancing astronomy in all aspects, including promoting ...
as
Universal Time#REDIRECT Universal Time Universal Time (UT) is a time standard based on Earth's rotation. There are several versions of Universal Time, which differ by up to a few seconds. The most commonly used are Coordinated Universal Time (UTC) and UT1 (see ...

Universal Time
(UT). This was to avoid confusion with the previous system in which the day had begun at noon. As the general public had always begun the day at midnight, the timescale continued to be presented to them as Greenwich Mean Time. By 1956, universal time had been split into various versions: UT2, which smoothed for polar motion and seasonal effects, was presented to the public as Greenwich Mean Time. Later, UT1 (which smooths only for polar motion) became the default form of UT used by astronomers and hence the form used in navigation, sunrise and sunset and moonrise and moonset tables where the name Greenwich Mean Time continues to be employed. Greenwich Mean Time is also the preferred method of describing the timescale used by legislators. Even to the present day, UT is still based on an international telescopic system. Observations at the Greenwich Observatory itself ceased in 1954, though the location is still used as the basis for the coordinate system. Because the rotational period of Earth is not perfectly constant, the duration of a second would vary if calibrated to a telescope-based standard like GMT, where the second is defined as 1/86 400 of the mean solar day. Until 1960, the methods and definitions of time-keeping that had been laid out at the International Meridian Conference proved to be adequate to meet the time tracking needs of science. Still, with the advent of the "electronic revolution" in the latter half of the 20th century, the technologies that had been available at the time of the Convention of the Metre proved to be in need of further refinement in order to meet the needs of the ever-increasing precision that the "electronic revolution" had begun to require.


Ephemeris second

An invariable second (the "ephemeris second") had been defined, use of which removed the errors in ephemerides resulting from the use of the variable mean solar second as the time argument. In 1960 this ephemeris second was made the basis of the "coordinated universal time" which was being derived from atomic clocks. It is a specified fraction of the mean tropical year as at 1900 and, being based on historical telescope observations, corresponds roughly to the mean solar second of the early nineteenth century.


SI second

In 1967 a further step was taken with the introduction of the SI second, essentially the ephemeris second as measured by atomic clocks and formally defined in atomic terms. The SI second (Standard Internationale second) is based directly on the measurement of the atomic-clock observation of the frequency oscillation of caesium atoms. It is the basis of all atomic timescales, e.g. coordinated universal time, GPS time, International Atomic Time, etc. Atomic clocks do not measure nuclear decay rates (a common misconception) but rather measure a certain natural vibrational frequency of caesium-133. Coordinated universal time is subject to one constraint which does not affect the other atomic timescales. As it has been adopted as the civil timescale by some countries (most countries have opted to retain mean solar time) it is not permitted to deviate from GMT by more than 0.9 second. This is achieved by the occasional insertion of a leap second.


Current application

Most countries use mean solar time. Australia, Canada (Quebec only), Colombia, France, Germany, New Zealand, Papua New Guinea (Bougainville only), Paraguay, Portugal, Switzerland, the United States and Venezuela use UTC. However, UTC is widely used by the scientific community in countries where mean solar time is official. UTC time is based on the SI second, which was first defined in 1967, and is based on the use of atomic clocks. Some other less used but closely related time-standards include International Atomic Time (TAI),
Terrestrial Time Terrestrial Time (TT) is a modern astronomical time standard defined by the International Astronomical Union The International Astronomical Union (IAU; french: link=yes, Union astronomique internationale, UAI) exists to promote and safeguard t ...
, and
Barycentric Dynamical TimeBarycentric Dynamical Time (TDB, from the French Temps Dynamique Barycentrique) is a relativistic coordinate time scale, intended for astronomical use as a time standard A time standard is a specification for measuring time: either the rate at whi ...
. Between 1967 and 1971, UTC was periodically adjusted by fractional amounts of a second in order to adjust and refine for variations in mean solar time, with which it is aligned. After 1 January 1972, UTC time has been defined as being offset from atomic time by a whole number of seconds, changing only when a
leap second A leap second is a one-second adjustment that is occasionally applied to Coordinated Universal Time (UTC), to accommodate the difference between precise time (International Atomic Time (TAI), as measured by atomic clocks) and imprecise solar tim ...

leap second
is added to keep radio-controlled clocks synchronized with the rotation of the Earth. The
Global Positioning System The Global Positioning System (GPS), originally Navstar GPS, is a satellite-based radionavigation system owned by the United States government The federal government of the United States (U.S. federal government) is the national ...
also broadcasts a very precise
time signal A time signal is a visible, audible, mechanical, or electronic signal In signal processing Signal processing is an electrical engineering subfield that focuses on analysing, modifying, and synthesizing signals such as audio signal pro ...
worldwide, along with instructions for converting GPS time to UTC. GPS-time is based on, and regularly synchronized with or from, UTC-time. The surface of the Earth is split up into a number of
time zone A time zone is an area that observes a uniform standard time Standard time is the synchronization of clock A clock or a timepiece is a device used to Measurement, measure and indicate time. The clock is one of the oldest Invent ...
s. Most time zones are exactly one hour apart, and by convention compute their local time as an offset from GMT. For example, time zones at sea are based on GMT. In many locations (but not at sea) these offsets vary twice yearly due to
daylight saving time#REDIRECT Daylight saving time Daylight saving time (DST), also daylight savings time or daylight time (the United States and Canada) and summer time (United Kingdom, European Union, and others), is the practice of advancing clocks (typically by ...

daylight saving time
transitions.


Conversions

These conversions are accurate at the millisecond level for time systems based on the rotation of the Earth (UT1 and TT). Conversions between atomic time systems (TAI, GPS, and UTC) are accurate at the microsecond level. Definitions: # LS = TAI – UTC = Leap Seconds fro
TAI to UTC
# DUT1 = UT1 – UTC fro
UT1 to UTC
or http://maia.usno.navy.mil/search/search.html


Sidereality

Unlike
solar time Solar time is a calculation of the passage of time Time is the continued of and that occurs in an apparently succession from the , through the , into the . It is a component quantity of various s used to events, to compare the dura ...
, which is relative to the
apparent positionThe apparent place of an object Object may refer to: General meanings * Object (philosophy), a thing, being, or concept ** Entity, something that is tangible and within the grasp of the senses ** Object (abstract), an object which does not exi ...
of the
Sun The Sun is the star A star is an astronomical object consisting of a luminous spheroid of plasma (physics), plasma held together by its own gravity. The List of nearest stars and brown dwarfs, nearest star to Earth is the Sun. Many othe ...

Sun
,
sidereal time Sidereal time () is a timekeeping system that astronomers use to locate celestial objects. Using sidereal time, it is possible to easily point a telescope to the proper coordinates in the night sky The term night sky, usually associated ...

sidereal time
is the measurement of time relative to that of a distant star. In
astronomy Astronomy (from el, ἀστρονομία, literally meaning the science that studies the laws of the stars) is a natural science that studies astronomical object, celestial objects and celestial event, phenomena. It uses mathematics, phys ...
, sidereal time is used to predict when a
star A star is an astronomical object consisting of a luminous spheroid of plasma Plasma or plasm may refer to: Science * Plasma (physics), one of the four fundamental states of matter * Plasma (mineral) or heliotrope, a mineral aggregate * Quark ...

star
will reach its
highest point A list of highest points typically contains the name, elevation, and location of the highest point in each of a set of geographical regions. Such a list is important in the sport of highpointing Highpointing is the sport of ascending to the point ...
in the sky. Due to
Earth's orbit Earth orbits the Sun at an average distance of 149.60 million km (92.96 million mi), and one complete orbit takes  days (1 sidereal year), during which time Earth has traveled 940 million km (584 million mi). Jean Meeus, ''Astron ...
al motion around the Sun, a mean solar day is about 3 minutes 56 seconds longer than a mean sidereal day, or more than a mean sidereal day.


Chronology

Another form of time measurement consists of studying the
past The past is the set of all Spacetime#Basic concepts, events that occurred before a given point in time. The past is contrasted with and defined by the present and the future. The concept of the past is derived from the linear fashion in which h ...

past
. Events in the past can be ordered in a sequence (creating a
chronology Chronology (from Latin ''chronologia'', from Ancient Greek , ''chrónos'', "time"; and , ''wikt:-logia, -logia'') is the science of arranging events in their order of occurrence in time. Consider, for example, the use of a timeline or sequence ...
), and can be put into chronological groups (
periodization Periodization is the process or study of categorizing the past into discrete, quantified named blocks of time.Adam Rabinowitz. It’s about time: historical periodization and Linked Ancient World Data'. Institute for the Study of the Ancient Wo ...
). One of the most important systems of periodization is the
geologic time scale The geologic time scale (GTS) is a system of chronological dating Chronological dating, or simply dating, is the process of attributing to an object or event a date in the past, allowing such object or event to be located in a previously establi ...

geologic time scale
, which is a system of periodizing the events that shaped the
Earth Earth is the third planet from the Sun and the only astronomical object known to harbour and support life. 29.2% of Earth's surface is land consisting of continents and islands. The remaining 70.8% is Water distribution on Earth, covered wi ...

Earth
and its life. Chronology, periodization, and interpretation of the past are together known as the study of
history History (from Greek#REDIRECT Greek Greek may refer to: Greece Anything of, from, or related to Greece Greece ( el, Ελλάδα, , ), officially the Hellenic Republic, is a country located in Southeast Europe. Its population is approxima ...

history
.


Terminology

The term "time" is generally used for many close but different concepts, including: *
instant In physics Physics is the natural science that studies matter, its Elementary particle, fundamental constituents, its Motion (physics), motion and behavior through Spacetime, space and time, and the related entities of energy and force. "Phy ...

instant
as an object – one point on the time axes. Being an object, it has no value; ** date as a quantity characterising an instant. As a quantity, it has a value which may be expressed in a variety of ways, for example "2014-04-26T09:42:36,75" in
ISO standard The International Organization for Standardization (ISO ) is an international standard-setting body composed of representatives from various national standards organizations. Founded on 23 February 1947, the organization develops and publish ...
format, or more colloquially such as "today, 9:42 a.m."; * time interval as an object – part of the time axes limited by two instants. Being an object, it has no value; ** duration as a quantity characterizing a time interval. As a quantity, it has a value, such as a number of minutes, or may be described in terms of the quantities (such as times and dates) of its beginning and end.


Philosophy


Religion


Linear and cyclical

Ancient cultures such as
Incan The Inca Empire, also known as Incan Empire and the Inka Empire, and at the time known as the Realm of the Four Parts,,  "four parts together" was the largest empire in pre-Columbian America. The administrative, political and military ce ...

Incan
,
Mayan Mayan most commonly refers to: * Maya peoples, various indigenous peoples of Mesoamerica and northern Central America * Maya civilization, pre-Columbian culture of Mesoamerica and northern Central America * Mayan languages, language family spoken i ...
,
Hopi The Hopi are a Native American tribe In the United States The United States of America (USA), commonly known as the United States (U.S. or US), or America, is a country Contiguous United States, primarily located in North America. It co ...

Hopi
, and other Native American Tribes – plus the
Babylonia Babylonia () was an and based in central-southern which was part of Ancient Persia (present-day and ). A small -ruled state emerged in 1894 BCE, which contained the minor administrative town of . It was merely a small provincial town dur ...
ns,
ancient Greeks Ancient Greece ( el, Ἑλλάς, Hellás) was a civilization belonging to a period of History of Greece, Greek history from the Greek Dark Ages of the 12th–9th centuries BC to the end of Classical Antiquity, antiquity ( AD 600). This era was ...
,
Hinduism Hinduism () is an Indian religion Indian religions, sometimes also termed Dharmic religions or Indic religions, are the religions that originated in the Indian subcontinent. These religions, which include Hinduism, Jainism, Buddhism, ...

Hinduism
,
Buddhism Buddhism (, ) is the world's fourth-largest religion Religion is a social Social organisms, including humans, live collectively in interacting populations. This interaction is considered social whether they are aware of it or not, and ...

Buddhism
,
Jainism Jainism (), traditionally known as ''Jain Dharma'', is an ancient Indian religion. It is one of the oldest Indian religions. The three main pillars of Jainism are ''Ahimsa in Jainism, ahiṃsā'' (non-violence), ''anekāntavāda'' (non-absolut ...

Jainism
, and others – have a concept of a
wheel of time The Wheel of time or wheel of history (also known as '' Kalachakra'') is a concept found in several religious traditions and philosophies, notably religions of Indian origin such as Hinduism Hinduism () is an Indian religion and ''dharma'' ...

wheel of time
: they regard time as cyclical and
quantic Quantic may refer to: * Quantic, an older name for a homogeneous polynomial. * Quantic Dream, a video game developer studio * Will Holland, musician and producer with stage name ''Quantic'' See also

* Quantum (disambiguation) {{disambig ...
, consisting of repeating ages that happen to every being of the Universe between birth and extinction. In general, the Islamic and
Judeo-Christian The term Judeo-Christian is used to group Christianity and Judaism Christianity is rooted in Second Temple Judaism Second Temple Judaism is Judaism Judaism ( he, יהדות, ''Yahadut''; originally from Hebrew , ''Yehudah'', "Kingdom of ...
world-view regards time as
linear Linearity is the property of a mathematical relationship (''function Function or functionality may refer to: Computing * Function key A function key is a key on a computer A computer is a machine that can be programmed to carry out se ...

linear
and directional, beginning with the act of
creation Creation may refer to: Religion * Creation ''ex nihilo'', the concept that matter was created by God out of nothing * Creation myth A creation myth (or cosmogonic myth) is a symbolic narrative of how the world began and how people first came t ...
by God. The traditional Christian view sees time ending, teleologically, with the
eschatological Eschatology is a part of theology Theology is the systematic study of the nature of the Divinity, divine and, more broadly, of religious belief. It is taught as an Discipline (academia), academic discipline, typically in universities and sem ...
end of the present order of things, the "
end time The end time (also called end times, end of time, end of days, last days, final days, doomsday, or eschaton) is a future described variously in the eschatologies of several world religion World religions is a category used in the study of ...
". In the
Old Testament The Old Testament (often abbreviated OT) is the first division of the Christian biblical canon, which is based primarily upon the 24 books of the Hebrew Bible or Tanakh, a collection of ancient religious Hebrew writings by the Israelites. The ...
book
Ecclesiastes Ecclesiastes (; Hebrew language, Hebrew: , , grc, Ἐκκλησιαστής, ) written , is one of the Ketuvim ("Writings") of the Hebrew Bible and one of the wisdom literature, "Wisdom" books of the Christianity, Christian Old Testament. Th ...

Ecclesiastes
, traditionally ascribed to
Solomon Solomon (; he, , ), ''Šlēmūn''; : سُلَيْمَان ', also : ' or '; el, Σολομών ''Solomōn''; : Salomon) also called Jedidiah (, ), was, according to the and Christian , a fabulously wealthy and wise monarch of the who suc ...

Solomon
(970–928 BC), time (as the Hebrew word עידן, זמן ''iddan (age, as in "Ice age") zĕman(time)'' is often translated) was traditionally regarded as a medium for the passage of
predestined Predestination, in Christian theology, is the doctrine that all events have been willed by God, usually with reference to the eventual fate of the individual soul. Explanations of predestination often seek to address the "Argument from free will, ...
events. (Another word, زمان" זמן" ''zamān'', meant ''time fit for an event'', and is used as the modern
Arabic Arabic (, ' or , ' or ) is a Semitic language The Semitic languages are a branch of the Afroasiatic language family originating in the Middle East The Middle East is a list of transcontinental countries, transcontinental region ...

Arabic
,
Persian Persian may refer to: * People and things from Iran, historically called ''Persia'' in the English language ** Persians, Persian people, the majority ethnic group in Iran, not to be conflated with the Iranian peoples ** Persian language, an Iranian ...
, and
Hebrew Hebrew (, , or ) is a Northwest Semitic languages, Northwest Semitic language of the Afroasiatic languages, Afroasiatic language family. Historically, it is regarded as one of the spoken languages of the Israelites and their longest-survivi ...
equivalent to the English word "time".)


Time in Greek mythology

The Greek language denotes two distinct principles,
Chronos Chronos (; grc-gre, Χρόνος, (Modern Greek: ); Meaning - "time"), also spelled Khronos or Chronus, is the of in and later literature. Chronos is frequently confused with, or perhaps consciously identified with, the in antiquity du ...

Chronos
and
Kairos Kairos ( grc, ) is an ancient word meaning 'the right, critical, or opportune moment'. In , ''kairos'' also means 'weather'. It is one of two words that the ancient Greeks had for ''; the other being (). Whereas the latter refers to or , '' ...

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. 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 An anachronism (from the Greek#REDIRECT Greek Greek may refer to: Greece Anything of, from, or related to Greece Greece ( el, Ελλάδα, , ), officially the Hellenic Republic, is a country located in Southeast Europe. Its population i ...
'', ''synchronise'', and ''chronicle''.


Time in Kabbalah

According to
Kabbalists Kabbalah ( he, קַבָּלָה, links=no ''Qabālā'', literally "reception, tradition" or "correspondence") is an esoteric method, discipline, and Jewish theology, school of thought in Jewish mysticism. A traditional Kabbalist in Judaism is ...

Kabbalists
, "time" is a
paradox A paradox is a logically self-contradictory statement or a statement that runs contrary to one's expectation. It is a statement that, despite apparently valid reasoning from true premises, leads to a seemingly self-contradictory or a logically un ...

paradox
and an
illusion An illusion is a distortion of the sense A sense is a biological system used by an organism for sensation, the process of gathering information about the world and responding to Stimulus (physiology), stimuli. (For example, in the human body, t ...
. Both the future and the past are recognised to be combined and simultaneously present.


In Western philosophy

Two contrasting viewpoints on time divide prominent philosophers. One view is that time is part of the fundamental structure of the
universe The universe ( la, universus) is all of space and time and their contents, including planets, stars, galaxy, galaxies, and all other forms of matter and energy. The Big Bang theory is the prevailing cosmological description of the development ...

universe
 – a
dimension In physics Physics is the that studies , its , its and behavior through , and the related entities of and . "Physical science is that department of knowledge which relates to the order of nature, or, in other words, to the regular s ...

dimension
independent of events, in which events occur in
sequence In mathematics Mathematics (from Greek: ) includes the study of such topics as numbers (arithmetic and number theory), formulas and related structures (algebra), shapes and spaces in which they are contained (geometry), and quantities and t ...

sequence
.
Isaac Newton Sir Isaac Newton (25 December 1642 – 20 March 1726/27) was an English mathematician A mathematician is someone who uses an extensive knowledge of mathematics Mathematics (from Greek: ) includes the study of such topics a ...

Isaac Newton
subscribed to this realist view, and hence it is sometimes referred to as Newtonian time. 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 Space is the boundless three-dimensional Three-dimensional space (also: 3-space or, rarely, tri-dimensional space) is a geometric setting in which three values (called parameter A parameter (from the Ancient Greek language, Ancient Gre ...

space
and number) within which humans sequence and compare events. This second view, in the tradition of
Gottfried Leibniz Gottfried Wilhelm (von) Leibniz ; see inscription of the engraving depicted in the "#1666–1676, 1666–1676" section. ( – 14 November 1716) was a German polymath active as a mathematician, philosopher, scientist, and diplomat. He is a promin ...
and
Immanuel Kant Immanuel Kant (, , ; 22 April 1724 – 12 February 1804) was a German philosopher A philosopher is someone who practices philosophy Philosophy (from , ) is the study of general and fundamental questions, such as those about r ...

Immanuel Kant
, 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
subjective Subjective may refer to: * Subjectivity, a subject's personal perspective, feelings, beliefs, desires or discovery, as opposed to those made from an independent, objective, point of view ** Subjective experience, the subjective quality of consciou ...
component to time, but whether or not time itself is "felt", as a sensation, or is a judgment, is a matter of debate. * * * * * * * * Lehar, Steve. (2000)
The Function of Conscious Experience: An Analogical Paradigm of Perception and Behavior
, ''Consciousness and Cognition''.
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
finite Finite is the opposite of Infinity, infinite. It may refer to: * Finite number (disambiguation) * Finite set, a set whose cardinality (number of elements) is some natural number * Finite verb, a verb form that has a subject, usually being inflected ...
. These
philosopher A philosopher is someone who practices philosophy Philosophy (from , ) is the study of general and fundamental questions, such as those about Metaphysics, existence, reason, Epistemology, knowledge, Ethics, values, Philosophy of mind, mi ...

philosopher
s had different ways of explaining time; for instance, ancient Indian philosophers had something called the It is believed that there was repeating ages over the lifespan of the universe. This led to beliefs like cycles of rebirth and
reincarnation Reincarnation, also known as rebirth or transmigration, is the philosophical Philosophy (from , ) is the study of general and fundamental questions, such as those about existence Existence is the ability of an entity to interact with p ...
. The Greek philosophers believe that the universe was infinite, and was an illusion to humans.
Plato Plato ( ; grc-gre, Πλάτων ; 428/427 or 424/423 – 348/347 BC) was an Classical Athens, Athenian philosopher during the Classical Greece, Classical period in Ancient Greece, founder of the Platonist school of thought and the Platoni ...

Plato
believed 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 the
heavenly bodies "Heavenly Bodies" is a song written by Elaine Lifton, Gloria Nissenson and Lee Ritenour, and recorded by American country music artist Earl Thomas Conley. It was released in May 1982 as the first single from the album ''Somewhere Between Right a ...
.
Aristotle Aristotle (; grc-gre, Ἀριστοτέλης ''Aristotélēs'', ; 384–322 BC) was a Greek philosopher A philosopher is someone who practices philosophy Philosophy (from , ) is the study of general and fundamental questio ...

Aristotle
believed 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 of
celestial bodies In astronomy Astronomy (from el, ἀστρονομία, literally meaning the science that studies the laws of the stars) is a natural science that studies astronomical object, celestial objects and celestial event, phenomena. It uses ...
; the reason that humans can tell time was because of orbital periods and therefore there was a duration on time. The ''
Vedas upright=1.2, The Vedas are ancient Sanskrit texts of Hinduism. Above: A page from the '' Atharvaveda''. The Vedas (, , ) are a large body of religious texts originating in ancient India. Composed in Vedic Sanskrit, the texts constitute the o ...

Vedas
'', the earliest texts on
Indian philosophy Indian philosophy refers to philosophical traditions of the Indian subcontinent. A traditional Hindu classification divides Āstika and nāstika, āstika and Āstika and nāstika, nāstika schools of philosophy, depending on one of three alter ...
and
Hindu philosophy Hindu philosophy encompasses the philosophies, world views and teachings of Hinduism Hinduism () is an Indian religion Indian religions, sometimes also termed Dharmic religions or Indic religions, are the religions that origina ...
dating back to the late
2nd millennium BC The 2nd millennium BC spanned the years 2000 through 1001 BC. In the Ancient Near East The ancient Near East was the home of early civilization A civilization (or civilisation) is any complex society that is characterized by urban de ...
, describe ancient
Hindu cosmology Hindu cosmology is the description of the universe and its states of matter, cycles within time, physical structure, and effects on living entities according to Hindu texts Hindus () are persons who regard themselves as culturally, ethnic ...
, in which the
universe The universe ( la, universus) is all of space and time and their contents, including planets, stars, galaxy, galaxies, and all other forms of matter and energy. The Big Bang theory is the prevailing cosmological description of the development ...

universe
goes through repeated cycles of creation, destruction and rebirth, with each cycle lasting 4,320 million years.
Ancient Ancient history is the aggregate of past eventsWordNet Search – 3.0
"History"
from ...
Greek philosophers Ancient Greek philosophy arose in the 6th century BC, at a time when the inhabitants of ancient Greece were struggling to repel devastating invasions from the east. Greek philosophy continued throughout the Hellenistic period The Hellenistic pe ...
, including
Parmenides Parmenides of Elea (; grc-gre, Παρμενίδης ὁ Ἐλεάτης; ) was a pre-Socratic Greek philosopher A philosopher is someone who practices philosophy. The term ''philosopher'' comes from the grc, φιλόσοφος, , translit ...

Parmenides
and
Heraclitus Heraclitus of Ephesus (; grc-gre, Ἡράκλειτος ; , ) was an Ancient Greek Ancient Greek includes the forms of the Greek language Greek ( el, label=Modern Greek Modern Greek (, , or , ''Kiní Neoellinikí Glóssa''), ...

Heraclitus
, wrote essays on the nature of time.
Plato Plato ( ; grc-gre, Πλάτων ; 428/427 or 424/423 – 348/347 BC) was an Classical Athens, Athenian philosopher during the Classical Greece, Classical period in Ancient Greece, founder of the Platonist school of thought and the Platoni ...

Plato
, in the ''Timaeus'', identified time with the period of motion of the heavenly bodies.
Aristotle Aristotle (; grc-gre, Ἀριστοτέλης ''Aristotélēs'', ; 384–322 BC) was a Greek philosopher A philosopher is someone who practices philosophy Philosophy (from , ) is the study of general and fundamental questio ...

Aristotle
, in Book IV of his ''Physica'' defined time as 'number of movement in respect of the before and after'. In Book 11 of his '' Confessions'', 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, an approach similar to that taken in other 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.
Isaac Newton Sir Isaac Newton (25 December 1642 – 20 March 1726/27) was an English mathematician A mathematician is someone who uses an extensive knowledge of mathematics Mathematics (from Greek: ) includes the study of such topics a ...

Isaac Newton
believed 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 famous
Leibniz–Clarke correspondence Image:Samuel Clarke.jpg, Samuel Clarke. The Leibniz–Clarke correspondence was a scientific, theological and philosophical debate conducted in an exchange of letters between the Germany, German thinker Gottfried Wilhelm Leibniz and Samuel Clarke, a ...
. 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 Newton Sir Isaac Newton (25 December 1642 – 20 March 1726/27) was an English mathematician A mathematician is someone who uses an extensive knowledge of mathematics Mathematics (from Greek: ) includes the study of such topics a ...

Isaac Newton
said that we are merely occupying time, he also says that humans can only understand relative 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 called relational time. René Descartes, John Locke, and David Hume said that one's mind needs to acknowledge time, in order to understand what time is.
Immanuel Kant Immanuel Kant (, , ; 22 April 1724 – 12 February 1804) was a German philosopher A philosopher is someone who practices philosophy Philosophy (from , ) is the study of general and fundamental questions, such as those about r ...

Immanuel Kant
believed that we can not know what something is unless we experience it first hand.
Immanuel Kant Immanuel Kant (, , ; 22 April 1724 – 12 February 1804) was a German philosopher A philosopher is someone who practices philosophy Philosophy (from , ) is the study of general and fundamental questions, such as those about r ...

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. translated by J.M.D. Meiklejohn, eBooks@Adelaide, 2004 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 Measurement is the quantification (science), quantification of variable and attribute (research), attributes of an object or event, which can be used to compare with other objects or events. The scope and application of measurement are dependen ...

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. 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. 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, If time tensed or tenseless, and is there a future to be? 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. 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. There is also something called imaginary time, this was from Stephen Hawking, he says that space and imaginary time are finite but have no boundaries. Imaginary time is 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 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 Parmenides of Elea (; grc-gre, Παρμενίδης ὁ Ἐλεάτης; ) was a pre-Socratic Greek philosopher A philosopher is someone who practices philosophy. The term ''philosopher'' comes from the grc, φιλόσοφος, , translit ...

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. Time as an illusion is also a common theme in Buddhism, Buddhist thought. J. M. E. McTaggart's 1908 ''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 (see also Philosophy of space and time#The flow of time, The flow of time). 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 in his book ''The End of Time (book), The End of Time'', argue that 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, called "Platonia (philosophy), platonia" by Barbour. A 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. 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

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. Non-relativistic classical mechanics is based on this Newtonian idea of time. Einstein, in his Special relativity, special 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 in
Minkowski space In mathematical physics, Minkowski space (or Minkowski spacetime) () is a combination of three-dimensional Euclidean space Euclidean space is the fundamental space of classical geometry. Originally it was the three-dimensional space of Euclid ...
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 his or her velocity).


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. 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.


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 General relativity, also known as the general theory of relativity, is the geometric Geometry (from the grc, γεωμετρία; '' geo-'' "earth", '' -metron'' "measurement") is, with arithmetic, one of the oldest branches of mathema ...
. 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 The past is the set of all Spacetime#Basic concepts, events that occurred before a given point in time. The past is contrasted with and defined by the present and the future. The concept of the past is derived from the linear fashion in which h ...

past
is the set of events that can send light signals to an entity and the
future The future is the time after the past and present. Its arrival is considered inevitable due to the existence of time and the laws of physics. Due to the apparent nature of reality and the unavoidability of the future, everything that currently ...

future
is the set of events to which an entity can send light signals.


Dilation

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 particles 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 Spacetime#Basic concepts, events taking place at the points A and B of a system K are simultaneous if they appear at the same instant when observed from the middle point, M, of the interval AB. Time is then defined as the ensemble of the indications of similar clocks, at rest 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.


Relativistic versus Newtonian

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 transformations 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. Note how in both pictures the view of spacetime changes when the observer accelerates. In the Newtonian description these changes are such that ''time'' is 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 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.


Arrow

Time appears to have a direction – the past lies behind, fixed and immutable, while the future lies ahead and is not necessarily fixed. Yet for the most part, the laws of physics do not specify an arrow of time, and allow any process to proceed both forward and in reverse. This is generally a consequence of time being modelled by a parameter in the system being analysed, where there is no "proper time": the direction of the arrow of time is sometimes arbitrary. Examples of this include the Physical cosmology, cosmological arrow of time, which points away from the Big Bang, CPT symmetry, and the radiative arrow of time, caused by light only travelling forwards in time (see light cone). In particle physics, the CP violation, violation of CP symmetry implies that there should be a small counterbalancing time asymmetry to preserve CPT symmetry as stated above. The standard description of
measurement Measurement is the quantification (science), quantification of variable and attribute (research), attributes of an object or event, which can be used to compare with other objects or events. The scope and application of measurement are dependen ...

measurement
in quantum mechanics is also time asymmetric (see Measurement in quantum mechanics). The second law of thermodynamics states that entropy must increase over time (see Entropy (arrow of time), Entropy). This can be in either direction – 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.


Quantization

Time quantization is a hypothetical concept. In the modern established physical theories (the Standard Model of Particles and Interactions and General Relativity) time is not quantized.
Planck time In particle physics Particle physics (also known as high energy physics) is a branch of that studies the nature of the particles that constitute and . Although the word ' can refer to various types of very small objects (e.g. , gas particl ...
(~ 5.4 × 10−44 seconds) is the unit of time in the system of natural units known as Planck units. Current established physical theories are believed to fail at this time scale, and many physicists expect that the Planck time might be the smallest unit of time that could ever be measured, even in principle. Tentative physical theories that describe this time scale exist; see for instance loop quantum gravity.


Travel

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 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 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 The universe ( la, universus) is all of space and time and their contents, including planets, stars, galaxy, galaxies, and all other forms of matter and energy. The Big Bang theory is the prevailing cosmological description of the development ...

universe
s. 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. As such, it would not be possible to enact the grandfather paradox 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 his or her past, resulting in the reality that the traveller moves from. More elaboration on this view can be found in the Novikov self-consistency principle.


Perception

The specious present refers to the time duration wherein one's perceptions 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.


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. Psychoactive drugs can impair the judgment of time. Stimulants can lead both humans and rats to overestimate time intervals, while depressants can have the opposite effect. The level of activity in the brain of neurotransmitters such as dopamine and norepinephrine may be the reason for this. Such chemicals will either excite or inhibit the firing of neurons 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). Mental chronometry is 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 by temporal illusions (like the kappa effect),Wada Y, Masuda T, Noguchi K, 2005, "Temporal illusion called 'kappa effect' in event perception" Perception 34 ECVP Abstract Supplement age, and hypnosis. The sense of time is impaired in some people with neurological diseases such as Parkinson's disease and attention 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 is Conceptual metaphor, conceptualized in 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, 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. 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 and Israeli Hebrew language, Israeli-Hebrew speakers 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 recent study of the indigenous Yupno people of Papua New Guinea focused 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, an Aboriginal groupings of Western Australia, aboriginal group in 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

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 and Norbert Elias 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 In mathematics Mathematics (from Greek: ) includes the study of such topics as numbers (arithmetic and number theory), formulas and related structures (algebra), shapes and spaces in which they are contained (geometry), and quantities and t ...

sequence
of items, facts, events, actions, changes, or procedural steps, arranged in time order (chronological order), often with causality relationships among the items. 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, charts, 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, historical events (
chronology Chronology (from Latin ''chronologia'', from Ancient Greek , ''chrónos'', "time"; and , ''wikt:-logia, -logia'') is the science of arranging events in their order of occurrence in time. Consider, for example, the use of a timeline or sequence ...
), directions and steps in procedures, 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., plans, 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 (sequence of events recorder). A specific example of a sequence of events is the timeline of the Fukushima Daiichi nuclear disaster.


See also

* List of UTC timing centers * Term (time) * 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


References


Further reading

* * Craig Callendar, ''Introducing Time'', Icon Books, 2010, * – Research bibliography * * * * Benjamin Gal-Or, ''Cosmology, Physics and Philosophy'', Springer Verlag, 1981, 1983, 1987, . * Charlie Gere, (2005) ''Art, Time and Technology: Histories of the Disappearing Body'', Berg * * * * * * * * * * * 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, . * * *


External links


Different systems of measuring time
*
Time
in the ''Internet Encyclopedia of Philosophy'', by Bradley Dowden. * {{Authority control Time, Concepts in metaphysics Physical phenomena Concepts in the philosophy of mind Concepts in the philosophy of science Metaphysical theories Ontology Perception Philosophy of time Scalar physical quantities Qualia Reality SI base quantities Spacetime Main topic articles