Time

# Time

Overview
Time is a part of the measuring system
Measurement
Measurement is the process or the result of determining the ratio of a physical quantity, such as a length, time, temperature etc., to a unit of measurement, such as the metre, second or degree Celsius...

used to sequence
Sequence
In mathematics, a sequence is an ordered list of objects . Like a set, it contains members , and the number of terms is called the length of the sequence. Unlike a set, order matters, and exactly the same elements can appear multiple times at different positions in the sequence...

events, to compare the durations of events
Event (philosophy)
In philosophy, events are objects in time or instantiations of properties in objects. However, a definite definition has not been reached, as multiple theories exist concerning events.-Kim’s Property-Exemplification Account of Events:...

and the intervals between them, and to quantify
Quantification
Quantification has several distinct senses. In mathematics and empirical science, it is the act of counting and measuring that maps human sense observations and experiences into members of some set of numbers. Quantification in this sense is fundamental to the scientific method.In logic,...

rates of change such as the motions
Motion (physics)
In physics, motion is a change in position of an object with respect to time. Change in action is the result of an unbalanced force. Motion is typically described in terms of velocity, acceleration, displacement and time . An object's velocity cannot change unless it is acted upon by a force, as...

of objects
Physical body
In physics, a physical body or physical object is a collection of masses, taken to be one...

.
The temporal position of events with respect to the transitory present
Present
Present is a time that is neither past nor future.Present may also refer to:- Time and timing :* Present tense, the grammatical tense of a verb* Before Present, radiocarbon dates relative to AD 1950* Presenting, a medical term* Presenteeism...

is continually changing; events happen, then are located further and further in the past
Past
Most generally, the past is a term used to indicate the totality of events which 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 human observers experience time, and is...

.
Time has been a major subject of religion
Religion
Religion is a collection of cultural systems, belief systems, and worldviews that establishes symbols that relate humanity to spirituality and, sometimes, to moral values. Many religions have narratives, symbols, traditions and sacred histories that are intended to give meaning to life or to...

, philosophy
Philosophy
Philosophy is the study of general and fundamental problems, such as those connected with existence, knowledge, values, reason, mind, and language. Philosophy is distinguished from other ways of addressing such problems by its critical, generally systematic approach and its reliance on rational...

, and science
Science
Science is a systematic enterprise that builds and organizes knowledge in the form of testable explanations and predictions about the universe...

, but defining it in a non-controversial manner applicable to all fields of study has consistently eluded the greatest scholars.
Discussion

Recent Discussions
Quotations

Time is an illusion, lunchtime doubly so.

Douglas Adams, The Hitchhiker's Guide to the Galaxy

How can I tell that the past isn't a fiction designed to account for the discrepancy between my immediate physical sensations and my state of mind?

The man in the shack, in The Restaurant at the End of the Universe|The Restaurant at the End of the Universe (1980) by Douglas Adams. Ch. 29

Time is not a reality [hupostasis], but a concept [noêma] or a measure [metron]…

Antiphon (person)|Antiphon the Sophist, Truth

Jacob Bronowski, Science and Human Values (1956)

With the magnificence of eternity before us, let time, with all its fluctuations, dwindle into its own littleness.

Thomas Chalmers, reported in Josiah Hotchkiss Gilbert, Dictionary of Burning Words of Brilliant Writers (1895), p. 584.

In the spirit of faith let us begin each day, and we shall be sure to " redeem the time " which it brings to us, by changing it into something definite and eternal. There is a deep meaning in this phrase of the apostle, to redeem time. We redeem time, and do not merely use it. We transform it into eternity by living it aright.

James Freeman Clarke, reported in Josiah Hotchkiss Gilbert, Dictionary of Burning Words of Brilliant Writers (1895), p. 583.

Time,— that black and narrow isthmus between two eternities.

Charles Caleb Colton, reported in Josiah Hotchkiss Gilbert, Dictionary of Burning Words of Brilliant Writers (1895), p. 582.

Time goes, you say? Ah no, alas, time stays, we go.

Henry Austin Dobson

The distinction between past, present, and future is only a stubbornly persistent illusion.

Albert Einstein

A butterflyFluttering over the vendor’sDry flowers of springOnly two days it fliesCaught by the lost boyYet still.

For Peng Fajardo by Shane Castro in The Now
Encyclopedia
Time is a part of the measuring system
Measurement
Measurement is the process or the result of determining the ratio of a physical quantity, such as a length, time, temperature etc., to a unit of measurement, such as the metre, second or degree Celsius...

used to sequence
Sequence
In mathematics, a sequence is an ordered list of objects . Like a set, it contains members , and the number of terms is called the length of the sequence. Unlike a set, order matters, and exactly the same elements can appear multiple times at different positions in the sequence...

events, to compare the durations of events
Event (philosophy)
In philosophy, events are objects in time or instantiations of properties in objects. However, a definite definition has not been reached, as multiple theories exist concerning events.-Kim’s Property-Exemplification Account of Events:...

and the intervals between them, and to quantify
Quantification
Quantification has several distinct senses. In mathematics and empirical science, it is the act of counting and measuring that maps human sense observations and experiences into members of some set of numbers. Quantification in this sense is fundamental to the scientific method.In logic,...

rates of change such as the motions
Motion (physics)
In physics, motion is a change in position of an object with respect to time. Change in action is the result of an unbalanced force. Motion is typically described in terms of velocity, acceleration, displacement and time . An object's velocity cannot change unless it is acted upon by a force, as...

of objects
Physical body
In physics, a physical body or physical object is a collection of masses, taken to be one...

.
The temporal position of events with respect to the transitory present
Present
Present is a time that is neither past nor future.Present may also refer to:- Time and timing :* Present tense, the grammatical tense of a verb* Before Present, radiocarbon dates relative to AD 1950* Presenting, a medical term* Presenteeism...

is continually changing; events happen, then are located further and further in the past
Past
Most generally, the past is a term used to indicate the totality of events which 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 human observers experience time, and is...

.
Time has been a major subject of religion
Religion
Religion is a collection of cultural systems, belief systems, and worldviews that establishes symbols that relate humanity to spirituality and, sometimes, to moral values. Many religions have narratives, symbols, traditions and sacred histories that are intended to give meaning to life or to...

, philosophy
Philosophy
Philosophy is the study of general and fundamental problems, such as those connected with existence, knowledge, values, reason, mind, and language. Philosophy is distinguished from other ways of addressing such problems by its critical, generally systematic approach and its reliance on rational...

, and science
Science
Science is a systematic enterprise that builds and organizes knowledge in the form of testable explanations and predictions about the universe...

, but defining it in a non-controversial manner applicable to all fields of study has consistently eluded the greatest scholars. A simple definition states that "time is what clocks measure".

Time is one of the seven fundamental physical quantities in the International System of Units
International System of Units
The International System of Units is the modern form of the metric system and is generally a system of units of measurement devised around seven base units and the convenience of the number ten. The older metric system included several groups of units...

. Time is used to define other quantities — such as velocity
Velocity
In physics, velocity is speed in a given direction. Speed describes only how fast an object is moving, whereas velocity gives both the speed and direction of the object's motion. To have a constant velocity, an object must have a constant speed and motion in a constant direction. Constant ...

— so defining time in terms of such quantities would result in circularity of definition
Circular definition
A circular definition is one that uses the term being defined as a part of the definition or assumes a prior understanding of the term being defined. Either the audience must already know the meaning of the key term, or the definition is deficient in including the term to be defined in the...

.
An operational definition
Operational definition
An operational definition defines something in terms of the specific process or set of validation tests used to determine its presence and quantity. That is, one defines something in terms of the operations that count as measuring it. The term was coined by Percy Williams Bridgman and is a part of...

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
Second
The second is a unit of measurement of time, and is the International System of Units base unit of time. It may be measured using a clock....

, is highly useful in the conduct of both advanced experiments and everyday affairs of life. The operational definition leaves aside the question whether there is something called time, apart from the counting activity just mentioned, that flows and that can be measured. Investigations of a single continuum called spacetime
Spacetime
In physics, spacetime is any mathematical model that combines space and time into a single continuum. Spacetime is usually interpreted with space as being three-dimensional and time playing the role of a fourth dimension that is of a different sort from the spatial dimensions...

Space
Space is the boundless, three-dimensional extent in which objects and events occur and have relative position and direction. Physical space is often conceived in three linear dimensions, although modern physicists usually consider it, with time, to be part of a boundless four-dimensional continuum...

into questions about time, questions that have their roots in the works of early students of natural philosophy
Natural philosophy
Natural philosophy or the philosophy of nature , is a term applied to the study of nature and the physical universe that was dominant before the development of modern science...

.

Two contrasting viewpoints on time divide many prominent philosophers.
One view is that time is part of the fundamental structure of the universe
Universe
The Universe is commonly defined as the totality of everything that exists, including all matter and energy, the planets, stars, galaxies, and the contents of intergalactic space. Definitions and usage vary and similar terms include the cosmos, the world and nature...

, a dimension
Dimension
In physics and mathematics, the dimension of a space or object is informally defined as the minimum number of coordinates needed to specify any point within it. Thus a line has a dimension of one because only one coordinate is needed to specify a point on it...

in which events occur in sequence
Sequence
In mathematics, a sequence is an ordered list of objects . Like a set, it contains members , and the number of terms is called the length of the sequence. Unlike a set, order matters, and exactly the same elements can appear multiple times at different positions in the sequence...

. Sir Isaac Newton subscribed to this realist
Philosophical realism
Contemporary philosophical realism is the belief that our reality, or some aspect of it, is ontologically independent of our conceptual schemes, linguistic practices, beliefs, etc....

view, and hence it is sometimes referred to as Newtonian time
Absolute time and space
Originally introduced by Sir Isaac Newton in the Philosophiæ Naturalis Principia Mathematica, the concepts of absolute time and space provided a theoretical foundation that facilitated Newtonian mechanics...

.
Time travel
Time travel
Time travel is the concept of moving between different points in time in a manner analogous to moving between different points in space. Time travel could hypothetically involve moving backward in time to a moment earlier than the starting point, or forward to the future of that point without the...

, in this view, becomes a possibility as other "times" persist like frames of a film strip, spread out across the time line.
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
Space is the boundless, three-dimensional extent in which objects and events occur and have relative position and direction. Physical space is often conceived in three linear dimensions, although modern physicists usually consider it, with time, to be part of a boundless four-dimensional continuum...

and number
Number
A number is a mathematical object used to count and measure. In mathematics, the definition of number has been extended over the years to include such numbers as zero, negative numbers, rational numbers, irrational numbers, and complex numbers....

) within which humans sequence and compare events. This second view, in the tradition of Gottfried Leibniz
Gottfried Leibniz
Gottfried Wilhelm Leibniz was a German philosopher and mathematician. He wrote in different languages, primarily in Latin , French and German ....

and Immanuel Kant
Immanuel Kant
Immanuel Kant was a German philosopher from Königsberg , researching, lecturing and writing on philosophy and anthropology at the end of the 18th Century Enlightenment....

,
holds that time is neither an event nor a thing, and thus is not itself measurable nor can it be travelled.

Temporal measurement has occupied scientists and technologist
Engineering technologist
An engineering technologist, is a specialist devoted to the implementation of existing technology within a field of engineering. Technologists often work with engineers in a wide variety of projects by applying basic engineering principles and technical skills...

s, and was a prime motivation in navigation
Navigation is the process of monitoring and controlling the movement of a craft or vehicle from one place to another. It is also the term of art used for the specialized knowledge used by navigators to perform navigation tasks...

and astronomy
Astronomy
Astronomy is a natural science that deals with the study of celestial objects and phenomena that originate outside the atmosphere of Earth...

. 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
Second
The second is a unit of measurement of time, and is the International System of Units base unit of time. It may be measured using a clock....

, is defined in terms of radiation emitted by caesium
Caesium
Caesium or cesium is the chemical element with the symbol Cs and atomic number 55. It is a soft, silvery-gold alkali metal with a melting point of 28 °C , which makes it one of only five elemental metals that are liquid at room temperature...

atoms (see below). Time is also of significant social importance, having economic value ("time is money
Time value of money
The time value of money is the value of money figuring in a given amount of interest earned over a given amount of time. The time value of money is the central concept in finance theory....

") as well as personal value, due to an awareness
Awareness
Awareness is the state or ability to perceive, to feel, or to be conscious of events, objects or sensory patterns. In this level of consciousness, sense data can be confirmed by an observer without necessarily implying understanding. More broadly, it is the state or quality of being aware of...

of the limited time in each day and in human life spans
Life expectancy
Life expectancy is the expected number of years of life remaining at a given age. It is denoted by ex, which means the average number of subsequent years of life for someone now aged x, according to a particular mortality experience...

.

Ray Cummings
Ray Cummings
Ray Cummings was an American author of science fiction, rated one of the "founding fathers of the science fiction pulp genre". He was born in New York and died in Mount Vernon, New York....

, an early writer of science fiction, wrote in 1922, "Time... is what keeps everything from happening at once",
a sentence repeated by scientists such as C. J. Overbeck,
and John Archibald Wheeler
John Archibald Wheeler
John Archibald Wheeler was an American theoretical physicist who was largely responsible for reviving interest in general relativity in the United States after World War II. Wheeler also worked with Niels Bohr in explaining the basic principles behind nuclear fission...

.

## Temporal measurement

Temporal measurement, or chronometry
Chronometry
Chronometry is the science of the measurement of time, or timekeeping.It should not to be confused with chronology, the science of locating events in time, which often relies upon it.-See also:...

, takes two distinct period forms: the calendar
Calendar
A calendar is a system of organizing days for social, religious, commercial, or administrative purposes. This is done by giving names to periods of time, typically days, weeks, months, and years. The name given to each day is known as a date. Periods in a calendar are usually, though not...

, a mathematical abstraction for calculating extensive periods of time,
and the clock
Clock
A clock is an instrument used to indicate, keep, and co-ordinate time. The word clock is derived ultimately from the Celtic words clagan and clocca meaning "bell". A silent instrument missing such a mechanism has traditionally been known as a timepiece...

, a physical mechanism that counts the ongoing passage of time. In day-to-day life, the clock is consulted for periods less than a day, the calendar, for periods longer than a day. 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 Palaeolithic suggest that the moon was used to reckon time as early as 6,000 years ago.
Lunar calendar
Lunar calendar
A lunar calendar is a calendar that is based on cycles of the lunar phase. A common purely lunar calendar is the Islamic calendar or Hijri calendar. A feature of the Islamic calendar is that a year is always 12 months, so the months are not linked with the seasons and drift each solar year by 11 to...

s were among the first to appear, either 12 or 13 lunar month
Lunar month
In lunar calendars, a lunar month is the time between two identical syzygies . There are many variations. In Middle-Eastern and European traditions, the month starts when the young crescent moon becomes first visible at evening after conjunction with the Sun one or two days before that evening...

s (either 354 or 384 days). Without intercalation
Intercalation
Intercalation is the insertion of a leap day, week or month into some calendar years to make the calendar follow the seasons or moon phases. Lunisolar calendars may require intercalations of both days and months.- Solar calendars :...

to add days or months to some years, seasons quickly drift in a calendar based solely on twelve lunar months. Lunisolar calendar
Lunisolar calendar
A lunisolar calendar is a calendar in many cultures whose date indicates both the moon phase and the time of the solar year. If the solar year is defined as a tropical year then a lunisolar calendar will give an indication of the season; if it is taken as a sidereal year then the calendar will...

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.

The reforms of Julius Caesar
Julius Caesar
Gaius Julius Caesar was a Roman general and statesman and a distinguished writer of Latin prose. He played a critical role in the gradual transformation of the Roman Republic into the Roman Empire....

in 45 BC put the Roman world
Roman Empire
The Roman Empire was the post-Republican period of the ancient Roman civilization, characterised by an autocratic form of government and large territorial holdings in Europe and around the Mediterranean....

on a solar calendar
Solar calendar
A solar calendar is a calendar whose dates indicate the position of the earth on its revolution around the sun .-Tropical solar calendars:...

. This Julian calendar
Julian calendar
The Julian calendar began in 45 BC as a reform of the Roman calendar by Julius Caesar. It was chosen after consultation with the astronomer Sosigenes of Alexandria and was probably designed to approximate the tropical year .The Julian calendar has a regular year of 365 days divided into 12 months...

was faulty in that its intercalation
Intercalation
Intercalation is the insertion of a leap day, week or month into some calendar years to make the calendar follow the seasons or moon phases. Lunisolar calendars may require intercalations of both days and months.- Solar calendars :...

still allowed the astronomical solstice
Solstice
A solstice is an astronomical event that happens twice each year when the Sun's apparent position in the sky, as viewed from Earth, reaches its northernmost or southernmost extremes...

s and equinox
Equinox
An equinox occurs twice a year, when the tilt of the Earth's axis is inclined neither away from nor towards the Sun, the center of the Sun being in the same plane as the Earth's equator...

es to advance against it by about 11 minutes per year. Pope Gregory XIII
Pope Gregory XIII
Pope Gregory XIII , born Ugo Boncompagni, was Pope from 1572 to 1585. He is best known for commissioning and being the namesake for the Gregorian calendar, which remains the internationally-accepted civil calendar to this date.-Youth:He was born the son of Cristoforo Boncompagni and wife Angela...

introduced a correction in 1582; the Gregorian calendar
Gregorian calendar
The Gregorian calendar, also known as the Western calendar, or Christian calendar, is the internationally accepted civil calendar. It was introduced by Pope Gregory XIII, after whom the calendar was named, by a decree signed on 24 February 1582, a papal bull known by its opening words Inter...

was only slowly adopted by different nations over a period of centuries, but is today by far the one in most common use around the world.

### History of time measurement devices

A large variety of device
Measuring instrument
In the physical sciences, quality assurance, and engineering, measurement is the activity of obtaining and comparing physical quantities of real-world objects and events. Established standard objects and events are used as units, and the process of measurement gives a number relating the item...

s have been invented to measure time. The study of these devices is called horology
Horology
Horology is the art or science of measuring time. Clocks, watches, clockwork, sundials, clepsydras, timers, time recorders and marine chronometers are all examples of instruments used to measure time.People interested in horology are called horologists...

.

An Egypt
Egypt
Egypt , officially the Arab Republic of Egypt, Arabic: , is a country mainly in North Africa, with the Sinai Peninsula forming a land bridge in Southwest Asia. Egypt is thus a transcontinental country, and a major power in Africa, the Mediterranean Basin, the Middle East and the Muslim world...

ian device dating to c.1500 BC, similar in shape to a bent T-square
T-square
A T-square is a technical drawing instrument used by draftsmen primarily as a guide for drawing horizontal lines on a drafting table. It may also guide a triangle to draw vertical or diagonal lines. Its name comes from the general shape of the instrument where the horizontal member of the T slides...

, 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
Noon
Noon is usually defined as 12 o'clock in the daytime. The word noon is also used informally to mean midday regarding the location of the sun not the middle of a persons day. Although this is a time around the middle of the day when people in many countries take a lunch break...

, the device was turned around so that it could cast its shadow in the evening direction.

A sundial
Sundial
A sundial is a device that measures time by the position of the Sun. In common designs such as the horizontal sundial, the sun casts a shadow from its style onto a surface marked with lines indicating the hours of the day. The style is the time-telling edge of the gnomon, often a thin rod or a...

uses a gnomon
Gnomon
The gnomon is the part of a sundial that casts the shadow. Gnomon is an ancient Greek word meaning "indicator", "one who discerns," or "that which reveals."It has come to be used for a variety of purposes in mathematics and other fields....

to cast a shadow on a set of markings which were calibrated to the hour
Hour
The hour is a unit of measurement of time. In modern usage, an hour comprises 60 minutes, or 3,600 seconds...

. The position of the shadow marked the hour in local time.

The most precise timekeeping devices of the ancient world were the water clock
Water clock
A water clock or clepsydra is any timepiece in which time is measured by the regulated flow of liquid into or out from a vessel where the amount is then measured.Water clocks, along with sundials, are likely to be the oldest time-measuring instruments, with the only exceptions...

or clepsydra, one of which was found in the tomb of Egyptian pharaoh Amenhotep I
Amenhotep I
Amenhotep I was the second Pharaoh of the 18th dynasty of Egypt. His reign is generally dated from 1526 to 1506 BC. He was born to Ahmose I and Ahmose-Nefertari, but had at least two elder brothers, Ahmose-ankh and Ahmose Sapair, and was not expected to inherit the throne...

(1525–1504 BC). They could be used to measure the hours even at night, but required manual upkeep to replenish the flow of water. The Greeks
Greeks
The Greeks, also known as the Hellenes , are a nation and ethnic group native to Greece, Cyprus and neighboring regions. They also form a significant diaspora, with Greek communities established around the world....

and Chaldea
Chaldea
Chaldea or Chaldaea , from Greek , Chaldaia; Akkadian ; Hebrew כשדים, Kaśdim; Aramaic: ܟܐܠܕܘ, Kaldo) was a marshy land located in modern-day southern Iraq which came to briefly rule Babylon...

ns 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
History of science and technology in China
The history of science and technology in China is both long and rich with many contributions to science and technology. In antiquity, independently of other civilizations, ancient Chinese philosophers made significant advances in science, technology, mathematics, and astronomy...

invented the first mechanical clocks to be driven by an escapement
Escapement
In mechanical watches and clocks, an escapement is a device that transfers energy to the timekeeping element and enables counting the number of oscillations of the timekeeping element...

mechanism.

The hourglass
Hourglass
An hourglass measures the passage of a few minutes or an hour of time. It has two connected vertical glass bulbs allowing a regulated trickle of material from the top to the bottom. Once the top bulb is empty, it can be inverted to begin timing again. The name hourglass comes from historically...

uses the flow of sand to measure the flow of time. They were used in navigation. Ferdinand Magellan
Ferdinand Magellan
Ferdinand Magellan was a Portuguese explorer. He was born in Sabrosa, in northern Portugal, and served King Charles I of Spain in search of a westward route to the "Spice Islands" ....

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
Richard of Wallingford
Richard of Wallingford was an English mathematician who made major contributions to astronomy/astrology and horology while serving as abbot of St Albans Abbey in Hertfordshire.-Biography:...

(1292–1336), abbot of St. Alban's abbey, famously built a mechanical clock as an astronomical orrery
Orrery
An orrery is a mechanical device that illustrates the relative positions and motions of the planets and moons in the Solar System in a heliocentric model. Though the Greeks had working planetaria, the first orrery that was a planetarium of the modern era was produced in 1704, and one was presented...

Galileo Galilei
Galileo Galilei , was an Italian physicist, mathematician, astronomer, and philosopher who played a major role in the Scientific Revolution. His achievements include improvements to the telescope and consequent astronomical observations and support for Copernicanism...

and especially Christiaan Huygens with the invention of pendulum driven clocks.

The English word clock
Clock
A clock is an instrument used to indicate, keep, and co-ordinate time. The word clock is derived ultimately from the Celtic words clagan and clocca meaning "bell". A silent instrument missing such a mechanism has traditionally been known as a timepiece...

probably comes from the Middle Dutch word "klocke" which is in turn derived from the mediaeval Latin word "clocca", which is ultimately derived from Celtic, and is cognate with French, Latin, and German words that mean bell
Bell (instrument)
A bell is a simple sound-making device. The bell is a percussion instrument and an idiophone. Its form is usually a hollow, cup-shaped object, which resonates upon being struck...

. The passage of the hours at sea were marked by bells, and denoted the time (see ship's bells
Ship's bells
A ship's bell is usually made of bronze, and often has the ship's name engraved or cast on it. The ship's cook traditionally has the job of shining the ship's bell.-Timing of duty periods:...

). The hours were marked by bells in the abbeys as well as at sea.

Clocks can range from watch
Watch
A watch is a small timepiece, typically worn either on the wrist or attached on a chain and carried in a pocket, with wristwatches being the most common type of watch used today. They evolved in the 17th century from spring powered clocks, which appeared in the 15th century. The first watches were...

es, to more exotic varieties such as the Clock of the Long Now
Clock of the Long Now
The Clock of the Long Now, also called the 10,000-year clock, is a proposed mechanical clock designed to keep time for 10,000 years. The project to build it is part of the Long Now Foundation....

. 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
Pendulum
A pendulum is a weight suspended from a pivot so that it can swing freely. When a pendulum is displaced from its resting equilibrium position, it is subject to a restoring force due to gravity that will accelerate it back toward the equilibrium position...

.

A chronometer
Chronometer
Chronometer may refer to:* Chronometer watch, a watch tested and certified to meet certain precision standards* Hydrochronometer, a water clock* Marine chronometer, a timekeeper used for celestial navigation...

is a portable timekeeper that meets certain precision standards. Initially, the term was used to refer to the marine chronometer
Marine chronometer
A marine chronometer is a clock that is precise and accurate enough to be used as a portable time standard; it can therefore be used to determine longitude by means of celestial navigation...

, a timepiece used to determine longitude
Longitude
Longitude is a geographic coordinate that specifies the east-west position of a point on the Earth's surface. It is an angular measurement, usually expressed in degrees, minutes and seconds, and denoted by the Greek letter lambda ....

Celestial navigation, also known as astronavigation, is a position fixing technique that has evolved over several thousand years to help sailors cross oceans without having to rely on estimated calculations, or dead reckoning, to know their position...

, a precision firstly achieved by John Harrison
John Harrison
John Harrison was a self-educated English clockmaker. He invented the marine chronometer, a long-sought device in solving the problem of establishing the East-West position or longitude of a ship at sea, thus revolutionising and extending the possibility of safe long distance sea travel in the Age...

. More recently, the term has also been applied to the chronometer watch
Chronometer watch
A chronometer watch is a specific type of watch tested and certified to meet certain precision standards. In Switzerland, only timepieces certified by the COSC may use the word 'Chronometer' on them....

, a wristwatch that meets precision standards set by the Swiss agency COSC
COSC
COSC aka C.O.S.C. is Contrôle Officiel Suisse des Chronomètres, the Official Swiss Chronometer Testing Institute, which is the institute responsible for certifying the accuracy and precision of wristwatches in Switzerland.-Background:...

.

The most accurate timekeeping devices are atomic clock
Atomic clock
An atomic clock is a clock that uses an electronic transition frequency in the microwave, optical, or ultraviolet region of the electromagnetic spectrum of atoms as a frequency standard for its timekeeping element...

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 spin property of atoms as their basis, and since 1967, the International System of Measurements bases its unit of time, the second, on the properties of caesium
Caesium
Caesium or cesium is the chemical element with the symbol Cs and atomic number 55. It is a soft, silvery-gold alkali metal with a melting point of 28 °C , which makes it one of only five elemental metals that are liquid at room temperature...

atoms. SI
International System of Units
The International System of Units is the modern form of the metric system and is generally a system of units of measurement devised around seven base units and the convenience of the number ten. The older metric system included several groups of units...

defines the second as 9,192,631,770 cycles of that radiation which corresponds to the transition between two electron spin energy levels of the ground state of the 133Cs atom.

Today, the Global Positioning System
Global Positioning System
The Global Positioning System is a space-based global navigation satellite system that provides location and time information in all weather, anywhere on or near the Earth, where there is an unobstructed line of sight to four or more GPS satellites...

in coordination with the Network Time Protocol
Network Time Protocol
The Network Time Protocol is a protocol and software implementation for synchronizing the clocks of computer systems over packet-switched, variable-latency data networks. Originally designed by David L...

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'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
Computus
Computus is the calculation of the date of Easter in the Christian calendar. The name has been used for this procedure since the early Middle Ages, as it was one of the most important computations of the age....

, the process of calculating the date of Easter
Easter
Easter is the central feast in the Christian liturgical year. According to the Canonical gospels, Jesus rose from the dead on the third day after his crucifixion. His resurrection is celebrated on Easter Day or Easter Sunday...

.

, the smallest unit of time that has been directly measured is on the attosecond (10−18 s) time scale, or around 1026 Planck time
Planck time
In physics, the Planck time, , is the unit of time in the system of natural units known as Planck units. It is the time required for light to travel, in a vacuum, a distance of 1 Planck length...

s.

## Definitions and standards

Units of time
Unit SizeNotes
yoctosecond  10−24 s
zeptosecond  10−21 s
attosecond  10−18 s shortest time now measurable
femtosecond  10−15 s pulse time on fastest lasers
picosecond
Picosecond
A picosecond is 10−12 of a second. That is one trillionth, or one millionth of one millionth of a second, or 0.000 000 000 001 seconds. A picosecond is to one second as one second is to 31,700 years....

10−12 s
nanosecond
Nanosecond
A nanosecond is one billionth of a second . One nanosecond is to one second as one second is to 31.7 years.The word nanosecond is formed by the prefix nano and the unit second. Its symbol is ns....

10−9 s time for molecules to fluoresce
microsecond
Microsecond
A microsecond is an SI unit of time equal to one millionth of a second. Its symbol is µs.A microsecond is equal to 1000 nanoseconds or 1/1000 millisecond...

10−6 s
millisecond
Millisecond
A millisecond is a thousandth of a second.10 milliseconds are called a centisecond....

0.001 s
second
Second
The second is a unit of measurement of time, and is the International System of Units base unit of time. It may be measured using a clock....

1 s SI
Si
Si, si, or SI may refer to :- Measurement, mathematics and science :* International System of Units , the modern international standard version of the metric system...

base unit
minute
Minute
A minute is a unit of measurement of time or of angle. The minute is a unit of time equal to 1/60th of an hour or 60 seconds. In the UTC time scale, a minute on rare occasions has 59 or 61 seconds; see leap second. The minute is not an SI unit; however, it is accepted for use with SI units...

60 seconds
hour
Hour
The hour is a unit of measurement of time. In modern usage, an hour comprises 60 minutes, or 3,600 seconds...

60 minutes
day
Day
A day is a unit of time, commonly defined as an interval equal to 24 hours. It also can mean that portion of the full day during which a location is illuminated by the light of the sun...

24 hours
week
Week
A week is a time unit equal to seven days.The English word week continues an Old English wice, ultimately from a Common Germanic , from a root "turn, move, change"...

7 days Also called sennight
fortnight
Fortnight
The fortnight is a unit of time equal to fourteen days, or two weeks. The word derives from the Old English fēowertyne niht, meaning "fourteen nights"....

14 days 2 weeks
lunar month
Lunar month
In lunar calendars, a lunar month is the time between two identical syzygies . There are many variations. In Middle-Eastern and European traditions, the month starts when the young crescent moon becomes first visible at evening after conjunction with the Sun one or two days before that evening...

27.2–29.5 days Various definitions of lunar month exist.
month
Month
A month is a unit of time, used with calendars, which was first used and invented in Mesopotamia, as a natural period related to the motion of the Moon; month and Moon are cognates. The traditional concept arose with the cycle of moon phases; such months are synodic months and last approximately...

28–31 days
quarter  3 months
year
Year
A year is the orbital period of the Earth moving around the Sun. For an observer on Earth, this corresponds to the period it takes the Sun to complete one course throughout the zodiac along the ecliptic....

12 months
common year
Year
A year is the orbital period of the Earth moving around the Sun. For an observer on Earth, this corresponds to the period it takes the Sun to complete one course throughout the zodiac along the ecliptic....

365 days 52 weeks + 1 day
leap year
Leap year
A leap year is a year containing one extra day in order to keep the calendar year synchronized with the astronomical or seasonal year...

366 days 52 weeks + 2 days
tropical year
Tropical year
A tropical year , for general purposes, is the length of time that the Sun takes to return to the same position in the cycle of seasons, as seen from Earth; for example, the time from vernal equinox to vernal equinox, or from summer solstice to summer solstice...

365.24219 days average
Gregorian year
Gregorian calendar
The Gregorian calendar, also known as the Western calendar, or Christian calendar, is the internationally accepted civil calendar. It was introduced by Pope Gregory XIII, after whom the calendar was named, by a decree signed on 24 February 1582, a papal bull known by its opening words Inter...

365.2425 days average
An Olympiad is a period of four years, associated with the Olympic Games of Classical Greece. In the Hellenistic period, beginning with Ephorus, Olympiads were used as calendar epoch....

4 year cycle
lustrum
Lustrum
A lustrum was a term for a five-year period in Ancient Rome.The lustration was originally a sacrifice for expiation and purification offered by one of the censors in the name of the Roman people at the close of the taking of the census...

A decade is a period of 10 years. The word is derived from the Ancient Greek dekas which means ten. This etymology is sometime confused with the Latin decas and dies , which is not correct....

10 years
Indiction
Indiction
An indiction is any of the years in a 15-year cycle used to date medieval documents throughout Europe, both East and West. Each year of a cycle was numbered: first indiction, second indiction, etc...

15 year cycle
generation
Generation
Generation , also known as procreation in biological sciences, is the act of producing offspring....

17–35 years approximate
jubilee (Biblical)
Jubilee (Biblical)
The Jubilee year is the year at the end of seven cycles of Sabbatical years , and according to Biblical regulations had a special impact on the ownership and management of land in the territory of the kingdoms of Israel and of Judah; there is some debate whether it was the 49th year The Jubilee...

50 years
century
Century
A century is one hundred consecutive years. Centuries are numbered ordinally in English and many other languages .-Start and end in the Gregorian Calendar:...

100 years
millennium
Millennium
A millennium is a period of time equal to one thousand years —from the Latin phrase , thousand, and , year—often but not necessarily related numerically to a particular dating system....

1,000 years
exasecond  1018 s roughly 32 billion years, more than twice
the age of the universe on current estimates
A cosmological decade is a division of the lifetime of the cosmos. The divisions are logarithmic in size, on base 10. Each successive cosmological decade represents a ten-fold increase in the total age of the universe....

varies 10 times the length of the previous
cosmological decade, with CÐ 1 beginning
either 10 seconds or 10 years after the
Big Bang, depending on the definition.

The SI base unit
SI base unit
The International System of Units defines seven units of measure as a basic set from which all other SI units are derived. These SI base units and their physical quantities are:* metre for length...

for time is the SI
Si
Si, si, or SI may refer to :- Measurement, mathematics and science :* International System of Units , the modern international standard version of the metric system...

second
Second
The second is a unit of measurement of time, and is the International System of Units base unit of time. It may be measured using a clock....

. From the second, larger units such as the minute
Minute
A minute is a unit of measurement of time or of angle. The minute is a unit of time equal to 1/60th of an hour or 60 seconds. In the UTC time scale, a minute on rare occasions has 59 or 61 seconds; see leap second. The minute is not an SI unit; however, it is accepted for use with SI units...

, hour
Hour
The hour is a unit of measurement of time. In modern usage, an hour comprises 60 minutes, or 3,600 seconds...

and day
Day
A day is a unit of time, commonly defined as an interval equal to 24 hours. It also can mean that portion of the full day during which a location is illuminated by the light of the sun...

are defined, though they are "non-SI" units because they do not use the decimal system, and also because of the occasional need for a leap second
Leap second
A leap second is a positive or negative one-second adjustment to the Coordinated Universal Time time scale that keeps it close to mean solar time. UTC, which is used as the basis for official time-of-day radio broadcasts for civil time, is maintained using extremely precise atomic clocks...

. They are, however, officially accepted for use with the International System. There are no fixed ratios between seconds and month
Month
A month is a unit of time, used with calendars, which was first used and invented in Mesopotamia, as a natural period related to the motion of the Moon; month and Moon are cognates. The traditional concept arose with the cycle of moon phases; such months are synodic months and last approximately...

s or year
Year
A year is the orbital period of the Earth moving around the Sun. For an observer on Earth, this corresponds to the period it takes the Sun to complete one course throughout the zodiac along the ecliptic....

s as months and years have significant variations in length.

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.
Previous to 1967, the second was defined as:
The current definition of the second, coupled with the current definition of the metre, is based on the special theory of relativity, which affirms our space-time to be a Minkowski space
Minkowski space
In physics and mathematics, Minkowski space or Minkowski spacetime is the mathematical setting in which Einstein's theory of special relativity is most conveniently formulated...

.

### World time

Time keeping is so critical to the functioning of modern societies that it is coordinated at an international level. The basis for scientific time is a continuous count of seconds based on atomic clock
Atomic clock
An atomic clock is a clock that uses an electronic transition frequency in the microwave, optical, or ultraviolet region of the electromagnetic spectrum of atoms as a frequency standard for its timekeeping element...

s around the world, known as the International Atomic Time (TAI)
International Atomic Time
International Atomic Time is a high-precision atomic coordinate time standard based on the notional passage of proper time on Earth's geoid...

. Other scientific time standards include Terrestrial Time
Terrestrial Time
Terrestrial Time is a modern astronomical time standard defined by the International Astronomical Union, primarily for time-measurements of astronomical observations made from the surface of the Earth....

and Barycentric Dynamical Time
Barycentric Dynamical Time
Barycentric Dynamical Time is a relativistic coordinate time scale, intended for astronomical use as a time standard to take account of time dilation when calculating orbits and astronomical ephemerides of planets, asteroids, comets and interplanetary spacecraft in the Solar system...

.

Coordinated Universal Time
Coordinated Universal Time
Coordinated Universal Time is the primary time standard by which the world regulates clocks and time. It is one of several closely related successors to Greenwich Mean Time. Computer servers, online services and other entities that rely on having a universally accepted time use UTC for that purpose...

(UTC) is the basis for modern civil time
Civil time
In modern usage, civil time refers to statutory time scales designated by civilian authorities, or to local time indicated by clocks. Modern civil time is generally standard time in a time zone at a fixed offset from Coordinated Universal Time or from Greenwich Mean Time , possibly adjusted by...

. Since January 1, 1972, it has been defined to follow TAI with an exact offset of an integer number of seconds, changing only when a leap second
Leap second
A leap second is a positive or negative one-second adjustment to the Coordinated Universal Time time scale that keeps it close to mean solar time. UTC, which is used as the basis for official time-of-day radio broadcasts for civil time, is maintained using extremely precise atomic clocks...

is added to keep clock time synchronized with the rotation of the Earth. In TAI and UTC systems, the duration of a second
Second
The second is a unit of measurement of time, and is the International System of Units base unit of time. It may be measured using a clock....

is constant, as it is defined by the unchanging transition period of the caesium atom.

Greenwich Mean Time
Greenwich Mean Time
Greenwich Mean Time is a term originally referring to mean solar time at the Royal Observatory in Greenwich, London. It is arguably the same as Coordinated Universal Time and when this is viewed as a time zone the name Greenwich Mean Time is especially used by bodies connected with the United...

(GMT) is an older standard, adopted starting with British railroads in 1847. Using telescopes instead of atomic clocks, GMT was calibrated to the mean solar time at the Royal Observatory, Greenwich
Royal Observatory, Greenwich
The Royal Observatory, Greenwich , in London, England played a major role in the history of astronomy and navigation, and is best known as the location of the prime meridian...

in the UK. Universal Time
Universal Time
Universal Time is a time scale based on the rotation of the Earth. It is a modern continuation of Greenwich Mean Time , i.e., the mean solar time on the Prime Meridian at Greenwich, and GMT is sometimes used loosely as a synonym for UTC...

(UT) is the modern term for the international telescope-based system, adopted to replace "Greenwich Mean Time" in 1928 by the International Astronomical Union
International Astronomical Union
The International Astronomical Union IAU is a collection of professional astronomers, at the Ph.D. level and beyond, active in professional research and education in astronomy...

. 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 or UT—in which a second was defined as a fraction of a day or year. The terms "GMT" and "Greenwich Mean Time" are sometimes used informally to refer to UT or UTC.

The Global Positioning System
Global Positioning System
The Global Positioning System is a space-based global navigation satellite system that provides location and time information in all weather, anywhere on or near the Earth, where there is an unobstructed line of sight to four or more GPS satellites...

also broadcasts a very precise time signal worldwide, along with instructions for converting GPS time to UTC.

Earth is split up into a number of time zone
Time zone
A time zone is a region on Earth that has a uniform standard time for legal, commercial, and social purposes. In order for the same clock time to always correspond to the same portion of the day as the Earth rotates , different places on the Earth need to have different clock times...

s. Most time zones are exactly one hour apart, and by convention compute their local time as an offset from UTC or GMT. In many locations these offsets vary twice yearly due to daylight saving time
Daylight saving time
Daylight saving time —also summer time in several countries including in British English and European official terminology —is the practice of temporarily advancing clocks during the summertime so that afternoons have more daylight and mornings have less...

transitions.

### Time conversions

The following time conversions are accurate at the millisecond level. Some are exact while others have differences at the microsecond level.
System Description UT1 UTC TT TAI GPS
UT1 Mean Solar Time UT1 UTC = UT1 - DUT1 TT = UT1 + 32.184 s + LS - DUT1 TAI = UT1 - DUT1 + LS GPS = UT1 - DUT1 + LS - 19 s
UTC Civil Time UT1 = UTC + DUT1 UTC TT = UTC + 32.184 s + LS TAI = UTC + LS GPS = UTC + LS - 19 s
TT Terrestrial (Ephemeris) Time UT1 = TT - 32.184 s - LS + DUT1 UTC = TT - 32.184 s - LS TT TAI = TT - 32.184 s GPS = TT - 51.184 s
TAI Atomic Time UT1 = TAI + DUT1 - LS UTC = TAI - LS TT = TAI + 32.184 s TAI GPS = TAI - 19 s
GPS GPS Time UT1 = GPS + DUT1 - LS + 19 s UTC = GPS - LS + 19 s TT = GPS + 51.184 s TAI = GPS + 19 s GPS

Definitions:
1. LS = TAI - UTC = Leap Seconds from http://maia.usno.navy.mil/ser7/tai-utc.dat
2. DUT1 = UT1 - UTC from http://maia.usno.navy.mil/ser7/ser7.dat or http://maia.usno.navy.mil/search/search.html

### Sidereal time

Sidereal time
Sidereal time
Sidereal time is a time-keeping system astronomers use to keep track of the direction to point their telescopes to view a given star in the night sky...

is the measurement of time relative to a distant star (instead of solar time that is relative to the sun). It is used in astronomy to predict when a star will be overhead. Due to the orbit of the earth around the sun a sidereal day is 4 minutes (1/366th) less than a solar day.

### Chronology

Another form of time measurement consists of studying the past
Past
Most generally, the past is a term used to indicate the totality of events which 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 human observers experience time, and is...

. Events in the past can be ordered in a sequence (creating a chronology
Chronology
Chronology is the science of arranging events in their order of occurrence in time, such as the use of a timeline or sequence of events. It is also "the determination of the actual temporal sequence of past events".Chronology is part of periodization...

), and can be put into chronological groups (periodization
Periodization
Periodization is the attempt to categorize or divide time into named blocks. The result is a descriptive abstraction that provides a useful handle on periods of time with relatively stable characteristics...

). One of the most important systems of periodization is geologic time, which is a system of periodizing the events that shaped the Earth
Earth
Earth is the third planet from the Sun, and the densest and fifth-largest of the eight planets in the Solar System. It is also the largest of the Solar System's four terrestrial planets...

and its life. Chronology, periodization, and interpretation of the past are together known as the study of history
History
History is the discovery, collection, organization, and presentation of information about past events. History can also mean the period of time after writing was invented. Scholars who write about history are called historians...

.

## Religion

### Linear and cyclical time

Ancient cultures such as Incan, Mayan
Maya civilization
The Maya is a Mesoamerican civilization, noted for the only known fully developed written language of the pre-Columbian Americas, as well as for its art, architecture, and mathematical and astronomical systems. Initially established during the Pre-Classic period The Maya is a Mesoamerican...

, Hopi
Hopi
The Hopi are a federally recognized tribe of indigenous Native American people, who primarily live on the Hopi Reservation in northeastern Arizona. The Hopi area according to the 2000 census has a population of 6,946 people. Their Hopi language is one of the 30 of the Uto-Aztecan language...

, and other Native American Tribes, plus the Babylonians, Ancient Greek
Ancient Greek
Ancient Greek is the stage of the Greek language in the periods spanning the times c. 9th–6th centuries BC, , c. 5th–4th centuries BC , and the c. 3rd century BC – 6th century AD of ancient Greece and the ancient world; being predated in the 2nd millennium BC by Mycenaean Greek...

s, Hinduism
Hinduism
Hinduism is the predominant and indigenous religious tradition of the Indian Subcontinent. Hinduism is known to its followers as , amongst many other expressions...

, Buddhism
Buddhism
Buddhism is a religion and philosophy encompassing a variety of traditions, beliefs and practices, largely based on teachings attributed to Siddhartha Gautama, commonly known as the Buddha . The Buddha lived and taught in the northeastern Indian subcontinent some time between the 6th and 4th...

, Jainism
Jainism
Jainism is an Indian religion that prescribes a path of non-violence towards all living beings. Its philosophy and practice emphasize the necessity of self-effort to move the soul towards divine consciousness and liberation. Any soul that has conquered its own inner enemies and achieved the state...

, and others have a concept of a wheel of time
Wheel of time
The Wheel of time or wheel of history is a concept found in several religious traditions and philosophies, notably religions of Indian origin such as Hinduism and Buddhism, which regard time as cyclical and consisting of repeating ages...

, that regards time as cyclical
Social cycle theory
Social cycle theories are among the earliest social theories in sociology. Unlike the theory of social evolutionism, which views the evolution of society and human history as progressing in some new, unique direction, sociological cycle theory argues that events and stages of society and history...

and quantic consisting of repeating ages that happen to every being of the Universe between birth and extinction.

In general, the Judaeo-Christian concept, based on the Bible
Bible
The Bible refers to any one of the collections of the primary religious texts of Judaism and Christianity. There is no common version of the Bible, as the individual books , their contents and their order vary among denominations...

, is that time is linear, beginning with the act of creation by God
God
God is the English name given to a singular being in theistic and deistic religions who is either the sole deity in monotheism, or a single deity in polytheism....

. The general Christian
Christian
A Christian is a person who adheres to Christianity, an Abrahamic, monotheistic religion based on the life and teachings of Jesus of Nazareth as recorded in the Canonical gospels and the letters of the New Testament...

view is that time will end with the end of the world
Christian eschatology
Christian eschatology is a major branch of study within Christian theology. Eschatology, from two Greek words meaning last and study , is the study of the end of things, whether the end of an individual life, the end of the age, or the end of the world...

. Others suggest that time is like a ray, having a beginning but going on forever into the future.

In the Old Testament
Old Testament
The Old Testament, of which Christians hold different views, is a Christian term for the religious writings of ancient Israel held sacred and inspired by Christians which overlaps with the 24-book canon of the Masoretic Text of Judaism...

book Ecclesiastes
Ecclesiastes
The Book of Ecclesiastes, called , is a book of the Hebrew Bible. The English name derives from the Greek translation of the Hebrew title.The main speaker in the book, identified by the name or title Qoheleth , introduces himself as "son of David, king in Jerusalem." The work consists of personal...

Solomon
Solomon , according to the Book of Kings and the Book of Chronicles, a King of Israel and according to the Talmud one of the 48 prophets, is identified as the son of David, also called Jedidiah in 2 Samuel 12:25, and is described as the third king of the United Monarchy, and the final king before...

(970–928 BC), time (as the Hebrew word עדן, זמן `iddan(time) zĕman(season) is often translated) was traditionally regarded as a medium for the passage of predestined
Predestination
Predestination, in theology is the doctrine that all events have been willed by God. John Calvin interpreted biblical predestination to mean that God willed eternal damnation for some people and salvation for others...

events. (Another word, زمان" זמן" zman, was current as meaning time fit for an event, and is used as the modern Arabic
Arabic language
Arabic is a name applied to the descendants of the Classical Arabic language of the 6th century AD, used most prominently in the Quran, the Islamic Holy Book...

and Hebrew
Hebrew language
Hebrew is a Semitic language of the Afroasiatic language family. Culturally, is it considered by Jews and other religious groups as the language of the Jewish people, though other Jewish languages had originated among diaspora Jews, and the Hebrew language is also used by non-Jewish groups, such...

equivalent to the English word "time".)

There is an appointed time (zman) for everything. And there is a time (’êth) for every event under heaven–

A time (’êth) to give birth, and a time to die; A time to plant, and a time to uproot what is planted.

A time to kill, and a time to heal; A time to tear down, and a time to build up.

A time to weep, and a time to laugh; A time to mourn, and a time to dance.

A time to throw stones, and a time to gather stones; A time to embrace, and a time to shun embracing.

A time to search, and a time to give up as lost; A time to keep, and a time to throw away.

A time to tear apart, and a time to sew together; A time to be silent, and a time to speak.

A time to love, and a time to hate; A time for war, and a time for peace.

### Numeric and Divine time

The Greek language denotes two distinct principles, Chronos
Chronos
In Greek mythology, Chronos in pre-Socratic philosophical works is said to be the personification of time. His name in Greek means "time" and is alternatively spelled Chronus or Khronos.Chronos was imagined as an incorporeal god, serpentine in form, with three heads—those of a man, a bull, and...

and Kairos
Kairos
Kairos is an ancient Greek word meaning the right or opportune moment . The ancient Greeks had two words for time, chronos and kairos. While the former refers to chronological or sequential time, the latter signifies a time in between, a moment of indeterminate time in which something special...

. 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.

## Philosophy

Two distinct viewpoints on time divide many prominent philosophers.
One view is that time is part of the fundamental structure of the universe
Universe
The Universe is commonly defined as the totality of everything that exists, including all matter and energy, the planets, stars, galaxies, and the contents of intergalactic space. Definitions and usage vary and similar terms include the cosmos, the world and nature...

, a dimension
Dimension
In physics and mathematics, the dimension of a space or object is informally defined as the minimum number of coordinates needed to specify any point within it. Thus a line has a dimension of one because only one coordinate is needed to specify a point on it...

in which events occur in sequence
Sequence
In mathematics, a sequence is an ordered list of objects . Like a set, it contains members , and the number of terms is called the length of the sequence. Unlike a set, order matters, and exactly the same elements can appear multiple times at different positions in the sequence...

. Sir Isaac Newton subscribed to this realist
Philosophical realism
Contemporary philosophical realism is the belief that our reality, or some aspect of it, is ontologically independent of our conceptual schemes, linguistic practices, beliefs, etc....

view, and hence it is sometimes referred to as Newtonian time
Absolute time and space
Originally introduced by Sir Isaac Newton in the Philosophiæ Naturalis Principia Mathematica, the concepts of absolute time and space provided a theoretical foundation that facilitated Newtonian mechanics...

.
An opposing view is that time does not refer to any kind of actually existing dimension that events and objects "move through", nor to any entity that "flows", but that it is instead an intellectual concept (together with space
Space
Space is the boundless, three-dimensional extent in which objects and events occur and have relative position and direction. Physical space is often conceived in three linear dimensions, although modern physicists usually consider it, with time, to be part of a boundless four-dimensional continuum...

and number
Number
A number is a mathematical object used to count and measure. In mathematics, the definition of number has been extended over the years to include such numbers as zero, negative numbers, rational numbers, irrational numbers, and complex numbers....

) that enables humans to sequence and compare events.
This second view, in the tradition of Gottfried Leibniz
Gottfried Leibniz
Gottfried Wilhelm Leibniz was a German philosopher and mathematician. He wrote in different languages, primarily in Latin , French and German ....

and Immanuel Kant
Immanuel Kant
Immanuel Kant was a German philosopher from Königsberg , researching, lecturing and writing on philosophy and anthropology at the end of the 18th Century Enlightenment....

,
holds that space and time "do not exist in and of themselves, but ... are the product of the way we represent things", because we can know objects only as they appear
Perception
Perception is the process of attaining awareness or understanding of the environment by organizing and interpreting sensory information. All perception involves signals in the nervous system, which in turn result from physical stimulation of the sense organs...

to us.

The Vedas
Vedas
The Vedas are a large body of texts originating in ancient India. Composed in Vedic Sanskrit, the texts constitute the oldest layer of Sanskrit literature and the oldest scriptures of Hinduism....

, the earliest texts on Indian philosophy
Indian philosophy
India has a rich and diverse philosophical tradition dating back to ancient times. According to Radhakrishnan, the earlier Upanisads constitute "...the earliest philosophical compositions of the world."...

and Hindu philosophy
Hindu philosophy
Hindu philosophy is divided into six schools of thought, or , which accept the Vedas as supreme revealed scriptures. Three other schools do not accept the Vedas as authoritative...

dating back to the late 2nd millennium BC
2nd millennium BC
The 2nd millennium BC marks the transition from the Middle to the Late Bronze Age.Its first half is dominated by the Middle Kingdom of Egypt and Babylonia. The alphabet develops. Indo-Iranian migration onto the Iranian plateau and onto the Indian subcontinent propagates the use of the chariot...

, describe ancient Hindu cosmology
Hindu cosmology
In Hindu cosmology the universe is, according to Hindu mythology and Vedic cosmology, cyclically created and destroyed.-Description:The Hindu cosmology and timeline is the closest to modern scientific timelines and even more which might indicate that the Big Bang is not the beginning of everything...

, in which the universe
Universe
The Universe is commonly defined as the totality of everything that exists, including all matter and energy, the planets, stars, galaxies, and the contents of intergalactic space. Definitions and usage vary and similar terms include the cosmos, the world and nature...

goes through repeated cycles of creation, destruction and rebirth, with each cycle lasting 4320 million years.
Ancient
Ancient philosophy
This page lists some links to ancient philosophy. In Western philosophy, the spread of Christianity through the Roman Empire marked the ending of Hellenistic philosophy and ushered in the beginnings of Medieval philosophy, whereas in Eastern philosophy, the spread of Islam through the Arab Empire...

Greek philosophers
Greek philosophy
Ancient Greek philosophy arose in the 6th century BCE and continued through the Hellenistic period, at which point Ancient Greece was incorporated in the Roman Empire...

, including Parmenides
Parmenides
Parmenides of Elea was an ancient Greek philosopher born in Elea, a Greek city on the southern coast of Italy. He was the founder of the Eleatic school of philosophy. The single known work of Parmenides is a poem, On Nature, which has survived only in fragmentary form. In this poem, Parmenides...

and Heraclitus
Heraclitus
Heraclitus of Ephesus was a pre-Socratic Greek philosopher, a native of the Greek city Ephesus, Ionia, on the coast of Asia Minor. He was of distinguished parentage. Little is known about his early life and education, but he regarded himself as self-taught and a pioneer of wisdom...

, wrote essays on the nature of time.
Plato
Plato
Plato , was a Classical Greek philosopher, mathematician, student of Socrates, writer of philosophical dialogues, and founder of the Academy in Athens, the first institution of higher learning in the Western world. Along with his mentor, Socrates, and his student, Aristotle, Plato helped to lay the...

, in the Timaeus
Timaeus (dialogue)
Timaeus is one of Plato's dialogues, mostly in the form of a long monologue given by the title character, written circa 360 BC. The work puts forward speculation on the nature of the physical world and human beings. It is followed by the dialogue Critias.Speakers of the dialogue are Socrates,...

, identified time with the period of motion of the heavenly bodies. Aristotle
Aristotle
Aristotle was a Greek philosopher and polymath, a student of Plato and teacher of Alexander the Great. His writings cover many subjects, including physics, metaphysics, poetry, theater, music, logic, rhetoric, linguistics, politics, government, ethics, biology, and zoology...

, in Book IV of his Physica
Physics (Aristotle)
The Physics of Aristotle is one of the foundational books of Western science and philosophy...

defined time as the number of change with respect to before and after.

In Book 11 of his Confessions
Confessions (St. Augustine)
Confessions is the name of an autobiographical work, consisting of 13 books, by St. Augustine of Hippo, written between AD 397 and AD 398. Modern English translations of it are sometimes published under the title The Confessions of St...

, St. Augustine of Hippo
Augustine of Hippo
Augustine of Hippo , also known as Augustine, St. Augustine, St. Austin, St. Augoustinos, Blessed Augustine, or St. Augustine the Blessed, was Bishop of Hippo Regius . He was a Latin-speaking philosopher and theologian who lived in the Roman Africa Province...

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
Negative theology
Apophatic theology —also known as negative theology or via negativa —is a theology that attempts to describe God, the Divine Good, by negation, to speak only in terms of what may not be said about the perfect goodness that is God...

. 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.

In contrast to ancient Greek philosophers who believed that the universe had an infinite past with no beginning, medieval philosophers
Medieval philosophy
Medieval philosophy is the philosophy in the era now known as medieval or the Middle Ages, the period roughly extending from the fall of the Western Roman Empire in the fifth century AD to the Renaissance in the sixteenth century...

and theologians
Theology
Theology is the systematic and rational study of religion and its influences and of the nature of religious truths, or the learned profession acquired by completing specialized training in religious studies, usually at a university or school of divinity or seminary.-Definition:Augustine of Hippo...

developed the concept of the universe having a finite past with a beginning.
This view is shared by Abrahamic faiths as they believe time started by creation, therefore the only thing being infinite is God and everything else, including time, is finite.

Isaac Newton
Isaac Newton
Sir Isaac Newton PRS was an English physicist, mathematician, astronomer, natural philosopher, alchemist, and theologian, who has been "considered by many to be the greatest and most influential scientist who ever lived."...

believed in absolute space and absolute time; Leibniz
Gottfried Leibniz
Gottfried Wilhelm Leibniz was a German philosopher and mathematician. He wrote in different languages, primarily in Latin , French and German ....

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.
Immanuel Kant
Immanuel Kant
Immanuel Kant was a German philosopher from Königsberg , researching, lecturing and writing on philosophy and anthropology at the end of the 18th Century Enlightenment....

, in the Critique of Pure Reason
Critique of Pure Reason
The Critique of Pure Reason by Immanuel Kant, first published in 1781, second edition 1787, is considered one of the most influential works in the history of philosophy. Also referred to as Kant's "first critique," it was followed by the Critique of Practical Reason and the Critique of Judgement...

, described time as an a priori
A priori and a posteriori (philosophy)
The terms a priori and a posteriori are used in philosophy to distinguish two types of knowledge, justifications or arguments...

intuition that allows us (together with the other a priori intuition, space
Space
Space is the boundless, three-dimensional extent in which objects and events occur and have relative position and direction. Physical space is often conceived in three linear dimensions, although modern physicists usually consider it, with time, to be part of a boundless four-dimensional continuum...

) to comprehend sense experience.
With Kant, neither space nor time are conceived as substance
Substance theory
Substance theory, or substance attribute theory, is an ontological theory about objecthood, positing that a substance is distinct from its properties. A thing-in-itself is a property-bearer that must be distinguished from the properties it bears....

s, 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
Abstract structure
An abstract structure in mathematics is a formal object that is defined by a set of laws, properties, and relationships in a way that is logically if not always historically independent of the structure of contingent experiences, for example, those involving physical objects...

conceptual framework, together with space
Space
Space is the boundless, three-dimensional extent in which objects and events occur and have relative position and direction. Physical space is often conceived in three linear dimensions, although modern physicists usually consider it, with time, to be part of a boundless four-dimensional continuum...

and number
Number
A number is a mathematical object used to count and measure. In mathematics, the definition of number has been extended over the years to include such numbers as zero, negative numbers, rational numbers, irrational numbers, and complex numbers....

, within which we sequence events, quantify
Quantity
Quantity is a property that can exist as a magnitude or multitude. Quantities can be compared in terms of "more" or "less" or "equal", or by assigning a numerical value in terms of a unit of measurement. Quantity is among the basic classes of things along with quality, substance, change, and relation...

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
Measurement is the process or the result of determining the ratio of a physical quantity, such as a length, time, temperature etc., to a unit of measurement, such as the metre, second or degree Celsius...

s are used to quantify
Quantity
Quantity is a property that can exist as a magnitude or multitude. Quantities can be compared in terms of "more" or "less" or "equal", or by assigning a numerical value in terms of a unit of measurement. Quantity is among the basic classes of things along with quality, substance, change, and relation...

the extent of and distances between object
Object (philosophy)
An object in philosophy is a technical term often used in contrast to the term subject. Consciousness is a state of cognition that includes the subject, which can never be doubted as only it can be the one who doubts, and some object or objects that may or may not have real existence without...

s, and temporal measurements are used to quantify the durations of and between event
Phenomenon
A phenomenon , plural phenomena, is any observable occurrence. Phenomena are often, but not always, understood as 'appearances' or 'experiences'...

s.
(See Ontology
Ontology
Ontology is the philosophical study of the nature of being, existence or reality as such, as well as the basic categories of being and their relations...

).

Henri Bergson
Henri Bergson
Henri-Louis Bergson was a major French philosopher, influential especially in the first half of the 20th century. Bergson convinced many thinkers that immediate experience and intuition are more significant than rationalism and science for understanding reality.He was awarded the 1927 Nobel Prize...

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

According to Martin Heidegger
Martin Heidegger
Martin Heidegger was a German philosopher known for his existential and phenomenological explorations of the "question of Being."...

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.

### Time as "unreal"

In 5th century BC Greece
Greece
Greece , officially the Hellenic Republic , and historically Hellas or the Republic of Greece in English, is a country in southeastern Europe....

, Antiphon
Antiphon (person)
Antiphon the Sophist lived in Athens probably in the last two decades of the 5th century BC. There is an ongoing controversy over whether he is one and the same with Antiphon of the Athenian deme Rhamnus in Attica , the earliest of the ten Attic orators...

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
Parmenides of Elea was an ancient Greek philosopher born in Elea, a Greek city on the southern coast of Italy. He was the founder of the Eleatic school of philosophy. The single known work of Parmenides is a poem, On Nature, which has survived only in fragmentary form. In this poem, Parmenides...

went further, maintaining that time, motion, and change were illusions, leading to the paradoxes
Zeno's paradoxes are a set of problems generally thought to have been devised by Greek philosopher Zeno of Elea to support Parmenides's doctrine that "all is one" and that, contrary to the evidence of our senses, the belief in plurality and change is mistaken, and in particular that motion is...

of his follower Zeno
Zeno of Elea
Zeno of Elea was a pre-Socratic Greek philosopher of southern Italy and a member of the Eleatic School founded by Parmenides. Aristotle called him the inventor of the dialectic. He is best known for his paradoxes, which Bertrand Russell has described as "immeasurably subtle and profound".- Life...

.
Time as an illusion is also a common theme in Buddhist thought.

J. M. E. McTaggart
J. M. E. McTaggart
John McTaggart was an idealist metaphysician. For most of his life McTaggart was a fellow and lecturer in philosophy at Trinity College, Cambridge. He was an exponent of the philosophy of Hegel and among the most notable of the British idealists.-Personal life:J. M. E. McTaggart was born in 1866...

's 1908 The Unreality of Time
The Unreality of Time
The Unreality of Time is the best-known philosophical work of the Cambridge idealist J. M. E. McTaggart. In the paper, first published in 1908 in Mind 17: 457-73, McTaggart argues that time is unreal because our descriptions of time are either contradictory, circular, or insufficient...

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 The flow of time).

These arguments often center around what it means for something to be "unreal". Modern physicists generally consider time to be as "real" as space, though others such as Julian Barbour
Julian Barbour
Julian Barbour is a British physicist with research interests in quantum gravity and the history of science.Since receiving his Ph.D. degree on the foundations of Einstein's general theory of relativity at the University of Cologne in 1968, Barbour has supported himself and his family without an...

in his book The End of Time, argue that quantum equations of the universe take their true form when expressed in the timeless configuration space
Configuration space
- Configuration space in physics :In classical mechanics, the configuration space is the space of possible positions that a physical system may attain, possibly subject to external constraints...

realm containing every possible "Now" or momentary configuration of the universe, which he terms 'platonia
Platonia (philosophy)
In Julian Barbour's book The End of Time: The Next Revolution in Our Understanding of the Universe, Platonia is the name given to his hypothetic entity of a timeless realm containing every possible "Now" or momentary configuration of the universe....

'.

## Physical definition

From the age of Newton
Isaac Newton
Sir Isaac Newton PRS was an English physicist, mathematician, astronomer, natural philosopher, alchemist, and theologian, who has been "considered by many to be the greatest and most influential scientist who ever lived."...

to Einstein's
Albert Einstein
Albert Einstein was a German-born theoretical physicist who developed the theory of general relativity, effecting a revolution in physics. For this achievement, Einstein is often regarded as the father of modern physics and one of the most prolific intellects in human history...

profound reinterpretation of the physical concepts associated with time and space, time was considered to be "absolute" and to flow "equably" (to use the words of Newton) for all observers. Non-relativistic classical mechanics
Classical mechanics
In physics, classical mechanics is one of the two major sub-fields of mechanics, which is concerned with the set of physical laws describing the motion of bodies under the action of a system of forces...

is based on this Newtonian idea of time.

Einstein, in his special theory of relativity
Special relativity
Special relativity is the physical theory of measurement in an inertial frame of reference proposed in 1905 by Albert Einstein in the paper "On the Electrodynamics of Moving Bodies".It generalizes Galileo's...

,
postulated the constancy and finiteness of the speed of light for all observers. He showed that this postulate, together with a reasonable definition for what it means for two events to be simultaneous, requires that distances appear compressed and time intervals appear lengthened for events associated with objects in motion relative to an inertial observer.

Einstein showed that if time and space is measured using electromagnetic phenomena (like light bouncing between mirrors) then due to the constancy of the speed of light, time and space become mathematically entangled together in a certain way (called Minkowski
Minkowski space
In physics and mathematics, Minkowski space or Minkowski spacetime is the mathematical setting in which Einstein's theory of special relativity is most conveniently formulated...

space
Space
Space is the boundless, three-dimensional extent in which objects and events occur and have relative position and direction. Physical space is often conceived in three linear dimensions, although modern physicists usually consider it, with time, to be part of a boundless four-dimensional continuum...

) which in turn results in Lorentz transformation
Lorentz transformation
In physics, the Lorentz transformation or Lorentz-Fitzgerald transformation describes how, according to the theory of special relativity, two observers' varying measurements of space and time can be converted into each other's frames of reference. It is named after the Dutch physicist Hendrik...

and in entanglement of all other important derivative physical quantities (like energy, momentum, mass, force, etc.) in a certain 4-vectorial way (see special relativity
Special relativity
Special relativity is the physical theory of measurement in an inertial frame of reference proposed in 1905 by Albert Einstein in the paper "On the Electrodynamics of Moving Bodies".It generalizes Galileo's...

for more details).

### Classical mechanics

In non-relativistic classical mechanics
Classical mechanics
In physics, classical mechanics is one of the two major sub-fields of mechanics, which is concerned with the set of physical laws describing the motion of bodies under the action of a system of forces...

, 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 behaviour of electricity and magnetism. Einstein resolved these problems by invoking a method of synchronizing clocks using the constant, finite speed of light as the maximum signal velocity. This led directly to the result that observers in motion relative to one another will measure different elapsed times for the same event.

### Spacetime

Time has historically been closely related with space
Space
Space is the boundless, three-dimensional extent in which objects and events occur and have relative position and direction. Physical space is often conceived in three linear dimensions, although modern physicists usually consider it, with time, to be part of a boundless four-dimensional continuum...

, the two together comprising spacetime
Spacetime
In physics, spacetime is any mathematical model that combines space and time into a single continuum. Spacetime is usually interpreted with space as being three-dimensional and time playing the role of a fourth dimension that is of a different sort from the spatial dimensions...

in Einstein's
Albert Einstein
Albert Einstein was a German-born theoretical physicist who developed the theory of general relativity, effecting a revolution in physics. For this achievement, Einstein is often regarded as the father of modern physics and one of the most prolific intellects in human history...

special relativity
Special relativity
Special relativity is the physical theory of measurement in an inertial frame of reference proposed in 1905 by Albert Einstein in the paper "On the Electrodynamics of Moving Bodies".It generalizes Galileo's...

and general relativity
General relativity
General relativity or the general theory of relativity is the geometric theory of gravitation published by Albert Einstein in 1916. It is the current description of gravitation in modern physics...

. According to these theories, the concept of time depends on the spatial reference frame of the observer
Inertial frame of reference
In physics, an inertial frame of reference is a frame of reference that describes time homogeneously and space homogeneously, isotropically, and in a time-independent manner.All inertial frames are in a state of constant, rectilinear motion with respect to one another; they are not...

, and the human perception as well as the measurement by instruments such as clocks are different for observers in relative motion. The past
Past
Most generally, the past is a term used to indicate the totality of events which 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 human observers experience time, and is...

is the set of events that can send light signals to the observer; the future
Future
The future is the indefinite time period after the present. Its arrival is considered inevitable due to the existence of time and the laws of physics. Due to the nature of the reality and the unavoidability of the future, everything that currently exists and will exist is temporary and will come...

is the set of events to which the observer can send light signals.

### Time dilation

Einstein showed in his thought experiments that people travelling at different speeds, while agreeing on cause and effect
Causality (physics)
Causality is the relationship between causes and effects. It is considered to be fundamental to all natural science, especially physics. Causality is also a topic studied from the perspectives of philosophy and statistics....

, will 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. Many subatomic particle
Subatomic particle
In physics or chemistry, subatomic particles are the smaller particles composing nucleons and atoms. There are two types of subatomic particles: elementary particles, which are not made of other particles, and composite particles...

s exist for only a fixed fraction of a second in a lab relatively at rest, but some that travel close to the speed of light can be measured to travel further and survive much longer than expected (a muon
Muon
The muon |mu]] used to represent it) is an elementary particle similar to the electron, with a unitary negative electric charge and a spin of ½. Together with the electron, the tau, and the three neutrinos, it is classified as a lepton...

is one example). According to the special theory of relativity
Special relativity
Special relativity is the physical theory of measurement in an inertial frame of reference proposed in 1905 by Albert Einstein in the paper "On the Electrodynamics of Moving Bodies".It generalizes Galileo's...

, in the high-speed particle's 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. Even in Newtonian terms time may be considered the fourth dimension of motion ; but Einstein showed how both temporal and spatial dimensions can be altered (or "warped") by high-speed motion.

Einstein (The Meaning of Relativity): "Two events taking place at the points A and B of a system K are simultaneous if they appear at the same instant when observed from the middle point, M, of the interval AB. Time is then defined as the ensemble of the indications of similar clocks, at rest relatively to K, which register the same simultaneously."

Einstein wrote in his book, Relativity, that simultaneity is also relative
Relativity of simultaneity
In physics, the relativity of simultaneity is the concept that simultaneity–whether two events occur at the same time–is not absolute, but depends on the observer's reference frame. According to the special theory of relativity, it is impossible to say in an absolute sense whether two events occur...

, i.e., two events that appear simultaneous to an observer in a particular inertial reference frame need not be judged as simultaneous by a second observer in a different inertial frame of reference.

### Relativistic time versus Newtonian time

The animations visualise the different treatments of time in the Newtonian and the relativistic descriptions. At the heart of these differences are the Galilean
Galilean transformation
The Galilean transformation is used to transform between the coordinates of two reference frames which differ only by constant relative motion within the constructs of Newtonian physics. This is the passive transformation point of view...

and Lorentz transformation
Lorentz transformation
In physics, the Lorentz transformation or Lorentz-Fitzgerald transformation describes how, according to the theory of special relativity, two observers' varying measurements of space and time can be converted into each other's frames of reference. It is named after the Dutch physicist Hendrik...

s applicable in the Newtonian and relativistic theories, respectively.

In the figures, the vertical direction indicates time. The horizontal direction indicates distance (only one spatial dimension is taken into account), and the thick dashed curve is the spacetime
Spacetime
In physics, spacetime is any mathematical model that combines space and time into a single continuum. Spacetime is usually interpreted with space as being three-dimensional and time playing the role of a fourth dimension that is of a different sort from the spatial dimensions...

trajectory ("world line
World line
In physics, the world line of an object is the unique path of that object as it travels through 4-dimensional spacetime. The concept of "world line" is distinguished from the concept of "orbit" or "trajectory" by the time dimension, and typically encompasses a large area of spacetime wherein...

") 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
Light cone
A light cone is the path that a flash of light, emanating from a single event and traveling in all directions, would take through spacetime...

" of the observer. Notice that with the change from a Newtonian to a relativistic description, the concept of absolute time is no longer applicable: events move up-and-down in the figure depending on the acceleration of the observer.

### Arrow of time

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
Arrow of time
The arrow of time, or time’s arrow, is a term coined in 1927 by the British astronomer Arthur Eddington to describe the "one-way direction" or "asymmetry" of time...

, and allow any process to proceed both forward and in reverse. This is generally a consequence of time being modeled by a parameter in the system being analyzed, where there is no "proper time": the direction of the arrow of time is arbitrary. The exceptions include the Second law of thermodynamics
Second law of thermodynamics
The second law of thermodynamics is an expression of the tendency that over time, differences in temperature, pressure, and chemical potential equilibrate in an isolated physical system. From the state of thermodynamic equilibrium, the law deduced the principle of the increase of entropy and...

, which states that entropy
Entropy
Entropy is a thermodynamic property that can be used to determine the energy available for useful work in a thermodynamic process, such as in energy conversion devices, engines, or machines. Such devices can only be driven by convertible energy, and have a theoretical maximum efficiency when...

must increase over time (see Entropy
Entropy (arrow of time)
Entropy is the only quantity in the physical sciences that requires a particular direction for time, sometimes called an arrow of time. As one goes "forward" in time, the second law of thermodynamics says, the entropy of an isolated system will increase...

); the cosmological
Physical cosmology
Physical cosmology, as a branch of astronomy, is the study of the largest-scale structures and dynamics of the universe and is concerned with fundamental questions about its formation and evolution. For most of human history, it was a branch of metaphysics and religion...

arrow of time, which points away from the Big Bang
Big Bang
The Big Bang theory is the prevailing cosmological model that explains the early development of the Universe. According to the Big Bang theory, the Universe was once in an extremely hot and dense state which expanded rapidly. This rapid expansion caused the young Universe to cool and resulted in...

, and the radiative arrow of time, caused by light
Light
Light or visible light is electromagnetic radiation that is visible to the human eye, and is responsible for the sense of sight. Visible light has wavelength in a range from about 380 nanometres to about 740 nm, with a frequency range of about 405 THz to 790 THz...

only traveling forwards in time (see light cone
Light cone
A light cone is the path that a flash of light, emanating from a single event and traveling in all directions, would take through spacetime...

). In particle physics
Particle physics
Particle physics is a branch of physics that studies the existence and interactions of particles that are the constituents of what is usually referred to as matter or radiation. In current understanding, particles are excitations of quantum fields and interact following their dynamics...

, the violation of CP symmetry implies that there should be a small counterbalancing time asymmetry to preserve CPT symmetry
CPT symmetry
CPT symmetry is a fundamental symmetry of physical laws under transformations that involve the inversions of charge, parity, and time simultaneously.-History:...

. The standard description of measurement
Measurement
Measurement is the process or the result of determining the ratio of a physical quantity, such as a length, time, temperature etc., to a unit of measurement, such as the metre, second or degree Celsius...

in quantum mechanics
Quantum mechanics
Quantum mechanics, also known as quantum physics or quantum theory, is a branch of physics providing a mathematical description of much of the dual particle-like and wave-like behavior and interactions of energy and matter. It departs from classical mechanics primarily at the atomic and subatomic...

is also time asymmetric (see Measurement in quantum mechanics
Measurement in quantum mechanics
The framework of quantum mechanics requires a careful definition of measurement. The issue of measurement lies at the heart of the problem of the interpretation of quantum mechanics, for which there is currently no consensus....

).

### Quantised time

Time quantization is a hypothetical concept. In the modern established physical theories (the Standard Model
Standard Model
The Standard Model of particle physics is a theory concerning the electromagnetic, weak, and strong nuclear interactions, which mediate the dynamics of the known subatomic particles. Developed throughout the mid to late 20th century, the current formulation was finalized in the mid 1970s upon...

of Particles and Interactions and General Relativity
General relativity
General relativity or the general theory of relativity is the geometric theory of gravitation published by Albert Einstein in 1916. It is the current description of gravitation in modern physics...

) time is not quantized.

Planck time
Planck time
In physics, the Planck time, , is the unit of time in the system of natural units known as Planck units. It is the time required for light to travel, in a vacuum, a distance of 1 Planck length...

(~ 5.4 × 10−44 seconds) is the unit of time in the system of natural units
Natural units
In physics, natural units are physical units of measurement based only on universal physical constants. For example the elementary charge e is a natural unit of electric charge, or the speed of light c is a natural unit of speed...

known as Planck units
Planck units
In physics, Planck units are physical units of measurement defined exclusively in terms of five universal physical constants listed below, in such a manner that these five physical constants take on the numerical value of 1 when expressed in terms of these units. Planck units elegantly simplify...

. 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
Loop quantum gravity
Loop quantum gravity , also known as loop gravity and quantum geometry, is a proposed quantum theory of spacetime which attempts to reconcile the theories of quantum mechanics and general relativity...

.

## Time and the Big Bang

Stephen Hawking
Stephen Hawking
Stephen William Hawking, CH, CBE, FRS, FRSA is an English theoretical physicist and cosmologist, whose scientific books and public appearances have made him an academic celebrity...

in particular has addressed a connection between time and the Big Bang
Big Bang
The Big Bang theory is the prevailing cosmological model that explains the early development of the Universe. According to the Big Bang theory, the Universe was once in an extremely hot and dense state which expanded rapidly. This rapid expansion caused the young Universe to cool and resulted in...

. In A Brief History of Time
A Brief History of Time
A Brief History of Time is a popular science book written by renown physicist Stephen Hawking and first published by the Bantam Dell Publishing Group in 1988. It became a best-seller and has sold more than 10 million copies...

and elsewhere, Hawking says that even if time did not begin with the Big Bang and there were another time frame before the Big Bang, no information from events then would be accessible to us, and nothing that happened then would have any effect upon the present time-frame.
Upon occasion, Hawking has stated that time actually began with the Big Bang, and that questions about what happened before the Big Bang are meaningless.
This less-nuanced, but commonly repeated formulation has received criticisms from philosophers such as Aristotelian
Aristotelianism
Aristotelianism is a tradition of philosophy that takes its defining inspiration from the work of Aristotle. The works of Aristotle were initially defended by the members of the Peripatetic school, and, later on, by the Neoplatonists, who produced many commentaries on Aristotle's writings...

Scientists have come to some agreement on descriptions of events that happened 10−35 seconds after the Big Bang, but generally agree that descriptions about what happened before one Planck time
Planck time
In physics, the Planck time, , is the unit of time in the system of natural units known as Planck units. It is the time required for light to travel, in a vacuum, a distance of 1 Planck length...

(5 × 10−44 seconds) after the Big Bang are likely to remain pure speculation.

### Speculative physics beyond the Big Bang

While the Big Bang model is well established in cosmology, it is likely to be refined in the future. Little is known about the earliest moments of the universe's history. The Penrose-Hawking singularity theorems
Penrose-Hawking singularity theorems
The Penrose–Hawking singularity theorems are a set of results in general relativity which attempt to answer the question of when gravitation produces singularities.A singularity in solutions of the Einstein field equations is one of two things:...

require the existence of a singularity at the beginning of cosmic time. However, these theorems assume that general relativity
General relativity
General relativity or the general theory of relativity is the geometric theory of gravitation published by Albert Einstein in 1916. It is the current description of gravitation in modern physics...

is correct, but general relativity must break down before the universe reaches the Planck temperature
Planck temperature
Planck temperature is the greatest physically-possible temperature, according the set of theories proposed by the German physicist Max Planck. It's part of a system of five natural units known as Planck units, based on universal physical constants....

, and a correct treatment of quantum gravity
Quantum gravity
Quantum gravity is the field of theoretical physics which attempts to develop scientific models that unify quantum mechanics with general relativity...

may avoid the singularity.

There may also be parts of the universe well beyond what can be observed in principle. If inflation occurred this is likely, for exponential expansion would push large regions of space beyond our observable horizon.

Some proposals, each of which entails untested hypotheses, are:
• models including the Hartle–Hawking boundary condition in which the whole of space-time is finite; the Big Bang does represent the limit of time, but without the need for a singularity.
• brane cosmology
Brane cosmology
Brane cosmology refers to several theories in particle physics and cosmology motivated by, but not exclusively derived from, superstring theory and M-theory.-Brane and bulk:...

models in which inflation is due to the movement of branes in string theory
String theory
String theory is an active research framework in particle physics that attempts to reconcile quantum mechanics and general relativity. It is a contender for a theory of everything , a manner of describing the known fundamental forces and matter in a mathematically complete system...

; the pre-big bang model; the ekpyrotic
Ekpyrotic
The ekpyrotic universe, or ekpyrotic scenario, is a cosmological model of the origin and shape of the universe. The name comes from a Stoic term ekpyrosis meaning conflagration or in Stoic usage "conversion into fire"...

model, in which the Big Bang is the result of a collision between branes; and the cyclic model
Cyclic model
A cyclic model is any of several cosmological models in which the universe follows infinite, self-sustaining cycles. For example, the oscillating universe theory briefly considered by Albert Einstein in 1930 theorized a universe following an eternal series of oscillations, each beginning with a...

, a variant of the ekpyrotic model in which collisions occur periodically.
• chaotic inflation, in which inflation events start here and there in a random quantum-gravity foam, each leading to a bubble universe expanding from its own big bang.

Proposals in the last two categories see the Big Bang as an event in a much larger and older universe, or multiverse
Multiverse
The multiverse is the hypothetical set of multiple possible universes that together comprise all of reality.Multiverse may also refer to:-In fiction:* Multiverse , the fictional multiverse used by DC Comics...

, and not the literal beginning.

## Time travel

Time travel is the concept of moving backwards and/or forwards to different points in time, in a manner analogous to moving through space
Space
Space is the boundless, three-dimensional extent in which objects and events occur and have relative position and direction. Physical space is often conceived in three linear dimensions, although modern physicists usually consider it, with time, to be part of a boundless four-dimensional continuum...

, 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
Plot device
A plot device is an object or character in a story whose sole purpose is to advance the plot of the story, or alternatively to overcome some difficulty in the plot....

in fiction
Fiction
Fiction is the form of any narrative or informative work that deals, in part or in whole, with information or events that are not factual, but rather, imaginary—that is, invented by the author. Although fiction describes a major branch of literary work, it may also refer to theatrical,...

since the 19th century. Traveling backwards in time has never been verified, presents many theoretic problems, and may be an impossibility. Any technological device, whether fictional or hypothetical, that is used to achieve time travel is known as a time machine
Time travel
Time travel is the concept of moving between different points in time in a manner analogous to moving between different points in space. Time travel could hypothetically involve moving backward in time to a moment earlier than the starting point, or forward to the future of that point without the...

.

A central problem with time travel to the past is the violation of causality
Causality
Causality is the relationship between an event and a second event , where the second event is understood as a consequence of the first....

; should an effect precede its cause, it would give rise to the possibility of a temporal paradox
Temporal paradox is a theoretical paradoxical situation that happens because of time travel. A time traveler goes to the past, and does something that would prevent him from time travel in the first place...

. Some interpretations of time travel resolve this by accepting the possibility of travel between branch points
Many-worlds interpretation
The many-worlds interpretation is an interpretation of quantum mechanics that asserts the objective reality of the universal wavefunction, but denies the actuality of wavefunction collapse. Many-worlds implies that all possible alternative histories and futures are real, each representing an...

, parallel realities
Multiverse
The multiverse is the hypothetical set of multiple possible universes that together comprise all of reality.Multiverse may also refer to:-In fiction:* Multiverse , the fictional multiverse used by DC Comics...

, or universe
Universe
The Universe is commonly defined as the totality of everything that exists, including all matter and energy, the planets, stars, galaxies, and the contents of intergalactic space. Definitions and usage vary and similar terms include the cosmos, the world and nature...

s.

Theory would point toward there having to be a physical dimension
Dimension
In physics and mathematics, the dimension of a space or object is informally defined as the minimum number of coordinates needed to specify any point within it. Thus a line has a dimension of one because only one coordinate is needed to specify a point on it...

in which one could travel to, where the present
Present
Present is a time that is neither past nor future.Present may also refer to:- Time and timing :* Present tense, the grammatical tense of a verb* Before Present, radiocarbon dates relative to AD 1950* Presenting, a medical term* Presenteeism...

(i.e. the point which one is leaving) would exist at a fixed point relative in either the past or future. Seeing as this theory would be dependent upon the theory of a multiverse
Multiverse
The multiverse is the hypothetical set of multiple possible universes that together comprise all of reality.Multiverse may also refer to:-In fiction:* Multiverse , the fictional multiverse used by DC Comics...

, it is uncertain how or if it would be possible to just prove the possibility of time travel.

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
Free will
"To make my own decisions whether I am successful or not due to uncontrollable forces" -Troy MorrisonA pragmatic definition of free willFree will is the ability of agents to make choices free from certain kinds of constraints. The existence of free will and its exact nature and definition have long...

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
The grandfather paradox is a proposed paradox of time travel first described by the science fiction writer René Barjavel in his 1943 book Le Voyageur Imprudent . The paradox is this: suppose a man traveled back in time and killed his biological grandfather before the latter met the traveler's...

because it is a historical fact that your grandfather was not killed before his child (your parent) was conceived. This view simply holds that history is an unchangeable constant. More elaboration on this view can be found in the Novikov self-consistency principle
Novikov self-consistency principle
The Novikov self-consistency principle, also known as the Novikov self-consistency conjecture, is a principle developed by Russian physicist Igor Dmitriyevich Novikov in the mid-1980s to solve the problem of paradoxes in time travel, which is theoretically permitted in certain solutions of general...

.

## Judgement of time

The specious present
Specious present
The specious present is the time duration wherein one's perceptions are considered to be in the present. Time perception studies the sense of time, which differs from other senses since time cannot be directly perceived but must be reconstructed by the brain....

refers to the time duration wherein one's perception
Perception
Perception is the process of attaining awareness or understanding of the environment by organizing and interpreting sensory information. All perception involves signals in the nervous system, which in turn result from physical stimulation of the sense organs...

s are considered to be in the present
Present
Present is a time that is neither past nor future.Present may also refer to:- Time and timing :* Present tense, the grammatical tense of a verb* Before Present, radiocarbon dates relative to AD 1950* Presenting, a medical term* Presenteeism...

. 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
William James
William James was a pioneering American psychologist and philosopher who was trained as a physician. He wrote influential books on the young science of psychology, educational psychology, psychology of religious experience and mysticism, and on the philosophy of pragmatism...

.

### Biopsychology

The brain's judgement of time is known to be a highly distributed system, including at least the cerebral cortex
Cerebral cortex
The cerebral cortex is a sheet of neural tissue that is outermost to the cerebrum of the mammalian brain. It plays a key role in memory, attention, perceptual awareness, thought, language, and consciousness. It is constituted of up to six horizontal layers, each of which has a different...

, cerebellum
Cerebellum
The cerebellum is a region of the brain that plays an important role in motor control. It may also be involved in some cognitive functions such as attention and language, and in regulating fear and pleasure responses, but its movement-related functions are the most solidly established...

and basal ganglia
Basal ganglia
The basal ganglia are a group of nuclei of varied origin in the brains of vertebrates that act as a cohesive functional unit. They are situated at the base of the forebrain and are strongly connected with the cerebral cortex, thalamus and other brain areas...

as its components. One particular component, the suprachiasmatic nuclei
Suprachiasmatic nucleus
The suprachiasmatic nucleus or nuclei, abbreviated SCN, is a tiny region on the brain's midline, situated directly above the optic chiasm. It is responsible for controlling circadian rhythms...

, is responsible for the circadian (or daily) rhythm
A circadian rhythm, popularly referred to as body clock, is an endogenously driven , roughly 24-hour cycle in biochemical, physiological, or behavioural processes. Circadian rhythms have been widely observed in plants, animals, fungi and cyanobacteria...

, while other cell clusters appear to be capable of shorter-range (ultradian
Ultradian rhythms are recurrent periods or cycles repeated throughout a 24-hour circadian day. In contrast, infradian rhythms, such as the human menstrual cycle, have periods longer than a day....

) timekeeping.

Psychoactive drugs can impair the judgement 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
Dopamine
Dopamine is a catecholamine neurotransmitter present in a wide variety of animals, including both vertebrates and invertebrates. In the brain, this substituted phenethylamine functions as a neurotransmitter, activating the five known types of dopamine receptors—D1, D2, D3, D4, and D5—and their...

and norepinephrine
Norepinephrine
Norepinephrine is the US name for noradrenaline , a catecholamine with multiple roles including as a hormone and a neurotransmitter...

may be the reason for this.

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

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

### Alterations

In addition to psychoactive drugs, judgements of time can be altered by temporal illusions (like the kappa effect
Kappa effect
-Introduction:The Kappa Effect is a psychological phenomenon related to the perception of distance, time and speed. It is a temporal illusion that, in some cases, can alter one’s judgement of time. The Kappa effect arises when observers judge the amount of elapsed time between two stimuli in a...

), age,
and hypnosis
Hypnosis
Hypnosis is "a trance state characterized by extreme suggestibility, relaxation and heightened imagination."It is a mental state or imaginative role-enactment . It is usually induced by a procedure known as a hypnotic induction, which is commonly composed of a long series of preliminary...

.
The sense of time is impaired in some people with neurological diseases such as Parkinson's disease
Parkinson's disease
Parkinson's disease is a degenerative disorder of the central nervous system...

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.
As an example, one day to an eleven-year-old person would be approximately 1/4,000 of their life, while one day to a 55-year-old would be approximately 1/20,000 of their life. According to such an interpretation, a day would appear much longer to a young child than to an adult, even though the measure of time is the same.

## Use of time

In sociology
Sociology
Sociology is the study of society. It is a social science—a term with which it is sometimes synonymous—which uses various methods of empirical investigation and critical analysis to develop a body of knowledge about human social activity...

and anthropology
Anthropology
Anthropology is the study of humanity. It has origins in the humanities, the natural sciences, and the social sciences. The term "anthropology" is from the Greek anthrōpos , "man", understood to mean mankind or humanity, and -logia , "discourse" or "study", and was first used in 1501 by German...

, time discipline
Time discipline
In sociology and anthropology, time discipline is the general name given to 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...

is the general name given to social
Society
A society, or a human society, is a group of people related to each other through persistent relations, or a large social grouping sharing the same geographical or virtual territory, subject to the same political authority and dominant cultural expectations...

and economic
Economics
Economics is the social science that analyzes the production, distribution, and consumption of goods and services. The term economics comes from the Ancient Greek from + , hence "rules of the house"...

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
Arlie Russell Hochschild
Arlie Russell Hochschild is a professor of sociology at the University of California, Berkeley. She is the author of several prize-winning books and numerous articles on the balancing acts of modern two-job couples at home and at work...

and Norbert Elias
Norbert Elias
Norbert Elias was a German sociologist of Jewish descent, who later became a British citizen.-Biography:...

have written on the use of time from a sociological perspective.

The use of time is an important issue in understanding human behaviour
Human Behaviour
"Human Behaviour" is Icelandic singer Björk's first solo single, taken from the album Debut. It contains a sample of "Go Down Dying" by Antonio Carlos Jobim. The lyrics reflect on human nature and emotion from a non-human animal's point of view. The song is the first part of a series of songs that...

, education
Education
Education in its broadest, general sense is the means through which the aims and habits of a group of people lives on from one generation to the next. Generally, it occurs through any experience that has a formative effect on the way one thinks, feels, or acts...

, and travel behaviour. Time use research
Time use research
Time use research is a developing interdisciplinary field of study dedicated to knowing how people allocate their time during an average day. Work Intensity is the umbrella topic that incorporates Time Use, specifically time poverty....

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
Technology
Technology is the making, usage, and knowledge of tools, machines, techniques, crafts, systems or methods of organization in order to solve a problem or perform a specific function. It can also refer to the collection of such tools, machinery, and procedures. The word technology comes ;...

, as the television
Television
Television is a telecommunication medium for transmitting and receiving moving images that can be monochrome or colored, with accompanying sound...

or the Internet
Internet
The Internet is a global system of interconnected computer networks that use the standard Internet protocol suite to serve billions of users worldwide...

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
Transport
Transport or transportation is the movement of people, cattle, animals and goods from one location to another. Modes of transport include air, rail, road, water, cable, pipeline, and space. The field can be divided into infrastructure, vehicles, and operations...

, has been observed to be about 20–30 minutes one-way for a large number of cities over a long period of time.

Time management
Time management
Time management is the act or process of exercising conscious control over the amount of time spent on specific activities, especially to increase efficiency or productivity. Time management may be aided by a range of skills, tools, and techniques used to manage time when accomplishing specific...

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

A sequence of events, or series of events, is a sequence
Sequence
In mathematics, a sequence is an ordered list of objects . Like a set, it contains members , and the number of terms is called the length of the sequence. Unlike a set, order matters, and exactly the same elements can appear multiple times at different positions in the sequence...

of items, facts, events, actions, changes, or procedural steps, arranged in time order (chronological order), often with causality
Causality
Causality is the relationship between an event and a second event , where the second event is understood as a consequence of the first....

relationships among the items. Because of causality
Causality
Causality is the relationship between an event and a second event , where the second event is understood as a consequence of the first....

, cause precedes effect
Result
A result is the final consequence of a sequence of actions or events expressed qualitatively or quantitatively. Possible results include advantage, disadvantage, gain, injury, loss, value and victory. There may be a range of possible outcomes associated with an event depending on the point of...

, 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
Table (information)
A table is a means of arranging data in rows and columns.Production % of goalNorth 4087102%South 4093110% The use of tables is pervasive throughout all communication, research and data analysis. Tables appear in print media, handwritten notes, computer software, architectural...

s, chart
Chart
A chart is a graphical representation of data, in which "the data is represented by symbols, such as bars in a bar chart, lines in a line chart, or slices in a pie chart"...

s, or timeline
Timeline
A timeline is a way of displaying a list of events in chronological order, sometimes described as a project artifact . It is typically a graphic design showing a long bar labeled with dates alongside itself and events labeled on points where they would have happened.-Uses of timelines:Timelines...

s. The description of the items or events may include a timestamp
Timestamp
A timestamp is a sequence of characters, denoting the date or time at which a certain event occurred. A timestamp is the time at which an event is recorded by a computer, not the time of the event itself...

. 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
World line
In physics, the world line of an object is the unique path of that object as it travels through 4-dimensional spacetime. The concept of "world line" is distinguished from the concept of "orbit" or "trajectory" by the time dimension, and typically encompasses a large area of spacetime wherein...

.

Uses of a sequence of events include stories
Stories
Stories may refer to:* Height of more than one Storey * Stories , a greatest hits compilation album by Randy Stonehill...

, historical
History
History is the discovery, collection, organization, and presentation of information about past events. History can also mean the period of time after writing was invented. Scholars who write about history are called historians...

events (chronology
Chronology
Chronology is the science of arranging events in their order of occurrence in time, such as the use of a timeline or sequence of events. It is also "the determination of the actual temporal sequence of past events".Chronology is part of periodization...

), directions and steps in procedure
Procedure
Procedure may refer to:* Procedure * Medical procedure* Algorithm, in mathematics and computing, a set of operations or calculations that accomplish some goal* Instructions or recipes, a set of commands that show how to prepare or make something...

s, and timetables for scheduling activities. A sequence of events may also be used to help describe 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 need to be in a predetermined order (e.g., plan
Plan
A plan is typically any diagram or list of steps with timing and resources, used to achieve an objective. See also strategy. It is commonly understood as a temporal set of intended actions, through which one expects to achieve a goal...

s, schedule
Schedule
Schedule generally refers to:* a timetable * an airline timetable* the act of Scheduling Schedule may also refer to:* Schedule , a list of actions from a set of transactions in databases...

s, procedures, timetables), or focused on the observation of past events with the expectation that the events will occur in the future (e.g., processes). The use of a sequence of events occurs in fields as diverse as machine
Machine
A machine manages power to accomplish a task, examples include, a mechanical system, a computing system, an electronic system, and a molecular machine. In common usage, the meaning is that of a device having parts that perform or assist in performing any type of work...

s (cam timer
Cam timer
A cam timer is an electromechanical system for controlling a sequence of events automatically.-Description:An electric motor drives, through a reduction gearbox, a shaft on which is arranged a series of cams. Associated with each cam is one or more switches. The motor rotates at a fixed speed, and...

), documentaries
Documentary
A documentary is a creative work of non-fiction, including:* Documentary film, including television* Radio documentary* Documentary photographyRelated terms include:...

(Seconds From Disaster
Seconds From Disaster
-By original broadcast date:National Geographic Channel has broadcast many episodes under multiple titles. The title currently or most recently listed on the NGC Calendar is shown first...

), law
Law
Law is a system of rules and guidelines which are enforced through social institutions to govern behavior, wherever possible. It shapes politics, economics and society in numerous ways and serves as a social mediator of relations between people. Contract law regulates everything from buying a bus...

(choice of law), computer simulation
Computer simulation
A computer simulation, a computer model, or a computational model is a computer program, or network of computers, that attempts to simulate an abstract model of a particular system...

(discrete event simulation
Discrete Event Simulation
In discrete-event simulation, the operation of a system is represented as a chronological sequence of events. Each event occurs at an instant in time and marks a change of state in the system...

), and electric power transmission
Electric power transmission
Electric-power transmission is the bulk transfer of electrical energy, from generating power plants to Electrical substations located near demand centers...

(sequence of events recorder
Sequence of events recorder
A sequence of events recorder is an intelligent standalone microprocessor based system, which monitors external inputs and records the time and sequence of the changes. Sequence of events recorders usually have an external time source such as a GPS or radio clock...

). A specific example of a sequence of events is the timeline of the Fukushima Daiichi nuclear disaster.

• Term (time)
Term (time)
A term is a period of duration, time or occurrence, in relation to an event. To differentiate an interval or duration, common phrases are used to distinguish the observance of length are near-term or short-term, medium-term or mid-term and long-term....

• Horology
Horology
Horology is the art or science of measuring time. Clocks, watches, clockwork, sundials, clepsydras, timers, time recorders and marine chronometers are all examples of instruments used to measure time.People interested in horology are called horologists...

• Kairos
Kairos
Kairos is an ancient Greek word meaning the right or opportune moment . The ancient Greeks had two words for time, chronos and kairos. While the former refers to chronological or sequential time, the latter signifies a time in between, a moment of indeterminate time in which something special...

### Books

• A Brief History of Time
A Brief History of Time
A Brief History of Time is a popular science book written by renown physicist Stephen Hawking and first published by the Bantam Dell Publishing Group in 1988. It became a best-seller and has sold more than 10 million copies...

by Stephen Hawking
Stephen Hawking
Stephen William Hawking, CH, CBE, FRS, FRSA is an English theoretical physicist and cosmologist, whose scientific books and public appearances have made him an academic celebrity...

• About Time: Einstein's Unfinished Revolution
About Time is the second book written by Paul Davies, regarding the subject of time. The intended audience is the general public, rather than science academics....

by Paul Davies
Paul Davies
Paul Charles William Davies, AM is an English physicist, writer and broadcaster, currently a professor at Arizona State University as well as the Director of BEYOND: Center for Fundamental Concepts in Science...

• From Eternity to Here: The Quest for the Ultimate Theory of Time
From Eternity to Here
From Eternity to Here is a book in which Sean Carroll attempts to address the question of what time is what what what processes make it come into existence.- Part One - Time, Experience, and the Universe :Chapter 1 - The Past is Present Memory...

by Sean M. Carroll
Sean M. Carroll
Sean Michael Carroll is a senior research associate in the Department of Physics at the California Institute of Technology. He is a theoretical cosmologist specializing in dark energy and general relativity...

• The Physical Basis of The Direction of Time by Heinz-Dieter Zeh
• An Experiment with Time
An Experiment with Time
An Experiment with Time is a long essay by the Irish aeronautical engineer J. W. Dunne on the subjects of precognition and the human experience of time. First published in March 1927, it was very widely read, and his ideas promoted by several other authors, in particular by J. B. Priestley. Other...

by J. W. Dunne
• Einstein's Dreams
Einstein's Dreams
Einstein's Dreams is a 1992 novel by Alan Lightman that was an international bestseller and has been translated into thirty languages. It was runner up for the 1994 L. L. Winship/PEN New England Award. Einstein's Dreams was also the March 1998 selection for National Public Radio's "Talk of the...

by Alan Lightman
Alan Lightman
Alan Lightman is an American physicist, writer, and social entrepreneur. He is a professor at the Massachusetts Institute of Technology and the author of the international bestseller Einstein's Dreams. He was the first professor at MIT to receive a joint appointment in the sciences and the...

### Organizations

Leading scholarly organizations for researchers on the history and technology of time and timekeeping
• Antiquarian Horological Society
Antiquarian Horological Society
The Antiquarian Horological Society, abbreviated as AHS, is the British organization of scholars and enthusiasts of horology.The association was founded in 1953 and unites collectors, scholars and museum professionals interested in the historical aspects of horology – the study of the art,...

– AHS (United Kingdom)
• Chronometrophilia
Chronometrophilia
Chronometrophilia is described in its byline as the "Swiss Association for the History of Timekeeping / Association suisse pour l'histoire de la mesure du temps / Schweizerische Gesellschaft für die Geschichte der Zeitmessung"...

(Switzerland)
• Deutsche Gesellschaft für Chronometrie
Deutsche Gesellschaft für Chronometrie
Deutsche Gesellschaft für Chronometrie , today is an organization of scholars, collectors and enthusiasts in Germany interested in the science, art and history of horology.-History:...

– DGC (Germany)
• National Association of Watch and Clock Collectors
National Association of Watch and Clock Collectors
The National Association of Watch and Clock Collectors is an American non-profit organization with about 18,000 members.The NAWCC was founded in 1943 by members of the Horological Society of New York and the Philadelphia Watchmakers' Guild who wished to create a national organization...

- NAWCC (United States of America)

### Miscellaneous arts and sciences

• Anachronistic
• Date and time notation by country
Date and time notation by country
Different conventions exist around the world for date and time representation, both written and spoken.- Differences :Differences can exist in:*The calendar that is used.*The order in which the year, month and day are represented....

• List of cycles
• Network Time Protocol
Network Time Protocol
The Network Time Protocol is a protocol and software implementation for synchronizing the clocks of computer systems over packet-switched, variable-latency data networks. Originally designed by David L...

(NTP)
• Nonlinear (arts)
Nonlinear (arts)
Nonlinear narrative, disjointed narrative or disrupted narrative is a narrative technique, sometimes used in literature, film, hypertext websites and other narratives, wherein events are portrayed out of chronological order...

• Philosophy of physics
Philosophy of physics
In philosophy, the philosophy of physics studies the fundamental philosophical questions underlying modern physics, the study of matter and energy and how they interact. The philosophy of physics begins by reflecting on the basic metaphysical and epistemological questions posed by physics:...

• Rate (mathematics)

### Miscellaneous units of time

• Fiscal year
• Half-life
Half-life
Half-life, abbreviated t½, is the period of time it takes for the amount of a substance undergoing decay to decrease by half. The name was originally used to describe a characteristic of unstable atoms , but it may apply to any quantity which follows a set-rate decay.The original term, dating to...

Hexadecimal time is the representation of the time of day as a hexadecimal number in the interval [0,1). The day is divided in 1016 hexadecimal hours, each hour in 10016 hexadecimal minutes and each minute in 1016 hexadecimal seconds.This time format was first proposed by the...

• Season
Season
A season is a division of the year, marked by changes in weather, ecology, and hours of daylight.Seasons result from the yearly revolution of the Earth around the Sun and the tilt of the Earth's axis relative to the plane of revolution...

• Tithi
Tithi
In vedic timekeeping, a tithi is a lunar day, or the time it takes for the longitudinal angle between the moon and the sun to increase by 12°. Tithis begin at varying times of day and vary in duration from approximately 19 to approximately 26 hours. There are 30 tithis in each lunar month, named...

• Unix epoch

### Philosophy

Eastern Philosophy

Western Philosophy