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Time

For other uses of "Time", see Time (disambiguation).

File:Wooden hourglass 3.jpg
The flow of sand in an hourglass can be used to keep track of elapsed time. It also concretely represents the present as being between the past and the future.

Time is a measure in which events can be ordered from the past through the present into the future, and also the measure of durations of events and the intervals between them.[1][2][3][4][5][6][7] Time is often referred to as the fourth dimension, along with the three spatial dimensions.[8]

Time has long been a major subject of study in religion, philosophy, and science, but defining it in a manner applicable to all fields without circularity has consistently eluded scholars.[2][6][7][9][10][11] Nevertheless, diverse fields such as business, industry, sports, the sciences, and the performing arts all incorporate some notion of time into their respective measuring systems.[12][13][14] Some simple definitions of time include "time is what clocks measure",[6][15] which is a problematically vague and self-referential definition that utilizes the device used to measure the subject as the definition of the subject, and "time is what keeps everything from happening at once", which is without substantive meaning in the absence of the definition of simultaneity in the context of the limitations of human sensation, observation of events, and the perception of such events.[16][17][18][19]

Two contrasting viewpoints on time divide many prominent philosophers. One view is that time is part of the fundamental structure of the universe—a dimension independent of events, in which events occur in sequence. Sir Isaac Newton subscribed to this realist view, and hence it is sometimes referred to as Newtonian time.[20][21] The opposing view is that time does not refer to any kind of "container" that events and objects "move through", nor to any entity that "flows", but that it is instead part of a fundamental intellectual structure (together with space and number) within which humans sequence and compare events. This second view, in the tradition of Gottfried Leibniz[15] and Immanuel Kant,[22][23] holds that time is neither an event nor a thing, and thus is not itself measurable nor can it be travelled.

Time is one of the seven fundamental physical quantities in both the International System of Units and International System of Quantities. Time is used to define other quantities—such as velocity—so defining time in terms of such quantities would result in circularity of definition.[24] An operational definition of time, wherein one says that observing a certain number of repetitions of one or another standard cyclical event (such as the passage of a free-swinging pendulum) constitutes one standard unit such as the second, is highly useful in the conduct of both advanced experiments and everyday affairs of life. 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 bring questions about space into questions about time, questions that have their roots in the works of early students of natural philosophy.

Furthermore, it may be that there is a subjective component to time, but whether or not time itself is "felt", as a sensation, or is a judgment, is a matter of debate.[2][6][7][25][26]

Temporal measurement has occupied scientists and technologists, and was a prime motivation in navigation and astronomy. Periodic events and periodic motion have long served as standards for units of time. Examples include the apparent motion of the sun across the sky, the phases of the moon, the swing of a pendulum, and the beat of a heart. Currently, the international unit of time, the second, is defined in terms of radiation emitted by caesium atoms (see below). Time is also of significant social importance, having economic value ("time is money") as well as personal value, due to an awareness of the limited time in each day and in human life spans.

Temporal measurement and history

Temporal measurement, chronometry, takes two distinct period forms: the calendar, a mathematical tool for organizing intervals of time,[27] and the clock, a physical mechanism that counts the passage of time. In day-to-day life, the clock is consulted for periods less than a day, 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

Main article: Calendar

Artifacts from the Paleolithic suggest that the moon was used to reckon time as early as 6,000 years ago.[28] Lunar calendars were among the first to appear, either 12 or 13 lunar months (either 354 or 384 days). Without intercalation to add days or months to some years, seasons quickly drift in a calendar based solely on twelve lunar months. Lunisolar calendars 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 in 45 BC put the Roman world on a solar calendar. This Julian calendar was faulty in that its intercalation still allowed the astronomical solstices and equinoxes to advance against it by about 11 minutes per year. Pope Gregory XIII introduced a correction in 1582; the Gregorian calendar was only slowly adopted by different nations over a period of centuries, but it is now the most commonly used calendar around the world, by far.

History of time measurement devices

See also: Clock

A large variety of devices has been invented to measure time. The study of these devices is called horology.

An Egyptian device that dates to c.1500 BC, similar in shape to a bent T-square, measured the passage of time from the shadow cast by its crossbar on a nonlinear rule. The T was orientated eastward in the mornings. At noon, the device was turned around so that it could cast its shadow in the evening direction.[29]

A sundial uses a gnomon to cast a shadow on a set of markings calibrated to the hour. The position of the shadow marks the hour in local time.

The most precise timekeeping device of the ancient world was the water clock, or clepsydra, one of which was found in the tomb of Egyptian pharaoh Amenhotep I (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 Ancient Greeks and the people from Chaldea (southeastern Mesopotamia) regularly maintained timekeeping records as an essential part of their astronomical observations. Arab inventors and engineers in particular made improvements on the use of water clocks up to the Middle Ages.[30] In the 11th century, Chinese inventors and engineers invented the first mechanical clocks driven by an escapement mechanism.

The hourglass uses the flow of sand to measure the flow of time. They were used in navigation. Ferdinand Magellan used 18 glasses on each ship for his circumnavigation of the globe (1522).[31] 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 (1292–1336), abbot of St. Alban's abbey, famously built a mechanical clock as an astronomical orrery about 1330.[32][33] Great advances in accurate time-keeping were made by Galileo Galilei and especially Christiaan Huygens with the invention of pendulum driven clocks.

The English word clock probably comes from the Middle Dutch word klocke which, in turn, derives from the medieval Latin word clocca, which ultimately derives from Celtic and is cognate with French, Latin, and German words that mean bell. The passage of the hours at sea were marked by bells, and denoted the time (see ship's bell). The hours were marked by bells in abbeys as well as at sea.

File:ChipScaleClock2 HR.jpg
Chip-scale atomic clocks, such as this one unveiled in 2004, are expected to greatly improve GPS location.[34]

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

A chronometer is a portable timekeeper that meets certain precision standards. Initially, the term was used to refer to the marine chronometer, a timepiece used to determine longitude by means of celestial navigation, a precision firstly achieved by John Harrison. More recently, the term has also been applied to the chronometer watch, a watch that meets precision standards set by the Swiss agency COSC.

The most accurate timekeeping devices are atomic clocks, which are accurate to seconds in many millions of years,[35] 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 atoms. SI defines the second as 9,192,631,770 cycles of the radiation that corresponds to the transition between two electron spin energy levels of the ground state of the 133Cs atom.

Today, the Global Positioning System in coordination with the Network Time Protocol 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,[36] where it was defined as 1/564 of a momentum (1½ minutes),[37] and thus equal to 15/94 of a second. It was used in the computus, the process of calculating the date of Easter.

As of May 2010, the smallest time interval uncertainty in direct measurements is on the order of 12 attoseconds (1.2 × 10−17 seconds), about 3.7 × 1026 Planck times.[38]

List of units

Units of time
Unit Length, Duration and Size Notes
instant varies loosely speaking, zero time (colloquially the term may be used in other ways)
Planck time unit 5.39 x 10−44 s The duration light takes to travel one Planck length. Theorized to be the smallest duration measurement that will ever be possible, roughly 10−43 seconds.
yoctosecond 10−24 s
jiffy varies in quantum physics, the duration light takes to travel one fermi (10−15m, about the size of a nucleon) in a vacuum: about 3 × 10−24s.
In electronics, the duration for one alternating current power cycle (1/60 or 1/50 of a second).
Also, an informal term for any unspecified short duration.
zeptosecond 10−21 s
attosecond 10−18 s shortest duration now measurable
femtosecond 10−15 s pulse duration on fastest lasers
picosecond 10−12 s
nanosecond 10−9 s duration for molecules to fluoresce
shake 10−8 s 10 nanoseconds. Also a casual term for a short duration.
microsecond 10−6 s
millisecond 0.001 s shortest duration unit used on stopwatches
centisecond 0.01 s used on some stopwatches
decisecond 0.1 s used on some stopwatches
jiffy (electronics) ~1/50s to 1/60s Used to measure the duration between alternating power cycles. Also a casual term for a short duration
second 1 sec SI base unit
decasecond 10 seconds
minute 60 seconds
moment (historical) 1/40th of an hour used by Medieval Western European computists.[39]
hectosecond 100 seconds 1 minute and 40 seconds
ke 864 seconds traditional Chinese unit of decimal time duration, usually 1/100 of a day. 14 minutes and 24 seconds. (Nearly 1/4 of an hour.)
kilosecond 1,000 seconds 16 minutes and 40 seconds
hour 60 minutes
day 24 hours longest unit used on stopwatches and countdowns
week 7 days Also called sennight
megasecond 1,000,000 seconds About 11.6 days
fortnight 14 days 2 weeks (more common in Great Britain)
lunar month 27.2–29.5 days Various definitions of lunar month exist.
month 28–31 days Often 30 days for financial and other calculations.
quarter and season 3 months The duration of any of the four calendar seasons; winter, spring, summer and autumn.
year 12 months
common year 365 days 52 weeks + 1 day
tropical year 365.24219 days[40] average
Gregorian year 365.2425 days[41] average
Julian year 365.25 days
sidereal year 365.256363004 days
leap year 366 days 52 weeks + 2 days
biennium 2 years A unit of time duration commonly used by legislatures
triennium 3 years
Olympiad 4 year cycle
lustrum 5 years
decade 10 years
Indiction 15 year cycle
generation varies about 17–35 years for humans
gigasecond 1,000,000,000 seconds About 31.7 years
jubilee 50 years
century 100 years
millennium 1,000 years also called "kiloannum"
terasecond 1012 seconds About 31,700 years
megaannum 1,000,000 years 1 million years
age varies on the geological timescale, some millions of years[42]
epoch varies on the geological timescale, tens of millions of years[42]
petasecond 1015 seconds About 31.7 million years
era varies on the geological timescale, several hundred millions of years[42]
galactic year Approximately 230 million years[43] The duration it takes the Sun to orbit the center of the Milky Way galaxy once.
eon varies on the geological timescale, 500 million years or more.[42] Also "an indefinite and very long period of time".[44]
gigaannum 1,000,000,000 years 109 years
exasecond 1018 seconds roughly 31.7 x 109 years, more than twice the age of the universe (on current estimates)
zettasecond 1021 seconds About 31.7 x 1012 years
yottasecond 1024 seconds About 31.7 x 1015 years
cosmological decade 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.

Definitions and standards

The SI base unit for time is the SI second. The International System of Quantities, which incorporates the SI, also defines larger units of time equal to fixed integer multiples of one second (1 s), such as the minute, hour and day. These are not part of the SI, but may be used alongside the SI. Other units of time such as the month and the year are not equal to fixed multiples of 1 s, and instead exhibit significant variations in duration.[45]

The official SI definition of the second is as follows:[45][46]

The second is the duration of 9,192,631,770 periods of the radiation corresponding to the transition between the two hyperfine levels of the ground state of the caesium 133 atom.

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.[45] Previous to 1967, the second was defined as:

the fraction 1/31,556,925.9747 of the tropical year for 1900 January 0 at 12 hours ephemeris time.

The current definition of the second, coupled with the current definition of the metre, is based on the special theory of relativity, which affirms our space-time to be a Minkowski space.

World time

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 clocks around the world, known as the International Atomic Time (TAI). Other scientific time standards include Terrestrial Time and Barycentric Dynamical Time.

Coordinated Universal Time (UTC) is the basis for modern civil time. Since 1 January 1972, it has been defined to follow TAI with an exact offset of an integer number of seconds, changing only when a leap second is added to keep clock time synchronized with the rotation of the Earth. In TAI and UTC systems, the duration of a second is constant, as it is defined by the unchanging transition period of the caesium atom.

Greenwich Mean Time (GMT) is an older standard, adopted starting with British railways in 1847. Using telescopes instead of atomic clocks, GMT was calibrated to the mean solar time at the Royal Observatory, Greenwich in the UK. Universal Time (UT) is the modern term for the international telescope-based system, adopted to replace "Greenwich Mean Time" in 1928 by the International Astronomical Union. 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 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 zones. 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 transitions.

Time conversions

These conversions are accurate at the millisecond level for time systems involving earth rotation (UT1 & TT). Conversions between atomic time systems (TAI, GPS, and UTC) are accurate 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 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

Main article: Chronology

Another form of time measurement consists of studying the past. Events in the past can be ordered in a sequence (creating a chronology), and can be put into chronological groups (periodization). One of the most important systems of periodization is the geologic time scale, which is a system of periodizing the events that shaped the Earth and its life. Chronology, periodization, and interpretation of the past are together known as the study of history.

Time-like concepts: terminology

The term "time" is generally used for many close but different concepts, including:

  • instant[47] as an object—one point on the time axes. Being an object, it has no value;
  • time interval[48] as an object—part of the time axes limited by two instants. Being an object, it has no value;
  • date[49] as a quantity characterizing an instant. As a quantity, it has a value which may be expressed in a variety of ways, for example "2014-04-26T09:42:36,75" in ISO standard format, or more colloquially such as "today, 9:42 a.m.";
  • duration[50] as a quantity characterizing a time interval.[51] As a quantity, it has a value, such as a number of minutes, or may be described in terms of the quantities (such as times and dates) of its beginning and end.

Religion

Further information: Time and fate deities

Linear and cyclical time

Ancient cultures such as Incan, Mayan, Hopi, and other Native American Tribes, plus the Babylonians, Ancient Greeks, Hinduism, Buddhism, Jainism, and others had the concept of a wheel of time, that regarded time as cyclical and quantic[clarification needed] consisting of repeating ages that happen to every being of the Universe between birth and extinction.

In general, the Islamic and Judeo-Christian concept, based on the Bible, is that time is linear, beginning with the act of creation by God. The general Christian view is that time will end with the end of the present order of things.

In the Old Testament book Ecclesiastes, traditionally ascribed to Solomon (970–928 BC), time (as the Hebrew word עדן, זמן `iddan(time) zĕman(season) is often translated) was traditionally regarded as a medium for the passage of predestined events.[citation needed] (Another word, زمان" זמן" zman, was current as meaning time fit for an event, and is used as the modern Arabic, Persian, and Hebrew equivalent to the English word "time".)

Time in Greek mythology

The Greek language denotes two distinct principles, Chronos and Kairos. The former refers to numeric, or chronological, time. The latter, literally "the right or opportune moment", relates specifically to metaphysical or Divine time. In theology, Kairos is qualitative, as opposed to quantitative.

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

Time in Kabbalah

According to Kabbalists, “time” is a paradox[52] and an illusion.[53] Both the future and the past are recognized to be simultaneously present.

Philosophy

Two distinct viewpoints on time divide many prominent philosophers. One view is that time is part of the fundamental structure of the universe, a dimension in which events occur in sequence. Sir Isaac Newton subscribed to this realist view, and hence it is sometimes referred to as Newtonian time.[21] 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 and number) that enables humans to sequence and compare events.[54] This second view, in the tradition of Gottfried Leibniz[15] and Immanuel Kant,[22][23] 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 to us.

The Vedas, the earliest texts on Indian philosophy and Hindu philosophy dating back to the late 2nd millennium BC, describe ancient Hindu cosmology, in which the universe goes through repeated cycles of creation, destruction and rebirth, with each cycle lasting 4320 million years.[55] Ancient Greek philosophers, including Parmenides and Heraclitus, wrote essays on the nature of time.[56] Plato, in the Timaeus, identified time with the period of motion of the heavenly bodies. Aristotle, in Book IV of his Physica defined time as 'number of movement in respect of the before and after'.[57]

In Book 11 of his Confessions, St. Augustine of Hippo ruminates on the nature of time, asking, "What then is time? If no one asks me, I know: if I wish to explain it to one that asketh, I know not." He begins to define time by what it is not rather than what it is,[58] an approach similar to that taken in other negative definitions. However, Augustine ends up calling time a “distention” of the mind (Confessions 11.26) by which we simultaneously grasp the past in memory, the present by attention, and the future by expectation.

In contrast to ancient Greek philosophers who believed that the universe had an infinite past with no beginning, medieval philosophers and theologians 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 believed in absolute space and absolute time; Leibniz believed that time and space are relational.[59] The differences between Leibniz's and Newton's interpretations came to a head in the famous Leibniz-Clarke Correspondence.

Time is not an empirical concept. For neither co-existence nor succession would be perceived by us, if the representation of time did not exist as a foundation a priori. Without this presupposition we could not represent to ourselves that things exist together at one and the same time, or at different times, that is, contemporaneously, or in succession.

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

Immanuel Kant, in the Critique of Pure Reason, described time as an a priori intuition that allows us (together with the other a priori intuition, space) to comprehend sense experience.[60] With Kant, neither space nor time are conceived as substances, but rather both are elements of a systematic mental framework that necessarily structures the experiences of any rational agent, or observing subject. Kant thought of time as a fundamental part of an abstract conceptual framework, together with space and number, within which we sequence events, quantify their duration, and compare the motions of objects. In this view, time does not refer to any kind of entity that "flows," that objects "move through," or that is a "container" for events. Spatial measurements are used to quantify the extent of and distances between objects, and temporal measurements are used to quantify the durations of and between events. Time was designated by Kant as the purest possible schema of a pure concept or category.

Henri Bergson believed that time was neither a real homogeneous medium nor a mental construct, but possesses what he referred to as Duration. Duration, in Bergson's view, was creativity and memory as an essential component of reality.[61]

According to Martin Heidegger we do not exist inside time, we are time. Hence, the relationship to the past is a present awareness of having been, which allows the past to exist in the present. The relationship to the future is the state of anticipating a potential possibility, task, or engagement. It is related to the human propensity for caring and being concerned, which causes "being ahead of oneself" when thinking of a pending occurrence. Therefore, this concern for a potential occurrence also allows the future to exist in the present. The present becomes an experience, which is qualitative instead of quantitative. Heidegger seems to think this is the way that a linear relationship with time, or temporal existence, is broken or transcended.[62] 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.[63]

Time as "unreal"

In 5th century BC Greece, Antiphon the Sophist, in a fragment preserved from his chief work On Truth, held that: "Time is not a reality (hypostasis), but a concept (noêma) or a measure (metron)." Parmenides went further, maintaining that time, motion, and change were illusions, leading to the paradoxes of his follower Zeno.[64] Time as an illusion is also a common theme in Buddhist thought.[65][66]

J. M. E. McTaggart's 1908 The Unreality of Time argues that, since every event has the characteristic of being both present and not present (i.e., future or past), that time is a self-contradictory idea (see also The flow of time).

These arguments often center around what it means for something to be unreal. Modern physicists generally believe that time is as real as space—though others, such as Julian Barbour in his book The End of Time, argue that quantum equations of the universe take their true form when expressed in the timeless realm containing every possible now or momentary configuration of the universe, called 'platonia' by Barbour.[67] (See also: Eternalism (philosophy of time))

Physical definition