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Encyclopedia > Time
Sunrise shown in time lapse.
The motions of Sun and Moon have demonstrated and symbolized time throughout humanity's existence.[1]
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.

Among prominent philosophers, there are two distinct viewpoints on time. 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.[4][5] 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[6] and Immanuel Kant,[7][8] holds that time is neither an event nor a thing, and thus is not itself measurable. For other uses, see Universe (disambiguation). ... For other uses, see Dimension (disambiguation). ... For other senses of this word, see sequence (disambiguation). ... Sir Isaac Newton in Knellers portrait of 1689. ... Contemporary philosophical realism, also referred to as metaphysical realism, is the belief in a reality that is completely ontologically independent of our conceptual schemes, linguistic practices, beliefs, etc. ... In physics, the concept of absolute time and absolute space are hypothetical models in which time either runs at the same rate for all the observers in the universe or the rate of time of each observer can be scaled to the absolute time by multiplying the rate by a... This article is about the idea of space. ... For other uses, see Number (disambiguation). ... Leibniz redirects here. ... Kant redirects here. ...

## Temporal measurement

Temporal measurement, or chronometry, takes two distinct period forms: the calendar, a mathematical abstraction for calculating extensive periods of time,[9] and the clock, a concrete 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. Wikipedia does not yet have an article with this exact name. ... For other uses, see Calendar (disambiguation) A page from the Hindu calendar 1871â€“1872. ... For other uses, see Clock (disambiguation). ...

### History of the calendar

Main article: Calendar

Artifacts from the Palaeolithic suggest that the moon was used to calculate time as early as 12,000, and possibly even 30,000 BP.[1] For other uses, see Calendar (disambiguation) A page from the Hindu calendar 1871â€“1872. ... The Paleolithic or Palaeolithic – lit. ... Before Present (BP) years are the units of time (counted backwards to the past) used to report raw radiocarbon ages and dates referenced to the BP scale origin in the year AD 1950 (identical to 1950 CE). ...

The Sumerian civilization of approximately 2000 BC introduced the sexagesimal system based on the number 60. 60 seconds in a minute, 60 minutes in an hour – and possibly a calendar with 360 (60x6) days in a year (with a few more days added on). Twelve also features prominently, with roughly 12 hours of day and 12 of night, and 12 months in a year (with 12 being 1/5 of 60). Sumer ( Sumerian: KI-EN-GIR, Land of the Lords of Brightness[1], or land of the Sumerian tongue[2][3], Akkadian: Å umeru; possibly Biblical Shinar ), located in southern Mesopotamia, is the earliest known civilization in the world. ... The sexagesimal (base-sixty) is a numeral system with sixty as the base. ...

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 is today the one in most common use around the world. For other uses, see Julius Caesar (disambiguation). ... For other uses, see Roman Empire (disambiguation). ... A solar calendar is a calendar whose dates indicate the position of the earth on its revolution around the sun (or equivalently the apparent position of the sun moving on the celestial sphere). ... The Julian calendar was a reform of the Roman calendar which was introduced by Julius Caesar in 46 BC and came into force in 45 BC (709 ab urbe condita). ... Intercalation is the insertioffn of an extra day, week or month into some calendar years to make the calendar follow the seasons. ... For other uses, see Solstice (disambiguation). ... For other uses, see Equinox (disambiguation). ... Pope Gregory XIII (January 7, 1502 â€“ April 10, 1585), born Ugo Boncompagni, was Pope from 1572 to 1585. ... For the calendar of religious holidays and periods, see liturgical year. ...

### History of time measurement devices

Horizontal sundial in Taganrog (1833)

A large variety of devices have been invented to measure time. The study of these devices is called horology. Image File history File links Sundial_Taganrog. ... Image File history File links Sundial_Taganrog. ... For other uses, see Sundial (disambiguation). ... Taganrog (Russian: , IPA: ) is a seaport city located on Taganrog Bay in Rostov Oblast, Russia. ... For other uses, see Clock (disambiguation). ... Captain Nemo and Professor Aronnax contemplating measuring instruments in Twenty Thousand Leagues Under the Sea A Love Meter at a Framingham, Massachusetts Rest Stop. ... Horology is the study of the science and art of timekeeping devices. ...

An Egyptian device dating 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 non-linear rule. The T was oriented eastward in the mornings. At noon, the device was turned around so that it could cast its shadow in the evening direction.[10] A T-square is a technical drawing instrument primarily a guide for drawing horizontal lines on a drafting table. ... Noon is the time exactly halfway through the day, written 12:00 in the 24-hour clock and 12:00 noon in the 12-hour clock. ...

A sundial uses a gnomon to cast a shadow on a set of markings which were calibrated to the hour. The position of the shadow marked the hour in local time. For other uses, see Sundial (disambiguation). ... The cantilever spar of this cable-stay bridge, the Sundial Bridge at Turtle Bay, forms the gnomon of a large garden sundial The gnomon is the part of a sundial that casts the shadow. ... The hour (symbol: h) is a unit of time. ... Time zones are areas of the Earth that have adopted the same standard time, usually referred to as the local time. ...

The most accurate timekeeping devices of the ancient world were the waterclock 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 timekeeping to replenish the flow of water. The Greeks and Chaldeans regularly maintained timekeeping records as an essential part of their astronomical observations. Arab engineers in particular made improvements on the use of waterclocks up to the Middle Ages.[11] A water clock or clepsydra is a device for measuring time by letting water regularly flow out of a container usually by a tiny aperture. ... Djeserkare Holy is the Soul of Re[1] Nomen Amenhotep Amun is Satisfied Horus name Kanaftau Bull who subdues the lands Nebty name Aaneru Who inspires great fear Golden Horus Uahrenput Enduring of years Consort(s) Ahmose-Meritamon Issue Amenemhat (died young), possibly Ahmes Father Ahmose I Mother Ahmose-Nefertari... Map showing the location of Tel Kaif, Iraq and the neighboring areas. ...

A contemporary quartz watch

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).[12] Image File history File linksMetadata Size of this preview: 800 Ã— 600 pixelsFull resolution (2848 Ã— 2136 pixel, file size: 1. ... Image File history File linksMetadata Size of this preview: 800 Ã— 600 pixelsFull resolution (2848 Ã— 2136 pixel, file size: 1. ... A quartz clock. ... For other uses, see Hourglass (disambiguation). ... For the Presidential railcar named Ferdinand Magellan, see Ferdinand Magellan Railcar. ...

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.[13][14] Richard of Wallingford (1292â€“1336) was an English mathematician active in the 14th century, who made major contributions to astronomy and horology whilst serving as the abbot of St Albans Abbey. ... For other uses, see Clock (disambiguation). ... A small orrery showing earth and the inner planets An orrery is a mechanical device that illustrates the relative positions and motions of the planets and moons in the solar system in heliocentric model. ...

The English word clock 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. The passage of the hours at sea were marked by bells, and denoted the time (see ship's bells). The hours were marked by bells in the abbeys as well as at sea. For other uses, see Clock (disambiguation). ... A bell is a simple sound-making device. ... Ships bells are a system to indicate the hour by means of bells, used aboard a ship to regulate the sailors duty watches. ...

A chip-scale atomic clock

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. Download high resolution version (1465x1449, 1419 KB) Wikipedia does not have an article with this exact name. ... Download high resolution version (1465x1449, 1419 KB) Wikipedia does not have an article with this exact name. ... For other uses, see Watch (disambiguation). ... The first prototype, on display at the Science Museum in London. ... For other uses, see Pendulum (disambiguation). ...

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. More recently, the term has also been applied to the chronometer watch, a wristwatch that meets precision standards set by the Swiss agency COSC. A chronometer is a timekeeper precise enough to be used as a portable time standard, usually in order to determine longitude by means of celestial navigation. ... A marine chronometer is a timekeeper precise enough to be used as a portable time standard, used to determine longitude by means of celestial navigation. ... Longitude is the east-west geographic coordinate measurement most commonly utilized in cartography and global navigation. ... For the episode of The West Wing, see Celestial Navigation (The West Wing). ... A chronometer watch is a watch tested and certified to meet certain precision standards. ... This page is about timekeeping devices. ... COSC a/k/a C.O.S.C. is the Official Swiss Chronometer Testing Institute. ...

The most accurate timekeeping devices are atomic clocks, which are accurate to seconds in many millions of years,[15] 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 that radiation which corresponds to the transition between two electron spin energy levels of the ground state of the 133Cs atom. Nuclear clock redirects here. ... General Name, Symbol, Number caesium, Cs, 55 Chemical series alkali metals Group, Period, Block 1, 6, s Appearance silvery gold Standard atomic weight 132. ... SI redirects here. ...

Today, the Global Positioning System in coordination with the Network Time Protocol can be used to synchronize timekeeping systems across the globe.
GPS redirects here. ... The Network Time Protocol (NTP) is a protocol for synchronizing the clocks of computer systems over packet-switched, variable-latency data networks. ...

## Definitions and standards

Common units of time
Unit Size Notes
picosecond 0.000 000 000 001 seconds no way of accurately measuring
nanosecond 0.000 000 001 seconds
microsecond 0.000 001 seconds
millisecond 0.001 seconds
second SI base unit
minute 60 seconds
hour 60 minutes
day 24 hours
week 7 days
fortnight 14 days 2 weeks
month 28 to 31 days
quarter 3 months
year 12 months
common year 365 days 52 weeks + 1 day
leap year 366 days 52 weeks + 2 days
tropical year 365.24219 days average
Gregorian year 365.2425 days average
Olympiad 4 year cycle
lustrum 5 years
Indiction 15 year cycle
generation 20 - 30 years approximate
century 100 years
millennium 1,000 years
See also: Time standard and Orders of magnitude (time)

The official SI definition of the second is as follows:[16][17]

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. General Name, Symbol, Number caesium, Cs, 55 Chemical series alkali metals Group, Period, Block 1, 6, s Appearance silvery gold Standard atomic weight 132. ...

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.[16] 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. A tropical year is the length of time that the Sun, as viewed from the Earth, takes to return to the same position along the ecliptic (its path among the stars on the celestial sphere). ... Ephemeris Time (ET) is a now obsolete time scale used in ephemerides of celestial bodies, in particular the Sun (as observed from the Earth), Moon, planets, and other members of the solar system. ...

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. This article is about the unit of length. ... Special relativity (SR) or the special theory of relativity is the physical theory published in 1905 by Albert Einstein. ... In special relativity and general relativity, time and three-dimensional space are treated together as a single four-dimensional pseudo-Riemannian manifold called spacetime. ... In physics and mathematics, Minkowski space (or Minkowski spacetime) is the mathematical setting in which Einsteins theory of special relativity is most conveniently formulated. ...

### World time

The measurement of time 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). This is the yardstick for other time scales, including Coordinated Universal Time (UTC), which is the basis for civil time. Nuclear clock redirects here. ... International Atomic Time (TAI, from the French name Temps Atomique International) is a high-precision atomic time standard that tracks proper time on Earths geoid. ... UTC redirects here. ...

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 Greenwich Mean Time. In many locations these offsets vary twice yearly due to daylight saving time transitions. Timezone and TimeZone redirect here. ... GMT redirects here. ... Although DST is common in Europe and North America, most of the worlds people do not use it. ...

### 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 rotation of the earth around the sun a sidereal day is slightly less than a solar day. Sidereal time is time measured by the apparent diurnal motion of the vernal equinox, which is very close to, but not identical with, the motion of stars. ...

### 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 be put into chronological groups (periodization). One of the most important systems of periodization is geologic time, 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. For the novel by Michael Crichton, see Timeline (novel). ... The past is the portion of the timeline that has already occurred; it is the opposite of the future. ... For the novel by Michael Crichton, see Timeline (novel). ... Periodization is the attempt to categorize or divide time into discrete named blocks. ... The table and timeline of geologic periods presented here is in accordance with the dates and nomenclature proposed by the International Commission on Stratigraphy. ... This article is about Earth as a planet. ... For other uses, see History (disambiguation). ...

Allegorical woodcut of Time, who "revealeth all things", guiding his daughter Truth away from the demon of Hypocrisy. John Byddell, 1535.

Image File history File links Size of this preview: 451 Ã— 600 pixelsFull resolution (2000 Ã— 2660 pixel, file size: 1. ... Image File history File links Size of this preview: 451 Ã— 600 pixelsFull resolution (2000 Ã— 2660 pixel, file size: 1. ...

## Time in religion and mythology

Further information: Category:Time and fate deities

In the Old Testament book Ecclesiastes, traditionally but not critically 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. (Another word, זמן zman, was current as meaning time fit for an event, and is used as the modern Hebrew equivalent to the English word "time".) Topics in Christianity Movements Â· Denominations Â· Other religions Ecumenism Â· Preaching Â· Prayer Music Â· Liturgy Â· Calendar Symbols Â· Art Â· Criticism Important figures Apostle Paul Â· Church Fathers Constantine Â· Athanasius Â· Augustine Anselm Â· Aquinas Â· Palamas Â· Luther Calvin Â· Wesley Arius Â· Marcion of Sinope Archbishop of Canterbury Â· Catholic Pope Coptic Pope Â· Ecumenical Patriarch Christianity Portal This box:      Note: Judaism... Ecclesiastes, Qohelet in Hebrew, is a book of the Hebrew Bible. ... This article is about the Biblical character . ... Predestination (also linked with foreknowledge) is a religious concept, which involves the relationship between the beginning of things and their destinies. ... Hebrew redirects here. ...

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. – Ecclesiastes 3:1–8 Image File history File links HinduMeasurements. ... Image File history File links HinduMeasurements. ... // The astronomical time cycles mentioned in ancient Hindu astronomical and Puranic texts are remarkably similar to each other. ... A logarithmic scale is a scale of measurement that uses the logarithm of a physical quantity instead of the quantity itself. ...

### Linear and cyclical time

See also: Time Cycles and Wheel of time

Ancient cultures such as Incan, Mayan, Hopi, and other Native American Tribes, plus the Babylonian, Ancient Greek, Hindu, Buddhist, Jainist, and others have a concept of a wheel of time, that regards time as cyclical and quantic consisting of repeating ages that happen to every being of the Universe between birth and extinction.
For other meanings of Inca, see Inca (disambiguation). ... The adjective Mayan is sometimes used to refer to the indigenous peoples of parts of Mexico and Central America, their culture, language, and history. ... Moki redirects here. ... Babylonia was an ancient state in Iraq), combining the territories of Sumer and Akkad. ... Beginning of Homers Odyssey The Ancient Greek language is the historical stage of the Greek language[1] as it existed during the Archaic (9thâ€“6th centuries BC) and Classical (5thâ€“4th centuries BC) periods in Ancient Greece. ... This article discusses the adherents of Hinduism. ... A replica of an ancient statue found among the ruins of a temple at Sarnath Buddhism is a philosophy based on the teachings of the Buddha, SiddhÄrtha Gautama, a prince of the Shakyas, whose lifetime is traditionally given as 566 to 486 BCE. It had subsequently been accepted by... The hand with a wheel on the palm symbolizes the Jain Vow of Ahinsa, meaning non-injury and nonviolence. ... Wheel of time may refer to: The Wheel of time or history, a religious concept predominant in Buddhism and Hinduism The Wheel of Time, a fantasy book series by author Robert Jordan The Wheel of Time (computer game), an action first-person shooter based on the series The Timewheel, a... Social cycle theories are one of the earliest social theories in sociology. ... Quantic may refer to: Quantic is another name for the mathematical concept of algebraic forms. ...

## Time in philosophy

The earliest recorded western philosophy of time was expounded by Ptahhotep, who lived c. 2650–2600 BC. He said: "Do not lessen the time of following desire, for the wasting of time is an abomination to the spirit."[citation needed] Philosophy of space and time is the branch of philosophy concerned with the issues surrounding the ontology, epistemology, and character of space and time. ... Ptahhotep, sometimes known as Ptahhotpe or Ptah-Hotep, is the name of a 24th century BC vizier and philosopher. ...

Ancient Greek philosophers, including Parmenides and Heraclitus, wrote essays on the nature of time.[18] Parmenides of Elea (Greek: , early 5th century BC) was an ancient Greek philosopher born in Elea, a Hellenic city on the southern coast of Italy. ... Heraclitus of Ephesus (Ancient Greek - HerÃ¡kleitos ho EphÃ©sios (Herakleitos the Ephesian)) (about 535 - 475 BC), known as The Obscure (Ancient Greek - ho SkoteinÃ³s), was a pre-Socratic Greek philosopher, a native of Ephesus on the coast of Asia Minor. ...

In Book 11 of St. Augustine's Confessions, he 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 settles on time being defined more by what it is not than what it is.[19] St. ... The word Confessions has several meanings: Confessions is a series of books composed by St. ...

Isaac Newton believed time and space form a container for events, which is as real as the objects it contains. Sir Isaac Newton FRS (4 January 1643 â€“ 31 March 1727) [ OS: 25 December 1642 â€“ 20 March 1727][1] was an English physicist, mathematician, astronomer, natural philosopher, and alchemist. ... This article is about the idea of space. ... In philosophy, an object is a thing, an entity, or a being. ...

Absolute, true, and mathematical time, in and of itself and of its own nature, without reference to anything external, flows uniformly and by another name is called duration. Relative, apparent, and common time is any sensible and external measure (precise or imprecise) of duration by means of motion; such a measure – for example, an hour, a day, a month, a year – is commonly used instead of true time.

Principia[20]

In contrast to Newton's belief in absolute space, and a precursor to Kantian time, Leibniz believed that time and space are relational.[21] The differences between Leibniz's and Newton's interpretations came to a head in the famous Leibniz-Clarke Correspondence. Leibniz 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. Leibniz redirects here. ... Two giants of late-17th and early-18th Century philosophy, Clarke (left) and Leibniz (right) did battle between 1715 and 1716 over the very natures of space and time themselves The Leibniz-Clarke Correspondence is a collection of letters exchanged between Gottfried Leibniz and Samuel Clarke between 1715 and 1716... An abstract structure is a set of laws, properties and relationships that is defined independently of any physical objects. ... This article is about the idea of space. ... For other uses, see Number (disambiguation). ... Quantity is a kind of property which exists as magnitude or multitude. ...

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.[22] 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. Spatial measurements are used to quantify how far apart objects are, and temporal measurements are used to quantify how far apart events occur. Kant redirects here. ... Title page of the 1781 edition. ... The terms a priori and a posteriori are used in philosophy to distinguish between two different types of propositional knowledge. ... This article is about the idea of space. ... Substance theory, or substance attribute theory, is an ontological theory about objecthood, positing that a substance is distinct from its properties. ... Look up Framework in Wiktionary, the free dictionary. ... Measurement is the estimation of the magnitude of some attribute of an object, such as its length or weight, relative to a unit of measurement. ... Quantity is a kind of property which exists as magnitude or multitude. ... In philosophy, an object is a thing, an entity, or a being. ... For other uses, see Phenomena (disambiguation). ...

In Existentialism, time is considered fundamental to the question of being,[citation needed] in particular by the philosopher Martin Heidegger.[citation needed] (See Ontology). Existentialism is a philosophical movement that posits that individuals create the meaning and essence of their lives, as opposed to deities or authorities creating it for them. ... In ontology, a being is anything that can be said to be, either transcendantly or immanently. ... Martin Heidegger (September 26, 1889 â€“ May 26, 1976) (IPA ) was a highly influential German philosopher. ... In philosophy, ontology (from the Greek , genitive : of being (part. ...

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.[23] Henri-Louis Bergson (October 18, 1859â€“January 4, 1941) was a major French philosopher, influential in the first half of the 20th century. ...

### Time as "unreal"

However, these arguments often center around what it means for something to be "real". Modern physicists generally consider time to be as "real" as space, though others such as Julian Barbour in his The End of Time argue that quantum equations of the universe take their true form when expressed in the timeless configuration spacerealm containing every possible "Now" or momentary configuration of the universe, which he terms 'platonia'.[26] (See also: Eternalism (philosophy of time).) Julian Barbour (born 1937) is a British physicist. ... The End of Time: The Next Revolution in Physics is a book by Julian Barbour published in 1999 that denies time exists as anything but an illusion. ... In mathematics, a function space is a set of functions of a given kind from a set X to a set Y. It is called a space because in most applications, it is a topological space or/and a vector space. ... Eternalism is a philosophical approach to the ontological nature of time. ...

## Time in the physical sciences

Main article: Time in physics

From the age of Newton up until Einstein's 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.[27] The science of classical mechanics is based on this Newtonian idea of time. To meet Wikipedias quality standards, this article or section may require cleanup. ... Sir Isaac Newton FRS (4 January 1643 â€“ 31 March 1727) [ OS: 25 December 1642 â€“ 20 March 1727][1] was an English physicist, mathematician, astronomer, natural philosopher, and alchemist. ... â€œEinsteinâ€ redirects here. ...

Einstein, in his special theory of relativity,[28] 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. For a generally accessible and less technical introduction to the topic, see Introduction to special relativity. ...

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 space) which in turn results in Lorentz transformation 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 for more details). Einstein redirects here. ... In physics and mathematics, Minkowski space (or Minkowski spacetime) is the mathematical setting in which Einsteins theory of special relativity is most conveniently formulated. ... This article is about the idea of space. ... In physics, the Lorentz transformation converts between two different observers measurements of space and time, where one observer is in constant motion with respect to the other. ... For a generally accessible and less technical introduction to the topic, see Introduction to special relativity. ...

Classical mechanics
$vec{F} = frac{mathrm{d}}{mathrm{d}t}(m vec{v})$
Newton's Second Law
History of ...
Fundamental concepts
Space · Time · Mass · Force
Energy · Momentum
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### Time in classical mechanics

In classical mechanics Newton's concept of "relative, apparent, and common time" can be used in the formulation of a prescription for the synchronization of clocks. Events seen by two different observers in motion relative to each other produce a mathematical concept of time that works pretty well for describing the everyday phenomena of most people's experience. Classical mechanics (commonly confused with Newtonian mechanics, which is a subfield thereof) is used for describing the motion of macroscopic objects, from projectiles to parts of machinery, as well as astronomical objects, such as spacecraft, planets, stars, and galaxies. ...

### Time in modern physics

In the late nineteenth century, physicists encountered problems with the classical understanding of time, in connection with the behavior of electricity and magnetism. Einstein resolved these problems by invoking a method of synchronizing clocks using the constant, finite speed of light as the maximum signal velocity. This led directly to the result that time appears to elapse at different rates relative to different observers in motion relative to one another.

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

For other uses of this term, see Spacetime (disambiguation). ... In special relativity, a light cone is the pattern describing the temporal evolution of a flash of light in Minkowski spacetime. ...

### Spacetime

Main article: Spacetime

Modern physics views the curvature of spacetime around an object as much a feature of that object as are its mass and volume.[citation needed] For other uses of this term, see Spacetime (disambiguation). ... A magnet levitating above a high-temperature superconductor demonstrates the Meissner effect. ... For other uses of this term, see Spacetime (disambiguation). ... For other uses, see Mass (disambiguation). ... For other uses, see Volume (disambiguation). ...

Time has historically been closely related with space, the two together comprising spacetime in Einstein's special relativity and general relativity. According to these theories, the concept of time depends on the spatial reference frame of the observer, and the human perception as well as the measurement by instruments such as clocks are different for observers in relative motion.[citation needed] Even the temporal order of events can change, but the past and future are defined by the backward and forward light cones, which never change.[citation needed] The past is the set of events that can send light signals to the observer, the future the events to which the observer can send light signals. All else is non-observable and within that set of events the very time-order differs for different observers.[citation needed] This article is about the idea of space. ... For other uses of this term, see Spacetime (disambiguation). ... â€œEinsteinâ€ redirects here. ... For a generally accessible and less technical introduction to the topic, see Introduction to special relativity. ... For a generally accessible and less technical introduction to the topic, see Introduction to general relativity. ... An inertial frame of reference, or inertial reference frame, is one in which Newtons first and second laws of motion are valid. ... In special relativity, a light cone is the pattern describing the temporal evolution of a flash of light in Minkowski spacetime. ... The past is the portion of the timeline that has already occurred; it is the opposite of the future. ... For other uses, see Future (disambiguation). ...

### Time dilation

Relativity of simultaneity: Event B is simultaneous with A in the green reference frame, but it occurred before in the blue frame, and will occur later in the red frame.
Main article: Time dilation

"Time is nature's way of keeping everything from happening at once". This quote, attributed variously to Einstein, John Archibald Wheeler, and Woody Allen, says that time is what separates cause and effect. Einstein showed that people traveling at different speeds, whilst agreeing on cause and effect, will measure different time separations between events and can even observe different chronological orderings between non-causally related events. Though these effects are minute unless one is traveling at a speed close to that of light, the effect becomes pronounced for objects moving at speeds approaching the speed of light. Many subatomic particles 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 is one example). According to the special theory of relativity, 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 seems 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. The relativity of simultaneity is the dependence of the notion of simultaneity on the observer. ... Time dilation is the phenomenon whereby an observer finds that anothers clock which is physically identical to their own is ticking at a slower rate as measured by their own clock. ... Einstein redirects here. ... John Archibald Wheeler (July 9, 1911â€“April 13, 2008) was an eminent American theoretical physicist. ... Woody Allen (born Allen Stewart Konigsberg; December 1, 1935) is a three-time Academy Award-winning American film director, writer, actor, jazz musician, comedian and playwright. ... Causality describes the relationship between causes and effects, and is fundamental to all natural science, especially physics. ... Helium atom (schematic) Showing two protons (red), two neutrons (green) and two electrons (yellow). ... The muon (from the letter mu (Î¼)--used to represent it) is an elementary particle with negative electric charge and a spin of 1/2. ... For a generally accessible and less technical introduction to the topic, see Introduction to special relativity. ... In physics, an inertial frame of reference, or inertial frame for short (also descibed as absolute frame of reference), is a frame of reference in which the observers move without the influence of any accelerating or decelerating force. ... Given an assembly of elements, the number of which decreases ultimately to zero, the lifetime (also called the mean lifetime) is a certain number that characterizes the rate of reduction (decay) of the assembly. ...

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." For other uses of this term, see Spacetime (disambiguation). ...

Einstein wrote in his book, Relativity, that simultaneity is also relative, i.e., two events that appear simultaneous to an observer in a particular inertial reference frame need not be judged as simultaneous by a second observer in a different inertial frame of reference. The relativity of simultaneity is the dependence of the notion of simultaneity on the observer. ...

### Relativistic time versus Newtonian time

Views of spacetime along the world line of a rapidly accelerating observer in a Newtonian universe. The events ("dots") that pass the horizontal line are the events current to the observer.
Views of spacetime along the world line of a rapidly accelerating observer in a relativistic universe. The events ("dots") that pass the two diagonal lines in the bottom half of the image (the past light cone of the observer in the origin) are the events visible to the observer.

The animations on the left and the right visualise the different treatments of time in the Newtonian and the relativistic descriptions. At heart of these differences are the Galilean and Lorentz transformations applicable in the Newtonian and relativistic theories, respectively. Image File history File links No higher resolution available. ... In physics, the world line of an object is the unique path of that object as it travels through 4-dimensional spacetime. ... Image File history File links Source of program used to generate image: //GPL #include <stdio. ... In physics, the world line of an object is the unique path of that object as it travels through 4-dimensional spacetime. ... In special relativity, a light cone is the pattern describing the temporal evolution of a flash of light in Minkowski spacetime. ... 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. ... In physics, the Lorentz transformation converts between two different observers measurements of space and time, where one observer is in constant motion with respect to the other. ...

In both figures, the vertical direction indicates time. The horizontal direction indicates distance (only one spatial dimension is taken into account), and the thick dashed curve is the spacetime trajectory ("world line") of the observer. The small dots indicate specific (past and future) events in spacetime. For other uses of this term, see Spacetime (disambiguation). ... In physics, the world line of an object is the unique path of that object as it travels through 4-dimensional 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 influences whether an event passes the light cone of the observer. Notice that with the change from a Newtonian to a relativistic description, the concept of absolute time is no longer applicable: events move up-and-down in the figure depending on the acceleration of the observer.

### Arrow of time

Main article: Arrow of time

### Quantised time

Time quantization is a hypothetical concept. In the modern established physical theories (the Standard Model of Particles and Interactions and General Relativity) time is not quantized. A chronon is a hypothetical concept in theoretical physics intended to describe a quantum of time. ... The Standard Model of Fundamental Particles and Interactions For the Standard Model in Cryptography, see Standard Model (cryptography). ... For a generally accessible and less technical introduction to the topic, see Introduction to general relativity. ...

Planck time (~ 5.4 × 10−44 seconds) is the unit of time in the system of natural units known as Planck units. Current established physical theories are believed to fail at this time scale, and many physicists expect that the Planck time might be the smallest unit of time that could ever be measured, even in principle. Tentative physical theories that describe this time scale exist; see for instance loop quantum gravity. In physics, the Planck time (tP), is the unit of time in the system of natural units known as Planck units. ... To help compare different orders of magnitudes this page lists times between 10-44s and 10-43s. ... In physics, natural units are physical units of measurement defined in terms of universal physical constants in such a manner that some chosen physical constants take on the numerical value of one when expressed in terms of a particular set of natural units. ... In physics, Planck units are one of several systems of natural units, units of measurement that normalize certain fundamental physical constants to 1. ... Loop quantum gravity (LQG), also known as loop gravity and quantum geometry, is a proposed quantum theory of spacetime which attempts to reconcile the seemingly incompatible theories of quantum mechanics and general relativity. ...

## Time and the Big Bang

Stephen Hawking in particular has addressed a connection between time and the Big Bang. He has sometimes stated that we may as well assume that time began with the Big Bang because trying to answer any question about what happened before the Big Bang is trying to answer a question that is meaningless as those events would have been part of a different time frame and different universe outside of the scope of the Big Bang theory.[29][30][31] Stephen William Hawking, CH, CBE, FRS, FRSA, (born 8 January 1942) is a British theoretical physicist. ... For other uses, see Big Bang (disambiguation). ...

Aristotelian philosopher Mortimer J. Adler,[32][33] has criticized some expositions that Hawking has given stating that time didn't exist before the big bang. This article needs cleanup. ... Mortimer Jerome Adler (December 28, 1902 – June 28, 2001) was an American philosopher and author. ...

Hawking, in A Brief History of Time and elsewhere, along with several other modern physicists, has stated his position more clearly and less controversially: 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.[34] A Brief History of Time is a popular science book written by Professor Stephen Hawking and first published in 1988. ...

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 (5 × 10−44 seconds) after the Big Bang will likely remain pure speculation. In physics, the Planck time (tP), is the unit of time in the system of natural units known as Planck units. ...

### Speculative physics beyond the Big Bang

A graphical representation of the expansion of the universe with the inflationary epoch represented as the dramatic expansion of the metric seen on the left. Image from WMAP press release, 2006.

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 require the existence of a singularity at the beginning of cosmic time. However, these theorems assume that general relativity is correct, but general relativity must break down before the universe reaches the Planck temperature, and a correct treatment of quantum gravity may avoid the singularity.[35] Image File history File links Size of this preview: 800 Ã— 576 pixelsFull resolution (3000 Ã— 2159 pixel, file size: 1. ... Image File history File links Size of this preview: 800 Ã— 576 pixelsFull resolution (3000 Ã— 2159 pixel, file size: 1. ... In mathematics, in Riemannian geometry, the metric tensor is a tensor of rank 2 that is used to measure distance and angle in a space. ... Artist depiction of the WMAP satellite at the L2 point The Wilkinson Microwave Anisotropy Probe (WMAP) is a NASA satellite whose mission is to survey the sky to measure the temperature of the radiant heat left over from the Big Bang. ... This article or section is in need of attention from an expert on the subject. ... For a generally accessible and less technical introduction to the topic, see Introduction to general relativity. ... The Planck temperature, named after German physicist Max Planck, is the natural unit of temperature, denoted by TP. The Planck units, in general, represent limits of quantum mechanics. ... Quantum gravity is the field of theoretical physics attempting to unify quantum mechanics, which describes three of the fundamental forces of nature, with general relativity, the theory of the fourth fundamental force: gravity. ...

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.[36]
• brane cosmology models[37] in which inflation is due to the movement of branes in string theory; the pre-big bang model; the ekpyrotic model, in which the Big Bang is the result of a collision between branes; and the cyclic model, a variant of the ekpyrotic model in which collisions occur periodically.[38][39][40]
• 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.[41]

Proposals in the last two categories see the Big Bang as an event in a much larger and older universe, or multiverse, and not the literal beginning. In theoretical physics, the Hartle-Hawking state, named after James Hartle and Stephen Hawking, is a hypothetical vector in the Hilbert space of a theory of quantum gravity that describes the wave function of the Universe. ... Brane cosmology is a protoscience motivated by, but not rigorously derived from, superstring theory and M-theory. ... This box:      String theory is a still developing mathematical approach to theoretical physics, whose original building blocks are one-dimensional extended objects called strings. ... The ekpyrotic universe or ekpyrotic scenario is a cosmological theory of the origin of the universe. ... The cyclic model is a brane cosmology model of the creation of the universe, derived from the earlier ekpyrotic model. ... The theory created by Andrei Linde that states that our universe could have come from nothing more than hundred-thousandth of a gram of matter. ... For other uses, see Multiverse (disambiguation). ...

## Time travel

Main article: Time travel

A central problem with time travel to the past is the violation of causality; should an effect precede its cause, it would give rise to the possibility of temporal paradox. Some interpretations of time travel resolve this by accepting the possibility of travel between parallel realities or universes. Causality or causation denotes the relationship between one event (called cause) and another event (called effect) which is the consequence (result) of the first. ... A temporal paradox is an impossible situation in which a time traveler interferes with the timeline involved in his own existence. ... A multiverse (or meta-universe) is the hypothetical set of multiple possible universes (including our universe) that together comprise all of physical reality. ... For other uses, see Universe (disambiguation). ...

## Perception of time

### Time in psychology

See also: Mental chronometry and Sense of time

Even in the presence of timepieces, different individuals may judge an identical length of time to be passing at different rates.[citation needed] Commonly, this is referred to as time seeming to "fly" (a period of time seeming to pass faster than possible) or time seeming to "drag" (a period of time seeming to pass slower than possible). The psychologist Jean Piaget called this form of time perception "lived time."[citation needed] // Definition and history Psychologists have investigated mental chronometry for over 100 years. ... Although the sense of time is not associated with a specific sensory system, the work of psychologists and neuroscientists indicates that our brains do have a system governing the perception of time. ... Jean Piaget (August 9, 1896 â€“ September 16, 1980) was a Swiss philosopher, natural scientist and developmental psychologist, well known for his work studying children, his theory of cognitive development and for his epistemological view called genetic epistemology. He created in 1955 the International Centre for Genetic Epistemology in Geneva and...

This common experience was used to familiarize the general public to the ideas presented by Einstein's theory of relativity in a 1930 cartoon by Sidney "George" Strube:[42][43] Einstein redirects here. ...

Man: Well, it's like this,—supposing I were to sit next to a pretty girl for half an hour it would seem like half a minute,—
Einstein: Braffo! You the idea haf! [sic]
Man: But if I were to sit on a hot stove for two seconds then it would seem like two hours. For other uses, see SIC. Sic is a Latin word meaning thus, so, as such, or just as that. In writing, it is placed within square brackets and usually italicizedâ€”[sic]â€”to indicate that an incorrect or unusual spelling, phrase, punctuation, and/or other preceding quoted material has been reproduced...

A form of temporal illusion verifiable by experiment is the kappa effect,[44] whereby time intervals between visual events are perceived as relatively longer or shorter depending on the relative spatial positions of the events. In other words: the perception of temporal intervals appears to be directly affected, in these cases, by the perception of spatial intervals. This article is considered orphaned, since there are very few or no other articles that link to this one. ...

Time also appears to pass more quickly as one gets older. Stephen Hawking suggests that the perception of time is a ratio: Unit of Time : Time Lived.[citation needed] For example, one hour to a six-month-old person would be approximately "1:4032", while one hour to a 40-year-old would be "1:349,440". Therefore an hour appears much longer to a young child than to an aged adult, even though the measure of time is the same. Stephen William Hawking, CH, CBE, FRS, FRSA, (born 8 January 1942) is a British theoretical physicist. ...

### Time in altered states of consciousness

Altered states of consciousness are sometimes characterized by a different estimation of time. Some psychoactive substances – such as entheogens – may also dramatically alter a person's temporal judgement. When viewed under the influence of such substances as LSD, psychedelic mushrooms and peyote, a clock may appear to be a strange reference point and a useless tool for measuring the passage of events as it does not correlate with the user's experience. At higher doses, time may appear to slow down, stop, speed up, go backwards and even seem out of sequence. A typical thought might be "I can't believe it's only 8 o'clock, but then again, what does 8 o'clock mean?" As the boundaries for experiencing time are removed, so is its relevance. Many users claim this unbounded timelessness feels like a glimpse into spiritual infinity. To imagine that one exists somewhere "outside" of time is one of the hallmark experiences of a psychedelic voyage.[citation needed] Marijuana, a milder psychedelic, may also distort the perception of time to a lesser degree.[45] This entry covers entheogens as psychoactive substances used in a religious or shamanic context. ... Lysergic acid diethylamide, commonly called LSD, LSD-25, or acid. ... Magic mushrooms are also known as sacred mushrooms, psychedelic mushrooms, and, more generally, hallucinogenic mushrooms. ... Binomial name (Lem. ... Cannabis, also known as marijuana[1] or ganja (Hindi: à¤—à¤¾à¤‚à¤œà¤¾),[2] is a psychoactive product of the plant Cannabis sativa. ...

The practice of meditation, central to all Buddhist traditions, takes as its goal the reflection of the mind back upon itself, thus altering the subjective experience of time; the so called, 'entering the now', or 'the moment'.[citation needed] For other senses of this word, see Meditation (disambiguation). ...

### Culture

Culture is another variable contributing to the perception of time. Anthropologist Benjamin Lee Whorf reported after studying the Hopi cultures that: "… the Hopi language is seen to contain no words, grammatical forms, construction or expressions or that refer directly to what we call “time”, or to past, present, or future…"[46] Whorf's assertion has been challenged and modified. Pinker debunks Whorf's claims about time in the Hopi language, pointing out that the anthropologist Malotki (1983) has found that the Hopi do have a concept of time very similar to that of other cultures; they have units of time, and a sophisticated calendar.[47] Benjamin Lee Whorf (April 24, 1897 - July 26, 1941) was an American linguist. ... Moki redirects here. ...

## Use of time

See also: Time management and 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 customs by others. Time management is straightforwardly defined as the management of time in order to make the most out of it. ... 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 peoples expectations concerning the observance of these customs by others. ... Sociology (from Latin: socius, companion; and the suffix -ology, the study of, from Greek Î»ÏŒÎ³Î¿Ï‚, lÃ³gos, knowledge [1]) is the scientific or systematic study of society, including patterns of social relationships, social interaction, and culture[2]. Areas studied in sociology can range from the analysis of brief contacts between anonymous... This article is about the social science. ... 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 peoples expectations concerning the observance of these customs by others. ... Young people interacting within an ethnically diverse society. ... Face-to-face trading interactions on the New York Stock Exchange trading floor. ...

The use of time is an important issue in understanding human behaviour, education, and travel behaviour. Time use research is a developing field of study. The question concerns how time is allocated across a number of activities (such as time spent at home, at work, shopping, etc.). Time use changes with technology, as the television or the Internet created new opportunities to use time in different ways. However, some aspects of time use are relatively stable over long periods of time, such as the amount of time spent traveling to work, which despite major changes in transport, has been observed to be about 20-30 minutes one-way for a large number of cities over a long period of time. This has led to the disputed time budget hypothesis. Human Behaviour is Icelandic singer BjÃ¶rks first solo single, taken from the album Debut. ... Travel behavior is the study of what people do over space, and how people use transport. ... Time use research is a developing interdisciplinary field of study dedicated to knowing how people allocate their time during an average day. ... By the mid 20th century humans had achieved a mastery of technology sufficient to leave the surface of the Earth for the first time and explore space. ...

Time management 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. Time management is straightforwardly defined as the management of time in order to make the most out of it. ...

Arlie Russell Hochschild and Norbert Elias have written on the use of time from a sociological perspective. Arlie Russell Hochschild is professor of sociology at the University of California-Berkeley. ... Norbert Elias (born June 22, 1897 in Breslau, Germany (now WrocÅ‚aw, Poland); died August 1, 1990 in Amsterdam) was a German sociologist of Jewish descent, who later became a British citizen. ...

Time's mortal aspect is personified in this bronze statue by Charles van der Stappen
 Time portal
See the Time navigation templates below for an exhaustive list of related articles.

Charles van der Stappen (September , 1843 - 1910), Belgian sculptor, was born in Brussels. ... Image File history File links Download high resolution version (800x697, 123 KB) fr:: Montre gousset cs:: KapesnÃ­ hodinky de: Deutsch: Taschenuhr en: English: Pocket watch it: Italiano: Orologio da taschino (cipolla) es: EspaÃ±ol: Reloj de bolsillo Template:àª—à«àªœàª°àª¾àª¤à«€ àª—à«àªœàª°àª¾àª¤à«€: àª–àª¿àª¸à«àª¸àª¾àª®àª¾àª‚ àª°àª¾àª–àªµàª¾àª¨à«€ àª˜àª¡àª¿àª¯àª¾àª³ ja: æ—¥æœ¬èªž: æ‡ä¸­æ™‚è¨ˆ pl: Polski: Zegarek kieszonkowy pt: PortuguÃªs: RelÃ³gio de bolso...

### Books

A Brief History of Time is a popular science book written by Professor Stephen Hawking and first published in 1988. ... About Time (ISBN 0-68-481822-1) is the second book written by Paul Davies, regarding the subject of time. ... An Experiment with Time, by J.W. Dunne, was published first in March of 1927. ...

### Organizations

Leading scholarly organizations for researchers on the history and technology of time and timekeeping

### Miscellaneous units of time

Antiquatian Horlogical Society ,abreviated as AHS, is the british organization of scholars and enthusiasts of horology. ... This article, image, template or category should belong in one or more categories. ... The National Association of Watch and Clock Collectors (NAWCC) is an American non-profit organization with over 38,000 members. ... An anachronism (from Greek ana, back, and chronos, time) is an artifact that belongs to another time, a person who seems to be displaced in time (i. ... Wikiquote has a collection of quotations related to: Change For other uses, see Change (disambiguation). ... // Different style conventions and habits exist around the world for dates and times in writing and speaking. ... Below is a listing of cycles. ... The Network Time Protocol (NTP) is a protocol for synchronizing the clocks of computer systems over packet-switched, variable-latency data networks. ... In the arts, the word nonlinear is used to describe events portrayed in a non-chronological manner. ... Philosophy of physics is the study of the fundamental, philosophical questions underlying modern physics, the study of matter and energy and how they interact. ... Half-Life For a quantity subject to exponential decay, the half-life is the time required for the quantity to fall to half of its initial value. ... Hexadecimal time is the representation of the time of day as a hexadecimal number in the interval [0,1] or [0,2], regulary multipied by a multiple of 16. ... This article or section is in need of attention from an expert on the subject. ... In vedic timekeeping, a tithi (also spelled thithi) 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. ... Unix time, or POSIX time, is a system for describing points in time. ...

## Notes and references

1. ^ a b Rudgley, Richard (1999). The Lost Civilizations of the Stone Age. New York: Simon & Schuster, 86-105.
2. ^ Duff, Michael J.; Okun, Lev B.; Veneziano, Gabriele (March 2002). "Trialogue on the number of fundamental constants" (PDF). Institute of Physics Publishing for SISSA/ISAS. Retrieved on 2008-02-02. p. 17. "I only add to this the observation that relativity and quantum mechanics provide, in string theory, units of length and time which look, at present, more fundamental than any other."
3. ^ Duff, Okun, Veneziano, ibid. p. 3. "There is no well established terminology for the fundamental constants of Nature. … The absence of accurately defined terms or the uses (i.e. actually misuses) of ill-defined terms lead to confusion and proliferation of wrong statements."
4. ^ Rynasiewicz, Robert : Johns Hopkins University (2004-08-12). Newton's Views on Space, Time, and Motion. Stanford Encyclopedia of Philosophy. Stanford University. Retrieved on 2008-01-10. “Newton did not regard space and time as genuine substances (as are, paradigmatically, bodies and minds), but rather as real entities with their own manner of existence as necessitated by God's existence... To paraphrase: Absolute, true, and mathematical time, from its own nature, passes equably without relation the [sic~to] anything external, and thus without reference to any change or way of measuring of time (e.g., the hour, day, month, or year).”
5. ^ Markosian, Ned "Time". The Stanford Encyclopedia of Philosophy (Winter 2002 Edition). Ed. Edward N. Zalta. “The opposing view, normally referred to either as “Platonism with Respect to Time” or as “Absolutism with Respect to Time,” has been defended by Plato, Newton, and others. On this view, time is like an empty container into which events may be placed; but it is a container that exists independently of whether or not anything is placed in it.”
6. ^ Burnham, Douglas : Staffordshire University (2006). Gottfried Wilhelm Leibniz (1646-1716) Metaphysics - 7. Space, Time, and Indiscernibles. The Internet Encyclopedia of Philosophy. Retrieved on 2008-01-10. “First of all, Leibniz finds the idea that space and time might be substances or substance-like absurd (see, for example, "Correspondence with Clarke," Leibniz's Fourth Paper, §8ff). In short, an empty space would be a substance with no properties; it will be a substance that even God cannot modify or destroy.... That is, space and time are internal or intrinsic features of the complete concepts of things, not extrinsic.... Leibniz's view has two major implications. First, there is no absolute location in either space or time; location is always the situation of an object or event relative to other objects and events. Second, space and time are not in themselves real (that is, not substances). Space and time are, rather, ideal. Space and time are just metaphysically illegitimate ways of perceiving certain virtual relations between substances. They are phenomena or, strictly speaking, illusions (although they are illusions that are well-founded upon the internal properties of substances).... It is sometimes convenient to think of space and time as something "out there," over and above the entities and their relations to each other, but this convenience must not be confused with reality. Space is nothing but the order of co-existent objects; time nothing but the order of successive events. This is usually called a relational theory of space and time.”
7. ^ Mattey, G. J. : UC Davis (1997-01-22). Critique of Pure Reason, Lecture notes: Philosophy 175 UC Davis. Retrieved on 2008-01-10. “What is correct in the Leibnizian view was its anti-metaphysical stance. Space and time do not exist in and of themselves, but in some sense are the product of the way we represent things. The are ideal, though not in the sense in which Leibniz thought they are ideal (figments of the imagination). The ideality of space is its mind-dependence: it is only a condition of sensibility.... Kant concluded "absolute space is not an object of outer sensation; it is rather a fundamental concept which first of all makes possible all such outer sensation."...Much of the argumentation pertaining to space is applicable, mutatis mutandis, to time, so I will not rehearse the arguments. As space is the form of outer intuition, so time is the form of inner intuition.... Kant claimed that time is real, it is "the real form of inner intuition."”
8. ^ McCormick, Matt : California State University, Sacramento (2006). Immanuel Kant (1724-1804) Metaphysics : 4. Kant's Transcendental Idealism. The Internet Encyclopedia of Philosophy. Retrieved on 2008-01-10. “Time, Kant argues, is also necessary as a form or condition of our intuitions of objects. The idea of time itself cannot be gathered from experience because succession and simultaneity of objects, the phenomena that would indicate the passage of time, would be impossible to represent if we did not already possess the capacity to represent objects in time.... Another way to put the point is to say that the fact that the mind of the knower makes the a priori contribution does not mean that space and time or the categories are mere figments of the imagination. Kant is an empirical realist about the world we experience; we can know objects as they appear to us. He gives a robust defense of science and the study of the natural world from his argument about the mind's role in making nature. All discursive, rational beings must conceive of the physical world as spatially and temporally unified, he argues.”
9. ^ Richards, E. G. (1998). Mapping Time: The Calendar and its History. Oxford University Press, 3-5.
10. ^ Barnett, Jo Ellen Time's Pendulum: The Quest to Capture Time - from Sundials to Atomic Clocks Plenum, 1998 ISBN 0-306-45787-3 p.28
11. ^ Barnett, ibid, p.37
12. ^ Laurence Bergreen, Over the Edge of the World: Magellan's Terrifying Circumnavigation of the Globe, HarperCollins Publishers, 2003, hardcover 480 pages, ISBN 0-06-621173-5
13. ^ North, J. (2004) God's Clockmaker: Richard of Wallingford and the Invention of Time. Oxbow Books. ISBN 1-85285-451-0
14. ^ Watson, E (1979) "The St Albans Clock of Richard of Wallingford". Antiquarian Horology 372-384.
15. ^ "New atomic clock can keep time for 200 million years: Super-precise instruments vital to deep space navigation", Vancouver Sun, 2008-02-16. Retrieved on 2008-02-16.
16. ^ a b c Organisation Intergouvernementale de la Convention du Métre (1998). The International System of Units (SI), 7th Edition (PDF). Retrieved on 2006-06-13.
17. ^ Base unit definitions: Second. NIST. Retrieved on 2008-01-09.
18. ^ Dagobert Runes, Dictionary of Philosophy, p. 318
19. ^ St. Augustine, Confessions, Book 11. http://ccat.sas.upenn.edu/jod/augustine/Pusey/book11 (Accessed 5/26/07).
20. ^ Newton, Isaac (1726). The Principia, 3rd edition.  Translated by I. Bernard Cohen and Anne Whitman, University of California Press, Berkeley, 1999.
21. ^ Gottfried Martin, Kant's Metaphysics and Theory of Science
22. ^ Kant, Immanuel (1787). The Critique of Pure Reason, 2nd edition.  translated by J. M. D. Meiklejohn, [email protected], 2004 - http://ebooks.adelaide.edu.au/k/kant/immanuel/k16p/k16p15.html
23. ^ Bergson, Henri (1907) Creative Evolution. trans. by Arthur Mitchell. Mineola: Dover, 1998.
24. ^ Harry Foundalis. You are about to disappear. Retrieved on 2007-04-27.
25. ^ Tom Huston. Buddhism and the illusion of time. Retrieved on 2007-04-27.
26. ^ Time is an illusion?. Retrieved on 2007-04-27.
27. ^ Herman M. Schwartz, Introduction to Special Relativity, McGraw-Hill Book Company, 1968, hardcover 442 pages, see ISBN 0882754785 (1977 edition), pp. 10-13
28. ^ A. Einstein, H. A. Lorentz, H. Weyl, H. Minkowski, The Principle of Relativity, Dover Publications, Inc, 2000, softcover 216 pages, ISBN 0486600815, See pp. 37-65 for an English translation of Einstein's original 1905 paper.
29. ^ Hawking, Stephen. The Beginning of Time. University of Cambridge. Retrieved on 2008-01-10. “The conclusion of this lecture is that the universe has not existed forever. Rather, the universe, and time itself, had a beginning in the Big Bang, about 15 billion years ago.”
30. ^ Hawking, Stephen (2006-02-27). Professor Stephen Hawking lectures on the origin of the universe. University of Oxford. Retrieved on 2008-01-10. “Suppose the beginning of the universe was like the South Pole of the earth, with degrees of latitude playing the role of time. The universe would start as a point at the South Pole. As one moves north, the circles of constant latitude, representing the size of the universe, would expand. To ask what happened before the beginning of the universe would become a meaningless question because there is nothing south of the South Pole.'”
31. ^ Ghandchi, Sam : Editor/Publisher (2004-01-16). Space and New Thinking. Retrieved on 2008-01-10. “and as Stephen Hawking puts it, asking what was before Big Bang is like asking what is North of North Pole, a meaningless question.”
32. ^ Adler, Mortimer J., Ph.D.. Natural Theology, Chance, and God. Retrieved on 2008-01-10. “Hawking could have avoided the error of supposing that time had a beginning with the Big Bang if he had distinguished time as it is measured by physicists from time that is not measurable by physicists.... an error shared by many other great physicists in the twentieth century, the error of saying that what cannot be measured by physicists does not exist in reality.” "The Great Ideas Today". Encylopaedia Britannica. (1992).
33. ^ Adler, Mortimer J., Ph.D.. Natural Theology, Chance, and God. Retrieved on 2008-01-10. “Where Einstein had said that what is not measurable by physicists is of no interest to them, Hawking flatly asserts that what is not measurable by physicists does not exist -- has no reality whatsoever.
With respect to time, that amounts to the denial of psychological time which is not measurable by physicists, and also to everlasting time -- time before the Big Bang -- which physics cannot measure. Hawking does not know that both Aquinas and Kant had shown that we cannot rationally establish that time is either finite or infinite.” "The Great Ideas Today". Encylopaedia Britannica. (1992).
34. ^ Hawking, Stephen. The Beginning of Time. University of Cambridge. Retrieved on 2008-01-10. “Since events before the Big Bang have no observational consequences, one may as well cut them out of the theory, and say that time began at the Big Bang. Events before the Big Bang, are simply not defined, because there's no way one could measure what happened at them. This kind of beginning to the universe, and of time itself, is very different to the beginnings that had been considered earlier.”
35. ^ Hawking, Stephen; and Ellis, G. F. R. (1973). The Large Scale Structure of Space-Time. Cambridge: Cambridge University Press. ISBN 0-521-09906-4.
36. ^ J. Hartle and S. W. Hawking (1983). "Wave function of the universe". Phys. Rev. D 28: 2960. doi:10.1103/PhysRevD.28.2960.
37. ^ Langlois, David (2002). "Brane cosmology: an introduction". arΧiv:hep-th/0209261.
38. ^ Linde, Andre (2002). "Inflationary Theory versus Ekpyrotic/Cyclic Scenario". arΧiv:hep-th/0205259.
39. ^ "Recycled Universe: Theory Could Solve Cosmic Mystery", Space.com, 8 May 2006. Retrieved on 2007-07-03.
40. ^ What Happened Before the Big Bang?. Retrieved on 2007-07-03.
41. ^ A. Linde (1986). "Eternal chaotic inflation". Mod. Phys. Lett. A1: 81.
A. Linde (1986). "Eternally existing self-reproducing chaotic inflationary universe". Phys. Lett. B175: 395–400.
42. ^ Priestley, J. B. (1964). Man and Time. New York: Crescent Books, 96.
43. ^ Sunrise (2008). Unified Field Theory: A new interpretation. Chapter 2 - The Development of the Unified Field Theory, pg. 31. Sunrise Information Services.
44. ^ Wada Y, Masuda T, Noguchi K, 2005, "Temporal illusion called 'kappa effect' in event perception" Perception 34 ECVP Abstract Supplement
45. ^ Cannabis Effects. Erowid. Retrieved on 2008-02-15. “Time sense altered: cars seem like they are moving too fast, time dilation and compression are common at higher doses.”
46. ^ Carroll, John B. (ed.)(1956). Language Thought and Reality. Selected Writings of Benjamin Lee Whorf. MIT Press, Boston, Massachusetts. ISBN 0262730065 9780262730068
47. ^ Parr-Davies, Neil (April 2001), The Sapir-Whorf Hypothesis: A Critique, Aberystwyth University. Retrieved on

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Year 2004 (MMIV) was a leap year starting on Thursday of the Gregorian calendar. ... is the 16th day of the year in the Gregorian calendar. ... 2008 (MMVIII) is the current year, a leap year that started on Tuesday of the Common Era (or Anno Domini), in accordance with the Gregorian calendar. ... is the 10th day of the year in the Gregorian calendar. ... 2008 (MMVIII) is the current year, a leap year that started on Tuesday of the Common Era (or Anno Domini), in accordance with the Gregorian calendar. ... is the 10th day of the year in the Gregorian calendar. ... 2008 (MMVIII) is the current year, a leap year that started on Tuesday of the Common Era (or Anno Domini), in accordance with the Gregorian calendar. ... is the 10th day of the year in the Gregorian calendar. ... 2008 (MMVIII) is the current year, a leap year that started on Tuesday of the Common Era (or Anno Domini), in accordance with the Gregorian calendar. ... is the 10th day of the year in the Gregorian calendar. ... 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• Barbour, Julian (1999). The End of Time: The Next Revolution in Physics. ISBN 0-19-514592-5.
• Das, Tushar Kanti (1990). The Time Dimension: An Interdisciplinary Guide. New York: Praeger. ISBN 0275926818. - Research bibliography
• Davies, Paul (1996). About Time: Einstein's Unfinished Revolution. ISBN 0-684-81822-1.
• Feynman, Richard [1965] (1994). The Character of Physical Law. Cambridge (Mass): The MIT Press, 108-126. ISBN 0-262-56003-8.
• Galison, Peter (1992). Einstein's Clocks and Poincaré's Maps: Empires of Time. New York: W. W. Norton. ISBN 0-393-02001-0.
• Highfield, Roger (1992). Arrow of Time: A Voyage through Science to Solve Time's Greatest Mystery. Random House. ISBN 0-449-90723-6.
• Mermin, N. David (2005). It's About Time: Understanding Einstein's Relativity. Princeton University Press. ISBN 0-691-12201-6.
• Penrose, Roger [1989] (1999). The Emperor's New Mind: Concerning Computers, Minds, and the Laws of Physics. New York: Oxford University Press, 391-417. ISBN 0-19-286198-0.
• Price, Huw (1996). Time's Arrow and Archimedes' Point. Oxford University Press. ISBN 0-19-511798-0.
• Reichenbach, Hans [1956] (1999). The Direction of Time. New York: Dover. ISBN 0-486-40926-0.
• Stiegler, Bernard, Technics and Time, 1: The Fault of Epimetheus
• Whitrow, Gerald J. (1973). The Nature of Time. Holt, Rinehart and Wilson (New York).
• Whitrow, Gerald J. (1980). The Natural Philosophy of Time. Clarendon Press (Oxford).
• Whitrow, Gerald J. (1988). Time in History. The evolution of our general awareness of time and temporal perspective. Oxford University Press. ISBN 0-19-285211-6.
• Rovelli, Carlo (2006). What is time? What is space?. Rome: Di Renzo Editore. ISBN 8883231465.

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 The World Clock - Time Zones (137 words) UTC is Coordinated Universal Time, GMT is Greenwich Mean Time. Time Zone Converter - Convert time between two time zones. Fixed time clock - convert between many time zones - in past or future
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