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Time
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For other uses, see Time (disambiguation).
Sunrise shown in time lapse. The perceived 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.

Time is a component of the measuring system used to sequence events, to compare the durations of events and the intervals between them, and to quantify the motions of objects. Time has been a major subject of religion, philosophy, and science, but defining time in a non-controversial manner applicable to all fields of study has consistently eluded the greatest scholars.[2]
“

The Moving Finger writes: and, having writ,
Moves on: nor all thy Piety nor Wit,
Shall lure it back to cancel half a Line,
Nor all thy Tears wash out a Word of it. … Omar Khayam
”

In physics and other sciences, time is considered one of the few fundamental quantities.[3] Time is used to define other quantities – such as velocity – and defining time in terms of such quantities would result in circularity of definition.[4] 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 space-time brings the nature of time into association with related questions into the nature of space, questions that have their roots in the works of early students of natural philosophy.

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. Time travel, in this view, becomes a possibility as other "times" persist like frames of a film strip, spread out across the time line. Sir Isaac Newton subscribed to this realist view, and hence it is sometimes referred to as Newtonian time.[5][6] 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[7] and Immanuel Kant,[8][9] holds that time is neither an event nor a thing, and thus is not itself measurable nor can it be travelled.

Temporal measurement has occupied scientists and 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.
Contents
[hide]

* 1 Temporal measurement
o 1.1 History of the calendar
o 1.2 History of time measurement devices
* 2 Definitions and standards
o 2.1 World time
o 2.2 Sidereal time
o 2.3 Chronology
* 3 Religion
o 3.1 Linear and cyclical time
o 3.2 Numeric and Divine time
* 4 Philosophy
o 4.1 Time as "unreal"
* 5 Physical definition
o 5.1 Classical mechanics
o 5.2 Modern physics
o 5.3 Spacetime
o 5.4 Time dilation
o 5.5 Relativistic time versus Newtonian time
o 5.6 Arrow of time
o 5.7 Quantised time
* 6 Time and the Big Bang
o 6.1 Speculative physics beyond the Big Bang
* 7 Time travel
* 8 Perception of time
o 8.1 Psychology
o 8.2 Altered states of consciousness
o 8.3 Culture
* 9 Use of time
* 10 See also
o 10.1 Books
o 10.2 Organizations
o 10.3 Miscellaneous arts and sciences
o 10.4 Miscellaneous units of time
* 11 Notes and references
* 12 Further reading
* 13 External links
o 13.1 Perception of time
o 13.2 Physics
o 13.3 Philosophy
o 13.4 Timekeeping
o 13.5 Miscellaneous
* 14 Navigation templates

[edit] Temporal measurement

Temporal measurement, or chronometry, takes two distinct period forms: the calendar, a mathematical abstraction for calculating extensive periods of time,[10] 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. 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.

[edit] 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]

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).

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.

[edit] History of time measurement devices
Horizontal sundial in Taganrog (1833)
Main article: History of timekeeping devices
See also: Clock

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

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.[11]

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.

The most precise timekeeping devices of the ancient world were 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 timekeeping to replenish the flow of water. The Greeks and Chaldeans 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.[12]

The Arab engineers also invented the first mechanical clocks to be driven by weights and gears in the 11th century.[13][14][15] Also in the 11th century, the Chinese inventors and engineers invented the first mechanical clocks to be driven by an escapement mechanism.
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).[16]

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.[17][18]

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.
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.

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.

The most accurate timekeeping devices are atomic clocks, which are accurate to seconds in many millions of years,[19] 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.

Today, the Global Positioning System in coordination with the Network Time Protocol can be used to synchronize timekeeping systems across the globe.

As of 2006[update], the smallest unit of time that has been directly measured is on the attosecond (10−18 s) time scale, or around 1026 Planck times.[20][21][22]

[edit] Definitions and standards
Common units of time Unit Size Notes
attosecond 1/1018 s smallest measured time
femtosecond 1/1015 s
picosecond 1/1012 s
nanosecond 1/109 s
microsecond 1/106 s
millisecond 0.001 s
second SI base unit
minute 60 s
hour 60 minutes
day 24 hours
week 7 days Also called sennight
fortnight 14 days 2 weeks
lunar month 27.2 to 29.5 days Various definitions of "lunar month" exist
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 Also called pentad
decade 10 years
Indiction 15 year cycle
generation 17 - 25 years approximate
jubilee (Biblical) 50 years
century 100 years
millennium 1,000 years
See also: Time standard and Orders of magnitude (time)

The SI base unit for time is the SI second. From the second, larger units such as the minute, hour and day are defined, though they are "non-SI" units because they do not use the decimal system, and also because of the occasional need for a leap second. They are, however, officially accepted for use with the International System. There are no fixed ratios between seconds and months or years as months and years have significant variations in length.[23]

The official SI definition of the second is as follows:[23][24]

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.[23] 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.

[edit] 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). Other scientific time standards include Terrestrial Time and Barycentric Dynamical Time.

Coordinated Universal Time (UTC) is the basis for modern civil time. Since January 1, 1972, it has been defined to follow TAI with an exact offset of an integer number of seconds, changing only when a leap second 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 cesium atom.

Greenwich Mean Time (GMT) is an older standard, adopted starting with British railroads in 1847. Using telescopes instead of atomic clocks, GMT was calibrated to the mean solar time at the Royal Observatory, Greenwich in the UK. Universal Time 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 motion of the planet varies with time, the duration of a second varies if calibrated to a telescope-based standard like GMT or UT varies - it is 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.

[edit] 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 1/366 (4 minutes) less than a solar day.

[edit] 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 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.

[edit] Religion
Further information: Category:Time and fate deities

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. (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".)
Hindu units of time shown logarithmically

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

[edit] Linear and cyclical time
See also: Time Cycles and Wheel of time

In general, the Judaeo-Christian concept, based on the Bible, is that time is linear, with a beginning, the act of creation by God. The Christian view assumes also an end, the eschaton, expected to happen when Christ returns to earth in the Second Coming to judge the living and the dead. This will be the consummation of the world and time. St Augustine's City of God was the first developed application of this concept to world history. The Christian view is that God is uncreated and eternal so that He and the supernatural world are outside time and exist in eternity. Christian Science defines time as "error" or illusion.

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.

[edit] Numeric and Divine time

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.

[edit] Philosophy
Main articles: Philosophy of space and time and Temporal finitism

The earliest recorded African philosophy of time was expounded by the ancient Egyptian thinker Ptahhotep (c. 2650–2600 BC), who said: "Do not lessen the time of following desire, for the wasting of time is an abomination to the spirit."[citation needed] 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 4,320,000 years. Ancient Greek philosophers, including Parmenides and Heraclitus, wrote essays on the nature of time.[25]

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,[26] an approach similar to that taken in other negative definitions.

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 was inspired by the creation myth shared by the three Abrahamic religions: Judaism, Christianity and Islam. The Christian philosopher, John Philoponus, presented the first such argument against the ancient Greek notion of an infinite past. However, the most sophisticated medieval arguments against an infinite past were developed by the early Muslim philosopher, Al-Kindi (Alkindus); the Jewish philosopher, Saadia Gaon (Saadia ben Joseph); and the Muslim theologian, Al-Ghazali (Algazel). They developed two logical arguments against an infinite past, the first being the "argument from the impossibility of the existence of an actual infinite", which states:[27]

"An actual infinite cannot exist."
"An infinite temporal regress of events is an actual infinite."
"∴ An infinite temporal regress of events cannot exist."

The second argument, the "argument from the impossibility of completing an actual infinite by successive addition", states:[27]

"An actual infinite cannot be completed by successive addition."
"The temporal series of past events has been completed by successive addition."
"∴ The temporal series of past events cannot be an actual infinite."

Both arguments were adopted by later Christian philosophers and theologians, and the second argument in particular became more famous after it was adopted by Immanuel Kant in his thesis of the first antimony concerning time.[27]

Isaac Newton believed time and space form a container for events, which is as real as the objects it contains.

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[28]

In contrast to Newton's belief in absolute space, and a precursor to Kantian time, Leibniz believed that time and space are relational.[29] 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.

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.[30] 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.

In Existentialism, time is considered fundamental to the question of being,[citation needed] in particular by the philosopher Martin Heidegger.[citation needed] (See Ontology).

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.[31]

[edit] 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.[32] Time as illusion is also a common theme in Buddhist thought,[33] and some modern philosophers have carried on with this theme. J. M. E. McTaggart's 1908 The Unreality of Time, for example, argues that time is unreal (see also The flow of time).

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'.[34] (See also: Eternalism (philosophy of time).)

[edit] Physical definition
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.[35] The science of classical mechanics is based on this Newtonian idea of time.

Einstein, in his special theory of relativity,[36] postulated the constancy and finiteness of the speed of light for all observers. He showed that this postulate, together with a reasonable definition for what it means for two events to be simultaneous, requires that distances appear compressed and time intervals appear lengthened for events associated with objects in motion relative to an inertial observer.

Einstein showed that if time and space is measured using electromagnetic phenomena (like light bouncing between mirrors) then due to the constancy of the speed of light, time and space become mathematically entangled together in a certain way (called Minkowski 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).
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[edit] 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.

[edit] 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.

[edit] 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]

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. 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]

[edit] 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 travelling 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 typically minute in the human experience, the effect becomes much more 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.

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

Einstein wrote in his book, Relativity, that simultaneity is also relative, 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.

[edit] 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 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.

In the figures, the vertical direction indicates time. The horizontal direction indicates distance (only one spatial dimension is taken into account), and the thick dashed curve is the spacetime trajectory ("world line") of the observer. The small dots indicate specific (past and future) events in spacetime.

The slope of the world line (deviation from being vertical) gives the relative velocity to the observer. Note how in both pictures the view of spacetime changes when the observer accelerates.

In the Newtonian description these changes are such that time is absolute: the movements of the observer do not influence whether an event occurs in the 'now' (i.e. whether an event passes the horizontal line through the observer).

However, in the relativistic description the observability of events is absolute: the movements of the observer do not influence whether an event passes the "light cone" of the observer. Notice that with the change from a Newtonian to a relativistic description, the concept of absolute time is no longer applicable: events move up-and-down in the figure depending on the acceleration of the observer.

[edit] Arrow of time
Main article: Arrow of time

Time appears to have a direction – the past lies behind, fixed and incommutable, while the future lies ahead and is not necessarily fixed. Yet the majority of the laws of physics don't provide this arrow of time. The exceptions include the Second law of thermodynamics, which states that entropy must increase over time (see Entropy); the cosmological arrow of time, which points away from the Big Bang, and the radiative arrow of time, caused by light only traveling forwards in time. In particle physics, there is also the weak arrow of time, from CPT symmetry, and also measurement in quantum mechanics (see Measurement in quantum mechanics).

[edit] Quantised time
See also: Chronon

Time quantization is a hypothetical concept. In the modern established physical theories (the Standard Model of Particles and Interactions and General Relativity) time is not quantized.

Planck time (~ 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.

[edit] Time and the Big Bang

Stephen Hawking in particular has addressed a connection between time and the Big Bang. In A Brief History of Time and elsewhere, Hawking says that even if time did not begin with the Big Bang and there were another time frame before the Big Bang, no information from events then would be accessible to us, and nothing that happened then would have any effect upon the present time-frame.[37] Upon occasion, Hawking has stated that time actually began with the Big Bang, and that questions about what happened before the Big Bang are meaningless.[38][39][40] This less-nuanced, but commonly repeated formulation has received criticisms from philosophers such as Aristotelian philosopher Mortimer J. Adler.[41][42]

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.

[edit] 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.[43]

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.[44]
* brane cosmology models[45] 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.[46][47][48]
* 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.[49]

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.

[edit] Time travel
Main article: Time travel
See also: Time travel in fiction and Grandfather paradox

Time travel is the concept of moving backwards and/or forwards to different points in time, in a manner analogous to moving through space and different from the normal "flow" of time to an earthbound observer. Although time travel has been a plot device in fiction since the 19th century, and one-way travel into the future is arguably possible given the phenomenon of time dilation in the theory of relativity, it is currently unknown whether the laws of physics would allow time travel to the past. Any technological device, whether fictional or hypothetical, that is used to achieve time travel is known as a time machine.

A central problem with time travel to the past is the violation of causality; should an effect precede its cause, it would give rise to the possibility of temporal paradox. Some interpretations of time travel resolve this by accepting the possibility of travel between parallel realities or universes.

Theory would point toward there having to be a physical dimension in which one could travel to, where the present (i.e. the point that which you are leaving) would be present at a point fixed in either the past or future. Seeing as this theory would be dependent upon the theory of a multiverse, it is uncertain how or if it would be possible to just prove the possibility of time travel.

[edit] Perception of time

[edit] 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]

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:[50][51]

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 haf zee ideah! [sic]
Man: But if I were to sit on a hot stove for two seconds then it would seem like two hours.

A form of temporal illusion verifiable by experiment is the kappa effect,[52] 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.

Time also appears to pass more quickly as one gets older.[citation needed] 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 "1:4368", 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.

[edit] 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.[53]

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]

[edit] 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…"[54]

In Hindi and Punjabi (two of the official languages of India), there is only one word kal for both yesterday and tomorrow, the meaning determined by the context.

[edit] 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.

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.

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.

Arlie Russell Hochschild and Norbert Elias have written on the use of time from a sociological