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| format = [[PDF]]
| accessdate = 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."</ref> ''Time'' is used to define other quantities – such as [[velocity]] – and defining ''time'' in terms of such quantities would result in [[circular definition|circularity of definition]].<ref name="TrialogueP3">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."</ref> 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]], has a high utility value 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''.
holds that ''time'' is neither an event nor a thing, and thus is not itself measurable nor can it be traveled.
Temporal measurement has occupied scientists and [[technologist]]s, and was a prime motivation in [[navigation]] and [[astronomy]].
=== History of the calendar ===
Artifacts from the [[Palaeolithic]] suggest that the moon was used to calculate time as early as 12,000, and possibly even 30,000 [[Before Present|BP]].<ref name="Rudgley" />
The [[Sumer]]ian 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).<!--- use of 60 could not have appeared until people started using minutes - which they would not have done with sundials --->
The reforms of [[Julius Caesar]] in 45 BC put the [[Roman Empire|Roman world]] on a [[solar calendar]]. This [[Julian calendar]] was faulty in that its [[intercalation]] still allowed the astronomical [[solstice]]s and [[equinox]]es 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.
=== History of time measurement devices ===
[[Gambar:Sundial Taganrog.jpg|thumb|right|Horizontal [[sundial]] in [[Taganrog]] (1833)]]
{{mainutama|History of timekeeping devices}}{{seealso|Clock}}
A large variety of [[Measuring instrument|devices]] have been invented to measure time. The study of these devices is called [[horology]].
| [[score]] || 20 years||
| [[generation]] || 17 - 25 years ||approximate
| [[century]] || 100 years||
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 [[month]]s or [[year]]s as months and years have significant variations in length.<ref name="si_units">{{cite book | title = The International System of Units (SI), 7th Edition | url = http://www1.bipm.org/utils/en/pdf/si-brochure.pdf | format = [[PDF]] | year = 1998 | author = Organisation Intergouvernementale de la Convention du Métre | accessdate = 2006-06-13}}</ref>
The official SI definition of the second is as follows:<ref name="si_units">{{cite book | title = The International System of Units (SI), 7th Edition | url = http://www1.bipm.org/utils/en/pdf/si-brochure.pdf | format = [[PDF]] | year = 1998 | author = Organisation Intergouvernementale de la Convention du Métre | accessdate = 2006-06-13}}</ref><ref name="second">{{cite web | title = Base unit definitions: Second | url = http://physics.nist.gov/cuu/Units/second.html | publisher = [[NIST]] | accessdate = 2008-01-09}}</ref>
{{Bquote|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.}}
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The current definition of the second, coupled with the current definition of the [[metre]], is based on the [[special theory of relativity]], which affirms our [[space-time]] to be a [[Minkowski space]].
=== World time ===
=== 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]].
[[Gambar:John Bydell - Engraving from the Goodly Primer.png|thumb|200px|Allegorical woodcut of Time, who "revealeth all things", guiding his daughter Truth away from the demon of Hypocrisy. John Byddell, 1535.]]
== Time in philosophy ==
{{mainutama|Philosophy of space and time}}
The earliest recorded [[African philosophy]] of time was expounded by the [[ancient Egypt]]ian 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."{{Fact|date=May 2008}} 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 philosophy|Ancient]] [[Greek philosophy|Greek philosophers]], including [[Parmenides]] and [[Heraclitus]], wrote essays on the nature of time.<ref>Dagobert Runes, ''Dictionary of Philosophy'', p. 318</ref>
In Book 11 of [[St. Augustine of Hippo|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.<ref>St. Augustine, ''Confessions'', Book 11. http://ccat.sas.upenn.edu/jod/augustine/Pusey/book11 (Accessed 5/26/07).</ref>
In contrast to Newton's belief in absolute space, and a precursor to Kantian time, [[Gottfried Leibniz|Leibniz]] believed that time and space are relational.<ref>Gottfried Martin, ''Kant's Metaphysics and Theory of Science''</ref> The differences between Leibniz's and Newton's interpretations came to a head in the famous [[The Leibniz-Clarke Correspondence|Leibniz-Clarke Correspondence]]. Leibniz thought of time as a fundamental part of an [[Abstract structure|abstract]] conceptual framework, together with [[space]] and [[number]], within which we sequence events, [[quantity|quantify]] their duration, and compare the motions of objects. In this view, ''time'' does not refer to any kind of entity that "flows," that objects "move through," or that is a "container" for events.
[[Immanuel Kant]], in the ''[[Critique of Pure Reason]]'', described time as an ''[[A priori and a posteriori (philosophy)|a priori]]'' intuition that allows us (together with the other ''a priori'' intuition, [[space]]) to comprehend sense experience.<ref name="kant"> {{cite book | last = Kant | first = Immanuel | authorlink = Immanuel Kant | title = The Critique of Pure Reason, 2nd edition | year = 1787}} translated by J. M. D. Meiklejohn, eBooks@Adelaide, 2004 - http://ebooks.adelaide.edu.au/k/kant/immanuel/k16p/k16p15.html</ref> With Kant, neither space nor time are conceived as [[Substance theory|substances]], but rather both are elements of a systematic mental [[framework]] that necessarily structures the experiences of any rational agent, or observing subject. Spatial [[measurement]]s are used to [[quantity|quantify]] how far apart [[object (philosophy)|objects]] are, and temporal measurements are used to quantify how far apart [[Phenomenon|events]] occur.
In [[Existentialism]], time is considered fundamental to the question of [[being]],{{Fact|date=September 2007}} in particular by the philosopher [[Martin Heidegger]].{{Fact|date=September 2007}} (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.<ref>Bergson, Henri (1907) ''Creative Evolution''. trans. by Arthur Mitchell. Mineola: Dover, 1998.</ref>
== Time in the physical sciences ==
{{mainutama|Time in physics}}
From the age of [[Isaac Newton|Newton]] up until [[Albert Einstein|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.<ref>Herman M. Schwartz, ''Introduction to Special Relativity'', McGraw-Hill Book Company, 1968, hardcover 442 pages, see ISBN 0-88275-478-5 (1977 edition), pp. 10-13</ref> The science of classical mechanics is based on this Newtonian idea of time.
=== Spacetime ===
Modern [[physics]] views the curvature of [[spacetime]] around an object as much a feature of that object as are its [[mass]] and [[volume]].{{Fact|date=February 2008}}
[[Gambar:relativity of simultaneity (color).png|thumb|[[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.]]
{{mainutama|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 [[Causality (physics)|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 particle]]s exist for only a fixed fraction of a second in a lab relatively at rest, but some that travel close to the speed of light can be measured to travel further and survive much longer than expected (a [[muon]] is one example). According to the [[Special relativity|special theory of relativity]], in the high-speed particle's [[Inertial reference frame|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 [[Spacetime#Basic concepts|events]] taking place at the points A and B of a system K are simultaneous if they appear at the same instant when observed from the middle point, M, of the interval AB. Time is then defined as the ensemble of the indications of similar clocks, at rest relatively to K, which register the same simultaneously."
Einstein wrote in his book, ''Relativity'', that [[Relativity of simultaneity|simultaneity is also relative]], i.e., two events that appear simultaneous to an observer in a particular inertial reference frame need not be judged as simultaneous by a second observer in a different inertial frame of reference.
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.
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 ===
{{mainutama|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 (arrow of time)|Entropy]]); the [[Physical cosmology|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]]).
== Time travel ==
{{mainutama|Time travel}}
{{seealso|Time travel in fiction|Grandfather paradox}}
=== Time in psychology ===
{{see also|Mental chronometry|Sense of time}}
Even in the presence of timepieces, different individuals may judge an identical length of time to be passing at different rates.{{Fact|date=February 2008}} 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."{{Fact|date=February 2008}}
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]]:<ref name="Priestley">{{cite book | last = Priestley | first = J. B. | authorlink = J. B. Priestley | title = Man and Time | publisher = Crescent Books | location = New York | year = 1964 | pages = 96 | doi = | isbn = }}</ref><ref name="Sunrise">{{cite web | last = Sunrise | first = | title = Unified Field Theory: A new interpretation | work = Chapter 2 - The Development of the Unified Field Theory, pg. 31 | publisher = Sunrise Information Services | year = 2008 | url = http://www.sunrisepage.com/uft/history.pdf| format = | doi = | accessdate = }}</ref>
{{quotation|'''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,—<br />'''Einstein:''' Braffo! You the idea haf! [''[[sic]]'']<br />'''Man:''' But if I were to sit on a hot stove for two seconds then it would seem like two hours.}}
In [[sociology]] and [[anthropology]], [[time discipline]] is the general name given to [[society|social]] and [[economics|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 20-3020–30 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.


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