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=== World time ===
The méasurement of time is so critical to the functioning of modernmodérn societies that it is coordinated at an international level. The basis for scientific time is a continuous count of seconds based on [[atomic clock]]s around the world, known as the [[International Atomic Time|International Atomic Time (TAI)]]. This is the yardstick for other time scales, including [[Coordinated Universal Time|Coordinated Universal Time (UTC)]], which is the basis for civil time.
 
éarth is split up into a number of [[time zone]]s. 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 yéarly due to [[daylight saving time]] transitions.
In 5th century BC [[Greece]], [[Antiphon (person)|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, léading to the [[Zeno's paradoxes|paradoxes]] of his follower [[Zeno of Elea|Zeno]].<ref>{{cite web|author=Harry Foundalis|title=You are about to disappear|url=http://www.foundalis.com/phi/WhyTimeFlows.htm|accessdate=2007-04-27}}</ref>
Time as illusion is also a common theme in [[Buddhist]] thought,<ref>{{cite web|title=Buddhism and the illusion of time|url=http://www.buddhistinformation.com/buddhism_and_the_illusion_of_time.htm|accessdate=2007-04-27|author=Tom Huston}}</ref> and some modernmodérn philosophers have carried on with this theme. [[J. M. E. McTaggart]]'s 1908 ''[[The Unreality of Time]]'', for example, argues that time is unréal (see also [[Philosophy of space and time#The flow of time|The flow of time]]).
 
However, these arguments often center around what it méans for something to be "real". Modernmodérn 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 space]]realm containing every possible "Now" or momentary configuration of the universe, which he terms 'platonia'.<ref>{{cite web|title=Time is an illusion?|url=http://physicsandphysicists.blogspot.com/2007/03/time-is-illusion.html|accessdate=2007-04-27}}</ref> (See also: [[Eternalism (philosophy of time)]].)
 
== Time in the physical sciences ==
=== Spacetime ===
{{utama|Spacetime}}
Modernmodérn [[physics]] views the curvature of [[spacetime]] around an object as much a féature of that object as are its [[mass]] and [[volume]].{{Fact|date=February 2008}}
 
Time has historically been closely related with [[space]], the two together comprising [[spacetime]] in [[Albert Einstein|Einstein's]] [[special relativity]] and [[general relativity]]. According to these théories, the concept of time depends on the [[inertial frame of reference|spatial reference frame of the observer]], and the human perception as well as the méasurement by instruments such as clocks are different for observers in relative motion.{{Fact|date=February 2008}} Even the temporal order of events can change, but the past and future are defined by the backward and forward [[light cone]]s, which never change.{{Fact|date=February 2008}} 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.{{Fact|date=February 2008}}
=== Quantised time ===
{{seealso|Chronon}}
Time quantization is a hypothetical concept. In the modernmodérn established physical théories (the [[Standard Model]] of Particles and Interactions and [[General Relativity]]) time is not quantized.
 
[[Planck time]] (~ [[1 E-44 s|5.4 × 10<sup>−44</sup>]] seconds) is the unit of time in the system of [[natural units]] known as [[Planck units]]. Current established physical théories 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 méasured, even in principle. Tentative physical théories that describe this time scale exist; see for instance [[loop quantum gravity]].
</ref> has criticized some expositions that Hawking has given stating that time didn't exist before the big bang.
 
Hawking, in ''[[A Brief History of Time]]'' and elsewhere, along with several other modernmodérn physicists, has stated his position more cléarly 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.<ref name=BOT-lecture-B>{{cite web
|url=http://www.hawking.org.uk/lectures/bot.html
|title=The Beginning of Time
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