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There are two distinct views on the meaning of '''time'''.

There are two distinct views on the meaning of '''time'''.

One view is that time is part of the fundamental structure of the [[universe]], a [[dimension]] in which events occur in [[sequence]], and time itself is something that can be measured. This is the [[Philosophical realism|realist]]'s view, to which [[Sir Isaac Newton]] subscribed, and hence is sometimes referred to as [[Newtonian time]] (''Newton's Views on Space, Time, and Motion'' - Stanford University [http://plato.stanford.edu/entries/newton-stm/]

One view is that time is part of the fundamental structure of the [[universe]], a [[dimension]] in which events occur in [[sequence]], and time itself is something that can be measured. This is the [[Philosophical realism|realist]]'s view, to which [[Sir Isaac Newton]] subscribed, and hence is sometimes referred to as [[Newtonian time]] (''Newton's Views on Space, Time, and Motion'' - Stanford University [https://plato.stanford.edu/entries/newton-stm/]

<center>For lessons on the topic of '''''Time''''', follow [https://nordan.daynal.org/wiki/index.php?title=Category:Time/TeaM this link.]</center>

<center>For lessons on the topic of '''''Time''''', follow [https://nordan.daynal.org/wiki/index.php?title=Category:Time/TeaM this link.]</center>

A contrasting view is that time is part of the fundamental intellectual structure (together with [[space]] and [[number]]). Within this structure, humans sequence events, [[quantity|quantify]] the duration of events and the intervals between them, and compare the [[motion (physics)|motions]] of objects. In this second view, time does not refer to any kind of entity that "flows", that objects "move through", or that is a "container" for events. This view is in the tradition of [[Gottfried Leibniz]]<ref> Leibniz on Space, Time, and Indiscernibles - Against the Absolute Theory -- Internet Encyclopedia of Philosophy [http://www.iep.utm.edu/l/leib-met.htm#H7] and [[Immanuel Kant]], Critique of Pure Reason - Lecture notes of G. J. Mattey, UC Davis [http://www-philosophy.ucdavis.edu/mattey/kant/TIMELEC.HTM] Kant's Transcendental Idealism - Internet Encyclopedia of Philosophy [http://www.iep.utm.edu/k/kantmeta.htm#H4] in which time, rather than being an objective thing to be measured, is part of the [[mind|mental]] measuring system.

A contrasting view is that time is part of the fundamental intellectual structure (together with [[space]] and [[number]]). Within this structure, humans sequence events, [[quantity|quantify]] the duration of events and the intervals between them, and compare the [[motion (physics)|motions]] of objects. In this second view, time does not refer to any kind of entity that "flows", that objects "move through", or that is a "container" for events. This view is in the tradition of [[Gottfried Leibniz]]<ref> Leibniz on Space, Time, and Indiscernibles - Against the Absolute Theory -- Internet Encyclopedia of Philosophy [https://www.iep.utm.edu/l/leib-met.htm#H7] and [[Immanuel Kant]], Critique of Pure Reason - Lecture notes of G. J. Mattey, UC Davis [https://www-philosophy.ucdavis.edu/mattey/kant/TIMELEC.HTM] Kant's Transcendental Idealism - Internet Encyclopedia of Philosophy [https://www.iep.utm.edu/k/kantmeta.htm#H4] in which time, rather than being an objective thing to be measured, is part of the [[mind|mental]] measuring system.

In [[physics]], time and space are considered [[fundamental unit|fundamental quantities]] (i.e. they cannot be defined in terms of other quantities because other quantities - such as [[velocity]], [[force]], [[energy]], etc - are already defined in terms of them). Thus the only definition possible is an [[operational definition|operational]] one, in which time is defined by the process of [[measurement]] and by the [[unit]]s chosen. Periodic events and periodic motion have long served as standards for units of time. Examples are the apparent motion of the sun across the sky, the phases of the moon, the swing of a pendulum, heartbeats, etc. Currently, the unit of time interval (the [[second]]) is defined as a certain number of [[hyperfine]] transitions in [[Cesium]] atoms (see below). All properties of time follow from this definition.

In [[physics]], time and space are considered [[fundamental unit|fundamental quantities]] (i.e. they cannot be defined in terms of other quantities because other quantities - such as [[velocity]], [[force]], [[energy]], etc - are already defined in terms of them). Thus the only definition possible is an [[operational definition|operational]] one, in which time is defined by the process of [[measurement]] and by the [[unit]]s chosen. Periodic events and periodic motion have long served as standards for units of time. Examples are the apparent motion of the sun across the sky, the phases of the moon, the swing of a pendulum, heartbeats, etc. Currently, the unit of time interval (the [[second]]) is defined as a certain number of [[hyperfine]] transitions in [[Cesium]] atoms (see below). All properties of time follow from this definition.

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  

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. St.,Augustine, ''Confessions'', Book 11.  [http://ccat.sas.upenn.edu/jod/augustine/Pusey/book11]. [[Isaac Newton|Newton]] believed time and [[space]] form a [[container]] for [[event]]s, which is as [[real]] as the [[object]]s it contains.

it to one that asketh, I know not."  He settles on time being defined more by what it is not than what it is. St.,Augustine, ''Confessions'', Book 11.  [https://ccat.sas.upenn.edu/jod/augustine/Pusey/book11]. [[Isaac Newton|Newton]] believed time and [[space]] form a [[container]] for [[event]]s, which is as [[real]] as the [[object]]s 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'', Isaac Newton Translated by I. Bernard Cohen and Anne Whitman, University of California Press, Berkeley, 1999.

"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'', Isaac Newton Translated by I. Bernard Cohen and Anne Whitman, University of California Press, Berkeley, 1999.

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 (hupostasis), but a concept (noêma) or a measure (metron)."''

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 (hupostasis), but a concept (noêma) or a measure (metron)."''

[[Parmenides]] went further, maintaining that time, motion, and change were illusions, leading to the [[Zeno's paradoxes|paradoxes]] of his follower [[Zeno of Elea|Zeno]]. Harry Foundalis, You are about to disappear [http://www.cogsci.indiana.edu/farg/harry/phi/WhyTimeFlows.htm]

[[Parmenides]] went further, maintaining that time, motion, and change were illusions, leading to the [[Zeno's paradoxes|paradoxes]] of his follower [[Zeno of Elea|Zeno]]. Harry Foundalis, You are about to disappear [https://www.cogsci.indiana.edu/farg/harry/phi/WhyTimeFlows.htm]

Time as illusion is also a common theme in [[Buddhist]] thought, [http://www.buddhistinformation.com/buddhism_and_the_illusion_of_time.htm]

Time as illusion is also a common theme in [[Buddhist]] thought, [https://www.buddhistinformation.com/buddhism_and_the_illusion_of_time.htm]

Tom Huston}} 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 [[Philosophy of space and time#The flow of time|The flow of time]]).

Tom Huston}} 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 [[Philosophy of space and time#The flow of time|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 space]]realm containing every possible "Now" or momentary configuration of the universe, which he terms 'platonia'. [http://physicsandphysicists.blogspot.com/2007/03/time-is-illusion.html]

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 space]]realm containing every possible "Now" or momentary configuration of the universe, which he terms 'platonia'. [https://physicsandphysicists.blogspot.com/2007/03/time-is-illusion.html]

=== Linear time ===

=== Linear time ===

===Time and the Big Bang===

===Time and the Big Bang===

According to some of the latest scientific theories, time began with the [[Big Bang]]. [[Stephen Hawking]] (borrowing a line of thought from [[Augustine of Hippo]]) has commented that trying to ascertain what happened before time began is like trying to find out what is north of the North Pole, and that such questions are self-contradictory, and thus without meaning.[http://www.ghandchi.com/312-SpaceEng.htm] Hawking has also stated, along with other theorists, 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>Public lecture on the beginning of time by Hawking [http://www.hawking.org.uk/lectures/bot.html]. Scientists have come to some agreement on descriptions of events that happened seconds after the Big Bang, but generally agree that descriptions about what happened before one [[Planck time]] after the Big Bang will likely remain pure speculation.

According to some of the latest scientific theories, time began with the [[Big Bang]]. [[Stephen Hawking]] (borrowing a line of thought from [[Augustine of Hippo]]) has commented that trying to ascertain what happened before time began is like trying to find out what is north of the North Pole, and that such questions are self-contradictory, and thus without meaning.[https://www.ghandchi.com/312-SpaceEng.htm] Hawking has also stated, along with other theorists, 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>Public lecture on the beginning of time by Hawking [https://www.hawking.org.uk/lectures/bot.html]. Scientists have come to some agreement on descriptions of events that happened seconds after the Big Bang, but generally agree that descriptions about what happened before one [[Planck time]] after the Big Bang will likely remain pure speculation.

===Time travel in science fiction===

===Time travel in science fiction===