Time

Time is the continuous progression of existence that occurs in an apparently irreversible succession from the past, through the present, and into the future.^[1]^[2]^[3] Time dictates all forms of action, age, and causality, being a component quantity of various measurements used to sequence events, to compare the duration of events (or the intervals between them), and to quantify rates of change of quantities in material reality or in the conscious experience.^[4]^[5]^[6]^[7] Time is often referred to as a fourth dimension, along with three spatial dimensions.^[8]

Time is primarily measured in linear spans or periods, ordered from shortest to longest. Practical, human-scale measurements of time are performed using clocks and calendars, reflecting a 24-hour day collected into a 365-day year linked to the astronomical motion of the Earth. Scientific measurements of time instead vary from Planck time at the shortest to billions of years at the longest. Measurable time is believed to have effectively begun with the Big Bang 13.8 billion years ago, encompassed by the chronology of the universe. Modern physics understands time to be inextricable from space within the concept of spacetime described by general relativity.^[9] Time can therefore be dilated by velocity and matter to pass faster or slower for an external observer, though this is considered negligible outside of extreme conditions, namely relativistic speeds or the gravitational pulls of black holes.

Throughout history, time has been an important subject of study in religion, philosophy, and science. Temporal measurement has occupied scientists and technologists, and has been a prime motivation in navigation and astronomy. 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 ("carpe diem") and in human life spans.

^ "Time". Oxford Dictionaries. Oxford University Press. Archived from the original on 4 July 2012. Retrieved 18 May 2017. The indefinite continued progress of existence and events in the past, present, and future regarded as a whole
^ Cite error: The named reference DefRefs02 was invoked but never defined (see the help page).
^ "Time". The American Heritage Dictionary of the English Language (Fourth ed.). 2011. Archived from the original on 19 July 2012. A nonspatial continuum in which events occur in apparently irreversible succession from the past through the present to the future.
^ Merriam-Webster Dictionary Archived 8 May 2012 at the Wayback Machine the measured or measurable period during which an action, process, or condition exists or continues: duration; a nonspatial continuum which is measured in terms of events that succeed one another from past through present to future
^ Compact Oxford English Dictionary A limited stretch or space of continued existence, as the interval between two successive events or acts, or the period through which an action, condition, or state continues. (1971).
^ Cite error: The named reference DefRefs01 was invoked but never defined (see the help page).
^ Cite error: The named reference Poidevin was invoked but never defined (see the help page).
^ "Newton did for time what the Greek geometers did for space, idealized it into an exactly measurable dimension." About Time: Einstein's Unfinished Revolution, Paul Davies, p. 31, Simon & Schuster, 1996, ISBN 978-0-684-81822-1
^ Rendall, Alan D. (2008). Partial Differential Equations in General Relativity (illustrated ed.). Oxford University Press. p. 9. ISBN 978-0-19-921540-9.

[1] "Time". Oxford Dictionaries. Oxford University Press. Archived from the original on 4 July 2012. Retrieved 18 May 2017. The indefinite continued progress of existence and events in the past, present, and future regarded as a whole

[DefRefs02-2] Cite error: The named reference DefRefs02 was invoked but never defined (see the help page).

[3] "Time". The American Heritage Dictionary of the English Language (Fourth ed.). 2011. Archived from the original on 19 July 2012. A nonspatial continuum in which events occur in apparently irreversible succession from the past through the present to the future.

[4] Merriam-Webster Dictionary Archived 8 May 2012 at the Wayback Machine the measured or measurable period during which an action, process, or condition exists or continues: duration; a nonspatial continuum which is measured in terms of events that succeed one another from past through present to future

[5] Compact Oxford English Dictionary A limited stretch or space of continued existence, as the interval between two successive events or acts, or the period through which an action, condition, or state continues. (1971).

[DefRefs01-6] Cite error: The named reference DefRefs01 was invoked but never defined (see the help page).

[Poidevin-7] Cite error: The named reference Poidevin was invoked but never defined (see the help page).

[8] "Newton did for time what the Greek geometers did for space, idealized it into an exactly measurable dimension." About Time: Einstein's Unfinished Revolution, Paul Davies, p. 31, Simon & Schuster, 1996, ISBN 978-0-684-81822-1

[9] Rendall, Alan D. (2008). Partial Differential Equations in General Relativity (illustrated ed.). Oxford University Press. p. 9. ISBN 978-0-19-921540-9.

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Time

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