Timeis adimensionin which events can be ordered from thepastthrough thepresentinto thefuture,[1][2][3][4][5][6]and also the measure of durations of events and the intervals between them.[3][7][8]Time has long been a major subject of study inreligion,philosophy, andscience, but defining it in a manner applicable to all fields withoutcircularityhas consistently eluded scholars.[3][7][8][9][10][11]Nevertheless, diverse fields such as business,industry, sports, the sciences, and theperforming artsall incorporate some notion of time into their respectivemeasuring systems.[12][13][14]Some simple, relatively uncontroversial definitions of time include "time is what clocks measure"[7][15]and "time is what keeps everything from happening at once".[16][17][18][19]
Two contrasting viewpoints on time divide many prominent philosophers. One view is that time is part of the fundamental structure of theuniverse adimensionindependent of events, in which events occur insequence.Sir Isaac Newtonsubscribed to thisrealistview, and hence it is sometimes referred to asNewtonian time.[20][21]The opposing view is thattimedoes 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 withspaceandnumber) within which humans sequence and compare events. This second view, in the tradition ofGottfried Leibniz[15]andImmanuel Kant,[22][23]holds thattimeis neither an event nor a thing, and thus is not itself measurable nor can it be travelled.
Time is one of the seven fundamentalphysical quantitiesin theInternational System of Units. Time is used to define other quantities such asvelocity so defining time in terms of such quantities would result in circularity of definition.[24]Anoperational definitionof 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 thesecond, 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 calledspacetimebring questions aboutspaceinto questions about time, questions that have their roots in the works of early students ofnatural philosophy.
Furthermore, it may be that there is a subjective component to time, but whether or not time itself is "felt", as a sensation, or is a judgement, is a matter of debate.[3][7][8][25][26]
Temporal measurement has occupied scientists andtechnologists, and was a prime motivation innavigationandastronomy. 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, thesecond, is defined in terms of radiation emitted bycaesiumatoms (seebelow). Time is also of significant social importance, having economic value ("time is money") as well as personal value, due to anawarenessof the limited time in each day and inhuman life spans.
