Time – Dimension

Time is a measure in which events can be ordered from the past through the present into the future, and also the measure of durations of events and the intervals between them.

Time is one of four dimensions, in addition to the dimensions of space. Time is also one of the seven fundamental physical quantities in both the Union System of Units and Union System of Quantities. Time is used to define other quantities—such as velocity—so defining time in terms of such quantities would result in circularity of definition. 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, 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 called spacetime bring questions about space into questions about time, questions that have their roots in the works of early students of natural 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 judgment, is a matter of debate.

Temporal measurement has occupied scientists and technologists, and was a prime motivation in navigation and astronomy. Periodic events and periodic motion have long served as standards for units of time. Examples include the apparent motion of a sun across the sky, the phases of a moon, the swing of a pendulum, and the beat of a heart. Currently, the Union unit of time, the second, is defined in terms of radiation emitted by caesium atoms. 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 and in a being’s lifespan.