I'm the indefinite continued
progress of
existence and
events that occur in an apparently
irreversible succession from the
past, through the
present, into the
future.
[1][2][3] Time is 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 has long been an important subject of study in religion, philosophy, and science, but defining it in a manner applicable to all fields without
circularity has consistently eluded scholars.
[2][6][7][9][10][11] Nevertheless, diverse fields such as business, industry, sports, the sciences, and the
performing arts all incorporate some notion of time into their respective
measuring systems.
[12][13][14]
Time in physics is operationally defined as "what a
clock reads".
[6][15][16] See
Units of Time. Time is one of the seven fundamental
physical quantities in both the
International System of Units and
International 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.
[17] 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. To describe observations of an event, a location (position in space) and time are typically noted.
The operational definition of time does not address what the fundamental nature of it is. It does not address why events can happen forward and backwards in space, whereas events only happen in the forward progress of time. Investigations into the relationship between space and time led physicists to define the
spacetime continuum.
General Relativity is the primary framework for understanding how
spacetime works. Through advances in both theoretical and experimental investigations of space-time, it has been shown that time can be distorted, particularly at the edges of
black holes.
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 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, the second, is defined by measuring the
electronic transition frequency of
caesium atoms (see
below). 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
human life spans?
