There are two distinct views on the meaning of time. One view is
that time is linear and 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 realist's
view, to which Sir Isaac Newton subscribed.[1]
A contrasting view is that time is part of the fundamental
intellectual structure (together with space and number) within
which we sequence events, quantify the duration of events and the
intervals between them, and compare the motions of objects. In this
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[2] and Immanuel
Kant,[3][4] in which time, rather than being an objective thing to
be measured, is part of the mental measuring system. The question,
perhaps overly simplified and allowing for no middle ground, is
thus: is time a "real thing" that is "all around us", or is it
nothing more than a way of speaking about and measuring
events?
In physics, time and space are considered fundamental (i.e. they
cannot be defined in other terms). Thus the only definition
possible is an operational one in which time is defined by the
process of measurement and by the units 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.
Time has long been a major subject of science, philosophy, and art.
The measurement of time has occupied scientists and technologists,
and was a prime motivation in 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 and in human lifespans.
