Time is the continuing progression of
events occurring in apparently irreversible succession from the past
through the present to the future, and a measure of the durations and
frequencies of events and the intervals between them.
Time has long been a major subject of
study in religion, philosophy, and science, but defining it in a manner
applicable to all fields without circularity has consistently eluded
scholars.
Nevertheless, diverse fields such as
business, industry, sports, the sciences, music, dance, and the live
theater all incorporate some notion of time into their respective
measuring systems.
Some simple, relatively uncontroversial
definitions of time include “time is what clocks measure” and “time is
what keeps everything from happening at once”.
Two contrasting viewpoints on time divide
many prominent philosophers. One view is that time is part of the
fundamental structure of the universe, a dimension in which events occur
in sequence.
Sir Isaac Newton subscribed to this
realist view, and hence it is sometimes referred to as Newtonian time.
Time travel, in this view, becomes a possibility as other “times”
persist like frames of a film strip, spread out across the time line.
The opposing view is that time does 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 with space and number)
within which humans sequence and compare events.
This second view, in the tradition of
Gottfried Leibniz[14] and Immanuel Kant, holds that time is neither an
event nor a thing, and thus is not itself measurable nor can it be
travelled.
Time is one of the seven fundamental
physical quantities in the International System of Units. 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 an experience, has never been settled.
