Time is
the continued sequence of existence and events that
occurs in an apparently irreversible succession
from the past,
through the present,
and into the future.[1][2][3] It
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][9]
Time is one of the
seven fundamental physical
quantities in both the International
System of Units (SI) and International
System of Quantities. The SI base unit
of time is the second,
which is defined by measuring the electronic
transition frequency of caesium atoms. General
relativity is the primary framework for
understanding how spacetime works.[10] Through
advances in both theoretical and experimental
investigations of spacetime, it has been shown
that time can be distorted and dilated,
particularly at the edges of black
holes.
Throughout history, time has been an important
subject of study in religion, philosophy, and
science. Temporal measurement has occupied
scientists and technologists and
has been a prime motivation in navigation and 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
life spans.
Definition
The concept of
time can be complex. Multiple notions exist and
defining time in a manner applicable to all
fields without circularity has
consistently eluded scholars.[7][11][12] 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.[13][14][15] Traditional
definitions of time involved the observation of
periodic motion such as the apparent motion of
the sun across the sky, the phases of the moon,
and the passage of a free-swinging pendulum.
More modern systems include the Global
Positioning System, other satellite
systems, Coordinated
Universal Time and mean
solar time. Although these systems differ
from one another, with careful measurements they
can be synchronized.
In physics, time
is a fundamental concept to define other
quantities, such as velocity.
To avoid a circular definition,[16] time
in physics is operationally
defined as "what a clock reads",
specifically a count of repeating events such as
the SI
second.[6][17][18] Although
this aids in practical measurements, it does not
address the essence of time. Physicists
developed the concept of the spacetime continuum,
where events are assigned four coordinates:
three for space and one for time. Events like particle
collisions, supernovas,
or rocket
launches have coordinates that may vary for
different observers, making concepts like "now"
and "here" relative. In general
relativity, these coordinates do not
directly correspond to the causal structure of
events. Instead, the spacetime
interval is calculated and classified as
either space-like or time-like, depending on
whether an observer exists that would say the
events are separated by space or by time.[19] Since
the time required for light to travel a specific
distance is the same for all observers—a fact
first publicly demonstrated by the Michelson–Morley
experiment—all observers will consistently
agree on this definition of time as a causal
relation.[20]
General relativity
does not address the nature of time for
extremely small intervals where quantum
mechanics holds. In quantum mechanics, time is
treated as a universal and absolute parameter,
differing from general relativity's notion of
independent clocks. The problem
of time consists of reconciling these two
theories.[21] As
of 2024, there is no generally accepted theory
of quantum general relativity.[22]
Measurement
Generally
speaking, methods of temporal measurement, or chronometry,
take two distinct forms: the calendar,
a mathematical tool for organising intervals of
time,[23] and
the clock,
a physical mechanism that counts the passage of
time. In day-to-day life, the clock is consulted
for periods less than a day, whereas the
calendar is consulted for periods longer than a
day. Increasingly, personal electronic devices
display both calendars and clocks
simultaneously. The number (as on a clock dial
or calendar) that marks the occurrence of a
specified event as to hour or date is obtained
by counting from a fiducial epoch – a central
reference point.
