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Time has long been a major subject of philosophy,
art, poetry, and science. There are widely divergent views
about its meaning; hence it is difficult to provide an uncontroversial
definition of time. Many fields use an operational definition
in which the units of time are defined. Scholars disagree
on whether time itself can be measured or is itself part of
the measuring system.
The Oxford English Dictionary defines time
as "the indefinite continued progress of existence and
events in the past, present, and future, regarded as a whole."
Another standard dictionary definition is "a nonspatial
linear continuum wherein events occur in an apparently irreversible
order." The Latin word for time, tempus, came from the
Greek temnein meaning "to cut" (same root for atomos
?t?µ?? meaning "indivisible"), thus signifying
a division of the flowing duration.
The measurement of time has also occupied
scientists and technologists, and was a prime motivation in
astronomy. Time is also a matter 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 our lives. Units of time have been agreed
upon to quantify the duration of events and the intervals
between them. Regularly recurring events and objects with
apparent 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, and the swing of a
pendulum.
Philosophy of time
In 5th century BC Greece, Antiphon the Sophist
wrote, in his chief work Truth, "Time is a thought or
a measure, not a substance." This is similar to the later
statement by Kant.
In ancient thought, Zeno's paradoxes challenged
the conception of infinite divisibility, and eventually led
to the development of calculus. Parmenides (of whom Zeno was
a follower) believed that time, motion, and change were illusions,
basing this on a rather interesting argument. More recently,
McTaggart held a similar belief.
Many ancient philosophers wrote lengthy essays
on time, believing it to be the essence around which life
was based. A famous analogy was one comparing the time of
life to the passing of sand through an hourglass. The sand
at the top is the future, and, one tiny grain at a time, the
future flows through the present into the past. The past ever
expanding, the future ever decreasing, but the future grains
being moulded into the past through the present. This was
widely discussed in around the 3rd century CE.
The dharmic religions such as Buddhism and
Hinduism, have a concept of wheel of time, that regards time
as cyclical and consisting of repeating ages.
The Judaeo-Christian concept, however, based
on the Bible, is that time is not cyclical but linear, with
a beginning, the act of creation by God, and an end, the eschaton.
In the Christian view, the eschaton will happen when Christ
returns to earth in the Second Coming to judge the living
and the dead. This will be the consummation of the world and
time. St Augustine's City of God was the first developed application
of this concept to world history. The Christian view is that
God and the supernatural world are outside time and exist
in eternity.
Newton believed time and space form a container
for events, which is as real as the objects it contains. In
contrast, Leibniz believed that time and space are a conceptual
apparatus describing the interrelations between events. These
differences came to a head in the famous Leibniz-Clark Correspondence.
Leibniz and others thought of time as a fundamental
part of an abstract conceptual framework, together with space
and number, within which we sequence events, quantify their
duration, 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.
The bucket argument proved problematic for
Leibniz, and his account fell into disfavour, at least amongst
scientists, until the development of Mach's principle. Modern
physics views the curvature of spacetime around an object
as much a feature of that object as are its mass and volume.
Immanuel Kant, in the Critique of Pure Reason,
described time as an a priori notion that allows us (together
with other a priori notions such as space) to comprehend sense
experience. With Kant, neither space nor time are conceived
as substances, but rather both are elements of a systematic
framework necessarily structuring the experiences of any rational
agent. This is similar to the outlook of the Sophist Antiphon.
Spatial measurements are used to quantify how far apart objects
are, and temporal measurements are used to quantify how far
apart events occur.
Schopenhauer, in the preface to his On the
Will in Nature, stated that "Time is the condition of
the possibility of succession." This is in accordance
with Kant's understanding of time as a mental form in an observing
subject.
Nietzsche, inspired by the concept of eternal
return in his book Thus Spoke Zarathustra, argued that time
possesses a circular characteristic. Postulating an infinite
past, "all things" must have come to pass therein;
the same for an infinite future.
In Existentialism, time is considered fundamental
to the question of being, in particular by the philosopher
Martin Heidegger.
Time in physics
Time is currently one of the few fundamental
quantities. These are quantities which can not be defined
via other quantities because there is nothing more fundamental
known at present. Thus, similar to definition of other fundamental
quantities (like space and mass), time is defined via measurement.
Currently, the standard time interval (called conventional
second, or simply second) is defined as 9 192 631 770 oscillations
of a hyperfine transition in the 133caesium (Cs) atom.
This definition of time coupled to the current
definition of space in physics makes our space-time to be
Minkowski space-time - and thus makes special relativity absolutely
correct simply by definition.
 |
A tesseract, a cube in 3
dimensions extended to a fourth, as a description of time;
adhering to defined finite bounds, all possibilities for
this configuration are conceptually representable. |
Prior to Albert Einstein's relativistic physics,
time and space had been treated as distinct dimensions; Einstein
linked time and space into spacetime. He said that time was
basically what a clock reads; the clock can be any action
or change, like the movement of the sun. Einstein showed that
people traveling at different speeds will measure different
times for events and different distances between objects,
though these differences are minute unless one is traveling
at a speed close to that of light. Many subatomic particles
exist for only a fixed fraction of a second in a lab relatively
at rest, but some that travel close to the speed of light
can be measured to travel further and survive longer than
expected (a muon is one example). According to the special
theory of relativity, in the high-speed particle's frame of
reference, it exists for the same amount of time as usual,
and the distance it travels in that time is what would be
expected for that velocity. Relative to a frame of reference
at rest, time seems to "slow down" for the particle.
Relative to the high-speed particle, distances seems to shorten.
Even in Newtonian terms time may be considered the fourth
dimension of motion; but Einstein showed how both temporal
and spatial dimensions can be altered (or "warped")
by high-speed motion.
Einstein (The Meaning Of Relativity):
"Two events taking place at the points A and B of a system
K are simultaneous if they appear at the same instant when
observed from the middle point, M, of the interval AB. Time
is then defined as the ensemble of the indications of similar
clocks, at rest relatively to K, which register the same simultaneously."
Measurement
Present-day standards
The standard unit for time is the SI second,
from which larger units are defined such as the minute, hour,
and day. Because they do not use the decimal system, and because
of the occasional need for a leap-second, the minute, hour,
and day are "non-SI" units, but are officially accepted
for use with the International System. There are no fixed
ratios between seconds (or days) on the one hand and months
and years on the other hand -- months and years having significant
variations in length. Despite its great social importance,
the week is not mentioned even as a "non-SI" unit.
The same holds for a quarter (of an hour), despite its great
potential as candidate for a decimal day reform. (See external
pdf file: The International System of Units.)
The measurement of time is so critical to
the functioning of modern societies that it is coordinated
at an international level. The basis for scientific time is
a continuous count of seconds based on atomic clocks around
the world, known as International Atomic Time (TAI). This
is the yardstick for other time scales, including Coordinated
Universal Time (UTC), which is the basis for civil time.
The seconds and minutes are expressed using
a number consisting of two denary digits and having a modulo
of 60. It is not to be confused with base-60 which refers
to sexagesimal numerals.
Hours are expressed using a number consisting
of two denary digits and having a modulo of 24, but is commonly
also expressed using the 12-hour clock.
Chronology
Another form of time measurement consists
of studying the past. Events in the past can be ordered in
a sequence (creating a chronology), and be put into chronological
groups (periodization). One of the most important systems
of periodization is Geologic time, which is a system of periodizing
the events that shaped the Earth and its life. Chronology,
periodization, and interpretation of the past are together
known as the study of history.
Psychology
Different people may judge identical lengths
of time quite differently. Time can "fly"; that
is, a long period of time can seem to go by very quickly.
Likewise, time can seem to "drag," as in when one
performs a boring task. The psychologist Jean Piaget called
this form of time perception "lived time."
Time appears to go fast when sleeping, or,
to put it differently, time seems not to have passed while
asleep. Time also appears to pass more quickly as one gets
older. For example, a day for a child seems to last longer
than a day for an adult. One possible reason for this is that
with increasing age, each segment of time is a decreasing
percentage of the person's total experience.
Altered states of consciousness are sometimes
characterised by a different estimation of time. Some psychoactive
substances--such as entheogens--may also dramatically alter
a person's temporal judgement.
In explaining his theory of relativity, Albert
Einstein is often quoted as saying that although sitting next
to a pretty girl for an hour feels like a minute, placing
one's hand on a hot stove for a minute feels like an hour.
This is intended to introduce the listener to the concept
of the interval between two events being perceived differently
by different observers.
Use of time
The use of time is an important issue in understanding
human behaviour, education, and travel behaviour. Time use
research is a developing field of study. The question concerns
how time is allocated across a number of activities (such
as time spent at home, at work, shopping, etc.). Time use
changes with technology, as the television or the Internet
created new opportunities to use time in different ways. However,
some aspects of time use are relatively stable over long periods
of time, such as the amount of time spent traveling to work,
which despite major changes in transport, has been observed
to be about 20-30 minutes one-way for a large number of cities
over a long period of time. This has led to the disputed time
budget hypothesis.
Time management is the organization of tasks
or events by first estimating how much time a task will take
to be completed, when it must be completed, and then adjusting
events that would interfere with its completion so that completion
is reached in the appropriate amount of time. Calendars and
day planners are common examples of time management tools.
Arlie Russell Hochschild and Norbert Elias
have written on the use of time from a sociological perspective.
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