Time
For the United States news magazine, see Time (magazine).Time has long been a major subject of
science,
philosophy and
art. Though dictionaries present some (varied) definitions of time (some of which are presented below), it is difficult to provide an uncontroversial definition because there are widely divergent views about its meaning, and concerns about whether there are any simpler terms with which to define it. Scholars also disagree on whether time itself can be measured or is itself part of the measuring system. To avoid these definitional problems, many fields use an
operational definition in which only the units of measurement are defined.
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.
Time has historically been closely related with
space, most obviously with
spacetime in
Einstein's general relativity.
Time is currently one of the few
fundamental quantities. These are quantities which cannot be defined via other quantities because there is nothing more fundamental than what is presently known. Thus, similar to definition of other fundamental quantities (like
space and
mass), time is defined via
measurement.
Measurement devices
A large variety of devices have been invented to measure time. The study of these devices is called
horology.
The origins of our current measurement system go back to the
Sumerian and approximately 2000
BCE.This is known as the Sumerian
Sexagesimal System based on the number 60. 60 seconds in a minute, 60 minutes in an hour, 24 hours in a day (1 day divided in half is 12 hours). The 12 hours comes from, 2 x 12 = 24 (5 x 12 = 60).
The most common devices in day-to-day life are the
clock, for periods less than a day, and the
calendar, for periods longer than a day. Clocks can range from
watches, to more exotic varieties such as the
Clock of the Long Now. They can be driven by a variety of means, including gravity, springs, and various forms of electrical power, and regulated by a variety of means such as a
pendulum. There are also a variety of different
calendars, for example the
Lunar calendar and the
Solar calendar, although the
Gregorian calendar is the most commonly used.
A
chronometer is a timekeeper precise enough to be used as a portable time standard, usually in order to determine
longitude by means of
celestial navigation.
The most accurate type of measuring devices for time is the
atomic clock. More archaic devices include the
hourglass, the
sundial, the
tempometer and the
water clock.
Standards
The standard
unit for time is the
SI second, from which larger units such as the
minute,
hour and
day are defined. The minute, hour, and day are "non-SI" units because they do not use the decimal system, and also because of the occasional need for a
leap-second. They are, however, officially accepted for use
with the International System. There are no fixed ratios between seconds and
months or
years as months and years have significant variations in length.
The official SI definition of the second is as follows:
"
World time
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 the
International Atomic Time (TAI). This is the yardstick for other time scales, including
Coordinated Universal Time (UTC), which is the basis for civil time.
Earth is split up into a number of
time zones. Most time zones are exactly one hour apart, and by convention compute their local time as an offset from
Greenwich Mean Time.
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.
According to the
Oxford English Corpus, the word 'time' comes top in the list of most common nouns in the English language.
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."
defines time as "a nonspatial linear continuum in which events occur in an apparently irreversible succession." The Latin word for time,
tempus, came from the Greek
temnein meaning "to cut" (same root for atomos
άτομος meaning "indivisible"), thus signifying a
division of the flowing duration.
Many ancient philosophers wrote lengthy essays on time, believing it to be the essence around which life was based. A famous analogy was one that compares 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 earliest recorded philosophy of time was expounded by Ptahhotep, who lived c.2650 -2600 BC said:
"Do not lessen the time of following desire, for the wasting of time is an abomination to the spirit."In the
Old Testament time was regarded as a medium for the passage of
predestined events. King
Solomon (970-928 BC) wrote:
"There is an appointed time for everything. And there is a time for every event under heavenâ€" A time to give birth, and a time to die; A time to plant, and a time to uproot what is planted. A time to kill, and a time to heal; A time to tear down, and a time to build up. A time to weep, and a time to laugh; A time to mourn, and a time to dance. A time to throw stones, and a time to gather stones; A time to embrace, and a time to shun embracing. A time to search, and a time to give up as lost; A time to keep, and a time to throw away. A time to tear apart, and a time to sew together; A time to be silent, and a time to speak. A time to love, and a time to hate; A time for war, and a time for peace."Around 500 BC
Heraclitus, a
fatalist held that the passage of time and the future both lay beyond the possibility of human influence:
"Everything flows and nothing abides; everything gives way and nothing stays fixed. You cannot step twice into the same river, for other waters and yet others, go flowing on. Time is a child, moving counters in a game; the royal power is a child's."Time in philosophy
Newton believed time and
space form a container for events, which is as real as the objects it contains.
"Absolute, true, and mathematical time, in and of itself and of its own nature, without reference to anything external, flows uniformly and by another name is called duration. Relative, apparent, and common time is any sensible and external measure (precise or imprecise) of duration by means of motion; such a measure - for example, an hour, a day, a month, a year - is commonly used instead of true time." -Principia [ Translated by I. Bernard Cohen and Anne Whitman, University of California Press, Berkeley, 1999.]
In contrast to Newton's belief in absolute space, and closely related to Kantian time,
Leibniz believed that time and space are a conceptual apparatus describing the interrelations between events. The differences between Leibniz's and Newton's interpretations came to a head in the famous
Leibniz-Clark Correspondence. Leibniz 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.
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 mental
framework necessarily structuring the experiences of any rational agent, or observing subject. Spatial
measurements are used to
quantify how far apart
objects are, and temporal measurements are used to quantify how far apart
events occur. Similarly,
Schopenhauer stated in the preface to his
On the Will in Nature that "Time is the condition of the
possibility of succession."
In
Existentialism, time is considered fundamental to the question of
being, in particular by the philosopher
Martin Heidegger. See
Ontology.
Time as "unreal"
In
5th century BC Greece,
Antiphon the
Sophist, in a fragment preserved from his chief work
Truth held that:
"Time is not a reality (hupostasis), but a concept (noêma) or a measure (metron)."Similarly,
Parmenides believed that time, motion, and change were illusions, leading to
Zeno's paradoxes (
Zeno was a follower of Parmenides).
Emerson considers time as presentness, where past and future are but our present projections (of our memory, hope, etc.). For Emerson, time needs a qualitative measurement rather than a quantitative one.
Julian Barbour believes time to be an illusion which we interpret through what he calls "
time capsules," which are "any fixed pattern that creates or encodes the appearance of motion, change or history." One example of this is the
arrow of time.
Linear time versus circular time
In general, the
Judaeo-Christian concept, based on the
Bible, is that time is linear, with a beginning, the act of
creation by
God. The
Christian view assumes also an end, the eschaton, expected to 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. This view relies on interpretation however, for some Jewish and Christian sects believe time may in fact be cyclical.On the other hand, the
dharmic religions such as
Buddhism and
Hinduism, have a concept of a
wheel of time, that regards time as
cyclical and
quantic consisting of repeating ages that happen to every being of the Universe between birth and extinction. In recent years this cyclical vision of time has been embraced by theorists of
quantic space-time and
systems theory.
Time in physical sciences
Spacetime
|
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. |
Modern
physics views the curvature of
spacetime around an object as much a feature of that object as are its
mass and
volume.
Block time
Block time consists of an unchanging four-dimensional spacetime. This does away with the idea of past, present and future.
Time quanta
Planck time (~
5.4 × 10âˆ'44 seconds) is the smallest unit of time that theoretically could ever be measured.
Time dilation
Einstein 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."
Arrow of time
Time appears to have a direction to us - the past lies behind us, and is fixed and incommutable, while the future lies ahead and is not necessarily fixed. Yet the majority of the laws of physics don't provide this
arrow of time. The exceptions include the
Second law of thermodynamics, which states that
entropy must increase over time (see
Entropy (arrow of time)); the
cosmological arrow of time, which points away from the
Big Bang, and the radiative arrow of time, caused by
light only traveling forwards in time. In
particle physics, there is also the weak arrow of time, from
CPT symmetry, and also
measurement in
quantum mechanics (see
Measurement in quantum mechanics).
Time and the "Big Bang"
According to some of the latest scientific theories, time began with the
Big Bang, and any inquiry into what happened before the big bang is either meaningless or totally inaccessible to us.
Time travel in science fiction
Time travel is the concept of moving backward or forward to different points in time, in a manner analogous to moving through
space. Additionally, some interpretations of time travel take the form of travel between
parallel realities or
universes. A central problem with time travel is that of
causality - causes preceding effects - which has given rise to a number of paradoxes (see
grandfather paradox).
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 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 substancesmay 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.
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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Change*
Chronology*
List of cycles*
Date and time notation by country*
Duration*
Eternal recurrence*
Eternity*
Exponential time*
Event*
History*
Network Time Protocol (NTP)*
Periodization*
Peter Lynds*
Philosophy of physics*
Quality time*
Rate*
Sense of time*
Time scales and time standards*
Time zone*
A Brief History of TimeSpecial units of time
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Cosmological decade*
Eon*
Era*
Epoch*
Stage*
Fiscal year*
Galactic year*
Geologic timescale*
Half-life*
Hexadecimal Time*
Period*
Season*
Swatch Internet Time*
Tithi*
Shake (time)*
Ship's bells*
Unix epoch*
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Perception of time
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The Experience and Perception of Time from the
Stanford Encyclopedia of Philosophy.
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Time and Its Discontents*
Subjective Perception of Time and a Progressive Present Moment: The Neurobiological Key to Unlocking Consciousness*
Time Perception I and
II*
The Order of Time: Platform for an Alternative Time Consciousness*
What is Time? An elucidation of the
Lubavitcher Rebbe's comments on the topic.
Physics
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A walk through Time*
Time Travel and Multi-Dimensionality*
Time and classical and quantum mechanics: Indeterminacy vs. discontinuity*
Time as a universal consequence of quantaTimekeeping
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Different systems of measuring time*
non-SI units*
UTC/TAI Timeserver*
Leapsecond*
Hex Time*
Florencetime.net*
BBC article on shortest time ever measured*
American Watchmakers-Clockmakers Institute*
The World Clock - Time Zones*
World Local Times on Google Map by single clickMiscellaneous
*
Cycles Research Institute*
TimeTicker and the time tickers...*
World Time and Zones*
Official US timezh-yue:時é–"
