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Time

2007 Schools Wikipedia Selection. Related subjects: General Physics

          A pocket watch, a device used to measure time.

          Enlarge
          A pocket watch, a device used to measure time.

   Two distinct views exist on the meaning of time. One view is that time
   is part of the fundamental structure of the universe, a dimension in
   which events occur in sequence. This is the realist view, to which
   Isaac Newton subscribed, in which time itself is something that can be
   measured.

   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 and Immanuel Kant, in which time, rather than being
   an objective thing to be measured, is part of the mental 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." The American Heritage Dictionary defines time as
   "a nonspatial linear continuum in which events occur in an apparently
   irreversible succession." Encarta, Microsoft's Digital Multimedia
   Encyclopedia, gives the definition of time as a "System of
   distinguishing events: a dimension that enables two identical events
   occurring at the same point in space to be distinguished, measured by
   the interval between the events."
   Enlarge

   Many fields avoid the problem of defining time itself by using
   operational definitions that specify the units of measurement that
   quantify time. 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, the two together
   comprising spacetime in Einstein's special relativity and general
   relativity. According to these theories, the concept of time depends on
   the spatial reference frame of the observer(s), and the human
   perception as well as the measurement by instruments such as clocks are
   different for observers in relative motion. Even the temporal order of
   events can change, but the past and future are defined by the backward
   and forward light cones, which never change. The past is the set of
   events that can send light signals to the observer, the future the
   events to which she can send light signals. All else is the present and
   within that set of events the very time-order differs for different
   observers.

   Time has long been a major subject of science, philosophy and art. 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. This article looks at some of the main
   philosophical and scientific issues relating to time.

Usage and origins

   According to the Oxford English Corpus, the word 'time' comes top in
   the list of most common nouns in the English language. 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 .

Measurement

   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.

   The origins of our current measurement system go back to the Sumerian
   civilization of 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 - and possibly a calendar with 360 (60x6) days in a
   year (with a few more days added on). Twelve also features prominently,
   with roughly 12 hours of day and 12 of night, and roughly 12 months in
   a year (especially in a 360 day year).

Measurement devices

   Horizontal sundial in Taganrog (1833).
   Enlarge
   Horizontal sundial in Taganrog ( 1833).

   A large variety of devices have been invented to measure time. The
   study of these devices is called horology.

   An Egyptian device dating to c. 1500 BCE, similar in shape to a bent
   T-square, measured the passage of time from the shadow cast by its
   crossbar on a non-linear rule. The T was oriented eastward in the
   mornings. At noon, the device was turned around so that it could cast
   its shadow in the evening direction.

   A sundial uses a gnomon to cast a shadow on a set of markings which
   were calibrated to the hour. The position of the shadow marked the hour
   in local time. Pliny the Elder records that the first sundial in Rome
   was looted from Catania, Sicily ( 264 BCE), which gave the incorrect
   time for a century, until the markings appropriate for the latitude of
   Rome were used ( 164 BCE). Noontime was an event which could be marked
   by the time of the shortest shadow on a sundial. This was used in Rome
   to judge when a court of law was open; lawyers had to be at the court
   by that time.

   The most accurate timekeeping devices of the ancient world were the
   waterclock or clepsydra, first found in Egypt. A waterclock was found
   in the tomb of pharaoh Amenhotep I (1525 - 1504 BCE). Waterclocks were
   used in Alexandria, and then worldwide, for example in Greece, from c.
   400 BCE. They could be used to measure the hours even at night, but
   required manual timekeeping to replenish the flow of water. Plato is
   said to have invented a water-based alarm clock. It depended on the
   nightly overflow of a vessel containing lead balls, which would float
   in a columnar vat. The vat would hold an increasing supply of water
   supplied by a cistern. Eventually the vessel would float high enough to
   tip over. The lead balls would then cascade onto a copper platter. The
   resultant clangor would then awaken his students at the Academy ( 378
   BCE). The Greeks and Chaldeans regularly maintained timekeeping records
   as an essential part of their astronomical observations. In particular,
   Arab engineers improved on the use of waterclocks up to the Middle
   Ages.

   The hourglass uses the flow of sand to measure the flow of time. They
   were used in navigation. Ferdinand Magellan used 18 glasses on each
   ship for his circumnavigation of the globe ( 1522). The English word
   clock actually comes from French, Latin, and German words that mean
   bell. The passage of the hours at sea were marked by bells, and denoted
   the time (see ship's bells). The hours were marked by bells in the
   abbeys as well as at sea.

   Incense sticks and candles were, and are, commonly used to measure time
   in temples and churches across the globe. Waterclocks, and later,
   mechanical clocks, were used to mark the events of the abbeys and
   monasteries of the Middle Ages. Richard of Wallingford (1292–1336),
   abbot of St. Alban's abbey, famously built a mechanical clock as an
   astronomical orrery about 1330,.

   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, needed to determine longitude by means of celestial
   navigation. Nowadays over 1,000,000 "Officially Certified Chronometer"
   certificates, mostly for mechanical wrist-chronometers (wristwatches)
   with sprung balance oscillators, are being delivered each year, after
   passing the COSC's most severe tests and being singly identified by an
   officially recorded individual serial number. According to COSC, a
   chronometer is a high-precision watch capable of displaying the seconds
   and housing a movement that has been tested over several days, in
   different positions, and at different temperatures, by an official,
   neutral body (COSC). Each movement is individually tested for several
   consecutive days, in five positions and at three temperatures. Each
   movement is individually measured. Any watch with the denomination
   "chronometer" is provided with a certified movement.

   The most accurate type of timekeeping device is currently the atomic
   clock, which are used to calibrate other clock and timekeeping
   instruments.

   Today, the GPS global positioning systems in coordination with the NTP
   network time protocol can be used to synchronize timekeeping systems
   across the globe.

Standards

              Common units of time
        Unit            Size           Notes
   Femtosecond    10^–15 second
   Picosecond     10^–12 second
   Nanosecond     10^–9 second
   Microsecond    10^–6 second
   Millisecond    ^1/[1,000] second
   Second         SI base unit
   Minute         60 seconds
   Hour           60 minutes
   Day            24 hours
   Week, Sennight 7 days
   Fortnight      14 days; 2 weeks
   Month          28 to 31 days
   Quarter        3 months
   Year           12 months
   Tropical year  365.24219 days    Average
   Olympiad       4 years
   Lustrum        5 years           obsolete
   Decade         10 years
   Indict         15 years          obsolete
   Score          20 years
   Generation     25 years          approximate
   Century        100 years
   Millennium     1,000 years

   The SI base unit for time is the SI second. From the second, larger
   units such as the minute, hour and day are defined, though they 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:

          "The second is the duration of 9,192,631,770 periods of the
          radiation corresponding to the transition between the two
          hyperfine levels of the ground state of the caesium 133 atom."

   Previous to 1967, the second was defined as:

          the fraction 1/31,556,925.9747 of the tropical year for 1900
          January 0 at 12 hours ephemeris time.

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.

Interpretations

   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 book Ecclesiastes, traditionally thought to have
   been written by King Solomon (970-928 BC), time was regarded as a
   medium for the passage of predestined events.

     "There is an appointed time for everything. And there is a time for
     every event under heaven— 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." (Ecclesiastes 3:1–8)

   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

   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 intuition that allows us (together with the other a priori
   intuition, 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).

   Ralph Waldo 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.

   Writers such as J. M. E. McTaggart in his 1908 The Unreality of Time
   have argued that time is an illusion (see also The flow of time).

Linear 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.

Cyclical time

   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.
   Enlarge
   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 the past, present and future.

Natural unit of time

   Planck time (~ 5.4 × 10^−44 seconds) is the unit of time in the system
   of natural units known as Planck units. According to current theory, it
   is the smallest unit of time that could ever be measured.

Time quanta

   Time quanta is a hypothetical concept. In the modern quantum theory (
   Standard Model of particle physics) and in General relativity time is
   not quantized.

Time dilation

   Einstein said that the only reason for time is so that everything
   doesn't happen at once. In this regard, 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, on the
   average, for a standard amount of time known as its mean lifetime, and
   the distance it travels in that time is zero, because its velocity is
   zero. 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).

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 also appears to pass more quickly as one gets older. For example,
   a year for a five-year-old child is 20% of his entire life so far,
   however for a 50 year old adult a year is only 2% of his entire life so
   far; so 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.
   When viewed under the influence of such substances as LSD, magic
   mushrooms and peyote, a clock may appear to be a strange reference
   point and a useless tool for measuring the passage of events as it does
   not correlate with the user's experience. At higher doses, time may
   appear to slow down, stop, speed up and even go backward when under the
   influence of these agents. A typical thought might be "I can't believe
   it's only 8 o'clock, but then again, what does 8 o'clock mean?" As the
   boundaries for experiencing time are removed, so is it's relevance.
   Many users claim this unbounded timelessness feels like a glimpse into
   spiritual infinity. To imagine that one exists somewhere "outside" of
   time is one of the hallmark experiences of a psychedelic voyage.
   Marijuana may also distort the perception of time, although, to a
   lesser degree than psychedelics.

   The practice of meditation, central to all Buddhist traditions, takes
   as its goal the reflection of the mind back upon itself, thus altering
   the subjective experience of time; the so called, 'entering the now',
   or 'the moment'.

   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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