time - Measurement, Standards, Interpretations, Psychology, Use of time, Further reading

That which distinguishes sequential events from simultaneous events; symbol t, units s (second); the fourth dimension, in addition to the three spatial dimensions. It allows the assignment of cause and effect, and, according to our perception, the assignment of past, present, and future. In Newtonian mechanics, time is absolute, meaning that a second as measured by one observer is the same as a second measured by any other observer in the universe. Relativity explains that this view of the nature of time is false. In thermodynamic systems, the directionality of time derives from entropy. Time directionality in particle physics was first seen in 1998 via the properties of particles called kaons.

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.

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."

Many fields avoid the problem of defining time itself by using operational definitions that specify the units of measurement that quantify time.

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.

Time has long been a major subject of science, philosophy and art. 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.

Measurement

Time is currently one of the few fundamental quantities.

Measurement devices

A large variety of devices have been invented to measure time.

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 hourglass uses the flow of sand to measure the flow of time.

Incense sticks and candles were, and are, commonly used to measure time in temples and churches across the globe.

A chronometer is a timekeeper precise enough to be used as a portable time standard, needed to determine longitude by means of celestial navigation.

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

See also: Orders of magnitude (time)

The SI base unit for time is the SI second.

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:

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.

Interpretations

See also: Philosophy of space and time

Many ancient philosophers wrote lengthy essays on time, believing it to be the essence around which life was based.

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; 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. 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. 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.

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.

In Existentialism, time is considered fundamental to the question of being, in particular by the philosopher Martin Heidegger.

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.). 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.

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.

Time in physical sciences

Spacetime

See also: Time in physics

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.

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.

Time quanta

Time quanta is a hypothetical concept.

See also: Chronon

Time dilation

Einstein said that the only reason for time is so that everything doesn't happen at once. 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. 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.

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. 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.

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

See also: Time travel in fiction

Time travel is the concept of moving backward or forward to different points in time, in a manner analogous to moving through space.

Psychology

Different people may judge identical lengths of time quite differently. The psychologist Jean Piaget called this form of time perception "lived time."

Time also appears to pass more quickly as one gets older.

Altered states of consciousness are sometimes characterised by a different estimation of time.

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;

Use of time

See also: Time management and Time discipline

The use of time is an important issue in understanding human behaviour, education, and travel behaviour. 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.

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.

Arlie Russell Hochschild and Norbert Elias have written on the use of time from a sociological perspective.

Special units of time

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