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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. Scholars disagree on whether time itself can be measured or is itself part of the measuring system. Many fields use an operational definition in which the only definition attempted is that of the units used.

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.

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.

Measurement devices

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

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 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 are atomic clocks. More archaic devices include the Hourglass, the Sundial, the Tempometer and the Water clock.

Standards

Common units of time
Unit Size
Millisecond One-thousandth of a second
Second SI base unit
Minute 60 seconds
Hour 60 minutes
Day 24 hours
Week 7 days
Fortnight 14 days; 2 weeks
Month Between 28 and 31 days
Quarter 3 months
Year 12 months
Olympiad 4 years
Lustrum 5 years
Decade 10 years
Indict 15 years
Generation 25 years
Century 100 years
Millennium 1000 years

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:

"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."

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 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 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."

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."

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

Dictionary definitions

According to the Oxford English Corpus, the word 'time' comes top in the list of commonest 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." The American Heritage Dictionary 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 άτομον meaning "indivisible"), thus signifying a division of the flowing duration.

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 infact be cyclical.

Circular time

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. Thus if one could go far enough into the future, one would return again to the past.

Absolute time

"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."
Isaac Newton, Principia Translated by I. Bernard Cohen and Anne Whitman, University of California Press, Berkeley, 1999.

Newton believed time and space form a container for events, which is as real as the objects it contains.

Spacetime

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.

Quantized time

Unreal time

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 Parmendies).

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 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.

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.

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 travel

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 suggest the possibility of travel between parallel realities or universes. A central problem with time travel is that of causality - causes preceeding 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 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.

References

See also

Special units of time

Further reading

External links

Perception of time

Physics

Timekeeping

Miscellaneous

Time | Metrology

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This article is licensed under the GNU Free Documentation License. It uses material from the "Time".

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