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A pocket watch.

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 Greek temnein meaning "to cut" (same root for atomos άτομον 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

Main article: Philosophy of space and time

(also ontology)

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

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.

Main article: 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 caesium (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.

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 like 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 our 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.

See also

General units of time

Special units of time

Light-year is the distance light can travel in an Earth year and so is a unit of distance rather than time.

Time measurement and horology

Theory and study of time

References

  • Oxford English Dictionary -

Further reading

  • Whitrow, Gerald J. (1973). The Nature of Time. Holt, Rinehart and Wilson (New York).
  • Whitrow, Gerald J. (1980). The Natural Philosophy of Time. Clarendon Press (Oxford).
  • Whitrow, Gerald J. (1988). Time in History. The evolution of our general awareness of time and temporal perspective. Oxford University Press. ISBN 0-19-285211-6.
  • Highfield, Roger (1992). Arrow of Time: A Voyage through Science to Solve Time's Greatest Mystery. Random House. ISBN 0449907236.
  • Galison, Peter (1992). Einstein's Clocks and Poincaré's Maps: Empires of Time.
  • Prerau, David (2005). Seize the Daylight: The Curious and Contentious Story of Daylight Saving Time. Thunder's Mouth Press. ISBN 1-56025-655-9. seizethedaylight.com

External links

Perception of time

Physics

Timekeeping

Miscellaneous

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