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time
:For alternate uses of "time", see Time (disambiguation) or see TIME (magazine).

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Time quantifies or measures the interval between events, or the duration of events. Time has long been perceived as a dimension in which each event has a definite (but not necessarily unique) position in a linear sequence, but as differing from spatial dimensions in that "motion" through time appears restricted to having only a forward direction.

For everyday purposes, and even for quite accurate measurements, this view is sufficient. However, the scientific understanding of time underwent a revolution in the early part of the twentieth century with the development of relativity theory. Modern physics treats time as a feature of spacetime, a notion which challenges intuitive conceptions of simultaneity and the flow of time in a linear fashion.

Despite scientific advances, the everyday meaning of time is affected more by the social importance of time, its economic value ("time is money") and an awareness of the limited time in each day and in our lives. Thus, time has long been an important theme for writers, artists and philosophers.

Measurement of time

Main articles: Intellectual history of time, Timeline of time measurement technology

The study of time measurement is called horology. People have always sought accurate measurements of time. Ancient people found that the Sun, moon, and stars move in predictable cycles at regular intervals; they used this observation to produce accurate calendars for measuring days, months, seasons, and years. See also equation of time.

More complex societies have discovered ways to measure time even more precisely. Sundials enabled ancient people to divide the daytime up into smaller pieces. Civilizations in Egypt, China, and Greece invented water clocks that could keep fairly accurate time in the dark too. Traditionally, aboard ships, a system of hourglasses and ship's bells were used to mark watches and for navigation. Mechanical clocks were developed in Europe in the 14th Century. Today, time can be measured on very accurate clocks, often called chronometers. The best available clocks are atomic clocks.

At first, people set their clocks based on the noon sun in their locality. The invention of time zones, north-south strips of the Earth in which everyone's clocks are coordinated, made time measurement standardized worldwide. With only a few exceptions, every place on Earth is part of a standard time zone connected with Greenwich Mean Time (because the benchmark for the world's time zones is the time in Greenwich, England).

The development of human understanding of the nature and measurement of time, through the work of making and improving its measurements, (calendars, clocks) and its intuitive concepts (spacetime, general relativity), has been a major engine of scientific discovery since the beginnings of civilization.

Present day standards for time

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. (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.

Philosophy of time

Important questions in the philosophy of time include: Is time absolute or merely relational? Is time without change conceptually impossible or is there more to the idea? Does time "pass" or are the ideas of past, present and future entirely subjective, descriptions only of our deception by the senses?

Zeno's paradoxes fundamentally challenged the ancient conception of time, and thereby helped motivate the development of calculus. McTaggart believed, rather eccentrically, that time and change are illusions. Parmenides (of whom Zeno was a follower) held a similar belief based on a rather interesting argument.

A point of contention between Newton and Leibniz concerned the question of absolute time: the former believed time was, like space, a container for events, while the latter believed time was, like space, a conceptual apparatus describing the interrelations between events.

An issue of philosophical debate is whether time is an ontological entity itself, or simply a conceptual framework we need to think (and talk) about the world. Another way to frame this is to ask, "Can time itself be measured, or is time part of the measurement system?" The same debate applies also to space, and an important formulation in both areas was given by Immanuel Kant.

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 we use to structure our experience. Spatial measurements are used to quantify how far apart objects are, and temporal measurements are used to quantify how far apart events occur.

Einstein's linking of time and space into spacetime also had philosophical consequences, making the idea of block time more credible, and thus affecting ideas of free will, causality, and eternity (in one technical sense, eternal means "outside of time").

Existentially, time has been considered fundamental to the question of being, in particular by the philosopher Martin Heidegger.

Current Time

The Current time, according to the Wikipedia server is:

And beats high mountain down.
:- Riddle about time by J. R. R. Tolkien, The Hobbit''

"Time is money." - Benjamin Franklin

"Time is what prevents everything from happening at once.
Space is what prevents everything from happening to me." - attributed to John Archibald Wheeler

"I confess I do not believe in time." - Vladimir Nabokov

"Truth is always new, therefore timeless." - J. Krishnamurti

External links

- A walk through Time
- Cycles Research Institute
- Time Travel and Multi-Dimensionality
- Time conversion - milliseconds and microseconds to seconds - prefixes
- Conversions of international time units
- A paper on time
- Another paper on consciousness and the perception of time
- Time and Learning
- Different systems of measuring time
- Conversion of any Time units
- non-SI units
- UTC/TAI Timeserver
- Leapsecond
- Hex Time
- Florencetime.net
- BBC article on shortest time ever measured

Books

- Einstein's Clocks and Poincaré's Maps: Empires of Time. By Peter Galison. W.W. Norton; 256 pages

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

terror time

Measurement of time

Present day standards for time

Philosophy of time

Current Time

See also

General units of time

Special units of time

Time measurement and horology

Theory and study of time

External links

Books