User:FrankP/Time


 * For alternate uses of "time", see Time (disambiguation).



Time quantifies or measures the interval between events or the duration of events.

Time has long been perceived as a linear flow which can be used to sequence events, differing from spatial dimensions in that our movement in time appears restricted to a steady pace in one 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.

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

Time can be measured, just like other physical dimensions; the study of time measurement is horology. People have always sought to measure time accurately. 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.

More complex societies have discovered ways to measure time even more precisely. Hourglasses and sundials can measure smaller increments of time quite accurately, and have been known for centuries. The most consistent and accurate time measurement tool, however, is the clock. Civilizations in Egypt, China, and Greece invented water clocks that could keep fairly accurate time. 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. Traditionally, aboard ship a system of hourglasses and ship's bells are used to mark time.

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 standard unit for time is the SI second, from which larger units are defined like the minute, hour, day, week, month, year, decade, and century. Thus, up to the year the system does not use a decimal system. Also there are no fixed ratios between second, minute, hour, day and week on one hand and month and year on the other hand. The minute, hour and day are officially "non-SI units accepted for use with the International System". (The International System of Units)

There are several continuous time scales in current use: Universal Time, International Atomic Time (TAI), which is the basis for other time scales, Coordinated Universal Time (UTC), which is the basis for civil time, Terrestrial Time (TT), etc.

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.

Accurate time measurements are a fundamental need in many scientific disciplines. 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 its beginning.

Time in physics
Main article: Time in physics

In physics, time is defined as the distance between events along the fourth axis of the spacetime manifold. Special relativity showed that time cannot be understood except as part of spacetime, a combination of space and time. The distance between events now depends on the relative speed of the observers of the events. General relativity further changed the notion of time by introducing the idea of curved spacetime. An important unit of time in theoretical physics is the Planck time – see Planck units for more details.

Click herefor BBC article on shortest time ever measured as of 2004.

See also: Synchronization, ISO 8601, Allan variance



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. 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. 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.

Einstein's theory of relativity linked time and space into spacetime in a way that also had philosophical consequences, making the idea of block time more credible, and thus affecting ideas of free will and causality.

Perception of time
It is a fact that different people may perceive identical lengths of time quite differently. Time can "fly;" that is, a long period of time can seem to go by very quickly. This can be good or bad, depending on whether whatever one was doing during that time was pleasant or unpleasant. Likewise, time can seem to "drag," so that brief spells of time can feel like long eons.

The perceived speed of time depends on a number of factors. If a person has a very long list of tasks to accomplish on a certain day, the day never feels like it has enough hours to do everything. Likewise, even a short wait at a bus stop while running late can feel endless. A day filled with fun activities can feel very long due to the number of activities that fill it. A long trip can go by quickly if the traveler's mind is occupied.

Time also seems to go fast when sleeping. Time seems to go faster with age. In childhood a day is a long time; in adulthood, it seems to pass much quicker. Most likely this is because with increasing age, each period of (e.g. a day) is an increasingly smaller percentage of the person's total experience of time. Hallucinogenic drugs can also dramatically alter a person's perception of time.

Since different people perceive time differently, this begs the question of whether time is merely an illusion, a product of people's senses. However, the fact that instruments can measure time consistently implies that time is a physical reality, but that, like many physical traits, can be perceived differently by different people.

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

Arlie Russell Hochschild and Norbert Elias have written on the subject from a sociological perspective.

Philosophy of time
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. The same debate applies also to space, and was given an important formulation in both areas by Immanuel Kant.

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