Time

   

For alternate uses of "time", see Time (disambiguation).
8:15 am, August 6, 1945. Japan time
8:15 am, August 6, 1945. Japan time

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.

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 (http://www1.bipm.org/utils/en/pdf/si-brochure.pdf).)

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.

Time in physics

Main article: 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.
Enlarge
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.


Chronology: historical time

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.


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

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 the measurement system?" The same debate applies also to space, and was given an important formulation in both areas by Immanuel Kant.

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 and causality.

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

Quotes

"What is time? I know what it is, but when you ask me I don't." - Augustine of Hippo

"Time is an illusion, lunchtime doubly so." - Douglas Adams, The Hitchhiker's Guide to the Galaxy

This thing all things devours:
Birds, beasts, trees, flowers;
Gnaws iron, bites steel,
Grinds hard stones to meal;
Slays king, ruins town,
And beats high mountain down.
- Riddle by J. R. R. Tolkien, The Hobbit (The answer is time.)

"Time is money." - Benjamin Franklin

"Time exists so that everything doesn't happen at once. Space exists so that everything doesn't happen to me." - anonymous

See also

General units of time

Special units of time

Time measurement and Horology

Theory and study of time

External links

Books

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


bg:Време ca:Temps cs:Čas da:Tid de:Zeit et:Aeg el:Χρόνος es:Tiempo eo:Tempo fr:Temps fy:Tiid io:Tempo is:Tími it:Tempo he:זמן la:Tempus nl:Tijd ja:時間 no:Tid nds:Tied pl:Czas simple:Time sl:Čas fi:Aika sv:Tid zh:时间


Retrieved from "http://www.centipedia.com/articles/Time"

This page has been accessed 3031 times. This page was last modified 01:18, 25 Nov 2004. All text is available under the terms of the GNU Free Documentation License (see Copyrights for details).