Time

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

File:815watch.jpg

8:15 am, August 6, 1945. Japan time

Time is the dimension (or medium) in which change occurs, and which measures the duration of states. Modern physics treats time as a dimension of space-time whose precise direction is relative to the inertial frame of measurement.

The common way of defining the idea of 'before' and 'after' is based on the assumption of causality. One can typically say that a partcular event "occurs" after another event, and one can apply the extra-contextual relative metric of "time" to gauge the interval between the two. The term "time", however, describes both the measurable interval between two (perceived or conceived) events, and implies the truth of a continuous measurable system as the purposeful metric for conceptualizing relationships between the forementioned two events and all other events within the same universe.

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.

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 presicely. 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 and localities set their clocks in a haphazard fashion. 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.

Time in engineering and applied 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.

See also: Synchronization, ISO 8601, Allan variance

Time in philosophy and theoretical 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.

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 and on the basis of a very shaky argument, that time and change are illusions. Parmenides (of whom Zeno was a follower) held a similar belief based on a similarly shaky, but rather more 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 illision, 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 all 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.

Quotes

"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;

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

See also

• Event
• Duration
• Change
• Rate
• Causality
• Cycles and List of cycles

General units of time

• second
• minute
• hour
• day
• week
• fortnight
• month
• quarter
• year
• decade
• century
• millenium

Special units of time

• Geologic timescale
• Season
• Era
• Tithi
• Fiscal year
• Ship's bells
• Half-life
• Periodization and list of time periods
• Unix epoch
• Swatch Internet Time
• Hex Time

Time measurement and Horology

• calendar
• lunar calendar
• solar calendar
• chronometer
• Railroad chronometers
• clock
• water clock
• hourglass
• sundial
• time zone
• Time scales and time standards
• watch

Theory and study of time

• philosophy of physics
• spacetime
• time travel
• exponential time
• Planck time
• orders of magnitude (time)
• Eternity
• Peter Lynds
• A Brief History of Time
• Periodization
• Chronology
• History
• Time management
• Wikibooks:English:Time

External links

• A walk through Time
• 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

Books

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