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For other uses, see Time (disambiguation).

Watches are used to measure time

Trying to understand time has long been a prime occupation for philosophers, scientists and artists. There are widely divergent views about its meaning, hence it is difficult to provide an uncontroversial and clear definition of time.

The Oxford English Dictionary defines it 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."

This article looks at some of the main philosophical and scientific issues relating to time.

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, the swing of a pendulum.

1 Philosophy of time
- 1.1 Contemporary theses in the philosophy of time
2 Time in physics
3 Measurement
- 3.1 Present day standards
- 3.2 Chronology
4 Psychology
- 4.1 Use of time
5 See also
- 5.1 General units of time
- 5.2 Special units of time
- 5.3 Time measurement and horology
- 5.4 Theory and study of time
6 References
7 External links
- 7.1 Perception of time
- 7.2 Physics
- 7.3 Timekeeping
- 7.4 Miscellaneous
8 Further reading

Philosophy of time

Main article: Philosophy of space and time; Ontology

In Greece, during the fifth century B.C., 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.

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.

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.

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.

Contemporary theses in the philosophy of time

The neutrality of this section is disputed.
Please see discussion on the talk page.

In contemporary philosophy, for the past century there has been a very active debate over the nature of time, especially in light of the major changes in physics since the turn of the twentieth century. Recent contributors to the debate include Ned Markosian, Ted Sider, Quentin Smith, L. Nathan Oaklander, David Lewis, and John Bigelow. Two major approaches have been developed, along with some hybrids. There is no real consensus among philosophers about which, if any, is correct. The two major sets of theories can be summed up as follows:

1. A-theories of time: Presentism: Oaklander writes: "[A] version of the pure A-theory, known as "", purports to avoid… the problem of change... According to presentism, only the present exists. Thus, it is not the case that, say, O is green and [then] O is red [if, for example, O is a tomato]." (Oaklander, L. Nathan. In Smith, Quentin, and Oaklander, L. Nathan. 1995. Time, Change, and Freedom. New York: Routledge. 2004, 27.) However, not all presentist theories are this extreme.

2. B-theories of time: Eternalism: or Four-dimensionalism): In this view, the past and future are not present to us, but they are nonetheless real. The following passage from L. Nathan Oaklander sums up one way of thinking about such theories:

…[T]ime [involves] events strung out along a series united to one another by the relations of earlier than, later and simultaneity… The events in the temporal series are fixed in that they never change their position relative to each other… It has become customary to call the entire series of events spread out along the time-line from earlier to later, the “B-series.” When viewed solely in terms of the B-series, time is thought of as static or unchanging for there is nothing about temporal relations between events that changes... Time not only has a static aspect, it also has a transitory aspect. In addition to conceiving of time in terms of events standing in temporal relations, we also conceive of time and the events in time as moving or passing from the far future to the near future, from the near future to the present, and then from present they recede into the more and more distant past… When events are ordered in terms of the notions of past, present, or future they form what is called an “A-series.” It should be noted, of course, that the A- and B-series are not really “two” different series of events, but the same series ordered in two different ways. (Oaklander 2004,Page 69)

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 (quantities which cannot 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 ¹³³Cs atom.

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. 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, implying a modulo of 12 and adding a suffix of AM or PM denoting time before noon or after noon, respectively.

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 an increasingly smaller 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. 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.

References

Oxford English Dictionary - [1]

External links

Perception of time

The Experience and Perception of Time
Subjective Perception of Time and a Progressive Present Moment: The Neurobiological Key to Unlocking Consciousness
Time and Its Discontents
Time and Learning
Time Perception I and II
The Order of Time: Platform for an Alternative Time Consciousness
What is Time? An elucidation of the Lubavitcher Rebbe's comments on the topic.

Physics

A walk through Time
Time Travel and Multi-Dimensionality
Time and classical and quantum mechanics: Indeterminacy vs. discontinuity
Time as a universal consequence of quanta

Timekeeping

Different systems of measuring time
non-SI units
UTC/TAI Timeserver
Leapsecond
Hex Time
Florencetime.net
BBC article on shortest time ever measured
American Watchmakers-Clockmakers Institute
The World Clock - Time Zones
World Local Times on Google Map by single click

Miscellaneous

Boost Date-Time Library -- Powerful C++ Library for date-time manipulation
Cycles Research Institute
TimeTicker and the time tickers...
World Time and Zones
Time Servers NTP Time Servers
[2] official US time