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For other uses, see Time (disambiguation).
8:17 am, August 6, 1945, Japanese time.  This watch was stopped when the nuclear bomb nicknamed the "Little Boy" was detonated over Hiroshima, Japan, near the end of World War II.
8:17 am, August 6, 1945, Japanese time. This watch was stopped when the nuclear bomb nicknamed the "Little Boy" was detonated over Hiroshima, Japan, near the end of World War II.

Attempting 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 except its physical definition, which dictionaries give as "a non-spatial 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 apparently periodic motion have long served as standards for units of time - such as the apparent motion of the sun across the sky, the phases of the moon, the swing of a pendulum.


Philosophy of time

Main article: Philosophy of space and time; Ontology

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. 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.

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.
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 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 standard time interval (called conventional second, or simply second) is defined as 9 192 631 770 oscillations of specified transition in Cs-133 atom.

Prior to Albert Einstein's relativistic physics, time and space had been treated as distinct dimensions; Einstein linked time and space into spacetime. 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 - 1968): "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.


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. (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 60 base used for seconds, minutes and hours is all the remains of the ancient Phoenician counting base, using 60 as the equivalent of 10, or 100 in modern times. A 60 base is known as sexagesimal.


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.


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.

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.

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

Time management

Time can be managed with less disputes if it is organised better. This can stop a last minute rush and even reduce time related stress if you try to work out how you are going to do everything you need to do instead of panicking about it. The rota or rosta is the method that is used in order to manage time and is something that normally needs other people to help with if it involves them for negotiation. Time also needs to be set for different reasons like with staffs shift work for example. This will lead to a reduction in confusion and worrying about time and things are more likely to get done in the best way possible.

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See also

Look up time on Wiktionary, the free dictionary.

General units of time

Special units of time

Light-year is the distance light can travel in an Earth year and so is a unit of distance rather than time.

Time measurement and horology

Theory and study of time

External links

Perception of time




Further reading

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