- For alternate uses of "time", see Time (disambiguation) or see TIME (magazine).
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.
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.)
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.
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 part of the measurement system?" The same debate applies also to space, and an important formulation in both areas was given by Immanuel Kant.
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 we use to structure our experience. Spatial measurements are used to quantify how far apart objects are, and temporal measurements are used to quantify how far apart events occur.
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, causality, and eternity (in one technical sense, eternal means "outside of time").
Existentially, time has been considered fundamental to the question of being , in particular by the philosopher Martin Heidegger.
Time in physics
, 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
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
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 eons. (Einstein said, "Put your hand on a hot stove for a minute, and it seems like an hour. Sit with a pretty girl for an hour, and it seems like a minute. That's relativity.")
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 may feel like it has not 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, or, to put it differently, time appears suspended. Time seems to go faster with age. In childhood a day is a long time; in adulthood, it seems to pass much more quickly. 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. Adults may perceive time to pass more quickly because it provides the evolutionary advantage of bestowing more patience upon adults, so they are better at long term planning, which benefits all of society.
Hallucinogenic drugs can also dramatically alter a person's perception of time.
Time is also suspected to be perceived differently by people, due to neurological differences. This is believed to be the result of the differences between the various ways people tend to perceive their own world. The term 'absent-minded' carries with it the connotation of being chronically late or unaware of the "correct" passage of time on a more general scale, or, in other words, the passage of time as it is more generally perceived. The term itself, however, has nothing to do with a perception of time, and it simply is a descriptor applied to a person who is introverted. Since human thoughts often flow more quickly than the observable events of the world outside (human neurons spike in electrical activity 200 times every second), introverted individuals who are more focused on what happens within their own minds may find themselves unaware of and often completely disinterested in time as perceived by extroverted individuals, because mental events occur at a much more rapid pace than external events.
Thusly, introverted people often measure large periods of time happening in very short periods of time, in relation to the measurements which might be derived if one were to focus on the outside world as a means of time measurement. Because of this, introverted people are prone to 'losing track of time', as they may perceive that, on a general basis, large amounts of time pass between events in 'reality' which most people would consider to be temporally significant but which they themselves would not find any significance, relative to their own perception of the passage of time. Therefore, introverted people are more likely to spend ages thinking about 'something else' and to not realize when a large amount of time has actually passed in terms of a system of temporal measurement they have no interest in maintaining. If a large amount of time spent thinking is equal to a short amount of time in 'outside reality', then it becomes difficult to distinguish at which point a large amount of time has transpired in 'reality' without counting, equating and making differences with the apparent time-frame of outside reality. However, no experiments confirm or discount this hypothesis so far.
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.
"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 about time by J. R. R. Tolkien, The Hobbit
"Time is money." - Benjamin Franklin
"Time is what prevents everything from happening at once. Space is what prevents everything from happening to me." - attributed to John Archibald Wheeler
"I confess I do not believe in time." - Vladimir Nabokov
"Truth is always new, therefore timeless." - J. Krishnamurti
General units of time
Special units of time
Time measurement and horology
Theory and study of time
- Einstein's Clocks and Poincaré's Maps: Empires of Time. By Peter Galison. W.W. Norton; 256 pages