This article refers to the common conception of time as indicated by intervals or periods of duration, sometimes referred to as "the fourth dimension" of a space-time continuum.For other uses see Time (disambiguation)
One can say that one event occurs after another event. Furthermore one can measure how much one event occurs after another. The answer to how much is the amount of time between these two events. The separation of two events is an interval, the amount of interval is the duration.
One way of defining the idea of 'after' is based on the assumption of causality. The work humanity has done to increasingly understand the nature and measurement of time, through the work of making and improving calendars and clocks, has been a major engine of scientific discovery.
The standard unit for time is the SIsecond, 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)
Time can be measured, just like other physical dimensions. Measuring devices for time are clocks. Very accurate clocks are often called chronometers. The best available clocks are atomic clocks.
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.
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.
in the case of something of fixed duration which is relatively pleasant (to complete the common phrase, "when you're having fun"), which may be e.g.:
leisure time, holidays
(on the other hand, that the time has flown is considered a sign that it has been enjoyable)
if one has a lot to do
on a larger time scale, "getting old quickly"
Time also seems to go fast when sleeping, some of the above applies, e.g. it may be an advantage to sleep as train or car passenger, and sleep long in the case of boredom, while it may be wasteful to sleep long on holidays.
Time seems to go faster for a person as they get older. In childhood a day is a long time, in adulthood, it seems to pass much quicker. There is a continuing debate about why this might seem to be. One theory is that each time element (e.g. a day) is an increasingly smaller percentage of the person's total experience 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 dispute time budget hypothesis.
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.)
