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 the those two events.

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.

Measurement of time

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. 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.

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 standard for civil time, Terrestrial Time (TT), etc. Mankind has invented calendars to track the passages of days, weeks, months, and years.

Time in engineering and applied 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.

Time in philosophy and theoretical physics

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 the 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.

The engineer J. W. Dunne developed a theory of time whereby he considered our perception of time like notes being played on piano. Having had a number of prescient dreams, he monitored his dreams and found that they generally included as many past as future events. From this he concluded that in dreams we escape linear time. He published his ideas in An Experiment with Time in 1927 and followed this with other books.

Perception of time

One may perceive time to go fast ("time flies"), meaning that a duration seems less than it is; this may be considered an advantage:

• in the case of something of fixed duration which is relatively unpleasant, which may be e.g.:

  • work (perhaps not as pleasant as leisure time, but done for the money)

  • travel (if not done for its own sake, but to get somewhere)

  • waiting, boredom

it may be considered a disadvantage:

• in the case of something of fixed duration which is relatively pleasant, 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.

Books

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