From Academic Kids
Attempting to understand the nature of Time has always been a prime occupation for philosophers and scientists. Perhaps as a result of this considerable discussion, it is difficult to provide an uncontroversial and clear definition of the nature of time. This article begins by looking 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. Time has long been an important theme for writers, artists and philosophers. 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.
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 (http://www1.bipm.org/utils/en/pdf/si-brochure.pdf).)
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.
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.
Philosophy of time
A contrasting view is that time is part of a fundamental 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.
These opposing views are relevant also to definitions of space. Modern physics does not treat time and space as independent dimensions, but treats both as features of spacetime – a conception that challenges intuitive notions of simultaneity and the regularity of time.
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.
In ancient thought, Zeno's paradoxes fundamentally 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.
The understanding of time was a point of contention between Newton and Leibniz. Newton believed time was, like space, a container for events, while Leibniz believed time was, like space, a conceptual apparatus describing the interrelations between events. (Leibniz also held that this notion of time depended on motion.) Scientists have tended to accept the Newtonian notion, whilst some philosophers follow Leibniz. It should be noted however that these two views are not mutually exclusive, and that there are wide variations on these themes.
On the other hand, 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.
Nietzsche, inspired by the concept of eternal return in his book Thus Spake 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.
Time in physics
Main article: Time in physics
Prior to Einstein's relativistic physics, time and space had been treated as distinct dimensions; Einstein linked time and space into spacetime. Einstein showed that people travelling at different speeds will measure different times for events and different distances between objects, though these differences are minute unless one is travelling at a speed close to that of light. Many subatomic particle 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 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.
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 judged 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 traveller'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.
Adult humans and other animals 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 the community.
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.
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.
"What is time? I know what it is, but when you ask me I don't." - Augustine of Hippo
"Life holds one great but quite commonplace mystery. Though shared by each of us and known to all, seldom rates a second thought. That mystery, which most of us take for granted and never think twice about, is time." - Michael Ende
This thing all things devours:Template:- Birds, beasts, trees, flowers;Template:- Gnaws iron, bites steel, Template:- Grinds hard stones to meal; Template:- Slays king, ruins town,Template:- And beats high mountain down.
"Time is what prevents everything from happening at once. Space is what prevents everything from happening to me." - attributed to John Archibald Wheeler
"Time reflexes like a whore/falls wanking to the floor." - David Bowie "Time"
"How can I tell that the past isn't a fiction designed to account for the discrepancy between my immediate physical sensations and my state of mind?" - Douglas Adams, The Restaurant at the End of the Universe (by the man in the shack)
"Time is the school in which we learn, Time is the fire in which we burn." Delmore Schwartz (1913-1966) from "For Rhoda" 
General units of time
Special units of time
- Geologic timescale
- Cosmological decade
- Fiscal year
- Ship's bells
- Periodization and list of time periods
- Unix epoch
- Swatch Internet Time
- Hexadecimal Time
- Shake (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
- lunar calendar
- solar calendar
- Railroad chronometers
- water clock
- time zone
- Time scales and time standards
- Network Time Protocol (NTP)
Theory and study of time
- philosophy of physics
- time travel
- exponential time
- Planck time
- orders of magnitude (time)
- Peter Lynds
- A Brief History of Time
- Time discipline
- Time management
- wheel of time
- A walk through Time (http://physics.nist.gov/GenInt/Time/world.html)
- Cycles Research Institute (http://www.cyclesresearchinstitute.org/)
- Time Travel and Multi-Dimensionality (http://pages.britishlibrary.net/lobster/tmx)
- A paper on time (http://arxiv.org/abs/physics/0310055)
- Another paper on consciousness and the perception of time (http://cogprints.ecs.soton.ac.uk/archive/00003125/)
- Another paper on the consciousness and the perception of time (http://www.primitivism.com/time.htm)
- Time and Learning (http://www.ericdigests.org/2003-5/time.htm)
- Different systems of measuring time (http://tycho.usno.navy.mil/systime.html)
- non-SI units (http://physics.nist.gov/cuu/Units/outside.html)
- UTC/TAI Timeserver (http://www1.bipm.org/en/scientific/tai/time_server.html)
- Leapsecond (http://tycho.usno.navy.mil/leapsec.html)
- Hex Time (http://www.intuitor.com/hex/hexclock.html)
- Florencetime.net (http://www.florencetime.net)
- BBC article on shortest time ever measured (http://news.bbc.co.uk/2/hi/science/nature/3486160.stm)
- American Watchmakers-Clockmakers Institute (http://www.awi-net.org)
- Boost Date-Time Library -- Powerful C++ Library for date-time manipulation (http://www.boost.org/doc/html/date_time.html)
- Earth View (http://www.fourmilab.ch/cgi-bin/uncgi/Earth/action?opt=-p) - showing day and night on earth in real time
- The World Clock - Time Zones (http://www.timeanddate.com/worldclock/)
- TimeTicker and the time tickers... (http://www.timeticker.com/)
- World Time and Zones (http://www.welt-zeit-uhr.de/worldtime.php)