There are two distinct views on the meaning of time.
One view is that time is part of the fundamental structure of the universe, a dimension in which events occur in sequence, and time itself is something that can be measured. This is the realist's view, to which Sir Isaac Newton subscribed, and hence is sometimes referred to as Newtonian time (Newton's Views on Space, Time, and Motion - Stanford University 
A contrasting view is that time is part of the fundamental intellectual structure (together with space and number). Within this structure, humans sequence events, quantify the duration of events and the intervals between them, and compare the motions of objects. In this second view, time does not refer to any kind of entity that "flows", that objects "move through", or that is a "container" for events. This view is in the tradition of Gottfried Leibniz<ref> Leibniz on Space, Time, and Indiscernibles - Against the Absolute Theory -- Internet Encyclopedia of Philosophy  and Immanuel Kant, Critique of Pure Reason - Lecture notes of G. J. Mattey, UC Davis  Kant's Transcendental Idealism - Internet Encyclopedia of Philosophy  in which time, rather than being an objective thing to be measured, is part of the mental measuring system.
In physics, time and space are considered fundamental quantities (i.e. they cannot be defined in terms of other quantities because other quantities - such as velocity, force, energy, etc - are already defined in terms of them). Thus the only definition possible is an operational one, in which time is defined by the process of measurement and by the units chosen. Periodic events and periodic motion have long served as standards for units of time. Examples are the apparent motion of the sun across the sky, the phases of the moon, the swing of a pendulum, heartbeats, etc. Currently, the unit of time interval (the second) is defined as a certain number of hyperfine transitions in Cesium atoms (see below). All properties of time follow from this definition.
Time has long been a major subject of science, philosophy, and art. Its measurement has occupied scientists and technologists, and was a prime motivation in astronomy. Time is also 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 human lifespans.
- 1 Measurement
- 2 Interpretations
- 2.1 Time in philosophy
- 2.2 Linear time
- 2.3 Cyclical time
- 2.4 Time in the physical sciences
- 2.5 Time and the Big Bang
- 2.6 Time travel in science fiction
Time is currently one of the few fundamental quantities. These are quantities which cannot be defined via other quantities because there is nothing more fundamental that is presently known. Thus, similar to definitions of other fundamental quantities (like space and mass), time is defined by the units used to measure it and the method of its measurement. In essence, this definition defines time itself which otherwise is left undefined.
The origins of our current measurement system go back to the Sumerian civilization of approximately 2000 BC. This is known as the Sumerian Sexagesimal System based on the number 60. 60 seconds in a minute, 60 minutes in an hour - and possibly a calendar with 360 (60x6) days in a year (with a few more days added on). Twelve also features prominently, with roughly 12 hours of day and 12 of night, and 12 months in a year.
An Egyptian device dating to c.1500 BCE, similar in shape to a bent T-square, measured the passage of time from the shadow cast by its crossbar on a non-linear rule. The T was oriented eastward in the mornings. At noon, the device was turned around so that it could cast its shadow in the evening direction. Jo Ellen Barnett, Time's Pendulum ISBN 0-306-45787-3 p.28
A sundial uses a gnomon to cast a shadow on a set of markings which were calibrated to the hour. The position of the shadow marked the hour in local time. Pliny the Elder records that the first sundial in Rome was looted from Catania, Sicily (264 BCE), which gave the incorrect time for a century, until the markings appropriate for the latitude of Rome were used (164 BCE). Jo Ellen Barnett, Time's Pendulum p.31 Noontime was an event which could be marked by the time of the shortest shadow on a sundial. This was used in Rome to judge when a court of law was open; lawyers had to be at the court by that time.
The most accurate timekeeping devices of the ancient world were the waterclock or clepsydra, first found in Egypt. A waterclock was found in the tomb of pharaoh Amenhotep I (1525 - 1504 BCE). Waterclocks were used in Alexandria, and then worldwide, for example in Greece, from c.400 BCE. They could be used to measure the hours even at night, but required manual timekeeping to replenish the flow of water. Plato is said to have invented a water-based alarm clock. It depended on the nightly overflow of a vessel containing lead balls, which would float in a columnar vat. The vat would hold an increasing supply of water supplied by a cistern. Eventually the vessel would float high enough to tip over. The lead balls would then cascade onto a copper platter. The resultant clangor would then awaken his students at the Academy (378 BCE).Jo Ellen Barnett, Time's Pendulum p.38</ref> The Greeks and Chaldeans regularly maintained timekeeping records as an essential part of their astronomical observations. In particular, Arab engineers improved on the use of waterclocks up to the Middle Ages.Jo Ellen Barnett, Time's Pendulum p.37
The hourglass uses the flow of sand to measure the flow of time. They were used in navigation. Ferdinand Magellan used 18 glasses on each ship for his circumnavigation of the globe (1522). Laurence Bergreen, Over the Edge of the World: Magellan's Terrifying Circumnavigation of the Globe, HarperCollins Publishers, 2003, hardcover 480 pages, ISBN 0-06-621173-5</ref> The English word clock actually comes from French, Latin, and German words that mean bell. The passage of the hours at sea were marked by bells, and denoted the time (see ship's bells). The hours were marked by bells in the abbeys as well as at sea.
Incense sticks and candles were, and are, commonly used to measure time in temples and churches across the globe. Waterclocks, and later, mechanical clocks, were used to mark the events of the abbeys and monasteries of the Middle Ages. Richard of Wallingford (1292–1336), abbot of St. Alban's abbey, famously built a mechanical clock as an astronomical orrery about 1330.<ref>North, J. (2004) God's Clockmaker: Richard of Wallingford and the Invention of Time. Oxbow Books. ISBN 1-85285-451-0</ref><ref>Watson, E (1979) "The St Albans Clock of Richard of Wallingford". Antiquarian Horology 372-384.</ref>
The most common devices in day-to-day life are the clock, for periods less than a day, and the calendar, for periods longer than a day. Clocks can range from watches, to more exotic varieties such as the Clock of the Long Now. They can be driven by a variety of means, including gravity, springs, and various forms of electrical power, and regulated by a variety of means such as a pendulum. There are also a variety of different calendars, for example the Lunar calendar and the Solar calendar, although the Gregorian calendar is the most commonly used.
A "chronometer" is a portable timekeeper that meets certain precision standards. Initially, the term was used to refer to the marine chronometer, a timepiece used to determine longitude by means of celestial navigation. More recently, the term has also been applied to the chronometer watch, a wristwatch that meets precision standards set by the Swiss agency COSC. Over 1,000,000 "Officially Certified Chronometer" certificates, mostly for mechanical wrist-chronometers (wristwatches) with sprung balance oscillators, are being delivered each year, after passing the COSC's most severe tests and being singly identified by an officially recorded individual serial number. According to COSC, a chronometer is a high-precision watch capable of displaying the seconds and housing a movement that has been tested over several days, in different positions, and at different temperatures, by an official, neutral body (COSC). Each movement is individually tested for several consecutive days, in five positions and at three temperatures. Any watch with the denomination "chronometer" is provided with a certified movement.
The most accurate type of timekeeping device is currently the atomic clock, which are used to calibrate other clock and timekeeping instruments.
Many ancient philosophers wrote lengthy essays on time. A famous analogy compared the time of life to the passing of sand through an hourglass (a common measuring device for time in the past). The sand at the top is associated with the future, and, one tiny grain at a time, the future flows through the present into the past (associated with the sandpile at the bottom of hourglass). The past: ever expanding, the future: ever decreasing, but the future grains become amassed into the past through the present. This was widely discussed in around the 3rd century CE.
The earliest recorded philosophy of time was expounded by Ptahhotep, who lived c.2650–2600 BCE said: "Do not lessen the time of following desire, for the wasting of time is an abomination to the spirit."
In the Old Testament book Ecclesiastes, thought to have been written by Solomon (970–928 BCE), time (as the Hebrew word עת ’êth is often translated, as well as "season") was traditionally regarded as a medium for the passage of predestined events. (Another word, זמן zman, was current as meaning time fit for an event, and is used as the modern Hebrew equivalent to the English word "time".)
"There is an appointed time (zman) for everything. And there is a time (’êth) for every event under heaven
A time to give birth, and a time to die; A time to plant, and a time to uproot what is planted.
A time to kill, and a time to heal; A time to tear down, and a time to build up.
A time to weep, and a time to laugh; A time to mourn, and a time to dance.
A time to throw stones, and a time to gather stones; A time to embrace, and a time to shun embracing.
A time to search, and a time to give up as lost; A time to keep, and a time to throw away.
A time to tear apart, and a time to sew together; A time to be silent, and a time to speak.
A time to love, and a time to hate; A time for war, and a time for peace.Ecclesiastes
Around 500 BC Heraclitus, held that the passage of time and the future both lay beyond the possibility of human influence: "Everything flows and nothing abides; everything gives way and nothing stays fixed. You cannot step twice into the same river, for other waters and yet others, go flowing on. Time is a child, moving counters in a game; the royal power is a child's."
Time in philosophy
In Book 11 of St. Augustine's Confessions, he ruminates on the nature of time, asking, "What then is time? If no one asks me, I know: if I wish to explain it to one that asketh, I know not." He settles on time being defined more by what it is not than what it is. St.,Augustine, Confessions, Book 11. . Newton believed time and space form a container for events, which is as real as the objects it contains.
"Absolute, true, and mathematical time, in and of itself and of its own nature, without reference to anything external, flows uniformly and by another name is called duration. Relative, apparent, and common time is any sensible and external measure (precise or imprecise) of duration by means of motion; such a measure—for example, an hour, a day, a month, a year—is commonly used instead of true time.|Principia, Isaac Newton Translated by I. Bernard Cohen and Anne Whitman, University of California Press, Berkeley, 1999.
In contrast to Newton's belief in absolute space, and closely related to Kantian time, Leibniz believed that time and space are a conceptual apparatus describing the interrelations between events. The differences between Leibniz's and Newton's interpretations came to a head in the famous Leibniz-Clarke Correspondence. Leibniz 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.
Immanuel Kant, in the Critique of Pure Reason, described time as an a priori intuition that allows us (together with the other a priori intuition, space) to comprehend sense experience. With Kant, neither space nor time are conceived as substances, but rather both are elements of a systematic mental framework necessarily structuring the experiences of any rational agent, or observing subject. Spatial measurements are used to quantify how far apart objects are, and temporal measurements are used to quantify how far apart events occur. Similarly, Schopenhauer stated in the preface to his On the Will in Nature that "Time is the condition of the possibility of succession."
Einstein showed that if time and space is measured using electromagnetic phenomena (like light bouncing between mirrors) then due to the constancy of the speed of light, time and space become mathematically entangled together in a certain way (called Minkowski space) which in turn results in Lorentz transformation and in entanglement of all other important derivative physical quantities (like energy, momentum, mass, force, etc) in a certain 4-vectorial way (see special relativity for more details).
Henri Bergson believed that time was neither a real homogeneous medium nor a mental construct, but possesses what he referred to as Duration. Duration, in Bergson's view, was creativity and memory as an essential component of reality.Creative Evolution. Translated by Arthur Mitchell. Mineola: Dover, 1998.
Time as "unreal"
In 5th century BC Greece, Antiphon the Sophist, in a fragment preserved from his chief work On Truth held that: "Time is not a reality (hupostasis), but a concept (noêma) or a measure (metron)." Parmenides went further, maintaining that time, motion, and change were illusions, leading to the paradoxes of his follower Zeno. Harry Foundalis, You are about to disappear 
Time as illusion is also a common theme in Buddhist thought,  Tom Huston}} and some modern philosophers have carried on with this theme. J. M. E. McTaggart's 1908 The Unreality of Time, for example, argues that time is unreal (see also The flow of time).
However, these arguments often center around what it means for something to be "real". Modern physicists generally consider time to be as "real" as space, though others such as Julian Barbour in his The End of Time argue that quantum equations of the universe take their true form when expressed in the timeless configuration spacerealm containing every possible "Now" or momentary configuration of the universe, which he terms 'platonia'. 
In general, the Judaeo-Christian concept, based on the Bible, is that time is linear, with a beginning, the act of creation by God. The Christian view assumes also an end, the eschaton, expected to happen when Christ returns to earth in the Second Coming to judge the living and the dead. This will be the consummation of the world and time. St Augustine's City of God was the first developed application of this concept to world history. The Christian view is that God and the supernatural world are outside time and exist in eternity. This view relies on interpretation however, for some Jewish and Christian sects believe time may in fact be cyclical. It is also possible to see time as having more than one dimension. In this view, over time the universe branches into multiple alternative universes where different events have occurred.
The dharmic religions such as Hinduism, Buddhism and Jainism, have a concept of a wheel of time, that regards time as cyclical and quantic consisting of repeating ages that happen to every being of the Universe between birth and extinction. In recent years this cyclical vision of time has been embraced by theorists of quantic space-time and systems theory. This view has also not been scientifically verified.
Time in the physical sciences
From the age of Newton up until Einstein's profound reinterpretation of the physical concepts associated with time and space, time was considered to be "absolute" and to flow "equably" (to use the words of Newton) for all observers.<ref>Herman M. Schwartz, Introduction to Special Relativity, McGraw-Hill Book Company, 1968, hardcover 442 pages, see ISBN 0882754785 (1977 edition), The science of classical mechanics is based on this Newtonian idea of time.
Einstein, in his special theory of relativity, A. Einstein, H. A. Lorentz, H. Weyl, H. Minkowski, The Principle of Relativity, Dover Publications, Inc, 2000, softcover 216 pages, ISBN 0486600815, See pp. 37-65 for an English translation of Einstein's original 1905 paper.</ref> postulated the constancy and finiteness of the speed of light for all observers. He showed that this postulate, together with a reasonable definition for what it means for two events to be simultaneous, requires that distances appear compressed and time intervals appear lengthened for events associated with objects in motion relative to an inertial observer.
Time in classical mechanics
In classical mechanics Newton's concept of "relative, apparent, and common time" can be used in the formulation of a prescription for the synchronization of clocks. Events seen by two different observers in motion relative to each other produce a mathematical concept of time that works pretty well for describing the everyday phenomena of most people's experience.
Time in modern physics
In the late nineteenth century physicists encountered problems with the classical understanding of time, in connection with the behavior of electricity and magnetism. Einstein resolved these problems by invoking a method of synchronizing clocks using the constant, finite speed of light as the maximum signal velocity. This led directly to the result that time appears to elapse at different rates relative to different observers in motion relative to one another.
Time has historically been closely related with space, the two together comprising spacetime in Einstein's special relativity and general relativity. According to these theories, the concept of time depends on the spatial reference frame of the observer(s), and the human perception as well as the measurement by instruments such as clocks are different for observers in relative motion. Even the temporal order of events can change, but the past and future are defined by the backward and forward light cones, which never change. The past is the set of events that can send light signals to the observer, the future the events to which s/he can send light signals. All else is the present and within that set of events the very time-order differs for different observers.
Natural unit of time
Planck time (~ 5.4 × 10−44 seconds) is the unit of time in the system of natural units known as Planck units. Current established physical theories are believed to fail at this time scale, and many physicists expect that the Planck time might be the smallest unit of time that could ever be measured, even in principle. Tentative physical theories that describe this time scale exist; see for instance loop quantum gravity.
Einstein said that "The only reason for time is so that everything does not happen at once". In this regard, Einstein said that time was basically what a clock reads; the clock can be any action or change, like the movement of the sun. 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 (a muon is one example). According to the special theory of relativity, in the high-speed particle's frame of reference, it exists, on the average, for a standard amount of time known as its mean lifetime, and the distance it travels in that time is zero, because its velocity is zero. 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): "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."
Einstein wrote in his book, Relativity, that simultaneity is also relative, i.e., two events that appear simultaneous to an observer in a particular inertial reference frame need not be judged as simultaneous by a second observer in a different inertial frame of reference.
Arrow of time
Time appears to have a direction - the past lies behind, fixed and incommutable, while the future lies ahead and is not necessarily fixed. Yet the majority of the laws of physics don't provide this arrow of time. The exceptions include the Second law of thermodynamics, which states that entropy must increase over time (see Entropy); the cosmological arrow of time, which points away from the Big Bang, and the radiative arrow of time, caused by light only traveling forwards in time. In particle physics, there is also the weak arrow of time, from CPT symmetry, and also measurement in quantum mechanics (see Measurement in quantum mechanics).
Time and the Big Bang
According to some of the latest scientific theories, time began with the Big Bang. Stephen Hawking (borrowing a line of thought from Augustine of Hippo) has commented that trying to ascertain what happened before time began is like trying to find out what is north of the North Pole, and that such questions are self-contradictory, and thus without meaning. Hawking has also stated, along with other theorists, that even if time did not begin with the Big Bang and there were another time frame before the Big Bang, no information from events then would be accessible to us, and nothing that happened then would have any effect upon the present time-frame.<ref>Public lecture on the beginning of time by Hawking . Scientists have come to some agreement on descriptions of events that happened seconds after the Big Bang, but generally agree that descriptions about what happened before one Planck time after the Big Bang will likely remain pure speculation.
Time travel in science fiction
Time travel is the concept of moving backward or forward to different points in time, in a manner analogous to moving through space. Additionally, some interpretations of time travel take the form of travel between parallel realities or universes. A central problem with time travel is that of logic - say, violation of causality (when effect precedes the cause it is the consequence of) — which has given rise to a number of paradoxes (see grandfather paradox).