Explanation of the word time

Recent Examples on the Web

April is a great time to enjoy outdoor events that offer arts, crafts, live music, children’s activities and more.


—Mary Colurso | Mcolurso@al.com, al, 9 Apr. 2023




Yes, the classes felt slow at times.


—Currie Engel, Women’s Health, 9 Apr. 2023




What’s more, this moment isn’t the first time folks have seen Shemar in the role since his departure.


—Adrianna Freedman, Good Housekeeping, 9 Apr. 2023




To see more game results from your favorite team, download the ScoreStream app and join over 10 million users nationwide who share the scores of their favorite teams with one another in real-time.


—Indy Star Sports, The Indianapolis Star, 8 Apr. 2023




This is the third time Koepka has been at 12-under par through 36 holes in a major championship.


—Georgia News, ajc, 8 Apr. 2023




The longtime civil rights attorney’s nascent quest to reshape the local criminal justice system from the inside and significantly reduce incarceration was met with critical and, at times, misleading media coverage.


—Joshua Sharpe, San Francisco Chronicle, 8 Apr. 2023




Because of how the scripts were written between the first and second season, certain characters and storylines were focused on more than others at times.


—Juliana Ukiomogbe, ELLE, 8 Apr. 2023




Ramadan is a time for hard work (fasting isn’t easy!), worship, and prayer.


—Manal Aman, Woman’s Day Magazine, 8 Apr. 2023

Jonatan Berggren scored midway through the third period and David Perron one-timed a shot to even the game with a minute to go in regulation.


—Helene St. James, Detroit Free Press, 7 Apr. 2023




Good hitters — not the Tigers — will time that up.


—Julian Mcwilliams, BostonGlobe.com, 7 Apr. 2023




The Prime Video premiere, which will reach more than 240 countries and territories, is timed to Earth Day on April 22.


—Manori Ravindran, Variety, 6 Apr. 2023




Something that intrigues me about Meteora is timing.


—Vulture, 3 Apr. 2023




The issue is timing.


—Mark Zeigler, San Diego Union-Tribune, 31 Mar. 2023




As for that conference in Miami, conveniently timed to coincide with Art Basel, Brown never showed up.


—Tim Fernholz, Quartz, 31 Mar. 2023




The official unveiling on Friday March 31 is timed to coincide with the start of UK school holidays, and huge crowds are expected.


—Tom Page, CNN, 29 Mar. 2023




Chopard has been timing the scenic race from Brescia to Rome since 1988 and co-President Karl-Friedrich Scheufele has personally taken part in the race every year since, covering more than 30,000 competition miles.


—Justin Fenner, Robb Report, 27 Mar. 2023

We’re used to seeing the Doctor traveling through space and time, neither of which is possible for non-Time Lords at the moment.


—Clark Collis, EW.com, 24 June 2020




Cons: The Jazz feature two multi-time All-Stars — Gobert and guard Donovan Mitchell — and a roster that has more playoff experience than the Mavericks’ core.


—Dallas News, 5 Apr. 2022




Strowman, a multi-time champion in WWE, is a popular figure with fans for his imposing presence while also bringing an entertaining personality.


—Christopher Kuhagen, Journal Sentinel, 19 Dec. 2022




Nevertheless, that didn’t stop multi-time defending champions Scott Patterson of Anchorage and Christy Marvin of Palmer from extending their respective winning streaks.


—Josh Reed, Anchorage Daily News, 8 Aug. 2022




Stephens and Floyd are both coming off all-GNAC campaigns in 2021 and are multi-time all-conference honorees.


—Josh Reed, Anchorage Daily News, 17 Aug. 2022




A week later, multi-time senior champ Sumolong added another title to his name, taking the overall gross win at the State Senior Championship at Settler’s Bay in Palmer.


—Josh Reed, Anchorage Daily News, 21 July 2022




The draft kicked off with a pair of high school position players who were the sons of multi-time All-Stars.


—Globe Staff, BostonGlobe.com, 18 July 2022




There is no such thing as a winner, especially a multi-time winner, that’s disloyal.


—Sydney Bucksbaum, EW.com, 15 July 2022


See More

Time is the continued sequence of existence and events that occurs in an apparently irreversible succession from the past, through the present, into the future.^[1][2][3] It is a component quantity of various measurements used to sequence events, to compare the duration of events or the intervals between them, and to quantify rates of change of quantities in material reality or in the conscious experience.^[4][5][6][7] Time is often referred to as a fourth dimension, along with three spatial dimensions.^[8]

Time has long been an important subject of study in religion, philosophy, and science, but defining it in a manner applicable to all fields without circularity has consistently eluded scholars.^[7][9]
Nevertheless, diverse fields such as business, industry, sports, the sciences, and the performing arts all incorporate some notion of time into their respective measuring systems.^[10][11][12]

Time in physics is operationally defined as «what a clock reads».^[6][13][14]

The physical nature of time is addressed by general relativity with respect to events in spacetime. Examples of events are the collision of two particles, the explosion of a supernova, or the arrival of a rocket ship. Every event can be assigned four numbers representing its time and position (the event’s coordinates). However, the numerical values are different for different observers. In general relativity, the question of what time it is now only has meaning relative to a particular observer. Distance and time are intimately related, and the time required for light to travel a specific distance is the same for all observers, as first publicly demonstrated by Michelson and Morley. General relativity does not address the nature of time for extremely small intervals where quantum mechanics holds. As of 2023, there is no generally accepted theory of quantum general relativity.^[15]

Time is one of the seven fundamental physical quantities in both the International System of Units (SI) and International System of Quantities. The SI base unit of time is the second, which is defined by measuring the electronic transition frequency of caesium atoms. Time is used to define other quantities, such as velocity, so defining time in terms of such quantities would result in circularity of definition.^[16] An operational definition of time, wherein one says that observing a certain number of repetitions of one or another standard cyclical event (such as the passage of a free-swinging pendulum) constitutes one standard unit such as the second, is highly useful in the conduct of both advanced experiments and everyday affairs of life. To describe observations of an event, a location (position in space) and time are typically noted.

The operational definition of time does not address what the fundamental nature of time is. It does not address why events can happen forward and backward in space, whereas events only happen in the forward progress of time. Investigations into the relationship between space and time led physicists to define the spacetime continuum. General relativity is the primary framework for understanding how spacetime works.^[17] Through advances in both theoretical and experimental investigations of spacetime, it has been shown that time can be distorted and dilated, particularly at the edges of black holes.

Temporal measurement has occupied scientists and technologists and was a prime motivation in navigation and astronomy. Periodic events and periodic motion have long served as standards for units of time. Examples include the apparent motion of the sun across the sky, the phases of the moon, and the swing of a pendulum. 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 life spans.

There are many systems for determining what time it is, including the Global Positioning System, other satellite systems, Coordinated Universal Time and mean solar time. In general, the numbers obtained from different time systems differ from one another.

Measurement

Generally speaking, methods of temporal measurement, or chronometry, take two distinct forms: the calendar, a mathematical tool for organising intervals of time,^[18] and the clock, a physical mechanism that counts the passage of time. In day-to-day life, the clock is consulted for periods less than a day, whereas the calendar is consulted for periods longer than a day. Increasingly, personal electronic devices display both calendars and clocks simultaneously. The number (as on a clock dial or calendar) that marks the occurrence of a specified event as to hour or date is obtained by counting from a fiducial epoch – a central reference point.

History of the calendar

Artifacts from the Paleolithic suggest that the moon was used to reckon time as early as 6,000 years ago.^[19] Lunar calendars were among the first to appear, with years of either 12 or 13 lunar months (either 354 or 384 days). Without intercalation to add days or months to some years, seasons quickly drift in a calendar based solely on twelve lunar months. Lunisolar calendars have a thirteenth month added to some years to make up for the difference between a full year (now known to be about 365.24 days) and a year of just twelve lunar months. The numbers twelve and thirteen came to feature prominently in many cultures, at least partly due to this relationship of months to years. Other early forms of calendars originated in Mesoamerica, particularly in ancient Mayan civilization. These calendars were religiously and astronomically based, with 18 months in a year and 20 days in a month, plus five epagomenal days at the end of the year.^[20]

The reforms of Julius Caesar in 45 BC put the Roman world on a solar calendar. This Julian calendar was faulty in that its intercalation still allowed the astronomical solstices and equinoxes to advance against it by about 11 minutes per year. Pope Gregory XIII introduced a correction in 1582; the Gregorian calendar was only slowly adopted by different nations over a period of centuries, but it is now by far the most commonly used calendar around the world.

During the French Revolution, a new clock and calendar were invented in an attempt to de-Christianize time and create a more rational system in order to replace the Gregorian calendar. The French Republican Calendar’s days consisted of ten hours of a hundred minutes of a hundred seconds, which marked a deviation from the base 12 (duodecimal) system used in many other devices by many cultures. The system was abolished in 1806.^[21]

History of other devices

A large variety of devices have been invented to measure time. The study of these devices is called horology.^[22]

An Egyptian device that dates to c. 1500 BC, similar in shape to a bent T-square, measured the passage of time from the shadow cast by its crossbar on a nonlinear 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.^[23]

A sundial uses a gnomon to cast a shadow on a set of markings calibrated to the hour. The position of the shadow marks the hour in local time. The idea to separate the day into smaller parts is credited to Egyptians because of their sundials, which operated on a duodecimal system. The importance of the number 12 is due to the number of lunar cycles in a year and the number of stars used to count the passage of night.^[24]

The most precise timekeeping device of the ancient world was the water clock, or clepsydra, one of which was found in the tomb of Egyptian pharaoh Amenhotep I. They could be used to measure the hours even at night but required manual upkeep to replenish the flow of water. The ancient Greeks and the people from Chaldea (southeastern Mesopotamia) regularly maintained timekeeping records as an essential part of their astronomical observations. Arab inventors and engineers, in particular, made improvements on the use of water clocks up to the Middle Ages.^[25] In the 11th century, Chinese inventors and engineers invented the first mechanical clocks driven by an escapement mechanism.

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).^[26]

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.^[27][28]

Great advances in accurate time-keeping were made by Galileo Galilei and especially Christiaan Huygens with the invention of pendulum-driven clocks along with the invention of the minute hand by Jost Burgi.^[29]

The English word clock probably comes from the Middle Dutch word klocke which, in turn, derives from the medieval Latin word clocca, which ultimately derives from Celtic and is cognate with French, Latin, and German words that mean bell. The passage of the hours at sea was marked by bells and denoted the time (see ship’s bell). The hours were marked by bells in abbeys as well as at sea.

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.

Alarm clocks first appeared in ancient Greece around 250 BC with a water clock that would set off a whistle. This idea was later mechanized by Levi Hutchins and Seth E. Thomas.^[29]

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, a precision firstly achieved by John Harrison. More recently, the term has also been applied to the chronometer watch, a watch that meets precision standards set by the Swiss agency COSC.

The most accurate timekeeping devices are atomic clocks, which are accurate to seconds in many millions of years,^[31] and are used to calibrate other clocks and timekeeping instruments.

Atomic clocks use the frequency of electronic transitions in certain atoms to measure the second. One of the atoms used is caesium, most modern atomic clocks probe caesium with microwaves to determine the frequency of these electron vibrations.^[32] Since 1967, the International System of Measurements bases its unit of time, the second, on the properties of caesium atoms. SI defines the second as 9,192,631,770 cycles of the radiation that corresponds to the transition between two electron spin energy levels of the ground state of the ¹³³Cs atom.

Today, the Global Positioning System in coordination with the Network Time Protocol can be used to synchronize timekeeping systems across the globe.

In medieval philosophical writings, the atom was a unit of time referred to as the smallest possible division of time. The earliest known occurrence in English is in Byrhtferth’s Enchiridion (a science text) of 1010–1012,^[33] where it was defined as 1/564 of a momentum (11⁄2 minutes),^[34] and thus equal to 15/94 of a second. It was used in the computus, the process of calculating the date of Easter.

As of May 2010, the smallest time interval uncertainty in direct measurements is on the order of 12 attoseconds (1.2 × 10⁻¹⁷ seconds), about 3.7 × 10²⁶ Planck times.^[35]

Units

The second (s) is the SI base unit. A minute (min) is 60 seconds in length, and an hour is 60 minutes or 3600 seconds in length. A day is usually 24 hours or 86,400 seconds in length; however, the duration of a calendar day can vary due to Daylight saving time and Leap seconds.

Definitions and standards

A time standard is a specification for measuring time: assigning a number or calendar date to an instant (point in time), quantifying the duration of a time interval, and establishing a chronology (ordering of events). In modern times, several time specifications have been officially recognized as standards, where formerly they were matters of custom and practice. The invention in 1955 of the caesium atomic clock has led to the replacement of older and purely astronomical time standards such as sidereal time and ephemeris time, for most practical purposes, by newer time standards based wholly or partly on atomic time using the SI second.

International Atomic Time (TAI) is the primary international time standard from which other time standards are calculated. Universal Time (UT1) is mean solar time at 0° longitude, computed from astronomical observations. It varies from TAI because of the irregularities in Earth’s rotation. Coordinated Universal Time (UTC) is an atomic time scale designed to approximate Universal Time. UTC differs from TAI by an integral number of seconds. UTC is kept within 0.9 second of UT1 by the introduction of one-second steps to UTC, the «leap second». The Global Positioning System broadcasts a very precise time signal based on UTC time.

The surface of the Earth is split up into a number of time zones. Standard time or civil time in a time zone deviates a fixed, round amount, usually a whole number of hours, from some form of Universal Time, usually UTC. Most time zones are exactly one hour apart, and by convention compute their local time as an offset from UTC. For example, time zones at sea are based on UTC. In many locations (but not at sea) these offsets vary twice yearly due to daylight saving time transitions.

Some other time standards are used mainly for scientific work. Terrestrial Time is a theoretical ideal scale realized by TAI. Geocentric Coordinate Time and Barycentric Coordinate Time are scales defined as coordinate times in the context of the general theory of relativity. Barycentric Dynamical Time is an older relativistic scale that is still in use.

Philosophy

Religion

Religions which view time as cyclical

Ancient cultures such as Incan, Mayan, Hopi, and other Native American Tribes – plus the Babylonians, ancient Greeks, Hinduism, Buddhism, Jainism, and others – have a concept of a wheel of time: they regard time as cyclical and quantic,^{[clarification needed]} consisting of repeating ages that happen to every being of the Universe between birth and extinction.^[36]

Time as Linear for Abrahamic Religions

In general, the Islamic and Judeo-Christian world-view regards time as linear^[37]
and directional,^[38]
beginning with the act of creation by God. The traditional Christian view sees time ending, teleologically,^[39]
with the eschatological end of the present order of things, the «end time».

In the Old Testament book Ecclesiastes, traditionally ascribed to Solomon (970–928 BC), time (as the Hebrew word עידן, זמן iddan (age, as in «Ice age») zĕman(time) is often translated) was traditionally regarded^{[by whom?]} as a medium for the passage of predestined events.^{[citation needed]} (Another word, زمان» זמן» zamān, meant time fit for an event, and is used as the modern Arabic, Persian, and Hebrew equivalent to the English word «time».)

Time in Greek mythology

The Greek language denotes two distinct principles, Chronos and Kairos. The former refers to numeric, or chronological, time. The latter, literally «the right or opportune moment», relates specifically to metaphysical or Divine time. In theology, Kairos is qualitative, as opposed to quantitative.^[40]

In Greek mythology, Chronos (ancient Greek: Χρόνος) is identified as the Personification of Time. His name in Greek means «time» and is alternatively spelled Chronus (Latin spelling) or Khronos. Chronos is usually portrayed as an old, wise man with a long, gray beard, such as «Father Time». Some English words whose etymological root is khronos/chronos include chronology, chronometer, chronic, anachronism, synchronise, and chronicle.

Time in Kabbalah

According to Kabbalists, «time» is a paradox^[41] and an illusion.^[42] Both the future and the past are recognised to be combined and simultaneously present.^{[clarification needed]}

In Western philosophy

Two contrasting viewpoints on time divide prominent philosophers. One view is that time is part of the fundamental structure of the universe – a dimension independent of events, in which events occur in sequence. Isaac Newton subscribed to this realist view, and hence it is sometimes referred to as Newtonian time.^[43][44]
The opposing view is that time does not refer to any kind of «container» that events and objects «move through», nor to any entity that «flows», but that it is instead part of a fundamental intellectual structure (together with space and number) within which humans sequence and compare events. This second view, in the tradition of Gottfried Leibniz^[13] and Immanuel Kant,^[45][46] holds that time is neither an event nor a thing, and thus is not itself measurable nor can it be travelled.

Furthermore, it may be that there is a subjective component to time, but whether or not time itself is «felt», as a sensation, or is a judgment, is a matter of debate.^{[2][6][7][47][48]}

In Philosophy, time was questioned throughout the centuries; what time is and if it is real or not. Ancient Greek philosophers asked if time was linear or cyclical and if time was endless or finite.^[49] These philosophers had different ways of explaining time; for instance, ancient Indian philosophers had something called the Wheel of Time. It is believed that there was repeating ages over the lifespan of the universe.^[50] This led to beliefs like cycles of rebirth and reincarnation.^[50] The Greek philosophers believe that the universe was infinite, and was an illusion to humans.^[50] Plato believed that time was made by the Creator at the same instant as the heavens.^[50] He also says that time is a period of motion of the heavenly bodies.^[50] Aristotle believed that time correlated to movement, that time did not exist on its own but was relative to motion of objects.^[50] he also believed that time was related to the motion of celestial bodies; the reason that humans can tell time was because of orbital periods and therefore there was a duration on time.^[51]

The Vedas, the earliest texts on Indian philosophy and Hindu philosophy dating back to the late 2nd millennium BC, describe ancient Hindu cosmology, in which the universe goes through repeated cycles of creation, destruction and rebirth, with each cycle lasting 4,320 million years.^[52]
Ancient Greek philosophers, including Parmenides and Heraclitus, wrote essays on the nature of time.^[53]
Plato, in the Timaeus, identified time with the period of motion of the heavenly bodies. Aristotle, in Book IV of his Physica defined time as ‘number of movement in respect of the before and after’.^[54]

In Book 11 of his Confessions, St. Augustine of Hippo 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 begins to define time by what it is not rather than what it is,^[55]
an approach similar to that taken in other negative definitions. However, Augustine ends up calling time a «distention» of the mind (Confessions 11.26) by which we simultaneously grasp the past in memory, the present by attention, and the future by expectation.

Isaac Newton believed in absolute space and absolute time; Leibniz believed that time and space are relational.^[56]
The differences between Leibniz’s and Newton’s interpretations came to a head in the famous Leibniz–Clarke correspondence.

Philosophers in the 17th and 18th century questioned if time was real and absolute, or if it was an intellectual concept that humans use to understand and sequence events.^[49] These questions lead to realism vs anti-realism; the realists believed that time is a fundamental part of the universe, and be perceived by events happening in a sequence, in a dimension.^[57] Isaac Newton said that we are merely occupying time, he also says that humans can only understand relative time.^[57] Relative time is a measurement of objects in motion.^[57] The anti-realists believed that time is merely a convenient intellectual concept for humans to understand events.^[57] This means that time was useless unless there were objects that it could interact with, this was called relational time.^[57] René Descartes, John Locke, and David Hume said that one’s mind needs to acknowledge time, in order to understand what time is.^[51] Immanuel Kant believed that we can not know what something is unless we experience it first hand.^[58]

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.^[59]
With Kant, neither space nor time are conceived as substances, but rather both are elements of a systematic mental framework that necessarily structures the experiences of any rational agent, or observing subject. Kant 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. Spatial measurements are used to quantify the extent of and distances between objects, and temporal measurements are used to quantify the durations of and between events. Time was designated by Kant as the purest possible schema of a pure concept or category.

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.^[60]

According to Martin Heidegger we do not exist inside time, we are time. Hence, the relationship to the past is a present awareness of having been, which allows the past to exist in the present. The relationship to the future is the state of anticipating a potential possibility, task, or engagement. It is related to the human propensity for caring and being concerned, which causes «being ahead of oneself» when thinking of a pending occurrence. Therefore, this concern for a potential occurrence also allows the future to exist in the present. The present becomes an experience, which is qualitative instead of quantitative. Heidegger seems to think this is the way that a linear relationship with time, or temporal existence, is broken or transcended.^[61]
We are not stuck in sequential time. We are able to remember the past and project into the future – we have a kind of random access to our representation of temporal existence; we can, in our thoughts, step out of (ecstasis) sequential time.^[62]

Modern era philosophers asked: is time real or unreal, is time happening all at once or a duration, is time tensed or tenseless, and is there a future to be?^[49] There is a theory called the tenseless or B-theory; this theory says that any tensed terminology can be replaced with tenseless terminology.^[63] For example, «we will win the game» can be replaced with «we do win the game», taking out the future tense. On the other hand, there is a theory called the tense or A-theory; this theory says that our language has tense verbs for a reason and that the future can not be determined.^[63] There is also something called imaginary time, this was from Stephen Hawking, he says that space and imaginary time are finite but have no boundaries.^[63] Imaginary time is not real or unreal, it is something that is hard to visualize.^[63] Philosophers can agree that physical time exists outside of the human mind and is objective, and psychological time is mind-dependent and subjective.^[51]

Unreality

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 (hypostasis), 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.^[64] Time as an illusion is also a common theme in Buddhist thought.^[65][66]

J. M. E. McTaggart’s 1908 The Unreality of Time argues that, since every event has the characteristic of being both present and not present (i.e., future or past), that time is a self-contradictory idea (see also The flow of time).

These arguments often center on what it means for something to be unreal. Modern physicists generally believe that time is as real as space – though others, such as Julian Barbour in his book The End of Time, argue that quantum equations of the universe take their true form when expressed in the timeless realm containing every possible now or momentary configuration of the universe, called «platonia» by Barbour.^{[citation needed]}

A modern philosophical theory called presentism views the past and the future as human-mind interpretations of movement instead of real parts of time (or «dimensions») which coexist with the present. This theory rejects the existence of all direct interaction with the past or the future, holding only the present as tangible. This is one of the philosophical arguments against time travel. This contrasts with eternalism (all time: present, past and future, is real) and the growing block theory (the present and the past are real, but the future is not).

Physical definition

Until Einstein’s reinterpretation of the physical concepts associated with time and space in 1907, time was considered to be the same everywhere in the universe, with all observers measuring the same time interval for any event.^[67]
Non-relativistic classical mechanics is based on this Newtonian idea of time.

Einstein, in his special theory of relativity,^[68]
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.

The theory of special relativity finds a convenient formulation in Minkowski spacetime, a mathematical structure that combines three dimensions of space with a single dimension of time. In this formalism, distances in space can be measured by how long light takes to travel that distance, e.g., a light-year is a measure of distance, and a meter is now defined in terms of how far light travels in a certain amount of time. Two events in Minkowski spacetime are separated by an invariant interval, which can be either space-like, light-like, or time-like. Events that have a time-like separation cannot be simultaneous in any frame of reference, there must be a temporal component (and possibly a spatial one) to their separation. Events that have a space-like separation will be simultaneous in some frame of reference, and there is no frame of reference in which they do not have a spatial separation. Different observers may calculate different distances and different time intervals between two events, but the invariant interval between the events is independent of the observer (and his or her velocity).

Classical mechanics

In non-relativistic 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 sufficiently well for describing the everyday phenomena of most people’s experience. 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 conclusion that observers in motion relative to one another measure different elapsed times for the same event.

Spacetime

Time has historically been closely related with space, the two together merging into 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, and the human perception, as well as the measurement by instruments such as clocks, are different for observers in relative motion. For example, if a spaceship carrying a clock flies through space at (very nearly) the speed of light, its crew does not notice a change in the speed of time on board their vessel because everything traveling at the same speed slows down at the same rate (including the clock, the crew’s thought processes, and the functions of their bodies). However, to a stationary observer watching the spaceship fly by, the spaceship appears flattened in the direction it is traveling and the clock on board the spaceship appears to move very slowly.

On the other hand, the crew on board the spaceship also perceives the observer as slowed down and flattened along the spaceship’s direction of travel, because both are moving at very nearly the speed of light relative to each other. Because the outside universe appears flattened to the spaceship, the crew perceives themselves as quickly traveling between regions of space that (to the stationary observer) are many light years apart. This is reconciled by the fact that the crew’s perception of time is different from the stationary observer’s; what seems like seconds to the crew might be hundreds of years to the stationary observer. In either case, however, causality remains unchanged: the past is the set of events that can send light signals to an entity and the future is the set of events to which an entity can send light signals.^[69][70]

Dilation

Einstein showed in his thought experiments that people travelling at different speeds, while agreeing on cause and effect, measure different time separations between events, and can even observe different chronological orderings between non-causally related events. Though these effects are typically minute in the human experience, the effect becomes much more pronounced for objects moving at speeds approaching the speed of light. Subatomic particles exist for a well-known average fraction of a second in a lab relatively at rest, but when travelling close to the speed of light they are measured to travel farther and exist for much longer than when at rest. 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 seem to shorten. 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 relative 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.

Relativistic versus Newtonian

The animations visualise the different treatments of time in the Newtonian and the relativistic descriptions. At the heart of these differences are the Galilean and Lorentz transformations applicable in the Newtonian and relativistic theories, respectively.

In the figures, the vertical direction indicates time. The horizontal direction indicates distance (only one spatial dimension is taken into account), and the thick dashed curve is the spacetime trajectory («world line») of the observer. The small dots indicate specific (past and future) events in spacetime.

The slope of the world line (deviation from being vertical) gives the relative velocity to the observer. Note how in both pictures the view of spacetime changes when the observer accelerates.

In the Newtonian description these changes are such that time is absolute:^[71] the movements of the observer do not influence whether an event occurs in the ‘now’ (i.e., whether an event passes the horizontal line through the observer).

However, in the relativistic description the observability of events is absolute: the movements of the observer do not influence whether an event passes the «light cone» of the observer. Notice that with the change from a Newtonian to a relativistic description, the concept of absolute time is no longer applicable: events move up and down in the figure depending on the acceleration of the observer.

Arrow

Time appears to have a direction – the past lies behind, fixed and immutable, while the future lies ahead and is not necessarily fixed. Yet for the most part, the laws of physics do not specify an arrow of time, and allow any process to proceed both forward and in reverse. This is generally a consequence of time being modelled by a parameter in the system being analysed, where there is no «proper time»: the direction of the arrow of time is sometimes arbitrary. Examples of this include the cosmological arrow of time, which points away from the Big Bang, CPT symmetry, and the radiative arrow of time, caused by light only travelling forwards in time (see light cone). In particle physics, the violation of CP symmetry implies that there should be a small counterbalancing time asymmetry to preserve CPT symmetry as stated above. The standard description of measurement in quantum mechanics is also time asymmetric (see Measurement in quantum mechanics). The second law of thermodynamics states that entropy must increase over time (see Entropy). This can be in either direction – Brian Greene theorizes that, according to the equations, the change in entropy occurs symmetrically whether going forward or backward in time. So entropy tends to increase in either direction, and our current low-entropy universe is a statistical aberration, in a similar manner as tossing a coin often enough that eventually heads will result ten times in a row. However, this theory is not supported empirically in local experiment.^[72]

Quantization

Time quantization is a hypothetical concept. In the modern established physical theories (the Standard Model of Particles and Interactions and General Relativity) time is not quantized.

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.

Travel

Time travel is the concept of moving backwards or forwards to different points in time, in a manner analogous to moving through space, and different from the normal «flow» of time to an earthbound observer. In this view, all points in time (including future times) «persist» in some way. Time travel has been a plot device in fiction since the 19th century. Travelling backwards or forwards in time has never been verified as a process, and doing so presents many theoretical problems and contradictive logic which to date have not been overcome. Any technological device, whether fictional or hypothetical, that is used to achieve time travel is known as a time machine.

A central problem with time travel to the past is the violation of causality; should an effect precede its cause, it would give rise to the possibility of a temporal paradox. Some interpretations of time travel resolve this by accepting the possibility of travel between branch points, parallel realities, or universes.

Another solution to the problem of causality-based temporal paradoxes is that such paradoxes cannot arise simply because they have not arisen. As illustrated in numerous works of fiction, free will either ceases to exist in the past or the outcomes of such decisions are predetermined. As such, it would not be possible to enact the grandfather paradox because it is a historical fact that one’s grandfather was not killed before his child (one’s parent) was conceived. This view does not simply hold that history is an unchangeable constant, but that any change made by a hypothetical future time traveller would already have happened in his or her past, resulting in the reality that the traveller moves from. More elaboration on this view can be found in the Novikov self-consistency principle.

Perception

The specious present refers to the time duration wherein one’s perceptions are considered to be in the present. The experienced present is said to be ‘specious’ in that, unlike the objective present, it is an interval and not a durationless instant. The term specious present was first introduced by the psychologist E.R. Clay, and later developed by William James.^[73]

Biopsychology

The brain’s judgment of time is known to be a highly distributed system, including at least the cerebral cortex, cerebellum and basal ganglia as its components. One particular component, the suprachiasmatic nuclei, is responsible for the circadian (or daily) rhythm, while other cell clusters appear capable of shorter-range (ultradian) timekeeping.

Psychoactive drugs can impair the judgment of time. Stimulants can lead both humans and rats to overestimate time intervals,^[74][75] while depressants can have the opposite effect.^[76] The level of activity in the brain of neurotransmitters such as dopamine and norepinephrine may be the reason for this.^[77] Such chemicals will either excite or inhibit the firing of neurons in the brain, with a greater firing rate allowing the brain to register the occurrence of more events within a given interval (speed up time) and a decreased firing rate reducing the brain’s capacity to distinguish events occurring within a given interval (slow down time).^[78]

Mental chronometry is the use of response time in perceptual-motor tasks to infer the content, duration, and temporal sequencing of cognitive operations.

Early childhood education

Children’s expanding cognitive abilities allow them to understand time more clearly. Two- and three-year-olds’ understanding of time is mainly limited to «now and not now». Five- and six-year-olds can grasp the ideas of past, present, and future. Seven- to ten-year-olds can use clocks and calendars.^[79]

Alterations

In addition to psychoactive drugs, judgments of time can be altered by temporal illusions (like the kappa effect),^[80] age,^[81] and hypnosis.^[82] The sense of time is impaired in some people with neurological diseases such as Parkinson’s disease and attention deficit disorder.

Psychologists assert that time seems to go faster with age, but the literature on this age-related perception of time remains controversial.^[83] Those who support this notion argue that young people, having more excitatory neurotransmitters, are able to cope with faster external events.^[78]

Spatial conceptualization

Although time is regarded as an abstract concept, there is increasing evidence that time is conceptualized in the mind in terms of space.^[84] That is, instead of thinking about time in a general, abstract way, humans think about time in a spatial way and mentally organize it as such. Using space to think about time allows humans to mentally organize temporal events in a specific way.

This spatial representation of time is often represented in the mind as a Mental Time Line (MTL).^[85] Using space to think about time allows humans to mentally organize temporal order. These origins are shaped by many environmental factors^[84]––for example, literacy appears to play a large role in the different types of MTLs, as reading/writing direction provides an everyday temporal orientation that differs from culture to culture.^[85] In western cultures, the MTL may unfold rightward (with the past on the left and the future on the right) since people read and write from left to right.^[85] Western calendars also continue this trend by placing the past on the left with the future progressing toward the right. Conversely, Arabic, Farsi, Urdu and Israeli-Hebrew speakers read from right to left, and their MTLs unfold leftward (past on the right with future on the left), and evidence suggests these speakers organize time events in their minds like this as well.^[85]

This linguistic evidence that abstract concepts are based in spatial concepts also reveals that the way humans mentally organize time events varies across cultures––that is, a certain specific mental organization system is not universal. So, although Western cultures typically associate past events with the left and future events with the right according to a certain MTL, this kind of horizontal, egocentric MTL is not the spatial organization of all cultures. Although most developed nations use an egocentric spatial system, there is recent evidence that some cultures use an allocentric spatialization, often based on environmental features.^[84]

A recent study of the indigenous Yupno people of Papua New Guinea focused on the directional gestures used when individuals used time-related words.^[84] When speaking of the past (such as «last year» or «past times»), individuals gestured downhill, where the river of the valley flowed into the ocean. When speaking of the future, they gestured uphill, toward the source of the river. This was common regardless of which direction the person faced, revealing that the Yupno people may use an allocentric MTL, in which time flows uphill.^[84]

A similar study of the Pormpuraawans, an aboriginal group in Australia, revealed a similar distinction in which when asked to organize photos of a man aging «in order,» individuals consistently placed the youngest photos to the east and the oldest photos to the west, regardless of which direction they faced.^[86] This directly clashed with an American group that consistently organized the photos from left to right. Therefore, this group also appears to have an allocentric MTL, but based on the cardinal directions instead of geographical features.^[86]

The wide array of distinctions in the way different groups think about time leads to the broader question that different groups may also think about other abstract concepts in different ways as well, such as causality and number.^[84]

Use

In sociology and anthropology, time discipline is the general name given to social and economic rules, conventions, customs, and expectations governing the measurement of time, the social currency and awareness of time measurements, and people’s expectations concerning the observance of these customs by others. Arlie Russell Hochschild^[87][88] and Norbert Elias^[89] have written on the use of time from a sociological perspective.

The use of time is an important issue in understanding human behavior, education, and travel behavior. Time-use research is a developing field of study. 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.

Time management is the organization of tasks or events by first estimating how much time a task requires and when it must be completed, and adjusting events that would interfere with its completion so it is done in the appropriate amount of time. Calendars and day planners are common examples of time management tools.

Sequence of events

A sequence of events, or series of events, is a sequence of items, facts, events, actions, changes, or procedural steps, arranged in time order (chronological order), often with causality relationships among the items.^[90][91][92]
Because of causality, cause precedes effect, or cause and effect may appear together in a single item, but effect never precedes cause. A sequence of events can be presented in text, tables, charts, or timelines. The description of the items or events may include a timestamp. A sequence of events that includes the time along with place or location information to describe a sequential path may be referred to as a world line.

Uses of a sequence of events include stories,^[93]
historical events (chronology), directions and steps in procedures,^[94]
and timetables for scheduling activities. A sequence of events may also be used to help describe processes in science, technology, and medicine. A sequence of events may be focused on past events (e.g., stories, history, chronology), on future events that must be in a predetermined order (e.g., plans, schedules, procedures, timetables), or focused on the observation of past events with the expectation that the events will occur in the future (e.g., processes, projections). The use of a sequence of events occurs in fields as diverse as machines (cam timer), documentaries (Seconds From Disaster), law (choice of law), finance (directional-change intrinsic time), computer simulation (discrete event simulation), and electric power transmission^[95]
(sequence of events recorder). A specific example of a sequence of events is the timeline of the Fukushima Daiichi nuclear disaster.

See also

• List of UTC timing centers
• Time metrology

Organizations

• Antiquarian Horological Society – AHS (United Kingdom)
• Chronometrophilia (Switzerland)
• Deutsche Gesellschaft für Chronometrie – DGC (Germany)
• National Association of Watch and Clock Collectors – NAWCC (United States)

References

Further reading

External links

• Different systems of measuring time
• Time on In Our Time at the BBC
• Time in the Internet Encyclopedia of Philosophy, by Bradley Dowden.
• Le Poidevin, Robin (Winter 2004). «The Experience and Perception of Time». In Edward N. Zalta (ed.). The Stanford Encyclopedia of Philosophy. Retrieved 9 April 2011.

time

 (tīm)

n.

1.

a. A nonspatial continuum in which events occur in apparently irreversible succession from the past through the present to the future.

b. An interval separating two points on this continuum; a duration: a long time since the last war; passed the time reading.

c. A number, as of years, days, or minutes, representing such an interval: ran the course in a time just under four minutes.

d. A similar number representing a specific point on this continuum, reckoned in hours and minutes: checked her watch and recorded the time, 6:17 am.

e. A system by which such intervals are measured or such numbers are reckoned: solar time.

2.

a. often times An interval, especially a span of years, marked by similar events, conditions, or phenomena; an era: hard times; a time of troubles.

b. times The present with respect to prevailing conditions and trends: You must change with the times.

3. A suitable or opportune moment or season: a time for taking stock of one’s life.

4.

a. Periods or a period designated for a given activity: harvest time; time for bed.

b. Periods or a period necessary or available for a given activity: I have no time for golf.

c. A period at one’s disposal: Do you have time for a chat?

5. An appointed or fated moment, especially of death or giving birth: He died before his time. Her time is near.

6.

a. One of several instances: knocked three times; addressed Congress for the last time before retirement.

b. times Used to indicate the number of instances by which something is multiplied or divided: This tree is three times taller than that one. My library is many times smaller than hers.

7.

a. One’s lifetime.

b. One’s period of greatest activity or engagement.

c. A person’s experience during a specific period or on a certain occasion: had a good time at the party.

8.

a. A period of military service.

b. A period of apprenticeship.

c. Informal A prison sentence.

9.

a. The customary period of work: hired for full time.

b. The period spent working.

c. The hourly pay rate: earned double time on Sundays.

10. The period during which a radio or television program or commercial is broadcast: «There’s television time to buy» (Brad Goldstein).

11. The rate of speed of a measured activity: marching in double time.

12. Music

a. The meter of a musical pattern: three-quarter time.

b. The rate of speed at which a piece of music is played; the tempo.

13. Chiefly British The hour at which a pub closes.

14. Sports A time-out.

adj.

1. Of, relating to, or measuring time.

2. Constructed so as to operate at a particular moment: a time release.

3. Payable on a future date or dates.

4. Of or relating to installment buying: time payments.

tr.v. timed, tim·ing, times

1. To set the time for (an event or occasion).

2. To adjust to keep accurate time.

3. To adjust so that a force is applied or an action occurs at the desired time: timed his swing so as to hit the ball squarely.

4. To record the speed or duration of: time a runner.

5. To set or maintain the tempo, speed, or duration of: time a manufacturing process.

6. To speculate based on the anticipated short-term performance of (a market): time the stock market.

Phrasal Verb:

time out

Computers To be canceled if an expected input is not received after a specified time. Used of processes.

Idioms:

against time

With a quickly approaching time limit: worked against time to deliver the manuscript before the deadline.

any time

Used to acknowledge an expression of gratitude.

at one time

1. Simultaneously.

2. At a period or moment in the past.

at the same time

However; nonetheless.

at times

On occasion; sometimes.

behind the times

Out-of-date; old-fashioned.

for the time being

Temporarily.

from time to time

Once in a while; at intervals.

high time

The appropriate or urgent time: It’s high time that you started working.

in good time

1. In a reasonable length of time.

2. When or before due.

3. Quickly.

in no time

Almost instantly; immediately.

in time

1. Before a time limit expires.

2. Within an indefinite time; eventually: In time they came to accept the harsh facts.

3. Music

a. In the proper tempo.

b. Played with a meter.

on time

1. According to schedule; punctual or punctually.

2. By paying in installments.

time after time

Again and again; repeatedly.

time and again

Again and again; repeatedly.

time of (one’s) life

A highly pleasurable experience: We had the time of our lives at the beach.

time on (one’s) hands

An interval with nothing to do.

time was

There was once a time: «Time was when [urban gangs] were part of a … subculture that inner-city adolescence outgrew» (George F. Will).

---

American Heritage® Dictionary of the English Language, Fifth Edition. Copyright © 2016 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company. All rights reserved.

time

(taɪm)

n

1.

a. the continuous passage of existence in which events pass from a state of potentiality in the future, through the present, to a state of finality in the past

b. (as modifier): time travel. temporal

2. (General Physics) physics a quantity measuring duration, usually with reference to a periodic process such as the rotation of the earth or the vibration of electromagnetic radiation emitted from certain atoms. In classical mechanics, time is absolute in the sense that the time of an event is independent of the observer. According to the theory of relativity it depends on the observer’s frame of reference. Time is considered as a fourth coordinate required, along with three spatial coordinates, to specify an event. See caesium clock, second²¹, space-time

3. a specific point on this continuum expressed in terms of hours and minutes: the time is four o’clock.

4. a system of reckoning for expressing time: Greenwich mean time.

5.

a. a definite and measurable portion of this continuum

b. (as modifier): time limit.

6.

a. an accepted period such as a day, season, etc

b. (in combination): springtime.

7. an unspecified interval; a while: I was there for a time.

8. (often plural) a period or point marked by specific attributes or events: the Victorian times; time for breakfast.

9. a sufficient interval or period: have you got time to help me?.

10. an instance or occasion: I called you three times.

11. an occasion or period of specified quality: have a good time; a miserable time.

12. the duration of human existence

13. the heyday of human life: in her time she was a great star.

14. a suitable period or moment: it’s time I told you.

15. the expected interval in which something is done: the flying time from New York to London was seven hours.

16. a particularly important moment, esp childbirth or death: her time had come.

17. (Mathematics) (plural) indicating a degree or amount calculated by multiplication with the number specified: ten times three is thirty; he earns four times as much as me.

18. (Clothing & Fashion) (often plural) the fashions, thought, etc, of the present age (esp in the phrases ahead of one’s time, behind the times)

19. Brit (in bars, pubs, etc) short for closing time

20. (Law) informal a term in jail (esp in the phrase do time)

21. (Industrial Relations & HR Terms)

a. a customary or full period of work

b. the rate of pay for this period

22. (Music, other)

a. the system of combining beats or pulses in music into successive groupings by which the rhythm of the music is established

b. a specific system having a specific number of beats in each grouping or bar: duple time.

23. (Music, other) music short for time value

24. (Poetry) prosody a unit of duration used in the measurement of poetic metre; mora

25. against time in an effort to complete something in a limited period

26. ahead of time before the deadline

27. all in good time in due course

28. all the time continuously

29. at one time

a. once; formerly

b. simultaneously

30. at the same time

a. simultaneously

b. nevertheless; however

31. at times sometimes

32. (Classical Music) beat time (of a conductor, etc) to indicate the tempo or pulse of a piece of music by waving a baton or a hand, tapping out the beats, etc

33. before one’s time prematurely

34. for the time being for the moment; temporarily

35. from time to time at intervals; occasionally

36. gain time See gain¹⁹

37. have no time for to have no patience with; not tolerate

38. in good time

a. early

b. quickly

39. in no time very quickly; almost instantaneously

40. in one’s own time

a. outside paid working hours

b. at one’s own rate

41. early or at the appointed time

42. eventually

43. (Music, other) music at a correct metrical or rhythmic pulse

44. (Music, other) keep time to observe correctly the accent or rhythmic pulse of a piece of music in relation to tempo

45. lose time (of a timepiece) to operate too slowly

46. lose no time to do something without delay

47. make time

a. to find an opportunity

b. (often foll by with) US informal to succeed in seducing

49. in the nick of time at the last possible moment; at the critical moment

50. at the expected or scheduled time

51. (Banking & Finance) US payable in instalments

52. pass the time of day to exchange casual greetings (with an acquaintance)

53. time about Scot alternately; turn and turn about

54. time and again frequently

55. (Industrial Relations & HR Terms) time off a period when one is absent from work for a holiday, through sickness, etc

56. (General Sporting Terms) time on Austral an additional period played at the end of a match, to compensate for time lost through injury or (in certain circumstances) to allow the teams to achieve a conclusive result. Also called (in Britain and certain other countries): extra time

57. time out of mind from time immemorial

58. time of one’s life a memorably enjoyable time

59. (modifier) operating automatically at or for a set time, for security or convenience: time lock; time switch.

vb (tr)

60. to ascertain or calculate the duration or speed of

61. to set a time for

62. to adjust to keep accurate time

63. to pick a suitable time for

64. sport to control the execution or speed of (an action, esp a shot or stroke) so that it has its full effect at the right moment

interj

the word called out by a publican signalling that it is closing time

[Old English tīma; related to Old English tīd time, Old Norse tīmi, Alemannic zīme; see tide¹]

Collins English Dictionary – Complete and Unabridged, 12th Edition 2014 © HarperCollins Publishers 1991, 1994, 1998, 2000, 2003, 2006, 2007, 2009, 2011, 2014

time

(taɪm)

n., adj., v. timed, tim•ing. n.

1. the system of those sequential relations that any event has to any other, as past, present, or future; indefinite and continuous duration regarded as that in which events succeed one another.

2. duration regarded as an aspect of the present life as distinct from the life to come or from eternity; finite duration.

3. (sometimes cap.) a system or method of measuring or reckoning the passage of time: Greenwich Time.

4. a limited period or interval, as between two events: a long time.

5. a particular period: Youth is the best time of life.

6. Often, times.

a. a period in history, or one contemporaneous with a notable person: prehistoric times; in Lincoln’s time.

b. the period or era now or previously present: a sign of the times.

c. a period with reference to its conditions: hard times.

7. the end of a prescribed or allotted period, as of one’s life or a pregnancy.

8. a period experienced in a particular way: Have a good time.

9. a period of work of an employee, or the pay for it.

10. Informal. a term of enforced duty or imprisonment.

11. the period necessary for or occupied by something: The bus takes too much time, so I’ll take a plane.

12. leisure or spare time: I hope to take some time in August.

13. a definite point in time, as indicated by a clock: What time is it?

14. a particular period in a day, year, etc.: breakfast time.

15. an appointed or proper instant or period: There is a time for everything.

16. the particular point in time when an event is scheduled to take place: Curtain time is at 8.

17. an indefinite period extending into the future: Time will tell.

18. each occasion of a recurring action or event: to do something five times.

19. times, the number of instances a quantity or factor are taken together: Two goes into six three times; five times faster.

20. one of the three dramatic unities. Compare unity (def. 8).

21. a unit or a group of units in the measurement of poetic meter.

22. Music.

a. tempo; relative rapidity of movement.

b. meter; rhythm.

c. the metrical duration of a note or rest.

d. proper or characteristic rhythm or tempo.

e. the general movement of a particular kind of musical composition with reference to it rhythm, metrical structure, and tempo: waltz time.

23. rate of marching, calculated on the number of paces taken per minute: double time.

adj.

24. of or pertaining to the passage of time.

25. (of an explosive device) containing a clock so that it will detonate at the desired moment: a time bomb.

26. of an installment plan: time payments.

v.t.

27. to measure or record the speed, duration, or rate of: to time a race.

28. to fix the duration of: She timed the test at 15 minutes.

29. to fix the interval between (actions, events, etc.): They timed their strokes at six per minute.

30. to regulate (a train, clock, etc.) as to time.

31. to choose the moment or occasion for; schedule: He timed the attack perfectly.

v.i.

32. to keep time; sound or move in unison.

Idioms:

1. against time, in an effort to finish within a limited period.

2. ahead of time, before the time due; early.

3. at one time,

a. once; formerly.

b. at the same time; simultaneously.

4. at the same time,

a. nevertheless; yet: He’s young; at the same time, he’s quite responsible.

b. simultaneously.

5. at times, at intervals; occasionally.

6. behind the times, old-fashioned; dated.

7. for the time being, temporarily; for the present.

8. from time to time, occasionally; at intervals.

9. gain time, to achieve a delay or postponement.

10. in good time, at or in advance of the appointed time; punctually.

11. in no time, in a very brief time.

12. in time,

a. early enough: Come in time for dinner.

b. in the future; eventually: In time he’ll understand.

c. in the correct rhythm or tempo.

13. keep time,

a. to record time, as a watch or clock does.

b. to mark or observe the tempo, as by performing rhythmic movements.

14. kill time, to occupy oneself with some activity to make time pass more quickly.

15. make time, to move or travel quickly.

16. make time with, Slang. to pursue or take as a sexual partner.

17. many a time, again and again; frequently.

18. mark time,

a. to suspend progress temporarily, as to await developments; fail to advance.

b. to move the feet alternately as in marching, but without advancing.

19. on one’s own time, during one’s free time; while not being paid.

20. on time,

a. at the specified time; punctually.

b. to be paid for within a designated period of time, as in installments.

21. take one’s time, to act without hurry.

22. the time of one’s life, an extremely enjoyable experience.

23. time after time, again and again.

24. time and (time) again, repeatedly; often.

[before 900; Old English tīma, c. Old Norse tīmi; akin to tide¹]

Random House Kernerman Webster’s College Dictionary, © 2010 K Dictionaries Ltd. Copyright 2005, 1997, 1991 by Random House, Inc. All rights reserved.

time

(tīm)

1. A continuous, measurable quantity in which events occur in a sequence proceeding from the past through the present to the future. See Note at space-time.

2.

a. An interval separating two points of this quantity; a duration.

b. A system by which such intervals are measured or such numbers are calculated: standard time; daylight-saving time.

The American Heritage® Student Science Dictionary, Second Edition. Copyright © 2014 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company. All rights reserved.

Time

1. a person or a thing remaining or appearing after its own time period; archaism.
2. an error in chronology. Also called antichronism. — anachronistic, anachronistical, anachronous, adj.

the absence of concurrent time. Cf. synchronism. — asynchronic, adj. — asynchrony, n.

1. the science of arranging time in fixed periods for the purpose of dating events accurately and arranging them in order of occurrence.
2. a reference book organized according to the dates of past events. — chronologer, chronologist, n. — chronological, adj.

1. the art of measuring time accurately.
2. the measurement of time by periods or divisions. — chronometric, chronometrical, adj.

an abnormal discomfort concerning time.

an instrument for accurate measurement of very short periods of time, as the time of trajectory of missiles.

accurate measurement of short intervals of time by means of a chronoscope. — chronoscopic, adj.

an instrument for measuring time by the controlled flow of water or mercury through a small opening.

coevalneity. — coetaneous, adj.

the state of eternal coexistence; eternal coexistence with another eternal entity. — coetemal, adj.

the state or quality of being alike in age or duration; contemporaneity. Also called coetaneity. — coeval, adj.

the practice or habit of delay or tardiness; procrastination. — cunctator, n. — cunctatious, cunctatory, adj.

the science of fixing dates in the past by the study of growth rings in trees. — dendrochronologist, n. — dendrochronological, adj.

the comparative study of a development based on its history. — diachronic, diachronistic, diachronistical, adj.

Rare. the quality of long duration in time; length of time. — diuturnal, adj.

the chronology of the earth as induced from geologic data. — geochronologist, n. — geochronologic, geochronological, adj.

the study of two or more related but distinct languages in order to determine when they separated, by examining the lexicon they share and those parts of it that have been replaced. — glottochronologist, n. — glottochronological, adj.

the art or science of constructing dials, as sundials, which show the time of day by the shadow of the gnomon, a pin or triangle raised above the surface of the dial.

a treatise or other work on the subject of gnomics.

any instrument or device for telling time, especially a sundial and early forms of the clock.

1. the description of watches and clocks.
2. the art of making timepieces. — horologiographer, horologiographian, n. — horologiographic, adj.

the art or science of making timepieces or of measuring time. — horologist, n. — horological, adj.

an instrument for measuring time.

the art or science of measuring time. — horometrical, adj.

immediateness; the quality or condition of being immediate.

Obsolete, the state or condition of being untimely. — intempestive, adj.

1. the characteristic of having a uniform period of vibration.
2. the condition of occurring at the same time as another event. — isochronic, adj. — isochrony, n.

a calendar of months.

the state or condition of occurring monthly.

a chronological error in which an event is assigned a date after its real one. Cf. parachronism. — metachronic, adj.

an instrument for measuring extremely small time intervals. — microchronometric, adj.

the process or condition of going out of date or being no longer in use. — obsolescent, adj.

the dating of an event as later than its actual occurrence. Cf. prochronism. — parachronic, adj.

the study of natural phenomena that occur periodically, as migration or blossoming, and their relation to climate and changes of season. — phenologist, n. — phenological, adj.

1. a camera for recording motion by a series of photographs taken at brief intervals.
2. the photograph so produced.
3. a camera that records the exact time of the event it is photographing by exposing a moving sensitized plate to the tracing of a thin beam of light synchronized with the event.

the act or state of preceding or coming before. — prevenient, adj.

the dating of an event as earlier than its actual occurrence. Cf. parachronism. — prochronic, adj.

the proportionate frequency at which an event takes place. See also numbers.

the state or quality of being eternal, without beginning or end. — sempiternal, adj.

a coincidence in time; simultaneity. Cf. asynchronism. — synchronistic, synchronistical, adj.

an arrangement of events by date, grouping together all those of the same date; a comparative chronology. — synchronological, adj.

the state or quality of passing with time or being ephemeral or fleeting. — transient, adj.

-Ologies & -Isms. Copyright 2008 The Gale Group, Inc. All rights reserved.

Time

 

See Also: DAY, DEATH, LIFE

1. About as much time left as an ice cube in a frying pan —William Diehl
2. Any decent church service lasts forty-five minutes, like the sex act —Heinrich Böll
3. As the waves make toward the pebbled shore, so do our minutes hasten to their end —William Shakespeare
4. As the years go by me, my life keeps filling up with names like abandoned cemeteries —Yehuda Amichai
5. The day runs through me as water through a sieve —Samuel Butler
6. The days chase one another like kittens chasing their tails —H. L. Mencken
7. The days slipped by … like apple-parings under a knife —Stephen Vincent Benét
8. A decade falling like snow on top of another —Elizabeth Hardwick
9. Each class seemed endless to him, as if the hour were stuck to his back like his damp shirt —Helen Hudson
10. Each year is like a snake that swallows its tail —Robert Penn Warren

    This line is the curtain raiser for Warren’s poem, Paradigm.

11. Every day yawned like a week —Donald Seaman
12. Forty-five minutes passed, like a very slow cloud —Dylan Thomas
13. Here [at a country inn] time swings idly as a toy balloon —Phyllis McGinley
14. The hours weighed like centuries on his heart —Lawrence Durrell
15. If time seems to pass so quickly, this is because there are no landmarks. Like the moon when it is at its heights on the horizon —Albert Camus
16. The hours [with nothing to do] hunted him like a pack of bloodhounds —Edith Wharton
17. If you let slip time, like a neglected rose it withers on the stalk with languished head —John Milton
18. The lagging hours of the day went by like windless clouds over a tender sky —Percy Bysshe Shelley

    The word ‘over’ is spelled ‘o’er’ in the original.

19. Leisure is like a beautiful garment that will not do for constant wear —Anon
20. Life goes like the river —Clifford Odets
21. Like a run in a stocking. It [lost time] always got worse —Anne Morrow Lindbergh
22. Like January weather, the years will bite and smart —Dorothy Parker
23. Like sand poured in a careful measure from the hand, the weeks flowed down —Paule Marshall
24. Like the swell of some sweet tune, morning rises into noon, May glides onward into June —Henry Wadsworth Longfellow
25. Like the waves make towards the pebbled shore, so do our minutes hasten to their end —William Shakespeare
26. The minutes crawl like last year’s flies —Ridgely Torrence
27. The minutes ticked off like separate eternities —Dan Wakefield
28. The moment hung in time like a miner’s hat on an oaken peg in a saloon abandoned ninety years ago —Loren D. Estleman
29. The moment shimmered like a glass of full-bodied wine —Marge Piercy
30. The moments [between two people] were stretching longer and longer, like so many rubber bands —Elizabeth Spencer
31. My days are consumed like smoke —The Holy Bible/Psalms
32. The passing years are like a mist sweeping up from the sea of time so that my memories acquire new aspects —W. Somerset Maugham
33. Saw the days of the year stretching ahead like a series of bright, white boxes, and separating one box from another was sleep, like a black shade —Sylvia Plath
34. She was forever saving time, like bits of string —Helen Hudson
35. Slowly the generations pass, like sand through heaven’s blue hour-glass —Vachel Lindsay

    Lindsay used this simile as a repeated refrain for his poem Shantung.

36. The summer was melting away like the unfinished ice cream Sonny left on his plate —Dan Wakefield
37. That night and the next day swept past like the waters of a rapids —James Crumley
38. (Time seems thin, one-dimensional,) the hours long and slender, stretched like a wire —Dan Wakefield
39. There is a rhythm inside a year of time, like a great mainspring that keeps it ticking from spring to summer to fall to winter —Borden Deal
40. Time … a substance of some sort which existence burned up like a fire —Susan Fromberg Schaeffer
41. Time can be nibbled away as completely as a tray of canapés in an irresolute fat man’s reach, or grandly lost in victory like the great marlin in The Old Man and the Sea [by Hemingway] —Charles Poore
42. Time crawled like ants —Marge Piercy
43. Time crouched, like a great cat, motionless but for tail’s twitch —Robert Penn Warren
44. Time dripped like drops of blood —Yukio Mishima
45. Time drops sail like a ketch in a lagoon —Diane Ackerman
46. Time fled past us like a startled bird —James Crumley
47. Time flies … like an arrow —Amy Hempel
48. Time goes cooly through the funnel of his fingers … like water over stones —William H. Gass
49. Time has moved on like a great flock of geese —Stephen Minot
50. Time is a storm in which we are all lost —William Carlos Williams
51. Time is like an enterprising manager always bent on staging some new and surprising production, without knowing very well what it will be —George Santayana
52. Time is like a river made up of the events which happen, and its current is strong. No sooner does anything appear than it is swept away and another comes in its place, and will be swept away too —Marcus Aurelius
53. Time is like money; the less we have of it to spare the further we make it go —Josh Billings
54. Time is like some balked monster, waiting outside the valley, to pounce on the slackers who have managed to evade him longer than they should —James Hilton
55. Time, like a flurry of wild rain, shall drift across the darkened pane —Charles G. D. Roberts
56. Time like an ever-rolling stream bears all its sons away —Isaac Watts
57. Time, like a pulse, shakes fierce through all the worlds —Dante Gabriel Rossetti
58. Time looked like snow dropping silently into a black room or … like a silent film in an ancient theatre, one hundred billion faces falling like those New Year balloons, down and down into nothing —Ray Bradbury
59. Time moves … like a treacle —Hortense Calisher
60. Time passes as on a fast day —Anon
61. Time pleated like a fan —Julia O’Faolain
62. Time pulses from the afternoon like blood from a serious wound —Hilma Wolitzer
63. Time roared in his ears like wind —John Barth

    See Also: NOISE

64. Time roars in my ears like a river —Derek Walcott
65. Time rushes past us like the snowflake on the river —Gore Vidal
66. Time seemed to have slowed down, dividing itself into innumerable fractions, like Zeno’s space or marijuana hours —Ross Macdonald
67. Time … sounded like water running in a dark cave and voices crying and dirt dropping down upon hollow box lids, and rain —Ray Bradbury
68. Time sticking to her like cold grease —Marge Piercy
69. Time swells like a wave at a wall and bursts to eternity —George Barker
70. Time went on like an unchanging ribbon drawn across a turbulent background —Heinrich Boll
71. Upon his silver hairs, time, like a Panama hat, sits at a tilt and smiles —Karl Shapiro

    In his poem, Boy-Man, Shapiro expands on the simile as follows: “… and smiles. To him the world has just begun. And every city waiting to be built.”

72. The week is dealt out like a hand —Randall Jarrell
73. The week passed slowly … like a prolonged Sunday —Edith Wharton
74. When a man sits with a pretty girl for an hour, it seems like a minute. When he sits on a hot stove for a minute, then it’s longer than any hour —Albert Einstein
75. When you’re deeply absorbed in what you’re doing, time gives itself to you like a warm and willing lover —Brendan Francis
76. The years are crawling over him like wee red ants —Ogden Nash
77. The years come close around me like a crowd of the strangers I knew once —Randall Jarrell
78. The years dropped from Randstable [character in novel] like a heavy overcoat —James Morrow
79. The years like great black oxen tread the world, and God the herdsman goads them on behind —W. B. Yeats
80. The years peeled back like the skin of an onion, layer on top of layer —T. Coraghessan Boyle
81. The years rolled in against one another like a rush of water —Frieda Arkin
82. The years shall run like rabbits —W. H. Auden
83. The years ticked past like crabs —Randall Jarrell
84. Years which rushed over her like weathered leaves in a storm —Ellen Glasgow
85. A year that dragged like a terminal illness —Rosellen Brown

Similes Dictionary, 1st Edition. © 1988 The Gale Group, Inc. All rights reserved.

Time

 

(See also DURATION, FREQUENCY, OPPORTUNENESS, TIMELINESS.)

before one had nails on one’s toes Before one was born; long ago, in the distant past. This expression refers to the fact that a baby’s toenails develop prenatally. Thus, an event or other matter that occurred before a person’s toenails developed occurred before he was born. In its most common usage, the expression cites a younger person’s age as the basis for denigrating his status, experience, ideals, or philosophies.

There’s Ulysses and old Nestor, whose wit was moldy ere your grandsires had nails on their toes. (Shakespeare, Troilus and Cressida, II, i)

between dog and wolf Neither day nor night; dusk. The dog is a domesticated animal, and therefore associated with all that is civilized and ordered, such as the day. On the other hand, the wild and mysterious wolf is associated with the night, from the image of a wolf baying at the moon. Although they are of the same family, dogs and wolves are as different as day and night. And between dog and wolf, or day and night, is dusk.

blind man’s holiday Dusk; neither day nor night. This phrase, used as early as 1599, is said to refer to the time just before candles are lighted when it is too dark to work or read—a fitting time to rest, or “take a holiday.” However, this explanation does not account for the use of blind man in the phrase. Perhaps dusk is a holiday for a blind man because it offers him a brief respite from his aloneness. He has company because everyone is in the same state of semi-darkness until the candles are lit. In fact, being accustomed to the darkness, a blind man can enjoy an advantage. The phrase is rarely heard today.

D-day A deadline, the last hour, the moment of truth; a date established for any significant event, originally for a secret military operation. During World War II, the Allied invasion of Normandy was set for June 5, 1944. To avoid referring to the date, for security reasons, the code word D-day was adopted. Hostile weather conditions, however, forced the postponement of this famous D-day until the next day. The term is currently used in a similar way, especially in the academic world where students often refer to the due, date for the submission of work as D-day.

graveyard shift A work shift usually from twelve midnight until eight in the morning; any late-night shift; also the graveyard watch. Factories running 24 hours a day employ three shifts—day, swing, and midnight or. graveyard. The expression gained currency during World War II when so many factories were operating around-the-clock. The phrase, American slang and dating from the early part of this century, is an allusion to the late hour of the shift, which works in the dead of night when it is quiet and still as a graveyard.

A month later he and his fellows went on “graveyard” shift. (The Saturday Evening Post, November, 1908)

zero hour Deadline; an anticipated stressful or critical period of time; the precise time established for the commencement of a military operation. This phrase originated and was widely used during World War I. It was for the most part replaced by the analogous term H-hour during World War II. As currently used, the expression often carries an implication of dread.

Picturesque Expressions: A Thematic Dictionary, 1st Edition. © 1980 The Gale Group, Inc. All rights reserved.

time

1. ‘time’

Time is what we measure in hours, days, years, etc.

It seemed like a long period of time.

More time passed.

You don’t usually use time when you are saying how long something takes or lasts. Don’t say, for example, ‘The course took two years’ time‘ or ‘Each song lasts ten minutes’ time‘. Say ‘The course took two years‘ or ‘Each song lasts ten minutes‘.

The whole process probably takes twenty-five years.

The tour lasts 4 hours.

You can, however, use time when you are saying how long it will be before something happens. For example, you can say ‘We are getting married in two years’ time‘.

The exchange ends officially in a month’s time.

In a few days’ time, she may change her mind.

Time is usually an uncountable noun, so don’t use ‘a’ with it. Don’t say, for example, ‘I haven’t got a time to go shopping‘. Say ‘I haven’t got time to go shopping’.

I didn’t know if we’d have time for tea.

2. ‘a…time’

However, you can use a with an adjective and time when you are showing how long something takes or lasts. You can say, for example, that something takes a long time or takes a short time.

The proposal would take quite a long time to discuss in detail.

After a short time one of them said ‘It’s all right, we’re all friends here.’

You can also use expressions like these, with or without for, as adverbial phrases.

He’s going to have to wait a very long time.

They worked together for a short time.

You’ve only been in the firm quite a short time.

If you are enjoying yourself while you are doing something, you can say, for example, that you are having a good time.

Downstairs, Aneesa was having a wonderful time.

Did you have a good time in Edinburgh?

You must use a in sentences like these. Don’t say, for example, ‘Aneesa was having wonderful time‘.

3. meaning ‘occasion’

Time is used with the or that and a qualifier to refer to the occasion when something happened or will happen.

By the time the waiter brought their coffee, she was almost asleep.

Do you remember that time when Adrian phoned up?

When time has this meaning, you can use words like first or last in front of it.

It was the first time she spoke.

When was the last time I saw you?

Expressions such as the first time and the next time are often adverbial phrases.

The next time he would offer to pay.

The second time I hired a specialist firm.

Next time (without ‘the’) is also an adverbial.

You’ll see a difference next time.

Next time you will do everything right.

4. ‘on time’

If something happens on time, it happens at the right time or punctually.

He turned up on time for guard duty.

Their planes usually arrive on time.

5. ‘in time’

Don’t confuse on time with in time. If you are in time for a particular event, you are not late for it.

We’re just in time.

He returned to his hotel in time for a late supper.

If something such as a job or task is finished in time, it is finished at or before the time when it should be finished.

I can’t do it in time.

In time has another meaning. You use it to say that something happens eventually, after a lot of time has passed.

In time the costs will decrease.

In time I came to see how important this was.

Collins COBUILD English Usage © HarperCollins Publishers 1992, 2004, 2011, 2012

time

Past participle: timed
Gerund: timing

Imperative
time
time

Present
I time
you time
he/she/it times
we time
you time
they time

Preterite
I timed
you timed
he/she/it timed
we timed
you timed
they timed

Present Continuous
I am timing
you are timing
he/she/it is timing
we are timing
you are timing
they are timing

Present Perfect
I have timed
you have timed
he/she/it has timed
we have timed
you have timed
they have timed

Past Continuous
I was timing
you were timing
he/she/it was timing
we were timing
you were timing
they were timing

Past Perfect
I had timed
you had timed
he/she/it had timed
we had timed
you had timed
they had timed

Future
I will time
you will time
he/she/it will time
we will time
you will time
they will time

Future Perfect
I will have timed
you will have timed
he/she/it will have timed
we will have timed
you will have timed
they will have timed

Future Continuous
I will be timing
you will be timing
he/she/it will be timing
we will be timing
you will be timing
they will be timing

Present Perfect Continuous
I have been timing
you have been timing
he/she/it has been timing
we have been timing
you have been timing
they have been timing

Future Perfect Continuous
I will have been timing
you will have been timing
he/she/it will have been timing
we will have been timing
you will have been timing
they will have been timing

Past Perfect Continuous
I had been timing
you had been timing
he/she/it had been timing
we had been timing
you had been timing
they had been timing

Conditional
I would time
you would time
he/she/it would time
we would time
you would time
they would time

Past Conditional
I would have timed
you would have timed
he/she/it would have timed
we would have timed
you would have timed
they would have timed

Collins English Verb Tables © HarperCollins Publishers 2011

time

(min) A unit of time measurement equal to 60 seconds. 60 min = 1 hr.

Dictionary of Unfamiliar Words by Diagram Group Copyright © 2008 by Diagram Visual Information Limited

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Translations

Updated on November 26, 2019

Time is familiar to everyone, yet it’s hard to define and understand. Science, philosophy, religion, and the arts have different definitions of time, but the system of measuring it is relatively consistent.

Clocks are based on seconds, minutes, and hours. While the basis for these units has changed throughout history, they trace their roots back to ancient Sumeria. The modern international unit of time, the second, is defined by the electronic transition of the cesium atom. But what, exactly, is time?

Scientific Definition

Physicists define time as the progression of events from the past to the present into the future. Basically, if a system is unchanging, it is timeless. Time can be considered to be the fourth dimension of reality, used to describe events in three-dimensional space. It is not something we can see, touch, or taste, but we can measure its passage.

The Arrow of Time

Physics equations work equally well whether time is moving forward into the future (positive time) or backward into the past (negative time.) However, time in the natural world has one direction, called the arrow of time. The question of why time is irreversible is one of the biggest unresolved questions in science.

One explanation is that the natural world follows the laws of thermodynamics. The second law of thermodynamics states that within an isolated system, the entropy of the system remains constant or increases. If the universe is considered to be an isolated system, its entropy (degree of disorder) can never decrease. In other words, the universe cannot return to exactly the same state in which it was at an earlier point. Time cannot move backward.

Time Dilation

In classical mechanics, time is the same everywhere. Synchronized clocks remain in agreement. Yet we know from Einstein’s special and general relativity that time is relative. It depends on the frame of reference of an observer. This can result in time dilation, where the time between events becomes longer (dilated) the closer one travels to the speed of light. Moving clocks run more slowly than stationary clocks, with the effect becoming more pronounced as the moving clock approaches light speed. Clocks in jets or in orbit record time more slowly than those on Earth, muon particles decay more slowly when falling, and the Michelson-Morley experiment confirmed length contraction and time dilation.

Time Travel

Time travel means moving forward or backward to different points in time, much like you might move between different points in space. Jumping forward in time occurs in nature. Astronauts on the International Space Station jump forward in time when they return to Earth because of its slower movement relative to the station.

The idea of traveling back in time, however, poses problems. One issue is causality or cause and effect. Moving back in time could cause a temporal paradox. The «grandfather paradox» is a classic example. According to the paradox, if you travel back in time and kill your grandfather before your mother or father was born, you could prevent your own birth. Many physicists believe time travel to the past is impossible, but there are solutions to a temporal paradox, such as traveling between parallel universes or branch points.

Time Perception

The human brain is equipped to track time. The suprachiasmatic nuclei of the brain is the region responsible for daily or circadian rhythms. But neurotransmitters and drugs affect time perceptions. Chemicals that excite neurons so they fire more quickly than normal speed up time, while decreased neuron firing slows down time perception. Basically, when time seems to speed up, the brain distinguishes more events within an interval. In this respect, time truly does seem to fly when one is having fun.

Time seems to slow down during emergencies or danger. Scientists at Baylor College of Medicine in Houston say the brain doesn’t actually speed up, but the amygdala becomes more active. The amygdala is the region of the brain that makes memories. As more memories form, time seems drawn out.

The same phenomenon explains why older people seem to perceive time as moving faster than when they were younger. Psychologists believe the brain forms more memories of new experiences than that of familiar ones. Since fewer new memories are built later in life, time seems to pass more quickly.

The Beginning and End of Time

As far as the universe is concerned, time had a beginning. The starting point was 13.799 billion years ago when the Big Bang occurred. We can measure cosmic background radiation as microwaves from the Big Bang, but there isn’t any radiation with earlier origins. One argument for the origin of time is that if it extended backward infinitely, the night sky would be filled with light from older stars.

Will time end? The answer to this question is unknown. If the universe expands forever, time would continue. If a new Big Bang occurs, our time line would end and a new one would begin. In particle physics experiments, random particles arise from a vacuum, so it doesn’t seem likely the universe would become static or timeless. Only time will tell.

Key Points

• Time is the progression of events from the past into the future.
• Time moves only in one direction. It’s possible to move forward in time, but not backward.
• Scientists believe memory formation is the basis for human perception of time.

Sources

• Carter, Rita. The Human Brain Book. Dorling Kindersley Publishing, 2009, London.
• Richards, E. G. Mapping Time: The Calendar and its History. Oxford University Press, 1998, Oxford.
• Schwartz, Herman M. Introduction to Special Relativity, McGraw-Hill Book Company, 1968, New York.