Misplaced Pages

Latest revision as of 13:51, 7 January 2025

Time
Major concepts Past Present Future Eternity of the world
Fields of study Archaeology Chronology History Horology Metrology Paleontology Futurology
Philosophy Presentism Eternalism Event Fatalism
Religion Mythology Creation End time Day of Judgement Immortality Afterlife Reincarnation Kalachakra
Measurement Standards ISO 8601 Metric Hexadecimal
Science Naturalism Chronobiology Cosmogony Evolution Radiometric dating Ultimate fate of the universe Time in physics
Related topics Motion Space Spacetime Time travel
v t e

Time is the continuous progression of our changing existence that occurs in an apparently irreversible succession from the past, through the present, and into the future. 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. Time is often referred to as a fourth dimension, along with three spatial dimensions. Scientists have theorized a beginning of time in our universe (the Big Bang) and an end (e.g., heat death or the Big Crunch). A cyclic model describes a cyclical nature, whereas the philosophy of eternalism views the subject from a different angle.

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. General relativity is the primary framework for understanding how spacetime works. 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.

Throughout history, time has been an important subject of study in religion, philosophy, and science. Temporal measurement has occupied scientists and technologists and has been a prime motivation in navigation and astronomy. Time is also of significant social importance, having economic value ("time is money") as well as personal value, due to an awareness of the limited time in each day and in human life spans. Cultural attitudes towards the human use of time are apparent in the verbs used—from "kill" to "waste" to "pass"—and sayings (like carpe diem).

Definition

The concept of time can be complex. Multiple notions exist and defining time in a manner applicable to all fields without circularity has consistently eluded scholars. 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. Traditional definitions of time involved the observation of periodic motion such as the apparent motion of the sun across the sky, the phases of the moon, and the passage of a free-swinging pendulum. More modern systems include the Global Positioning System, other satellite systems, Coordinated Universal Time and mean solar time. Although these systems differ from one another, with careful measurements they can be synchronized.

In physics, time is a fundamental concept to define other quantities, such as velocity. To avoid a circular definition, time in physics is operationally defined as "what a clock reads", specifically a count of repeating events such as the SI second. Although this aids in practical measurements, it does not address the essence of time. Physicists developed the concept of the spacetime continuum, where events are assigned four coordinates: three for space and one for time. Events like particle collisions, supernovas, or rocket launches have coordinates that may vary for different observers, making concepts like "now" and "here" relative. In general relativity, these coordinates do not directly correspond to the causal structure of events. Instead, the spacetime interval is calculated and classified as either space-like or time-like, depending on whether an observer exists that would say the events are separated by space or by time. Since the time required for light to travel a specific distance is the same for all observers—a fact first publicly demonstrated by the Michelson–Morley experiment—all observers will consistently agree on this definition of time as a causal relation.

General relativity does not address the nature of time for extremely small intervals where quantum mechanics holds. In quantum mechanics, time is treated as a universal and absolute parameter, differing from general relativity's notion of independent clocks. The problem of time consists of reconciling these two theories. As of 2024, there is no generally accepted theory of quantum general relativity.

Measurement

Generally speaking, historical methods of temporal measurement, or chronometry, have taken two distinct forms: the calendar, a mathematical tool for organising long intervals of time, and the clock (e.g., watch), a physical mechanism that counts the passage of time. In day-to-day life, a clock was consulted for periods less than a day, whereas a calendar was 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 certain starting date (epoch), and relevant to a certain time zone (including daylight saving time). Precise measurements, as in astronomy, use 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. 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.

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 as part of the dechristianization of France and to 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.

History of other devices

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

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.

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.

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. 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).

Incense sticks and candles were, and are, commonly used to measure time in temples and churches across the globe. Water clocks, 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.

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.

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.

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 first 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, 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. 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 Cs 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, where it was defined as 1/564 of a momentum (11⁄2 minutes), 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 seconds), about 3.7 × 10 Planck times.

Units

The second (s) is the SI base unit. A minute (min) is 60 seconds in length (or, rarely, 59 or 61 seconds when leap seconds are employed), 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.

Time 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 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

Many ancient cultures, particularly in the East, had a cyclical view of time. In these traditions, time was often seen as a recurring pattern of ages or cycles, where events and phenomena repeated themselves in a predictable manner. One of the most famous examples of this concept is found in Hindu philosophy, where time is depicted as a wheel called the "Kalachakra" or "Wheel of Time." According to this belief, the universe undergoes endless cycles of creation, preservation, and destruction.

Similarly, in other ancient cultures such as those of the Mayans, Aztecs, and Chinese, there were also beliefs in cyclical time, often associated with astronomical observations and calendars. These cultures developed complex systems to track time, seasons, and celestial movements, reflecting their understanding of cyclical patterns in nature and the universe.

The cyclical view of time contrasts with the linear concept of time more common in Western thought, where time is seen as progressing in a straight line from past to future without repetition.

Time in Abrahamic religions

In general, the Islamic and Judeo-Christian world-view regards time as linear and directional, beginning with the act of creation by God. The traditional Christian view sees time ending, teleologically, 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) is a medium for the passage of predestined events. (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.

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 & Rabbinical thought

Rabbis sometimes saw time like "an accordion that was expanded and collapsed at will." According to Kabbalists, "time" is a paradox and an illusion.

Time in Advaita Vedanta

According to Advaita Vedanta, time is integral to the phenomenal world, which lacks independent reality. Time and the phenomenal world are products of maya, influenced by our senses, concepts, and imaginations. The phenomenal world, including time, is seen as impermanent and characterized by plurality, suffering, conflict, and division. Since phenomenal existence is dominated by temporality (kala), everything within time is subject to change and decay. Overcoming pain and death requires knowledge that transcends temporal existence and reveals its eternal foundation.

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.

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 and Immanuel Kant, 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.

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. 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. This led to beliefs like cycles of rebirth and reincarnation. The Greek philosophers believe that the universe was infinite, and was an illusion to humans. Plato believed that time was made by the Creator at the same instant as the heavens. He also says that time is a period of motion of the heavenly bodies. Aristotle believed that time correlated to movement, that time did not exist on its own but was relative to motion of objects. 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.

The Vedas, the earliest texts on Indian philosophy and Hindu philosophy dating 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. Ancient Greek philosophers, including Parmenides and Heraclitus, wrote essays on the nature of time. 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'.

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, 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. 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. 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. Isaac Newton said that we are merely occupying time, he also says that humans can only understand relative time. Relative time is a measurement of objects in motion. The anti-realists believed that time is merely a convenient intellectual concept for humans to understand events. This means that time was useless unless there were objects that it could interact with, this was called relational time. René Descartes, John Locke, and David Hume said that one's mind needs to acknowledge time, in order to understand what time is. Immanuel Kant believed that we can not know what something is unless we experience it first hand.

Time is not an empirical concept. For neither co-existence nor succession would be perceived by us, if the representation of time did not exist as a foundation a priori. Without this presupposition, we could not represent to ourselves that things exist together at one and the same time, or at different times, that is, contemporaneously, or in succession.

Immanuel Kant, in the Critique of Pure Reason, described time as an a priori intuition that allows us (together with the other a priori intuition, space) to comprehend sense experience. With Kant, neither space nor time are conceived as substances, but rather both are elements of a systematic mental framework 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.

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. 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.

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? There is a theory called the tenseless or B-theory; this theory says that any tensed terminology can be replaced with tenseless terminology. 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. There is also something called imaginary time, this was from Stephen Hawking, who said that space and imaginary time are finite but have no boundaries. Imaginary time is not real or unreal, it is something that is hard to visualize. Philosophers can agree that physical time exists outside of the human mind and is objective, and psychological time is mind-dependent and subjective.

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. Time as an illusion is also a common theme in Buddhist thought.

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, argue 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.

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

Part of a series on
Classical mechanics
${\displaystyle {\textbf {F}}={\frac {d\mathbf {p} }{dt}}}$Second law of motion
History Timeline Textbooks
Branches Applied Celestial Continuum Dynamics Field theory Kinematics Kinetics Statics Statistical mechanics
Fundamentals Acceleration Angular momentum Couple D'Alembert's principle Energy kinetic potential Force Frame of reference Inertial frame of reference Impulse Inertia / Moment of inertia Mass Mechanical power Mechanical work Moment Momentum Space Speed Time Torque Velocity Virtual work
Formulations Newton's laws of motion Analytical mechanics Lagrangian mechanics Hamiltonian mechanics Routhian mechanics Hamilton–Jacobi equation Appell's equation of motion Koopman–von Neumann mechanics
Core topics Damping Displacement Equations of motion Euler's laws of motion Fictitious force Friction Harmonic oscillator Inertial / Non-inertial reference frame Motion (linear) Newton's law of universal gravitation Newton's laws of motion Relative velocity Rigid body dynamics Euler's equations Simple harmonic motion Vibration
Rotation Circular motion Rotating reference frame Centripetal force Centrifugal force reactive Coriolis force Pendulum Tangential speed Rotational frequency Angular acceleration / displacement / frequency / velocity
Scientists Kepler Galileo Huygens Newton Horrocks Halley Maupertuis Daniel Bernoulli Johann Bernoulli Euler d'Alembert Clairaut Lagrange Laplace Poisson Hamilton Jacobi Cauchy Routh Liouville Appell Gibbs Koopman von Neumann
Physics portal  Category
v t e

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. Non-relativistic classical mechanics is based on this Newtonian idea of time.

Einstein, in his special theory of relativity, 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).

Arrow of time

Unlike space, where an object can travel in the opposite directions (and in 3 dimensions), time appears to have only one dimension and only one direction – the past lies behind, fixed and immutable, while the future lies ahead and is not necessarily fixed. Yet most laws of physics allow any process to proceed both forward and in reverse. There are only a few physical phenomena, that violate the reversibility of time. This time directionality is known as the arrow of time. Acknowledged examples of the arrow of time are:

1. Radiative arrow of time, manifested in waves (e.g. light and sound) travelling only expanding (rather than focusing) in time (see light cone);
2. Entropic arrow of time: according to the second law of thermodynamics an isolated system evolves toward a larger disorder rather than orders spontaneously;
3. Quantum arrow time, which is related to irreversibility of measurement in quantum mechanics according to the Copenhagen interpretation of quantum mechanics;
4. Weak arrow of time: preference for a certain time direction of weak force in particle physics (see violation of CP symmetry);
5. Cosmological arrow of time, which follows the accelerated expansion of the Universe after the Big Bang.

The relationship(s) between these different Arrows of Time is a hotly debated topic in theoretical physics.

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.

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. In both pictures the view of spacetime changes when the observer accelerates.

In the Newtonian description these changes are such that time is absolute: 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.

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 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.

Thermodynamics

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.

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.

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, while depressants can have the opposite effect. The level of activity in the brain of neurotransmitters such as dopamine and norepinephrine may be the reason for this. 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).

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.

Alterations

In addition to psychoactive drugs, judgments of time can be altered by temporal illusions (like the kappa effect), age, and hypnosis. 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. Those who support this notion argue that young people, having more excitatory neurotransmitters, are able to cope with faster external events.

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. 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). Using space to think about time allows humans to mentally organize temporal order. These origins are shaped by many environmental factors––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. 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. 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.

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.

A study of the indigenous Yupno people of Papua New Guinea focused on the directional gestures used when individuals used time-related words. 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.

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. 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.

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.

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 and Norbert Elias 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. 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, historical events (chronology), directions and steps in procedures, 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 (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
• Loschmidt's paradox
• 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

1. "Time". Oxford Dictionaries. Oxford University Press. Archived from the original on 4 July 2012. Retrieved 18 May 2017. The indefinite continued progress of existence and events in the past, present, and future regarded as a whole
2. ^ * "Webster's New World College Dictionary". 2010. Archived from the original on 5 August 2011. Retrieved 9 April 2011. 1.indefinite, unlimited duration in which things are considered as happening in the past, present, or future; every moment there has ever been or ever will be… a system of measuring duration 2.the period between two events or during which something exists, happens, or acts; measured or measurable interval
   • "The American Heritage Stedman's Medical Dictionary". 2002. Archived from the original on 5 March 2012. Retrieved 9 April 2011. A duration or relation of events expressed in terms of past, present, and future, and measured in units such as minutes, hours, days, months, or years.
   • "Collins Language.com". HarperCollins. 2011. Archived from the original on 2 October 2011. Retrieved 18 December 2011. 1. 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. 2. physics a quantity measuring duration, usually with reference to a periodic process such as the rotation of the earth or the frequency 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.
   • "The American Heritage Science Dictionary @dictionary.com". 2002. Archived from the original on 5 March 2012. Retrieved 9 April 2011. 1. A continuous, measurable quantity in which events occur in a sequence proceeding from the past through the present to the future. 2a. An interval separating two points of this quantity; a duration. 2b. A system or reference frame in which such intervals are measured or such quantities are calculated.
   • "Eric Weisstein's World of Science". 2007. Archived from the original on 29 November 2017. Retrieved 9 April 2011. A quantity used to specify the order in which events occurred and measure the amount by which one event preceded or followed another. In special relativity, ct (where c is the speed of light and t is time), plays the role of a fourth dimension.
3. "Time". The American Heritage Dictionary of the English Language (Fourth ed.). 2011. Archived from the original on 19 July 2012. A nonspatial continuum in which events occur in apparently irreversible succession from the past through the present to the future.
4. Merriam-Webster Dictionary Archived 8 May 2012 at the Wayback Machine the measured or measurable period during which an action, process, or condition exists or continues : duration; a nonspatial continuum which is measured in terms of events that succeed one another from past through present to future
5. Compact Oxford English Dictionary A limited stretch or space of continued existence, as the interval between two successive events or acts, or the period through which an action, condition, or state continues. (1971).
6. ^ * "Internet Encyclopedia of Philosophy". 2010. Archived from the original on 11 April 2011. Retrieved 9 April 2011. Time is what clocks measure. We use time to place events in sequence one after the other, and we use time to compare how long events last... Among philosophers of physics, the most popular short answer to the question "What is physical time?" is that it is not a substance or object but rather a special system of relations among instantaneous events. This working definition is offered by Adolf Grünbaum who applies the contemporary mathematical theory of continuity to physical processes, and he says time is a linear continuum of instants and is a distinguished one-dimensional sub-space of four-dimensional spacetime.
   • "Dictionary.com Unabridged, based on Random House Dictionary". 2010. Archived from the original on 5 March 2012. Retrieved 9 April 2011. 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.... 3. (sometimes initial capital letter) a system or method of measuring or reckoning the passage of time: mean time; apparent time; Greenwich Time. 4. a limited period or interval, as between two successive events: a long time.... 14. a particular or definite point in time, as indicated by a clock: What time is it? ... 18. an indefinite, frequently prolonged period or duration in the future: Time will tell if what we have done here today was right.
   • Ivey, Donald G.; Hume, J.N.P. (1974). Physics. Vol. 1. Ronald Press. p. 65. Archived from the original on 14 April 2021. Retrieved 7 May 2020. Our operational definition of time is that time is what clocks measure.
7. ^ Le Poidevin, Robin (Winter 2004). "The Experience and Perception of Time". In Edward N. Zalta (ed.). The Stanford Encyclopedia of Philosophy. Archived from the original on 22 October 2013. Retrieved 9 April 2011.
8. "Newton did for time what the Greek geometers did for space, idealized it into an exactly measurable dimension." About Time: Einstein's Unfinished Revolution, Paul Davies, p. 31, Simon & Schuster, 1996, ISBN 978-0-684-81822-1
9. Bunyadzade, Konul (15 March 2018). "Thoughts of Time" (PDF). Metafizika Journal (in Azerbaijani). 1. AcademyGate Publishing: 8–29. doi:10.33864/MTFZK.2019.0. ISSN 2616-6879. Archived (PDF) from the original on 5 April 2019. Retrieved 15 March 2018.
10. Rendall, Alan D. (2008). Partial Differential Equations in General Relativity (illustrated ed.). OUP Oxford. p. 9. ISBN 978-0-19-921540-9. Archived from the original on 14 April 2021. Retrieved 24 November 2020.
11. Carroll, Sean M. (2009). "From Eternity to Here: The Quest for the Ultimate Theory of Time". Physics Today. 63 (4). Dutton: 54–55. Bibcode:2010PhT....63d..54C. doi:10.1063/1.3397046. ISBN 978-0-525-95133-9.
12. "The Feynman Lectures on Physics Vol. I Ch. 5: Time and Distance". www.feynmanlectures.caltech.edu. Retrieved 15 December 2023.
13. "Official Baseball Rules – 8.03 and 8.04" (Free PDF download). Major League Baseball. 2011. Archived (PDF) from the original on 1 July 2017. Retrieved 18 May 2017. Rule 8.03 Such preparatory pitches shall not consume more than one minute of time...Rule 8.04 When the bases are unoccupied, the pitcher shall deliver the ball to the batter within 12 seconds...The 12-second timing starts when the pitcher is in possession of the ball and the batter is in the box, alert to the pitcher. The timing stops when the pitcher releases the ball.
14. "Guinness Book of Baseball World Records". Guinness World Records, Ltd. Archived from the original on 6 June 2012. Retrieved 7 July 2012. The record for the fastest time for circling the bases is 13.3 seconds, set by Evar Swanson at Columbus, Ohio in 1932...The greatest reliably recorded speed at which a baseball has been pitched is 100.9 mph by Lynn Nolan Ryan (California Angels) at Anaheim Stadium in California on 20 August 1974.
15. Zeigler, Kenneth (2008). Getting organized at work : 24 lessons to set goals, establish priorities, and manage your time. McGraw-Hill. ISBN 978-0-07-159138-6. Archived from the original on 18 August 2020. Retrieved 30 July 2019. 108 pages.
16. Duff, Okun, Veneziano, ibid. p. 3. "There is no well established terminology for the fundamental constants of Nature. ... The absence of accurately defined terms or the uses (i.e., actually misuses) of ill-defined terms lead to confusion and proliferation of wrong statements."
17. ^ Burnham, Douglas: Staffordshire University (2006). "Gottfried Wilhelm Leibniz (1646–1716) Metaphysics – 7. Space, Time, and Indiscernibles". The Internet Encyclopedia of Philosophy. Archived from the original on 14 May 2011. Retrieved 9 April 2011. First of all, Leibniz finds the idea that space and time might be substances or substance-like absurd (see, for example, "Correspondence with Clarke," Leibniz's Fourth Paper, §8ff). In short, an empty space would be a substance with no properties; it will be a substance that even God cannot modify or destroy.... That is, space and time are internal or intrinsic features of the complete concepts of things, not extrinsic.... Leibniz's view has two major implications. First, there is no absolute location in either space or time; location is always the situation of an object or event relative to other objects and events. Second, space and time are not in themselves real (that is, not substances). Space and time are, rather, ideal. Space and time are just metaphysically illegitimate ways of perceiving certain virtual relations between substances. They are phenomena or, strictly speaking, illusions (although they are illusions that are well-founded upon the internal properties of substances).... It is sometimes convenient to think of space and time as something "out there," over and above the entities and their relations to each other, but this convenience must not be confused with reality. Space is nothing but the order of co-existent objects; time nothing but the order of successive events. This is usually called a relational theory of space and time.
18. Considine, Douglas M.; Considine, Glenn D. (1985). Process instruments and controls handbook (3 ed.). McGraw-Hill. pp. 18–61. Bibcode:1985pich.book.....C. ISBN 978-0-07-012436-3. Archived from the original on 31 December 2013. Retrieved 1 November 2016.
19. Lilley, Sam (1981). Discovering Relativity for Yourself (illustrated ed.). Cambridge University Press Archive. p. 125. ISBN 978-0-521-23038-4. Extract of page 125.
20. Surya, Sumati (December 2019). "The causal set approach to quantum gravity". Living Reviews in Relativity. 22 (1): 5. arXiv:1903.11544. Bibcode:2019LRR....22....5S. doi:10.1007/s41114-019-0023-1. Thus, the causal structure poset (M, ≺) of a future and past distinguishing spacetime is equivalent to its conformal geometry.
21. Macías, Alfredo; Camacho, Abel (May 2008). "On the incompatibility between quantum theory and general relativity". Physics Letters B. 663 (1–2): 99–102. Bibcode:2008PhLB..663...99M. doi:10.1016/j.physletb.2008.03.052. In our opinion, it is not possible to reconciliate and integrate into a common scheme the absolute and non-dynamical character of Newtonian time of canonical quantization and path integral approaches with the relativistic and dynamical character of time in general relativity.
22. Shavit, Joseph (18 July 2024). "Revolutionary theory finally unites quantum mechanics and Einstein's theory of general relativity". The Brighter Side of News. The prevailing consensus has been that Einstein's theory of gravity must be modified to fit within the framework of quantum theory when it comes to merging these two theories into a single, comprehensive framework, the scientific community has hit a roadblock.
23. Richards, E. G. (1998). Mapping Time: The Calendar and its History. Oxford University Press. pp. 3–5. ISBN 978-0-19-850413-9.
24. Rudgley, Richard (1999). The Lost Civilizations of the Stone Age. New York: Simon & Schuster. pp. 86–105.
25. Van Stone, Mark (2011). "The Maya Long Count Calendar: An Introduction". Archaeoastronomy. 24: 8–11.
26. "French Republican Calendar | Chronology." Encyclopædia Britannica Online. Encyclopædia Britannica, n.d. Web. 21 February 2016.
27. "Education". Archived from the original on 1 May 2019. Retrieved 1 July 2018.
28. Barnett, Jo Ellen (1999). Time's Pendulum: From Sundials to Atomic Clocks, the Fascinating History of Timekeeping and how Our Discoveries Changed the World (reprinted ed.). Harcourt Brace. p. 28. ISBN 978-0-15-600649-1.
29. Lombardi, Michael A. "Why Is a Minute Divided into 60 Seconds, an Hour into 60 Minutes, Yet There Are Only 24 Hours in a Day?" Scientific American. Springer Nature, 5 March 2007. Web. 21 February 2016.
30. Barnett, ibid, p. 37.
31. Bergreen, Laurence. Over the Edge of the World: Magellan's Terrifying Circumnavigation of the Globe (HarperCollins Publishers, 2003), ISBN 0-06-621173-5
32. North, J. (2004) God's Clockmaker: Richard of Wallingford and the Invention of Time. Oxbow Books. ISBN 1-85285-451-0
33. Watson, E., (1979) "The St Albans Clock of Richard of Wallingford". Antiquarian Horology, pp. 372–384.
34. ^ "History of Clocks." About.com Inventors. About.com, n.d. Web. 21 February 2016.
35. "NIST Unveils Chip-Scale Atomic Clock". NIST. 27 August 2004. Archived from the original on 22 May 2011. Retrieved 9 June 2011.
36. "New atomic clock can keep time for 200 million years: Super-precise instruments vital to deep space navigation". Vancouver Sun. 16 February 2008. Archived from the original on 11 February 2012. Retrieved 9 April 2011.
37. "NIST-F1 Cesium Fountain Clock". Archived from the original on 25 March 2020. Retrieved 24 July 2015.
38. "Byrhtferth of Ramsey". Encyclopædia Britannica. 2008. Archived from the original on 14 June 2020. Retrieved 15 September 2008.
39. "atom", Oxford English Dictionary, Draft Revision September 2008 (contains relevant citations from Byrhtferth's Enchiridion)
40. "12 attoseconds is the world record for shortest controllable time". 12 May 2010. Archived from the original on 5 August 2011. Retrieved 19 April 2012.
41. Newman, John (1991). Geshe Lhundub Sopa (ed.). The Wheel of Time: Kalachakra in Context. Shambhala. pp. 51–54, 62–77. ISBN 978-1-55939-779-7.
42. "Chichén Itzá: Venus Cycle". Travel. 27 February 2012. Retrieved 8 May 2024.
43. Jennifer M. Gidley (2017). The Future: A Very Short Introduction. Oxford University Press. p. 13. ISBN 978-0-19-105424-2. Extract of page 13
44. Rust, Eric Charles (1981). Religion, Revelation and Reason. Mercer University Press. p. 60. ISBN 978-0-86554-058-3. Archived from the original on 3 April 2017. Retrieved 20 August 2015. Profane time, as Eliade points out, is linear. As man dwelt increasingly in the profane and a sense of history developed, the desire to escape into the sacred began to drop in the background. The myths, tied up with cyclic time, were not so easily operative. So secular man became content with his linear time. He could not return to cyclic time and re-enter sacred space though its myths. Just here, as Eliade sees it, a new religious structure became available. In the Judaeo-Christian religions – Judaism, Christianity, Islam – history is taken seriously, and linear time is accepted. The cyclic time of the primordial mythical consciousness has been transformed into the time of profane man, but the mythical consciousness remains. It has been historicized. The Christian mythos and its accompanying ritual are bound up, for example, with history and center in authentic history, especially the Christ-event. Sacred space, the Transcendent Presence, is thus opened up to secular man because it meets him where he is, in the linear flow of secular time. The Christian myth gives such time a beginning in creation, a center in the Christ-event, and an end in the final consummation.
45. Betz, Hans Dieter, ed. (2008). Religion Past & Present: Encyclopedia of Theology and Religion. Vol. 4 (4 ed.). Brill. p. 101. ISBN 978-90-04-14688-4. Archived from the original on 24 September 2015. Retrieved 20 August 2015. God produces a creation with a directional time structure .
46. Lundin, Roger; Thiselton, Anthony C.; Walhout, Clarence (1999). The Promise of Hermeneutics. Wm. B. Eerdmans Publishing. p. 121. ISBN 978-0-8028-4635-8. Archived from the original on 19 September 2015. Retrieved 20 August 2015. We need to note the close ties between teleology, eschatology, and utopia. In Christian theology, the understanding of the teleology of particular actions is ultimately related to the teleology of history in general, which is the concern of eschatology.
47. "(Dictionary Entry)". Henry George Liddell, Robert Scott, A Greek-English Lexicon. Archived from the original on 7 May 2022. Retrieved 13 July 2015.
48. New Myths and Meanings in Jewish New Moon Rituals David M. Rosen, Victoria P. Rosen Ethnology, Vol. 39, No. 3 (Summer, 2000), pp. 263–277 (referencing Yerushalmi 1989)
49. Hus, Boʿaz; Pasi, Marco; Stuckrad, Kocku von (2011). Kabbalah and Modernity: Interpretations, Transformations, Adaptations. BRILL. ISBN 978-90-04-18284-4. Archived from the original on 13 May 2016. Retrieved 27 February 2016.
50. Wolfson, Elliot R. (2006). Alef, Mem, Tau: Kabbalistic Musings on Time, Truth, and Death. University of California Press. p. 111. ISBN 978-0-520-93231-9. Archived from the original on 19 August 2020. Retrieved 7 May 2020. Extract of page 111 Archived 11 May 2022 at the Wayback Machine
51. Puligandla, R. (1974). "Time and History in the Indian Tradition". Philosophy East and West. 24 (2): 165–170. doi:10.2307/1398019. ISSN 0031-8221. JSTOR 1398019.
52. Rynasiewicz, Robert: Johns Hopkins University (12 August 2004). "Newton's Views on Space, Time, and Motion". Stanford Encyclopedia of Philosophy. Stanford University. Archived from the original on 16 July 2012. Retrieved 5 February 2012. Newton did not regard space and time as genuine substances (as are, paradigmatically, bodies and minds), but rather as real entities with their own manner of existence as necessitated by God's existence ... To paraphrase: Absolute, true, and mathematical time, from its own nature, passes equably without relation to anything external, and thus without reference to any change or way of measuring of time (e.g., the hour, day, month, or year).
53. Markosian, Ned. "Time". In Edward N. Zalta (ed.). The Stanford Encyclopedia of Philosophy (Winter 2002 Edition). Archived from the original on 14 September 2006. Retrieved 23 September 2011. The opposing view, normally referred to either as "Platonism with Respect to Time" or as "Absolutism with Respect to Time", has been defended by Plato, Newton, and others. On this view, time is like an empty container into which events may be placed; but it is a container that exists independently of whether or not anything is placed in it.
54. Mattey, G. J. (22 January 1997). "Critique of Pure Reason, Lecture notes: Philosophy 175 UC Davis". Archived from the original on 14 March 2005. Retrieved 9 April 2011. What is correct in the Leibnizian view was its anti-metaphysical stance. Space and time do not exist in and of themselves, but in some sense are the product of the way we represent things. The are ideal, though not in the sense in which Leibniz thought they are ideal (figments of the imagination). The ideality of space is its mind-dependence: it is only a condition of sensibility.... Kant concluded ... "absolute space is not an object of outer sensation; it is rather a fundamental concept which first of all makes possible all such outer sensation."...Much of the argumentation pertaining to space is applicable, mutatis mutandis, to time, so I will not rehearse the arguments. As space is the form of outer intuition, so time is the form of inner intuition.... Kant claimed that time is real, it is "the real form of inner intuition."
55. McCormick, Matt (2006). "Immanuel Kant (1724–1804) Metaphysics: 4. Kant's Transcendental Idealism". The Internet Encyclopedia of Philosophy. Archived from the original on 26 April 2011. Retrieved 9 April 2011. Time, Kant argues, is also necessary as a form or condition of our intuitions of objects. The idea of time itself cannot be gathered from experience because succession and simultaneity of objects, the phenomena that would indicate the passage of time, would be impossible to represent if we did not already possess the capacity to represent objects in time.... Another way to put the point is to say that the fact that the mind of the knower makes the a priori contribution does not mean that space and time or the categories are mere figments of the imagination. Kant is an empirical realist about the world we experience; we can know objects as they appear to us. He gives a robust defense of science and the study of the natural world from his argument about the mind's role in making nature. All discursive, rational beings must conceive of the physical world as spatially and temporally unified, he argues.
56. Carrol, Sean (2010). "Chapter One, Section Two, Plume". From Eternity to Here: The Quest for the Ultimate Theory of Time. Penguin. ISBN 978-0-452-29654-1. As human beings we 'feel' the passage of time.
57. Lehar, Steve. (2000). The Function of Conscious Experience: An Analogical Paradigm of Perception and Behavior Archived 21 October 2015 at the Wayback Machine, Consciousness and Cognition.
58. ^ "Philosophy of Time – Exactly What Is Time?". Archived from the original on 28 March 2019. Retrieved 28 March 2019.
59. ^ "Ancient Philosophy – Exactly What Is Time?". Archived from the original on 28 March 2019. Retrieved 28 March 2019.
60. ^ Bunnag, Anawat (August 2017). "The concept of time in philosophy: A comparative study between Theravada Buddhist and Henri Bergson's concept of time from Thai philosophers' perspectives". Kasetsart Journal of Social Sciences. doi:10.1016/j.kjss.2017.07.007. Archived from the original on 2 April 2019. Retrieved 11 April 2019.
61. Layton, Robert (1994). Who Needs the Past?: Indigenous Values and Archaeology. Psychology Press. p. 7. ISBN 978-0-415-09558-7.
62. Dagobert Runes, Dictionary of Philosophy, p. 318.
63. Hardie, R. P.; Gaye, R. K. "Physics by Aristotle". Massachusetts Institute of Technology. Archived from the original on 26 June 2014. Retrieved 4 May 2014. "Time then is a kind of number. (Number, we must note, is used in two senses – both of what is counted or the countable and also of that with which we count. Time obviously is what is counted, not that with which we count: there are different kinds of thing.) It is clear, then, that time is 'number of movement in respect of the before and after', and is continuous since it is an attribute of what is continuous."
64. Augustine of Hippo. Confessions. Archived from the original on 19 January 2012. Retrieved 9 April 2011. Book 11, Chapter 14.
65. Gottfried Martin, Kant's Metaphysics and Theory of Science.
66. ^ "Early Modern Philosophy – Exactly What Is Time?". Archived from the original on 28 March 2019. Retrieved 28 March 2019.
67. Jankowiak, Tim. "Immanuel Kant". Archived from the original on 23 May 2021. Retrieved 2 April 2019.
68. Kant, Immanuel (1787). The Critique of Pure Reason, 2nd edition. Archived from the original on 13 April 2011. Retrieved 9 April 2011. translated by J.M.D. Meiklejohn, eBooks@Adelaide, 2004
69. Bergson, Henri (1907) Creative Evolution. trans. by Arthur Mitchell. Mineola: Dover, 1998.
70. Balslev, Anindita N.; Jitendranath Mohanty (November 1992). Religion and Time. Studies in the History of Religions, 54. The Netherlands: Brill Academic Publishers. pp. 53–59. ISBN 978-90-04-09583-0. Archived from the original on 20 August 2020. Retrieved 30 July 2019.
71. Heidegger, Martin (1962). "V". Being and Time. Blackwell. p. 425. ISBN 978-0-631-19770-6. Archived from the original on 19 August 2020. Retrieved 30 July 2019.
72. ^ "Modern Philosophy – Exactly What Is Time?". Archived from the original on 28 March 2019. Retrieved 28 March 2019.
73. Foundalis, Harry. "You are about to disappear". Archived from the original on 12 May 2011. Retrieved 9 April 2011.
74. Huston, Tom. "Buddhism and the illusion of time". Archived from the original on 8 July 2011. Retrieved 9 April 2011.
75. Garfield, Jay L. (1995). The fundamental wisdom of the middle way: Nāgārjuna's Mūlamadhyamakakārikā. New York: Oxford University Press. ISBN 978-0-19-509336-0. Archived from the original on 19 August 2020. Retrieved 19 May 2018.
76. Herman M. Schwartz, Introduction to Special Relativity, McGraw-Hill Book Company, 1968, hardcover 442 pages, see ISBN 0-88275-478-5 (1977 edition), pp. 10–13
77. A. Einstein, H. A. Lorentz, H. Weyl, H. Minkowski, The Principle of Relativity, Dover Publications, Inc, 2000, softcover, 216 pages, ISBN 0-486-60081-5, See pp. 37–65 for an English translation of Einstein's original 1905 paper.
78. Quantum causality and the arrows of time and thermodynamics. 2020. Prog Part Nucl Phys. 115/13. J.F. Donoghue, G. Menezes. doi: 10.1016/j.ppnp.2020.103812.
79. The Direction of Time. 2015. Time: A Philosophical Introduction. pp. 191–214. J. Harrington, J. Harrington.
80. Time irreversibility in active matter, from micro to macro. 2022. Nat Rev Phys. 4/3, pp. 167–183.
81. J. O'byrne, Y. Kafri, J. Tailleur, F. Van Wijland. doi: 10.1038/s42254-021-00406-2.
82. The Nature and Origin of Time-Asymmetric Spacetime Structures. 2014. Springer Handbook of Spacetime. pp. 185–196. H.D. Zeh.
83. The Arrow of Time. 2016. Cosmological and Psychological Time. 285/155-62. M. Hemmo, O. Shenker. doi: 10.1007/978-3-319-22590-6_9.
84. Relativity Theory May not Have the Last Word on the Nature of Time: Quantum Theory and Probabilism. 2017. Space, Time and the Limits of Human Understanding. pp. 109214. N. Maxwell. doi: 10.1007/978-3-319-44418-5_9.
85. Complexity and the Arrow of Time. 2013. Complexity and the Arrow of Time. pp. 1–357. doi: 10.1017/cbo9781139225700.
86. Peter Coveney and Roger Highfield. The Arrow of Time : A Voyage Through Science to Solve Time's Greatest Mystery. Hardcover – May 14, 1991. https://www.publishersweekly.com/9780449906309
87. "Albert Einstein's Theory of Relativity". YouTube. 30 November 2011. Archived from the original on 17 October 2013. Retrieved 24 September 2013.
88. "Time Travel: Einstein's big idea (Theory of Relativity)". YouTube. 9 January 2007. Archived from the original on 17 October 2013. Retrieved 24 September 2013.
89. Knudsen, Jens M.; Hjorth, Poul G. (6 December 2012). Elements of Newtonian Mechanics. Springer Science & Business Media. p. 30. ISBN 978-3-642-97599-8.
90. Greene, Brian (2005). "Chapter 6: Chance and the Arrow". The Fabric of the Cosmos. Penguin Books Limited. ISBN 978-0-14-195995-5. Archived from the original on 20 August 2020. Retrieved 16 September 2017.
91. Andersen, Holly; Grush, Rick (2009). "A brief history of time-consciousness: historical precursors to James and Husserl" (PDF). Journal of the History of Philosophy. 47 (2): 277–307. doi:10.1353/hph.0.0118. S2CID 16379171. Archived from the original (PDF) on 16 February 2008. Retrieved 9 April 2011.
92. Wittmann, M.; Leland, D. S.; Churan, J.; Paulus, M. P. (8 October 2007). "Impaired time perception and motor timing in stimulant-dependent subjects". Drug Alcohol Depend. 90 (2–3): 183–192. doi:10.1016/j.drugalcdep.2007.03.005. PMC 1997301. PMID 17434690.
93. Cheng, Ruey-Kuang; Macdonald, Christopher J.; Meck, Warren H. (2006). "Differential effects of cocaine and ketamine on time estimation: Implications for neurobiological models of interval timing" (online abstract). Pharmacology Biochemistry and Behavior. 85 (1): 114–122. doi:10.1016/j.pbb.2006.07.019. PMID 16920182. S2CID 42295255. Archived from the original on 10 August 2011. Retrieved 9 April 2011.
94. Tinklenberg, Jared R.; Roth, Walton T.; Kopell, Bert S. (January 1976). "Marijuana and ethanol: Differential effects on time perception, heart rate, and subjective response". Psychopharmacology. 49 (3): 275–279. doi:10.1007/BF00426830. PMID 826945. S2CID 25928542.
95. Arzy, Shahar; Molnar-Szakacs, Istvan; Blanke, Olaf (18 June 2008). "Self in Time: Imagined Self-Location Influences Neural Activity Related to Mental Time Travel". The Journal of Neuroscience. 28 (25): 6502–6507. doi:10.1523/JNEUROSCI.5712-07.2008. PMC 6670885. PMID 18562621.
96. ^ Carter, Rita (2009). The Human Brain Book. Dorling Kindersley Publishing. pp. 186–187. ISBN 978-0-7566-5441-2. Archived from the original on 13 May 2016. Retrieved 27 February 2016.
97. Kennedy-Moore, Eileen (28 March 2014). "Time Management for Kids". Psychology Today. Archived from the original on 30 July 2022. Retrieved 26 April 2014.
98. Wada Y, Masuda T, Noguchi K, 2005, "Temporal illusion called 'kappa effect' in event perception" Perception 34 ECVP Abstract Supplement.
99. Adler, Robert. "Look how time flies". Archived from the original on 14 June 2011. Retrieved 9 April 2011.
100. Bowers, Kenneth; Brenneman, Heather A. (January 1979). "Hypnosis and the perception of time". International Journal of Clinical and Experimental Hypnosis. 27 (1): 29–41. doi:10.1080/00207147908407540. PMID 541126.
101. Gruber, Ronald P.; Wagner, Lawrence F.; Block, Richard A. (2000). "Subjective Time Versus Proper (Clock) Time". In Buccheri, R.; Di Gesù, V.; Saniga, Metod (eds.). Studies on the structure of time: from physics to psycho(patho)logy. Springer. p. 54. ISBN 978-0-306-46439-3. Archived from the original on 21 July 2011. Retrieved 9 April 2011. Extract of page 54 Archived 13 May 2016 at the Wayback Machine
102. ^ Núñez, Rafael; Cooperrider, Kensy; Doan, D; Wassmann, Jürg (1 July 2012). "Contours of time: Topographic construals of past, present, and future in the Yupno valley of Papua New Guinea". Cognition. 124 (1): 25–35. doi:10.1016/j.cognition.2012.03.007. PMID 22542697. S2CID 17215084.
103. ^ Bottini, Roberto; Crepaldi, Davide; Casasanto, Daniel; Crollen, Virgine; Collignon, Olivier (1 August 2015). "Space and time in the sighted and blind". Cognition. 141: 67–72. doi:10.1016/j.cognition.2015.04.004. hdl:2078.1/199842. PMID 25935747. S2CID 14646964.
104. ^ Boroditsky, Lera; Gaby, Alice (2010). "Remembrances of Times East". Psychological Science. 21 (11): 1635–1639. doi:10.1177/0956797610386621. PMID 20959511. S2CID 22097776.
105. Hochschild, Arlie Russell (1997). The time bind: when work becomes home and home becomes work. New York: Metropolitan Books. ISBN 978-0-8050-4471-3.
106. Russell Hochschild, Arlie (20 April 1997). "There's no place like work". The New York Times Magazine. Archived from the original on 23 March 2017. Retrieved 20 February 2017.
107. Elias, Norbert (1992). Time: an essay. Oxford, UK Cambridge, US: Blackwell. ISBN 978-0-631-15798-4.
108. "Sequence – Order of Important Events" (PDF). Austin Independent School District. 2009. Archived from the original (PDF) on 27 September 2011.
109. "Sequence of Events Worksheets". Reference.com. Archived from the original on 13 October 2010.
110. Compiled by David Luckham & Roy Schulte (23 August 2011). "Event Processing Glossary – Version 2.0". Complex Event Processing. Archived from the original on 15 October 2011.
111. Nordquist, Richard. "narrative". About.com. Archived from the original on 4 September 2011.
112. Piasecki, David J. "Inventory Accuracy Glossary". AccuracyBook.com (OPS Publishing). Archived from the original on 3 September 2011.
113. "Utility Communications Architecture (UCA) glossary". NettedAutomation. Archived from the original on 10 December 2011.

Further reading

• Barbour, Julian (1999). The End of Time: The Next Revolution in Our Understanding of the Universe. Oxford University Press. ISBN 978-0-19-514592-2.
• Craig Callendar, Introducing Time, Icon Books, 2010, ISBN 978-1-84831-120-6
• Das, Tushar Kanti (1990). The Time Dimension: An Interdisciplinary Guide. New York: Praeger. ISBN 978-0-275-92681-6. – Research bibliography
• Davies, Paul (1996). About Time: Einstein's Unfinished Revolution. New York: Simon & Schuster Paperbacks. ISBN 978-0-684-81822-1.
• Feynman, Richard (1994) . The Character of Physical Law. Cambridge (Mass): The MIT Press. pp. 108–126. ISBN 978-0-262-56003-0.
• Galison, Peter (1992). Einstein's Clocks and Poincaré's Maps: Empires of Time. New York: W.W. Norton. ISBN 978-0-393-02001-4.
• Benjamin Gal-Or, Cosmology, Physics and Philosophy, Springer Verlag, 1981, 1983, 1987, ISBN 0-387-90581-2, 0-387-96526-2.
• Charlie Gere, (2005) Art, Time and Technology: Histories of the Disappearing Body, Berg
• Highfield, Roger (1992). Arrow of Time: A Voyage through Science to Solve Time's Greatest Mystery. Random House. ISBN 978-0-449-90723-8.
• Landes, David (2000). Revolution in Time. Harvard University Press. ISBN 978-0-674-00282-1.
• Lebowitz, Joel L. (2008). "Time's arrow and Boltzmann's entropy". Scholarpedia. 3 (4): 3448. Bibcode:2008SchpJ...3.3448L. doi:10.4249/scholarpedia.3448.
• Mermin, N. David (2005). It's About Time: Understanding Einstein's Relativity. Princeton University Press. ISBN 978-0-691-12201-4.
• Morris, Richard (1985). Time's Arrows: Scientific Attitudes Toward Time. New York: Simon and Schuster. ISBN 978-0-671-61766-0.
• Penrose, Roger (1999) . The Emperor's New Mind: Concerning Computers, Minds, and the Laws of Physics. New York: Oxford University Press. pp. 391–417. ISBN 978-0-19-286198-6. Archived from the original on 26 December 2010. Retrieved 9 April 2011.
• Price, Huw (1996). Time's Arrow and Archimedes' Point. Oxford University Press. ISBN 978-0-19-511798-1. Retrieved 9 April 2011.
• Reichenbach, Hans (1999) . The Direction of Time. New York: Dover. ISBN 978-0-486-40926-9.
• Rovelli, Carlo (2006). What is time? What is space?. Rome: Di Renzo Editore. ISBN 978-88-8323-146-9. Archived from the original on 27 January 2007.
• Rovelli, Carlo (2018). The Order of Time. New York: Riverhead. ISBN 978-0735216105.
• Stiegler, Bernard, Technics and Time, 1: The Fault of Epimetheus
• Roberto Mangabeira Unger and Lee Smolin, The Singular Universe and the Reality of Time, Cambridge University Press, 2014, ISBN 978-1-107-07406-4.
• Whitrow, Gerald J. (1973). The Nature of Time. Holt, Rinehart and Wilson (New York).
• Whitrow, Gerald J. (1980). The Natural Philosophy of Time. Clarendon Press (Oxford).
• Whitrow, Gerald J. (1988). Time in History. The evolution of our general awareness of time and temporal perspective. Oxford University Press. ISBN 978-0-19-285211-3.

External links

• Different systems of measuring time (archived 16 October 2015).
• Time on In Our Time at the BBC.
• "Time". Merriam-Webster.com Dictionary. Merriam-Webster.
• 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 articles: Detailed navigation

v t e Time measurement and standards Chronometry Orders of magnitude Metrology International standards Coordinated Universal Time offset UT ΔT DUT1 International Earth Rotation and Reference Systems Service ISO 31-1 ISO 8601 International Atomic Time 12-hour clock 24-hour clock Barycentric Coordinate Time Barycentric Dynamical Time Civil time Daylight saving time Geocentric Coordinate Time International Date Line IERS Reference Meridian Leap second Solar time Terrestrial Time Time zone 180th meridian Obsolete standards Ephemeris time Greenwich Mean Time Prime meridian Time in physics Absolute space and time Spacetime Chronon Continuous signal Coordinate time Cosmological decade Discrete time and continuous time Proper time Theory of relativity Time dilation Gravitational time dilation Time domain Time-translation symmetry T-symmetry Horology Clock Astrarium Atomic clock Complication History of timekeeping devices Hourglass Marine chronometer Marine sandglass Radio clock Watch stopwatch Water clock Sundial Dialing scales Equation of time History of sundials Sundial markup schema Calendar Gregorian Hebrew Hindu Holocene Islamic (lunar Hijri) Julian Solar Hijri Astronomical Dominical letter Epact Equinox Intercalation Julian day Leap year Lunar Lunisolar Solar Solstice Tropical year Weekday determination Weekday names Archaeology and geology Chronological dating Geologic time scale International Commission on Stratigraphy Astronomical chronology Galactic year Nuclear timescale Precession Sidereal time Other units of time Instant Flick Shake Jiffy Second Minute Moment Hour Day Week Fortnight Month Year Olympiad Lustrum Decade Century Saeculum Millennium Related topics Chronology Duration music Mental chronometry Decimal time Metric time System time Time metrology Time value of money Timekeeper v t e Chronology Key topics Archaeology Astronomy Geology History Big History Paleontology Time Periods Eras Epochs Calendar eras Human Era Ab urbe condita Anno Domini / Common Era Anno Mundi Bosporan era Bostran era Byzantine era Seleucid era Era of Caesar (Iberia) Before present Hijri Egyptian Sothic cycle Hindu units of time (Yuga) Mesoamerican Long Count Short Count Tzolk'in Haab' Regnal year Anka year Canon of Kings English and British regnal year Lists of kings Limmu Era names Chinese Japanese Korean Vietnamese Calendars Pre-Julian / Julian Pre-Julian Roman Original Julian Proleptic Julian Revised Julian Gregorian Gregorian Proleptic Gregorian Old Style and New Style dates Adoption of the Gregorian calendar Dual dating Astronomical Lunisolar (Hebrew, Hindu) Solar Lunar (Islamic) Astronomical year numbering Others Chinese sexagenary cycle Geologic Calendar Iranian ISO week date Mesoamerican Maya Aztec Winter count New Earth Time Astronomic time Cosmic Calendar Ephemeris Galactic year Metonic cycle Milankovitch cycles Geologic time Concepts Deep time Geological history of Earth Geological time units Standards Global Standard Stratigraphic Age (GSSA) Global Boundary Stratotype Section and Point (GSSP) Methods Chronostratigraphy Geochronology Isotope geochemistry Law of superposition Luminescence dating Samarium–neodymium dating Chronological dating Absolute dating Amino acid racemisation Archaeomagnetic dating Dendrochronology Ice core Incremental dating Lichenometry Paleomagnetism Radiometric dating Lead–lead Potassium–argon Radiocarbon Uranium–lead Tephrochronology Luminescence dating Thermoluminescence dating Relative dating Fluorine absorption Nitrogen dating Obsidian hydration Seriation Stratigraphy Genetic methods Molecular clock Linguistic methods Glottochronology Related topics Chronicle New Chronology Synchronoptic view Timeline Year zero Floruit Terminus post quem ASPRO chronology v t e Time in religion and mythology Aeon Calendar Deities Destiny Divination Eschatology Eternity Eviternity Heortology Golden Age Prophecy Wheel of the Year Yuga v t e Philosophy of time Concepts in time Time A priori and a posteriori A series and B series Action Duration Eternal return Eternity Event Eviternity Imaginary time Multiple time dimensions Temporal parts Temporality Theories of time A-theory of time B-theory of time Endurantism Eternalism Four-dimensionalism Fatalism Finitism Growing block universe Perdurantism Presentism Related articles Etiology Metaphysics Post hoc ergo propter hoc Teleology "The Unreality of Time" The Singular Universe and the Reality of Time An Experiment with Time v t e Metaphysics Theories Abstract object theory Action theory Anti-realism Determinism Dualism Enactivism Essentialism Existentialism Free will Idealism Libertarianism Liberty Materialism Meaning of life Monism Naturalism Nihilism Phenomenalism Realism Physicalism Relativism Scientific realism Solipsism Spiritualism Subjectivism Substance theory Theory of forms Truthmaker theory Type theory Concepts Abstract object Anima mundi Category of being Causality Causal closure Cogito, ergo sum Concept Embodied cognition Entity Essence Existence Experience Hypostatic abstraction Idea Identity Information Data Insight Intelligence Intention Linguistic modality Matter Meaning Mental representation Mind Motion Nature Necessity Object Ontology Pattern Perception Physical object Principle Property Qualia Quality Reality Relation Self Soul Subject Substantial form Thought Time Truth Type–token distinction Universal Unobservable Value more ... Metaphysicians Parmenides Plato Aristotle Lucretius Proclus Plotinus Avicenna Scotus Aquinas Suárez Descartes Spinoza Locke Malebranche Newton Leibniz Wolff Reid Berkeley Hume Kant Hegel Schopenhauer Bolzano Kierkegaard Lotze Peirce Nietzsche Meinong Bergson Whitehead Russell Moore Collingwood Wittgenstein Heidegger Carnap Ryle Sartre Quine Davidson Strawson Anscombe Deleuze Dummett Armstrong Putnam Plantinga Kripke Lewis Baudrillard Parfit more ... Notable works Sophist (c. 350 BC) Timaeus (c. 350 BC) Nyāya Sūtras (c. 200 BC) De rerum natura (c. 80 BC) Metaphysics (c. 50) Enneads (c. 270) Daneshnameh-ye Alai (c. 1000) Meditations on First Philosophy (1641) Ethics (1677) A Treatise Concerning the Principles of Human Knowledge (1710) Monadology (1714) Critique of Pure Reason (1781) Prolegomena to Any Future Metaphysics (1783) The Phenomenology of Spirit (1807) The World as Will and Representation (1818) Concluding Unscientific Postscript to Philosophical Fragments (1846) Being and Time (1927) Being and Nothingness (1943) Simulacra and Simulation (1981) Related topics Axiology Cosmology Epistemology Feminist metaphysics Interpretations of quantum mechanics Mereology Meta- Phenomenology Philosophy of mind Philosophy of psychology Philosophy of self Philosophy of space and time Teleology Category Philosophy portal

v t e SI base quantities
Base quantities	Quantity SI unit Name Symbol Dimension symbol Unit name Unit symbol time, duration t T second s length l, x, r, etc. L metre m mass m M kilogram kg electric current  I , i  I  ampere A thermodynamic temperature T Θ kelvin K amount of substance n N mole mol luminous intensity Iv J candela cd
See also	History of the metric system International System of Quantities 2019 revision Systems of measurement
Category Outline

• Physics
• Astronomy
• Stars
• Science

• Time
• Main topic articles
• Concepts in aesthetics
• Concepts in epistemology
• Concepts in metaphysics
• Concepts in the philosophy of mind
• Concepts in the philosophy of science
• Ontology
• Perception
• Physical phenomena
• Reality
• Scalar physical quantities
• SI base quantities
• Spacetime