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The horizontal direction indicates distance only one spatial dimension is taken into account , and the thick dashed curve is the spacetime trajectory " world line " of the observer. The small dots indicate specific past and future events in spacetime. The slope of the world line deviation from being vertical gives the relative velocity to the observer. Note how in both pictures the view of spacetime changes when the observer accelerates. In the Newtonian description these changes are such that time is absolute:  the movements of the observer do not influence whether an event occurs in the 'now' i.
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 Notice that with the change from a Newtonian to a relativistic description, the concept of absolute time is no longer applicable: events move up-and-down in the figure depending on the acceleration of the observer.
Yet for the most part the laws of physics do not specify an arrow of time , and allow any process to proceed both forward and in reverse. This is generally a consequence of time being modelled by a parameter in the system being analysed, where there is no "proper time": the direction of the arrow of time is sometimes arbitrary. Examples of this include the cosmological arrow of time, which points away from the Big Bang , CPT symmetry , and the radiative arrow of time, caused by light only travelling forwards in time see light cone.
In particle physics , the violation of CP symmetry implies that there should be a small counterbalancing time asymmetry to preserve CPT symmetry as stated above. The standard description of measurement in quantum mechanics is also time asymmetric see Measurement in quantum mechanics. The second law of thermodynamics states that entropy must increase over time see Entropy.
This can be in either direction — Brian Greene theorizes that, according to the equations, the change in entropy occurs symmetrically whether going forward or backward in time. So entropy tends to increase in either direction, and our current low-entropy universe is a statistical aberration, in the 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. 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.
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. 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 in time has never been verified, presents many theoretical problems, and may be an impossibility. 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 your grandfather was not killed before his child your parent was conceived. This view doesn't 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. 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. Clay , and later developed by William James.
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.
Mental chronometry is the use of response time in perceptual-motor tasks to infer the content, duration, and temporal sequencing of cognitive operations. 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.
Seven- to ten-year-olds can use clocks and calendars. In addition to psychoactive drugs, judgments of time can be altered by temporal illusions like the kappa effect ,  age,  and hypnosis. Psychologists assert that time seems to go faster with age, but the literature on this age-related perception of time remains controversial. 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.
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- Definition of Time – Exactly What Is Time?.
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. 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. 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. The use of a sequence of events occurs in fields as diverse as machines cam timer , documentaries Seconds From Disaster , law choice of law , 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. Although time is regarded as an abstract concept, there is increasing evidence that time is conceptualized in the mind in terms of space. Using space to think about time allows humans to mentally organize temporal events in a specific way.
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.
Verbs in time clauses and 'if' clauses
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 recent 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 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.
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. Leading scholarly organisations for researchers on the history and technology of time and timekeeping.
Physics of Time – Exactly What Is Time?
From Wikipedia, the free encyclopedia. For the concept in physics, see Time in physics.
For the magazine, see Time magazine. For other uses, see Time disambiguation. Major concepts.