Everything about Time totally explained
Time is a basic component of the measuring system used to sequence events, to compare the durations of events and the intervals between them, and to quantify the motions of objects. Time has been a major subject of
religion,
philosophy, and
science, but defining time in a non-controversial manner applicable to all fields of study has consistently eluded the greatest scholars.
In
physics and other sciences,
time is considered one of the few
fundamental quantities.
Time is used to define other quantities – such as
velocity – and defining
time in terms of such quantities would result in
circularity of definition. An
operational definition of time, wherein one says that observing a certain number of repetitions of one or another standard cyclical event (such as the passage of a free-swinging pendulum) constitutes one standard unit such as the
second, has a high utility value in the conduct of both advanced experiments and everyday affairs of life. The operational definition leaves aside the question whether there's something called time, apart from the counting activity just mentioned, that flows and that can be measured. Investigations of a single continuum called
space-time brings the nature of time into association with related questions into the nature of
space, questions that have their roots in the works of early students of
natural philosophy.
Among prominent philosophers, there are two distinct viewpoints on
time.
One view is that time is part of the fundamental structure of the
universe, a
dimension in which events occur in
sequence.
Sir Isaac Newton subscribed to this
realist view, and hence it's sometimes referred to as
Newtonian time. The opposing view is that
time doesn't refer to any kind of "container" that events and objects "move through", nor to any entity that "flows", but that it's 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 isn't itself measurable.
Temporal measurement has occupied scientists and
technologists, and was a prime motivation in
navigation and
astronomy.
Periodic events and periodic motion have long served as standards for units of time. Examples include the apparent motion of the sun across the sky, the phases of the moon, the swing of a pendulum, and the beat of a heart. Currently, the international unit of time, the
second, is defined as a certain number of
hyperfine transitions in
caesium atoms (see below).
Time is also of significant social importance, having economic value ("
time is money") as well as personal value, due to an
awareness of the limited time in each day and in
human lifespans.
Temporal measurement
Temporal measurement, or
chronometry, takes two distinct period forms. The
calendar, a mathematical abstraction for calculating extensive periods of time, and the
clock, a concrete mechanism that counts the ongoing passage of time. In day-to-day life, the clock is consulted for periods less than a day, the calendar, for periods longer than a day.
History of the calendar
Artifacts from the
Palaeolithic suggest that the moon was used to calculate time as early as 12,000, and possibly even 30,000
BP.
A
sundial uses a
gnomon to cast a shadow on a set of markings which were calibrated to the
hour. The position of the shadow marked the hour in
local time.
The most accurate timekeeping devices of the ancient world were the
waterclock or
clepsydra, one of which was found in the tomb of Egyptian pharaoh
Amenhotep I (1525–1504 BC). They could be used to measure the hours even at night, but required manual timekeeping to replenish the flow of water. The
Greeks and
Chaldeans regularly maintained timekeeping records as an essential part of their astronomical observations.
Arab engineers in particular made improvements on the use of waterclocks up to the Middle Ages.
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. Waterclocks, and later, mechanical clocks, were used to mark the events of the abbeys and monasteries of the Middle Ages.
Richard of Wallingford (1292–1336), abbot of St. Alban's abbey, famously built a
mechanical clock as an astronomical
orrery about 1330.
The English word
clock probably comes from the Middle Dutch word "klocke" which is in turn derived from the mediaeval Latin word "clocca", which is ultimately derived from Celtic, and is cognate with French, Latin, and German words that mean
bell. The passage of the hours at sea were marked by bells, and denoted the time (see
ship's bells). The hours were marked by bells in the abbeys as well as at sea.
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.
A
chronometer is a portable timekeeper that meets certain precision standards. Initially, the term was used to refer to the
marine chronometer, a timepiece used to determine
longitude by means of
celestial navigation. More recently, the term has also been applied to the
chronometer watch, a
wristwatch that meets precision standards set by the Swiss agency
COSC.
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 spin property of atoms as their basis, and 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 that radiation which corresponds to the transition between two electron spin energy levels of the ground state of the
133Cs atom.
Today, the
Global Positioning System in coordination with the
Network Time Protocol can be used to synchronize timekeeping systems across the globe.
Definitions and standards
The
SI base unit for time is the
SI second. From the second, larger units such as the
minute,
hour and
day are defined, though they're "non-SI" units because they don't use the decimal system, and also because of the occasional need for a
leap-second. They are, however, officially accepted for use
with the International System. There are no fixed ratios between seconds and
months or
years as months and years have significant variations in length.
The official SI definition of the second is as follows:
At its 1997 meeting, the CIPM affirmed that this definition refers to a caesium atom in its ground state at a temperature of 0 K.
In Book 11 of
St. Augustine's Confessions, he ruminates on the nature of time, asking, "What then is time? If no one asks me, I know: if I wish to explain it to one that asketh, I know not." He settles on time being defined more by what it isn't than what it is.
Isaac Newton believed time and
space form a container for events, which is as real as the
objects it contains.
Absolute, true, and mathematical time, in and of itself and of its own nature, without reference to anything external, flows uniformly and by another name is called duration. Relative, apparent, and common time is any sensible and external measure (precise or imprecise) of duration by means of motion; such a measure – for example, an hour, a day, a month, a year – is commonly used instead of true time. |
In contrast to Newton's belief in absolute space, and a precursor to Kantian 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. Leibniz thought of time as a fundamental part of an
abstract conceptual framework, together with
space and
number, within which we sequence events,
quantify their duration, and compare the motions of objects. In this view,
time doesn't refer to any kind of entity that "flows," that objects "move through," or that's a "container" for events.
Immanuel Kant, in the
Critique of Pure Reason, described time as an
a priori intuition that allows us (together with the other
a priori intuition,
space) to comprehend sense experience. With Kant, neither space nor time are conceived as
substances, but rather both are elements of a systematic mental
framework that necessarily structures the experiences of any rational agent, or observing subject. Spatial
measurements are used to
quantify how far apart
objects are, and temporal measurements are used to quantify how far apart
events occur.
In
Existentialism, time is considered fundamental to the question of
being, in particular by the philosopher
Martin Heidegger. See
Ontology.
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.
Time as "unreal"
In 5th century BC
Greece,
Antiphon the
Sophist, in a fragment preserved from his chief work
On Truth held that:
"Time isn't 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 illusion is also a common theme in
Buddhist thought, and some modern philosophers have carried on with this theme.
J. M. E. McTaggart's 1908
The Unreality of Time, for example, argues that time is unreal (see also
The flow of time).
However, these arguments often center around what it means for something to be "real". Modern physicists generally consider time to be as "real" as space, though others such as
Julian Barbour in his
The End of Time argue that quantum equations of the universe take their true form when expressed in the timeless
configuration spacerealm containing every possible "Now" or momentary configuration of the universe, which he terms 'platonia'. (See also:
Eternalism (philosophy of time).)
Time in the physical sciences
From the age of
Newton up until
Einstein's profound reinterpretation of the physical concepts associated with time and space, time was considered to be "absolute" and to flow "equably" (to use the words of Newton) for all observers. The science of 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.
Einstein showed that if time and space is measured using electromagnetic phenomena (like light bouncing between mirrors) then due to the constancy of the speed of light, time and space become mathematically entangled together in a certain way (called
Minkowski space) which in turn results in
Lorentz transformation and in entanglement of all other important derivative physical quantities (like energy, momentum, mass, force, etc) in a certain 4-vectorial way (see
special relativity for more details).
Time in classical mechanics
In
classical mechanics Newton's concept of "relative, apparent, and common time" can be used in the formulation of a prescription for the synchronization of clocks. Events seen by two different observers in motion relative to each other produce a mathematical concept of time that works pretty well for describing the everyday phenomena of most people's experience.
Time in modern physics
In the late nineteenth century, physicists encountered problems with the classical understanding of time, in connection with the behavior of electricity and magnetism. Einstein resolved these problems by invoking a method of synchronizing clocks using the constant, finite speed of light as the maximum signal velocity. This led directly to the result that time appears to elapse at different rates relative to different observers in motion relative to one another.
Spacetime
physics views the curvature of
spacetime around an object as much a feature of that object as are its
mass and
volume.
Time has historically been closely related with
space, the two together comprising
spacetime in
Einstein's special relativity and
general relativity. According to these theories, the concept of time depends on the
spatial reference frame of the observer, and the human perception as well as the measurement by instruments such as clocks are different for observers in relative motion. Even the temporal order of events can change, but the past and future are defined by the backward and forward
light cones, which never change. The
past is the set of events that can send light signals to the observer, the
future the events to which the observer can send light signals. All else is non-observable and within that set of events the very time-order differs for different observers.
Time dilation
"Time is nature's way of keeping everything from happening at once". This quote, attributed variously to
Einstein,
John Archibald Wheeler, and
Woody Allen, says that time is what separates
cause and effect. Einstein showed that people traveling at different speeds, whilst agreeing on cause and effect, will measure different time separations between events and can even observe different chronological orderings between non-causally related events. Though these effects are minute unless one is traveling at a speed close to that of light, the effect becomes pronounced for objects moving at speeds approaching the speed of light. Many
subatomic particles exist for only a fixed fraction of a second in a lab relatively at rest, but some that travel close to the speed of light can be measured to travel further and survive much longer than expected (a
muon is one example). According to the
special theory of relativity, in the high-speed particle's
frame of reference, it exists, on the average, for a standard amount of time known as its
mean lifetime, and the distance it travels in that time is zero, because its velocity is zero. Relative to a frame of reference at rest, time seems to "slow down" for the particle. Relative to the high-speed particle, distances seems to shorten. Even in Newtonian terms time may be considered the fourth dimension of motion; but Einstein showed how both temporal and spatial dimensions can be altered (or "warped") by high-speed motion.
Einstein (
The Meaning of Relativity): "Two
events taking place at the points A and B of a system K are simultaneous if they appear at the same instant when observed from the middle point, M, of the interval AB. Time is then defined as the ensemble of the indications of similar clocks, at rest relatively to K, which register the same simultaneously."
Einstein wrote in his book,
Relativity, that simultaneity is also relative, for example, 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 time versus Newtonian time
The animations on the left and the right visualise the different treatments of time in the Newtonian and the relativistic descriptions. At heart of these differences are the
Galilean and
Lorentz transformations applicable in the Newtonian and relativistic theories, respectively.
In both figures, the vertical direction indicates time. The horizontal direction indicates distance (only one spatial dimension is taken into account), and the thick dashed curve is the
spacetime trajectory ("
world line") of the observer. The small dots indicate specific (past and future) events in spacetime.
The slope of the world line (deviation from being vertical) gives the relative velocity to the observer. Note how in both pictures the view of spacetime changes when the observer accelerates.
In the Newtonian description these changes are such that
time is absolute: the movements of the observer don't influence whether an event occurs in the 'now' (for example 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 influences whether an event passes the light cone of the observer. Notice that with the change from a Newtonian to a relativistic description, the concept of
absolute time is no longer applicable: events move up-and-down in the figure depending on the acceleration of the observer.
Arrow of time
Time appears to have a direction – the past lies behind, fixed and incommutable, while the future lies ahead and isn't necessarily fixed. Yet the majority of the laws of physics don't provide this
arrow of time. The exceptions include the
Second law of thermodynamics, which states that
entropy must increase over time (see
Entropy); the
cosmological arrow of time, which points away from the
Big Bang, and the radiative arrow of time, caused by
light only traveling forwards in time. In
particle physics, there's also the weak arrow of time, from
CPT symmetry, and also
measurement in
quantum mechanics (see
Measurement in quantum mechanics).
Quantised time
Time quantization is a hypothetical concept. In the modern established physical theories (the
Standard Model of Particles and Interactions and
General Relativity) time isn't quantized.
Planck time (~
5.4 × 10−44 seconds) is the unit of time in the system of
natural units known as
Planck units. Current established physical theories are believed to fail at this time scale, and many physicists expect that the Planck time might be the smallest unit of time that could ever be measured, even in principle. Tentative physical theories that describe this time scale exist; see for instance
loop quantum gravity.
Time and the Big Bang
Stephen Hawking in particular has addressed a connection between time and the
Big Bang. He has sometimes stated that we may as well assume that time began with the Big Bang because trying to answer any question about what happened
before the Big Bang is trying to answer a question that's meaningless
as those events would have been part of a different time frame and different universe outside of the scope of the Big Bang theory.
Aristotelian philosopher
Mortimer J. Adler, has criticized some expositions that Hawking has given stating that time didn't exist before the big bang.
Hawking, in
A Brief History of Time and elsewhere, along with several other modern physicists, has stated his position more clearly and less controversially: that even if time didn't begin with the Big Bang and there were another time frame before the Big Bang, no information from events then would be accessible to us, and nothing that happened then would have any effect upon the present time-frame.
Scientists have come to some agreement on descriptions of events that happened 10
−35 seconds after the Big Bang, but generally agree that descriptions about what happened before one
Planck time (5 × 10
−44 seconds) after the Big Bang will likely remain pure speculation.
Speculative physics beyond the Big Bang
While the Big Bang model is well established in cosmology, it's likely to be refined in the future. Little is known about the earliest moments of the universe's history. The
Penrose-Hawking singularity theorems require the existence of a singularity at the beginning of cosmic time. However, these theorems assume that
general relativity is correct, but general relativity must break down before the universe reaches the
Planck temperature, and a correct treatment of
quantum gravity may avoid the singularity.
There may also be parts of the universe well beyond what can be observed in principle. If inflation occurred this is likely, for exponential expansion would push large regions of space beyond our observable horizon.
Some proposals, each of which entails untested hypotheses, are:
- models including the Hartle-Hawking boundary condition in which the whole of space-time is finite; the Big Bang does represent the limit of time, but without the need for a singularity.
- brane cosmology models in which inflation is due to the movement of branes in string theory; the pre-big bang model; the ekpyrotic model, in which the Big Bang is the result of a collision between branes; and the cyclic model, a variant of the ekpyrotic model in which collisions occur periodically.
- chaotic inflation, in which inflation events start here and there in a random quantum-gravity foam, each leading to a bubble universe expanding from its own big bang.
Proposals in the last two categories see the Big Bang as an event in a much larger and older universe, or
multiverse, and not the literal beginning.
Time travel
space and different than the "normal" flow of time to an earthbound observer. Although time travel has been a
plot device in
fiction since the 19th century, and one-way travel into the future is arguably possible given the phenomenon of
time dilation in the
theory of relativity, it's currently unknown whether the
laws of physics would allow time travel to the past. Any technological device, whether fictional or hypothetical, that's 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
temporal paradox. Some interpretations of time travel resolve this by accepting the possibility of travel between
parallel realities or
universes.
Perception of time
Time in psychology
Even in the presence of timepieces, different individuals may judge an identical length of time to be passing at different rates. Commonly, this is referred to as time seeming to "fly" (a period of time seeming to pass faster than possible) or time seeming to "drag" (a period of time seeming to pass slower than possible). The psychologist
Jean Piaget called this form of time perception "lived time."
This common experience was used to familiarize the general public to the ideas presented by
Einstein's theory of relativity in a 1930 cartoon by
Sidney "George" Strube:
Man: Well, it's like this,—supposing I were to sit next to a pretty girl for half an hour it would seem like half a minute,—
Einstein: Braffo! You the idea haf! [sic]
Man: But if I were to sit on a hot stove for two seconds then it would seem like two hours. |
A form of temporal illusion verifiable by experiment is the
kappa effect, whereby time intervals between visual events are perceived as relatively longer or shorter depending on the relative spatial positions of the events. In other words: the perception of temporal intervals appears to be directly affected, in these cases, by the perception of spatial intervals.
Time also appears to pass more quickly as one gets older.
Stephen Hawking suggests that the perception of time is a ratio:
Unit of Time : Time Lived. For example, one hour to a six-month-old person would be approximately "1:4032", while one hour to a 40-year-old would be "1:349,440". Therefore an hour appears much longer to a young child than to an aged adult, even though the measure of time is equal.
Time in altered states of consciousness
Altered states of consciousness are sometimes characterized by a different estimation of time. Some psychoactive substances – such as
entheogens – may also dramatically alter a person's temporal judgement. When viewed under the influence of such substances as
LSD,
psychedelic mushrooms and
peyote, a clock may appear to be a strange reference point and a useless tool for measuring the passage of events as it doesn't correlate with the user's experience. At higher doses, time may appear to slow down, stop, speed up, go backwards and even seem out of sequence. A typical thought might be "I can't believe it's only 8 o'clock, but then again, what does 8 o'clock mean?" As the boundaries for experiencing time are removed, so is its relevance. Many users claim this unbounded timelessness feels like a glimpse into spiritual infinity. To imagine that one exists somewhere "outside" of time is one of the hallmark experiences of a psychedelic voyage.
Marijuana, a milder psychedelic, may also distort the perception of time to a lesser degree.
The practice of
meditation, central to all Buddhist traditions, takes as its goal the reflection of the mind back upon itself, thus altering the subjective experience of time; the so called, 'entering the now', or 'the moment'.
Culture
Culture is another variable contributing to the perception of time. Anthropologist
Benjamin Lee Whorf reported after studying the
Hopi cultures that: "… the Hopi language is seen to contain no words, grammatical forms, construction or expressions or that refer directly to what we call “time”, or to past, present, or future…" Whorf's assertion has been challenged and modified. Pinker debunks Whorf's claims about time in the Hopi language, pointing out that the anthropologist Malotki (1983) has found that the Hopi do have a concept of time very similar to that of other cultures; they've units of time, and a sophisticated calendar.
Use of time
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.
The use of time is an important issue in understanding
human behaviour,
education, and
travel behaviour.
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 of time. This has led to the disputed
time budget hypothesis.
Time management is the organization of tasks or events by first estimating how much time a task will take to be completed, when it must be completed, and then adjusting events that would interfere with its completion so that completion is reached in the appropriate amount of time. Calendars and day planners are common examples of time management tools.
Arlie Russell Hochschild and
Norbert Elias have written on the use of time from a sociological perspective.
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