Time is a part of the measuring system used to sequence events, to compare the durations of eventsand the intervals
between them, and to quantify rates of change such as
the motions of objects.[1] The temporal position of events with respect to
the transitory present is continually changing;
events happen, then are located further and further in the past. Time has been a major subject of religion,philosophy, and science, but defining it in a non-controversial manner
applicable to all fields of study has consistently eluded the greatest
scholars. A simple definition states that "time is what clocks
measure".
Time
is one of the seven fundamental physical quantities in the International System of Units. Time is used to define other quantities — such as velocity — so defining time in terms of such quantities
would result in circularity of definition.[2] An operational definition of time, wherein one says that observing a certain number of
repetitions of one or another standard cyclical event (such as the passage of a
free-swinging pendulum) constitutes one standard unit such as the second, is highly useful in the conduct of both advanced experiments and
everyday affairs of life. The operational definition leaves aside the question
whether there is something called time, apart from the counting activity just
mentioned, that flows and that can be measured. Investigations of a single
continuum called spacetime bring questions about space into questions about time, questions that have their roots in the
works of early students ofnatural philosophy.
Two
contrasting viewpoints on time divide many prominent philosophers. 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 is
sometimes referred to as Newtonian time.[3][4]Time travel, in this view, becomes a possibility as other
"times" persist like frames of a film strip, spread out across the
time line. 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[5] and Immanuel Kant,[6][7] holds that time is neither an event nor
a thing, and thus is not itself measurable nor can it be travelled.
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, thesecond, is defined in terms of radiation emitted by 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 life spans.
Ray
Cummings, an early writer of science fiction, wrote in 1922, "Time... is
what keeps everything from happening at once",[8] a sentence repeated by
scientists such as C. J. Overbeck,[9] and John Archibald Wheeler.[10][11]
Temporal measurement
Temporal
measurement, or chronometry,
takes two distinct period forms: the calendar, a mathematical abstraction for calculating
extensive periods of time,[12] and the clock, a physical 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. Increasingly, personal electronic
devices display both calendars and clocks simultaneously. The number (as on a
clock dial or calendar) that marks the occurrence of a specified event as to
hour or date is obtained by counting from a fiducial epoch — a central
reference point.
Artifacts
from the Palaeolithic suggest that the moon
was used to reckon time as early as 6,000 years ago.[13] Lunar calendars were among the first to appear, 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.
The
reforms of Julius Caesar in 45 BC put the Roman world on a solar calendar. This Julian calendar was faulty in that its intercalationstill allowed the astronomical solstices and equinoxes to advance against it by about 11 minutes per
year. Pope Gregory XIIIintroduced a correction in 1582; the Gregorian calendar was only slowly adopted by different nations over a period of
centuries, but is today by far the one in most common use around the world.
A
large variety of devices have been invented to
measure time. The study of these devices is called horology.
An Egyptian device dating 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.[14]
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 precise timekeeping devices of the ancient world were the water clock 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 upkeep to
replenish the flow of water. The Greeks and Chaldeans 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.[15] In the 11th century, Chinese inventors and engineers invented the first mechanical clocks to be driven by an escapementmechanism.
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).[16] 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.[17][18] Great advances in accurate time-keeping were
made by Galileo Galilei and especially Christiaan Huygenswith the invention of pendulum driven clocks.
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.
Chip-scale atomic clocks, such as this one unveiled in 2004, are
expected to greatly improve GPS location.[19]
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 determinelongitude by means of celestial navigation, a precision firstly achieved by John Harrison. More recently, the term has also been applied
to the chronometer watch, a wristwatch that meets precision standards set by the Swiss
agency COSC.
The 555 timer IC is an integrated circuit (chip)
used in a variety of timer, pulse generation and oscillator applications.
The
most accurate timekeeping devices areatomic clocks, which are accurate to seconds in many millions
of years,[20] 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.
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,[21] where it was defined as 1/564 of a momentum(1½ minutes),[22] 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 2006, the smallest unit of time that has been directly measured is on the attosecond (10−18 s) time scale, or around 1026Planck times.[23][24][25]
|
Unit
|
Size
|
Notes
|
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10−24 s
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||
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10−21 s
|
||
|
10−18 s
|
shortest time now measurable
|
|
|
10−15 s
|
pulse time of ultrafast lasers
|
|
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10−12 s
|
||
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10−9 s
|
time for molecules to fluoresce
|
|
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10−6 s
|
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|
0.001 s
|
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1 s
|
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60 seconds
|
||
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60 minutes
|
||
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24 hours
|
||
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7 days
|
Also called sennight
|
|
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14 days
|
2 weeks
|
|
|
27.2–29.5 days
|
Various definitions of lunar month exist.
|
|
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28–31 days
|
||
|
3 months
|
||
|
12 months
|
||
|
365 days
|
52 weeks + 1 day
|
|
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366 days
|
52 weeks + 2 days
|
|
|
average
|
||
|
average
|
||
|
4 year cycle
|
||
|
5 years
|
Also called pentad
|
|
|
10 years
|
||
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15 year cycle
|
||
|
17–35 years
|
approximate
|
|
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50 years
|
||
|
100 years
|
||
|
1,000 years
|
||
|
1018 s
|
roughly 32 billion years, more than twice
the age of the universe on current estimates |
|
|
varies
|
10 times the length of the previous
cosmological decade, with CÐ 1 beginning either 10 seconds or 10 years after the Big Bang, depending on the definition. |
|
The SI base unit for time is the SI second. From the second, larger units such as the minute, hour andday are defined, though they are "non-SI" units because they
do not 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.[28]
The second is the duration of 9,192,631,770
periods of the radiation corresponding to the transition between the two
hyperfine levels of the ground state of the caesium 133 atom.
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.[28] Previous to 1967, the second was defined as:
The
current definition of the second, coupled with the current definition of the
metre, is based on thespecial theory of relativity, which affirms our space-time to be a Minkowski space.
Time
keeping is so critical to the functioning of modern societies that it is
coordinated at an international level. The basis for scientific time is a
continuous count of seconds based on atomic clocks around the world, known as the International Atomic Time (TAI). Other scientific time standards include Terrestrial Time and Barycentric Dynamical Time.
Coordinated Universal Time (UTC) is the basis for modern civil time. Since January 1, 1972, it has been defined to
follow TAI with an exact offset of an integer number of seconds, changing only
when a leap second is added to keep clock time synchronized with
the rotation of the Earth. In TAI and UTC systems, the duration of a second is constant, as it is defined by the unchanging transition period
of the caesium atom.
Greenwich Mean Time (GMT) is an older standard, adopted starting with British railways
in 1847. Using telescopes instead of atomic clocks, GMT was calibrated to the mean solar time at the Royal Observatory, Greenwich in the UK. Universal Time (UT) is the modern term for the international telescope-based
system, adopted to replace "Greenwich Mean Time" in 1928 by the International Astronomical Union. Observations at the Greenwich Observatory
itself ceased in 1954, though the location is still used as the basis for the
coordinate system. Because the rotational period of Earth is not perfectly
constant, the duration of a second would vary if calibrated to a
telescope-based standard like GMT or UT—in which a second was defined as a
fraction of a day or year. The terms "GMT" and "Greenwich Mean
Time" are sometimes used informally to refer to UT or UTC.
The Global Positioning System also broadcasts a very precise time signal worldwide, along with
instructions for converting GPS time to UTC.
Earth
is split up into a number of time zones.
Most time zones are exactly one hour apart, and by convention compute their
local time as an offset from UTC or GMT. In many locations these offsets vary
twice yearly due to daylight saving time transitions.
The
following time conversions are accurate at the millisecond level. Some are
exact while others have differences at the microsecond level.
|
System
|
Description
|
UT1
|
UTC
|
TT
|
TAI
|
GPS
|
|
UT1
|
Mean Solar Time
|
UT1
|
UTC = UT1 - DUT1
|
TT = UT1 + 32.184 s + LS - DUT1
|
TAI = UT1 - DUT1 + LS
|
GPS = UT1 - DUT1 + LS - 19 s
|
|
UTC
|
Civil Time
|
UT1 = UTC + DUT1
|
UTC
|
TT = UTC + 32.184 s + LS
|
TAI = UTC + LS
|
GPS = UTC + LS - 19 s
|
|
TT
|
Terrestrial (Ephemeris) Time
|
UT1 = TT - 32.184 s - LS + DUT1
|
UTC = TT - 32.184 s - LS
|
TT
|
TAI = TT - 32.184 s
|
GPS = TT - 51.184 s
|
|
TAI
|
Atomic Time
|
UT1 = TAI + DUT1 - LS
|
UTC = TAI - LS
|
TT = TAI + 32.184 s
|
TAI
|
GPS = TAI - 19 s
|
|
GPS
|
GPS Time
|
UT1 = GPS + DUT1 - LS + 19 s
|
UTC = GPS - LS + 19 s
|
TT = GPS + 51.184 s
|
TAI = GPS + 19 s
|
GPS
|
Definitions:
2.
DUT1 = UT1 - UTC from http://maia.usno.navy.mil/ser7/ser7.dat or http://maia.usno.navy.mil/search/search.html
Sidereal time is the measurement of time relative to a distant
star (instead of solar time that is relative to the sun). It is used in
astronomy to predict when a star will be overhead. Due to the orbit of the
earth around the sun a sidereal day is 4 minutes (1/366th) less than a solar
day.
Another
form of time measurement consists of studying the past. Events in the past can be ordered in a sequence (creating achronology), and can be put into chronological groups (periodization). One of the most important systems of periodization
is geologic time, which is a system of periodizing the events
that shaped the Earth and its life. Chronology, periodization, and
interpretation of the past are together known as the study of history.
Ancient
cultures such as Incan, Mayan, Hopi, and other Native American Tribes, plus the Babylonians,Ancient Greeks, Hinduism, Buddhism, Jainism, and others have a
concept of a wheel of time,
that regards time as cyclical and quantic consisting of repeating ages that happen to
every being of the Universe between birth and extinction.
In
general, the Judaeo-Christian concept, based on the Bible, is that time is linear, beginning with the act ofcreation by God. The general Christian view is that time will
end with the end of the world. Others suggest[who?] that time is like a ray, having a beginning but going on forever
into the future.
In
the Old Testament book Ecclesiastes, traditionally ascribed to Solomon (970–928 BC), time (as the Hebrew word עדן, זמן `iddan(time) zĕman(season) is often translated) was traditionally regarded
as a medium for the passage of predestined events. (Another word, زمان" זמן" zman, was current as meaning time fit for an event, and is used as the modern Arabic and Hebrew equivalent to the English word
"time".)
There is an appointed time (zman) for
everything. And there is a time (’êth) for every event under heaven–
A time (’êth) to give birth, and a time to die; A time to plant, and a time to uproot what is planted.
A time to kill, and a time to heal; A time to tear down, and a time to build up.
A time to weep, and a time to laugh; A time to mourn, and a time to dance.
A time to throw stones, and a time to gather stones; A time to embrace, and a time to shun embracing.
A time to search, and a time to give up as lost; A time to keep, and a time to throw away.
A time to tear apart, and a time to sew together; A time to be silent, and a time to speak.
A time to love, and a time to hate; A time for war, and a time for peace. – Ecclesiastes 3:1–8
A time (’êth) to give birth, and a time to die; A time to plant, and a time to uproot what is planted.
A time to kill, and a time to heal; A time to tear down, and a time to build up.
A time to weep, and a time to laugh; A time to mourn, and a time to dance.
A time to throw stones, and a time to gather stones; A time to embrace, and a time to shun embracing.
A time to search, and a time to give up as lost; A time to keep, and a time to throw away.
A time to tear apart, and a time to sew together; A time to be silent, and a time to speak.
A time to love, and a time to hate; A time for war, and a time for peace. – Ecclesiastes 3:1–8
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, synchronize, and chronicle.
Two
distinct viewpoints on time divide many prominent philosophers. One view is
that time is part of the fundamental structure of theuniverse, a dimension in which events occur in sequence. Sir Isaac Newton subscribed to this realist view, and hence it is
sometimes referred to as Newtonian time.[4] An opposing view is that time does not refer to any kind of actually existing dimension that
events and objects "move through", nor to any entity that
"flows", but that it is instead an intellectual concept (together
with space andnumber) that enables humans to sequence and compare
events.[30] This second view, in the tradition of Gottfried Leibniz[5] andImmanuel Kant,[6][7] holds that space and time "do not exist in
and of themselves, but ... are the product of the way we represent
things", because we can know objects only as they appear to us.
The Vedas, the earliest texts on Indian philosophy and Hindu philosophy dating back to the late 2nd millennium BC, describe ancientHindu cosmology, in which the universe goes through repeated cycles of creation,
destruction and rebirth, with each cycle lasting 4320 million years.[31] Ancient Greek philosophers, including Parmenides and Heraclitus, wrote essays on the nature of time.[32]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 the
number of change with respect to 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,[33] 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.
In
contrast to ancient Greek philosophers who believed that the universe had an
infinite past with no beginning, medieval philosophersand theologians developed the concept of
the universe having a finite past with a beginning. This view is shared by
Abrahamic faiths as they believe time started by creation, therefore the only
thing being infinite is God and everything else, including time, is finite.
Isaac Newton believed in absolute space and absolute time; Leibniz believed that time and space are relational.[34] The differences between Leibniz's and Newton's
interpretations came to a head in the famous Leibniz-Clarke Correspondence.
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 prioriintuition that allows us (together with the other a priori intuition, space) to comprehend sense experience.[35] 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 abstractconceptual
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 toquantify the extent of and distances between objects, and temporal measurements
are used to quantify the durations of and between events. (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.[36]
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.[37] 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.[38]
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.[39] Time as an illusion is also a common theme in Buddhistthought.[40][41]
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 around what it means for something to be
"unreal". Modern physicists generally consider time to be as
"real" as space, though others such as Julian Barbour in his book The End of Time, argue that quantum equations of the universe
take their true form when expressed in the timeless configuration spacerealm containing every possible "Now" or momentary configuration
of the universe, which he terms 'platonia'.[42] (See also: Eternalism (philosophy of time).)
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Classical mechanics
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Space · Time · Velocity · Speed ·Mass · Acceleration · Gravity · Force ·Impulse · Torque / Moment / Couple ·Momentum · Angular momentum ·Inertia · Moment of inertia ·Reference frame · Energy ·Kinetic energy · Potential energy ·Mechanical work · Virtual work ·D'Alembert's principle
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From
the age of Newton to 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.[43] Non-relativistic classical mechanics is based on this Newtonian idea of time.
Einstein,
in his special theory of relativity,[44] 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, space
and time can be measured in the same units,[vague] with the conversion
fixed by the speed of light.
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 behaviour 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 observers in
motion relative to one another will measure different elapsed times for the
same event.
Two-dimensional space depicted in
three-dimensional spacetime.
The past and future light cones are absolute, the
"present" is a relative concept different for observers in relative
motion.
Time
has historically been closely related with space, the two together comprisingspacetime 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. The past is the set of events that can send light signals
to the observer; the future is the set of events to which the observer can
send light signals.
Relativity of simultaneity: Event B is simultaneous with A in the green reference frame, but
it occurred before in the blue frame, and will occur later in the red frame.
Einstein
showed in his thought experiments that people travelling at different speeds,
while 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 typically minute in the human experience, the
effect becomes much more 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 seem to shorten. Even in Newtonian terms time
may be considered the fourth dimension of motion[citation needed]; but Einstein showed how both temporal and spatial dimensions can
be altered (or "warped") by high-speed motion.
Einstein
(The Meaning of Relativity): "Two events taking place at the points A and B of a system K
are simultaneous if they appear at the same instant when observed from the
middle point, M, of the interval AB. Time is then defined as the ensemble of
the indications of similar clocks, at rest relatively to K, which register the
same simultaneously."
Einstein
wrote in his book, Relativity, that simultaneity is also relative, i.e., two events that appear simultaneous to an observer in a
particular inertial reference frame need not be judged as simultaneous by a
second observer in a different inertial frame of reference.
Views of spacetime along the world lineof a rapidly accelerating observer in a
relativistic universe. The events ("dots") that pass the two diagonal
lines in the bottom half of the image (the past light cone of the observer in the origin) are the events
visible to the observer.
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 transformationsapplicable 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 thespacetime 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.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 ofabsolute
time is no longer applicable:
events move up-and-down in the figure depending on the acceleration of the
observer.
Time
appears to have a direction – the past lies behind, fixed and immutable, while
the future lies ahead and is not necessarily fixed. Yet for the most part the
laws of physics do not specify an arrow of time, and allow any process to proceed both forward
and in reverse. This is generally a consequence of time being modeled by a
parameter in the system being analyzed, where there is no "proper
time": the direction of the arrow of time is sometimes arbitrary. Examples
of this 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, CPT symmetry, and the radiative arrow of time, caused by light only traveling 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 (seeMeasurement in quantum mechanics).
Time
quantization is a hypothetical concept. In the modern established physical
theories (the Standard Model of Particles and Interactions and General Relativity) time is not quantized.
Planck time (~ 5.4 × 10−44 seconds) is the unit of time in the system of natural units known as Planck units. Current established physical theories are
believed to fail at this time scale, and many physicists expect that the Planck
time might be the smallest unit of time that could ever be measured, even in
principle. Tentative physical theories that describe this time scale exist; see
for instanceloop quantum gravity.
Stephen Hawking in particular has addressed a connection between
time and the Big Bang.
In A Brief History of Time and elsewhere, Hawking says that even if time did not begin with
the Big Bang and there were another time frame before the Big Bang, no
information from events then would be accessible to us, and nothing that
happened then would have any effect upon the present time-frame.[45]Upon occasion, Hawking has stated that time
actually began with the Big Bang, and that questions about what happened before the Big Bang are meaningless.[46][47][48] This less-nuanced, but commonly repeated
formulation has received criticisms from philosophers such as Aristotelian philosopher Mortimer J. Adler.[49][50]
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 are likely to remain
pure speculation.
A graphical representation of the expansion of the universe with the inflationary epoch represented as the
dramatic expansion of the metric seen on the left.
While
the Big Bang model is well established in cosmology, it is 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.[51]
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.[52]
§ brane cosmology models[53] in which inflation is due to the movement of
branes in string theory;
the pre-big bang model; theekpyrotic 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.[54][55][56]
§ 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.[57]
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 is the concept of moving backwards and/or forwards to different points
in time, in a manner analogous to moving throughspace, 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. Traveling backwards in
time has never been verified, presents many theoretic problems, and may be an
impossibility.[58] 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 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 traveler would already have happened in his or her
past, resulting in the reality that the traveler 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.R. Clay, and later developed by William James.[59]
The
brain's judgement 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 to be capable of shorter-range (ultradian) timekeeping.
Psychoactive
drugs can impair the judgement of time. Stimulants can lead both humans and rats to overestimate
time intervals,[60][61]while depressants can have the opposite effect.[62] The level of activity in the brain of neurotransmitters such as dopamine andnorepinephrine may be the reason for this.[63]
Mental chronometry is the use of response time in perceptual-motor tasks to infer the
content, duration, and temporal sequencing of cognitive operations.
In
addition to psychoactive drugs, judgements of time can be altered by temporal illusions (like the kappa effect[64] ), age,[65] andhypnosis.[66] 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.[67] As an example, one day to an eleven-year-old
person would be approximately 1/4,000 of their life, while one day to a
55-year-old would be approximately 1/20,000 of their life. According to such an
interpretation, a day would appear much longer to a young child than to an
adult, even though the measure of time is the same.[original research?]
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 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.
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.
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.[68][69][70] Because of causality, cause precedeseffect, 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,[71] historical events (chronology), directions and steps in procedures,[72] 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 need to 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). The use
of a sequence of events occurs in fields as diverse asmachines (cam timer), documentaries (Seconds From Disaster), law (choice of law), computer simulation (discrete event simulation), and electric power transmission[73] (sequence of events recorder). A specific example of a sequence of events is the timeline of the
Fukushima Daiichi nuclear disaster.
