When the U.S. football field–size, cigar-shaped object ‘Oumuamua
entered our solar system last year, it didn’t just give us our first
glimpse of an interstellar piece of rock. It also bolstered the
plausibility of space rocks spreading life among the stars by ferrying
microbes between distant star systems, according to a new study. “Life
could potentially be exchanged over thousands of light-years,” says
author Idan Ginsburg, a postdoc at the Harvard-Smithsonian Center for
Astrophysics in Cambridge, Massachusetts.
The idea, known as panspermia, has been around for centuries. Some
astronomers have even speculated that life on Earth was seeded by
microbes that hitched a ride on debris ejected from another
life-harboring world in the solar system, perhaps on meteorites from
Mars. But it seemed improbable that life could have come from
interstellar space.
Take computer simulations in 2003 by planetary scientist H. Jay
Melosh, now at Purdue University in Lafayette, Indiana. The analysis
revealed that about a third of the meteorites shot off Earth were eventually thrown out of the solar system by Jupiter or Saturn,
but that the process took millions or tens of millions of years—a long
stretch for even the toughest bugs or spores to be exposed to the vacuum
and radiation of space. And vanishingly few rocks would ever be
captured by some distant system, Melosh found.
The outlook improves if the receiving system is a binary star, which has a more complex gravitational field than the solar system.
Yet any system that’s good at capturing is also good at ejecting,
meaning refugees from another system are far more likely to be tossed
back out in a game of interstellar hot potato than to settle on a
hospitable world.
‘Oumuamua is providing fresh hope for the idea of galactic
panspermia. For the telescopic survey that found it, the Panoramic
Survey Telescope and Rapid Response System, to have detected such an
object in the region it had scanned, our Galaxy needs to have 1 trillion of them per cubic light-year,
according to study published earlier this year. To fill space like
this, every star in the Milky Way would have to eject 10 quadrillion
such objects, and a few should be passing through our solar system at
any given moment.
In the new study, Ginsburg, along with astrophysicists Manasvi Lingam
and Abraham Loeb, also of the Harvard-Smithsonian Center for
Astrophysics, calculated the chances of such objects delivering life to
an alien world. A binary star such as Alpha Centauri would ensnare a few
thousand rocks of ‘Oumuamua’s size every year, and our solar system
might snag one a century, the team estimates in a preprint posted last week on arXiv and in a forthcoming paper in The Astronomical Journal.
The researchers then multiplied this capture rate by the number of
stars an interstellar object will encounter before whatever bugs it
carries all die. If the objects move, like ‘Oumuamua, at a velocity of
26 kilometers per second through interstellar space, 10 million of them
will be captured somewhere in the Milky Way in a million years. “If you
look at the galaxy as a whole, you expect this to happen fairly often,”
Ginsburg says.
Astronomer Jason Wright of Pennsylvania State University in State
College says the analysis has merit: “For reasonable numbers, this
suggests that planets and asteroids are commonly exchanged between
stars.”
But astronomer Ed Turner of Princeton University says the authors may
be reading too much into the single example of ‘Oumuamua. “There’s no
rigorous mathematical argument you can write about one event evaluated a
posteriori,” he says.
And even if our galaxy is thick with ‘Oumuamuas, they are unlikely
vectors of panspermia, Melosh says. ‘Oumuamua is way too big to have
been ejected from an inhabited planet, he says.
Still, Loeb says more data could settle where galactic panspermia is
plausible. Additional discoveries of interstellar interlopers would
clarify their prevalence, and Loeb says the detection of life on other
worlds would show whether it tends to cluster,
as it would if it arose in one place and spread elsewhere by
panspermia. If so, he says, our entire galaxy might be considered
biologically interconnected, its vast distances offset by vast spans of
time and the vast number of objects that set out to cross the void.
*Correction, 16 October, 10:55 a.m.: This story has been updated to note that the new study will appear in The Astronomical Journal. A link to the study has been added.
The flow of sand in an hourglass can be used to measure the passage of time. It also concretely represents the present as being between the past and the future.
Time has long been an important subject of study in religion,
philosophy, and science, but defining it in a manner applicable to all
fields without circularity has consistently eluded scholars.
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.
Time in physics is unambiguously operationally defined as "what a clock reads". Time is one of the seven fundamental physical quantities in both the International System of Units and International System of Quantities. Time is used to define other quantities – such as velocity – so 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, 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 of natural philosophy.
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 by measuring the electronic transitionfrequency of 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.
Temporal measurement and history
Generally speaking, methods of temporal measurement, or chronometry, take two distinct forms: the calendar, a mathematical tool for organising intervals of time,
and the clock,
a physical mechanism that counts the passage of time. In day-to-day
life, the clock is consulted for periods less than a day whereas the
calendar is consulted for periods longer than a day. Increasingly,
personal electronic devices display both calendars and clocks
simultaneously. The number (as on a clock dial or calendar) that marks
the occurrence of a specified event as to hour or date is obtained by
counting from a fiducial epoch – a central reference point.
History of the calendar
Artifacts from the Paleolithic suggest that the moon was used to reckon time as early as 6,000 years ago.
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. 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 the most commonly used calendar around the
world, by far.
During the French Revolution,
a new clock and calendar were invented in attempt to de-Christianize
time and create a more rational system in order to replace the Gregorian
calendar. The French Republican Calendar's days consisted of ten hours of a hundred minutes of a hundred seconds, which marked a deviation from the 12-based duodecimal system used in many other devices by many cultures. The system was later abolished in 1806.
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 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. 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.
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 were marked by bells, and denoted the time (see ship's bell). The hours were marked by bells in abbeys as well as at sea.
Chip-scale atomic clocks, such as this one unveiled in 2004, are expected to greatly improve GPS location.
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 firstly achieved by John Harrison. More recently, the term has also been applied to the chronometer watch, a watch that meets precision standards set by the Swiss agency COSC.
The most accurate timekeeping devices are atomic clocks, which are accurate to seconds in many millions of years, 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 most common 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 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, where it was defined as 1/564 of a momentum (1½ 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 × 10−17 seconds), about 3.7 × 1026Planck times.
Units of time
The second (s) is the SI base unit. A minute (min) is 60 seconds in length, and an hour is 60 minutes in length. A day is 24 hours or 86,400 seconds in length.
Definitions and standards
The Mean Solar Time system defines the second as 1/86,400 of the mean solar day,
which is the year-average of the solar day. The solar day is the time
interval between two successive solar noons, i.e., the time interval
between two successive passages of the Sun across the local meridian.
The local meridian is an imaginary line that runs from celestial north
pole to celestial south pole passing directly over the head of the
observer. At the local meridian the Sun reaches its highest point on its
daily arc across the sky.
In 1874 the British Association for the Advancement of Science
introduced the CGS (centimetre/gramme/second system) combining
fundamental units of length, mass and time. The second is "elastic",
because tidal friction is slowing the earth's rotation rate. For use in
calculating ephemerides of celestial motion, therefore, in 1952
astronomers introduced the "ephemeris second", currently defined as
The CGS system has been superseded by the Système international. The SI base unit for time is the SI second. The International System of Quantities,
which incorporates the SI, also defines larger units of time equal to
fixed integer multiples of one second (1 s), such as the minute, hour
and day. These are not part of the SI, but may be used alongside the SI.
Other units of time such as the month and the year are not equal to
fixed multiples of 1 s, and instead exhibit significant variations in
duration.
The official SI definition of the second is as follows:
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.
The current definition of the second, coupled with the current definition of the metre, is based on the special theory of relativity, which affirms our spacetime to be a Minkowski space. The definition of the second in mean solar time, however, is unchanged.
World time
While in theory, the concept of a single worldwide universal
time-scale may have been conceived of many centuries ago, in
practicality the technical ability to create and maintain such a
time-scale did not become possible until the mid-19th century. The
timescale adopted was Greenwich Mean Time, created in 1847. A few
countries have replaced it with Coordinated Universal Time, UTC.
History of the development of UTC
With the advent of the industrial revolution,
a greater understanding and agreement on the nature of time itself
became increasingly necessary and helpful. In 1847 in Britain, Greenwich Mean Time
(GMT) was first created for use by the British railways, the British
navy, and the British shipping industry. Using telescopes, GMT was
calibrated to the mean solar time at the Royal Observatory, Greenwich in the UK.
As international commerce continued to increase throughout
Europe, in order to achieve a more efficiently functioning modern
society, an agreed upon, and highly accurate international standard of time measurement became necessary. In order to find or determine such a time-standard, three steps had to be followed:
An internationally agreed upon time-standard had to be defined.
This new time-standard then had to be consistently and accurately measured.
The new time-standard then had to be freely shared and distributed around the world.
The development of what is now known as UTC time
came about historically as an effort which first began as a
collaboration between 41 nations, officially agreed to and signed at the
International Meridian Conference,
in Washington D.C. in 1884. At this conference, the local mean solar
time at the Royal Observatory, Greenwich in England was chosen to define
the "universal day", counted from 0 hours at Greenwich mean midnight.
This agreed with the civil Greenwich Mean Time used on the island of
Great Britain since 1847. In contrast astronomical GMT began at mean
noon, i.e. astronomical day X began at noon of civil day X.
The purpose of this was to keep one night's observations under one
date. The civil system was adopted as of 0 hours (civil) 1 January 1925.
Nautical GMT began 24 hours before astronomical GMT, at least until
1805 in the Royal Navy,
but persisted much later elsewhere because it was mentioned at the 1884
conference. In 1884, the Greenwich meridian was used for two-thirds of
all charts and maps as their Prime Meridian.
Among the 41 nations represented at the conference, the advanced
time-technologies that had already come into use in Britain were
fundamental components of the agreed upon method of arriving at a
universal and agreed upon international time. In 1928 Greenwich Mean
Time was rebranded for scientific purposes by the International Astronomical Union as Universal Time
(UT). This was to avoid confusion with the previous system where the
day had begun at noon. As the general public had always begun the day at
midnight the timescale continued to be presented to them as Greenwich
Mean Time. By 1956, universal time had been split into various versions –
UT2, which smoothed for polar motion and seasonal effects, was
presented to the public as Greenwich Mean Time. Later, UT1 (which
smooths only for polar motion) became the default form of UT used by
astronomers and hence the form used in navigation, sunrise and sunset
and moonrise and moonset tables where the name Greenwich Mean Time
continues to be employed. Greenwich Mean Time is also the preferred
method of describing the timescale used by legislators. Even to the
present day, UT is still based on an international telescopic system.
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, where the second is defined as 1/86 400 of the mean
solar day.
For the better part of the first century following the
"International Meridian Conference," until 1960, the methods and
definitions of time-keeping that had been laid out at the conference
proved to be adequate to meet time tracking needs of science. Still,
with the advent of the "electronic revolution" in the latter half of the
20th century, the technologies that had been available at the time of
the Convention of the Metre proved to be in need of further refinement
in order to meet the needs of the ever-increasing precision that the
"electronic revolution" had begun to require.
The ephemeris second
An invariable second (the "ephemeris second") had been defined, use
of which removed the errors in ephemerides resulting from the use of the
variable mean solar second as the time argument. In 1960 this ephemeris
second was made the basis of the "coordinated universal time" which was
being derived from atomic clocks. It is a specified fraction of the
mean tropical year as at 1900 and, being based on historical telescope
observations, corresponds roughly to the mean solar second of the early
nineteenth century.
The SI second
In 1967 a further step was taken with the introduction of the SI
second, essentially the ephemeris second as measured by atomic clocks
and formally defined in atomic terms.
The SI second (Standard Internationale second) is based directly on the
measurement of the atomic-clock observation of the frequency oscillation
of caesium atoms. It is the basis of all atomic timescales, e.g.
coordinated universal time, GPS time, International Atomic Time, etc.
Atomic clocks do not measure nuclear decay rates, which is a common
misconception, but rather measure a certain natural vibrational
frequency of caesium-133.
Coordinated universal time is subject to one constraint which does not
affect the other atomic timescales. As it has been adopted as the civil
timescale by some countries (most countries have opted to retain mean
solar time) it is not permitted to deviate from GMT by more than 0.9
second. This is achieved by the occasional insertion of a leap second.
Current application of UTC
Most countries use mean solar time. Australia, Canada (Quebec only),
Colombia, France, Germany, New Zealand, Papua New Guinea (Bougainville
only), Paraguay, Portugal, Switzerland, the United States and Venezuela
use UTC. However, UTC is widely used by the scientific community in
countries where mean solar time is official. UTC time
is based on the SI second, which was first defined in 1967, and is
based on the use of atomic clocks. Some other less used but closely
related time-standards include International Atomic Time (TAI), Terrestrial Time, and Barycentric Dynamical Time.
Between 1967 and 1971, UTC was periodically adjusted by
fractional amounts of a second in order to adjust and refine for
variations in mean solar time, with which it is aligned. After 1 January
1972, UTC time has been defined as being offset from atomic time by a
whole number of seconds, changing only when a leap second is added to keep radio-controlled clocks synchronized with the rotation of the Earth.
The Global Positioning System
also broadcasts a very precise time signal worldwide, along with
instructions for converting GPS time to UTC. GPS-time is based on, and
regularly synchronized with or from, UTC-time.
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 GMT. For example, time zones at sea
are based on GMT. In many locations (but not at sea) these offsets vary
twice yearly due to daylight saving time transitions.
Sidereal time
Unlike solar time, which is relative to the apparent position of the Sun, sidereal time is the measurement of time relative to that of a distant star. In astronomy, sidereal time is used to predict when a star will reach its highest point in the sky. Due to Earth's orbital motion around the Sun, a mean solar day is about 3 minutes 56 seconds longer than a mean sidereal day, or 1⁄366 more than a mean sidereal day.
Chronology
Another form of time measurement consists of studying the past. Events in the past can be ordered in a sequence (creating a chronology), and can be put into chronological groups (periodization). One of the most important systems of periodization is the geologic time scale, 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.
Time-like concepts: terminology
The term "time" is generally used for many close but different concepts, including:
instant as an object – one point on the time axes. Being an object, it has no value;
time interval as an object – part of the time axes limited by two instants. Being an object, it has no value;
date
as a quantity characterising an instant. As a quantity, it has a value
which may be expressed in a variety of ways, for example
"2014-04-26T09:42:36,75" in ISO standard format, or more colloquially such as "today, 9:42 a.m.";
duration as a quantity characterizing a time interval.
As a quantity, it has a value, such as a number of minutes, or may be
described in terms of the quantities (such as times and dates) of its
beginning and end.
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) was traditionally regarded[by whom?] as 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
According to Kabbalists, "time" is a paradox and an illusion. Both the future and the past are recognised to be combined and simultaneously present.
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 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.
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.[63]
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 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.
Time as "unreal"
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 in his book The End of Time, argue that quantum equations of the universe take their true form when expressed in the timeless realm containing every possible now or momentary configuration of the universe, called 'platonia' by Barbour.
A modern philosophical theory called presentism
views the past and the future as human-mind interpretations of movement
instead of real parts of time (or "dimensions") which coexist with the
present. This theory rejects the existence of all direct interaction
with the past or the future, holding only the present as tangible. This
is one of the philosophical arguments against time travel. This
contrasts with eternalism (all time: present, past and future, is real) and the growing block theory (the present and the past are real, but the future is not).
Physical definition
Until Einstein's
reinterpretation of the physical concepts associated with time and
space, 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).
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 result
that observers in motion relative to one another 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.
Spacetime
Time has historically been closely related with space, the two together merging into spacetime in Einstein'sspecial 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.
Time dilation
Relativity of simultaneity:
Event B is simultaneous with A in the green reference frame, but it
occurred before in the blue frame, and occurs later in the red frame.
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 time versus Newtonian time
Views of spacetime along the world line
of 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 transformations applicable in the Newtonian and relativistic theories, respectively.
In the figures, the vertical direction indicates time. The
horizontal direction indicates distance (only one spatial dimension is
taken into account), and the thick dashed curve is the spacetime
trajectory ("world line") of the observer. The small dots indicate specific (past and future) events in spacetime.
The slope of the world line (deviation from being vertical) gives
the relative velocity to the observer. Note how in both pictures the
view of spacetime changes when the observer accelerates.
In the Newtonian description these changes are such that time is absolute:
the movements of the observer do not influence whether an event occurs
in the 'now' (i.e., whether an event passes the horizontal line through
the observer).
However, in the relativistic description the observability of events is absolute: the movements of the observer do not influence whether an event passes the "light cone" of the observer. Notice that with the change from a Newtonian to a relativistic description, the concept of absolute time is no longer applicable: events move up-and-down in the figure depending on the acceleration of the observer.
Arrow of time
Time appears to have a direction – the past lies behind, fixed and
immutable, while the future lies ahead and is not necessarily fixed. Yet
for the most part the laws of physics do not specify an arrow of time,
and allow any process to proceed both forward and in reverse. This is
generally a consequence of time being modelled by a parameter in the
system being analysed, where there is no "proper time": the direction of
the arrow of time is sometimes arbitrary. Examples of this include the cosmological arrow of time, which points away from the Big Bang, CPT symmetry, and the radiative arrow of time, caused by light only travelling forwards in time (see light cone). In particle physics, the violation of CP symmetry implies that there should be a small counterbalancing time asymmetry to preserve CPT symmetry as stated above. The standard description of measurement in quantum mechanics is also time asymmetric. The second law of thermodynamics states that entropy must increase over time.
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.
Quantized time
Time quantization is a hypothetical concept. In the modern established physical theories (the Standard Model of Particles and Interactions and General Relativity) time is not quantized.
Planck time (~ 5.4 × 10−44 seconds) is the unit of time in the system of natural units known as Planck units.
Current established physical theories are believed to fail at this time
scale, and many physicists expect that the Planck time might be the
smallest unit of time that could ever be measured, even in principle.
Tentative physical theories that describe this time scale exist; see for
instance loop quantum gravity.
Time 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 in time has
never been verified, presents many theoretical problems, and may be an
impossibility. 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 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.R. Clay, and later developed by William James.
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.
Development of awareness and understanding of time in children
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.
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.
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. 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.
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), 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.
Spatial conceptualization of time
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 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 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 which 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.
