Evolutionary biology is a subfield of biology that analyzes the four mechanisms of evolution: natural selection, mutation, genetic drift, and gene flow. The purpose of evolutionary biology is to observe the diversity of life on Earth. The idea of natural selection was first researched by Charles Darwin as he studied bird beaks. The discipline of evolutionary biology emerged through what Julian Huxley called the modern synthesis of understanding, from previously unrelated fields of biological research, such as genetics and ecology, systematics, and paleontology. Huxley was able to take what Charles Darwin discovered and elaborate to build on his understandings.
Evolutionary
biology explains diversity between species by analyzing changes in a
few individuals within a population over multiple generations. The purpose of this subfield is to determine how genetic variation
develops, how it is inherited, and how the evolutionary mechanisms shape
a population's genetic composition. Researchers study the traits of
organisms to identify which characteristics enhance or reduce survival
and reproduction. Advantageous traits tend to be passed on to offspring,
contributing to evolutionary change as those traits become more common.
These processes are studied at different levels of complexity
from observing features in living or fossilized species to analyzing DNA
genomic sequencing between species.
The idea of evolution by natural selection was proposed by Charles Darwin in 1859, but evolutionary biology, as an academic discipline in its own right, emerged during the period of the modern synthesis in the 1930s and 1940s. It was not until the 1980s that many universities had departments of evolutionary biology.
Microbiology too is becoming an evolutionary discipline now that microbial physiology and genomics are better understood. The quick generation time of bacteria and viruses such as bacteriophages makes it possible to explore evolutionary questions.
More recently, the merge between biological science and applied
sciences gave birth to new fields that are extensions of evolutionary
biology, including evolutionary robotics, engineering, algorithms, economics, and architecture. The basic mechanisms of evolution are applied directly or indirectly to
come up with novel designs or solve problems that are difficult to
solve otherwise. The research generated in these applied fields,
contribute towards progress, especially from work on evolution in computer science and engineering fields such as mechanical engineering.
In evolutionary developmental biology,
scientists look at how the different processes in development play a
role in how a specific organism reaches its current body plan. The
genetic regulation of ontogeny and the phylogenetic process is what
allows for this kind of understanding of biology. By looking at
different processes during development, and going through the
evolutionary tree, one can determine at which point a specific structure
came about.
Some evolutionary biologists ask the most straightforward
evolutionary question: "what happened and when?". This includes fields
such as paleobiology,
where paleobiologists and evolutionary biologists, including Thomas
Halliday and Anjali Goswami, studied the evolution of early mammals
going far back in time during the Mesozoic and Cenozoic eras (between
299 million to 12,000 years ago). Other fields related to generic exploration of evolution ("what happened and when?" ) include systematics and phylogenetics.
The modern evolutionary synthesis was devised at a time when the
molecular basis of genes was unknown. Today, evolutionary biologists try
to determine the genetic architecture underlying visible evolutionary phenomena such as adaptation
and speciation. They seek answers to questions such as which genes are
involved, how interdependent are the effects of different genes, what do
the genes do, and what changes happen to them (e.g., point mutations vs. gene duplication or even genome duplication). They try to reconcile the high heritability seen in twin studies with the difficulty in finding which genes are responsible for this heritability using genome-wide association studies. The modern evolutionary synthesis involved agreement about which forces
contribute to evolution, but not about their relative importance.
In physics and cosmology, the mathematical universe hypothesis (MUH), also known as the ultimate ensemble theory, is a speculative "theory of everything" (TOE) proposed by cosmologist Max Tegmark. According to the hypothesis, the universeis a mathematical object
in and of itself. Tegmark extends this idea to hypothesize that all
mathematical objects exist, which he describes as a form of Platonism or modal realism.
Tegmark replies that not only is the universe mathematical, but it is also computable.
In 2014, Tegmark published a popular science book about the topic, titled Our Mathematical Universe.
Description
Tegmark's MUH is the hypothesis that our external physical reality is a mathematical structure. That is, the physical universe is not merely described by mathematics, but is mathematics — specifically, a mathematical structure.
Mathematical existence equals physical existence, and all structures
that exist mathematically exist physically as well. Observers, including
humans, are "self-aware substructures (SASs)". In any mathematical
structure complex enough to contain such substructures, they "will
subjectively perceive themselves as existing in a physically 'real'
world".
The theory can be considered a form of Pythagoreanism or Platonism in that it proposes the existence of mathematical entities; a form of mathematicism in that it denies that anything exists except mathematical objects; and a formal expression of ontic structural realism.
Tegmark claims that the hypothesis has no free parameters and is
not observationally ruled out. Thus, he reasons, it is preferred over
other theories-of-everything by Occam's Razor. Tegmark also considers augmenting the MUH with a second assumption, the computable universe hypothesis (CUH), which says that the mathematical structure that is our external physical reality is defined by computable functions.
The MUH is related to Tegmark's categorization of four levels of the multiverse. This categorization posits a nested hierarchy of increasing diversity, with worlds corresponding to different sets of initial conditions (level 1), physical constants (level 2), quantum branches (level 3), and altogether different equations or mathematical structures (level 4).
Criticisms and responses
Andreas Albrecht when at Imperial College
in London called it a "provocative" solution to one of the central
problems facing physics. Although he "wouldn't dare" go so far as to say
he believes it, he noted that "it's actually quite difficult to
construct a theory where everything we see is all there is".
Definition of the ensemble
Jürgen Schmidhuber argues that "Although Tegmark suggests that '... all mathematical
structures are a priori given equal statistical weight,' there is no way
of assigning equal non-vanishing probability to all (infinitely many)
mathematical structures." Schmidhuber puts forward a more restricted
ensemble which admits only universe representations describable by constructive mathematics, that is, computer programs; e.g., the Global Digital Mathematics Library and Digital Library of Mathematical Functions, linked open data representations of formalized
fundamental theorems intended to serve as building blocks for
additional mathematical results. He explicitly includes universe
representations describable by non-halting programs whose output bits
converge after finite time, although the convergence time itself may not
be predictable by a halting program, due to the undecidability of the halting problem.
In response, Tegmark notes that a constructive mathematicsformalized
measure of free parameter variations of physical dimensions, constants,
and laws over all universes has not yet been constructed for the string theory landscape either, so this should not be regarded as a "show-stopper".
It has also been suggested that the MUH is inconsistent with Gödel's incompleteness theorem. In a three-way debate between Tegmark and fellow physicists Piet Hut and Mark Alford, the "secularist" (Alford) states that "the methods allowed by
formalists cannot prove all the theorems in a sufficiently powerful
system... The idea that math is 'out there' is incompatible with the
idea that it consists of formal systems."
Tegmark's response is to offer a new hypothesis "that only Gödel-complete (fully decidable)
mathematical structures have physical existence. This drastically
shrinks the Level IV multiverse, essentially placing an upper limit on
complexity, and may have the attractive side effect of explaining the
relative simplicity of our universe." Tegmark goes on to note that
although conventional theories in physics are Gödel-undecidable, the
actual mathematical structure describing our world could still be
Gödel-complete, and "could in principle contain observers capable of
thinking about Gödel-incomplete mathematics, just as finite-state digital computers can prove certain theorems about Gödel-incomplete formal systems like Peano arithmetic." In he gives a more detailed response, proposing as an alternative to MUH
the more restricted "Computable Universe Hypothesis" (CUH) which only
includes mathematical structures that are simple enough that Gödel's
theorem does not require them to contain any undecidable or uncomputable
theorems. Tegmark admits that this approach faces "serious challenges",
including (a) it excludes much of the mathematical landscape; (b) the
measure on the space of allowed theories may itself be uncomputable; and
(c) "virtually all historically successful theories of physics violate
the CUH".
Observability
Stoeger, Ellis, and Kircher
note that in a true multiverse theory, "the universes are then
completely disjoint and nothing that happens in any one of them is
causally linked to what happens in any other one. This lack of any
causal connection in such multiverses really places them beyond any
scientific support". Ellis
specifically criticizes the MUH, stating that an infinite ensemble of
completely disconnected universes is "completely untestable, despite
hopeful remarks sometimes made, see, e.g., Tegmark (1998)." Tegmark
maintains that MUH is testable,
stating that it predicts (a) that "physics research will uncover
mathematical regularities in nature", and (b) by assuming that we occupy
a typical member of the multiverse of mathematical structures, one
could "start testing multiverse predictions by assessing how typical our
universe is".
The MUH is based on the radical Platonist view that math is an external reality. However, Jannes argues that "mathematics is at least in part a human construction", on
the basis that if it is an external reality, then it should be found in
some other animals
as well: "Tegmark argues that, if we want to give a complete
description of reality, then we will need a language independent of us
humans, understandable for non-human sentient entities, such as aliens
and future supercomputers". Brian Greene argues similarly:
"The deepest description of the universe should not require concepts
whose meaning relies on human experience or interpretation. Reality
transcends our existence and so shouldn't, in any fundamental way,
depend on ideas of our making."
However, there are many non-human entities, plenty of which are
intelligent, and many of which can apprehend, memorise, compare and even
approximately add numerical quantities. Several animals have also
passed the mirror test of self-consciousness.
But a few surprising examples of mathematical abstraction
notwithstanding (for example, chimpanzees can be trained to carry out
symbolic addition with digits, or the report of a parrot understanding a
"zero-like concept"), all examples of animal intelligence
with respect to mathematics are limited to basic counting abilities. He
adds, "non-human intelligent beings should exist that understand the
language of advanced mathematics. However, none of the non-human
intelligent beings that we know of confirm the status of (advanced)
mathematics as an objective language." In the paper "On Math, Matter and
Mind" the secularist viewpoint examined argues
that math is evolving over time, there is "no reason to think it is
converging to a definite structure, with fixed questions and established
ways to address them", and also that "The Radical Platonist position is
just another metaphysical theory like solipsism... In the end the
metaphysics just demands that we use a different language for saying
what we already knew." Tegmark responds that "The notion of a mathematical structure is rigorously defined in any book on Model Theory",
and that non-human mathematics would only differ from our own "because
we are uncovering a different part of what is in fact a consistent and
unified picture, so math is converging in this sense." In his 2014 book
on the MUH, Tegmark argues that the resolution is not that we invent the
language of mathematics, but that we discover the structure of
mathematics.
Coexistence of all mathematical structures
Don Page has argued that "At the ultimate level, there can be only one world and, if mathematical structures are broad enough to include all possible worlds
or at least our own, there must be one unique mathematical structure
that describes ultimate reality. So I think it is logical nonsense to
talk of Level 4 in the sense of the co-existence of all mathematical
structures." This means there can only be one mathematical corpus.
Tegmark responds
that "This is less inconsistent with Level IV than it may sound, since
many mathematical structures decompose into unrelated substructures, and
separate ones can be unified."
Consistency with our "simple universe"
Alexander Vilenkin comments
that "The number of mathematical structures increases with increasing
complexity, suggesting that 'typical' structures should be horrendously
large and cumbersome. This seems to be in conflict with the beauty and
simplicity of the theories describing our world". He goes on to note that Tegmark's solution to this problem, the assigning of lower "weights" to the more complex structures
seems arbitrary ("Who determines the weights?") and may not be
logically consistent ("It seems to introduce an additional mathematical
structure, but all of them are supposed to be already included in the
set").
Occam's razor
Tegmark has been criticized as misunderstanding the nature and application of Occam's razor; Massimo Pigliucci reminds that "Occam's razor is just a useful heuristic, it should never be used as the final arbiter to decide which theory is to be favored".
Time is the continuous progression of existence that occurs in an apparently irreversible succession from the past, through the present, and into the future.[1][2][3] Time dictates all forms of action, age, and causality, being a component quantity of various measurements used to sequence events, to compare the duration of events (or the intervals between them), and to quantify rates of change of quantities in material reality or in the conscious experience. Time is often referred to as a fourth dimension, along with three spatial dimensions.
Time is primarily measured in linear spans or periods, ordered
from shortest to longest. Practical, human-scale measurements of time
are performed using clocks and calendars, reflecting a 24-hour day collected into a 365-day year linked to the astronomical motion of the Earth. Scientific measurements of time instead vary from Planck time at the shortest to billions of years at the longest. Measurable time is believed to have effectively begun with the Big Bang 13.8 billion years ago, encompassed by the chronology of the universe. Modern physics understands time to be inextricable from space within the concept of spacetime described by general relativity. Time can therefore be dilated
by velocity and matter to pass faster or slower for an external
observer, though this is considered negligible outside of extreme
conditions, namely relativistic speeds or the gravitational pulls of black holes.
Throughout history, time has been an important subject of study
in religion, philosophy, and science. Temporal measurement has occupied
scientists and technologists,
and has been a prime motivation in navigation and astronomy. Time is
also of significant social importance, having economic value ("time is money") as well as personal value, due to an awareness of the limited time in each day ("carpe diem") and in human life spans.
Definition
The concept of time can be complex. Multiple notions exist, and defining time in a manner applicable to all fields without circularity has consistently eluded scholars. Nevertheless, diverse fields such as business, industry, sports, the
sciences, and the performing arts all incorporate some notion of time
into their respective measuring systems. Traditional definitions of time involved the observation of periodic
motion such as the apparent motion of the sun across the sky, the phases
of the moon, and the passage of a free-swinging pendulum. More modern
systems include the Global Positioning System, other satellite systems, Coordinated Universal Time and mean solar time. Although these systems differ from one another, with careful measurements they can be synchronized.
In physics, time is a fundamental concept to define other quantities, such as velocity. To avoid a circular definition, time in physics is operationally defined as "what a clock reads", specifically a count of repeating events such as the SI second. Although this aids in practical measurements, it does not address the essence of time. Physicists developed the concept of the spacetime continuum, where events are assigned four coordinates: three for space and one for time. Events like particle collisions, supernovas, or rocket launches have coordinates that may vary for different observers, making concepts like "now" and "here" relative. In general relativity, these coordinates do not directly correspond to the causal structure of events. Instead, the spacetime interval
is calculated and classified as either space-like or time-like,
depending on whether an observer exists that would say the events are
separated by space or by time. Since the time required for light to travel a specific distance is the
same for all observers—a fact first publicly demonstrated by the Michelson–Morley experiment—all observers will consistently agree on this definition of time as a causal relation.
General relativity does not address the nature of time for
extremely small intervals where quantum mechanics holds. In quantum
mechanics, time is treated as a universal and absolute parameter,
differing from general relativity's notion of independent clocks. The problem of time consists of reconciling these two theories. As of 2025, there is no generally accepted theory of quantum general relativity.
Measurement
The flow of sand in an hourglass can be used to measure the passage of one hour of time. It also concretely represents the present as being between the past and the future.
Methods of temporal measurement, or chronometry, generally take two forms. The first is a calendar, a mathematical tool for organising intervals of time on Earth, consulted for periods longer than a day. The second is a clock,
a physical mechanism that indicates the passage of time, consulted for
periods less than a day. The combined measurement marks a specific
moment in time from a reference point, or epoch.
Artifacts from the Paleolithic suggest that the moon was used to reckon time as early as 6,000 years ago. Lunar calendars were among the first to appear, with years of either 12 or 13 lunar months (either 354 or 384 days). Without intercalation to add days or months to some years, seasons quickly drift in a calendar based solely on twelve lunar months. Lunisolar calendars
have a thirteenth month added to some years to make up for the
difference between a full year (now known to be about 365.24 days) and a
year of just twelve lunar months. The numbers twelve and thirteen came
to feature prominently in many cultures, at least partly due to this
relationship of months to years.
Other early forms of calendars originated in Mesoamerica, particularly in ancient Mayan civilization, in which they developed the Maya calendar, consisting of multiple interrelated calendars. These calendars were religiously and astronomically based; the Haab' calendar has 18 months in a year and 20 days in a month, plus five epagomenal days at the end of the year. In conjunction, the Maya also used a 260-day sacred calendar called the Tzolk'in.
The reforms of Julius Caesar in 45 BC put the Roman world on a solar calendar. This Julian calendar was faulty in that its intercalation still allowed the astronomical solstices and equinoxes to advance against it by about 11 minutes per year. Pope Gregory XIII introduced a correction in 1582; the Gregorian calendar
was only slowly adopted by different nations over a period of
centuries, but it is now by far the most commonly used calendar around
the world.
During the French Revolution, a new clock and calendar were invented as part of the dechristianization of France and to create a more rational system in order to replace the Gregorian calendar. The French Republican Calendar's days consisted of ten hours of a hundred minutes of a hundred seconds, which marked a deviation from the base 12 (duodecimal) system used in many other devices by many cultures. The system was abolished in 1806.
History of other devices
Horizontal sundial in Canberra24-hour clock face in Florence
A large variety of devices have been invented to measure time. The study of these devices is called horology. 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.
A sundial is any device that uses the direction of sunlight to cast shadows from a gnomon onto a set of markings calibrated to indicate the local time,
usually to the hour. The idea to separate the day into smaller parts is
credited to Egyptians because of their sundials, which operated on a
duodecimal system. The importance of the number 12 is due to the number
of lunar cycles in a year and the number of stars used to count the
passage of night. Obelisks made as a gnomon were built as early as c. 3500 BC. An Egyptian device that dates to c. 1500 BC, similar in shape to a bent T-square,
also 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.
Alarm clocks reportedly first appeared in ancient Greece c. 250 BC with a water clock made by Plato that would set off a whistle. The hydraulic alarm worked by gradually filling a series of vessels with water. After some time, the water emptied out of a siphon. Inventor Ctesibius
revised Plato's design; the water clock uses a float as the power drive
system and uses a sundial to correct the water flow rate.
In medieval philosophical writings, the atom was a unit of time
referred to as the smallest possible division of time. The earliest
known occurrence in English is in Byrhtferth's Enchiridion (a science text) of 1010–1012, where it was defined as 1/564 of a momentum (11⁄2 minutes), and thus equal to 15/94 of a second. It was used in the computus, the process of calculating the date of Easter. 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.
Incense sticks and candles were, and are, commonly used to measure
time in temples and churches across the globe. Water clocks, and, later,
mechanical clocks, were used to mark the events of the abbeys and
monasteries of the Middle Ages. The passage of the hours at sea can also
be marked by bell. The hours were marked by bells in abbeys as well as at sea. Richard of Wallingford (1292–1336), abbot of St. Alban's abbey, famously built a mechanical clock as an astronomical orrery about 1330.The hourglass uses the flow of sand to measure the flow of time. They were also used in navigation. Ferdinand Magellan used 18 glasses on each ship for his circumnavigation of the globe (1522). 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.
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. There is also a clock that was designed to keep time for 10,000 years called the Clock of the Long Now. Alarm clock devices were later mechanized. Levi Hutchins's alarm clock has been credited as the first American alarm clock, though it can only ring at 4 a.m. Antoine Redier was also credited as the first person to patent an adjustable mechanical alarm clock in 1847. Digital forms of alarm clocks became more accessible through digitization and integration with other technologies, such as smartphones.
Chip-scale atomic clocks, such as this one unveiled in 2004, are expected to greatly improve GPS location.
The most accurate timekeeping devices are atomic clocks, which are accurate to seconds in many millions of years, and are used to calibrate other clocks and timekeeping instruments. Atomic clocks use the frequency of electronic transitions in certain atoms to measure the second. One of the atoms used is caesium; most modern atomic clocks probe caesium with microwaves to determine the frequency of these electron vibrations. Since 1967, the International System of Measurements bases its unit of time, the second, on the properties of caesium atoms. SI
defines the second as 9,192,631,770 cycles of the radiation that
corresponds to the transition between two electron spin energy levels of
the ground state of the 133Cs atom. A portable timekeeper that meets certain precision standards is called a chronometer. Initially, the term was used to refer to the marine chronometer, a timepiece used to determine longitude by means of celestial navigation, a precision first achieved by John Harrison. More recently, the term has also been applied to the chronometer watch, a watch that meets precision standards set by the Swiss agency COSC.
In modern times, the Global Positioning System in coordination with the Network Time Protocol can be used to synchronize timekeeping systems across the globe. 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. The time measured was the delay caused by out-of-sync electron waves' interference patterns.
The second (s) is the SI base unit. A minute (min) is 60 seconds in length (or, rarely, 59 or 61 seconds when leap seconds are employed), and an hour
is 60 minutes or 3600 seconds in length. A day is usually 24 hours or
86,400 seconds in length; however, the duration of a calendar day can
vary due to daylight saving time and leap seconds.
A time standard is a specification for measuring time: assigning a number or calendar date to an instant (point in time), quantifying the duration of a time interval, and establishing a chronology
(ordering of events). In modern times, several time specifications have
been officially recognized as standards, where formerly they were
matters of custom and practice. The invention in 1955 of the caesium atomic clock has led to the replacement of older and purely astronomical time standards such as sidereal time and ephemeris time, for most practical purposes, by newer time standards based wholly or partly on atomic time using the SI second.
International Atomic Time (TAI) is the primary international time standard from which other time standards are calculated. Universal Time
(UT1) is mean solar time at 0° longitude, computed from astronomical
observations. It varies from TAI because of the irregularities in
Earth's rotation. Coordinated Universal Time
(UTC) is an atomic time scale designed to approximate Universal Time.
UTC differs from TAI by an integral number of seconds. UTC is kept
within 0.9 second of UT1 by the introduction of one-second steps to UTC,
the leap second. The Global Positioning System broadcasts a very precise time signal based on UTC time.
The surface of the Earth is split into a number of time zones. Standard time or civil time
in a time zone deviates a fixed, round amount, usually a whole number
of hours, from some form of Universal Time, usually UTC. Most time zones
are exactly one hour apart, and by convention compute their local time
as an offset from UTC. For example, time zones at sea are based on UTC.
In many locations (but not at sea) these offsets vary twice yearly due
to daylight saving time transitions.
Many ancient cultures, particularly in the East, had a cyclical view
of time. In these traditions, time was often seen as a recurring pattern
of ages or cycles, where events and phenomena repeated themselves in a
predictable manner. One of the most famous examples of this concept is
found in Hindu philosophy, where time is depicted as a wheel called the "Kalachakra"
or "Wheel of Time." According to this belief, the universe undergoes
endless cycles of creation, preservation, and destruction.
Similarly, in other ancient cultures such as those of the Mayans,
Aztecs, and Chinese, there were also beliefs in cyclical time, often
associated with astronomical observations and calendars. These cultures developed complex systems to track time, seasons, and
celestial movements, reflecting their understanding of cyclical patterns
in nature and the universe.
The cyclical view of time contrasts with the linear concept of
time more common in Western thought, where time is seen as progressing
in a straight line from past to future without repetition.
Time in Abrahamic religions
In general, the Islamic and Judeo-Christian world-view regards time as linear and directional, beginning with the act of creation by God. The traditional Christian view sees time ending, teleologically, with the eschatological end of the present order of things, the "end time". Though some Christian theologians (such as Augustine of Hippo and Aquinas)
believe that God is outside of time, seeing all events simultaneously,
that time did not exist before God, and that God created time.
In the Old Testament book Ecclesiastes, traditionally ascribed to Solomon (970–928 BC), time is depicted as cyclical and beyond human control. The book wrote that there is an appropriate season or time for every activity.[57]
Time in Greek mythology
The Greek language denotes two distinct principles, Chronos and Kairos.
The former refers to numeric, or chronological, time. The latter,
literally "the right or opportune moment", relates specifically to
metaphysical or Divine time. In theology, Kairos is qualitative, as
opposed to quantitative.
In Greek mythology, Chronos (ancient Greek: Χρόνος) is identified
as the personification of time. His name in Greek means "time" and is
alternatively spelled Chronus (Latin spelling) or Khronos. Chronos is
usually portrayed as an old, wise man with a long, gray beard, such as
"Father Time". Some English words whose etymological root is
khronos/chronos include chronology, chronometer, chronic, anachronism, synchronise, and chronicle.
Time in Kabbalah & Rabbinical thought
Rabbis sometimes saw time like "an accordion that was expanded and collapsed at will." According to Kabbalists, "time" is a paradox and an illusion.
Time in Advaita Vedanta
According to Advaita Vedanta, time is integral to the phenomenal world, which lacks independent reality. Time and the phenomenal world are products of maya,
influenced by our senses, concepts, and imaginations. The phenomenal
world, including time, is seen as impermanent and characterized by
plurality, suffering, conflict, and division. Since phenomenal existence
is dominated by temporality (kala),
everything within time is subject to change and decay. Overcoming pain
and death requires knowledge that transcends temporal existence and
reveals its eternal foundation.
Two contrasting viewpoints on time divide prominent philosophers. One
view is that time is part of the fundamental structure of the universe—a dimension independent of events, in which events occur in sequence. Isaac Newton subscribed to this realist view, and hence it is sometimes referred to as Newtonian time.
The opposing view is that time does not refer to any kind
of "container" that events and objects "move through", nor to any entity
that "flows", but that it is instead part of a fundamental intellectual
structure (together with space and number) within which humans sequence and compare events. This second view, in the tradition of Gottfried Leibniz and Immanuel Kant, holds that time
is neither an event nor a thing, and thus is not itself measurable nor
can it be travelled. Furthermore, it may be that there is a subjective component to time, but whether or not time itself is "felt", as a sensation, or is a judgment, is a matter of debate.
In philosophy, time was questioned throughout the centuries; what
time is and if it is real or not. Ancient Greek philosophers asked if
time was linear or cyclical and if time was endless or finite. These philosophers had different ways of explaining time; for instance, ancient Indian philosophers had something called the Wheel of Time. It is believed that there was repeating ages over the lifespan of the universe. This led to beliefs like cycles of rebirth and reincarnation. The Greek philosophers believe that the universe was infinite, and was an illusion to humans. Plato believed that time was made by the Creator at the same instant as the heavens. He also says that time is a period of motion of the heavenly bodies. Aristotle believed that time correlated to movement, that time did not exist on its own but was relative to motion of objects. He also believed that time was related to the motion of celestial bodies; the reason that humans can tell time was because of orbital periods and therefore there was a duration on time.
The Vedas, the earliest texts on Indian philosophy and Hindu philosophy dating to the late 2nd millennium BC, describe ancient Hindu cosmology, in which the universe goes through repeated cycles of creation, destruction and rebirth, with each cycle lasting 4,320 million years. AncientGreek philosophers, including Parmenides and Heraclitus, wrote essays on the nature of time. Plato, in the Timaeus, identified time with the period of motion of the heavenly bodies. Aristotle, in Book IV of his Physica defined time as 'number of movement in respect of the before and after'. In Book 11 of his Confessions, St. Augustine of Hippo
ruminates on the nature of time, asking, "What then is time? If no one
asks me, I know: if I wish to explain it to one that asketh, I know
not." He begins to define time by what it is not rather than what it is, an approach similar to that taken in other negative definitions.
However, Augustine ends up calling time a "distention" of the mind
(Confessions 11.26) by which we simultaneously grasp the past in memory,
the present by attention, and the future by expectation.
Philosophers in the 17th and 18th century questioned if time was
real and absolute, or if it was an intellectual concept that humans use
to understand and sequence events. These questions lead to realism vs anti-realism; the realists believed
that time is a fundamental part of the universe, and be perceived by
events happening in a sequence, in a dimension. Isaac Newton said that we are merely occupying time, he also says that humans can only understand relative time. Isaac Newton believed in absolute space and absolute time; Leibniz believed that time and space are relational. The differences between Leibniz's and Newton's interpretations came to a head in the famous Leibniz–Clarke correspondence. Relative time is a measurement of objects in motion. The anti-realists believed that time is merely a convenient intellectual concept for humans to understand events. This means that time was useless unless there were objects that it could interact with, this was called relational time. René Descartes, John Locke, and David Hume said that one's mind needs to acknowledge time, in order to understand what time is. Immanuel Kant believed that we can not know what something is unless we experience it first hand.
Time is not an empirical concept. For neither co-existence nor
succession would be perceived by us, if the representation of time did
not exist as a foundation a priori. Without this presupposition,
we could not represent to ourselves that things exist together at one
and the same time, or at different times, that is, contemporaneously, or
in succession.
Immanuel Kant, in the Critique of Pure Reason, described time as an a priori intuition that allows us (together with the other a priori intuition, space) to comprehend sense experience. With Kant, neither space nor time are conceived as substances,
but rather both are elements of a systematic mental framework that
necessarily structures the experiences of any rational agent, or
observing subject. Kant thought of time as a fundamental part of an abstract conceptual framework, together with space and number, within which we sequence events, quantify their duration, and compare the motions of objects. In this view, time does not refer to any kind of entity that "flows," that objects "move through," or that is a "container" for events. Spatial measurements are used to quantify the extent of and distances between objects, and temporal measurements are used to quantify the durations of and between events. Time was designated by Kant as the purest possible schema of a pure concept or category.
Henri Bergson believed that time was neither a real homogeneous medium nor a mental construct, but possesses what he referred to as Duration. Duration, in Bergson's view, was creativity and memory as an essential component of reality.
According to Martin Heidegger we do not exist inside time, we are time. Hence, the relationship to the past is a present awareness of having been,
which allows the past to exist in the present. The relationship to the
future is the state of anticipating a potential possibility, task, or
engagement. It is related to the human propensity for caring and being
concerned, which causes "being ahead of oneself" when thinking of a
pending occurrence. Therefore, this concern for a potential occurrence
also allows the future to exist in the present. The present becomes an
experience, which is qualitative instead of quantitative. Heidegger
seems to think this is the way that a linear relationship with time, or
temporal existence, is broken or transcended. We are not stuck in sequential time. We are able to remember the past
and project into the future; we have a kind of random access to our
representation of temporal existence; we can, in our thoughts, step out
of (ecstasis) sequential time.
Modern era philosophers asked: is time real or unreal, is time
happening all at once or a duration, is time tensed or tenseless, and is
there a future to be? There is a theory called the tenseless or B-theory; this theory says that any tensed terminology can be replaced with tenseless terminology. For example, "we will win the game" can be replaced with "we do win the
game", taking out the future tense. On the other hand, there is a
theory called the tense or A-theory; this theory says that our language has tense verbs for a reason and that the future can not be determined. There is also something called imaginary time, this was from Stephen Hawking, who said that space and imaginary time are finite but have no boundaries. Imaginary time is not real or unreal, it is something that is hard to visualize. Philosophers can agree that physical time exists outside of the human
mind and is objective, and psychological time is mind-dependent and
subjective.
Unreality
In 5th century BC Greece, Antiphon the Sophist, in a fragment preserved from his chief work On Truth, held that: "Time is not a reality (hypostasis), but a concept (noêma) or a measure (metron)." Parmenides went further, maintaining that time, motion, and change were illusions, leading to the paradoxes of his follower Zeno. Time as an illusion is also a common theme in Buddhist thought.
These arguments often center on what it means for something to be unreal. Modern physicists generally believe that time is as real as space—though others, such as Julian Barbour, argue quantum equations of the universe take their true form when expressed in the timeless realm containing every possible now or momentary configuration of the universe. J. M. E. McTaggart's 1908 article 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.
Another 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).
Until Einstein's
reinterpretation of the physical concepts associated with time and
space in 1907, time was considered to be the same everywhere in the
universe, with all observers measuring the same time interval for any
event. Non-relativistic classical mechanics is based on this Newtonian idea of time. Einstein, in his special theory of relativity, postulated the constancy and finiteness of the speed of light for all
observers. He showed that this postulate, together with a reasonable
definition for what it means for two events to be simultaneous, requires
that distances appear compressed and time intervals appear lengthened
for events associated with objects in motion relative to an inertial
observer.
The theory of special relativity finds a convenient formulation in Minkowski spacetime,
a mathematical structure that combines three dimensions of space with a
single dimension of time. In this formalism, distances in space can be
measured by how long light takes to travel that distance, e.g., a light-year is a measure of distance, and a meter is now defined in terms of how far light travels in a certain amount of time. Two events in Minkowski spacetime are separated by an invariant interval, which can be either space-like, light-like, or time-like. Events that have a time-like separation cannot be simultaneous in any frame of reference,
there must be a temporal component (and possibly a spatial one) to
their separation. Events that have a space-like separation will be
simultaneous in some frame of reference, and there is no frame of
reference in which they do not have a spatial separation. Different
observers may calculate different distances and different time intervals
between two events, but the invariant interval between the events is independent of the observer and their velocity.
Unlike space, where an object can travel in the opposite directions
(and in 3 dimensions), time appears to have only one dimension and only
one direction—the past lies behind, fixed and immutable, while the
future lies ahead and is not necessarily fixed. Yet most laws of physics
allow any process to proceed both forward and in reverse. There are
only a few physical phenomena that violate the reversibility of time.
This time directionality is known as the arrow of time. Acknowledged examples of the arrow of time are:
Radiative arrow of time, manifested in waves (e.g., light and
sound) travelling only expanding (rather than focusing) in time (see light cone);
The relationships between these different arrows of time is a hotly debated topic in theoretical physics.
The second law of thermodynamics states that entropy must increase over time. Brian Greene
theorizes that, according to the equations, the change in entropy
occurs symmetrically whether going forward or backward in time. So
entropy tends to increase in either direction, and our current
low-entropy universe is a statistical aberration, in a similar manner as
tossing a coin often enough that eventually heads will result ten times
in a row. However, this theory is not supported empirically in local
experiment.
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. The 1860s Maxwell's equations described that light always travels at a constant speed (in a vacuum). However, classical mechanics assumed that motion was measured relative to a fixed reference frame. The Michelson–Morley experiment
contradicted the assumption. Einstein later proposed a method of
synchronizing clocks using the constant, finite speed of light as the
maximum signal velocity. This led directly to the conclusion that
observers in motion relative to one another measure different elapsed
times for the same event.
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 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.
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.
According to general relativity, time also runs slower in stronger gravitational fields; this is gravitational time dilation. The effect of the dilation becomes more noticeable in a mass-dense
object. A famous example of time dilation is a thought experiment of a
subject approaching the event horizon of a black hole.
As a consequence of how gravitational fields warp spacetime, the
subject will experience gravitational time dilation. From the
perspective of the subject itself, they will experience time normally.
Meanwhile, an observer from the outside will see the subject move closer
to the black hole until the extreme, in which the subject appears
'frozen' in time and eventually fades to nothingness due to the
diminishing amount of light returning.
Relativistic versus Newtonian
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.
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.
Time quantization refers to the theory that time has a smallest
possible unit. Time quantization is a hypothetical concept. In the
modern established physical theories like the Standard Model of particle physics 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.
Though tentative physical theories that attempt to describe phenomena at
this scale exist; an example is loop quantum gravity. Loop quantum gravity suggests that time is quantized; if gravity is quantized, spacetime is also quantized.
Time travel is the concept of moving backwards or forwards to
different points in time, in a manner analogous to moving through space,
and different from the normal "flow" of time to an earthbound observer.
In this view, all points in time (including future times) "persist" in
some way. Time travel has been a plot device
in fiction since the 19th century. Travelling backwards or forwards in
time has never been verified as a process, and doing so presents many
theoretical problems and contradictory logic which to date have not been
overcome. Any technological device, whether fictional or hypothetical,
that is used to achieve time travel is known as a time machine.
A central problem with time travel to the past is the violation of causality; should an effect precede its cause, it would give rise to the possibility of a temporal paradox. Some interpretations of time travel resolve this by accepting the possibility of travel between branch points, parallel realities, or universes.
The many-worlds interpretation has been used as a way to solve
causality paradoxes arising from time travel. Any quantum event creates
another branching timeline, and all possible outcomes coexist without
any wave function collapse. This interpretation was an alternative but is opposite from the Copenhagen interpretation, which suggests that wave functions do collapse. In science, hypothetical faster-than-light particles are known as tachyons; the mathematics of Einstein's relativity suggests that they would have an imaginary rest mass. Some interpretations suggest that it might move backward in time. General relativity permits the existence of closed timelike curves, which could allow an observer to travel back in time to the same space. Though for the Gödel metric, such an occurrence requires a globally rotating universe, which has been contradicted by observations of the redshifts of distant galaxies and the cosmic background radiation. However, it has been suggested that a slowly rotating universe model may solve the Hubble tension, so it can not yet be ruled out.
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. A famous example is the grandfather paradox,
in which a person is supposed to travel back in time to kill their own
grandfather. This would not be possible to enact because it is a
historical fact that one's grandfather was not killed before his child
(one's parent) was conceived. This view does not simply hold that
history is an unchangeable constant, but that any change made by a
hypothetical future time traveller would already have happened in their
past, resulting in the reality that the traveller moves from. The Novikov self-consistency principle asserts that due to causality constraints, time travel to the past is impossible.
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).
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. Some also argued that the perception of time is also influenced by
memory and how much one have experienced; for example, as one get older,
they will have spent less part of their total life waiting a month. Meanwhile, children's expanding cognitive abilities allow them to
understand time in a different way. 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. Socioemotional selectivity theory proposed that when people perceive their time as open-ended and nebulous, they focus more on future-oriented goals.
Spatial conceptualization
Although time is regarded as an abstract concept, there is increasing evidence that time is conceptualized in the mind in terms of space. That is, instead of thinking about time in a general, abstract way,
humans think about time in a spatial way and mentally organize it as
such. Using space to think about time allows humans to mentally organize
temporal events in a specific way. This spatial representation of time
is often represented in the mind as a mental timeline (MTL). These origins are shaped by many environmental factors. 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 mostly 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, speakers
of Arabic, Farsi, Urdu, and Hebrew read from right to left, and their
MTLs unfold leftward (past on the right with future on the left);
evidence suggests these speakers organize time events in their minds
like this as well.
There is also evidence that some cultures use an allocentric spatialization, often based on environmental features. A study of the indigenous Yupno people of Papua New Guinea
found that they may use an allocentric MTL, in which time flows uphill;
when speaking of the past, 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. A similar study of the Pormpuraawans, an aboriginal group
in Australia, reported that when they were asked to organize photos of a
man aging "in order," individuals consistently placed the youngest
photos to the east and the oldest photos to the west, regardless of
which direction they faced. This directly clashed with an American group that consistently
organized the photos from left to right. Therefore, this group also
appears to have an allocentric MTL, but based on the cardinal directions
instead of geographical features. The wide array of distinctions in the way different groups think about
time leads to the broader question that different groups may also think
about other abstract concepts in different ways as well, such as
causality and number.
In sociology and anthropology, time discipline is the general name given to social and economic rules, conventions, customs, and expectations governing the measurement of time, the social currency and awareness of time measurements, and people's expectations concerning the observance of these customs by others. Arlie Russell Hochschild and Norbert Elias have written on the use of time from a sociological perspective.
The use of time is an important issue in understanding human behavior, education, and travel behavior. Time-use research
is a developing field of study. The question concerns how time is
allocated across a number of activities (such as time spent at home, at
work, shopping, etc.). Time use changes with technology, as the
television or the Internet created new opportunities to use time in
different ways. However, some aspects of time use are relatively stable
over long periods of time, such as the amount of time spent traveling to
work, which despite major changes in transport, has been observed to be
about 20–30 minutes one-way for a large number of cities over a long
period.
Time management
is the organization of tasks or events by first estimating how much
time a task requires and when it must be completed, and adjusting events
that would interfere with its completion so it is done in the
appropriate amount of time. Calendars and day planners are common
examples of time management tools.
Sequence of events
A sequence of events, or series of events, is a sequence of items, facts, events, actions, changes, or procedural steps, arranged in time order (chronological order), often with causality relationships among the items.Because of causality, cause precedes effect,
or cause and effect may appear together in a single item, but effect
never precedes cause. A sequence of events can be presented in text, tables, charts, or timelines. The description of the items or events may include a timestamp.
A sequence of events that includes the time along with place or
location information to describe a sequential path may be referred to as
a world line.
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