Talk:TFNR - 3.2.3 Time and Temporality

abs - rel What is time? What is the nature of time? Time has always been playing a strange (no role in vast part of physics) role in conventional physics. As we can easily understand, time is so central in an evolutionary constructive perspective. The history is so important to understand the present and the whole in causal, space and temporal sense, because we intent the incessant formation of Reality as a causal exploration process of the possible that can become actual. Co-evolution of the possible and the actual Reality. Time is strictly connected with relation. This is crucial to understand the differences, between action and energy/information, between absolute and relative. We can see/measure anything in acting, during the changing of a system. We can only report/measure the changes of the states of a system. We cannot measure the action, we can only measure differences in Information, in the energy levels/configurations of a system. In the elementary domains, Action is what happens (an event, a variation in the space dimension) in the time dimension produced by the expression of forces (causal dimension). Action happens in the absolute side of Reality A gravity field is defined as force per unit mass. Gravity Field is relative to Time. Therefore a relativity exist between the two.. That means without Time gravity would not exist. Time is relative to the different Time potentials that exist in the constricted space levels of the substance of Space in the Universe. Therefore, Gravity is variable as a function of the Time potentials of the Universe. Note; Acceleration is defined as Space displacement divided by square of time. .. In the absence of" Time ",acceleration would not be defined.. It was J. Kepler who established the relationship between space and time..in terms of a Space Time equation. His posited on the basis of astronomy data that the square of Time was proportional to a particular volume of space which was occupied by the sun and a planet. The equation of time relative to a gravity field can be expressed as follows; Is it time “real” (physically real) or “meta-real” (psychological / perceived)? The answer may be: both the two! Time is one of the fundamental dimensions of Reality, that is physical and meta-physical. . T^2 = R/Ag Where T is time measured in seconds R is the radius vector of the Space containing the sun and the Earth. Ag is the Gravity Field between the Sun and the Earth. Hence the radial velocity of the space between the Earth and the Sun is Vr= R/T. Problem of time From Wikipedia, the free encyclopedia This article needs attention from an expert on the subject. Please add a reason or a talk parameter to this template to explain the issue with the article. Consider associating this request with a WikiProject. (June 2015) In quantum gravity, the problem of time is a conceptual conflict between general relativity and quantum mechanics. Roughly speaking, the problem of time is that there is none in general relativity. This is because in general relativity, the Hamiltonian is an energy constraint that must vanish to allow for general covariance. However, in theories of quantum mechanics, the Hamiltonian generates the time evolution of quantum states. Therefore, we arrive at the conclusion that "nothing moves" ("there is no time") in general relativity. Since "there is no time", the usual interpretation of quantum mechanics measurements at given moments of time breaks down. This problem of time is the broad banner for all interpretational problems of the formalism. Contents [hide] 1 Time in quantum mechanics 2 Overturning of absolute time in general relativity 3 Proposed solutions to the problem of time 4 The thermal time hypothesis 5 References 6 Further reading Time in quantum mechanics[edit] In classical mechanics, a special status is assigned to time in the sense that it is treated as a classical background parameter, external to the system itself. This special role is seen in the standard formulation of quantum mechanics. It is regarded as part of an a priori given classical background with a well defined value. In fact, the classical treatment of time is deeply intertwined with the Copenhagen interpretation of quantum mechanics, and, thus, with the conceptual foundations of quantum theory: all measurements of observables are made at certain instants of time and probabilities are only assigned to such measurements. Special relativity has modified the notion of time. But from a fixed Lorentz observer's viewpoint time remains a distinguished, absolute, external, global parameter. The Newtonian notion of time essentially carries over to special relativistic systems, hidden in the spacetime structure. Overturning of absolute time in general relativity[edit] Though classically spacetime appears to be an absolute background, general relativity reveals that spacetime is actually dynamical; gravity is a manifestation of spacetime geometry. Matter reacts with spacetime: Spacetime tells matter how to move; matter tells spacetime how to curve. — John Archibald Wheeler, Geons, Black Holes, and Quantum Foam, p. 235[1] Also, spacetime can interact with itself (e.g. gravitational waves). The dynamical nature of spacetime has a vast array of consequences. This section may be confusing or unclear to readers. In particular, vocabulary needs to be explained: diffeomorphism, Hamiltonian, Dirac observables, perennials, partial observables. Need to illustrate with example. (September 2014) (Learn how and when to remove this template message) The dynamical nature of spacetime, via the Hole argument, implies that the theory is diffeomorphism invariant. The constraints are the imprint in the canonical theory of the diffeomorphism invariance of the four-dimensional theory. They also contain the dynamics of the theory, since the Hamiltonian identically vanishes. The quantum theory has no explicit dynamics; wavefunctions are annihilated by the constraints and Dirac observables commute with the constraints and hence are constants of motion. Kuchar introduces the idea of "perennials" and Rovelli the idea of "partial observables". The expectation is that in physical situations some of the variables of the theory will play the role of a "time" with respect to which other variables would evolve and define dynamics in a relational way. This runs into difficulties and is a version of the "problem of time" in the canonical quantization.[2] Proposed solutions to the problem of time[edit] Main article: Multiple histories The quantum concept of time was invented by physicist Bryce DeWitt in 1960's:[3] "Different times are special cases of different universes" In other words, time is an entanglement phenomenon, which places all equal clock readings (of correctly prepared clocks - or of any objects usable as clocks) into the same history. This was first understood by physicist Don Page and William Wootters in 1983.[4] They made a proposal to address the problem of time in systems like general relativity called conditional probabilities interpretation.[5] It consists in promoting all variables to quantum operators, one of them as a clock, and asking conditional probability questions with respect to other variables. They made a solution based on the quantum phenomenon of entanglement. Page and Wootters showed how quantum entanglement can be used to measure time.[6] In 2013, at the Istituto Nazionale di Ricerca Metrologica (INRIM) in Turin, Italy, Ekaterina Moreva, together with Giorgio Brida, Marco Gramegna, Vittorio Giovannetti, Lorenzo Maccone, and Marco Genovese performed the first experimental test of Page and Wootters' ideas. They confirmed that time is an emergent phenomenon for internal observers but absent for external observers of the universe just as the Wheeler-DeWitt equation predicts.[7][8] Consistent discretizations approach developed by Jorge Pullin and Rodolfo Gambini have no constraints. These are lattice approximation techniques for quantum gravity. In the canonical approach if one discretizes the constraints and equations of motion, the resulting discrete equations are inconsistent: they cannot be solved simultaneously. To address this problem one uses a technique based on discretizing the action of the theory and working with the discrete equations of motion. These are automatically guaranteed to be consistent. Most of the hard conceptual questions of quantum gravity are related to the presence of constraints in the theory. Consistent discretized theories are free of these conceptual problems and can be straightforwardly quantized, providing a solution to the problem of time. It is a bit more subtle than this. Although without constraints and having "general evolution", the latter is only in terms of a discrete parameter that isn't physically accessible. The way out is addressed in a way similar to the Page–Wooters approach. The idea is to pick one of the physical variables to be a clock and asks relational questions. These ideas where the clock is also quantum mechanical have actually led to a new interpretation of quantum mechanics — the Montevideo interpretation of quantum mechanics.[9][10] This new interpretation solves the problems of the use of environmental decoherence as a solution to the problem of measurement in quantum mechanics by invoking fundamental limitations, due to the quantum mechanical nature of clocks, in the process of measurement in quantum mechanics. These limitations are very natural in the context of generally covariant theories as quantum gravity where the clock must be taken as one of the degrees of freedom of the system itself. They have also put forward this fundamental decoherence as a way to resolve the black hole information paradox.[11][12] In certain circumstances, a matter field is used to de-parametrize the theory and introduce a physical Hamiltonian. This generates physical time evolution, not a constraint. Reduced phase space quantization constraints are solved first then quantized. This approach was considered for some time to be impossible as it seems to require first finding the general solution to Einstein's equations. However, with use of ideas involved in Dittrich's approximation scheme (built on ideas of Rovelli) a way to explicitly implement, at least in principle, a reduced phase space quantization was made viable.[13] The thermal time hypothesis[edit] Main articles: Thermal time hypothesis and thermodynamics Generally covariant theories do not have a notion of a distinguished physical time with respect to which everything evolves. However, it is not needed for the full formulation and interpretation of the theory. The dynamical laws are determined by correlations which are sufficient to make predictions. But then a mechanism is needed which explains how the familiar notion of time eventually emerges from the timeless structure to become such an important ingredient of the macroscopic world we live in as well as of our conscious experience. A possible solution to this problem has been put forward by Carlo Rovelli and Alain Connes, both in the classical and quantum theory, and goes by the name of the thermal time hypothesis. It postulates that physical time flow is not a priori given fundamental property of the theory, but is a macroscopic feature of thermodynamical origin.[14] Time as a primary expression of existence is the indefinite continued progress of existence and events that occur in apparently irreversible succession from the past through the present to the future.[1][2][3] Time is 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.[4][5][6][7] Time is often referred to as the fourth dimension, along with the three spatial dimensions.[8] Time as part of the fundamental structure Reality and the Universe 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. 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 holds that time is neither an event nor a thing, and thus is not itself measurable nor can it be travelled. Two distinct viewpoints on time divide many prominent philosophers. One view is that time is part of the fundamental structure of the universe, a dimension in which events occur in sequence. Sir Isaac Newton subscribed to this realist view, and hence it is sometimes referred to as Newtonian time.[16] An opposing view is that time does not refer to any kind of actually existing dimension that events and objects "move through", nor to any entity that "flows", but that it is instead an intellectual concept (together with space and number) that enables humans to sequence and compare events.[64] This second view, in the tradition of Gottfried Leibniz[17] and Immanuel Kant,[18][19] holds that space and time "do not exist in and of themselves, but ... are the product of the way we represent things", because we can know objects only as they appear to us. IOs time a physical quantity or a metaphysical perception? time as an a priori intuition that allows us (together with the other a priori intuition, space) to comprehend sense experience.[70] 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.[71] 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.[72] 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.[73] 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.[74] Time as an illusion is also a common theme in Buddhist thought.[75][76] J. M. E. McTaggart's 1908 The Unreality of Time argues that, since every event has the characteristic of being both present and not present (i.e., future or past), that time is a self-contradictory idea (see also The flow of time).

These arguments often center around what it means for something to be unreal. Modern physicists generally 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.[77] 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). Time as a physical quantity: Time in physics is unambiguously operationally defined as "what a clock reads". Time is one of the seven fundamental physical 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.[21] 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. Physical definition Classical mechanics F → = m a → {\displaystyle {\vec {F}}=m{\vec {a}}} {\vec {F}}=m{\vec {a}} Second law of motion

   Acceleration Angular momentum Couple D'Alembert's principle Energy
       kinetic potential Force Frame of reference Impulse Inertia / Moment of inertia Mass
   Mechanical power Mechanical work
   Moment Momentum Space Speed Time Torque Velocity Virtual work 

Time as a measurement 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 transition frequency 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. Generally speaking, methods of temporal measurement, or chronometry, take two distinct forms: the calendar, a mathematical tool for organizing intervals of time,[24] 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. Time principles The Greek language denotes two distinct principles, Chronos and Kairos. The former refers to numeric, or chronological, time. The latter, literally "the right or opportune moment", relates specifically to metaphysical or Divine time. In theology, Kairos is qualitative, as opposed to quantitative. In Greek mythology, Chronos (Ancient Greek: Χρόνος) is identified as the Personification of Time. His name in Greek means "time" and is alternatively spelled Chronus (Latin spelling) or Khronos. Chronos is usually portrayed as an old, wise man with a long, gray beard, such as "Father Time". Some English words whose etymological root is khronos/chronos include chronology, chronometer, chronic, anachronism, synchronize, and chronicle. Time as an illusion According to Kabbalists, “time” is a paradox[62] and an illusion.[63] Both the future and the past are recognized to be combined and simultaneously present. Time as finite or infinite In contrast to ancient Greek philosophers who believed that the universe had an infinite past with no beginning, medieval philosophers and theologians developed the concept of the universe having a finite past with a beginning. This view is shared by Abrahamic faiths as they believe time started by creation, therefore the only thing being infinite is God and everything else, including time, is finite. Time as absolute or relative Isaac Newton believed in absolute space and absolute time; Leibniz believed that time and space are relational.[69] The differences between Leibniz's and Newton's interpretations came to a head in the famous Leibniz–Clarke correspondence. 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.[78] Non-relativistic classical mechanics is based on this Newtonian idea of time.

Einstein, in his special theory of relativity,[79] 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 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 Main article: Spacetime Time in space time continuum Time has historically been closely related with space, the two together merging into spacetime in Einstein's special relativity and general relativity. According to these theories, the concept of time depends on the spatial reference frame of the observer, and the human perception as well as the measurement by instruments such as clocks are different for observers in relative motion. 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.[80][81][82] Simultaneity and 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. Main article: Time dilation 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. Many subatomic particles exist for only a fixed fraction of a second in a lab relatively at rest, but some that travel close to the speed of light can be measured to travel farther and survive much longer than expected (a muon is one example). According to the special theory of relativity, in the high-speed particle's frame of reference, it exists, on the average, for a standard amount of time known as its mean lifetime, and the distance it travels in that time is zero, because its velocity is zero. Relative to a frame of reference at rest, time seems to "slow down" for the particle. Relative to the high-speed particle, distances seem to shorten. Einstein showed how both temporal and spatial dimensions can be altered (or "warped") by high-speed motion. Einstein (The Meaning of Relativity): "Two events taking place at the points A and B of a system K are simultaneous if they appear at the same instant when observed from the middle point, M, of the interval AB. Time is then defined as the ensemble of the indications of similar clocks, at rest relatively to K, which register the same simultaneously." Einstein wrote in his book, Relativity, that simultaneity is also relative, i.e., two events that appear simultaneous to an observer in a particular inertial reference frame need not be judged as simultaneous by a second observer in a different inertial frame of reference. 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:[83] 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 Main article: 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 modeled by a parameter in the system being analyzed, where there is no "proper time": the direction of the arrow of time is sometimes arbitrary. Examples of this include the Second law of thermodynamics, which states that entropy must increase over time (see Entropy); the cosmological arrow of time, which points away from the Big Bang, CPT symmetry, and the radiative arrow of time, caused by light only traveling forwards in time (see light cone). In particle physics, the violation of CP symmetry implies that there should be a small counterbalancing time asymmetry to preserve CPT symmetry as stated above. The standard description of measurement in quantum mechanics is also time asymmetric (see Measurement in quantum mechanics). Time as a continuum or as a discrete, quantized dimension 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 and the beginning / creation Time and the Big Bang theory Stephen Hawking in particular has addressed a connection between time and the Big Bang. In A Brief History of Time and elsewhere, Hawking says that even if time did not begin with the Big Bang and there were another time frame before the Big Bang, no information from events then would be accessible to us, and nothing that happened then would have any effect upon the present time-frame.[84] Upon occasion, Hawking has stated that time actually began with the Big Bang, and that questions about what happened before the Big Bang are meaningless.[85][86][87] This less-nuanced, but commonly repeated formulation has received criticisms from philosophers such as Aristotelian philosopher Mortimer J. Adler.[88][89] Scientists have come to some agreement on descriptions of events that happened 10−35 seconds after the Big Bang, but generally agree that descriptions about what happened before one Planck time (5 × 10−44 seconds) after the Big Bang are likely to remain pure speculation. Speculative physics beyond the Big Bang A graphical representation of the expansion of the universe with the inflationary epoch represented as the dramatic expansion of the metric seen on the left While the Big Bang model is well established in cosmology, it is likely to be refined in the future. Little is known about the earliest moments of the universe's history. The Penrose–Hawking singularity theorems require the existence of a singularity at the beginning of cosmic time. However, these theorems assume that general relativity is correct, but general relativity must break down before the universe reaches the Planck temperature, and a correct treatment of quantum gravity may avoid the singularity.[90] If inflation has indeed occurred, it is likely that there are parts of the universe so distant that they cannot be observed in principle, as exponential expansion would push large regions of space beyond our observable horizon. Some proposals, each of which entails untested hypotheses, are:

   Models including the Hartle–Hawking boundary condition in which the whole of space-time is finite; the Big Bang does represent the limit of time, but without the need for a singularity.[91]
   Brane cosmology models[92] in which inflation is due to the movement of branes in string theory; the pre-big bang model; the ekpyrotic model, in which the Big Bang is the result of a collision between branes; and the cyclic model, a variant of the ekpyrotic model in which collisions occur periodically.[93][94][95]
   Chaotic inflation, in which inflation events start here and there in a random quantum-gravity foam, each leading to a bubble universe expanding from its own big bang.[96]

Proposals in the last two categories see the Big Bang as an event in a much larger and older universe, or multiverse, and not the literal beginning. Reversing the time Moving in the time dimension Time travel Main article: Time travel See also: Time travel in fiction, Wormhole, and Twin paradox 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. Traveling backwards in time has never been verified, presents many theoretic problems, and may be an impossibility.[97] 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. Time and causality Another solution to the problem of causality-based temporal paradoxes is that such paradoxes cannot arise simply because they have not arisen. As illustrated in numerous works of fiction, free will either ceases to exist in the past or the outcomes of such decisions are predetermined. As such, it would not be possible to enact the grandfather paradox because it is a historical fact that your grandfather was not killed before his child (your parent) was conceived. This view doesn't simply hold that history is an unchangeable constant, but that any change made by a hypothetical future time traveler would already have happened in his or her past, resulting in the reality that the traveler moves from. More elaboration on this view can be found in the Novikov self-consistency principle. Time as a perception in the metaphysical domain 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.[2][6][7][22][23] 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. Time perception 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.[98] Biopsychology The brain's judgment of time is known to be a highly distributed system, including at least the cerebral cortex, cerebellum and basal ganglia as its components. One particular component, the suprachiasmatic nuclei, is responsible for the circadian (or daily) rhythm, while other cell clusters appear capable of shorter-range (ultradian) timekeeping. Psychoactive drugs can impair the judgment of time. Stimulants can lead both humans and rats to overestimate time intervals,[99][100] while depressants can have the opposite effect.[101] The level of activity in the brain of neurotransmitters such as dopamine and norepinephrine may be the reason for this.[102] 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).[103] 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.[104] Alterations In addition to psychoactive drugs, judgments of time can be altered by temporal illusions (like the kappa effect),[105] age,[106] and hypnosis.[107] The sense of time is impaired in some people with neurological diseases such as Parkinson's disease and attention deficit disorder. Psychologists assert that time seems to go faster with age, but the literature on this age-related perception of time remains controversial.[108] Those who support this notion argue that young people, having more excitatory neurotransmitters, are able to cope with faster external events.[103] Use of time See also: Time management and Time discipline

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[109][110] and Norbert Elias[111] 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. [112][113][114] 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, [115] historical events (chronology), directions and steps in procedures,[116] 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). 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 [117] (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.[118] 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).[119] Using space to think about time allows humans to mentally organize temporal order. These origins are shaped by many environmental factors[118]––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.[119] 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.[119] Western calendars also continue this trend by placing the past on the left with the future progressing toward the right. Conversely, 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.[119] 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.[118] A recent study of the indigenous Yupno people of Papua New Guinea focused on the directional gestures used when individuals used time-related words.[118] 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.[118] 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.[120] 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.[120] 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.