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48-411: Now most commonly refers to the present time. Now , NOW , or The Now may also refer to: Present It is sometimes represented as a hyperplane in space-time , typically called "now", although modern physics demonstrates that such a hyperplane cannot be defined uniquely for observers in relative motion. The present may also be viewed as a duration . Contemporary history describes

96-464: A quantum algorithm that reverses a given quantum state through complex conjugation of the state. Note that quantum decoherence merely allows the process of quantum wave collapse; it is a matter of dispute whether the collapse itself actually takes place or is redundant and apparent only. However, since the theory of quantum decoherence is now widely accepted and has been supported experimentally, this dispute can no longer be considered as related to

144-481: A consequence of the initial conditions in the early universe. Therefore, they ultimately result from the cosmological set-up. Waves, from radio waves to sound waves to those on a pond from throwing a stone, expand outward from their source, even though the wave equations accommodate solutions of convergent waves as well as radiative ones. This arrow has been reversed in carefully worked experiments that created convergent waves, so this arrow probably follows from

192-414: A different associated light cone. One has to conclude that in relativistic models of physics there is no place for "the present" as an absolute element of reality, and only refers to things that are close to us. Einstein phrased this as: "People like us, who believe in physics, know that the distinction between past, present, and future is only a stubbornly persistent illusion" . In physical cosmology ,

240-470: A given event, can not be in direct cause-effect relationship . Such collections of events are perceived differently by different observers. Instead, when focusing on "now" as the events perceived directly, not as a recollection or a speculation, for a given observer "now" takes the form of the observer's past light cone . The light cone of a given event is objectively defined as the collection of events in causal relationship to that event, but each event has

288-464: A system and its surrounding are thought to increase with entropy, and have been shown to be equivalent to it in a simplified case of a finite system interacting with the environment. The assumption of low initial entropy is indeed equivalent to assuming no initial correlations in the system; thus correlations can only be created as we move forward in time, not backwards. Controlling the future , or causing something to happen, creates correlations between

336-400: A team of Russian scientists reported the reversal of the quantum arrow of time on an IBM quantum computer , in an experiment supporting the understanding of the quantum arrow of time as emerging from the thermodynamic one. By observing the state of the quantum computer made of two and later three superconducting qubits , they found that in 85% of the cases, the two-qubit computer returned to

384-437: Is not time-reversible . According to the statistical notion of increasing entropy, the "arrow" of time is identified with a decrease of free energy. In his book The Big Picture , physicist Sean M. Carroll compares the asymmetry of time to the asymmetry of space: While physical laws are in general isotropic , near Earth there is an obvious distinction between "up" and "down", due to proximity to this huge body, which breaks

432-412: Is a time-reversible force. A ball that is tossed up, slows to a stop, and falls is a case where recordings would look equally realistic forwards and backwards. The system is T-symmetrical. However, the process of the ball bouncing and eventually coming to a stop is not time-reversible. While going forward, kinetic energy is dissipated and entropy is increased. Entropy may be one of the few processes that

480-480: Is normally impossible, by the second law of thermodynamics. In the language of relational quantum mechanics, the observer becomes entangled with the measured state, where this entanglement increases entropy. As stated by Seth Lloyd , "the arrow of time is an arrow of increasing correlations". However, under special circumstances, one can prepare initial conditions that will cause a decrease in decoherence and in entropy. This has been shown experimentally in 2019, when

528-465: The Anthropic bias ), with this arrow reversing as gravity pulls everything back into a Big Crunch . If this arrow of time is related to the other arrows of time, then the future is by definition the direction towards which the universe becomes bigger. Thus, the universe expands—rather than shrinks—by definition. The thermodynamic arrow of time and the second law of thermodynamics are thought to be

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576-656: The accelerating expansion of the universe has removed the local supercluster beyond the cosmological horizon (at about 150 billion years). In radiocarbon dating , the "present" is defined as AD 1950 . In English grammar , actions are classified according to one of the following twelve verb tenses: past ( past , past continuous , past perfect , or past perfect continuous ), present (present, present continuous , present perfect , or present perfect continuous ), or future ( future , future continuous , future perfect , or future perfect continuous ). The present tense refers to things that are currently happening or are always

624-494: The ego to be in front of or behind the past. There are no languages that place the past and future on a left–right axis (e.g., there is no expression in English such as *the meeting was moved to the left ), although at least English speakers associate the past with the left and the future with the right, which seems to have its origin in the left-to-right writing system. The words "yesterday" and "tomorrow" both translate to

672-431: The historical timeframe immediately relevant to the present time and is a certain perspective of modern history . You shouldn't chase after the past or place expectations on the future. What is past is left behind. The future is as yet unreached. Whatever quality is present you clearly see right there, right there. What we perceive as present is the vivid fringe of memory tinged with anticipation. "The present" raises

720-415: The inorganic nature of the earth and its thermodynamic processes. The cosmological arrow of time points in the direction of the universe's expansion. It may be linked to the thermodynamic arrow , with the universe heading towards a heat death (Big Chill) as the amount of Thermodynamic free energy becomes negligible. Alternatively, it may be an artifact of our place in the universe's evolution (see

768-407: The many-worlds interpretation and relational quantum mechanics interpretation). The theory of quantum decoherence explains why wave function collapse happens in a time-asymmetric fashion due to the second law of thermodynamics, thus deriving the quantum arrow of time from the thermodynamic arrow of time . In essence, following any particle scattering or interaction between two larger systems,

816-545: The Big Bang and the special circumstances that existed then. (Strictly speaking, the weak interactions are asymmetric to both spatial reflection and to flipping of the time direction. However, they do obey a more complicated symmetry that includes both.) In the 1928 book The Nature of the Physical World , which helped to popularize the concept, Eddington stated: Let us draw an arrow arbitrarily. If as we follow

864-838: The Second Law breaks down under any circumstances." However, there are a number of paradoxes regarding violation of the second law of thermodynamics , one of them due to the Poincaré recurrence theorem . This arrow of time seems to be related to all other arrows of time and arguably underlies some of them, with the exception of the weak arrow of time . Harold Blum 's 1951 book Time's Arrow and Evolution discusses "the relationship between time's arrow (the second law of thermodynamics) and organic evolution ." This influential text explores " irreversibility and direction in evolution and order, negentropy , and evolution ." Blum argues that evolution followed specific patterns predetermined by

912-416: The amount of order in an isolated system: as a system advances through time, it becomes more statistically disordered. This asymmetry can be used empirically to distinguish between future and past, though measuring entropy does not accurately measure time. Also, in an open system, entropy can decrease with time. An interesting thought experiment would be to ask: "if entropy was increased in an open system, would

960-436: The arrow of time flip in polarity and point towards the past." [citation required] British physicist Sir Alfred Brian Pippard wrote: "There is thus no justification for the view, often glibly repeated, that the Second Law of Thermodynamics is only statistically true, in the sense that microscopic violations repeatedly occur, but never violations of any serious magnitude. On the contrary, no evidence has ever been presented that

1008-429: The arrow of time in how their eyes perceived different stimuli. The arrow of time is the "one-way direction" or "asymmetry" of time. The thermodynamic arrow of time is provided by the second law of thermodynamics , which says that in an isolated system, entropy tends to increase with time. Entropy can be thought of as a measure of microscopic disorder; thus the second law implies that time is asymmetrical with respect to

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1056-448: The arrow of time question. Certain subatomic interactions involving the weak nuclear force violate the conservation of both parity and charge conjugation , but only very rarely. An example is the kaon decay . According to the CPT theorem , this means they should also be time-irreversible, and so establish an arrow of time. Such processes should be responsible for matter creation in

1104-606: The arrow we find more and more of the random element in the state of the world, then the arrow is pointing towards the future; if the random element decreases the arrow points towards the past. That is the only distinction known to physics . This follows at once if our fundamental contention is admitted that the introduction of randomness is the only thing which cannot be undone. I shall use the phrase 'time's arrow' to express this one-way property of time which has no analogue in space. Eddington then gives three points to note about this arrow: A related mental arrow arises because one has

1152-401: The case. For example, in the sentence, "she walks home everyday," the verb "walks" is in the present tense because it refers to an action that is regularly occurring in the present circumstances. Verbs in the present continuous tense indicate actions that are currently happening and will continue for a period of time. In the sentence, "she is walking home," the verb phrase "is walking" is in

1200-445: The causal relation per se cannot be perceived; one only perceives sequences of events. Furthermore, it is surprisingly difficult to provide a clear explanation of what the terms cause and effect really mean, or to define the events to which they refer. However, it does seem evident that dropping a cup of water is a cause while the cup subsequently shattering and spilling the water is the effect. Physically speaking, correlations between

1248-526: The cup, as well as the spilled water, and the object that caused the cup to drop. Quantum evolution is governed by equations of motions that are time-symmetric (such as the Schrödinger equation in the non-relativistic approximation), and by wave function collapse , which is a time-irreversible process, and is either real (by the Copenhagen interpretation of quantum mechanics ) or apparent only (by

1296-400: The diagram on the right was to portray a 3-dimensional object having access to the past, present, and future in the present moment (4th dimension). It follows from Albert Einstein 's Special Theory of Relativity that there is no such thing as absolute simultaneity . When care is taken to operationalise "the present", it follows that the events that can be labeled as "simultaneous" with

1344-453: The direction of time were to reverse, the theoretical statements that describe them would remain true. Yet at the macroscopic level it often appears that this is not the case: there is an obvious direction (or flow ) of time. The symmetry of time ( T-symmetry ) can be understood simply as the following: if time were perfectly symmetrical, a video of real events would seem realistic whether played forwards or backwards. Gravity , for example,

1392-402: The doer and the effect, and therefore the relation between cause and effect is a result of the thermodynamic arrow of time, a consequence of the second law of thermodynamics. Indeed, in the above example of the cup dropping, the initial conditions have high order and low entropy, while the final state has high correlations between relatively distant parts of the system – the shattered pieces of

1440-442: The future without us being determined to do it) since at least Boethius . Thomas Aquinas offers the metaphor of a watchman, representing God, standing on a height looking down on a valley to a road where past, present and future, represented by the individuals and their actions strung out along its length, are all visible simultaneously to God. Therefore, God's knowledge is not tied to any particular date. The original intent of

1488-411: The initial state. The state's reversal was made by a special program, similarly to the random microwave background fluctuation in the case of the electron . However, according to the estimations, throughout the age of the universe (13.7 billion years) such a reversal of the electron's state would only happen once, for 0.06  nanoseconds . The scientists' experiment led to the possibility of

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1536-418: The latter contradictory to the second law of thermodynamics in usual circumstances. A cause precedes its effect: the causal event occurs before the event it causes or affects. Birth, for example, follows a successful conception and not vice versa. Thus causality is intimately bound up with time's arrow. An epistemological problem with using causality as an arrow of time is that, as David Hume maintained,

1584-485: The organization of atoms , molecules , and bodies , and might be drawn upon a four-dimensional relativistic map of the world ("a solid block of paper"). The arrow of time paradox was originally recognized in the 1800s for gases (and other substances) as a discrepancy between microscopic and macroscopic description of thermodynamics / statistical Physics : at the microscopic level physical processes are believed to be either entirely or mostly time-symmetric : if

1632-517: The outer world and our brain (see correlations and the arrow of time ); and our present volitions and actions are causes of future events. This is because the increase of entropy is thought to be related to increase of both correlations between a system and its surroundings and of the overall complexity, under an appropriate definition; thus all increase together with time. Past and future are also psychologically associated with additional notions. English , along with other languages, tends to associate

1680-456: The past and is finished as of the current reference to the action. Finally, verbs in the present perfect continuous tense refer to actions that have been continuing up until the current time, thus combining the characteristics of both the continuous and perfect tenses. An example of a present perfect continuous verb phrase can be found in the sentence, "she has been walking this route for a week now," where "has been walking" indicates an action that

1728-479: The past being observed and the future being unobserved. Similarly, the Chinese term for "the day after tomorrow" 後天 ("hòutiān") literally means "after (or behind) day", whereas "the day before yesterday" 前天 ("qiántiān") is literally "preceding (or in front) day", and Chinese speakers spontaneously gesture in front for the past and behind for the future, although there are conflicting findings on whether they perceive

1776-462: The past with "behind" and the future with "ahead", with expressions such as "to look forward to welcoming you", "to look back to the good old times", or "to be years ahead". However, this association of "behind ⇔ past" and "ahead ⇔ future" is culturally determined. For example, the Aymara language associates "ahead ⇔ past" and "behind ⇔ future" both in terms of terminology and gestures, corresponding to

1824-416: The present continuous tense because it refers to a current action that will continue until a certain endpoint (when "she" reaches home). Verbs in the present perfect tense indicate actions that started in the past and is completed at the time of speaking. For example, in the sentence, "She has walked home," the verb phrase "has walked" is in the present perfect tense because it describes an action that began in

1872-429: The present moment are the happiest. A number of meditative techniques aim to help the practiser live in the present moment. Christianity views God as being outside of time and, from the divine perspective past, present and future are actualized in the now of eternity . This trans-temporal conception of God has been proposed as a solution to the problem of divine foreknowledge (i.e. how can God know what we will do in

1920-489: The present time in the chronology of the universe is estimated at 13.8 billion years after the singularity determining the arrow of time . In terms of the cosmic expansion history , it is in the dark-energy-dominated era , after the universe's matter content has become diluted enough for dark energy to dominate the total energy density. It is also in the universe's Stelliferous Era , after enough time for superclusters to have formed (at about 5 billion years), but before

1968-646: The question: "How is it that all sentient beings experience now at the same time?" There is no logical reason why this should be the case and no easy answer to the question. Buddhism and many of its associated paradigms emphasize the importance of living in the present moment—being fully aware of what is happening, and not dwelling on the past or worrying about the future . This does not mean that they encourage hedonism , but merely that constant focus on one's current position in space and time (rather than future considerations, or past reminiscence) will aid one in relieving suffering. They teach that those who live in

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2016-673: The realm of volition and action. We plan and often execute actions intended to affect the course of events in the future. From the Rubaiyat : The Moving Finger writes; and, having writ,   Moves on: nor all thy Piety nor Wit. Shall lure it back to cancel half a Line,   Nor all thy Tears wash out a Word of it. — Omar Khayyam (translation by Edward Fitzgerald ). In June 2022, researchers reported in Physical Review Letters finding that salamanders were demonstrating counter-intuitive responses to

2064-437: The relative phases of the two systems are at first orderly related, but subsequent interactions (with additional particles or systems) make them less so, so that the two systems become decoherent. Thus decoherence is a form of increase in microscopic disorder – in short, decoherence increases entropy. Two decoherent systems can no longer interact via quantum superposition , unless they become coherent again, which

2112-453: The same word in Hindi : कल ("kal"), meaning "[one] day remote from today." The ambiguity is resolved by verb tense. परसों ("parson") is used for both "day before yesterday" and "day after tomorrow", or "two days from today". तरसों ("tarson") is used for "three days from today" and नरसों ("narson") is used for "four days from today". The other side of the psychological passage of time is in

2160-445: The sense that one's perception is a continuous movement from the known past to the unknown future. This phenomenon has two aspects: memory (we remember the past but not the future) and volition (we feel we can influence the future but not the past). The two aspects are a consequence of the causal arrow of time: past events (but not future events) are the cause of our present memories, as more and more correlations are formed between

2208-497: The symmetry of space. Similarly, physical laws are in general symmetric to the flipping of time direction, but near the Big Bang (i.e., in the first many trillions of years following it ), there is an obvious distinction between "forward" and "backward" in time, due to relative proximity to this special event, which breaks the symmetry of time. Under this view, all the arrows of time are a result of our relative proximity in time to

2256-421: The thermodynamic arrow in that meeting the conditions to produce a convergent wave requires more order than the conditions for a radiative wave. Put differently, the probability for initial conditions that produce a convergent wave is much lower than the probability for initial conditions that produce a radiative wave. In fact, normally a radiative wave increases entropy, while a convergent wave decreases it, making

2304-554: Was happening continuously in the past and continues to happen continuously in the present. [REDACTED] Quotations related to present at Wikiquote Arrow of time The arrow of time , also called time's arrow , is the concept positing the "one-way direction" or " asymmetry " of time . It was developed in 1927 by the British astrophysicist Arthur Eddington , and is an unsolved general physics question . This direction, according to Eddington, could be determined by studying

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