Editing Arrow of time (section)

== Arrows ==
=== Psychological/perceptual arrow of time ===
{{Main|Time perception}}

A related mental arrow arises because one has 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 (psychology)|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 the outer world and our brain (see [[Entropy (arrow of time)#Correlations|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<ref name=elv/> and of the overall complexity, under an appropriate definition;<ref>Ladyman, J.; Lambert, J.; Weisner, K.B. What is a Complex System? Eur. J. Philos. Sci. 2013, 3, 33–67.</ref> thus all increase together with time.

Past and future are also psychologically associated with additional notions. [[English language|English]], along with other languages, tends to associate 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.<ref name=":0">{{Cite journal|last1=Ulrich|first1=Rolf|last2=Eikmeier|first2=Verena|last3=de la Vega|first3=Irmgard|last4=Ruiz Fernández|first4=Susana|last5=Alex-Ruf|first5=Simone|last6=Maienborn|first6=Claudia|date=2012-04-01|title=With the past behind and the future ahead: Back-to-front representation of past and future sentences|journal=Memory & Cognition|language=en|volume=40|issue=3|pages=483–495|doi=10.3758/s13421-011-0162-4|pmid=22160871|issn=1532-5946|doi-access=free}}</ref> For example, the [[Aymara language idiosyncrasies|Aymara language]] associates "ahead ⇔ past" and "behind ⇔ future" both in terms of terminology and gestures, corresponding to the past being observed and the future being unobserved.<ref>{{Cite web |title=(6/13/2006) For Andes Tribe, It's Back To The Future |url=http://www.albionmonitor.com/0606a/aymara.html |access-date=2023-09-13 |website=www.albionmonitor.com}}</ref><ref>{{cite web |last1=Núñez |first1=Rafael E. |last2=Sweetser |first2=Eve |title=With the Future Behind Them: Convergent Evidence From Aymara Language and Gesture in the Crosslinguistic Comparison of Spatial Construals of Time |url=http://www.cogsci.ucsd.edu/~nunez/web/FINALpblshd.pdf |archive-url=https://web.archive.org/web/20200121131545/http://www.cogsci.ucsd.edu/%7Enunez/web/FINALpblshd.pdf |archive-date=21 January 2020 |access-date=8 March 2020 |website=Department of Cognitive Science, University of California at San Diego}}</ref> Similarly, the [[Chinese language|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 the [[Ego (Freudian)|ego]] to be in front of or behind the past.<ref>{{Cite journal|last1=Gu|first1=Yan|last2=Zheng|first2=Yeqiu|last3=Swerts|first3=Marc|date=2019|title=Which Is in Front of Chinese People, Past or Future? The Effect of Language and Culture on Temporal Gestures and Spatial Conceptions of Time|journal=Cognitive Science|language=en|volume=43|issue=12|pages=e12804|doi=10.1111/cogs.12804|issn=1551-6709|pmc=6916330|pmid=31858627}}</ref><ref>[https://www.mdbg.net/chindict/chindict.php?page=worddict&wdrst=0&wdqb=day+before+yesterday mbdg.net Chinese-English Dictionary] — accessed 2017-01-11</ref> There are no languages that place the past and future on a [[x-axis|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.<ref name=":0" />

The words "yesterday" and "tomorrow" both translate to the same word in [[Hindi]]: कल ("kal"),<ref>{{cite book|last=Bahri|first=Hardev|title=Learners' Hindi-English Dictionary|year=1989|publisher=Rajpal & Sons|location=Delhi|isbn=978-81-7028-002-6|page=95}}</ref> meaning "[one] day remote from today."<ref>{{cite book|last=Alexiadou|first=Artemis|title=Adverb placement: a case study in antisymmetric syntax|year=1997|publisher=Benjamins|location=Amsterdam [u.a.]|isbn=978-90-272-2739-3|page=108}}</ref> 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".<ref>[http://hindi-english.org/index.php?input=%E0%A4%AA%E0%A4%B0%E0%A4%B8%E0%A5%8B%E0%A4%82&trans=Translate&direction=AU Hindi-English.org Hindi English Dictionary परसों] — accessed 2017-01-11</ref>

तरसों ("tarson") is used for "three days from today"<ref>{{Cite web |url=https://shabdkosh.raftaar.in/Meaning-of-%E0%A4%A4%E0%A4%B0%E0%A4%B8%E0%A5%8B%E0%A4%82-in-Hindi#gsc.tab=0 |title=Meaning of तरसों in Hindi &#124; Hindi meaning of तरसों (तरसों ka Hindi Matlab) |access-date=2021-09-11 |archive-date=2021-09-11 |archive-url=https://web.archive.org/web/20210911213835/https://shabdkosh.raftaar.in/Meaning-of-%E0%A4%A4%E0%A4%B0%E0%A4%B8%E0%A5%8B%E0%A4%82-in-Hindi#gsc.tab=0 |url-status=dead }}</ref> and नरसों ("narson") is used for "four days from today".

The other side of the psychological passage of time is in 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 of Omar Khayyam|Rubaiyat]]:
<blockquote>
<poem>
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.
</poem>
— [[Omar Khayyam]] (translation by [[Edward FitzGerald (poet)|Edward Fitzgerald]]).
</blockquote>

In June 2022, researchers reported<ref name="2022-06-22_PRL">{{Cite journal |last1=Lynn |first1=Christopher W. |last2=Holmes |first2=Caroline M. |last3=Bialek |first3=William |last4=Schwab |first4=David J. |date=2022-09-06 |title=Decomposing the Local Arrow of Time in Interacting Systems |journal=Physical Review Letters |volume=129 |issue=11 |pages=118101 |doi=10.1103/PhysRevLett.129.118101|pmid=36154397 |pmc=9751844 |arxiv=2112.14721 |bibcode=2022PhRvL.129k8101L }}</ref> in ''[[Physical Review Letters]]'' finding that [[salamander]]s were demonstrating [[counter-intuitive]] responses to the arrow of time in how their eyes perceived different stimuli.{{clarify|date=June 2023}}
=== Thermodynamic arrow of time ===
{{Main|Entropy as an arrow of time}}
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 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 the arrow of time flip in polarity and point towards the past." [citation required]

British physicist [[Brian Pippard|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 the Second Law breaks down under any circumstances."<ref>A. B. Pippard, Elements of Classical Thermodynamics for Advanced Students of Physics (1966), p. 100.</ref> However, there are a number of [[paradox]]es{{which|date=June 2023}} regarding [[second law of thermodynamics#Irreversibility|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 [[#Particle physics (weak) arrow of time|weak arrow of time]].{{clarify|date=April 2019|reason=anchor lost results in unhelpful self-reference, clarification of the weak arrow is needed}}

[[Harold F. Blum|Harold Blum]]'s 1951 book ''Time's Arrow and Evolution''<ref name="Blum1951">{{cite book|last=Blum|first=Harold F.|author-link=Harold F. Blum|title=Time's Arrow and Evolution|url=https://books.google.com/books?id=tmcNnwEACAAJ|edition=First|year=1951|publisher=Princeton University Press |isbn=978-0-691-02354-0}}</ref> 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]]."<ref name=arrow-review>{{cite journal | doi = 10.1016/0019-1035(69)90059-1 | title= Book review: Time's arrow and evolution: Third Edition | journal = Icarus | volume = 11 | issue = 2 | pages = 278–279 | last = Morowitz | first = Harold J. | date = September 1969|bibcode = 1969Icar...11..278M | pmc = 2599115 }}</ref> Blum argues that evolution followed specific patterns predetermined by the [[inorganic chemistry|inorganic]] nature of the earth and its thermodynamic processes.<ref name=yale>{{cite journal | pmc = 2599115 | title= Book reviews: Time's Arrow and Evolution | journal = [[Yale Journal of Biology and Medicine]] | volume = 24 | issue = 2 | pages = 164 | last = McN. | first = W. P. | date = November 1951}}</ref>

=== Cosmological arrow of time ===
{{See also|Entropy|Entropy as an arrow of time|Past hypothesis}}
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 of the Universe|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 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 a consequence of the [[initial conditions]] in the early universe.<ref>{{cite web|last1=Susskind|first1=Leonard|title=Boltzmann and the Arrow of Time: A Recent Perspective|url=http://www.cornell.edu/video/leonard-susskind-1-boltzmann-and-the-arrow-of-time|website=Cornell University|access-date=June 1, 2016}}</ref> Therefore, they ultimately result from the cosmological set-up.

=== Radiative arrow of time ===
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 equation]]s accommodate solutions of convergent waves as well as radiative ones. This arrow has been reversed in carefully worked experiments that created convergent waves,<ref>{{cite web |url=http://www4.ncsu.edu/~fouque/fink.pdf|archive-url=https://web.archive.org/web/20051231022842/http://www4.ncsu.edu/~fouque/fink.pdf |archive-date=31 December 2005 |title=Time-Reversed Acoustic |author=Mathias Fink |date=30 November 1999 |access-date=27 May 2016 |author-link=Mathias Fink }}</ref> so this arrow probably follows from 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,{{Citation needed|date=May 2010}}<!-- ref>{{cite book |author1=Nikolai Chernov |author2=Roberto Markarian |year=2006 |title=Chaotic Billiards |publisher=American Mathematical Soc. |page=207 |isbn=978-0-8218-4096-2}}</ref --> making the latter contradictory to the second law of thermodynamics in usual circumstances.

=== Causal arrow of time ===
A [[causality|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, 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 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.<ref name=elv>Esposito, M., [[Katja Lindenberg|Lindenberg, K.]], & Van den Broeck, C. (2010). Entropy production as correlation between system and reservoir. New Journal of Physics, 12(1), 013013.</ref> 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 [[Entropy (arrow of time)#Correlations|correlations]] between the doer and the effect,<ref>''Physical Origins of Time Asymmetry'', pp. 109–111.</ref> 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.<ref>''Physical Origins of Time Asymmetry'', chapter 6</ref> 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 the cup, as well as the spilled water, and the object that caused the cup to drop.

=== Quantum arrow of time ===
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 only physically real in explicit collapse interpretations of quantum theory, such as the [[ Diósi–Penrose model]], the [[Ghirardi–Rimini–Weber theory]], or the [[Transactional interpretation]], which uses the direct-action or "absorber" theory of fields. 

The conventional approach is to assume that [[quantum decoherence]] explains irreversibility and the second law of thermodynamics, thus claiming to derive the quantum arrow of time from the [[Entropy (arrow of time)|thermodynamic arrow of time]]; however this is a matter of some debate, since the underlying dynamics is assumed to be unitary and thus reversible.
<ref name="Kastner 2014">Kastner, R. E. (2014). ‘Einselection’ of pointer observables: The new H-theorem? Stud. Hist. Philos. Mod. Phys. 48, 56-58.</ref> 
A conventional account of decoherence is to say that following any particle [[scattering]] or interaction between two larger systems, the relative [[phase (waves)|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{{snd}} in short, decoherence increases entropy. Two decoherent systems can no longer interact via [[quantum superposition]], unless they become coherent again, which is normally impossible, by the second law of thermodynamics.<ref name="Schlosshauer">Schlosshauer, M. (2005). Decoherence, the measurement problem, and interpretations of quantum mechanics. Reviews of Modern physics, 76(4), 1267.</ref> 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".<ref name = wolchover >{{cite magazine |url=https://www.wired.com/2014/04/quantum-theory-flow-time/ |title=New Quantum Theory Could Explain the Flow of Time |first=Natalie |last=Wolchover |magazine=Wired |date=25 April 2014 |via=www.wired.com}}</ref><ref name="iqoqi">Univ of Bristol [https://scitechdaily.com/time-reversal-phenomenon-in-the-quantum-realm-not-even-time-flows-as-you-might-expect/ (26 Nov 2021) Time-Reversal Phenomenon: In the Quantum Realm, Not Even Time Flows As You Might Expect] Lead: Professor Caslav Brukner: "quantum systems can simultaneously evolve along two opposite time arrows — both forward and backward in time".</ref>

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 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.<ref name="nature">{{cite journal |author=Lesovik |first1=G. B. |last2=Sadovskyy |first2=I. A. |last3=Suslov |first3=M. V. |last4=Lebedev |first4=A. V. |last5=Vinokur |first5=V. M. |date=13 March 2019 |title=Arrow of time and its reversal on the IBM quantum computer |journal=Nature |volume=9 |issue=1 |page=4396 |arxiv=1712.10057 |bibcode=2019NatSR...9.4396L |doi=10.1038/s41598-019-40765-6 |pmc=6416338 |pmid=30867496 |s2cid=3527627}}</ref> By observing the state of the quantum computer made of two and later three [[Superconducting quantum computing#Qubit archetypes|superconducting qubits]], they found that in 85% of the cases, the two-qubit computer returned to the initial state.<ref name="phys">{{cite web |url=https://phys.org/news/2019-03-physicists-reverse-quantum.html |title=Physicists reverse time using quantum computer |publisher=[[Phys.org]] |date=13 March 2019 |access-date=13 March 2019}}</ref> The state's reversal was made by a special program, similarly to the random [[microwave background]] fluctuation in the case of the [[electron]].<ref name="phys"/> However, according to the estimations, throughout the [[age of the universe]] (13.7&nbsp;billion years) such a reversal of the electron's state would only happen once, for 0.06&nbsp;[[nanoseconds]].<ref name="phys" /> The scientists' experiment led to the possibility of a [[quantum algorithm]] that reverses a given [[quantum state]] through [[complex conjugation]] of the state.<ref name="nature"/>

Note that quantum decoherence merely allows the appearance of quantum wave collapse (based on the vanishing of diagonal elements of the density matrix); it is a matter of dispute whether the collapse itself actually takes place or is redundant and apparent only. While the theory of quantum decoherence is widely accepted and has been supported experimentally at the level of the applicable density matrix, the conventional theory's inability to predict actual measurement outcomes via non-unitary collapse remains. That is, the density matrix obtained from standard unitary-only decoherence (without actual collapse) is an improper mixture that cannot be interpreted as reflecting a determinate measurement outcome. Thus the arrow of time question continues to be addressed by way of explicit collapse approaches.<ref name="Kastner 2017">Kastner, R. E. (2017). On Quantum Collapse as a Basis for the Second Law of Thermodynamics, Entropy 19(3)106.</ref>

=== Particle physics (weak) arrow of time ===
{{Main|CP violation}}
Certain subatomic interactions involving the [[weak nuclear force]] violate the conservation of both [[Parity (physics)|parity]] and [[charge conjugation]], but only very rarely. An example is the [[kaon]] [[Particle decay|decay]].<ref>{{cite web|url=https://physicsworld.com/p/|title=Home|website=Physics World|date=11 March 2008}}</ref> According to the [[CPT symmetry|CPT theorem]], this means they should also be time-irreversible, and so establish an arrow of time. Such processes should be responsible for [[Baryogenesis|matter creation]] in the early universe.

That the combination of parity and charge conjugation is broken so rarely means that this arrow only "barely" points in one direction, setting it apart from the other arrows whose direction is much more obvious. This arrow had not been linked to any large-scale temporal behaviour until the work of [[Joan Vaccaro]], who showed that T violation could be responsible for conservation laws and dynamics.<ref>{{Cite journal|last=Vaccaro|first=Joan|date=2016|title=Quantum asymmetry between time and space|journal=Proceedings of the Royal Society A|doi=10.1098/rspa.2015.0670|pmid=26997899|pmc=4786044|volume=472|issue=2185|pages=20150670|arxiv = 1502.04012 |bibcode = 2016RSPSA.47250670V }}</ref>