Arrow of time

Source: Wikipedia, the free encyclopedia.
For other uses, see Arrow of time (disambiguation).
This article is an overview of the subject. For a more technical discussion and for information related to current research, see
Entropy (arrow of time)
.
"The Arrow of Time" redirects here. For the soundtrack for Timelapse of the Future, see Timelapse of the Future § Soundtrack.
Sir Arthur Stanley Eddington (1882–1944)

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 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").[1]

The arrow of time paradox was originally recognized in the 1800s for gases (and other substances) as a discrepancy between

macroscopic
level it often appears that this is not the case: there is an obvious direction (or flow) of time.

Overview

The symmetry of time (

not time-reversible. According to the statistical notion of increasing entropy, the "arrow" of time is identified with a decrease of free energy.[3]

In his book

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 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.)[citation needed
]

Conception by Eddington

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 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:

  1. It is vividly recognized by consciousness.
  2. It is equally insisted on by our reasoning faculty, which tells us that a reversal of the arrow would render the external world nonsensical.
  3. It makes no appearance in
    arising from a system
    .)

Arrows

Psychological/perceptual arrow of time

Main article: 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:

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[4] and of the overall complexity, under an appropriate definition;[5]
thus all increase together with time.

Past and future are also psychologically associated with additional notions.

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.[6]

The words "yesterday" and "tomorrow" both translate to the same word in Hindi: कल ("kal"),[11] meaning "[one] day remote from today."[12] 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".[13]

तरसों ("tarson") is used for "three days from today"[14] 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:

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

counter-intuitive responses to the arrow of time in how their eyes perceived different stimuli.[clarification needed
]

Thermodynamic arrow of time

Main article: 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.

British physicist

which?] 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.[clarification needed]

irreversibility and direction in evolution and order, negentropy, and evolution."[18] Blum argues that evolution followed specific patterns predetermined by the inorganic nature of the earth and its thermodynamic processes.[19]

Cosmological arrow of time

See also: Entropy, Entropy as an arrow of time, and Past hypothesis

The cosmological arrow of time points in the direction of the universe's expansion. It may be linked to 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.[20]
Therefore, they ultimately result from the cosmological set-up.

Radiative arrow of time

Waves, from

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,[21] 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
] making the latter contradictory to the second law of thermodynamics in usual circumstances.

Causal arrow of time

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, 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.

thermodynamic arrow of time, a consequence of the second law of thermodynamics.[23]
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 either real (by the Copenhagen interpretation of quantum mechanics) or apparent only (by the many-worlds interpretation and relational quantum mechanics interpretation).

The theory of

thermodynamic arrow of time. In essence, following any particle scattering or interaction between two larger systems, 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 is normally impossible, by the second law of thermodynamics.[24] 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".[25][26]

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

complex conjugation of the state.[27]

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 the arrow of time question.[24]

Particle physics (weak) arrow of time

Main article: CP violation

Certain subatomic interactions involving the

charge conjugation, but only very rarely. An example is the kaon decay.[29] 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 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.[30]

See also

References

  1. ISBN 978-3-540-21374-1
    .
  2. ^ David Albert on Time and Chance
  3. doi:10.1098/rspa.2008.0494
    .
  4. ^ a b Esposito, M., Lindenberg, K., & Van den Broeck, C. (2010). Entropy production as correlation between system and reservoir. New Journal of Physics, 12(1), 013013.
  5. ^ Ladyman, J.; Lambert, J.; Weisner, K.B. What is a Complex System? Eur. J. Philos. Sci. 2013, 3, 33–67.
  6. ^
    PMID 22160871
    .
  7. ^ "(6/13/2006) For Andes Tribe, It's Back To The Future". www.albionmonitor.com. Retrieved 2023-09-13.
  8. ^ Núñez, Rafael E.; Sweetser, Eve. "With the Future Behind Them: Convergent Evidence From Aymara Language and Gesture in the Crosslinguistic Comparison of Spatial Construals of Time" (PDF). Department of Cognitive Science, University of California at San Diego. Archived from the original (PDF) on 21 January 2020. Retrieved 8 March 2020.
  9. PMID 31858627
    .
  10. ^ mbdg.net Chinese-English Dictionary — accessed 2017-01-11
  11. ISBN 978-81-7028-002-6
    .
  12. ISBN 978-90-272-2739-3
    .
  13. ^ Hindi-English.org Hindi English Dictionary परसों — accessed 2017-01-11
  14. ^ "Meaning of तरसों in Hindi | Hindi meaning of तरसों (तरसों ka Hindi Matlab)". Archived from the original on 2021-09-11. Retrieved 2021-09-11.
  15. PMID 36154397
    .
  16. ^ A. B. Pippard, Elements of Chemical Thermodynamics for Advanced Students of Physics (1966), p. 100.
  17. ISBN 978-0-691-02354-0
    .
  18. PMC 2599115
    .
  19. PMC 2599115
    .
  20. ^ Susskind, Leonard. "Boltzmann and the Arrow of Time: A Recent Perspective". Cornell University. Retrieved June 1, 2016.
  21. ^ Mathias Fink (30 November 1999). "Time-Reversed Acoustic" (PDF). Archived from the original (PDF) on 31 December 2005. Retrieved 27 May 2016.
  22. ^ Physical Origins of Time Asymmetry, pp. 109–111.
  23. ^ Physical Origins of Time Asymmetry, chapter 6
  24. ^ a b Schlosshauer, M. (2005). Decoherence, the measurement problem, and interpretations of quantum mechanics. Reviews of Modern physics, 76(4), 1267.
  25. ^ Wolchover, Natalie (25 April 2014). "New Quantum Theory Could Explain the Flow of Time". Wired – via www.wired.com.
  26. ^ Univ of Bristol (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".
  27. ^
    S2CID 3527627
    .
  28. ^ a b c "Physicists reverse time using quantum computer". Phys.org. 13 March 2019. Retrieved 13 March 2019.
  29. ^ "Home". Physics World. 11 March 2008.
  30. PMID 26997899
    .

Further reading

External links

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