La « '''flèche du temps''' » est l'expression introduite en 1927 par Eddington pour décrire le fait que le temps nous semble s'écouler toujours dans la même direction. La question de la flèche du temps est compliquée par le fait que les équations fondamentales de la physique sont bien souvent invariantes lorsqu'on renverse la direction du temps. Un problème fondamental de la physique théorique est d'expliquer pourquoi, à partir d'une dynamique microscopique réversible, notre perception macroscopique du temps choisit une direction particulière. 

== Aspects thermodynamiques ; entropie ==

Le Boltzmann, avoir une entropie faible est une chose à très faible probabilité.  L'entropie plutôt basse de notre présent Univers serait le résultat d'une entropie encore plus basse, d'un état encore moins probable.  La flèche du temps est donc bel et bien parallèle à l'entropie : Celle-ci a considérablement plus de chance de grandir.

== Aspects quantiques ==

Selon la physique quantique, le temps serait une succession de "clichés" de l'univers, qui mis bout à bout, formeraient l'histoire de l'univers, le "film cosmique". La flèche du temps serait le fait que ces "clichés" se passent dans un certain sens.  Si la flèche du temps n'existait pas, tous les changements ne pourraient plus avoir lieu car le temps ne s'écoulerait plus régulièrement.  Le temps s'écoulerait dans un sens ou dans l'autre, très rapidement ou très lentement avec des variations énormes.  Un peu comme sur le Mur de Planck

== Voir aussi ==

:''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).''

In the natural sciences, '''arrow of time''', or '''time’s arrow''', is a term coined in 1927 by British astronomer Arthur Eddington used to distinguish a direction of time on a four-dimensional relativistic map of the world; which, according to Eddington, can be determined by a study of organizations of atoms, molecules, and bodies.  

Physical processes at the time symmetric, meaning that the theoretical statements that describe them remain true if the direction of time is reversed; yet when we describe things at the macroscopic level it often appears that this is not the case: there is an obvious direction (or ''flow'') of time.  An '''arrow of time''' is anything that exhibits such time-asymmetry.

==History of term==
From the 1928 book ''The Nature of the Physical World'', which helped to popularize the term, Eddington states: 

Eddington then gives four points to note about this arrow:

#It is vividly recognized by consciousness.
#It is equally insisted on by our reasoning faculty, which tells us that a reversal of the arrow would render the external world nonsensical.
#It makes no appearance in physical science except in the study of organization of a number of individuals.
#It is definite within the make up of the Universe.

Here, according to Eddington, the arrow indicates the direction of progressive increase of the random element.  Following a lengthy argument into the nature of thermodynamics, Eddington concludes that in so far as physics is concerned time's arrow is a property of entropy alone.

== Overview ==
The symmetry of time (T-symmetry) can be understood by a simple analogy: if time were perfectly symmetric then it would be possible to watch a movie taken of real events and everything that happens in the movie would seem realistic whether it was played forwards or backwards.

For example, a movie showing a cup falling off a table seems realistic when run forwards, but seems unrealistic if run backwards. On the other hand, a movie of the planets orbiting the sun would look equally realistic run forwards or backwards; either way the orbital motions would appear to conform to physical laws.

=== An example of irreversibility ===

Consider the situation in which a large container is filled with two separated liquids, for example a dye on one side and water on the other. With no barrier between the two liquids, the random jostling of their molecules will result in them becoming more mixed as time passes. However, if the dye and water are mixed then one does not expect them to separate out again when left to themselves.  A movie of the mixing would seem realistic when played forwards, but unrealistic when played backwards.

If the large container is observed early on in the mixing process, it might be found to be only partially mixed. It would be reasonable to conclude that, without outside intervention, the liquid reached this state because it was more ordered in the past, when there was greater separation, and will be more disordered, or mixed, in the future.

Now imagine that the experiment is repeated, this time with only a few molecules, perhaps ten, in a very small container. One can easily imagine that by watching the random jostling of the molecules it might occur &mdash; by chance alone &mdash; that the molecules became neatly segregated, with all dye molecules on one side and all water molecules on the other. That this can be expected to occur from time to time can be concluded from the fluctuation theorem; thus it is not impossible for the molecules to segregate themselves. However, for a large numbers of molecules it is so unlikely that one would have to wait, on average, many times longer than the age of the universe for it to occur. Thus a movie that showed a large number of molecules segregating themselves as described above would appear unrealistic and one would be inclined to say that the movie was being played in reverse.

See also another example.

== Arrows ==
=== The thermodynamic arrow of time ===

The '''thermodynamic arrow of time''' is provided by the disorder; thus the Second Law implies that time is asymmetrical with respect to the amount of order in an isolated system: as time increases, a system will always become more disordered. This asymmetry can be used empirically to distinguish between future and past.

Since the Second Law is statistical, it does not hold with strict universality: any system can fluctuate to a state of lower entropy (see the Poincaré recurrence theorem). However, the Second Law seems to accurately describe the overall trend in real systems toward higher entropy.

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.

=== The cosmological arrow of time ===

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 usable 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. Therefore they ultimately result from the cosmological set-up.