Editing Loschmidt's paradox (section)

== Arrow of time ==
{{main|Arrow of time}}

Any process that happens regularly in the forward direction of time but rarely or never in the opposite direction, such as entropy increasing in an isolated system, defines what physicists call an [[arrow of time]] in nature. This term only refers to an observation of an asymmetry in time; it is not meant to suggest an explanation for such asymmetries. Loschmidt's paradox is equivalent to the question of how it is possible that there could be a [[Entropy (arrow of time)|thermodynamic arrow of time]] given time-symmetric fundamental laws, since time-symmetry implies that for any process compatible with these fundamental laws, a reversed version that looked exactly like a film of the first process played backwards would be equally compatible with the same fundamental laws, and would even be equally probable if one were to pick the system's initial state randomly from the [[phase space]] of all possible states for that system.

Although most of the arrows of time described by physicists are thought to be special cases of the thermodynamic arrow, there are a few that are believed to be unconnected, like the cosmological arrow of time based on the fact that the universe is expanding rather than contracting, and the fact that a few processes in particle physics actually violate time-symmetry, while they respect a related symmetry known as [[CPT symmetry]]. In the case of the cosmological arrow, most physicists believe that entropy would continue to increase even if the universe began to contract{{Citation needed|date=April 2022}} (although the physicist [[Thomas Gold]] once proposed a model in which the thermodynamic arrow would reverse in this phase). In the case of the violations of time-symmetry in particle physics, the situations in which they occur are rare and are only known to involve a few types of [[meson]] particles. Furthermore, due to [[CPT symmetry]], reversal of the direction of time is equivalent to renaming particles as [[antiparticle]]s and ''vice versa''. Therefore, this cannot explain Loschmidt's paradox.