Editing Arrow of time (section)

== Overview ==
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.<ref>[http://www.isepp.org/Pages/01-02%20Pages/Albert.html David Albert on ''Time and Chance'']</ref> [[Gravity]], for example, 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 is [[Irreversibility|not time-reversible]]. According to the statistical notion of increasing entropy, the "arrow" of time is identified with a decrease of free energy.<ref>{{cite journal
| author=Tuisku, P. |author2=Pernu, T.K. |author3=Annila, A. | title=In the light of time
| journal=Proceedings of the Royal Society A | volume=465 |issue=2104 | pages=1173–1198 | year=2009
|doi=10.1098/rspa.2008.0494|bibcode = 2009RSPSA.465.1173T | doi-access=free }}</ref>

In his book ''[[The Big Picture (Carroll 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 the symmetry of space. Similarly, [[physical law]]s are in general symmetric to the flipping of time direction, but near the [[Big Bang]] (i.e., in the [[Future of an expanding universe|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 interaction]]s are asymmetric to both spatial reflection and to flipping of the time direction. However, they do obey a [[CPT symmetry|more complicated symmetry]] that includes both.){{cn|date=June 2023}}