Editing Arrow of time (section)

=== 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.