Editing Causality (physics) (section)

== Macroscopic causality ==
In classical physics, an effect cannot occur ''before'' its cause which is why solutions such as the advanced time solutions of the [[Liénard–Wiechert potential]] are discarded as physically meaningless. In both Einstein's theory of special and general relativity, causality means that an effect cannot occur from a cause that is not in the back (past) [[light cone]] of that event. Similarly, a cause cannot have an effect outside its front (future) light cone. These restrictions are consistent with the constraint that [[mass]] and [[energy]] that act as causal influences cannot travel faster than the speed of light and/or backwards in time. In [[quantum field theory]], observables of events with a [[Spacelike curve|spacelike]] relationship, "elsewhere", have to [[Commutative property|commute]], so the order of observations or measurements of such observables do not impact each other.

Another requirement of causality is that cause and effect be mediated across space and time (requirement of ''contiguity''). This requirement has been very influential in the past, in the first place as a result of direct observation of causal processes (like pushing a cart), in the second place as a problematic aspect of Newton's theory of gravitation (attraction of the earth by the sun by means of [[Action at a distance (physics)|action at a distance]]) replacing mechanistic proposals like [[Mechanical explanations of gravitation#Vortex Theory|Descartes' vortex theory]]; in the third place as an incentive to develop dynamic [[Classical field theory|field theories]] (e.g., [[Electromagnetism#Classical electrodynamics|Maxwell's electrodynamics]] and [[Einstein field equations|Einstein's general theory of relativity]]) restoring contiguity in the transmission of influences in a more successful way than in Descartes' theory.