Editing Arrow of time (section)

=== Radiative arrow of time ===
Waves, from [[radio waves]] to [[sound waves]] to those on a pond from throwing a stone, expand outward from their source, even though the [[wave equation]]s accommodate solutions of convergent waves as well as radiative ones. This arrow has been reversed in carefully worked experiments that created convergent waves,<ref>{{cite web |url=http://www4.ncsu.edu/~fouque/fink.pdf|archive-url=https://web.archive.org/web/20051231022842/http://www4.ncsu.edu/~fouque/fink.pdf |archive-date=31 December 2005 |title=Time-Reversed Acoustic |author=Mathias Fink |date=30 November 1999 |access-date=27 May 2016 |author-link=Mathias Fink }}</ref> so this arrow probably follows from the thermodynamic arrow in that meeting the conditions to produce a convergent wave requires more order than the conditions for a radiative wave. Put differently, the probability for initial conditions that produce a convergent wave is much lower than the probability for initial conditions that produce a radiative wave. In fact, normally a radiative wave increases entropy, while a convergent wave decreases it,{{Citation needed|date=May 2010}}<!-- ref>{{cite book |author1=Nikolai Chernov |author2=Roberto Markarian |year=2006 |title=Chaotic Billiards |publisher=American Mathematical Soc. |page=207 |isbn=978-0-8218-4096-2}}</ref --> making the latter contradictory to the second law of thermodynamics in usual circumstances.