Editing Gun (cellular automaton) (section)

{{short description|Cellular automaton pattern that emits spaceships}}
[[Image:Gospers glider gun.gif|thumb|Gosper glider gun shooting ''gliders'']]

In a [[cellular automaton]], a '''gun''' is a pattern with a main part that repeats periodically, like an [[Oscillator (cellular automaton)|oscillator]], and that also periodically emits [[spaceship (cellular automaton)|spaceship]]s. There are then two periods that may be considered: the period of the spaceship output, and the period of the gun itself, which is necessarily a multiple of the spaceship output's period. A gun whose period is larger than the period of the output is a '''pseudoperiod gun'''.

[[Image:Day and night.gif|thumb|left|A gun and an "antigun" in the Life variation [[Day and Night (cellular automaton)|Day & Night]]]]
[[Image:game of life glider gun.svg|thumb|The first gun to be found in [[Conway's Game of Life]] was the Gosper glider gun]]

In the [[Conway's Game of Life|Game of Life]], for every ''p'' greater than or equal to 14, it is possible to construct a [[glider (Conway's Life)|glider]] gun in which the gliders are emitted with period ''p''.<ref>{{cite web |url=http://entropymine.com/jason/life/status.html |title=Game of Life Status page |work=Entropymine.com |first=Jason |last=Summers |access-date=February 5, 2011}}</ref>

Since guns continually emit spaceships, the existence of guns in Life means that initial patterns with finite numbers of cells can eventually lead to configurations with limitless numbers of cells, something that [[John Horton Conway|John Conway]] himself originally conjectured to be impossible.  However, according to Conway's later testimony,<ref>{{cite web |url=https://www.youtube.com/watch?v=E8kUJL04ELA  |archive-url=https://ghostarchive.org/varchive/youtube/20211222/E8kUJL04ELA |archive-date=2021-12-22 |url-status=live|title=Does John Conway hate his Game of Life? |access-date=April 16, 2015}}{{cbignore}}</ref> this conjecture was explicitly intended to encourage someone to disprove it – i.e., Conway hoped that infinite-growth patterns did exist.

[[Bill Gosper]] discovered the first glider gun in 1970, earning $50 from Conway. The discovery of the glider gun eventually led to the proof that Conway's Game of Life could function as a [[Turing machine]].<ref>{{cite book |title=The Colossal Book of Mathematics |publisher=W. W. Norton |location=New York |first=Martin |last=Gardner |year=2001 |isbn=0-393-02023-1}}</ref> For many years this glider gun was the smallest one known in Life,<ref>{{cite web|url=http://www.conwaylife.com/ref/lexicon/lex_g.htm#gosperglidergun|title=Gosper glider gun|author=Stephen A. Silver|publisher=The Life Lexicon|access-date=July 12, 2009}}</ref>  although other rules had smaller guns.
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