A rake in a cellular automaton is a puffer that, instead of leaving behind a trail of debris, emits a stream of spaceships.[1] In Conway's Game of Life, the discovery of rakes was one of the key components needed to form the breeder, the first known patterns in Life in which the number of live cells exhibits quadratic growth. A breeder is formed by arranging several rakes so that the gliders they generate interact to form a sequence of glider guns, the gliders from which fill a growing triangle of the plane.[2] More generally, when a rake exists for a cellular automaton rule, one can often construct puffers which leave trails of many other kinds of objects, by colliding the streams of spaceships emitted by multiple rakes moving in parallel.[3] As David Bell writes,[4]
| “ | They are extremely important in Life because the output can be used to construct other objects and can pass signals around to perform logic operations. Whenever any new puffer engine is found an important goal is to "tame" it so that its useless "dirty" exhaust is converted into "clean" exhaust, particularly gliders. | ” |
The first rake to be discovered, in the early 1970s, was the "space rake", which moves with speed c/2, emitting a glider every 20 steps.[5] For Life, rakes are now known that move orthogonally with speeds c/2, c/3, c/4, c/5, 2c/5, and 17c/45, and diagonally with speeds c/4 and c/12, with many different periods.[6] Rakes are also known for some other Life-like cellular automata including HighLife,[7] Day & Night,[8] and Seeds.[9]
Gotts (1980) shows that the space rake in Life can be formed by a "standard collision sequence" in which a single glider interacts with a widely separated set of 3-cell initial seeds (blinkers and blocks). As a consequence, he finds lower bounds on the probability that these patterns form in any sufficiently sparse and sufficiently large random initial condition for Life. This result leads to standard collision sequences for many other patterns such as breeders.[10]
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