In a cellular automaton

Cellular automaton

A cellular automaton is a discrete model studied in computability theory, mathematics, physics, complexity science, theoretical biology and microstructure modeling. It consists of a regular grid of cells, each in one of a finite number of states, such as "On" and "Off"...

 a puffer train, or simply puffer, is a finite pattern that moves itself across the "universe", leaving debris behind. Thus a pattern consisting of only a puffer will grow arbitrarily large over time. Puffers differ from spaceships in that spaceships do not leave debris behind. Nonetheless, puffers are considered to have periods and speeds in the same way that spaceships have.

The period of a puffer can be considered as the combination of two periods; the first is the period of the puffer itself, while the second is the apparent period of the pattern of debris produced. This is often the same as the period of the puffer, but sometimes is a factor of the period. A puffer for which the apparent period deduced from the debris is smaller than the period of the engine is a pseudoperiod puffer. Such puffers are typically produced by artificial means. A true period puffer is one in which the period of the debris matches that of the puffer.

Puffers are divided into two classes, dirty puffers and clean puffers. While there is no precise distinction, a dirty puffer is one in which there is little apparent order in the debris (although the debris will still eventually be periodic). A clean puffer, conversely, has a small amount of debris that appears much more organized. A dirty puffer can sometimes be turned into a clean puffer by adding spaceships of the same velocity as the puffer that affect what debris results.

A puffer whose debris consists entirely of spaceships is called a rake

Rake (CA)

A rake in a cellular automaton is a puffer that, instead of leaving behind a trail of debris, emits a stream of spaceships. 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...

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In Conway's Game of Life

The first known puffer, in Conway's Game of Life

Conway's Game of Life

The Game of Life, also known simply as Life, is a cellular automaton devised by the British mathematician John Horton Conway in 1970....

, was discovered by Bill Gosper

Bill Gosper

Ralph William Gosper, Jr. , known as Bill Gosper, is an American mathematician and programmer from Pennsauken Township, New Jersey...

; it is a dirty puffer, but eventually stabilizes to leave a pattern of debris that repeats every 140 generations. Since then, many puffers have been discovered for this cellular automaton, with many different speeds and periods. Puffers are significant for Life and related rules

Life-like cellular automaton

A cellular automaton is Life-like if it meets the following criteria:* The array of cells of the automaton has two dimensions....

 for three reasons: First, if they can be stabilized in such a way that they produce only gliders (that is, turned into rakes) they can be used as part of many more complex patterns such as breeders

Breeder (cellular automaton)

In Conway's Game of Life, a breeder is a pattern that exhibits quadratic growth, by generating multiple copies of a secondary pattern, each of which then generates multiple copies of a tertiary pattern.-Classification:...

. Second, stabilizations of puffers that eliminate all of their output debris can be used to produce spaceships with arbitrarily large periods. And third, puffers can sometimes be tamed or combined to form spaceships with speeds that do not seem to be achievable in other ways; for instance, in Life, the switch engine is a puffer train discovered by Charles Corderman that moves diagonally at speed c/12, and in 1991 Dean Hickerson showed how to combine several switch engines to form a c/12 spaceship that he called the Cordership.