# Spaceship

A **spaceship** (much less commonly referred to as a **glider**^{[1]} or a **fish**^{[2]}) is a finite pattern that returns to its initial state after a number of generations (known as its period) but in a different location.

## Spaceship Speed

*Main article: Speed*

The speed of a spaceship is the number of cells that the pattern moves during its period divided by the period length. This is expressed in terms of *c* (the metaphorical "speed of light") which is one cell per generation; thus, a spaceship with a period of five that moves two cells to the left during its period travels at the speed of 2c/5. Until the construction of Gemini in May 2010, all known spaceships in Life traveled either orthogonally (only horizontal or vertical displacement) or diagonally (equal horizontal and vertical displacement). Other recently constructed large spaceships such as the waterbear and the Parallel HBK. It is known that Life has spaceships that travel in all rational directions at arbitrarily slow speeds (see universal constructor). Spaceships traveling in other directions and at different speeds have also been constructed in other two dimensional cellular automata.^{[3]}

## Spaceship types

*Main article: Spaceship types*

Although Spaceships are most commonly categorized by their speed and direction, other categorizations have been applied to spaceships based on their appearances, components, or other properties. One such categorization is the symmetry of spaceships: spaceships can be bilaterally symmetric, Such as Tim Coe's snail, may exhibit glide symmetry, such as the glider, or may be asymmetric, such as the caterpillar. Other, somewhat subjective categorizations have also been made, such as greyships, spaceships filled with large amounts of static, live cells, or smoking ships, which produce large sparks, a notable example being the Schick engine. A spaceship may also support other components which would not function as spaceships on their own. Given a freestanding spaceship, such additional components are often referred to as tagalongs; perhaps even pushalongs. Unstable spaceships immersed in a sustaining cloud are known as flotillae. A well known example is that of the overweight spaceships, which are unstable alone but may be 'escorted' by two or more smaller spaceships.

Speed | Direction | Smallest known | Minimum # of cells | |
---|---|---|---|---|

c/2 | orthogonal | lightweight spaceship | 9 | |

2c/5 | orthogonal | 30P5H2V0 | 30 | |

17c/45 | orthogonal | caterpillar | 11,880,063 | |

c/3 | orthogonal | 25P3H1V0.2 | 25 | |

2c/7 | orthogonal | weekender | 36 | |

c/4 | orthogonal | 37P4H1V0 | 37 | |

2c/9 | orthogonal | caterloopillar 2c/9 | ? | |

c/5 | orthogonal | spider | 58 | |

2c/11 | orthogonal | caterloopillar 2c/11 | ? | |

c/6 | orthogonal | 56P6H1V0 | 56 | |

c/7 | orthogonal | loafer | 20 | |

31c/240 | orthogonal | centipede | 620,901 | |

c/8 | orthogonal | caterloopillar | 232,815 | |

c/9 | orthogonal | caterloopillar c/9 | ? | |

c/10 | orthogonal | copperhead | 28 | |

c/11 | orthogonal | caterloopillar c/11 | ? | |

c/12 | orthogonal | c/12 caterloopillar | ? | |

(5120,1024)c/16849793 | slope 5 | Gemini | 846,278 | |

(23,5)c/79 | slope 23/5 | waterbear | 197,896 | |

(3072,1024)c/36000001 | slope 3 | Gemini 3 | <=850932 | |

(4096,2048)c/17783745 | slope 2 | Gemini 2 | <=872252 | |

(6,3)c/245912 | slope 2 | Parallel HBK | 132,945 | |

(6,3)c/2621440 | slope 2 | Half-baked knightship | 1,049,395 | |

c/4 | diagonal | glider | 5 | |

c/5 | diagonal | 58P5H1V1 | 58 | |

c/6 | diagonal | 77P6H1V1 | 77 | |

c/7 | diagonal | lobster | 83 | |

c/12 | diagonal | 4-engine Cordership | 134 | |

65c/438856 | diagonal | 0hd Demonoid | 27,250 | |

65c/818356 | diagonal | 10hd Demonoid | 47,701 |

## History

The four smallest spaceships in life, the glider, lightweight spaceship, middleweight spaceship and heavyweight spaceship, were all found by hand in 1970. For almost twenty years spaceship development was limited to adding tagalongs to the known c/2 spaceships. Significant advances in spaceship technology came in 1989, when Dean Hickerson began using automated searches based on a depth-first backtracking algorithm. These searches found orthogonal spaceships with speeds of c/3, c/4, and 2c/5, as well as the first spaceship other than the glider to travel at the speed of c/4 diagonally, dubbed the big glider. Hickerson also found several ways to combine switch engines to create the first diagonal c/12 spaceships, named Corderships in honour of Charles Corderman. The next spaceship speed to be discovered was that of the orthogonal c/5 snail, found by Tim Coe in 1996, with a program he had designed using breadth-first searching, and which could split tasks between multiple CPUs.^{[4]} In the following year David Bell found the much smaller c/5 spider using lifesrc, a program based on Hickerson's search algorithm.^{[5]}

In March of 1998 David Eppstein created gfind, a breadth-first program that uses a depth-first search to limit the size of the search queue.^{[6]} In 2000 he used this program to find the first spaceship that travels at the speed of 2c/7 orthogonally, the weekender. A search by Paul Tooke using the same program found the first c/6 orthogonal spaceship, the dragon, later that year. Also in 2000, Jason Summers found the first c/5 diagonal spaceship using David Bell's lifesrc program.

In 2004 Gabriel Nivasch, with the help of Jason Summers and David Bell, finished construction on the caterpillar, the first known orthogonal 17c/45 spaceship, which made use of the 17c/45 reaction.

In May 2010 Andrew J. Wade created a universal constructor-based spaceship, Gemini, which travels at a speed of (5120,1024)c/33699586.^{[7]} This was the first explicitly constructed spaceship in Life to travel in an oblique direction, and also yielded the first explicit method of constructing arbitrarily slow spaceships.

In Febuary 2013, the first c/7 orthogonal spaceship, loafer, was discovered by Josh Ball.

In July 2014 several half-bakery-based knightships were constructed using a new technique that did not require universal-constructor circuitry. These produced spaceships that were both much slower and much smaller than the Gemini variants.

In September 2014, Dave Greene and Chris Cain completed two 31c/240 orthogonal spaceships, along the same general lines as the original Caterpillar but using a number of new mechanisms.

In December 2014, an oblique caterpillar dubbed the Waterbear was completed by Brett Berger, traveling at (23,5)c/79.

In December 2015, Chris Cain completed a diagonal self-constructing spaceship -- a "0hd Demonoid" -- based on Geminoid technology, adapted from a larger 10hd version constructed in November in collaboration with Dave Greene.

In March 2016, forum user zdr discovered an extremely small c/10 spaceship, named the Copperhead.

## See also

- List of spaceships
- Spaceship flotilla
- Spaceship periods status table
- Spaceship terminology
- Types of spaceships
- Velocities of spaceships and related objects status table

## Notes

- ↑ "Glider".
*The Life Lexicon*. Stephen Silver. Retrieved on April 18, 2009. - ↑ "Fish".
*The Life Lexicon*. Stephen Silver. Retrieved on April 18, 2009. - ↑ "Gliders in Life-Like Cellular Automata". David Eppstein. Retrieved on April 18, 2009.
- ↑ Tim Coe. "c/5 Orthogonal spaceship". Paul's Page of Conway's Life Miscellany. Retrieved on April 18, 2009.
- ↑ David Bell. "New c/5 spaceship". Paul's Page of Conway's Life Miscellany. Retrieved on April 18, 2009.
- ↑ David Eppstein. "Searching for Spaceships (PDF)". Retrieved on April 18, 2009.
- ↑ Adam P. Goucher (May 19, 2010). "Oblique Life spaceship created".
*Game of Life News*. Retrieved on May 21, 2010.

## External links

- Spaceship at the Life Lexicon