Spaceship

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The lightweight spaceship in Conway's Game of Life
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. 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. All known spaceships in life travel either orthogonally (only horizontal or vertical displacement) or diagonally (equal horizontal and vertical displacement) at one of the twelve known speeds; however, spaceships traveling in other directions and at different speeds have been constructed in other two dimensional cellular automata[3], and it is known that Life has spaceships that travel in all rational directions at arbitrarily slow speeds (see universal constructor).

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 program that uses iterative deepening depth-first searching[6], and 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. The caterpillar's design is unique in that it is made entirely of simple component parts and reactions, including many gliders and small orthogonal c/2 spaceships[7].

The most recently discovered spaceship speed is that of the diagonal c/6 seal, found by Nicolay Beluchenko in 2005[8].

Speed First discovered Discoverer Year of discovery
c/2 lightweight spaceship John Conway 1970
c/3  ? Dean Hickerson 1989
c/4  ? Dean Hickerson 1989
c/5 snail Tim Coe 1996
2c/5  ? Dean Hickerson 1991
c/6 dragon Paul Tooke 2000
2c/7 weekender David Eppstein 2000
17c/45 caterpillar Gabriel Nivasch 2004
c/4 Diagonal glider Richard Guy 1970
c/5 Diagonal 295P5H1V1.1 Jason Summers 2000
c/6 Diagonal seal Nicolay Beluchenko 2005
c/12 Diagonal 13-engine Cordership Dean Hickerson 1991

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; these additional components are often referred to as tagalongs, and spaceships created using these reactions are known as flotilla. A well known example of spaceship tagalong is the overweight spaceship, which is unstable on its own but may be 'escorted' by two smaller spaceships.

Speed Direction Smallest known Minimum # of cells
c/4 diagonal glider 5
c/5 diagonal 67P5H1V1 67
c/6 diagonal seal 170
c/12 diagonal 4-engine Cordership 134
c/2 orthogonal lightweight spaceship 9
c/3 orthogonal 25P3H1V0.1, 25P3H1V0.2 25
c/4 orthogonal 46P4H1V0 46
2c/5 orthogonal 30P5H2V0 30
c/5 orthogonal spider 58
c/6 orthogonal dragon 102
2c/7 orthogonal weekender 36
17c/45 orthogonal caterpillar 11880063

See also

Notes

  1. "Glider". The Life Lexicon. Stephen Silver. Retrieved on April 18, 2009.
  2. "Fish". The Life Lexicon. Stephen Silver. Retrieved on April 18, 2009.
  3. "Gliders in Life-Like Cellular Automata". David Eppstein. Retrieved on April 18, 2009.
  4. Tim Coe. "c/5 Orthogonal spaceship". Paul's Page of Conway's Life Miscellany. Retrieved on April 18, 2009.
  5. David Bell. "New c/5 spaceship". Paul's Page of Conway's Life Miscellany. Retrieved on April 18, 2009.
  6. David Eppstein. "Searching for Spaceships (PDF)". Retrieved on April 18, 2009.
  7. Gabriel Nivasch. "The 17c/45 Caterpillar spaceship". Gabriel Nivasch's Game of Life page.
  8. "c/6 Diagonal Spaceship". Game of Life News. Heinrich Koenig.

External links