Glider synthesis (or glider construction) is the construction of an object by means of glider collisions. It is generally assumed that the gliders should be arranged so that they could come from infinity - that is, gliders should not have had to pass through one another to achieve the initial arrangement.
Glider syntheses for all still lifes and known oscillators with at most 14 cells were found by David Buckingham. A collaborative effort then completed glider syntheses of all still lifes with 17 or fewer cells in 2014.
Features of syntheses
Four main characterizing features of a synthesis are the geometry, reaction speed, reaction multiplicity, and glider cost.
The geometry is the number of directions of incoming gliders:
- four-directional: gliders collide from all four directions
- three-directional: gliders collide from all directions but one
- two-directional; further divisible in head-on and 90° syntheses. All two-glider syntheses are necessarily two-directional.
- unidirectional, which assumes the initial presence of a target (usually a still life or an oscillator) to be hit with gliders.
Since gliders are themselves glider-constructible, any multidirectional synthesis can be technically downgraded to a fewer-directional one, usually at the cost of increasing the speed, multiplicity and cost of the synthesis. More challenging is finding a two- or three-directional synthesis for a particular object where few or no parts of the synthesis reactions extend outside the final pattern's bounding box in a particular direction. This is especially important for the synthesis of temporary catalysts, which will need to be placed sometimes quite close to other components without perturbing them. For especially tight locations, sometimes it will be useful to construct an LWSS (or another standard c/2 spaceship) some ways away from the synthesis nexus and let that collide with a glider in the final stages; this allows synthesis at a 45° angle, rather than a 90° angle as required for synthesis by gliders from separate directions.
The speed is simply the number of generations it takes to complete a synthesis. For multi-stage syntheses, each stage has its own speed.
The multiplicity is the number of stages a synthesis operates in. Often a particular synthesis operation cannot be achieved by a direct collision of gliders, and a synthesis procedure instead requires first synthesizing a number of catalysts, and then hitting these with gliders to produce the final result.
The cost is the number of gliders expended over the course of the synthesis. Much as speed, it can be defined also for individual synthesis stages.
Of particular interest is slow salvo synthesis: unidirectional synthesis where every stage has a glider cost of one. Perhaps surprisingly, anything that is glider synthesizable is also slow salvo synthesizable; a result that crucially depends on the existence of movable targets and splitters.
Syntheses of notespaceships, because these can be used to create corresponding guns and rakes. Many of the c/2 spaceships that are based on standard spaceships have been synthesized, mostly by Mark Niemiec. In June 1998, Stephen Silver found syntheses for some of the Corderships (although it was not until July 1999 that Jason Summers used this to build a Cordership gun). In May 2000, Noam Elkies suggested that a 2c/5 spaceship (60P5H2V0) found by Tim Coe in May 1996 might be a candidate for glider synthesis. Initial attempts to construct a synthesis for this spaceship got fairly close, but it was only in March 2003 that Summers and Elkies managed to find a way to perform the crucial last step. Summers then used the new synthesis to build a c/2 forward rake for the 2c/5 spaceship; this was the first example in Life of a rake which fires spaceships that travel in the same direction as the rake but more slowly.
During late 2014 and early 2015, a number of new spaceship syntheses were found, including the dart, crab, 25P3H1V0.2, 30P5H2V0, x66, and weekender. Pushalong 1 was reduced to glider collisions with a constellation, though a full syntheses is not yet available. Most of this was due to the work of Martin Grant.
A 3-glider synthesis of a pentadecathlon was found in April 1997 by Heinrich Koenig, which came as a surprise because it was widely assumed that anything using just three gliders would already be known.
There are 71 distinct 2-glider collisions, of which 28 produce nothing, six produce a block, five produce a honey farm, three produce a B-heptomino, three produce a pi-heptomino, three produce a blinker, three produce a traffic light, two produce a glider, two produce a pond, two produce a loaf and a blinker, one produces a boat, one produces a beehive, one produces a loaf, one produces an eater 1, one produces lumps of muck, one produces a teardrop, one produces an interchange, one produces a traffic light and a glider, one produces an octomino, one produces a bi-block, one produces four blocks, one produces two blocks, one produces a blinker, loaf, tub and block, and one produces the so-called two-glider mess, a methuselah stabilizing after 530 generations and consisting of four gliders, eight blinkers (including a traffic light), four blocks, a beehive and a ship.
- "Constructions Known for All Still Lifes up to 17 Bits". Game of Life News. Dave Greene. Retrieved on September 17, 2014.
- "17-bit SL Syntheses (100% Complete!)". ConwayLife.com forums. Martin Grant. Retrieved on September 17, 2014.
- Dean Hickerson's Life page with four pages of glider syntheses
- Glider synthesis at Eric Weisstein's Treasure Trove of Life
- Glider synthesis at the Life Lexicon