:0hd Demonoid See Demonoid.

:101 (p5) Found by Achim Flammenkamp in August 1994. The name was suggested by Bill Gosper, noting that the phase shown below displays the period in binary.

:10hd Demonoid See Demonoid.

:119P4H1V0 (c/4 orthogonally, p4) A spaceship discovered by Dean Hickerson in December 1989, the first spaceship of its kind to be found. Hickerson then found a small tagalong for this spaceship which could be attached to one side or both. These three variants of 119P4H1V0 were the only known c/4 orthogonal spaceships until July 1992 when Hartmut Holzwart discovered a larger spaceship, 163P4H1V0.

:1-2-3 (p3) Found by Dave Buckingham, August 1972. This is one of only three essentially different p3 oscillators with only three cells in the rotor. The others are stillater and cuphook.

:1-2-3-4 (p4) See also Achim's p4.

:135-degree MWSS-to-G The following converter, discovered by Matthias Merzenich in July 2013. It accepts an MWSS as input, and produces an output glider travelling at a 135-degree angle relative to the input direction.

:14-ner = fourteener

:17c/45 spaceship A spaceship travelling at seventeen forty-fifths of the speed of light. This was the first known macro-spaceship speed. See Caterpillar for details.

:180-degree kickback The only other two-glider collision besides the standard kickback that produces a clean output glider with no leftover ash. The 180-degree change in direction is occasionally useful in glider synthesis, but is rarely used in signal circuitry or in self-supporting patterns like the Caterpillar or Centipede, because 90-degree collisions are generally much easier to arrange.

:1G seed See seed.

:(2,1)c/6 spaceship A knightship that travels obliquely at the fastest possible speed. To date the only known example of a spaceship with this velocity is Sir Robin.

:(23,5)c/79 Herschel climber The following glider-supported Herschel climber reaction used in the self-supporting waterbear knightship, which can be repeated every 79 ticks, moving the Herschel 23 cells to the right and 5 cells upward, and releasing two gliders to the northwest and southwest. As the diagram shows, it is possible to substitute a loaf or other still lifes for some or all of the support gliders. This fact is used to advantage at the front end of the waterbear.

:24-cell quadratic growth A 39786×143 quadratic growth pattern found by Michael Simkin in October 2014, two days after 25-cell quadratic growth and a week before switch-engine ping-pong.

:25-cell quadratic growth A 25-cell quadratic growth pattern found by Michael Simkin in October 2014, with a bounding box of 21372×172. It was the smallest-population quadratic growth pattern for two days, until the discovery of 24-cell quadratic growth. It superseded wedge, which had held the record for eight years. See switch-engine ping-pong for the lowest-population superlinear growth pattern as of July 2018, along with a list of the record-holders.

:25P3H1V0.1 (c/3 orthogonally, p3) A spaceship discovered by Dean Hickerson in August 1989. It was the first c/3 spaceship to be discovered. In terms of its 25 cells, it is tied with 25P3H1V0.2 as the smallest c/3 spaceship. Unlike 25P3H1V0.2, it has a population of 25 in all of its phases, as well as a smaller bounding box.

:25P3H1V0.2 (c/3 orthogonally, p3) A spaceship discovered by David Bell in early 1992, with a minimum of 25 cells - the lowest number of cells known for any c/3 spaceship. A note in Spaceships in Conway's Life indicates that it was found with a search that limited the number of live cells in each column, and possibly also the maximum cross-section (4 cells in this case). See also edge-repair spaceship for a very similar c/3 spaceship with a minimum population of 26.

:26-cell quadratic growth = wedge.

:295P5H1V1 (c/5 diagonally, p5) The first spaceship of its type to be discovered, found by Jason Summers on 22 November 2000.

:2c/3 Two thirds of the speed of light - the speed of signals in a 2c/3 wire or of some against the grain negative spaceship signals in the zebra stripes agar, and also the speed of burning of the blinker fuse and the bi-block fuse.

:2c/3 wire A wire discovered by Dean Hickerson in March 1997, using his dr search program. It supports signals that travel through the wire diagonally at two thirds of the speed of light.

Each 2c/3 signal is made up of two half-signals that can be separated from each other by an arbitrary number of ticks.

Considerable effort has been spent on finding a way to turn a 2c/3 signal 90 or 180 degrees, since this would by one way to prove Life to be omniperiodic. There is a known 2c/3 converter shown under signal elbow, which converts a standard 2c/3 signal into a double-length signal. This is usable in some situations, but unfortunately it fails when its input is a double-length signal, so it can't be used to complete a loop.

Noam Elkies discovered a glider synthesis of a reaction that can repeatably insert a signal into the upper end of a 2c/3 wire. See stable pseudo-Heisenburp for details. On 11 September 2017, Martin Grant reduced the input reaction to five gliders, or three gliders plus a Herschel. With the Herschel option the recovery time is 152 ticks.

See also 5c/9 wire.

:2c/5 spaceship A spaceship travelling at two fifths of the speed of light. The only such spaceships that are currently known travel orthogonally. Examples include 30P5H2V0, 44P5H2V0, 60P5H2V0, and 70P5H2V0. As of June 2018, only 30P5H2V0 and 60P5H2V0 have known glider synthesis recipes.

:2c/7 spaceship A spaceship travelling at two sevenths of the speed of light. The only such spaceships that are currently known travel orthogonally. The first to be found was the weekender, found by David Eppstein in January 2000. See also weekender distaff.

:2 eaters = two eaters

:2-engine Cordership The smallest known Cordership, with a minimum population of 100 cells, discovered by Aidan F. Pierce on 31 December 2017. Luka Okanishi produced a 9-glider synthesis of the spaceship on the same day.

:2-glider collision Two gliders can react with each other in many different ways, either at right angles, or else head-on. A large number of the reactions cleanly destroy both gliders leaving nothing. Many of the remaining reactions cleanly create some common objects, and so are used as the first steps in glider synthesis or as part of constructing interesting objects using rakes. Only a small number of collisions can be considered dirty due to creating multiple objects or a mess.

Here is a list of the possible results along with how many different ways they can occur (ignoring reflections and rotations).

	-------------------------------
	result     right-angle  head-on
	-------------------------------
	nothing             11       17
	beehive              1        0
	B-heptomino          1        2
	bi-block             1        0
	blinker              2        1
	block                3        3
	boat                 0        1
	eater1               1        0
	glider               1        1
	honey farm           3        2
	interchange          1        0
	loaf                 0        1
	lumps of muck        1        0
	octomino             0        1
	pi-heptomino         2        1
	pond                 1        1
	teardrop             1        0
	traffic light        2        1
	four skewed blocks   0        1
	dirty                6        0
	-------------------------------

:2-glider mess A constellation made up of eight blinkers, four blocks, a beehive and a ship, plus four emitted gliders, created by the following 2-glider collision.

:30P5H2V0 (2c/5 orthogonally, p5) A spaceship discovered by Paul Tooke on 7 December 2000. With just 30 cells, it is currently the smallest known 2c/5 spaceship. A glider synthesis for 30P5H2V0 was found by Martin Grant in January 2015, based on a predecessor by Tanner Jacobi.

:31c/240 The rate of travel of the 31c/240 Herschel-pair climber reaction, and Caterpillar-type spaceships based on that reaction. Each Herschel travels 31 cells orthogonally every 240 ticks.

:31c/240 Herschel-pair climber The mechanism defining the rate of travel of the Centipede and shield bug spaceships. Compare pi climber. It consists of a pair of Herschels climbing two parallel chains of blocks. Certain spacings between the block chains allow gliders from each Herschel to delete the extra ash objects produced by the other Herschel. Two more gliders escape, one to each side, leaving only an exact copy of the original block chains, but shifted forward by 9 cells:

:3c/7 spaceship A spaceship travelling at three sevenths of the speed of light. The only such spaceships that are currently known travel orthogonally. The first to be found was the spaghetti monster, found by Tim Coe in June 2016.

:3-engine Cordership See Cordership.

:44P5H2V0 (2c/5 orthogonally, p5) A spaceship discovered by Dean Hickerson on 23 July 1991, the first 2c/5 spaceship to be found. Small tagalongs were found by Robert Wainwright and David Bell that allowed the creation of arbitrarily large 2c/5 spaceships. These were the only known 2c/5 spaceships until the discovery of 70P5H2V0 in December 1992.

:45-degree LWSS-to-G = 45-degree MWSS-to-G.

:45-degree MWSS-to-G The following small converter, which accepts an MWSS or LWSS as input and produces an output glider travelling at a 45-degree angle relative to the input direction.

:4-8-12 diamond The following pure glider generator.

:4 boats (p2)

:4F = Fast Forward Force Field. This term is no longer in common use.

:4g-to-5g reaction A reaction involving 4 gliders which cleanly produces 5 gliders. The one shown below was found by Dieter Leithner in July 1992:

The first two gliders collide to produce a traffic light and glider. The other two gliders react symmetrically with the evolving traffic light to form four gliders. A glider gun can be built by using reflectors to turn four of the output gliders so that they repeat the reaction.

:56P6H1V0 (c/6 orthogonally, p6) A 56-cell spaceship discovered by Hartmut Holzwart in 2009, the smallest known c/6 orthogonal spaceship as of July 2018.

:58P5H1V1 (c/5 diagonally, p5) A spaceship discovered by Matthias Merzenich on 5 September 2010. In terms of its minimum population of 58 cells it is the smallest known c/5 diagonal spaceship. It provides sparks at its trailing edge which can perturb gliders, and this property was used to create the first c/5 diagonal puffers. These sparks also allow the attachment of tagalongs which was used to create the first c/5 diagonal wickstretcher in January 2011.

:5c/9 wire A wire discovered by Dean Hickerson in April 1997, using his dr search program. It supports signals that travel through the wire diagonally at five ninths of the speed of light. See also 2c/3 wire.

:60P312 (p312) Found by Dave Greene, 1 November 2004, based on 92P156.

:60P5H2V0 (2c/5 orthogonally, p5) A 60-cell spaceship discovered by Tim Coe in May 1996. It was the first non-c/2 orthogonal spaceship to be successfully constructed via glider synthesis.

:67P5H1V1 (c/5 diagonally, p5) A spaceship discovered by Nicolay Beluchenko in July 2006. It was the smallest known c/5 diagonal spaceship until the discovery of 58P5H1V1 in September 2010.

:70P5H2V0 (2c/5 orthogonally, p5) A spaceship discovered by Hartmut Holzwart on 5 December 1992.

:7x9 eater A high-clearance eater5 variant that can suppress passing gliders in tight spaces, such as on the inside corner of an R64 Herschel conduit. Like the eater5 and sidesnagger, the 7×9 eater is able to eat gliders coming from two directions, though this ability is not commonly used.

:83P7H1V1 = lobster

:86P5H1V1 (c/5 diagonally, p5) A spaceship discovered by Jason Summers on January 8, 2005. It was the smallest known c/5 diagonal spaceship until the discovery of 67P5H1V1 in July 2006.

:90-degree kickback See kickback reaction.

:92P156 (p156) Discovered by Jason Summers on October 31, 2004. It is actually an eight-barrel glider gun, with all output gliders suppressed by eater1s. Replacing each pair of eater1s with a beehive doubles the period and produces 60P312.

:9hd Separated by 9 half diagonals. Specifically used to describe the distance between the two construction lanes in the linear propagator.

:Achim's p144 (p144) This was found (minus the blocks shown below) on a cylinder of width 22 by Achim Flammenkamp in July 1994. Dean Hickerson reduced it to a finite form using figure-8s the same day. The neater finite form shown here, replacing the figure-8s with blocks, was found by David Bell in August 1994. See factory for a use of this oscillator.

:Achim's p16 (p16) Found by Achim Flammenkamp, July 1994.

:Achim's p4 (p4) Dave Buckingham found this in a less compact form (using two halves of sombreros) in 1976. The form shown here was found by Achim Flammenkamp in 1988. The rotor is two copies of the rotor of 1-2-3-4, so the oscillator is sometimes called the "dual 1-2-3-4".

:Achim's p5 = pseudo-barberpole

:Achim's p8 (p8) Found by Achim Flammenkamp, July 1994.

:acorn (stabilizes at time 5206) A methuselah found by Charles Corderman. It has a final population of 633 and covers an area of 215 by 168 cells, not counting the 13 gliders it produces. Its ash consists of typical stable objects and blinkers, along with the relatively rare mango and a temporary eater1.

:A for all (p6) Found by Dean Hickerson in March 1993.

:against the grain A term used for negative spaceships travelling in zebra stripes agar, perpendicular to the stripes, and also for against-the-grain grey ships.

Below is a sample signal, found by Hartmut Holzwart in April 2006, that travels against the grain at 2c/3. This "negative spaceship" travels upward and will quickly reach the edge of the finite patch of stabilized agar shown here.

Holzwart proved in 2006 that 2c/3 is the maximum speed at which signals can move non-destructively against the grain through zebra stripes agar.

:against-the-grain grey ship A grey ship in which the region of density 1/2 consists of lines of ON cells lying perpendicular to the direction in which the spaceship moves. See also with-the-grain grey ship.

:agar Any pattern covering the whole plane that is periodic in both space and time. The simplest (nonempty) agar is the stable one extended by the known spacefillers. For some more examples see chicken wire, houndstooth agar, onion rings, squaredance and Venetian blinds. Tiling the plane with the pattern O......O produces another interesting example: a p6 agar which has a phase of density 3/4, which is the highest yet obtained for any phase of an oscillating pattern. See lone dot agar for an agar composed of isolated cells.

:aircraft carrier (p1) This is the smallest still life that has more than one island.

:airforce (p7) Found by Dave Buckingham in 1972. The rotor consists of two copies of that used in the burloaferimeter.

:AK47 reaction The following reaction (found by Rich Schroeppel and Dave Buckingham) in which a honey farm predecessor, catalysed by an eater and a block, reappears at another location 47 generations later, having produced a glider and a traffic light. This was in 1990 the basis for the Dean Hickerson's construction of the first true p94 gun, and for a very small (but pseudo) p94 glider gun found by Paul Callahan in July 1994. (The original true p94 gun was enormous, and has now been superseded by comparatively small Herschel loop guns and Mike Playle's tiny AK94 gun.)

:AK94 gun The smallest known gun using the AK47 reaction, found by Mike Playle in May 2013 using his Bellman program.

:Al Jolson = Jolson

:almost knightship A promising partial result discovered by Eugene Langvagen in March 2004. This was an early near miss in the ongoing search for a small elementary (2,1)c/6 knightship. After six generations, only two cells are incorrect.

:almosymmetric (p2) Found in 1971.

:ambidextrous A type of Herschel transceiver where the receiver can be used in either of two mirror-image orientations. See also chirality.

:anteater A pattern that consumes ants. Matthias Merzenich discovered a c/5 anteater on 15 April 2011. See wavestretcher for details.

:antlers = moose antlers

:ants (p5 wick) The standard form is shown below. It is also possible for any ant to be displaced by one or two cells relative to either or both of its neighbouring ants. Dean Hickerson found fenceposts for both ends of this wick in October 1992 and February 1993. See electric fence, and also wickstretcher.

:antstretcher Any wickstretcher or wavestretcher that stretches ants. Nicolay Beluchenko and Hartmut Holzwart constructed the following small extensible antstretcher in January 2006:

:anvil The following induction coil.

:apgluxe See apgsearch

:apgmera See apgsearch.

:apgnano See apgsearch.

:apgsearch One of several versions of a client-side Ash Pattern Generator soup search script by Adam P. Goucher, for use with Conway's Life and a wide variety of other rules. Development of the original Golly-based Python script started in August 2014. After the addition in 2016 of apgnano (native C++) and apgmera (self-modifying, 256-bit SIMD compatibility), development continues in 2017 with apgluxe (Larger Than Life and Generations rules, more soup shapes). Several customized variants of the Python script have also been created by other programmers, to perform types of searches not supported by Goucher's original apgsearch 1.×.

All of these versions of the search utility work with a "haul" that usually consists of many thousands or millions of random soup patterns. Each soup is run to stability, and detailed object census results are reported to Catagolue. For any rare objects discovered in the ash, the source soup can be easily retrieved from the Catagolue server.

:APPS (c/5 orthogonally, p30) An asymmetric PPS. The same as the SPPS, but with the two halves 15 generations out of phase with one another. Found by Alan Hensel in May 1998.

:ark A pair of mutually stabilizing switch engines. The archetype is Noah's ark. The diagram below shows an ark found by Nick Gotts that takes until generation 736692 to stabilize, and can therefore be considered as a methuselah.

:arm A long extension, sometimes also called a "wing", hanging off from the main body of a spaceship or puffer perpendicular to the direction of travel. For example, here is a sparking c/3 spaceship which contains two arms.

For an alternate meaning see construction arm.

:armless A method of generating slow salvos across a wide range of lanes without using a construction arm with a movable elbow. Instead, streams of gliders on two fixed opposing lanes collide with each other to produce clean 90-degree output gliders. Slowing down one of the streams by 8N ticks will move the output lanes of the gliders toward the source of that stream by N full diagonals. This construction method was used to create the supporting slow salvos in the half-baked knightships, and also in the Parallel HBK gun.

:ash The stable or oscillating objects left behind when a chaotic reaction stabilizes, or "burns out". Experiments show that for random soups with moderate initial densities (say 0.25 to 0.5) the resulting ash has a density of about 0.0287. (This is, of course, based on what happens in finite fields. In infinite fields the situation may conceivably be different in the long run because of the effect of certain initially very rare objects such as replicators.)

:asynchronous Indicates that precise relative timing is not needed for two or more input signals entering a circuit, or two or more sets of gliders participating in a glider synthesis. In some cases the signals or sets of gliders can arrive in any order at all - i.e., they have non-overlapping effects.

However, in some cases such as slow salvo constructions, there is a required order for some of the incoming signals. These signals can still be referred to as "asynchronous" because the number of ticks between them is infinitely adjustable: arbitrarily long delays can be added with no change to the final result. Compare synchronized.

:aVerage (p5) Found by Dave Buckingham, 1973. The average number of live rotor cells is five (V), which is also the period.

:B = B-heptomino

:B29 (c/4 diagonally, p4) The following spaceship, found by Hartmut Holzwart in April 2004. A glider synthesis of this spaceship was completed by Tanner Jacobi in April 2015.

:B-52 bomber The following p104 double-barrelled glider gun. It uses a B-heptomino and emits one glider every 52 generations. It was found by Noam Elkies in March 1996, except that Elkies used blockers instead of molds, the improvement being found by David Bell later the same month.

:B60 A Herschel conduit discovered by Michael Simkin in 2015 using his search program, CatForce. It is one of two known Blockic elementary conduits. After 60 ticks, it produces a Herschel rotated 180 degrees at (-6,-10) relative to the input. It can most easily be connected to another B60 conduit, producing a closed loop, the Simkin glider gun.

:babbling brook Any oscillator whose rotor consists of a string of cells each of which is adjacent to exactly two other rotor cells, except for the endpoints which are adjacent to only one other rotor cell. Compare muttering moat. Examples include the beacon, the great on-off, the light bulb and the spark coil. The following less trivial example (by Dean Hickerson, August 1997) is the only one known with more than four cells in its rotor. It is p4 and has a 6-cell rotor.

:backrake Another term for a backwards rake. A p8 example by Jason Summers is shown below. See total aperiodic for a p12 example.

:backward glider A glider which moves at least partly in the opposite direction to the puffer(s) or spaceship(s) under consideration.

:bait An object in a converter, usually a small still life, that is temporarily destroyed by an incoming signal, but in such a way that a usable output signal is produced. In general such a converter produces multiple output signals (or a signal splitter is added) and one branch of the output is routed to a factory mechanism that rebuilds the bait object so that the converter can be re-used.

:baker (c p4 fuse) A fuse by Keith McClelland.

:baker's dozen (p12) A loaf hassled by two blocks and two caterers. The original form (using p4 and p6 oscillators to do the hassling) was found by Robert Wainwright in August 1989.

:bakery (p1) A common formation of two bi-loaves.

:banana spark A common three-bit polyplet spark used in glider synthesis and signal circuitry. The buckaroo is an oscillator that produces this spark. It can be used to turn a glider 90 degrees:

:barberpole Any p2 oscillator in the infinite sequence bipole, tripole, quadpole, pentapole, hexapole, heptapole ... (It wasn't my idea to suddenly change from Latin to Greek.) This sequence of oscillators was found by the MIT group in 1970. The term is also used (usually in the form "barber pole") to describe other extensible sections of oscillators or spaceships, especially those (usually of period 2) in which all generations look alike except for a translation and/or rotation/reflection. Any barberpole can be lengthened by the reaction shown in barbershop. See also pseudo-barberpole.

:barberpole intersection = quad

:barbershop An object created by Jason Summers in 1999 which builds an infinite barberpole. It uses slide guns to repeatedly lengthen a barberpole at a speed of c/124. The key lengthening reaction from Mark Niemiec is shown below:

:barber's pole = barberpole

:barge (p1)

:basic shuttle = queen bee shuttle

:beacon (p2) The third most common oscillator. Found by Conway, March 1970.

:beacon maker (c p8 fuse)

:beehive (p1) The second most common still life.

:beehive and dock (p1)

:beehive on big table = beehive and dock

:beehive pusher = hivenudger

:beehive stopper A Spartan logic circuit discovered by Tanner Jacobi on 12 May 2015. It converts an input glider signal into a beehive, in such a way that the beehive can cleanly absorb a single glider from a perpendicular glider stream. The circuit can't be re-used until the beehive "bit" is cleared by the passage of at least one perpendicular input.

This term has sometimes been used for the beehive catalyst variant of SW-2, and also for Paul Callahan's larger glider stopper, which also provides optional 0-degree and 180-degree glider outputs.

:beehive wire See lightspeed wire.

:beehive with tail (p1)

:Bellman A program for searching catalytic reactions, developed by Mike Playle, which successfully found the Snark.

:belly spark The spark of a MWSS or HWSS other than the tail spark.

:Beluchenko's p37 (p37) Found by Nicolay Beluchenko on April 14, 2009. It was the first period 37 oscillator to be found, and remains the smallest.

:Beluchenko's p51 (p51) Found by Nicolay Beluchenko on February 17, 2009. It was the first non-trivial period 51 oscillator to be found.

:bent keys (p3) Found by Dean Hickerson, August 1989. See also odd keys and short keys.

:BFx59H One of the earliest and most remarkable converters, discovered by Dave Buckingham in July 1996. In 59 generations it transforms a B-heptomino into a clean Herschel with very good clearance, allowing easy connections to other conduits. It forms the final stage of many of the known composite conduits, including the majority of the original sixteen Herschel conduits. Here a ghost Herschel marks the output location:

:B-heptomino (stabilizes at time 148) This is a very common methuselah that evolves into three blocks, two gliders and a ship after 148 generations. Compare with Herschel, which appears at generation 20 of the B-heptomino's evolution. B-heptominoes acquired particular importance in 1996 due to Dave Buckingham's work on B tracks. See in particular My Experience with B-heptominos in Oscillators.

This pattern often arises with the cell at top left shifted one space to the left, producing a seven-bit polyplet that shares the same eight-bit descendant but is not technically a heptomino at all. This alternate form is shown as the input for elementary converter patterns such as BFx59H and BRx46B. This is standard practice for elementary conduits, since many of these conduits do in fact produce this alternate form as output.

The B-heptomino is considered a failed puffer or failed spaceship, since on its own it travels at c/2 for only a short time before being affected by its own trailing debris. However, it can be stabilized into a c/2 puffer or into a clean c/2 rake or spaceship. See, e.g., ecologist.

:B-heptomino shuttle = twin bees shuttle

:bi-block (p1) The smallest pseudo still life.

:bi-block fuse A clean fuse made by a row of bi-blocks separated by 2 cell gaps. The bi-block row wick is usually created by a bi-block puffer. The burning advances 8 cells every 12 generations making its speed 2c/3.

:bi-block puffer Any puffer whose output is bi-blocks. The term is particularly used for p8 c/2 puffers, in which case a bi-block fuse is created. A bi-block puffer is easily made using two backrakes whose gliders impact symmetrically. Jason Summers welded two backrakes to form a more compact puffer, as shown below.

:bi-boat = boat-tie

:biclock The following pure glider generator consisting of two clocks.

:big beacon = figure-8

:big fish = HWSS

:big glider (c/4 diagonally, p4) This was found by Dean Hickerson in December 1989 and was the first known diagonal spaceship other than the glider.

:big S (p1)

:big table = dock

:billiard table = billiard table configuration.

:billiard table configuration Any oscillator in which the rotor is enclosed within the stator. Examples include airforce, cauldron, clock II, Hertz oscillator, negentropy, pinwheel, pressure cooker and scrubber.

:bi-loaf This term has been used in at least three different senses. A bi-loaf can be half a bakery:

:bipole (p2) The barberpole of length 2.

:bi-pond (p1)

:bi-ship = ship-tie

:bistable switch A Spartan memory cell found by Paul Callahan in 1994. It can be in one of two states, containing either a boat or a block. Input gliders on the appropriate paths can change the boat to a block, or vice-versa, while also emitting an output glider. Unlike many memory cells, attempts to change the state to the one it is already in are ignored with the glider passing through with no reaction. This makes it easy to reset the memory cell to a known state. Which of the two states is considered the SET and which considered the RESET is just a matter of convention.

The pattern below shows the "boat" state of the memory cell in its original 1994 form. Two gliders are also shown to indicate the input paths used to change the states. A smaller version is shown under century eater, with the circuit in its "block" state.

As shown, the rightmost glider changes the state from a boat to a block and emits a glider to the upper right, while the leftmost glider passes through unchanged. Alternatively, when the state contains a block, then the leftmost glider changes the state from a block to a boat, and emits a glider to the lower right, while the rightmost glider passes through unchanged.

:bit A live cell, if used in reference to still life population. For example, a beehive is a 6-bit still life. Other uses generally involve information storage: a memory cell such as a honey bit that can hold one binary bit of information for later retrieval.

:biting off more than they can chew (p3) Found by Peter Raynham, July 1972.

:Black&White = Immigration

:blasting cap The pi-heptomino (after the shape at generation 1). A term used at MIT and still occasionally encountered.

:blinker (p2) The smallest and most common oscillator. Found by Conway, March 1970.

:blinker fuse A clean fuse made from a row of blinkers separated by one cell gaps. The blinker row wick is usually created by a blinker puffer. The fuse can burn in at least three different ways at a speed of 2c/3 depending on the method used to ignite the end of the row of blinkers. This variant has found the most use. The burning advances 12 cells every 18 generations.

:blinker puffer Any puffer whose output is blinkers. However, the term is particularly used for p8 c/2 puffers. The first such blinker puffer was found by Robert Wainwright in 1984, and was unexpectedly simple:

:blinker pull The following glider/blinker collision, which moves a blinker (-1,3) toward the glider source:

:blinkers bit pole (p2) Found by Robert Wainwright, June 1977.

:blinker ship A growing spaceship in which the wick consists of a line of blinkers. An example by Paul Schick based on his Schick engine is shown below. Here the front part is p12 and moves at c/2, while the back part is p26 and moves at 6c/13. Every 156 generations 13 blinkers are created and 12 are destroyed, so the wick becomes one blinker longer.

:block (p1) The most common still life, and also the most common object produced by 2-glider collisions (six different ways).

:blockade (p1) A common formation of four blocks. The final form of lumps of muck.

:block and dock (p1)

:block and glider (stabilizes at time 106)

:blocker (p8) Found by Robert Wainwright. See also filter.

:block factory Any factory circuit that produces a block in response to an input signal. For a useful high-clearance example see keeper.

:Blockic Adjective for constellations consisting entirely of blocks. It's possible to arrange blocks in a way that can be triggered by a single glider to produce any glider constructible pattern. A simple example of a Blockic pattern is shown under fuse. See also seed.

:block keeper See keeper.

:block-laying switch engine See stabilized switch engine.

:block on big table = block and dock

:block on table (p1)

:block pull The following glider/block collision, which moves a block (2,1) toward the glider source. Performing this reaction twice using a salvo of two gliders can move a block diagonally back by three cells, which can be of use for a sliding block memory.

:block pusher A pattern emitting streams of gliders which can repeatedly push a block further away. This can be used as part of a sliding block memory.

The following pattern, in which three gliders push a block one cell diagonally, is an example of how a block pusher works.

A universal construction elbow recipe library is also likely to contain one or more block-pushing reactions, since blocks are commonly used as elbows.

:blom (stabilizes at time 23314) The following methuselah, found by Dean Hickerson in July 2002.

:blonk A block or a blinker. This term is mainly used in the context of sparse Life and was coined by Rich Schroeppel in September 1992.

:blonker (p6) The following oscillator, found by Nicolay Beluchenko in April 2004.

:BLSE = block-laying switch engine

:BNE14T30 A B-heptomino to glider converter found by Tanner Jacobi on 26 May 2016. This converter has the unusual property of being an edge shooter where no part of the reaction's envelope extends beyond the glider's output lane. It can be easily connected to Herschel circuitry via HFx58B or other known elementary conduits.

:boat (p1) The only 5-cell still life.

:boat-bit A binary digit represented by the presence of a boat next to a snake (or other suitable object, such as an aircraft carrier). The bit can be toggled by a glider travelling along a certain path. A correctly timed glider on a crossing path can detect whether the transition was from 1 to 0 (in which case the crossing glider is deleted) or from 0 to 1 (in which case it passes unharmed). Three gliders therefore suffice for a non-destructive read. The mechanisms involved are shown in the diagram below. Here the bit is shown in state 0. It is about to be set to 1 and then switched back to 0 again. The first crossing glider will survive, but the second will be destroyed.

In January 1997 David Bell found a method of reading the bit while setting it to 0. A MWSS is fired at the boat-bit. If it is already 0 (absent) then the MWSS passes unharmed, but if it is 1 (present) then the boat and the MWSS are destroyed and, with the help of an eater1, converted into a glider which travels back along exactly the same path that is used by the gliders that toggle the boat-bit.

:boat maker (c p4 fuse)

:boat on boat = boat-tie

:boat-ship-tie = ship tie boat

:boatstretcher See tubstretcher.

:boat-tie (p1) A 10-cell still life consisting of two boats placed tip-to-tip. The name is a pun on "bow tie".

:bobsled = switch engine channel.

:boojum reflector (p1) Dave Greene's name for the following 180-degree glider reflector which he found in April 2001, winning $100 bounties offered by Alan Hensel and Dieter Leithner. The name is taken from Lewis Carroll's _The Hunting of the Snark_, referring to the fact that a small 90-degree stable reflector was really what was wanted. 180-degree reflectors are relatively undesirable and have limited use in larger circuitry constructions.

The boojum reflector was the smallest and fastest known stable reflector until the discovery of the rectifier in 2009, followed by the Snark in 2013.

:bookend The following induction coil. It is generation 1 of century.

:bookends (p1)

:boss (p4) Found by Dave Buckingham, 1972.

:bottle (p8) Found by Achim Flammenkamp in August 1994. The name is a back-formation from ship in a bottle.

:bouncer A label used for the small periodic colour-changing glider reflectors discovered mainly by Noam Elkies in the late 1990s. See p5 bouncer, p6 bouncer, p7 bouncer, p8 bouncer, or p15 bouncer.

:bounding box The smallest rectangular array of cells that contains the whole of a given pattern. For oscillators and guns this usually is meant to include all phases of the pattern, but in the case of guns, the outgoing stream(s) are excluded. The bounding box is one of the standard ways to measure the size of an object; the other standard metric is the population.

:bow tie = boat-tie

:brain (c/3 orthogonally, p3) Found by David Bell, May 1992.

:branching spaceship An extensible spaceship containing components which can be attached in multiple ways so that the result can contain arbitrarily many arms arranged like a binary tree. Here is an example of a period 2 c/2 branching spaceship, which also includes a wicktrailer:

:breeder Any pattern whose population grows at a quadratic rate, although it is usual to exclude spacefillers. It is easy to see that this is the fastest possible growth rate.

The term is also sometimes used to mean specifically the breeder created by Bill Gosper's group at MIT, which was the first known pattern exhibiting superlinear growth.

There are four common types of breeder, known as MMM, MMS, MSM and SMM (where M=moving and S=stationary). Typically an MMM breeder is a rake puffer, an MMS breeder is a puffer producing puffers which produce stationary objects (still lifes and/or oscillators), an MSM breeder is a gun puffer and an SMM breeder is a rake gun. There are, however, less obvious variants of these types. Other less common breeder categories (SSS, hybrid MSS/MSM, etc.) can be created with some difficulty, based on universal constructor technology; see Pianola breeder.

The original breeder was of type MSM (a p64 puffer puffing p30 glider guns). The known breeder with the smallest initial population is switch-engine ping-pong.

:bridge A term used in naming certain still lifes (and the stator part of certain oscillators). It indicates that the object consists of two smaller objects joined edge to edge, as in snake bridge snake.

:broken lines A pattern constructed by Dean Hickerson in May 2005 which produces complex broken lines of gliders and blocks.

:broth = soup

:BRx46B A Spartan elementary conduit discovered by Michael Simkin on 25 April 2016, one of the relatively few known conduits that can move a B-heptomino input to a B-heptomino output without an intervening Herschel stage.

:BTC = billiard table configuration

:B track A track for B-heptominoes. A B-heptomino becomes a Herschel plus a block in twenty generations, so this term was nearly synonymous with Herschel track until the discovery of elementary conduits that convert a B directly to another B, or to some other non-Herschel signal output. See for example BRx46B.

:BTS A 19-cell still life made up of a bookend, a table, and a snake. Starting in 2015, with the help of Mike Playle's Bellman search program, Tanner Jacobi discovered a surprising number of ways to use this object as a catalyst for signal circuitry. One example can be seen in the CC semi-cenark entry.

:buckaroo (p30) A queen bee shuttle stabilized at one end by an eater in such a way that it can turn a glider, as shown below. The glider turning reaction uses a banana spark and is colour-preserving. The mechanism was found by Dave Buckingham in the 1970s. The name is due to Bill Gosper.

:bullet heptomino Generation 1 of the T-tetromino.

:bumper One of several periodic colour-preserving glider reflectors discovered by Tanner Jacobi on 6 April 2016. See p3 bumper, p4 bumper, p5 bumper, p6 bumper, p7 bumper, p8 bumper, p9 bumper, p11 bumper, and p15 bumper.

:bun The following induction coil. By itself this is a common predecessor of the honey farm. See also cis-mirrored R-bee.

:bunnies (stabilizes at time 17332) This is a parent of rabbits and was found independently by Robert Wainwright and Andrew Trevorrow.

:burloaf = loaf

:burloaferimeter (p7) Found by Dave Buckingham in 1972. See also airforce.

:burn A reaction which travels indefinitely as a wave through the components of a wick or an agar. A burning wick is known as a fuse.

If the object being burned has a spatial periodicity, then the active area of the burning usually remains bounded and so eventually develops a periodicity too. It is unknown whether this will always occur.

The speed of burning can range from arbitrarily slow up to the speed of light. The results of burning can be clean (leaving no debris), or leaving debris usually much different from the original object. In rare cases, a reburnable fuse produces an exact copy of the original object, allowing the creation of objects such as the telegraph.

In many useful cases burning can be initiated by impacting an object with gliders or other spaceships. An object might be able to burn in more than one way, depending on how the burn is initiated.

:bushing That part of the stator of an oscillator which is adjacent to the rotor. Compare casing.

:butterfly The following pattern, or the formation of two beehives that it evolves into after 33 generations. (Compare teardrop, where the beehives are five cells closer together.)

:Bx125 An elementary conduit, one of the original sixteen Herschel conduits, discovered by Paul Callahan in November 1998. After 125 ticks, it produces an inverted Herschel rotated 180 degrees at (-9, -17) relative to the input. Its recovery time is 166 ticks. A ghost Herschel in the pattern below marks the output location:

:Bx222 A composite conduit, one of the original sixteen Herschel conduits, discovered by Paul Callahan in October 1998. It is made up of three elementary conduits, HF95P + PB68B + BFx59H. After 222 ticks, it produces a mirror-reflected Herschel rotated 180 degrees, at (6, -16) relative to the input. Its recovery time is 271 ticks. A ghost Herschel in the pattern below marks the output location:

:by flops (p2) Found by Robert Wainwright.

:c = speed of light

:c/10 spaceship A spaceship travelling at one tenth of the speed of light. The first such spaceship to be discovered was the orthogonally travelling copperhead, found by 'zdr' on 5 March 2016. Simon Ekström found the related fireship two weeks later. A Caterloopillar can theoretically be configured to move at c/10, but there are technical difficulties with speeds of the form 4n+2, and as of June 2018 this has not been done in practice.

:c/12 spaceship A spaceship travelling at one twelfth of the speed of light. The only diagonal spaceships that are currently known to move at this speed are the Corderships. An orthogonal Caterloopillar has been configured to move at c/12.

:c/2 spaceship A spaceship travelling at half the speed of light. Such spaceships necessarily move orthogonally. The first to be discovered was the LWSS. For other examples see Coe ship, ecologist, flotilla, hammerhead, hivenudger, HWSS, MWSS, puffer train, puff suppressor, pushalong, Schick engine, sidecar, still life tagalong and x66.

:c/3 spaceship A spaceship travelling at one third of the speed of light. All known c/3 spaceships travel orthogonally. The first was 25P3H1V0.1, found in August 1989 by Dean Hickerson. For further examples see brain, dart, edge-repair spaceship, fly, turtle and wasp.

:c/4 spaceship A spaceship travelling at one quarter of the speed of light. The first such spaceship to be discovered was, of course, the glider, and this remained the only known example until December 1989, when Dean Hickerson found the first orthogonal example, 119P4H1V0, and also a new diagonal example (the big glider). For other examples see B29, Canada goose, crane, Enterprise, edge-repair spaceship (third pattern), non-monotonic, Orion, quarter, sparky, swan and tagalong. It is known that c/4 is the fastest possible speed for a (45-degree) diagonal spaceship.

:c/5 spaceship A spaceship travelling at one fifth of the speed of light. The first such spaceship to be discovered was the snail, found by Tim Coe in January 1996. The first diagonally moving example, 295P5H1V1, was found by Jason Summers in November 2000. For other c/5 ships see 58P5H1V1, 67P5H1V1, 86P5H1V1 and spider. A Caterloopillar has also been configured to move at c/5.

:c/6 spaceship A spaceship travelling at one sixth of the speed of light. The first such spaceship to be discovered was the dragon, found by Paul Tooke in April 2000. The first diagonally moving example was the seal, found by Nicolay Beluchenko in September 2005. Another orthogonal c/6 spaceship, found by Paul Tooke in March 2006, is shown below. For the smallest known c/6 spaceship see 56P6H1V0.

:c/7 spaceship A spaceship travelling at one seventh of the speed of light. The first such spaceship to be discovered was the diagonally travelling lobster, found by Matthias Merzenich in August 2011. The first known orthogonal c/7 spaceship was the loafer, discovered by Josh Ball in February 2013. A Caterloopillar has been configured to move at c/7.

:CA = cellular automaton

:caber tosser Any pattern whose population is asymptotic to c.log(t) for some constant c, and which contains a glider (or other spaceship) bouncing between a slower receding spaceship and a fixed reflector which emits a spaceship (in addition to the reflected one) whenever the bouncing spaceship hits it.

As the receding spaceship gets further away the bouncing spaceship takes longer to complete each cycle, and so the extra spaceships emitted by the reflector are produced at increasingly large intervals. More precisely, if v is the speed of the bouncing spaceship and u the speed of the receding spaceship, then each interval is (v+u)/(v-u) times as long as the previous one. The population at time t is therefore n.log(t)/log((v+u)/(v-u)) + O(1), where n is the population of one of the extra spaceships (assumed constant).

The first caber tosser was built by Dean Hickerson in May 1991.

:Callahan G-to-H A stable glider reflector and glider-to-Herschel converter discovered by Paul Callahan in November 1998. Its recovery time is 575 ticks. The initial stage converts two gliders into a Herschel. A ghost Herschel in the pattern below marks the output location:

The glider from the southeast can be supplied by an Fx77 + L112 + Fx77 Herschel track, or by reflecting the output Herschel's FNG as in the p8 G-to-H. See also Silver reflector, Silver G-to-H.

:Cambridge pulsar CP 48-56-72 = pulsar (The numbers refer to the populations of the three phases. The Life pulsar was indeed discovered at Cambridge, like the first real pulsar a few years earlier.)

:Canada goose (c/4 diagonally, p4) Found by Jason Summers, January 1999. It consists of a glider plus a tagalong.

:candelabra (p3) By Charles Trawick. See also the note under cap.

:candlefrobra (p3) Found by Robert Wainwright in November 1984.

:canoe (p1)

:cap The following induction coil. It can also easily be stabilized to form a p3 oscillator. See candelabra for a slight variation on this.

:carnival shuttle (p12) Found by Robert Wainwright in September 1984 (using MW emulators at the end, instead of the monograms shown here).

:carrier = aircraft carrier

:casing That part of the stator of an oscillator which is not adjacent to the rotor. Compare bushing.

:catacryst A 58-cell quadratic growth pattern found by Nick Gotts in April 2000. This was formerly the smallest such pattern known, but has since been superseded by the related metacatacryst. See switch-engine ping-pong for the lowest-population superlinear growth pattern as of July 2018, along with a list of the record-holders.

The catacryst consists of three arks plus a glider-producing switch engine. It produces a block-laying switch engine every 47616 generations. Each block-laying switch engine has only a finite life, but the length of this life increases linearly with each new switch engine, so that the pattern overall grows quadratically, as an unusual type of MMS breeder.

:Catagolue An online database of objects in Conway's Game of Life and similar cellular automata, set up by Adam P. Goucher in 2015 at http://catagolue.appspot.com. It gathers data from a distributed search of random initial configurations and records the eventual decay products. Within a year of operation it had completed a census of the ash objects from over two trillion asymmetric 16×16 soups. As of June 2018, well over two hundred trillion ash objects have been counted, from over a trillion asymmetric soups.

It is often possible to use Catagolue search results find equivalent glider synthesis recipes for selected parts of long-running active reactions. These random soup searches have made it possible to find efficient construction methods for thousands of increasingly rare still lifes and oscillators, and the occasional puffer or spaceship. In many of these cases a glider synthesis was previously very difficult or unknown.

:catalyst An object that participates in a reaction but emerges from it unharmed. All eaters are catalysts. Some small still lifes can act as catalysts in some situations, such as the block, ship, and tub. The still lifes and oscillators that form a conduit are examples of catalysts.

A relatively rare form of catalysis occurs in a transparent debris effect, where the catalyst in question is completely destroyed and then rebuilt. The term is also sometimes used for a modification of an active reaction in a rake by passing spaceships.

:catch and throw A technology used (e.g., in the Caterpillar) to adjust the timing of a glider by turning it into a stationary object using one interaction, and then later restoring it using a second interaction. The interactions are caused by passing objects which are not otherwise affected. The direction of the glider is not usually changed.

Here is an example where a glider is turned into a boat by the first LWSS, and is then restored by the remaining spaceships:

:caterer (p3) Found by Dean Hickerson, August 1989. Compare with jam. In terms of its minimum population of 12 this is the smallest p3 oscillator. See also double caterer and triple caterer.

:Caterloopillar A family of adjustable-speed spaceships constructed by Michael Simkin in 2016, based on an "engineless caterpillar" idea originally proposed by David Bell. The front and back halves of Caterloopillars each function as universal constructors, with each half constructing the building blocks of the other half, while also reading and moving a construction tape. The overall design is reminiscent of M.C. Escher's lithograph "Drawing Hands". The name "Caterloopillar" is a reference to Douglas Hofstader's Strange Loop concept.

Simkin has written an automated script that can construct a Caterloopillar for any rational speed strictly less than c/4, with some exceptions. Speeds closer to the c/4 limit in general require larger constructions, and for any given computer system it is easy to choose a speed that makes it impractical to construct a Caterloopillar.

As of June 2018 one significant remaining exception is that Caterloopillars with periods c/(6+4N) can't be constructed. This is only a limitation of the current construction script, not of the underlying Caterloopillar toolkit. For technical reasons, the lowest speed that the current script can produce is around c/95. The slowest Caterloopillars that have been explicitly constructed to date are c/87 and c/92. These are among the smallest in terms of population, though their bounding boxes are larger than some of the higher-speed Caterloopillars.

:Caterpillar A spaceship that works by laying tracks at its front end. The first example constructed was a p270 17c/45 spaceship built by Gabriel Nivasch in December 2004, based on work by himself, Jason Summers and David Bell. This Caterpillar has a population of about 12 million in each generation and was put together by a computer program that Nivasch wrote. At the time it was by far the largest and most complex Life object ever constructed, and it is still one of the largest in terms of population.

The 17c/45 Caterpillar is based on the following reaction between a pi-heptomino and a blinker:

A different Caterpillar may be possible based on the following reaction, in which the pattern at top left reappears after 31 generations displaced by (13,1), having produced a new NW-travelling glider. In this case the tracks would be waves of backward-moving gliders.

:CatForce An optimized search program written by Michael Simkin in 2015, using brute-force enumeration of small Spartan objects in a limited area, instead of a depth-first tree search. One major purpose of CatForce is to find glider-constructible completions for signal conduits. An early CatForce discovery was the B60 conduit, which enabled a record-breaking new glider gun.

:Catherine wheel = pinwheel

:cauldron (p8) Found in 1971 independently by Don Woods and Robert Wainwright. Compare with Hertz oscillator.

:cavity = eater plug

:CC semi-cenark The colour-changing version of Tanner Jacobi's century-based semi-Snark mechanism, using a C-to-G consisting of a BTS catalyst and a block. See CP semi-cenark for the colour-preserving version, or semi-cenark for repeat time details and an alternate initial catalyst.

:CC semi-Snark A small 90-degree colour-changing glider reflector requiring two input gliders on the same lane for each output glider. It was discovered by Sergei Petrov on 1 July 2013, using a custom-written search utility. It functions as a very compact period doubler in some signal circuitry, for example the linear propagator. The semi-Snark can period-double a regular glider stream of period 51 or more, or an intermittent stream with two gliders every 67 ticks or more, since the block reset glider can be sent just 16 ticks before its partner.

:cell The fundamental unit of space in the Life universe. The term is often used to mean a live cell - the sense is usually clear from the context.

:cellular automaton A certain class of mathematical objects of which Life is an example. A cellular automaton consists of a number of things. First there is a positive integer n which is the dimension of the cellular automaton. Then there is a finite set of states S, with at least two members. A state for the whole cellular automaton is obtained by assigning an element of S to each point of the n-dimensional lattice Zⁿ (where Z is the set of all integers). The points of Zⁿ are usually called cells. The cellular automaton also has the concept of a neighbourhood. The neighbourhood N of the origin is some finite (nonempty) subset of Zⁿ. The neighbourhood of any other cell is obtained in the obvious way by translating that of the origin. Finally there is a transition rule, which is a function from S^N to S (that is to say, for each possible state of the neighbourhood the transition rule specifies some cell state). The state of the cellular automaton evolves in discrete time, with the state of each cell at time t+1 being determined by the state of its neighbourhood at time t, in accordance with the transition rule.

There are some variations on the above definition. It is common to require that there be a quiescent state, that is, a state such that if the whole universe is in that state at generation 0 then it will remain so in generation 1. (In Life the OFF state is quiescent, but the ON state is not.) Other variations allow spaces other than Zⁿ, neighbourhoods that vary over space and/or time, probabilistic or other non-deterministic transition rules, etc.

It is common for the neighbourhood of a cell to be the 3×...×3 (hyper)cube centred on that cell. (This includes those cases where the neighbourhood might more naturally be thought of as a proper subset of this cube.) This is known as the Moore neighbourhood.

:census A count of the number of different individual Life objects within one larger object, most often the final ash of a random soup experiment. This includes the number of blocks, blinkers, gliders, and other common objects, as well as any rarer larger still lifes, oscillators or spaceships.

:centinal (p100) Found by Bill Gosper. This combines the mechanisms of the p46 and p54 shuttles (see twin bees shuttle and p54 shuttle).

:Centipede (31c/240 orthogonally, p240) The smallest known 31c/240 spaceship, constructed by Chris Cain in September 2014 as a refinement of the shield bug.

:century (stabilizes at time 103) This is a common pattern which evolves into three blocks and a blinker. In June 1996 Dave Buckingham built a neat p246 gun using a century as the engine. See also bookend and diuresis.

:century eater A 20-cell still life that functions as an eater for the active reaction produced by any century relative. The most well-known use is to replace a four-object constellation in Paul Callahan's bistable switch, as shown below. In September 2014 Josh Ball showed that a variant of this still life has a relatively inexpensive slow glider construction recipe.

:century-to-glider converter Any signal circuit that accepts a century as input and produces a clean output glider. For example, in November 2017 Adam P. Goucher noticed that this previously known C-to-G converter can replace the century eater in Paul Callahan's bistable switch, producing an extra glider output.

:channel A lane or signal path used in construction circuitry. Until the invention of single-channel construction arms, signals in a channel would usually be synchronized with one or more coordinated signals on other paths, as in the Gemini, which used twelve channels to run three construction arms simultaneously, or the 10hd Demonoid which needed only two channels. See also Geminoid.

:chaotic growth An object whose fate is unknown, except that it appears to grow forever in an unpredictable manner. In Life, no pattern has yet been found that is chaotic. This is in contrast to many other Life-like rules, where even small objects can appear to grow chaotically.

It is possible that chaotic growth may occur rarely or even regularly for large enough random Life objects, but if so the minimum size of such patterns must be larger than what can currently be experimentally simulated (but see novelty generator).

In any case, it is not decidable whether a pattern that apparently grows randomly forever is in fact displaying chaotic growth. Continuing to evolve such a pattern might at any time result in it suddenly cleaning itself up and becoming predictable.

:chemist (p5)

:C-heptomino Name given by Conway to the following heptomino, a less common variant of the B-heptomino.

:Cheshire cat A block predecessor by C. R. Tompkins that unaccountably appeared both in Scientific American and in Winning Ways. See also grin.

:chicken wire A type of stable agar of density 1/2. The simplest version is formed from the tile:

:chirality A term borrowed from chemistry to describe asymmetrical patterns with two distinct mirror-image orientations. One common use is in relation to Herschel transmitters, where the spacing between the two gliders in the tandem glider output can limit the receiver to a single chirality.

:cigar = mango

:circuit Any combination of conduits or converters that moves or processes an active signal. This includes components with multiple states such as period multipliers or switches, which can be used to build guns, logic gates, universal constructors, and other computation or construction circuitry.

:cis-beacon on anvil (p2)

:cis-beacon on table (p2)

:cis-boat with tail (p1)

:cis fuse with two tails (p1) See also pulsar quadrant.

:cis-mirrored R-bee (p1)

:cis snake = canoe

:clean Opposite of dirty. A reaction which produces a small number of different products which are desired or which are easily deleted is said to be clean. For example, a puffer which produces just one object per period is clean. Clean reactions are useful because they can be used as building blocks in larger constructions.

When a fuse is said to be clean, or to burn cleanly, this usually means that no debris at all is left behind.

:clearance In signal circuitry, the distance from an edge shooter output lane to the last unobstructed lane adjacent to the edge-shooter circuitry. For example, an Fx119 inserter has an unusually high 27hd clearance.

Also, oscillator and eater variants may be said to have better clearance if they allow gliders or other signals to pass closer to them than the standard variant allows. The following high-clearance eater1 variant by Karel Suhajda allows gliders to pass one lane closer on the southeast side, than is allowed by the standard fishhook shape.

:clock (p2) Found by Simon Norton, May 1970. This is the fifth or sixth most common oscillator, being about as frequent as the pentadecathlon, but much less frequent than the blinker, toad, beacon or pulsar. It is surprisingly rare considering its small size.

The protruding cells at the edges can perturb some reactions by inhibiting the birth of a cell in a 3-cell corner. For example, a clock can be used to suppress the surplus blinker produced by an F171 conduit, significantly improving the recovery time of the circuit:

:clock II (p4) Compare with pinwheel.

:clock inserter = clock insertion.

:clock insertion A uniquely effective method of adding a glider to the front edge of a salvo, by first constructing a clock, then converting it to a glider using a one-bit spark. Here it rebuilds a sabotaged glider in a deep pocket between other gliders:

In 2015 Chris Cain used this reaction to demonstrate conclusively that any unidirectional glider salvo, no matter how large or tightly packed, can be constructed by collisions between gliders that are initially separated by any finite distance. As a corollary, because all glider syntheses are made up of two to four unidirectional salvos, any glider-constructible object has a synthesis that starts with every glider at least N cells away from every other glider (for any chosen N).

:cloud of smoke = smoke

:cloverleaf This name was given by Robert Wainwright to his p2 oscillator washing machine. But Achim Flammenkamp also gave this name to Achim's p4.

:cluster Any pattern in which each live cell is connected to every other live cell by a path that does not pass through two consecutive dead cells. This sense is due to Nick Gotts, but the term has also been used in other senses, often imprecise.

:CNWH Conweh, creator of the Life universe.

:Coe ship (c/2 orthogonally, p16) A puffer engine discovered by Tim Coe in October 1995.

In December 2015, the Coe ship was discovered in an asymmetric random soup on Catagolue. This was the first time any non-p4 ship was discovered in a random asymmetric soup experiment, winning Adam P. Goucher a 50-euro prize offered by Ivan Fomichev.

:Coe's p8 (p8) Found by Tim Coe in August 1997.

:Collatz 5N+1 simulator An unknown fate pattern constructed by David Bell in December 2017 that simulates the Collatz 5N+1 algorithm using sliding block memory and p1 technology, while always having a population below 32000.

The algorithm is simple. Starting with a number, if it is even divide it by 2, otherwise multiply it by 5 and add 1. When this process is iterated a sequence of numbers is generated. When starting with the value of 7, it is currently unknown whether or not the sequence ever forms a cycle.

Because of this the fate of the simulator is also currently unknown. It may become stable, or become an oscillator with a high period, or have a bounding box which grows irregularly.

:colour = colour of a glider

:colour-changing See colour of a glider. The reflector shown in p8 bouncer is colour-changing, as are its 5/6/7/8 and higher-period versions.

:colour-changing semi-Snark = CC semi-Snark.

:colourised Life A cellular automaton that is the same as Life except for the use of a number of different ON states ("colours"). All ON states behave the same for the purpose of applying the Life rule, but additional rules are used to specify the colour of the resulting ON cells. Examples are Immigration and QuadLife.

:colour of a glider The colour of a glider is a property of the glider that remains constant while the glider is moving along a straight path, but that can be changed when the glider bounces off a reflector. It is an important consideration when building something using reflectors.

The colour of a glider can be defined as follows. First choose some cell to be the origin. This cell is then considered to be white, and all other cells to be black or white in a checkerboard pattern. (So the cell with coordinates (m,n) is white if m+n is even, and black otherwise.) Then the colour of a glider is the colour of its leading cell when it is in a phase that can be rotated to look like this:

A reflector that does not change the colour of gliders obviously cannot be used to move a glider onto a path of different colour than it started on. But a 90-degree reflector that does change the colour of gliders is similarly limited, as the colour of the resulting glider will depend only on the direction of the glider, no matter how many reflectors are used. For maximum flexibility, therefore, both types of reflector are required.

Small periodic colour-changing glider reflectors (bouncers) are known, and also small periodic colour-preserving glider reflectors (bumpers). Among stable patterns, only a small colour-preserving reflector (Snark) is known. The smallest known 90-degree colour-changing reflector is given at the end of the reflector entry.

:colour-preserving See colour of a glider. Snarks and bumpers are colour-preserving reflectors.

:colour-preserving semi-Snark = CP semi-Snark

:complementary blinker = fore and back

:component A partial glider synthesis that can be used in the same way in multiple glider recipes. A component transforms part of an object under construction in a well-defined way, without affecting the rest of the object. For example, this well-known component can be used to add a hook to any object that includes a protruding table end, converting it to a long bookend:

"Component" is also used to specify any piece of an object - spaceship, oscillator, etc. - that can be combined with other components in specific ways according to a grammar to produce a variety of objects. The components can either be independent objects that only occasionally react with each other, or else they can be fused together to support each other. For example, any branching spaceship is made up of several components, and there is a single repeating component in any wicktrailer.

:composite See composite conduit.

:composite conduit A signal-processing conduit that can be subdivided into two or more elementary conduits.

:compression = repeat time, recovery time.

:computational universality See universal computer.

:conduit Any arrangement of still lifes and/or oscillators that moves an active object to another location, perhaps also transforming it into a different active object at the same time, but without leaving any permanent debris (except perhaps gliders, or other spaceships) and without any of the still lifes or oscillators being permanently damaged. Probably the most important conduit is the following remarkable one (Dave Buckingham, July 1996) in which a B-heptomino is transformed into a Herschel in 59 generations.

:conduit 1 = BFx59H.

:confused eaters (p4) Found by Dave Buckingham before 1973.

:constellation A general term for a group of two or more separate objects, usually small still lifes and low-period oscillators. Compare pseudo still life.

:construction arm An adjustable mechanism in a universal constructor that allows new objects to be constructed in any chosen location that the arm can reach. A construction arm generally consists of a shoulder containing fixed guns or edge shooters, a movable construction elbow that slides forward and backward along the construction lane(s), and in the case of single-arm universal constructors, a hand target object at the construction site that can be progressively modified by a slow salvo to produce each desired object.

:construction elbow One of the components of a construction arm in a universal constructor. The elbow usually consists of a single Spartan still life or small constellation. It accepts elbow operation recipes, in the form of salvos coming from the construction arm's shoulder.

These recipes may do one of several things: 1) pull the elbow closer to the shoulder, 2) push the elbow farther from the shoulder, 3) emit a glider on a particular output lane (while also optionally pushing or pulling the elbow); 4) create a "hand" target block or other useful object as a target for output gliders, to one side of the construction lane; 5) duplicate the elbow, or 6) destroy the elbow.

Elbows that receive and emit orthogonally-travelling spaceships instead of gliders are technically possible, but no working examples are currently known. The discussion below assumes that gliders are used to communicate between the shoulder, elbow, and hand locations.

If a mechanism can be programmed to generate recipes for at least the first three options listed above, it is generally capable of functioning as a universal constructor. The main requirement is that push and pull elbow operations should be available that are either minimal (1fd) or the distances should be relatively prime.

Depending on the elbow operation library, there may be only one type of elbow, or there may be two or more elbow objects, with recipes that convert between them. The 9hd library had just one elbow type, a block. The original 10hd library had two elbows, blocks in mirror-symmetric locations; this was expanded to a larger list for the 10hd Demonoid. The 0hd Demonoid also has a multi-elbow recipe library. A slow elbow toolkit may make use of an even larger number of glider output recipes, because the target elbow object in that case is not restricted to a single diagonal line.

If only one colour, parity, or phase of glider can be emitted, then the mechanism will be limited to producing monochromatic salvos or monoparity salvos. These are less efficient at most construction tasks, but are still generally accepted to enable universal toolkits. See also half-baked knightship.

:construction envelope The region affected by an active reaction, such as a glider synthesis of an object. The envelope corresponds to the state-2 blue cells in LifeHistory. See also edgy.

:construction lane Part of a construction arm between the shoulder and the elbow - in particular, one of the fixed lanes that elbow operation signals travel on. All known universal constructors have used arms with two or more construction lanes, except for the ones in the 0hd Demonoid and in recent single-channel construction recipes.

:construction recipe One or more streams of gliders or other signals fed into a universal constructor to create a target object. Compare glider recipe.

:construction universality See universal constructor.

:converter A conduit in which the input object is not of the same type as the output object. This term tends to be preferred when either the input object or the output object is a spaceship.

The following diagram shows a p8 pi-heptomino-to-HWSS converter. This was originally found by Dave Buckingham in a larger form (using a figure-8 instead of the boat). The improvement shown here is by Bill Gosper (August 1996). Dieter Leithner has since found (much larger) oscillators of periods 44, 46 and 60 that can be used instead of the Kok's galaxy.

For another periodic converter, see the glider-to-LWSS example in queen bee shuttle pair. However, many converters are stable. Examples of elementary conduit converters include BFx59H, 135-degree MWSS-to-G, and 45-degree MWSS-to-G.

The earliest and simplest stable converters known are shown below. These are an HWSS-to-loaf, MWSS-to-beehive, and LWSS-to-blinker. These can serve as memory cells, or as the first steps in constructing objects using salvos.

:convoy A collection of spaceships all moving in the same direction at the same speed. Convoys are usually not destroyed by the reactions that they cause. Compare salvo. For examples, see reanimation, fly-by deletion and glider turner.

:copperhead (c/10 orthogonally, p10) The following small c/10 spaceship, discovered by conwaylife.com forum user 'zdr' on 5 March 2016, using a simple depth-first search program. A glider synthesis was found on the same day.

:Corder- Prefix used for things involving switch engines, after Charles Corderman.

:Corder engine = switch engine

:Cordergun A gun firing Corderships. The first was built by Jason Summers in July 1999, using a glider synthesis by Stephen Silver.

:Cordership Any spaceship based on switch engines. These necessarily move at a speed of c/12 diagonally with a period of 96 or a multiple thereof. The first Cordership was constructed by Dean Hickerson in April 1991, using 13 switch engines. He soon reduced this to 10, and in August 1993 to 7. In July 1998 he reduced it to 6. In January 2004, Paul Tooke found the 3-engine glide symmetric Cordership shown below.

At the end of 2017, Aidan F. Pierce discovered a clean 2-engine Cordership. There is also an adjustable-length 4-engine Cordership found by Michael Simkin, made up of two identical or mirror-image 2-engine components. The leading pair of switch engines builds a block trail, which are then deleted by the trailing pair.

Corderships generate sparks which can perturb other objects in many ways, especially gliders which can reach them from the side or from behind. Some perturbations reflect gliders back the way they came, and can be used for constructions such as the caber tosser and the infinite glider hotel.

:cousins (p3) This contains two copies of the stillater rotor.

:cover The following induction coil. See scrubber for an example of its use.

:covered table = cap

:cow (c p8 fuse)

:CP pulsar = pulsar

:CP semi-cenark A colour-preserving variant of Tanner Jacobi's century-based semi-Snark mechanism, the semi-cenark. See CC semi-cenark for the colour-changing version, or semi-cenark for repeat time details and an alternate initial catalyst.

:CP semi-Snark A period-multiplying colour-preserving signal conduit found by Tanner Jacobi in October 2017, producing one output glider for every two input gliders. It is made by replacing one of the eaters in a Snark with a catalyst found using Bellman. The catalyst causes the formation of a tub which requires a second glider to delete. However, this adds 5 ticks to the repeat time, so that it becomes 48. This is still 3 ticks faster than the CC semi-Snark.

:crab = quarter.

:crane (c/4 diagonally, p4) The following spaceship found by Nicolay Beluchenko in September 2005, a minor modification of a tubeater found earlier by Hartmut Holzwart. The wing is of the same form as in the swan and Canada goose.

:cross (p3) Found by Robert Wainwright in October 1989. The members of this family are all polyominoes.

:crowd (p3) Found by Dave Buckingham in January 1973.

:crown The p12 part of the following p12 oscillator, where it is hassled by a caterer, a jam and a HW emulator. This oscillator was found by Noam Elkies in January 1995.

:crucible = cauldron

:crystal A regular growth that is sometimes formed when a stream of gliders, or other spaceships, is fired into some junk.

The most common example is initiated by the following collision of a glider with a block. With a glider stream of even period at least 82, this gives a crystal which forms a pair of beehives for every 11 gliders which hit it.

:C-to-G = century-to-glider converter

:cuphook (p3) Found by Rich Schroeppel, October 1970. This is one of only three essentially different p3 oscillators with only three cells in the rotor. The others are 1-2-3 and stillater.

:curl = loop

:dart (c/3 orthogonally, p3) Found by David Bell, May 1992. A 25-glider recipe for the dart was found in December 2014 by Martin Grant and Chris Cain, making it the first glider-constructible c/3 spaceship.

:dead spark coil (p1) Compare spark coil.

:debris = ash.

:de Bruijn diagram = de Bruijn graph

:de Bruijn graph As applied to Life, a de Bruijn graph is a graph showing which pieces can be linked to which other pieces to form a valid part of a Life pattern of a particular kind.

For example, if we are interested in still lifes, then we could consider 2×3 rectangular pieces and the de Bruijn graph would show which pairs of these can be overlapped to form 3×3 squares in which the centre cell remains unchanged in the next generation.

David Eppstein's search program gfind is based on de Bruijn graphs.

:Deep Cell A pattern by Jared James Prince, based on David Bell's unit Life cell, in which each unit cell simulates two Life cells, in such a way that a Life universe filled with Deep Cells simulates two independent Life universes running in parallel.

In fact, a Life universe filled with Deep Cells can simulate infinitely many Life universes, as follows. Let P₁, P₂, P₃, ... be a sequence of Life patterns. Set the Deep Cells to run a simulation of P₁ in parallel with a simulation of a universe filled with Deep Cells, with these simulated Deep Cells running a simulation of P₂ in parallel with another simulation of a universe filled with Deep Cells, with these doubly simulated Deep Cells simulating P₃ in parallel with yet another universe of Deep Cells, and so on.

Deep Cell is available from http://psychoticdeath.com/life.htm.

:Demonoid The first self-constructing diagonal spaceship. A 0hd Demonoid was completed by Chris Cain in December 2015, shortly after a much larger 10hd version was constructed the previous month in collaboration with Dave Greene. The 0hd spaceship fits in a bounding box about 55,000 cells square, and displaces itself by 65 cells diagonally every 438,852 generations.

The first 0hd Demonoid was fired by a gun. No spaceship gun pattern had previously been completed before the first appearance of the actual spaceship.

In June 2017 Dave Greene completed a much simpler single-channel Demonoid using a temporary lossless elbow, which displaces itself 79 cells diagonally every 1,183,842 ticks. This was an improvement in terms of design complexity, but not in terms of speed, population, or bounding box. However, all of these could be further optimized. A smaller Hashlife-friendly single-channel Demonoid design was completed in 2018.

:demultiplexer A simple Herschel circuit consisting of three eater1s, found by Brice Due in August 2006. An input Herschel places a boat in a location accessible to an input glider. If the boat is present, a one-time turner reaction occurs and the glider is turned 90 degrees onto a new lane.

:demuxer = demultiplexer

:density The density of a pattern is the limit of the proportion of live cells in a (2n+1)×(2n+1) square centred on a particular cell as n tends to infinity, when this limit exists. (Note that it does not make any difference what cell is chosen as the centre cell. Also note that if the pattern is finite then the density is zero.) There are other definitions of density, but this one will do here.

In 1994 Noam Elkies proved that the maximum density of a stable pattern is 1/2, which had been the conjectured value. See the paper listed in the bibliography. Marcus Moore provided a simpler proof in 1995, and in fact proves that a still life with an m × n bounding box has at most (mn+m+n)/2 cells.

But what is the maximum average density of an oscillating pattern? The answer is conjectured to be 1/2 again, but this remains unproved. The best upper bound so far obtained is 8/13 (Hartmut Holzwart, September 1992).

The maximum possible density for a phase of an oscillating pattern is also unknown. An example with a density of 3/4 is known (see agar), but densities arbitrarily close to 1 may perhaps be possible.

:dependent conduit A Herschel conduit in which the input Herschel interacts with catalysts in the first few ticks. The standard interaction actually starts at T=-3, before the Herschel is completely formed. Compare independent conduit. The Herschel is prevented from emitting its first natural glider. This is useful in cases where the previous conduit cannot survive a first natural glider emitted from its output Herschel.

This term is somewhat confusing, since it is actually the previous conduit that depends on the dependent conduit to suppress the problematic glider. Dependent conduits such as the F166 and Lx200 do not actually depend on anything. They can be freely connected to any other conduits that fit, as long as the output Herschel evolves from its standard great-grandparent. As of this writing, the Fx158 is the only known case where a conduit's output Herschel has an alternate great-grandparent, which is incompatible with dependent conduits' initial transparent block.

:destructive read The most common type of test reaction in memory cell circuitry. Information is stored in a memory cell by placing objects in known positions, or by changing the state of a stable or periodic toggle circuit. A destructive-read test consists of sending one or more signals to the memory cell. A distinct output signal is produced for each possible state of the memory cell, which is reset to a known "zero" or "rest" state. See for example boat-bit, keeper, and demultiplexer.

To permanently store information in a destructive-read memory cell, the output signal(s) must be used, in part, to send appropriate signals back to the memory cell to restore its state to its previous value. With output looped back to input, this larger composite circuit then effectively becomes a non-destructive read memory cell.

:destructor arm A dedicated construction arm in the Gemini spaceship, used only for removing previously active circuitry once it is no longer needed. More generally, any circuitry in a self-constructing pattern dedicated exclusively to cleanup.

:D-heptomino = Herschel

:diamond = tub

:diamond ring (p3) Found by Dave Buckingham in 1972.

:diehard Any pattern that vanishes, but only after a long time. The following example vanishes in 130 generations, which is probably the limit for patterns of 7 or fewer cells. Note that there is no limit for higher numbers of cells. E.g., for 8 cells we could have a glider heading towards an arbitrarily distant blinker.

:dinner table (p12) Found by Robert Wainwright in 1972.

:dirty Opposite of clean. A reaction which produces a large amount of complicated junk which is difficult to control or use is said to be dirty. Many basic puffer engines are dirty and need to be tamed by accompanying spaceships in order to produce clean output. Similarly, a dirty conduit is one that does not recover perfectly after the passage of a signal; one or more extra ash objects are left behind (or more rarely a catalyst is damaged) and additional signals must be used to clean up the circuit before it can be re-used.

:diuresis (p90) Found by David Eppstein in October 1998. His original stabilization used pentadecathlons. The stabilization with complicated still lifes shown here (in two slightly different forms) was found by Dean Hickerson the following day. The name is due to Bill Gosper (see kidney).

:dock The following induction coil.

:domino The 2-cell polyomino. A number of objects, such as the HWSS and pentadecathlon, produce domino sparks.

:dormant An object that is either stable or oscillates without producing any output, until it is triggered by an appropriate signal, which then produces some desired action. For example, freeze-dried objects are dormant until the arrival of a particular glider.

:do-see-do The following reaction, found by David Bell in 1996, in which two gliders appear to circle around each other as they are reflected 90 degrees by a twin bees shuttle. Four copies of the reaction can be used to create a p92 glider loop which repeats the do-see-do reaction forever.

:double-barrelled Of a gun, emitting two streams of spaceships (or rakes) every period. For examples, see B-52 bomber, Simkin glider gun, and p246 gun. In most cases, the two streams are alternately emitted 1/2 period apart. It is also possible for the two streams to be emitted simultaneously, as in this double-barrelled glider gun by Bill Gosper:

:double block reaction A certain reaction that can be used to stabilize the twin bees shuttle (qv). This was discovered by David Bell in October 1996.

The same reaction sometimes works in other situations, as shown in the following diagram where a pair of blocks eats an R-pentomino and a LWSS. (The LWSS version was known at least as early 1994, when Paul Callahan saw it form spontaneously as a result of firing an LWSS stream at some random junk.)

:double caterer (p3) Found by Dean Hickerson, October 1989. Compare caterer and triple caterer.

:double ewe (p3) Found by Robert Wainwright before September 1971.

:double wing = moose antlers. This term is no longer in use.

:dove The following induction coil, found in 2015 to be a possible active reaction for the input or output of a converter.

:down boat with tail = cis-boat with tail

:dr Short identifier for Dean Hickerson's 'drifter' search program, used at various times to find wires, eaters, higher-period billiard table configurations, and related signal-carrying and signal-processing mechanisms. See also drifter.

:dragon (c/6 orthogonally, p6) This spaceship, discovered by Paul Tooke in April 2000, was the first known c/6 spaceship. With 102 cells, it was the smallest known orthogonal c/6 spaceship until Hartmut Holzwart discovered 56P6H1V0 in April 2009.

:drain trap = paperclip. This term is no longer in use.

:D read = destructive read

:dried = freeze-dried.

:drifter A perturbation moving within a stable pattern. Dean Hickerson has written a search program to search for drifters, with the hope of finding one which could be moved around a track. Because drifters can be very small, they could be packed more tightly than Herschels, and so allow the creation of oscillators of periods not yet attained, and possibly prove that Life is omniperiodic. Hickerson has found a number of components towards this end, but it has proved difficult to change the direction of movement of a drifter, and so far no complete track has been found. However, Hickerson has had success using the same program to find eaters with novel properties, such as sparking eaters and the ones shown in diuresis.

:dual 1-2-3-4 = Achim's p4

:duoplet A diagonal two-bit spark produced by many oscillators and eater reactions. Among other uses, it can reflect gliders 90 degrees. The following pattern shows an eater5 eating gliders and producing duoplets which are then used to reflect a separate glider stream. If only one glider is present, the eater5 successfully absorbs it, so this mechanism may be considered to be a simple AND gate.

:dying spark See spark. A spark by definition dies out completely after some number of ticks.

:early universe Conway's somewhat confusing term for sparse Life.

:eater Any still life that has the ability to interact with certain patterns without suffering any permanent damage. (If it doesn't suffer even temporary damage then it may be referred to as a rock.) The eater1 is a very common eater, and the term "eater" is often used specifically for this object. Other eaters include eater2, eater3, eater4, and eater5, and many hundreds of others are known. Below is a complex eater found by Dean Hickerson in 1998 using his dr search program. It takes 25 ticks to recover after feasting on a glider:

Some common still lifes can act as eaters in some situations, such as the block, ship, and tub. In fact the block was the first known eater, being found capable of eating beehives from a queen bee.

:eater1 (p1) Usually simply called an eater, and also called a fishhook.

This eater can be constructed using a simple two-glider collision, as shown in stamp collection. It is often modified in various ways, or welded to other objects, to allow tighter packing of circuits or to allow a signal stream to pass close by. See clearance for an eater1 variant that is 1hd shorter to the southeast than the standard fishhook form. An eater1 can also be used as a 90-degree one-time turner.

Its ability to eat various objects was discovered by Bill Gosper in 1971. The fishhook eater can consume a glider, a LWSS, and a MWSS as shown below. It is not able to consume an HWSS, however. See honey bit or killer toads for that.

:eater2 (p1) This eater was found by Dave Buckingham in the 1970s. Mostly it works like the ordinary eater1 but with two slight differences that make it useful despite its size: it takes longer to recover from each bite, and it can eat objects appearing at two different positions.

The following eater2 variant (Stephen Silver, May 1998) can be useful for obtaining smaller bounding boxes. A more compact variant with the same purpose can be seen under gliderless.

:eater3 (p1) This large symmetric eater, found by Dave Buckingham, has a very different eating action from the eater1 and eater2. The loaf can take bites out things, being flipped over in the process. The rest of the object merely flips it back again.

:eater4 (p1) Another eater by Dave Buckingham, which he found in 1971, but did not recognize as an eater until 1975 or 1976. It can't eat gliders, but it can be used for various other purposes. The four NE-most centre cells regrow in a few generations after being destroyed by taking a bite out of something, such as suppressing half of a developing traffic light as it does in the p29 pentadecathlon hassler.

:eater5 (p1) A compound eater that can eat gliders coming from two different directions. Also called the tub-with-tail eater (TWIT), it is often placed along the edges of glider lanes to suppress unwanted gliders in conduits. Below is the standard form, a compact form with a long hook, and an often-useful conjoined form found with Bellman. The sidesnagger is a Spartan constellation that has a similar glider-absorbing function, using a loaf. See also 7x9 eater.

With gliders from either direction, the eater5's eating reaction creates a spark that can be used to reflect other gliders. See the example pattern in duoplet, or advance any of the topmost three gliders in the above pattern by two ticks.

:eater/block frob (p4) Found by Dave Buckingham in 1976 or earlier.

:eater-bound pond = biting off more than they can chew

:eater-bound Z-hexomino = pentoad

:eater eating eater = two eaters

:eater plug (p2) Found by Robert Wainwright, February 1973.

:eaters plus = French kiss

:ecologist (c/2 orthogonally, p20) This consists of the classic puffer train with a LWSS added to suppress the debris. See also space rake.

:edge-repair spaceship A spaceship which has an edge that possesses no spark and yet is able to perturb things because of its ability to repair certain types of damage to itself. The most useful examples are the following two small p3 c/3 spaceships:

:edge shooter A gun or signal circuit that fires its gliders (or whatever) right at the edge of the pattern, so that it can be used to fire them closely parallel to others. This is useful for constructing complex guns. Compare glider pusher, which can in fact be used for making edge shooters.

The following diagram shows a p46 edge shooter found by Paul Callahan in June 1994.

Stable edge shooters became possible with the development of Herschel circuitry. For example, NW31, BNE14T30, RNE-19T84, and the high-clearance Fx119 inserter are often used in shotguns for complex salvos. Composite edge-shooter circuits with arbitrarily high clearance can be constructed.

:edge spark A spark at the side of a spaceship that can be used to perturb things as the spaceship passes by.

:edge sparker A spaceship that produces one or more edge sparks.

:edgy In slow salvo terminology, an edgy glider construction recipe is one that places its final product at or very near the edge of its construction envelope. Similarly, an edgy factory will place its output object in an accessible location near the edge of its reaction envelope.

:egg = non-spark. This term is no longer in use.

:E-heptomino Name given by Conway to the following heptomino.

:elbow Depending on context, this term may refer to a signal elbow or a construction elbow. See also elbow ladder.

:elbow ladder Scot Ellison's name for the type of pattern he created in which one or more gliders shuttle back and forth (using the kickback reaction) deleting the output gliders from a pair of slide guns.

:elbow operation A recipe, usually a salvo of gliders travelling on one or more construction lanes, that collides with an elbow constellation and performs one of the standard transformations on it: push, pull, or fire for simple construction arms, along with possible construct, duplicate-elbow, or delete-elbow ops for more complicated systems. See construction elbow.

:electric fence (p5) A stabilization of ants. Dean Hickerson, February 1993.

:elementary Not reducible to a combination of smaller parts. Elementary spaceships in particular are usually those found by search programs, and they can't be subdivided into smaller spaceships, tagalongs, and supporting reactions, as contrasted with engineered macro-spaceships.

:elementary conduit A conduit with no recognizable active signal stage besides its input and output. An early example still very commonly used is Buckingham's BFx59H, which transforms a B-heptomino into an inverted Herschel in 59 ticks. The BFx59H elementary conduit is a component in many of the original universal toolkit of Herschel conduits. An extension of the same naming convention is used for elementary conduits, with the first and last letters of the name specifying the input and output signal objects. As with Herschels, an arbitrary orientation and center point is chosen for each object. "Fx" means the signal moves forward and produces a mirror-image output. See Herschel conduit for further details.

Theoretically an elementary conduit may become a composite conduit, if another conduit can be found that shares the beginning or end of the conduit in question. In practice this happens only rarely, because many of the most likely branch points have already been identified: glider (G), LWSS (L) or MWSS (M), Herschel (H), B-heptomino (B), R-pentomino (R), pi-heptomino (P), queen bee shuttle (Q), century or bookend (C), dove (D), and wing (W). A Herschel descendant might qualify, due to the elementary conduit that can be seen in the p184 gun. However, there are very few simple conduits that produce Herschel descendants without Herschels, so in practice this is not a useful branch point.

:elevener (p1)

:Elkies' p5 (p5) Found by Noam Elkies in 1997.

:emu Dave Buckingham's term for a Herschel loop that does not emit gliders (and so is "flightless"). All known Herschel loops of periods 52, 57, 58, 59 and 61 are emus. See also Quetzal.

:emulator Any one of three p4 oscillators that produce sparks similar to those produced by LWSS, MWSS and HWSS. See LW emulator, MW emulator and HW emulator. Larger emulators are also possible, but they require stabilizing objects to suppress their non-sparks and so are of little use. The emulators were discovered by Robert Wainwright in June 1980.

:engine The active portion of an object (usually a puffer or gun) which is considered to actually produce its output, and which generally permits no variation in how it works. The other parts of the object are just there to support the engine. For examples, see puffer train, Schick engine, blinker puffer, frothing puffer and line puffer.

:engineless A rake or puffer which does not contain a specific engine for its operation. Instead it depends on perturbations of gliders or other objects by passing spaceships. The period of such objects is often adjustable, and in some cases the speed as well. An early example was the creation of c/5 rakes in September 1997, using gliders circulating among a convoy of c/5 spaceships. More recently, the passing spaceships themselves are also constructed, as in the Caterloopillar.

:en retard (p3) Found by Dave Buckingham, August 1972.

:Enterprise (c/4 diagonally, p4) Found by Dean Hickerson, March 1993.

:envelope See construction envelope, reaction envelope.

:Eureka (p30) A pre-pulsar shuttle found by Dave Buckingham in August 1980. A variant is obtained by shifting the top half two spaces to either side.

:evolution The process or result of running one or more generations of an object. For example, a row of 10 cells evolves into a pentadecathlon.

:evolutionary factor For an unstable pattern, the time to stabilization divided by the initial population. For example, the R-pentomino has an evolutionary factor of 220.6, while bunnies has an evolutionary factor of 1925.777... The term is no longer in use.

:exhaust The debris or smoke left behind by a puffer, especially if the debris is dirty and takes many generations to settle. The term is not usually used for the objects created by clean puffers.

:exponential filter A toolkit developed by Gabriel Nivasch in 2006, enabling the construction of patterns with asymptotic population growth matching O((log log ... log(t))) for any number of nested log operations. See also quadratic filter, recursive filter.

:exposure = underpopulation

:extensible A pattern is said to be extensible if arbitrarily large patterns of the same type can be made by repeating parts of the original pattern in a regular way. For examples, see p6 shuttle, pentoad, pufferfish spaceship, snacker, wavestretcher, wicktrailer and branching spaceship.

:extra extra long = long^4

:extra long = long^3

:extremely impressive (p6) Found by Dave Buckingham, August 1976.

:extruder See traffic lights extruder. A single-channel constructor arm has also been programmed to extrude a growing wick consisting of a chain of Snarks, again working from the stationary fencepost end of the wick with no need for a wickstretcher component.

:F116 An elementary conduit, one of the original sixteen Herschel conduits, discovered by Paul Callahan in February 1997. After 116 ticks, it produces a Herschel at (32, 1) relative to the input. Its recovery time is 138 ticks; this can be reduced to 120 ticks by adding extra mechanisms to suppress the internal glider. It is Spartan only if the following conduit is a dependent conduit, so that the welded FNG eater can be removed. A ghost Herschel in the pattern below marks the output location:

:F117 A composite conduit, one of the original sixteen Herschel conduits, discovered by Dave Buckingham in July 1996. It is made up of two elementary conduits, HFx58B + BFx59H. After 117 ticks, it produces a Herschel at (40, -6) relative to the input. Its recovery time is 63 ticks. It can be made Spartan by replacing the snake with an eater1 in one of two orientations. A ghost Herschel in the pattern below marks the output location:

:F166 A composite conduit, one of the original sixteen Herschel conduits, discovered by Paul Callahan in May 1997. It is composed of two elementary conduits, HFx107B + BFx59H. The F166 and Lx200 conduits are the two original dependent conduits (several more have since been discovered). After 166 ticks, it produces a Herschel at (49, 3) relative to the input. Its recovery time is 116 ticks. A ghost Herschel in the pattern below marks the output location:

:F171 An elementary conduit, the seventeenth Herschel conduit, discovered by Brice Due in August 2006 in a search using only eaters as catalysts. This was the first new Herschel conduit discovery since 1998. After 171 ticks, it produces a Herschel at (29, -17) relative to the input. A ghost Herschel in the pattern below marks the output location:

The conduit's recovery time is 227 ticks, slower than many of the original sixteen conduits because of the delayed destruction of a temporary blinker, though the circuit itself is clearly Spartan. The recovery time can be improved to 120 ticks by adding sparkers of various periods to suppress the blinker. See clock for a period-2 example.

The central eater in the group of three to the northwest can be removed to release an additional glider output signal on a transparent lane.

:factory Another word for gun, but not used in the case of glider guns. The term is also used for a pattern that repeatedly manufactures objects other than spaceships or rakes. In this case the new objects do not move out of the way, and therefore must be used up in some way before the next one is made. The following shows an example of a p144 gun which consists of a p144 block factory whose output is converted into gliders by a p72 oscillator.

:familiar fours Common patterns of four identical objects. The five commonest are traffic light (4 blinkers), honey farm (4 beehives), blockade (4 blocks), fleet (4 ships, although really 2 ship-ties) and bakery (4 loaves, although really 2 bi-loaves). Also sometimes included is four skewed blocks.

:fanout A mechanism that emits two or more objects of some type for each one that it receives. Typically the objects are gliders or Herschels; glider duplicators are a special case.

:Fast Forward Force Field The following reaction found by Dieter Leithner in May 1994. In the absence of the incoming LWSS the gliders would simply annihilate one another, but as shown they allow the LWSS to advance 11 spaces in the course of the next 6 generations.

The illusion of super-light-speed travel is caused by an LWSS that is always created, but is then destroyed in some cases, by a signal catching up to it from behind that necessarily never travels faster than the speed of light. It is not possible to make any use of the apparent super-light-speed signal. The front end of an output LWSS can't be distinguished from the alternative dying spark output until several more ticks have passed. Not surprisingly, this extra time is enough to drop the average speed of information transmission safely below c.

Leithner named the Fast Forward Force Field in honour of his favourite science fiction writer, the physicist Robert L. Forward. See also star gate and speed booster.

:fate The result of evolving a pattern until its final behaviour is known. This answers such questions such as whether or not the pattern remains finite, what its growth rate is, what period the final state may settle into, and what its final census is. All small Life objects seem to eventually settle down into a mix of oscillators, simple spaceships, and occasionally small puffers. See methuselah, soup, ash.

Most sufficiently large random patterns are expected to grow forever due to the production of switch engines at their boundary. Engineered Life objects - and therefore also sufficiently large and unlikely random patterns - can have more interesting behaviour, such as breeders, sawtooths, and prime calculators. Some objects have even been constructed or designed having an unknown fate.

:father = parent

:fd Abbreviation for full diagonals.

:featherweight spaceship = glider

:fencepost Any pattern that stabilizes one end of a wick.

:Fermat prime calculator A pattern constructed by Jason Summers in January 2000 that exhibits infinite growth if and only if there are no Fermat primes greater than 65537. The question of whether or not it really does exhibit infinite growth is therefore equivalent to a well-known and long-standing unsolved mathematical problem. It will, however, still be growing at generation 10^2585827975. The pattern is based on Dean Hickerson's primer and caber tosser patterns and a p8 beehive puffer by Hartmut Holzwart.

:F-heptomino Name given by Conway to the following heptomino.

:figure-8 (p8) A domino sparker found by Simon Norton in 1970.

:filter Any oscillator used to delete some but not all of the spaceships in a stream. An example is the blocker, which can be positioned so as to delete every other glider in a stream of period 8n+4, and can also do the same for LWSS streams. Other examples are the MW emulator and T-nosed p4 (either of which can be used to delete every other LWSS in a stream of period 4n+2), the fountain (which does the same for MWSS streams) and a number of others, such as the p6 pipsquirter, the pentadecathlon and the p72 oscillator shown under factory. Another example, a p4 oscillator deleting every other HWSS in a stream of period 4n+2, is shown below. (The p4 oscillator here was found, with a slightly larger stator, by Dean Hickerson in November 1994.)

:filter stream A stream of spaceships in which there are periodic gaps in the stream. This can thin out another crossing stream by deleting the spaceships in the second stream except where the gaps occur. The filter stream is not affected by the deletions so that the same stream can thin out multiple other streams. The Caterpillar uses filter streams of MWSSs in which there is a gap every 6 spaceships. Here is part of a filter stream that thins a glider stream by 2/3:

:finger A protruding cell in an oscillator or dying spark, with the ability to modify a nearby active reaction. Like a thumb, a finger cell appears at the edge of a reaction envelope and is the only live cell in its row or column. The finger spark remains alive for two ticks before dying, whereas a thumb cell dies after one tick. Because the key cell is kept alive for an extra tick, an alternate technical term is "held (orthogonal) bit spark". A "held diagonal bit spark" is not possible in B3/S23 for obvious reasons.

:fire An encoded signal used in combination with push and pull elbow operations in a simple construction arm. When a FIRE signal is sent, the construction-arm elbow produces an output glider, usually at 90 degrees from the construction arm. This terminology is generally used when there is only a single recipe for such a glider output, or only one recipe for each glider colour (e.g., FIRE WHITE, FIRE BLACK).

:fireship (c/10 orthogonally, p10) A variant of the copperhead with a trailing component that emits several large sparks, discovered by Simon Ekström on 20 March 2016. The interaction between the copperhead and the additional component is minimal enough that the extension technically fits the definition of a tagalong. However, the extension slightly modifies two of the phases of the spaceship, starting two ticks after the phase shown below, so it's also valid to classify the fireship as a distinct spaceship.

:fire-spitting (p3) Found by Nicolay Beluchenko, September 2003.

:first natural glider The glider produced at T=21 during the evolution of a Herschel. This is the most common signal output from a Herschel conduit.

:fish A generic term for LWSS, MWSS and HWSS, or, more generally, for any spaceship. In recent years *WSS is much more commonly used to refer to the small orthogonal c/2 spaceships.

:fishhook = eater1

:fleet (p1) A common formation of two ship-ties.

:flip-flop Any p2 oscillator. However, the term is also used in two more specific (and non-equivalent) senses: (a) any p2 oscillator whose two phases are mirror images of one another, and (b) any p2 oscillator in which all rotor cells die from underpopulation. In the latter sense it contrasts with on-off. The term has also been used even more specifically for the 12-cell flip-flop shown under phoenix.

:flip-flops Another name for the flip-flop shown under phoenix.

:flipper Any oscillator or spaceship that forms its mirror image halfway through its period.

:flotilla A spaceship composed of a number of smaller interacting spaceships. Often one or more of these is not a true spaceship and could not survive without the support of the others. The following example shows an OWSS escorted by two HWSS.

:fly A certain c/3 tagalong found by David Bell, April 1992. Shown here attached to the back of a small spaceship (also by Bell).

:fly-by deletion A reaction performed by a passing convoy of spaceships which deletes a common stationary object without harming the convoy. Fly-by deletion is often used in the construction of puffers and spaceships to clean up unwanted debris.

For c/2 convoys this is not usually difficult since the LWSS, MWSS, and HWSS spaceships have such useful sparks. However, some objects are more difficult to delete. For example, deleting a tub appears to require an unusual p4 spaceship.

The deletion of a pond appears to require a convoy which is 89 cells in width containing a very unusual p4 spaceship which has 273 cells. There are small objects which have no known fly-by deletion reactions. However, as in the case of reanimation, hitting them with the output of rakes is an effective brute force method.

:flying machine = Schick engine

:FNG = first natural glider.

:fore and back (p2) Compare snake pit. Found by Achim Flammenkamp, July 1994.

:forward glider A glider which moves at least partly in the same direction as the puffer(s) or spaceship(s) under consideration.

:fountain (p4) Found by Dean Hickerson in November 1994, and named by Bill Gosper. See also filter and superfountain.

:four skewed blocks (p1) The following constellation, sometimes considered to be one of the familiar fours.

:fourteener (p1)

:fox (p2) This is the smallest asymmetric p2 oscillator. Found by Dave Buckingham, July 1977.

:freeze-dried A term used for a glider constructible seed that can activated in some way to produce a complex object. For example, a "freeze-dried salvo" is a constellation of constructible objects which, when triggered by a single glider, produces a unidirectional glider salvo, and nothing else. Freeze-dried salvos can be useful in slow salvo constructions, especially when an active circuit has to destroy or reconstruct itself in a limited amount of time. Gradual modification by a construction arm may be too slow, or the circuit doing the construction may itself be the object that must be modified.

The concept may be applied to other types of objects. For example, one possible way to build a gun for a waterbear would be to program a construction arm to build a freeze-dried waterbear seed, and then trigger it when the construction is complete.

:French kiss (p3) Found by Robert Wainwright, July 1971.

:frog II (p3) Found by Dave Buckingham, October 1972.

:frothing puffer A frothing puffer (or a frothing spaceship) is a puffer (or spaceship) whose back end appears to be unstable and breaking apart, but which nonetheless survives. The exhaust festers and clings to the back of the puffer/spaceship before breaking off. The first known frothing puffers were c/2, and most were found by slightly modifying the back ends of p2 spaceships. A number of these have periods which are not a multiple of 4 (as with some line puffers). Paul Tooke has also found c/3 frothing puffers.

The following p78 c/2 frothing puffer was found by Paul Tooke in April 2001.

:frothing spaceship See frothing puffer.

:frozen = freeze-dried.

:full diagonal Diagonal distance measurement, abbreviated "fd", often appropriate when a construction arm elbow or similar diagonally-adjustable mechanism is present.

:fumarole (p5) Found by Dean Hickerson in September 1989. In terms of its 7×8 bounding box this is the smallest p5 oscillator.

:fuse A wick burning at one end. For examples, see baker, beacon maker, blinker ship, boat maker, cow, harvester, lightspeed wire, pi ship, reverse fuse, superstring and washerwoman. Useful fuses are usually clean, but see also reburnable fuse.

A fuse can burn arbitrarily slowly, as demonstrated by the example Blockic fuse below. A signal, alternating between glider and MWSS form, travels up and down between two rows of blocks in a series of one-time turner reactions. The spacing shown here causes the fuse to burn 24 cells to the right every 240 generations, for a speed of c/10. Moving the bottom half further from the top half by any even number of cells will slow down the burning even further.

:Fx119 An elementary conduit, one of the original sixteen Herschel conduits, discovered by Dave Buckingham in September 1996. After 119 ticks, it produces an inverted Herschel at (20, 14) relative to the input. Its recovery time is 231 ticks; this can be reduced somewhat by suppressing the output Herschel's glider, or by adding extra catalysts to make the reaction settle more quickly. A ghost Herschel in the pattern below marks the output location:

:Fx119 inserter A Herschel-to-glider converter and edge shooter based on an Fx119 Herschel conduit:

This edge shooter has an unusually high 27hd clearance, one of the highest known for a single small component. The only known higher-clearance edge shooters are injectors making use of multiple interacting spaceships. This makes the Fx119 inserter ideal for the construction of wide convoys whose total width can fit within its clearance distance.

The component creates a large cloud of smoke behind its emitted glider which lasts for over 90 generations. In spite of this, many tightly packed convoys can be made by injecting later gliders behind others in the convoy, helped along by the insertion reaction which is able to catch up to the existing gliders. The Fx119 inserter can place a glider on the same lane as a passing glider and as close as 15 ticks behind, which is only one step away from the minimum possible following distance.

:Fx153 A composite conduit, one of the original sixteen Herschel conduits, discovered by Paul Callahan in February 1997. It is made up of two elementary conduits, HF94B + BFx59H. After 153 ticks, it produces an inverted Herschel at (48, -4) relative to the input. Its recovery time is 69 ticks. It can be made Spartan by replacing the snake with an eater1 in one of two orientations. A ghost Herschel in the pattern below marks the output location:

:Fx158 An elementary conduit, one of the original sixteen Herschel conduits, discovered by Dave Buckingham in July 1996. After 158 ticks, it produces an inverted Herschel at (27, -5) relative to the input. Its recovery time is 176 ticks. It is the only known small conduit that does not produce its output Herschel via the usual Herschel great-grandparent, so it cannot be followed by a dependent conduit. A ghost Herschel in the pattern below marks the output location:

:Fx176 A composite conduit, one of the original sixteen Herschel conduits, discovered by Paul Callahan in October 1997. It is made up of three elementary conduits, HF95P + PF35W + WFx46H. After 176 ticks, it produces an inverted Herschel at (45, 0) relative to the input. The recovery time of the standard form shown here is 92 ticks, but see the PF35W entry for a variant discovered in November 2017 that lowers the repeat time to 73 ticks. A ghost Herschel in the pattern below marks the output location.

:Fx77 An elementary conduit, one of the original sixteen Herschel conduits, discovered by Dave Buckingham in August 1996. After 77 ticks, it produces an inverted Herschel at (25, -8) relative to the input. Its recovery time is 61 ticks; this can be reduced slightly by suppressing the output Herschel's glider, as in the L112 case. A pipsquirter can replace the blinker-suppressing eater to produce an extra glider output. It is one of the simplest known Spartan conduits, and one of the few elementary conduits in the original set of sixteen.

In January 2016, Tanner Jacobi discovered a Spartan method of extracting an extra glider output (top variant below). A ghost Herschel marks the output location for each variant.

:G4 receiver An alternate Herschel receiver discovered by Sergei Petrov on 28 December 2011, using his previous glider to 2 blocks converter. In the pattern below the Herschel output is marked by a ghost Herschel. A glider also escapes to the northwest. For an explanation of the "G4" describing the tandem glider input, see Gn.

:Gabriel's p138 (p138) The following oscillator found by Gabriel Nivasch in October 2002.

:galaxy = Kok's galaxy

:Game of Life = Life

:Game of Life News A blog reporting on new Life discoveries, started by Heinrich Koenig in December 2004, currently found at http://pentadecathlon.com/lifenews/.

:Garden of Eden A configuration of ON and OFF cells that can only occur in generation 0. (This term was first used in connection with cellular automata by John W. Tukey, many years before Life.) It was known from the start that there are Gardens of Eden in Life, because of a theorem by Edward Moore that guarantees their existence in a wide class of cellular automata. Explicit examples have since been constructed, the first by Roger Banks, et al. at MIT in 1971. This example was 9 × 33. In 1974 J. Hardouin-Duparc et al. at the University of Bordeaux 1 produced a 6 × 122 example. The following shows a 12 × 12 example found by Nicolay Beluchenko in February 2006, based on a 13 × 12 one found by Achim Flammenkamp in June 2004.

Below is a 10×10 Garden of Eden found by Marijn Heule, Christiaan Hartman, Kees Kwekkeboom, and Alain Noels in 2013 using SAT-solver techniques. An exhaustive search of 90-degree rotationally symmetric 10×10 patterns was possible because the symmetry reduces the number of unknown cells by a factor of four.

Steven Eker has since found several asymmetrical Gardens of Eden that are slightly smaller than this in terms of bounding box area. Patterns have also been found that have only Garden of Eden parents. For related results see grandparent.

:Gemini ((5120,1024)c/33699586 obliquely, p33699586) The first self-constructing spaceship, and also the first oblique spaceship. It was made public by Andrew Wade on 18 May 2010. It was the thirteenth explicitly constructed spaceship velocity in Life, and made possible an infinite family of related velocities. The Gemini spaceship derives its name from the Latin "gemini", meaning twins, describing its two identical halves, each of which contains three Chapman-Greene construction arms. A tape of gliders continually relays between the two halves, instructing each to delete its parent and construct a daughter configuration.

:Gemini puffer See Pianola breeder.

:Geminoid A type of self-constructing circuitry that borrows key ideas from Andrew Wade's Gemini spaceship, but with several simplifications. The main feature common to the Gemini spaceship is the construction recipe encoding method. Information is stored directly, and much more efficiently, in the timings of moving gliders, rather than in a static tape with 1s and 0s encoded by the presence of small stationary objects.

Unlike the original Gemini, Geminoids have ambidextrous construction arms, initially using glider pairs on two lanes separated by 9hd, 10hd, or 0hd. The design was the basis for the linear propagator and the Demonoids. A more recent development is a Geminoid toolkit using a single-channel construction arm, which allows for the possibility of multiple elbows with no loss of efficiency, or the construction of temporary lossless elbows. Compare slow elbow.

Other new developments that could be considered part of the extended "Geminoid" toolkit include freeze-dried construction salvos and seeds, used when objects must be built within a short time window, and self-destruct circuits, which are used as an alternative to a destructor arm to clean up temporary objects in a similarly short window.

:generation The fundamental unit of time. The starting pattern is generation 0.

:germ (p3) Found by Dave Buckingham, September 1972.

:gfind A program by David Eppstein which uses de Bruijn graphs to search for new spaceships. It was with gfind that Eppstein found the weekender, and Paul Tooke later used it to find the dragon. It is available at http://www.ics.uci.edu/~eppstein/ca/gfind.c (C source code only).

Compare lifesrc.

:ghost Herschel A dying spark made by removing one cell from the Herschel heptomino. This particular spark has the advantage that, when placed in a conduit to mark the location of an input or output Herschel, it disappears cleanly without damaging adjacent catalysts, even in dependent conduits with a block only two cells away.

:GIG A glider injection gate. This is a device for injecting a glider into a glider stream. The injected glider is synthesized from one or more incoming spaceships assisted by the presence of the GIG. (This contrasts with some other glider injection reactions which do not require a GIG, as in inject.) Gliders already in the glider stream pass through the GIG without interfering with it. A GIG usually consists of a small number of oscillators.

For example, in July 1996 Dieter Leithner found the following reaction which allows the construction of a pseudo-period 14 glider stream. It uses two LWSS streams, a pentadecathlon and a volcano.

Glider injection gates are useful for building glider guns with pseudo-periods that are of the form nd, where n is a positive integer, and d is a proper divisor of some convenient base gun period (such as 30 or 46), with d > 13.

:glasses (p2) Compare scrubber and spark coil.

:glider (c/4 diagonally, p4) The smallest, most common and first discovered spaceship. This was found by Richard Guy in 1970 while Conway's group was attempting to track the evolution of the R-pentomino. The name is due in part to the fact that it is glide symmetric. (It is often stated that Conway discovered the glider, but he himself has said it was Guy. See also the cryptic reference ("some guy") in Winning Ways.)

:glider-block cycle An infinite oscillator based on the following reaction (a variant of the rephaser). The oscillator consists of copies of this reaction displaced 2n spaces from one another (for some n>6) with blocks added between the copies in order to cause the reaction to occur again halfway through the period. The period of the resulting infinite oscillator is 8n-20. (Alternatively, in a cylindrical universe of width 2n the oscillator just consists of two gliders and two blocks.)

:glider constructible See glider synthesis.

:glider construction = glider synthesis.

:glider duplicator Any reaction in which one input glider is converted into two output gliders. This can be done by oscillators or spaceships, or by Herschel conduits or other signal circuitry such as the stable example shown under splitter. The most useful glider duplicators are those with low periods.

The following period 30 glider duplicator demonstrates a simple mechanism found by Dieter Leithner. The input glider stream comes in from the upper left, and the output glider streams leave at the upper and lower right. One of the output glider streams is inverted, so an inverting reflector is required to complete the duplicator. To produce non-parallel output, an inline inverter could be substituted for the northmost p30 glider gun.

Spaceship convoys that can duplicate gliders are very useful since they (along with glider turners) provide a means to clean up many dirty puffers by duplicating and turning output gliders so as to impact into the exhaust to clean it up.

Glider duplicators and turners are known for backward gliders using p2 c/2 spaceships, and for forward gliders using p3 c/3 spaceships. These are the most general duplicators for these speeds.

:glider gun A gun that fires gliders. For examples, see Gosper glider gun, Simkin glider gun, new gun, p45 gun.

True-period glider guns are known for some low periods, and for all periods over 53 using Herschel conduit technology. See true for a list of known true-period guns. The lowest true-period gun possible is the p14 gun since that is the lowest possible period for any glider stream, but no example has yet been found.

Pseudo-period glider guns are known for every period above 13. These are made by using multiple true-period guns of some multiple of the period, and glider injection methods to fill in the gaps.

:glider injection gate = GIG

:glider lane See lane.

:gliderless A gun is said to be gliderless if it does not use gliders. The purist definition would insist that a glider does not appear anywhere, even incidentally. For a long time the only known way to construct LWSS, MWSS and HWSS guns involved gliders, and it was not until April 1996 that Dieter Leithner constructed the first gliderless gun (a p46 LWSS gun).

In October 2017 Matthias Merzenich used two copies of Tanner's p46 to create a p46 MWSS gun. This is the smallest known gliderless gun, and also the smallest known MWSS gun.

:glider pair Two gliders travelling in the same direction with a specific spacetime offset. In a transceiver the preferred term is tandem glider. For several years, glider pairs on lanes separated by 9 or 10 half diagonals were the standard building blocks in Geminoid construction arm recipes. In more recent 0hd and single-channel construction toolkits, all gliders share the same lane, but glider pairs and singletons are still important concepts.

:glider-producing switch engine See stabilized switch engine.

:glider pusher An arrangement of a queen bee shuttle and a pentadecathlon that can push the path of a passing glider out by one half-diagonal space. This was found by Dieter Leithner in December 1993 and is shown below. It is useful for constructing complex guns where it may be necessary to produce a number of gliders travelling on close parallel paths. See also edge shooter.

:glider recipe = glider synthesis.

:glider reflector See reflector.

:gliders by the dozen (stabilizes at time 184) In early references this is usually shown in a larger form whose generation 1 is generation 8 of the form shown here.

:glider stopper A Spartan logic circuit discovered by Paul Callahan in 1996. It allows a glider signal to pass through the circuit, leaving behind a beehive that can cleanly absorb a single glider from a perpendicular glider stream. Two optional glider outputs are also shown. The circuit can't be re-used until the beehive "bit" is cleared by the passage of at least one perpendicular input. A similar mechanism discovered more recently is shown in the beehive stopper entry.

:glider synthesis 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 Dave Buckingham. As of June 2018, this limit has been increased to 18 cells.

Perhaps the most interesting glider syntheses are those of spaceships, 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 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.

A 3-glider synthesis of a pentadecathlon is shown in the diagram below. This was found in April 1997 by Heinrich Koenig and came as a surprise, as it was widely assumed that anything using just three gliders would already be known.

:glider to 2 blocks A converter discovered by Sergei Petrov on 8 October 2011, used in his later G4 receiver.

:glider to block A converter discovered by Sergei Petrov that places a block at its right edge in response to a single glider input. This has a variety of uses in Herschel circuitry and other signal-processing applications.

:glider train A certain p64 c/2 orthogonal puffer that produces two rows of blocks and two backward glider waves. Ten of these were used to make the first breeder.

:glider turner Any reaction in which a glider is turned onto a new path by a spaceship, oscillator, or still life constellation. In the last two cases, the glider turner is usually called a reflector if the reaction is repeatable, or a one-time turner if the reaction can only happen once.

Glider turners are easily built using standard spaceships. The following diagram shows a convoy which turns a forward glider 90 degrees, with the new glider also moving forwards.

See also glider duplicator and reflector.

:glide symmetric Undergoing simultaneous reflection and translation. A glide symmetric spaceship is sometimes called a flipper.

:Gn An abbreviation specific to converters that produce multiple gliders. A "G" followed by any integer value means that the converter produces a tandem glider - two parallel glider outputs with lanes separated by the specified number of half diagonals.

:gnome = fox

:GoE = Garden of Eden

:GoL = Game of Life

:Golly A cross-platform open source Life program by Andrew Trevorrow and Tomas Rokicki. Unlike most Life programs it includes the ability to run patterns using the hashlife algorithm. It is available from http://golly.sourceforge.net.

:Gosper glider gun The first known gun, and indeed the first known finite pattern displaying infinite growth, found by Bill Gosper in November 1970. This period 30 gun remains the smallest known gun in terms of its bounding box, though some variants of the p120 Simkin glider gun have a lower population. Gosper later constructed several other guns, such as new gun and the p144 gun shown under factory. See also p30 gun.

:Gotts dots A 41-cell 187×39 superlinear growth pattern found by Bill Gosper in March 2006, who named it in honour of Nick Gotts, discoverer of many other low-population superlinear patterns, such as Jaws, the mosquitoes, teeth, catacryst and metacatacryst. See switch-engine ping-pong for the lowest-population superlinear growth pattern as of July 2018, along with a list of the record-holders.

Collisions within the pattern cause it to sprout its Nth switch engine at generation T = ~224n-6. The population of the pattern at time t is asymptotically proportional to t times log(t), so the growth rate is O(t ln(t)), faster than linear growth but slower than quadratic growth.

:gourmet (p32) Found by Dave Buckingham in March 1978. Compare with pi portraitor and popover.

:gp = glider pair

:GPSE = glider-producing switch engine

:grammar A set of rules for connecting components together to make an object such as a spaceship, oscillator or still life. For example, in August 1989 Dean Hickerson found a grammar for constructing an infinite number of short wide c/3 period 3 spaceships, using 33 different components and a table showing the ways that they can be joined together.

:grandfather = grandparent

:grandfatherless A traditional name for a pattern with one or more parents but no grandparent. This was a hypothetical designation until May 2016. See grandparent for details.

:grandparent A pattern is said to be a grandparent of the pattern it gives rise to after two generations. For over thirty years, a well-known open problem was the question of whether any pattern existed that had a parent but no grandparent. In 1972, LifeLine Volume 6 mentioned John Conway's offer of a $50 prize for a solution to the problem, but it remained open until May 2016 when a user with the conwaylife.com forum handle 'mtve' posted an example.

Other patterns have since been found that have a grandparent but no great-grandparent, or a great-grandparent but no great-great-grandparent. Further examples in this series almost certainly exist, but as of July 2018 none have yet been found.

:Gray counter (p4) Found in 1971. If you look at this in the right way you will see that it cycles through the Gray codes from 0 to 3. Compare with R2D2.

:gray ship = grey ship

:great on-off (p2)

:grey counter = Gray counter (This form is erroneous, as Gray is surname, not a colour.)

:grey ship A spaceship that contains a region with an average density of 1/2, and which is extensible in such a way that the region of average density 1/2 can be made larger than any given square region.

See also with-the-grain grey ship, against-the-grain grey ship and hybrid grey ship.

:grin The following common parent of the block. This name relates to the infamous Cheshire cat. See also pre-block.

:grow-by-one object A pattern whose population increases by one cell every generation. The smallest known grow-by-one object is the following 44-cell pattern (David Bell's one-cell improvement of a pattern found by Nicolay Beluchenko, September 2005).

:growing/shrinking line ship A line ship in which the line repeatedly grows and shrinks, resulting in a high-period spaceship.

:growing spaceship An object that moves like a spaceship, except that its front part moves faster than its back part and a wick extends between the two. Put another way, a growing spaceship is a puffer whose output is burning cleanly at a slower rate than the puffer is producing it. Examples include blinker ships, pi ships, and some wavestretchers.

:G-to-H A converter that takes a glider as an input signal and produces a Herschel output, which can then be used by other conduits. G-to-Hs are frequently used in stable logic circuitry. Early examples include Callahan G-to-H, Silver G-to-H, and p8 G-to-H for periodic circuits. A more compact recent example is the syringe.

:gull = elevener

:gun Any stationary pattern that emits spaceships (or rakes) forever. For examples see double-barrelled, edge shooter, factory, gliderless, Gosper glider gun, Simkin glider gun, new gun and true.

:gunstar Any of a series of glider guns of period 144+72n (for all non-negative integers n) constructed by Dave Buckingham in 1990 based on his transparent block reaction and Robert Wainwright's p72 oscillator (shown under factory).

:gutter A single straight line of cells along the axis of symmetry of a mirror-symmetric pattern. Most commonly this is an orthogonal line, and the pattern is then odd-symmetric (as opposed to even-symmetric, where the axis of symmetry follows the boundary between two rows or columns of cells).

The birth rule for Conway's Life trivially implies that if there are no live cells in the gutter of a symmetric pattern, new cells can never be born there. For examples, see 44P5H2V0, 60P5H2V0, Achim's p4, brain, c/6 spaceship, centinal, p54 shuttle, pufferfish, snail, spider, and pulsar (in two orientations).

:half-baked knightship ((6,3)c/2621440, p2621440) A self-supporting macro-spaceship with adjustable period but fixed direction, based on the half-bakery reaction. This was the first spaceship based on this reaction, constructed in December 2014 by Adam P. Goucher. It moves 6 cells horizontally and 3 cells vertically every 2621440+8N ticks, depending on the relative spacing of the two halves. It is one of the slowest known knightships, and the first one that was not a Geminoid. Chris Cain optimized the design a few days later to create the Parallel HBK.

The spaceship produces gliders from near-diagonal lines of half-bakeries, which collide with each other at 180 degrees. These collisions produce monochromatic salvos that gradually build and trigger seeds, which in turn eventually construct small synchronized salvos of gliders. These re-activate the lines of half-bakeries, thus closing the cycle and moving the entire spaceship obliquely by (6,3).

:half bakery = bi-loaf.

:half-bakery reaction The key reaction used in the half-baked knightship and Parallel HBK, where a half-bakery is moved by (6,3) when a glider collides with it, and the glider continues on a new lane. Ivan Fomichev noticed in May 2014 that pairs of these reactions at the correct relative spacing can create 90-degree output gliders:

:half diagonal A natural measurement of distance between parallel glider lanes, or between elbow locations in a universal construction arm elbow operation library. If two gliders are in the same phase and exactly lined up vertically or horizontally, N cells away from each other, then the two glider lanes are considered to be N half diagonals (hd) apart. Gliders that are an integer number of full diagonals apart must be the same colour, whereas integer half diagonals allow for both glider colours. See colour of a glider, linear propagator.

:half fleet = ship-tie

:Halfmax A pattern that acts as a spacefiller in half of the Life plane, found by Jason Summers in May 2005. It expands in three directions at c/2, producing a triangular region that grows to fill half the plane.

:hammer To hammer a LWSS, MWSS or HWSS is to smash things into the rear end of it in order to transform it into a different type of spaceship. A hammer is the object used to do the hammering. In the following example by Dieter Leithner an LWSS is hammered by two more LWSS to make it into an MWSS.

:hammerhead A certain front end for c/2 spaceships. The central part of the hammerhead pattern is supported between two MWSS. The picture below shows a small example of a spaceship with a hammerhead front end (the front 9 columns).

:hand Any object used as a slow salvo target by a construction arm.

:handshake An old MIT name for lumps of muck, from the following form (2 generations on from the stairstep hexomino):

:harbor (p5) Found by Dave Buckingham in September 1978. The name is by Dean Hickerson.

:harvester (c p4 fuse) Found by David Poyner, this was the first published example of a fuse. The name refers to the fact that it produces debris in the form of blocks which contain the same number of cells as the fuse has burnt up.

:hashlife A Life algorithm by Bill Gosper that is designed to take advantage of the considerable amount of repetitive behaviour in many large patterns of interest. It provides a means of evolving repetitive patterns millions (or even billions or trillions) of generations further than normal Life algorithms can manage in a reasonable amount of time.

The hashlife algorithm is described by Gosper in his paper listed in the bibliography at the end of this lexicon. Roughly speaking, the idea is to store subpatterns in a hash table so that the results of their evolution do not need to be recomputed if they arise again at some other place or time in the evolution of the full pattern. This does, however, mean that complex patterns can require substantial amounts of memory.

Tomas Rokicki and Andrew Trevorrow implemented Hashlife into Golly in 2005. See also macrocell.

:hassle See hassler.

:hassler An oscillator that works by hassling (repeatedly moving or changing) some object. For some examples, see Jolson, baker's dozen, toad-flipper, toad-sucker and traffic circle. Also see p24 gun for a good use of a traffic light hassler.

:hat (p1) Found in 1971. See also twinhat and sesquihat.

:HBK = half-baked knightship

:hd Abbreviation for half diagonal. This metric is used primarily for relative measurements of glider lanes, often in relation to self-constructing circuitry; compare Gn.

:heat For an oscillator or spaceship, the average number of cells which change state in each generation. For example, the heat of a glider is 4, because 2 cells are born and 2 die every generation.

For a period n oscillator with an r-cell rotor the heat is at least 2r/n and no more than r(1-(n mod 2)/n). For n=2 and n=3 these bounds are equal.

:heavyweight emulator = HW emulator

:heavyweight spaceship = HWSS

:heavyweight volcano = HW volcano

:hebdarole (p7) Found by Noam Elkies, November 1997. Compare fumarole. The smaller version shown below was found soon after by Alan Hensel using a component found by Dave Buckingham in June 1977. The top ten rows can be stabilized by their mirror image (giving an inductor) and this was the original form found by Elkies.

:hectic (p30) Found by Robert Wainwright in September 1984.

:Heisenburp device A pattern which can detect the passage of a glider without affecting the glider's path or timing. The first such device was constructed by David Bell in December 1992. The term, coined by Bill Gosper, refers to the fact that Heisenberg's Uncertainty Principle fails to apply in the Life universe. See also stable pseudo-Heisenburp and natural Heisenburp.

The following is an example of the kind of reaction used at the heart of a Heisenburp device. The glider at bottom right alters the reaction of the other two gliders without itself being affected in any way.

:Heisenburp effect See Heisenburp device.

:helix A convoy of standard spaceships used in a Caterpillar to move some piece of debris at the speed of the Caterpillar. The following diagram illustrates the idea. The leading edge of this example helix, represented by the glider at the upper right in the pattern below, moves at a speed of 65c/213, or slightly faster than c/4.

Adjustable-speed helices can produce a very wide range of spaceship speeds; see Caterloopillar.

:heptaplet Any 7-cell polyplet.

:heptapole (p2) The barberpole of length 7.

:heptomino Any 7-cell polyomino. There are 108 such objects. Those with names in common use are the B-heptomino, the Herschel and the pi-heptomino.

:Herschel (stabilizes at time 128) The following pattern which occurs at generation 20 of the B-heptomino.

The name is commonly ascribed to the Herschel heptomino's similarity to a planetary symbol. William Herschel discovered Uranus in 1781. However, in point of fact a Herschel bears no particular resemblance to either of the symbols used for Uranus, but does closely resemble the symbol for Saturn. So the appropriate name might actually be "Huygens", but "Herschel" is now universally used by tradition.

Herschels are one of the most versatile types of signal in stable circuitry. R-pentominoes and B-heptominoes naturally evolve into Herschels, and converters have also been found that change pi-heptominoes and several other signal types into Herschels, and vice versa. See elementary conduit.

:Herschel circuit A series of Herschel conduits or other components, connected by placing them so that the output Herschels from early conduits become the input Herschels for later conduits. Often the initial component is a converter accepting some other signal type as input - usually a glider, in which case a syringe is most commonly used. The Silver reflector is a well-known early Spartan Herschel circuit from before the syringe was discovered, where the initial converter is a Callahan G-to-H.

Sometimes a direct connection between two conduits is not possible due to unwanted gliders that destroy required catalysts, or wanted gliders that are not able to escape. In this case, small "spacer" conduits such as F116, F117, Fx77, R64, L112, or L156 can be inserted between the other conduits to solve the problem.

Some converter or factory conduits do not produce a Herschel as output, instead generating other useful results such as gliders, boats or MWSSes. See Herschel-to-glider, demultiplexer, and H-to-MWSS respectively for examples of these. For those conduits which do produce an unwanted Herschel, an eater such as SW-2 can be added to delete it.

If the first and last conduits of a chain connect to each other in a loop then there is no need for a syringe to generate the first Herschel, or an eater to consume the last one. The circuit becomes a self-supporting Herschel loop. A loop is also formed by a syringe connected to a Herschel-to-glider converter, with the glider reflected back to the syringe's input with glider reflectors of the appropriate colour, usually Snarks. In either case, if the loop has a surplus glider output, it becomes a gun; if no output is available it is an emu.

:Herschel climber Any reburnable fuse reaction involving Herschels. May refer specifically to the (23,5)c/79 Herschel climber used in the waterbear, or one of several similar reactions with various velocities. See also Herschel-pair climber.

:Herschel component = Herschel conduit

:Herschel conduit A conduit that moves a Herschel from one place to another. See also Herschel loop.

Well over a hundred simple stable Herschel conduits are currently known. As of June 2018 the number is approximately 150, depending on the precise definition of "simple" - e.g., fitting inside a 100×100 bounding box, and producing output in no more than 300 ticks. In general a Herschel conduit can be called "simple" if its active reaction does not return to a Herschel stage except at its output. Compare elementary conduit, composite conduit. A description of common usage in complex circuitry, using syringes and Snarks to make compact connections, can be found in Herschel circuit.

The original universal set consisted of sixteen stable Herschel conduits, discovered between 1995 and 1998 by Dave Buckingham (DJB) and Paul Callahan (PBC). These are shown in the following table. In this table, the number in "name/steps" is the number of ticks needed to produce an output Herschel from the input Herschel. "m" tells how the Herschel is moved (R = turned right, L = turned left, B = turned back, F = unturned, f = flipped), and "dx" and "dy" give the displacement of the centre cell of the Herschel (assumed to start in the orientation shown above).

	-----------------------------------------
	name/steps  m     dx   dy     discovery
	-----------------------------------------
	R64         R     11    9   DJB, Sep 1995
	Fx77        Fflip 25   -8   DJB, Aug 1996
	L112        L     12  -33   DJB, Jul 1996
	F116        F     32    1   PBC, Feb 1997
	F117        F     40   -6   DJB, Jul 1996
	Bx125       Bflip -9  -17   PBC, Nov 1998
	Fx119       Fflip 20   14   DJB, Sep 1996
	Fx153       Fflip 48   -4   PBC, Feb 1997
	L156        L     17  -41   DJB, Aug 1996
	Fx158       Fflip 27   -5   DJB, Jul 1996
	F166        F     49    3   PBC, May 1997
	Fx176       Fflip 45    0   PBC, Oct 1997
	R190        R     24   16   DJB, Jul 1996
	Lx200       Lflip 17  -40   PBC, Jun 1997
	Rx202       Rflip  7   32   DJB, May 1997
	Bx222       Bflip -6  -16   PBC, Oct 1998
	-----------------------------------------

See also Herschel transceiver.

:Herschel descendant A common active pattern occurring at generation 22 of a Herschel's evolution:

:Herschel great-grandparent A specific three-tick predecessor of a Herschel, commonly seen in Herschel conduit collections that contain dependent conduits. In some situations it is helpful to display the input reaction in this form instead of the standard Herschel form.

Dependent conduit inputs are catalysed by a transparent block before the Herschel's standard form can appear, and before the Herschel's first natural glider is produced. This means that these conduits will fail if an actual Herschel is placed in the "correct" input location for a dependent conduit. Refer to F166 or Lx200 to see the correct relative placement of the standard transparent block catalyst.

Almost all known Herschel conduits produce a Herschel great-grandparent near the end of their evolutionary sequence. In the original universal set of Herschel conduits, Fx158 is the only exception.

:Herschel loop A cyclic Herschel track. Although no loop of length less than 120 generations has been constructed it is possible to make oscillators of smaller periods by putting more than one Herschel in a higher-period track. In this way oscillators, and in most cases guns, of all periods from 54 onwards can now be constructed (although the p55 case is a bit strange, shooting itself with gliders in order to stabilize itself). A mechanism for a period-52 loop was found in April 2018, but it includes a stage where the signal is carried by a triplet of gliders so it may not be considered to be a pure Herschel loop. The missing period, 53, is a difficult case simply because 53 is prime and so no small sparkers or reflectors are available.

See Simkin glider gun and p256 gun for the smallest known Herschel loops. See also emu and omniperiodic.

:Herschel-pair climber Any reburnable fuse reaction involving pairs of Herschels. May refer specifically to the 31c/240 Herschel-pair climber used in the Centipede, or one of several similar reactions with various velocities. See also Herschel climber.

:Herschel receiver Any circuit that converts a tandem glider into a Herschel signal. The following diagram shows a pattern found by Paul Callahan in 1996, as part of the first stable glider reflector. Used as a receiver, it converts two parallel input gliders (with path separations of 2, 5, or 6) to an R-pentomino. The signal is then converted to a Herschel by one of several known mechanisms, the first of which was found by Dave Buckingham way back in 1972. The second is elementary conduit RF48H, found by Stephen Silver in October 1997. The receiver version shown below uses Buckingham's R-to-Herschel converter, which is made up of elementary conduit RF28B followed by BFx59H.

:Herschel stopper A method of cleanly suppressing a Herschel signal with an asynchronous boat-bit, discovered by Dean Hickerson. Here a ghost Herschel marks the location of the output signal, in cases where the boat-bit is not present. Other boat-bit locations that allow for clean suppression of a Herschel are also known.

This term is also sometimes used to refer to any mechanism that cleanly suppresses a Herschel. These usually allow the Herschel's first natural glider to escape, so they are more commonly classified as converters. See SW-2.

:Herschel-to-glider The largest category of elementary conduit. Gliders are very common and self-supporting, so it's much easier to find these than any other type of output signal. A large collection of these H-to-G converters has been compiled, with many different output lanes and timings. These can be used to synchronize multiple signals to produce gun patterns or complex logic circuitry. See NW31T120 for an example.

:Herschel track A track for Herschels. An equivalent term is Herschel circuit. See also B track.

:Herschel transceiver An adjustable Herschel conduit made up of a Herschel transmitter and a Herschel receiver. The intermediate stage consists of a tandem glider - two gliders on parallel lanes - so that the transmitter and receiver can be separated by any required distance. The conduit may be stable, or may contain low-period oscillators.

:Herschel transmitter Any Herschel-to-two-glider converter that produces a tandem glider that can be used as input to a Herschel receiver. If the gliders are far enough apart, and if one of the gliders is used only for cleanup, then the transmitter is ambidextrous: with a small modification to the receiver, a suitably oriented mirror image of the receiver will also work.

The following diagram shows a stable Herschel transmitter found by Paul Callahan in May 1997:

:Hertz oscillator (p8) Compare negentropy, and also cauldron. Found by Conway's group in 1970.

:hexadecimal = beehive and dock

:hexaplet Any 6-cell polyplet.

:hexapole (p2) The barberpole of length 6.

:hexomino Any 6-cell polyomino. There are 35 such objects. For some examples see century, stairstep hexomino, table, toad and Z-hexomino.

:HF = honey farm

:HFx58B A common Herschel to B-heptomino converter, used as the first stage of F117 and many other Herschel conduits. There are two variants, both shown in the pattern below.

:H-heptomino Name given by Conway to the following heptomino. After one generation this is the same as the I-heptomino.

:high-bandwidth telegraph (p960, p30 circuitry) A variant of the telegraph constructed by Louis-François Handfield in February 2017, using periodic components to achieve a transmission rate of one bit per 192 ticks. The same ten signals are sent as in the original telegraph and the p1 telegraph, but information is encoded more efficiently in the timing of those signals. Specifically, the new transmitter sends five bits every 960 ticks by adjusting the relative timings inside each of the five mirror-image paired subunits of the composite signal in the beehive-chain lightspeed wire fuse.

:high-clearance See clearance.

:highway robber Any mechanism that can retrieve a signal from a spaceship lane while allowing spaceships on nearby lanes to pass by unaffected. In practice the spaceship is generally a glider. The signal is removed from the lane, an output signal is generated elsewhere, and the highway robber returns to its original state. A competent highway robber does not affect gliders even on the lane adjacent to the affected glider stream, except during its recovery period.

A perfect highway robber doesn't affect later gliders even in the lane to which it is attached, even during its recovery period. Below is a near-perfect highway robber "bait" that requires three synchronized signals to rebuild (the Herschel, B-heptomino, and glider.) The glider at the top right passes by unharmed, but another glider following on the same lane 200 ticks later will be cleanly reflected to a new path, and another glider following that one will also pass by unharmed. The only imperfection is a few ticks at the very end of the reconstruction, as the beehive is being rebuilt:

:hive = beehive

:hivenudger (c/2 orthogonally, p4) A spaceship found by Hartmut Holzwart in July 1992. (The name is due to Bill Gosper.) It consists of a pre-beehive escorted by four LWSS. In fact any LWSS can be replaced by a MWSS or an HWSS, so that there are 45 different single-hive hivenudgers.

:honey bit A block and pond constellation used in the OTCA metapixel by Brice Due in 2006, to store and retrieve a bit of data - specifically, the presence or absence of a neighbor metacell. The "0" state of the honey bit memory unit is a simple beehive, which is also the source of the name.

An input glider collides with the beehive to convert it into the honey bit constellation, which can be thought of as a value of "1" stored in the memory unit. A passing LWSS can then test for the presence of the pond. If a collision occurs, the LWSS and the honey bit constellation are mutually annihilated, leaving just the original beehive. Below is the honeybit constellation with the two reactions occurring in the opposite order - test, then reset.

The honey bit is also an interesting eater for the HWSS as shown below. An HWSS colliding with the pond happens to create the exact same reset glider used in the above memory unit.

:honeycomb (p1)

:honey farm (p1) A common formation of four beehives.

:hook Another term for a bookend. It is also used for other hook-shaped things, such as occur in the eater1 and the hook with tail, for example.

:hook with tail (p1) For a long time this was the smallest still life without a well-established name. It is now a vital component of the smallest known HWSS gun, where it acts as a rock.

:houndstooth agar The p2 agar that results from tiling the plane with the following pattern.

:house The following induction coil. It is generation 3 of the pi-heptomino. See spark coil and dead spark coil.

:H-to-G A Herschel-to-glider converter.

:H-to-MWSS A Spartan converter found by Tanner Jacobi in October 2015, which converts an input Herschel to a middleweight spaceship. The key discovery was a very small but slightly dirty H-to-MWSS conduit, where a Herschel is catalyzed to produce an MWSS but also leaves behind a beehive. Prefixing two R64 conduits to this produces a composite converter that successfully deletes the beehive in advance, using the input Herschel's first natural glider.

:hustler (p3) Found by Robert Wainwright, June 1971.

:hustler II (p4)

:HW emulator (p4) Found by Robert Wainwright in June 1980. See also emulator.

:HWSS (c/2 orthogonally, p4) A heavyweight spaceship, the fourth most common spaceship. Found by Conway in 1970 by modifying a LWSS. See also MWSS.

The HWSS possesses both a tail spark and a domino belly spark which can easily perturb other objects as it passes by. The spaceship can also perturb some objects in additional ways. For examples, see puffer and glider turner.

Dave Buckingham found that the HWSS can be synthesized using three gliders as shown below:

:HWSS emulator = HW emulator

:HW volcano (p5) A p5 domino sparker, found by Dean Hickerson in February 1995.

:hybrid grey ship A grey ship containing more than one type of region of density 1/2, usually a combination of a with-the-grain grey ship and an against-the-grain grey ship.

:I-heptomino Name given by Conway to the following heptomino. After one generation this is the same as the H-heptomino.

:IMG = intermitting glider gun

:Immigration A form of colourised Life in which there are two types of ON cell, a newly-born cell taking the type of the majority of its three parent cells and surviving cells remaining of the same type as in the previous generation.

:independent conduit A Herschel conduit in which the input Herschel produces its first natural glider. Compare dependent conduit.

:induction coil Any object used to stabilize an edge (or edges) without touching. The tubs used in the Gray counter are examples, as are the blocks and snakes used in the Hertz oscillator and the heptomino at the bottom of the mathematician.

:inductor Any oscillator with a row of dead cells down the middle and whose two halves are mirror images of one another, both halves being required for the oscillator to work. The classic examples are the pulsar and the tumbler. If still lifes are considered as p1 oscillators then there are numerous simple examples that include this kind of central gutter, such as table on table, dead spark coil and cis-mirrored R-bee. Some spaceships, such as the brain, the snail and the spider, use the same principle.

:infinite glider hotel A pattern by David Bell, named after Hilbert's "infinite hotel" scenario in which a hotel with an infinite number of rooms has room for more guests even if it is already full, simply by shuffling the old guests around.

In this pattern, two pairs of Corderships moving at c/12 are pulling apart such that there is an ever-lengthening glider track between them. Every 128 generations another glider is injected into the glider track (see LWSS-glider bounce), joining the gliders already circulating there. The number of gliders in the track therefore increases without limit.

The tricky part of this construction is that even though all the previously injected gliders are repeatedly flying through the injection point, that point is guaranteed to be empty when it is time for the next glider to be injected.

:infinite growth Growth of a finite pattern such that the population tends to infinity, or at least is unbounded. Sometimes the term is used for growth of something other than population (for example, length), but here we will only consider infinite population growth. The first known pattern with infinite growth in this sense was the Gosper glider gun, created in a response to a $50 prize challenge by John Conway. Martin Gardner's October 1970 article described the challenge as "Conway conjectures that no pattern can grow without limit", but Conway later explained that he had always expected that this would be disproved. The original purpose in investigating CA rules including B3/S23 was to show that a very simple two-state rule could support a universal computer and/or universal constructor. If all finite patterns could be proven to be bounded, neither of these would be possible.

An interesting question is: What is the minimum population of a pattern that exhibits infinite growth? In 1971 Charles Corderman found that a switch engine could be stabilized by a pre-block in a number of different ways, giving 11-cell patterns with infinite growth. This record stood for more than quarter of a century until Paul Callahan found, in November 1997, two 10-cell patterns with infinite growth. The following month he found the one shown below, which is much neater, being a single cluster. This produces a stabilized switch engine of the block-laying type.

In October 2014, Michael Simkin discovered a three-glider collision that produces a glider-producing stabilized switch engine and thus produces infinite growth from the smallest possible number of gliders (since all 71 2-glider collisions have a finite limit population).

Also of interest is the following pattern (again found by Callahan), which is the only 5×5 pattern with infinite growth. This too emits a block-laying switch engine.

Following a conjecture of Nick Gotts, Stephen Silver produced, in May 1998, a pattern of width 1 which exhibits infinite growth. This pattern was very large (12470×1 in the first version, reduced to 5447×1 the following day). In October 1998 Paul Callahan did an exhaustive search, finding the smallest example, the 39×1 pattern shown below. This produces two block-laying switch engines, stability being achieved at generation 1483.

Although the simplest infinite growth patterns grow at a rate that is (asymptotically) linear, many other types of growth rate are possible, quadratic growth (see also breeder) being the fastest. Dean Hickerson has found many patterns with unusual growth rates, such as sawtooths and a caber tosser. Another pattern with superlinear but non-quadratic growth is Gotts dots.

See also Fermat prime calculator.

:initials = monogram

:inject A reaction in which a hole in a regular spaceship stream is filled partially or fully by adding a new spaceship of the same type without affecting the existing spaceships in the stream. Depending on the period of the stream, different mechanisms can be used. For adding a spaceship to an existing multi-lane convoy, see inserter.

For large period glider streams, simple reactions such as LWSS-LWSS bounce and LWSS-glider bounce suffice. If Herschel technology is used, a large number of edge shooters and transparent conduits are known. Simple examples include the NW31 Herschel-to-glider converter and the Fx119 inserter.

Shown below is an injector found by Dave Buckingham that can fill a hole in a p15 glider stream:

:inline inverter The following reaction in which a p30 gun can be used to invert the presence or absence of gliders in a p30 stream, with the output glider stream being in the same direction as the input glider stream.

:inserter A mechanism that can add another spaceship into a stream or convoy of other spaceships without affecting the existing spaceships. For examples see Fx119 inserter, tee, GIG, clock insertion and inject.

:integral = integral sign

:integral sign (p1)

:intentionless = elevener

:interchange (p2) A common formation of six blinkers.

:intermediate target A temporary product of a partial slow salvo, elbow operation, or glider synthesis. An intermediate target is a useful step toward a desired outcome, but will not appear in the final construction.

:intermittent stream A stream of spaceships which is based on a periodic stream, but which can contain holes where some of the spaceships are not present. There is a base period for the intermittent stream such that if a spaceship arrives at a specific location, then it always does so at a generation which is a multiple of the base period. For example, the output from a period 30 glider gun where every third glider is deleted is an intermittent stream. A pseudo-random glider generator can produce a complicated intermittent stream with no obvious pattern.

Intermittent streams can be used to transmit signals, where holes in the stream can also convey information. For example, the stream can be processed by an inverter having the same period.

:intermitting glider gun Despite the name, an intermitting glider gun (IMG) is more often an oscillator than a gun. There are two basic types. A type 1 IMG consists of two guns firing at one another in such a way that each gun is temporarily disabled on being hit by a glider from the other gun. A type 2 IMG consists of a single gun firing at a 180-degree glider reflector in such a way that returning gliders temporarily disable the gun.

Both types of IMG can be used to make glider guns of periods that are multiples of the base period. This is done by firing another gun across the two-way intermittent stream of gliders in the IMG in such a way that gliders only occasionally escape.

:inverter A device which can be used to invert the presence or absence of spaceships in an intermittent stream of spaceships. The device must be a gun whose period matches the base period of the stream, since if there are no input spaceships then the device must produce spaceships as the result of the inversion. Typically the spaceships are gliders, and the inverter is made from a glider gun. Inverters provide a way to produce a NOT logic operation on a stream.

There are several ways to produce an inverter. The simplest method is to simply hit the output of a gun with the input stream to delete its spaceships, producing an output stream that is always turned 90 degrees from the input stream. An example is the northernmost p30 gun in the glider duplicator example pattern. For one way to produce an inverted output stream which is not turned, see inline inverter.

:inverting reflector See inverter.

:island The individual polyplets of which a stable pattern consists are sometimes called islands. So, for example, a boat has only one island, while an aircraft carrier has two, a honey farm has four and the standard form of the eater3 has five.

:Iwona (stabilizes at time 28786) The following methuselah found by Andrzej Okrasinski in August 2004.

:J = Herschel

:jack (p4) Found by Robert Wainwright, April 1984.

:jagged lines A pattern constructed by Dean Hickerson in May 2005 that uses puffers to produce a line of bi-blocks that weaves back and forth in a complicated way.

:jam (p3) Found by Achim Flammenkamp in 1988, but not widely known about until its independent discovery (and naming) by Dean Hickerson in September 1989. Compare with mold. In fact this is really very like caterer. In terms of its 7×7 bounding box it ties with trice tongs as the smallest p3 oscillator.

:JavaLifeSearch See lifesrc.

:Jaws A breeder constructed by Nick Gotts in February 1997. In the original version Jaws had an initial population of 150, which at the time was the smallest for any known pattern with superlinear growth. In November 1997 Gotts produced a 130-cell Jaws using some switch engine predecessors found by Paul Callahan. See switch-engine ping-pong for the lowest-population superlinear growth pattern as of July 2018, along with a list of the record-holders.

Jaws consists of eight pairs of switch engines which produce a new block-laying switch engine (plus masses of junk) every 10752 generations. It is therefore an MMS breeder.

:JC = dead spark coil

:JHC John Horton Conway. Also another name for monogram.

:J-heptomino = Herschel

:JLS = JavaLifeSearch

:Jolson (p15) Two blocks hassled by two pentadecathlons. Found by Robert Wainwright in November 1984 and named by Bill Gosper. A p9 version using snackers instead of pentadecathlons is also possible.

:junk = ash.

:Justyna (stabilizes at time 26458) The following methuselah found by Andrzej Okrasinski in May 2004.

:Karel's p15 (p15) An oscillator discovered by Karel Suhajda on December 11, 2002. It consists of a period 15 rotor supported by the domino spark of a pentadecathlon. It provides accessible sparks that can be used to perturb reactions or thin signal streams.

:keeper A type of factory circuit that always results in the presence of an object in the output location, whether or not the object was previously present. In many cases it is easy to construct examples by connecting multiple circuits to shoot down an object with a glider, then rebuild the object again later. The smallest keeper circuits accomplish the same thing more directly with a lucky preliminary spark from the active reaction, which removes the existing object (if any) just before the construction occurs. Below is a useful block keeper with a Herschel input.

:keys See short keys, bent keys and odd keys.

:kickback = kickback reaction or 180-degree kickback.

:kickback reaction The following collision of two gliders whose product is a single glider travelling in the opposite direction to one of the original gliders. This is important in the proof of the existence of a universal constructor, and in Bill Gosper's total aperiodic, as well as a number of other constructions.

:kidney A Gosperism for century. See also diuresis.

:killer toads A pair of toads acting together so that they can eat things. Here, for example, are some killer toads eating an HWSS. Similarly they can eat a MWSS (but not a LWSS). For another example see twirling T-tetsons II. See also candlefrobra.

:Klein bottle As an alternative to a torus, it's possible to make a finite Life universe in the form of a Klein bottle. The simplest way to do this is to use an m × n rectangle with the top edge joined to the bottom edge (as for a torus) and the left edge twisted and joined to the right.

:knightship Any spaceship of type (2m,m)/n - that is, a spaceship of any speed that moves obliquely in a (2,1) direction. The first Conway's Life knightship was a variant of Andrew Wade's Gemini spaceship, constructed in May 2010. The next was an even slower knightship based on the half-bakery reaction.

A knightship must be asymmetric and its period must be at least 6. This is barely within the range of current search programs, as proven by the discovery on March 6, 2018 of an elementary knightship, Sir Robin, by Adam P. Goucher and Tomas Rokicki.

By analogy with the corresponding fairy chess pieces, spaceships of types (3m,m)/n, (3m,2m)/n and (4m,m)/n would presumably be called camelships, zebraships and giraffeships, respectively. Such spaceships do exist (see universal constructor) but small elementary versions are even more difficult to search for. Any of these ship types could be constructed by trivially modifying a Gemini spaceship, or less trivially by reprogramming one of the more recent small Geminoid construction arms, but as of July 2018 a camelship Gemini is the only example that has been explicitly built.

Alternatively, the term "knightship" is regularly used to refer to any oblique spaceship, such as the original Gemini or the waterbear.

:Kok's galaxy (p8) An oscillator found by Jan Kok in 1971, currently serving as the icon for Golly. See converter for a use of this sparker.

:L112 A composite conduit, one of the original sixteen Herschel conduits, discovered by Dave Buckingham in July 1996. It is made up of two elementary conduits, HLx53B + BFx59H. After 112 ticks, it produces a Herschel turned 90 degrees counterclockwise at (12, -33) relative to the input. Its recovery time is 61 ticks; this can be reduced slightly by removing the output glider, either with a specialized eater (as in the original true p59 gun), or with a sparker as in most of the Quetzal guns. It can be made Spartan by replacing the aircraft carrier with an eater1. A ghost Herschel in the pattern below marks the output location:

:L156 A composite conduit, one of the original sixteen Herschel conduits, discovered by Dave Buckingham in August 1996. It is made up of three elementary conduits, HLx69R + RF28B + BFx59H. After 156 ticks, it produces a Herschel turned 90 degrees counterclockwise at (17, -41) relative to the input. Its recovery time is 62 ticks. It can be made Spartan by replacing the snake with an eater1 in one of two orientations. Additional gliders can be produced by removing the southeasternmost eater, or by replacing the RF28B elementary conduit by an alternate version. A ghost Herschel in the pattern below marks the output location:

:lake (p1) Any still life consisting of a simple closed curve made from diagonally connected dominoes. The smallest example is the pond, and the next smallest is this (to which the term is sometimes restricted):

:lane A path traveled by a glider, or less commonly a spaceship such as a loafer. The lane is centered on the line of symmetry (if any) of the spaceship in question. If a lane is clear, then the spaceship can travel along it without colliding or interfering with any other objects.

Diagonal lanes are often numbered consecutively, in half-diagonals (hd). Occasionally diagonal lane measurements are given in quarter-diagonals (qd), in part because diagonally symmetric spaceships have a line of symmetry 1qd away from the lines available for gliders. It's also convenient that moving a glider forward by 100qd (for example) has the same effect as evolving the same glider for 100 ticks.

:Laputa (p2) Found by Rich Schroeppel, September 1992.

:large prime oscillator Any oscillator with a relatively small bounding box whose period is a very large prime. (If the bounding-box restriction is removed, then eight gliders travelling in a four-Snark loop would provide a trivial example for any chosen prime.) The first such oscillator was built by Gabriel Nivasch in 2003. The current record holder is an oscillator constructed by Adam P. Goucher with a period that is a Mersenne prime with 13,395 digits (2⁴⁴⁴⁹⁷-1).

The next higher Mersenne-prime oscillator, period 2⁸⁶²⁴³-1, could be constructed with quadri-Snarks and semi-Snarks. It would actually be significantly smaller than the current record holder. As of June 2018 the construction of this pattern has not yet been completed.

:large S = big S

:Lidka (stabilizes at time 29053) A methuselah found by Andrzej Okrasinski in July 2005.

:Life A 2-dimensional 2-state cellular automaton discovered by John Conway in 1970. The states are referred to as ON and OFF (or live and dead). The transition rule is as follows: a cell that is ON will remain ON in the next generation if and only if exactly 2 or 3 of the 8 adjacent cells are also ON, and a cell that is OFF will turn ON if and only if exactly 3 of the 8 adjacent cells are ON. (This is more succinctly stated as: "If 2 of your 8 nearest neighbours are ON, don't change. If 3 are ON, turn ON. Otherwise, turn OFF.")

:Life32 A freeware Life program by Johan Bontes for Microsoft Windows 95/98/ME/NT/2000/XP: https://github.com/JBontes/Life32/.

:LifeHistory A multistate CA rule supported by Golly, equivalent to two-state B3/S23 Life but with several additional states intended for annotation purposes. A "history" state records whether an off cell has ever turned on in the past, and other states allow on and off cells to be permanently or temporarily marked, without affecting the evolution of the pattern.

:LifeLab A shareware Life program by Andrew Trevorrow for the Macintosh (MacOS 8.6 or later): http://www.trevorrow.com/lifelab/.

:LifeLine A newsletter edited by Robert Wainwright from 1971 to 1973. During this period it was the main forum for discussions about Life. The newsletter was nominally quarterly, but the actual dates of its eleven issues were as follows:

	Mar, Jun, Sep, Dec 1971
	Sep, Oct, Nov, Dec 1972
	Mar, Jun, Sep 1973

:Lifenthusiast A Life enthusiast. Term coined by Robert Wainwright.

:lifesrc David Bell's Life search program for finding new spaceships and oscillators. This is a C implementation of an algorithm developed by Dean Hickerson in 6502 assembler.

Although lifesrc itself is a command-line program, Jason Summers has made a GUI version called WinLifeSearch for Microsoft Windows. A Java version, JavaLifeSearch, was written in November 2012 by Karel Suhajda.

The lifesrc algorithm is only useful for very small periods, as the amount of computing power required rises rapidly with increasing period. For most purposes, period 7 is the practical limit with current hardware.

Lifesrc is available from http://tip.net.au/~dbell/ (source code only). Compare gfind.

:LifeViewer A scriptable Javascript Life pattern viewer written by Chris Rowett, used primarily on the conwaylife.com discussion forums.

:light bulb (p2) Found in 1971.

:lightspeed bubble A type of negative spaceship travelling through the zebra stripes agar. The center of the bubble is simple empty space, and the length and/or width of the bubble can usually be extended to any desired size.

Below is a small stabilized section of agar containing a sample lightspeed bubble, found by Gabriel Nivasch in August 1999. The bubble travels to the left at the speed of light, so it will eventually reach the edge of any finite patch and destroy itself and its supporting agar.

An open problem related to lightspeed bubbles was whether large extensible empty areas could be created whose length was not proportional to the width (as it must be in the above case, due to the tapering back edge). This was solved in February 2017 by Arie Paap; a simple period-2 solution is shown below.

:lightspeed ribbon = superstring

:lightspeed telegraph = telegraph.

:lightspeed wire Any wick that can burn non-destructively at the speed of light. Lightspeed wires are a type of reburnable fuse. These are potentially useful for various things, but so far the necessary mechanisms are very large and unwieldy. In October 2002, Jason Summers discovered a lightspeed reaction travelling through an orthogonal chain of beehives. Summers completed a period-1440 lightspeed telegraph based on this reaction in 2003.

A stable lightspeed transceiver mechanism using this same signal reaction, the p1 telegraph, was constructed by Adam P. Goucher in 2010; the bounding boxes of both the transmitter and receiver are over 5000 cells on a side. A more compact periodic high-bandwidth telegraph with a much improved transmission rate was completed by Louis-François Handfield in 2017.

The following diagram shows an older example of a lightspeed wire, with a small defect that travels along it at the speed of light. As of June 2018, no method has been found of creating such a defect in the upstream end of this particular stable wire, or of non-destructively detecting the arrival of the defect and repairing the wire at the downstream end.

:lightweight emulator = LW emulator

:lightweight spaceship = LWSS

:lightweight volcano = toaster

:linear growth A growth rate proportional to T, where T is the number of ticks that a pattern has been run. Compare superlinear growth, quadratic growth.

:linear propagator A self-replicating pattern in which each copy of a pattern produces one child that is an exact copy of itself. The child pattern then blocks the parent from any further replication. An example was constructed by Dave Greene on 23 November 2013, with a construction arm using two glider lanes separated by 9hd. By some definitions, due to its limited one-dimensional growth pattern, the linear propagator is not a true replicator. Compare quadratic replicator.

:line crosser A pattern which is able to send a signal across an infinite diagonal line of live cells without destroying the line. David Bell built one in August 2006. It uses many one-shot period 44160 glider guns on both sides of the line having the proper synchronization to create the reactions shown in line-cutting reaction and line-mending reaction.

An input glider can arrive at any multiple of 44160 generations to first cut the line, then send a glider through the gap, and finally mend the line while leaving an output glider on the other side.

A line crosser whose complete mechanism is on one side of the line is theoretically possible, using single-channel construction methods for example.

:line-cutting reaction A reaction that can cut an infinite diagonal line of cells, leaving a gap with both ends sealed. Such a reaction is demonstrated below. In actual use the reaction should be spread out so that the incoming LWSSes don't conflict. See line-mending reaction for a way to mend the gap.

:line-mending reaction A reaction which can fully mend a sealed gap in an infinite diagonal line of cells, such as the one produced by a line-cutting reaction. Such a reaction is demonstrated below. See the line cutting reaction for a way of creating the gliders travelling parallel to the line.

This reaction uses spaceships on both sides of the line which need to be synchronized to each other, for example by passing a glider through the gap to trigger the creation of the required spaceships and gliders.

No simple mechanism is known to mend the gap which lies completely on one side of the line. However, it is technically possible to use construction arm technology to push objects through the gap to build and trigger a seed for the required synchronized signals on the other side.

:line puffer A puffer which produces its output by means of an orthogonal line of cells at right angles to the direction of travel. The archetypal line puffer was found by Alan Hensel in March 1994, based on a spaceship found earlier that month by Hartmut Holzwart. The following month Holzwart found a way to make extensible c/2 line puffers, and Hensel found a much smaller stabilization the following day. But in October 1995 Tim Coe discovered that for large widths these were often unstable, although typically lasting millions of generations. In May 1996, however, Coe found a way to fix the instability. The resulting puffers appear to be completely stable and to exhibit an exponential increase in period as a function of width, although neither of these things has been proved.

Line puffers have enabled the construction of various difficult periods for c/2 spaceships and puffers, including occasionally periods which are not multiples of 4 and which would therefore be impossible to attain with the usual type of construction based on standard spaceships. (See frothing puffer for another method of constructing such periods.) In particular, the first c/2 rake with period not divisible by 4 was achieved in January 2000 when David Bell constructed a p42 backrake by means of line puffers.

See also hivenudger and puff suppressor.

:line ship A spaceship in which the front end is a linestretcher, the line being eaten by the back end.

:linestretcher A wickstretcher that stretches a single diagonal line of cells. The first example was constructed by Jason Summers in March 1999; this was c/12 and used switch engine based puffers found earlier by Dean Hickerson. The first c/4 example was found by Hartmut Holzwart in November 2004.

:loading dock (p3) Found by Dave Buckingham, September 1972.

:loaf (p1)

:loafer (c/7 orthogonally, p7) A small c/7 spaceship discovered by Josh Ball on 17 February 2013:

It has a known 8-glider construction recipe, probably not minimal, discovered on the following day:

:loaflipflop (p15) Here four pentadecathlons hassle a loaf. Found by Robert Wainwright in 1990.

:loaf on loaf = bi-loaf

:loaf pull The following glider/loaf collision, which pulls a loaf (3,1) toward the glider source:

:loaf siamese barge (p1)

:lobster (c/7 diagonally, p7) A spaceship discovered by Matthias Merzenich in August 2011, the first diagonally travelling c/7 spaceship to be found. It consists of two gliders pulling a tagalong that then rephases them.

:logarithmic growth A pattern whose population or bounding box grows no faster than logarithmically, asymptotic to n.log(t) for some constant n. The first such pattern constructed was the caber tosser whose population is logarithmic, but whose bounding box still grows linearly. The first pattern whose bounding box and population both grow logarithmically was constructed by Jason Summers with Gabriel Nivasch in 2003. For a pattern with a slower growth rate than this, see Osqrtlogt.

:LoM = lumps of muck

:lone dot agar An agar in which every live cell is isolated in every generation. There are many different lone dot agars. All of them are phoenixes. In 1995 Dean Hickerson and Alan W. Hensel found stabilizations for finite patches of ten lone dot agars to create period 2 oscillators. One of these is shown below:

:lonely bee = worker bee

:long A term applied to an object that is of the same basic form as some standard object, but longer. For examples see long barge, long boat, long bookend, long canoe, long shillelagh, long ship and long snake.

:long^3 The next degree of longness after long long. Some people prefer "extra long".

:long^4 The next degree of longness after long^3. Some people prefer "extra extra long".

:long barge (p1)

:long boat (p1)

:long bookend The following induction coil, longer than a bookend.

:long canoe (p1)

:long hat = loop

:long hook = long bookend

:long house = dock

:long integral (p1)

:long long The next degree of longness after long. Some people prefer "very long".

:long long barge (p1)

:long long boat (p1)

:long long canoe (p1)

:long long ship (p1)

:long long snake (p1)

:long shillelagh (p1)

:long ship (p1)

:long sinking ship = long canoe

:long snake (p1)

:loop (p1)

:looping spaceship = reflectorless rotating oscillator

:lossless elbow A stationary elbow in a construction arm toolkit that allows a recipe to turn a corner with no exponential increase in construction cost. Compare slow elbow. It is theoretically possible to construct lossless elbows for early construction arms such as the one in the 10hd Demonoid, but these would currently have to be very large.

The lossless elbow that has been used the most in practice is the Snark, which can be constructed directly on a single-channel construction lane using a Snarkmaker recipe. Controlled demolition of a Snark is also possible, to remove a temporary elbow that is no longer needed, and leave a hand target in its place if necessary for further construction.

A Silver reflector was used as a lossless elbow in the first spiral growth pattern, attached to a separate universal constructor component.

:low-density Life = sparse Life

:lumps of muck The common evolutionary sequence that ends in the blockade. The name is sometimes used of the blockade itself, and can in general be used of any stage of the evolution of the stairstep hexomino.

:LW emulator (p4) The smallest (and least useful) emulator, found by Robert Wainwright in June 1980.

:LWSS (c/2 orthogonally, p4) A lightweight spaceship, the smallest known orthogonally moving spaceship, and the second most common (after the glider). Found by Conway when one formed from a random soup in 1970. See also MWSS and HWSS.

The LWSS possesses a tail spark which can easily perturb other objects which grow into its path. The spaceship can also perturb some objects in additional ways. For examples, see blinker ship, hivenudger, and puffer train.

Dave Buckingham found that the LWSS can be synthesized in several different ways using three gliders, and can be constructed from two gliders and another small object in several more ways. Here is the fastest synthesis:

:LWSS emulator = LW emulator

:LWSS-glider bounce The following reaction in which a LWSS and a glider collide to form a glider heading back between the two input paths:

:LWSS-LWSS bounce The following symmetric reaction in which two LWSSs collide head-on to form two gliders heading apart at 90 degrees from each other. Compare LWSS-LWSS deflection.

:LWSS-LWSS deflection The following symmetric reaction in which two LWSSs collide nearly head-on to form two gliders heading apart at 180 degrees from each other. Compare LWSS-LWSS bounce.

:LWSS-to-G See 135-degree MWSS-to-G.

:LWTDS Life Worker Time Deficiency Syndrome. Term coined by Dieter Leithner to describe the problem of having to divide scarce time between Life and real life.

:LW volcano = toaster

:Lx200 A composite conduit, one of the original sixteen Herschel conduits, discovered by Paul Callahan in June 1997. It is made up of two elementary conduits, HL141B + BFx59H. The Lx200 and F166 conduits are the two original dependent conduits (several more have since been discovered.) After 200 ticks, it produces an inverted Herschel turned 90 degrees counterclockwise at (17, -40) relative to the input. Its recovery time is 90 ticks. It can be made Spartan by replacing the snakes with eater1s in one of two orientations. A ghost Herschel in the pattern below marks the output location:

:macrocell A format used by Golly and its hashlife algorithm, capable of storing repetitive patterns very efficiently, even if they contain a large number of cells. For example, a filled square 2¹⁶⁷ cells on a side can be stored in less than three kilobytes in macrocell format, or about 800 bytes in compressed macrocell format. The square's total population is over a googol, 10¹⁰⁰; the number of atoms in the observable universe is only about 10⁸⁰.

This high level of compression is obtained by defining a tree structure composed of increasingly large cell "tiles" with power-of-two dimensions. Tile definitions of any size are re-used whenever they appear multiple times in a large pattern (at the same power-of-two offset). For example, the following is a macrocell encoding of a complex pseudo still life arrangement of ships, with a total population over 2500 cells:

	[M2] (golly 3.0)
	#R B3/S23
	.OO.OO$O.O.O.O$OO...OO$$OO...OO$O.O.O.O$.OO.OO$
	4 0 1 1 1
	5 2 0 2 2
	6 3 3 0 3
	7 4 4 4 4

The first line after the #R rule line defines a quadtree tile at the lowest level - a level-3 tile in this case, meaning a 2³ square area. At this level the pattern is encoded in a modified ASCII format with dollar signs as line separators. The next line, #2, defines a level-4 quadtree tile, made from one empty level-3 tile in the northwest corner (0), and three copies of the level-3 tile that was defined on the previous line (1). Lines 3, 4, and 5 similarly define level 5, 6, and 7 quadtree tiles by giving the line numbers of four tiles of the next lower size.

Many patterns are only moderately repetitive, so macrocell format is somewhat less successful at compressing them. Certainly most patterns are not nearly as regular as the artificial example above: there are usually many different tiles defined at each level, not just one. Chaotic patterns, such as ash from random soups, usually need so many different tile definitions that they can be stored more efficiently using rle format.

:macro-spaceship A self-constructing or self-supporting spaceship, such as the Caterpillar, Centipede, half-baked knightship, waterbear, Demonoid, Orthogonoid, and Caterloopillar. Engineered spaceships of these types tend to be much larger and more complex than elementary spaceships.

:mango (p1) A relatively rare 180-degree rotationally symmetric 8-bit still life. The acorn produces a mango as part of its ash.

:mathematician (p5) Found by Dave Buckingham, 1972.

:Max A name for the smallest known spacefiller. The name represents the fact that the growth rate is the fastest possible. (This has not quite been proved, however. There remains the possibility, albeit not very likely, that a periodic agar could have an average density greater than 1/2, and a spacefiller stretching such an agar at the same speed as the known spacefillers would have a faster average growth rate.)

:mazing (p4) In terms of its minimum population of 12 this ties with mold as the smallest p4 oscillator. Found by Dave Buckingham in December 1973. For some constructions using mazings, see popover and sixty-nine.

:mc = macrocell

:medium fish = MWSS

:megacell = p1 megacell.

:memory cell A type of information storage circuit useful in many patterns that perform complex logical operations. Most commonly a memory cell can store a single bit of information. See for example demultiplexer, honey bit, and boat-bit. Depending on the application, the circuit may be a toggle circuit or a permanent switch, or it may be possible to send one or more signals to set the circuit to a "1" state, as can be done with a keeper mechanism. In that case a different input signal must be used to test the current state, usually with a destructive read reaction.

A more complicated example can be found in the Osqrtlogt pattern, which destructively reads a growing 2-dimensional array of minimal memory cells. Each memory cell may either contain a boat (below left) or empty space (below right), with no permanent circuitry anywhere near:

:Merzenich's p11 (p11) Found by Matthias Merzenich in December 2010.

:Merzenich's p18 (p18) Found by Matthias Merzenich in June 2011.

:metacatacryst A 52-cell pattern exhibiting quadratic growth. Found by Nick Gotts, December 2000. This was for some time the smallest known pattern (in terms of initial population) with superlinear growth. See switch-engine ping-pong for the lowest-population superlinear growth pattern as of July 2018, along with a list of the record-holders.

:metacell CA logic circuitry that emulates the behavior of a single cell. The circuitry is hard-wired to emulate a particular CA rule, but changing the rule is usually a matter of making simple adjustments. Known examples include David Bell's original 500×500 unit Life cell, Jared Prince's Deep Cell, Brice Due's OTCA metapixel, and Adam P. Goucher's megacell.

:metamorphosis An oscillator built by Robert Wainwright that uses the following reaction (found by Bill Gosper) to turn gliders into LWSS, and converts these LWSS back into gliders by colliding them head on using an LWSS-LWSS bounce. There are two ways to do the following reaction, because the twin bees shuttle spark is symmetric.

:metamorphosis II An oscillator built by Robert Wainwright in December 1994 based on the following p30 glider-to-LWSS converter using a queen bee shuttle pair. This converter was first found by Paul Rendell, January 1986 or earlier, but wasn't widely known about until Paul Callahan rediscovered it in December 1994.

:metapixel See metacell, OTCA metapixel.

:methuselah Any small pattern that stabilizes only after a long time. Term coined by Conway. Examples include rabbits, acorn, the R-pentomino, blom, Iwona, Justyna and Lidka. See also ark.

:Mickey Mouse (p1) The following still life, named by Mark Niemiec:

:middleweight emulator = MW emulator

:middleweight spaceship = MWSS

:middleweight volcano = MW volcano

:mini pressure cooker (p3) Found by Robert Wainwright before June 1972. Compare pressure cooker.

:M.I.P. value The maximum population divided by the initial population for an unstable pattern. For example, the R-pentomino has an M.I.P. value of 63.8, since its maximum population is 319. The term is no longer in use.

:MIT oscillator = cuphook

:MMM breeder See breeder.

:MMS breeder See breeder.

:mod The smallest number of generations it takes for an oscillator or spaceship to reappear in its original form, possibly subject to some rotation or reflection. The mod may be equal to the period, but it may also be a quarter of the period (for oscillators that rotate 90 degrees every quarter period) or half the period (for other oscillators which rotate 180 degrees every half period, and also for flippers).

:mold (p4) Found by Achim Flammenkamp in 1988, but not widely known until Dean Hickerson rediscovered it (and named it) in August 1989. Compare with jam. In terms of its minimum population of 12 it ties with mazing as the smallest p4 oscillator. But in terms of its 6×6 bounding box it wins outright. In fact, of all oscillators that fit in a 6×7 box it is the only one with period greater than 2.

:monochromatic salvo A slow salvo that uses gliders of only one colour. For example, the slow salvos generated by half-baked knightships are monochromatic, because they are generated by a single type of reaction which can happen at any position along a diagonal line. The smallest possible step size is one full diagonal (1fd), which is two half diagonals (2hd), which means that any single glider-producing reaction can only reach half of the available glider lanes. See colour of a glider.

:monogram (p4) Found by Dean Hickerson, August 1989.

:monoparity salvo A slow salvo that uses gliders of only one parity. Compare monochromatic salvo.

:Moore neighbourhood The set of all cells that are orthogonally or diagonally adjacent to a cell or group of cells. The Moore neighbourhood of a cell can be thought of as the points at a Chebyshev distance of 1 from that cell. Compare von Neumann neighbourhood. The Conway's Life rule is based on the Moore neighborhood, as are all the "Life-like" rules and many other commonly investigated rule families.

Cell neighbourhoods can also be defined with a higher range. The Moore neighbourhood of range n can be defined recursively as the Moore neighbourhood of the Moore neighbourhood of range n-1. For example, the Moore neighbourhood of range 2 includes all cells that are orthogonally or diagonally adjacent to the standard Moore neighbourhood.

:moose antlers (p1)

:mosquito See mosquito1, mosquito2. mosquito3, mosquito4 and mosquito5.

:mosquito1 A breeder constructed by Nick Gotts in September 1998. The original version had an initial population of 103, which was then the smallest for any known pattern with superlinear growth (beating the record previously held by Jaws). This was reduced to 97 by Stephen Silver the following month, but was then almost immediately superseded by mosquito2.

Mosquito1 consists of the classic puffer train plus four LWSS and four MWSS (mostly in predecessor form, to keep the population down). Once it gets going it produces a new block-laying switch engine (plus a lot of junk) every 280 generations. It is therefore an MMS breeder, albeit a messy one.

:mosquito2 A breeder constructed by Nick Gotts in October 1998. Its initial population of 85 was for a couple of hours the smallest for any known pattern with superlinear growth, but was then beaten by mosquito3.

Mosquito2 is very like mosquito1, but uses two fewer MWSS and one more LWSS.

:mosquito3 A breeder constructed by Nick Gotts in October 1998. Its initial population of 75 was at the time the smallest for any known pattern with superlinear growth, but was beaten a few days later by mosquito4.

Mosquito3 has one less LWSS than mosquito2. It is somewhat different from the earlier mosquitoes in that the switch engines it makes are glider-producing rather than block-laying.

:mosquito4 A slightly improved version of mosquito3 which Stephen Silver produced in October 1998 making use of another discovery of Nick Gotts (September 1997): an 8-cell pattern that evolves into a LWSS plus some junk. Mosquito4 is a breeder with an initial population of 73, at the time the smallest for any known pattern with superlinear growth, but superseded a few days later by mosquito5.

:mosquito5 A slightly improved version of mosquito4 which Nick Gotts produced in October 1998. The improvement is of a similar nature to the improvement of mosquito4 over mosquito3. Mosquito5 is a breeder with an initial population of 71. This was the smallest population for any known pattern with superlinear growth until it was superseded by teeth. See switch-engine ping-pong for the smallest such pattern as of July 2018, along with a list of the record-holders.

:mould = mold

:moving sawtooth A sawtooth such that no cell is ON for more than a finite number generations. David Bell constructed the first pattern of this type, with a c/2 front end and a c/3 back end. The front end is a blinker puffer. The back end ignites the blinker fuse.

The smallest currently known moving sawtooth was constructed in April 2011 by a conwaylife.com forum user with the handle 'cloudy197'. The c/2 front end is a bi-block puffer. The 2c/5 back end ignites the bi-block fuse.

:MSM breeder See breeder.

:multiple roteightors (p8) An extensible oscillator family consisting of one or more roteightor rotors, discovered by Dean Hickerson in 1990.

:multiplicity In a reflectorless rotating oscillator, the maximum number n of independent patterns that can orbit a single point, in a way that reduces the period of the combined oscillator by a factor of n.

:multi-state Life = colourised Life

:multum in parvo (stabilizes at time 3933) A methuselah found by Charles Corderman, but not as long-lasting as his acorn.

:muttering moat Any oscillator whose rotor consists of a closed chain of cells each of which is adjacent to exactly two other rotor cells. Compare babbling brook. Examples include the bipole, the blinker, the clock, the cuphook, the Gray counter, the quad, the scrubber, the skewed quad and the p2 snake pit. The following diagram shows a p2 example (by Dean Hickerson, May 1993) with a larger rotor. See ring of fire for a very large one.

:MW emulator (p4) Found by Robert Wainwright in June 1980. See also emulator and filter.

:MWSS (c/2 orthogonally, p4) A middleweight spaceship, the third most common spaceship. Found by Conway in 1970 by modifying a LWSS. See also HWSS.

The MWSS possesses both a tail spark and a belly spark which can easily perturb other objects as it passes by. The spaceship can also perturb some objects in additional ways. For examples see blinker puffer and glider turner.

Dave Buckingham found that the MWSS can be synthesized using three gliders, and can be constructed from two gliders and another small object in several more ways. Here is the glider synthesis:

:MWSS emulator = MW emulator

:MWSS out of the blue The following reaction, found by Peter Rott in November 1997, in which a LWSS passing by a p46 oscillator creates a MWSS travelling in the opposite direction. Together with some reactions found by Dieter Leithner, and an LWSS-turning reaction which Rott had found in November 1993 (but which was not widely known until Paul Callahan rediscovered it in June 1994) this can be used to prove that there exist gliderless guns for LWSS, MWSS and HWSS for every period that is a multiple of 46.

:MWSS-to-G See 135-degree MWSS-to-G, 45-degree MWSS-to-G.

:MW volcano (p5) Found by Dean Hickerson in April 1992.

:My Experience with B-heptominos in Oscillators An article by Dave Buckingham (October 1996) available from http://conwaylife.com/ref/lifepage/patterns/bhept/bhept.html. It describes his discovery of Herschel conduits, including sufficient (indeed ample) stable conduits to enable, for the first time, the construction of period n oscillators and true period n guns for every sufficiently large integer n. See Herschel loop and emu.

:natural Occurring often in random patterns. There is no precise measure of naturalness, since the most useful definition of "random" in this context is open to debate. Nonetheless, it is clear that objects such as blocks, blinkers, beehives and gliders are very natural, while eater2s, darts, guns, etc., are not.

:natural Heisenburp (p46) A twin bees shuttle pair arrangement found by Brice Due in 2006. A glider passes through the reaction area of the shuttle pair completely unaffected. However, a Heisenburp effect causes a second glider to be created "out of the blue", following behind the first at a 2hd offset.

:ND read = non-destructive read

:negative spaceship A type of signal travelling through a periodic agar such as zebra stripes. The leading edge of the signal removes the agar, and the trailing edge rebuilds the agar some time later. The distance between the two edges is sometimes adjustable, as shown in lightspeed bubble. The central part of the "spaceship" may consist of dying sparks or even simple empty space.

Below is a sample period-5 negative spaceship, found by Hartmut Holzwart in March 2007, in a small stabilized section of zebra stripes agar:

:negentropy (p2) Compare Hertz oscillator.

:neighbour Any of the eight cells adjacent to a given cell. A cell is therefore not considered to be a neighbour of itself, although the neighbourhood used in Life does in fact include this cell (see cellular automaton).

:new five (p3) Found by Dean Hickerson, January 1990.

:new gun (p46) An old name for the period 46 glider gun show below. This was found by Bill Gosper in 1971, and was the second basic glider gun found (after the Gosper glider gun). It produces a period 46 glider stream.

A number of other ways of constructing a gun from two twin bees shuttles have since been found. See edge shooter for one of these, and see also p46 gun.

:Noah's ark The following diagonal puffer consisting of two switch engines. This was found by Charles Corderman in 1971. The name comes from the variety of objects it leaves behind: blocks, blinkers, beehives, loaves, gliders, ships, boats, long boats, beacons and block on tables.

See also ark.

:n-omino Any polyomino with exactly n cells.

:non-destructive read A type of test reaction in memory cell circuitry, where the information in the memory cell is unchanged and can be read again to produce the same result. One simple type of non-destructive read reaction is a signal sent to a permanent switch. Memory cells with destructive read reactions are generally simpler and more commonly used.

:nonfiller = space nonfiller

:non-monotonic A spaceship is said to be non-monotonic if its leading edge falls back in some generations. The first example (shown below) was found by Hartmut Holzwart in August 1992. This is p4 and travels at c/4. In April 1994, Holzwart found examples of p3 spaceships with this property, and this is clearly the smallest possible period. Another non-monotonic spaceship is the weekender.

:nonomino switch engine predecessor This is the unique nonomino (a polyomino having 9 cells) whose evolution results in a switch engine, and the smallest polyomino to do so.

Charles Corderman may have found this object in 1971 while exhaustively investigating the fate of all the small polyominoes. Records indicate that he found the switch engine while investigating the decominoes (polyominoes having 10 cells). However, there do not appear to be decominoes which result in a clean switch engine. If Corderman was examining polyominoes in order of size, then this smaller predecessor should have been found first in any case.

:non-spark Something that looks like a spark, but isn't. An OWSS produces one of these instead of a belly spark, and is destroyed by it.

:non-standard spaceship Any spaceship other than a glider, LWSS, MWSS or HWSS.

:non-trivial A non-trivial period-N oscillator contains at least one cell that oscillates at the full period. In other words, it is not made up solely of separate oscillators with smaller periods. Usually it includes a spark or other reaction that would not occur if all lower-period subpatterns were separated from each other, but some exceptions are given under trivial. See also omniperiodic.

:novelty generator A pattern that appears to have an unknown fate due to complex feedback loops, for example involving waves of gliders shuttling between perpendicular rakes. Novelty generator patterns fall short of counting as chaotic growth, since the rakes continue to be predictable, and much of their ash generally remains stable.

It has not been proven conclusively that any particular pattern is in fact an infinite novelty generator, since it is always possible that periodicity will spontaneously arise if the simulation is continued far enough. In fact this happens quite regularly. But conversely, it has not been proven that periodicity must spontaneously arise for all such patterns. Bill Gosper, Nick Gotts and others have done extensive experiments along these lines using Golly.

:NW31 One of the most common stable edge shooters. This Herschel-to-glider converter suppresses the junk ordinarily left behind by an evolving Herschel while allowing both the first natural glider and second natural glider to escape on transparent lanes:

The complete name for this converter is "NW31T120", where 31 is the output glider lane number. In the above orientation, lane numbers get bigger toward the upper right and smaller toward the lower left (and may easily be negative).

The T120 timing measurement means that a canonical NW glider placed on lane 31 at time T=120, at (+31, +0) relative to the input Herschel, would in theory reach the exact same spacetime locations as the converter's output glider does.

Most converters are not edge shooters and their output lanes are not transparent, so they usually have catalysts that would interfere with this theoretically equivalent glider. This is the case for the optional third glider output created by the lower eater1 catalyst: the upper eater1 overlaps its lane. For the alternate block catalyst suppressing this glider output, see transparent lane.

:NW31T120 = NW31

:oblique Neither diagonal nor orthogonal. See also knightship.

:obo spark A spark of the form O.O (so called after its rle encoding).

:octagon II (p5) The first known p5 oscillator, discovered in 1971 independently by Sol Goodman and Arthur Taber. The name is due to the latter.

:octagon IV (p4) Found by Robert Wainwright, January 1979.

:octomino Any 8-cell polyomino. There are 369 such objects. The word is particularly applied to the following octomino (or its two-generation successor), which is fairly common but lacks a proper name:

:odd keys (p3) Found by Dean Hickerson, August 1989. See also short keys and bent keys.

:omino = polyomino

:omniperiodic A cellular automaton is said to be omniperiodic if it has oscillators of all periods. It is not known if Life is omniperiodic, although this seems likely. Dave Buckingham's work on Herschel conduits in 1996 (see My Experience with B-heptominos in Oscillators) left only a short list of unresolved cases, all with periods of 58 or below. The list has been progressively reduced since then. Most recently, period 43 and 53 oscillators were made possible in 2013 by Mike Playle's Snark. As of June 2018, no oscillators are known for periods 19, 23, 38, or 41. If we insist that the oscillator must be non-trivial, then 34 should be added to this list.

Note that if we were to allow infinite oscillators, then all periods are certainly possible, as any period of 14 or more can be obtained using a glider (or LWSS) stream, or an infinitely long 2c/3 wire containing signals with the desired separation.

:one per generation See grow-by-one object.

:one-sided spaceship synthesis A glider synthesis of a spaceship in which all gliders come from the same side of the spaceship's path. Such syntheses are used extensively in the 17c/45 Caterpillar. For example, here is a one-sided way to create an LWSS.

:one-time A term used for turners and splitters, specifying that the reaction in question is not repeatable as it would be in a reflector or fanout device. Instead, the constellation is used up, usually in a clean reaction, but the much more common dirty turners and splitters are also very useful in some situations.

:onion rings For each integer n>1 onion rings of order n is a stable agar of density 1/2 obtained by tiling the plane with a certain 4n × 4n pattern. The tile for order 3 onion rings is shown below. The reader should be able to deduce the form of tiles of other orders.

:Online Life-Like CA Soup Search = The Online Life-Like CA Soup Search.

:on-off Any p2 oscillator in which all rotor cells die from overpopulation. The simplest example is a beacon. Compare flip-flop.

:O-pentomino Conway's name for the following pentomino, a traffic light predecessor, although not one of the more common ones.

:orbit A term proposed by Jason Summers to refer to a natural stabilization of a puffer. For example, the switch engine has two (known) orbits, the block-laying one and the glider-producing one.

:Orion (c/4 diagonally, p4) Found by Hartmut Holzwart, April 1993.

:orphan Conway's preferred term for a Garden of Eden. According to some definitions, an orphan consists of just the minimum living and dead cells needed to ensure that no parent is possible, whereas a GoE is an entire infinite Life plane that contains an orphan.

:Orthogonoid (256c/3476016, p3476016) A self-constructing spaceship analogous to the Demonoids, with a slow orthogonal direction of travel. The first example was completed by Dave Greene on 29 June 2017, with a top speed of 16c/217251 (this is just 256c/3476016 in lowest terms).

The construction recipe is a stream of MWSSes, with the recovery time limited to 90 ticks by the Lx200 dependent conduit that follows the initial syringe converter. The design is hashlife-friendly, meaning that the spaceship can be trivially adjusted so that spatial and temporal offsets are exact powers of two; period 4194304 and period 8388608 versions have been constructed, with speeds of c/16384 and c/32768 respectively.

The MWSSes are converted to Herschels, which produce a standard single-channel glider stream that runs the Orthogonoid's single construction arm. After the child circuitry is complete, a previously constructed Snark in the parent is removed from the construction arm lane, converting it to a "destruction arm" that shoots down the previous constructor/reflector in the series.

:oscillator Any pattern that is a predecessor of itself. The term is usually restricted to non-stable finite patterns; period 1 oscillators are stable and are usually just called still lifes. The blinker is the smallest non-stable oscillator, having period 2. There are oscillators of almost all higher periods (see omniperiodic). In general cellular automaton theory the term "oscillator" usually covers spaceships as well, but this usage is not normal in Life.

Oscillators consisting of separate objects which do not react in any phase are usually ignored. For example, a separated blinker and pulsar makes a period 6 oscillator, but is considered trivial.

An oscillator can be divided into a rotor and a stator, and the stator can be further subdivided into bushing and casing. Some oscillators have no casing cells, and a few 100%-volatility oscillators also have no bushing cells.

An oscillator can be constructed from any gun as long as eaters can be added to consume its output. If it is a true gun then the period of the oscillator will be the same as the gun - unless the eating mechanism multiplies the period, as in the case of gliders caught by a boat-bit.

With the discovery of reflectors, relays provide an easy way to create oscillators of all large periods. For example, eight gliders travelling in a loop created by four Snarks can create any period above 42, with a population never exceeding 356 live cells.

For the very lowest periods, whole families of extensible oscillators are known. Examples of this are barberpole, cross, pentoad, p6 shuttle, snacker, and multiple roteightors. Any of the shuttles are oscillators by definition, for example the queen bee shuttle. Many of these are also extensible. Other oscillators such as figure-8 and tumbler have unique mechanisms that are not part of an extended family.

Some oscillators are useful because of the perturbations they can cause to other objects. This is especially true if they provide a spark on their boundary. Some oscillators are explicitly found by search programs in order to produce these sparks, such as pipsquirters.

Some higher period oscillators have been found while running random soups. This is especially true if the soup is run on a cylindrical or torus universe. Sometimes the found objects can be moved to the normal universe and supported there by added catalysts. Achim's p144 is an example.

:Osqrtlogt (p1 circuitry) A pattern constructed by Adam P. Goucher in 2010, which uses an unbounded triangular region as memory for a binary counter. Empty space is read as a zero, and a boat as a one, as shown in the example pattern in memory cell. The pattern's diametric growth rate is O(sqrt(log(t))), which is the slowest possible for any Life pattern, or indeed any 2D Euclidean cellular automaton. The population returns infinitely often to its initial minimum value (during carry operations from 11111...1 to 100000...0, so it can be considered to be an unusual form of sawtooth.

:OTCA metapixel (p46 circuitry) A 2048 × 2048 period 35328 metacell constructed by Brice Due in 2006. It contains a large "pixel" area that contains a large population of LWSSes when the metacell state is ON, but is empty when it is OFF. This allows the state of the metacell to be visible at high zoom levels, unlike previous unit cells where the state was signaled by the presence or absence of a single glider in a specific location.

:out of the blue See natural Heisenburp. Other similar mechanisms, particularly the method of LWSS creation used in the pixel part of the OTCA metapixel, may also be referred to as "out of the blue" reactions.

:overclocking A term used when a circuit can accept a signal at a specific period which it cannot accept at a higher period. A syringe is a simple example.

Some staged recovery circuits also permit overclocking, and can function successfully at a rate faster than their recovery time. A Silver reflector has a recovery time of 497 ticks, but can be overclocked to reflect a period 250 glider stream, or any nearby period above 248, simply by removing a beehive after the first glider enters the reflector. However, a continuous stream of gliders is then required to maintain the circuit, with timing within a tightly bounded range.

:overcrowding = overpopulation

:over-exposure = underpopulation

:overpopulation Death of a cell caused by it having more than three neighbours. See also underpopulation.

:over-unity reaction An important concept in gun and macro-spaceship construction. To be a good candidate for building one of these types of patterns with a new period or speed, a stationary reaction (for a gun) or a moving reaction (for a macro-spaceship) must be able to produce some number of output signals, strictly greater than the number of input signals required to maintain the reaction. The extra signal becomes a gun's output stream, or may be used in a variety of ways to construct the supporting track for a macro-spaceship. By implication, "over-unity" refers to the ratio of output signals to input signals.

If all signal outputs must be used up to sustain a stationary reaction, a high-period oscillator may still be possible. See emu for example.

:overweight spaceship = OWSS

:OWSS A would-be spaceship similar to LWSS, MWSS and HWSS but longer. On its own an OWSS is unstable, but it can be escorted by true spaceships to form a flotilla.

:Ox A 1976 novel by Piers Anthony which involves Life.

:p = period

:p1 Period 1, i.e., stable. In the context of logic circuitry, this tends to mean that a mechanism is constructed from Herschel conduits that contain only still lifes as catalysts. In the context of slow glider construction, a P1 slow salvo is one in which there are no constraints on the parity of gliders in the salvo, because the intermediate targets are all stable constellations. (The usual alternative is a "P2 slow salvo", where the relative timing between adjacent gliders can be increased arbitrarily, but only by multiples of two ticks.)

:p104 gun A glider gun with period 104, found by Noam Elkies on 21 March 1996. It is based on an R-pentomino shuttle reaction.

:p11 bumper (p11) A periodic colour-preserving glider reflector with a minimum repeat time of 44 ticks. Unlike the p5 through p8 cases where Noam Elkies' domino-spark based reflectors are available, no small period-22 colour-changing reflector is known. A stable Snark reflector can be substituted for any bumper. This changes the timing of the output glider, which can be useful for rephasing periodic glider streams.

In practice this reflector is not useful with input streams below period 121, because lower-period bumpers can be used to reflect all smaller multiples of 11 for which the bumper reaction can be made to work.

:p130 shuttle A shuttle found in March 2004 by David Eppstein, which originally needed several period 5 oscillators for support. David Bell found a reaction between two of the shuttles to produce a p130 glider gun. On 18 November 2017 Tanner Jacobi found that the stable sidesnagger can be used to support the shuttle instead, and this is shown here.

:p144 gun A glider gun with true period 144. The first one was found by Bill Gosper in July 1994. For a full description and pattern see factory.

:p14 gun A glider gun which emits a period 14 glider stream. This is the smallest possible period for any stream, so such a gun is of great interest. There is no known true-period p14 glider gun, and finding a small direct example is well beyond current search algorithms' abilities. However, pseudo-period p14 guns have been created by injecting gliders into a higher period glider stream. The first pseudo p14 gun was built by Dieter Leithner in 1995. Smaller pseudo p14 guns have since been constructed, but they are still much too large to show here. The essential mechanism used by them is demonstrated in GIG.

:p15 bouncer Noam Elkies' colour-changing glider reflector, with Karel's p15 providing the necessary domino spark. Compare to the colour-preserving Snark. The minimum repeat time is 30 ticks.

:p15 bumper A periodic colour-preserving glider reflector with Karel's p15 providing the necessary spark. The minimum repeat time is 45 ticks. For an equivalent colour-changing periodic glider reflector see p15 bouncer. A stable Snark reflector can be substituted for any bumper. This changes the timing of the output glider, which can be useful for rephasing periodic glider streams.

:p15 reflector An ambiguous term that may refer to PD-pair reflector, p15 bouncer, or the more recently discovered p15 bumper.

:p184 gun A true period 184 double-barrelled glider gun found by Dave Buckingham in July 1996. The engine in this gun is a Herschel descendant. Unlike previous glider guns, the reaction flips on a diagonal so that both gliders travel in the same direction.

:p1 megacell (p1 circuitry) A metacell constructed by Adam P. Goucher in 2008, capable of being programmed to emulate any Moore neighborhood rule, including isotropic and anisotropic non-totalistic rules. It fits in a 32768 by 32768 bounding box, with the resulting metacell grid at 45 degrees to the underlying Life grid. Like the OTCA metapixel, it includes a large "pixel" area so that the state of the megacell can easily be seen even at extremely small-scale zoom levels.

:p1 telegraph (p1 circuitry) A variant of Jason Summers' telegraph pattern, constructed in 2010 by Adam P. Goucher using only stable circuitry. A single incoming glider produces the entire ten-part composite lightspeed signal that restores the beehive-chain lightspeed wire to its original position. The signal is detected at the other end of the telegraph and converted back into a single output signal. This simplification came at the cost of a much slower transmission speed, one bit per 91080 ticks. In this mechanism, sending the entire ten-part signal constitutes a '1' bit, and not sending the signal means '0'. See also high-bandwidth telegraph.

:p22 gun A true period 22 glider gun constructed by David Eppstein in August 2000, using two interacting copies of a p22 oscillator found earlier the same day by Jason Summers.

:p246 gun A true period glider gun with period 246, discovered by Dave Buckingham in June 1996. The 180-degree mod-123 symmetry of its bookend-based engine makes it trivial to modify it into a double-barrelled gun. Its single-barreled form is shown below.

:p24 gun A glider gun with true period 24. The first one was found by Noam Elkies in June 1997. It uses three p4 oscillators to hassle a pair of traffic lights. One of the oscillators was very large and custom-made. Shown below is a much smaller version built by Jason Summers and Karel Suhajda in December 2002, using the same mechanism but with a smaller oscillator:

:p256 gun A true period 256 four-barrelled glider gun found by Dave Buckingham in September 1995. It uses four R64 conduits to make the second smallest known Herschel loop (after the Simkin glider gun). The p256 gun was an early "teaser" from Dave Buckingham before he released his full Herschel technology.

:p29 pentadecathlon hassler A hassler where two copies of a period 29 oscillator (which is itself a pre-pulsar hassler) change the period of a pentadecathlon.

:p30 gun A glider gun with true period 30. The first one, found by Bill Gosper in November 1970 (see Gosper glider gun), was also the first gun found of any period. All known p30 glider guns are made from two or more interacting queen bee shuttles. Paul Callahan found 30 different ways that three queen bee shuttles can react to form a period 30 glider gun. One of the most interesting of these is shown below in which the gliders emerge in an unexpected direction.

:p30 reflector = buckaroo

:p30 shuttle = queen bee shuttle

:p36 gun A glider gun with true period 36. The first one was found by Jason Summers in 2004. Shown below is a smaller version using improvements by Adam P. Goucher and Scot Ellison:

:p3 bumper A variant of Tanner Jacobi's bumper found by Arie Paap in April 2018. Two forms of the period 3 oscillator catalyst are shown below.

For bounding box optimization purposes, it's also possible to replace the eater1 in a p3, p6 or p9 bumper with another period 3 oscillator, saving one row along the south edge at the cost of a higher population.

The repeat time for all these variants is 36 ticks, as shown.

:p44 gun A glider gun with a true period of 44. The first one was found by Dave Buckingham in April 1992. It uses two interacting copies of an oscillator which he also found. In 1996 he found a gun which only used one copy of the oscillator. Paul Callahan improved it in 1997, resulting in the gun shown below:

:p44 MWSS gun A gun discovered by Dieter Leithner in April 1997, in a somewhat larger form. This was the smallest known gliderless gun and smallest known MWSS gun until the construction in 2017 of the gun shown under gliderless, based on Tanner's p46.

The p44 MWSS gun is based on a p44 oscillator discovered by Dave Buckingham in early 1992, shown here in an improved form found in January 2005 by Jason Summers using a new p4 sparker by Nicolay Beluchenko. A glider shape appears in this gun for three consecutive generations, but always as part of a larger cluster, so even a purist would regard this gun as gliderless.

:p45 gun A true-period glider gun discovered by Matthias Merzenich in April 2010. By most measures this is the smallest known odd-period gun of any type, either true-period or pseudo-period:

:p46 gun A glider gun which has true-period 46. The first one found was the new gun by Bill Gosper in 1971. Prior to the discovery of Tanner's p46 in October 2017, all known p46 guns were made from two or more twin bees shuttles that interact (e.g., see twin bees shuttle pair). See edge shooter and double-barrelled for two more of these.

On 21 October 2017 Heinrich Koenig found a glider gun using two copies of Tanner's p46 placed at right angles to each other. This is the first p46 gun found which makes no use of the twin bees shuttle.

See gliderless for a MWSS gun also made using two copies of Tanner's p46.

:p46 shuttle = twin bees shuttle

:p48 gun A true period compound glider gun based on the p24 gun, using a Rich's p16 oscillator as a filter to remove half of the gliders from the stream.