A catalyst is a stable object (usually a still life) which can interact with an active region and later return to its original state. Catalysts are very useful in conduits and in many other applications. Here is an example of a stable catalyst:
The green still life interacts with the white object, and recovers to its original state by generation 15. In this tutorial, you'll learn the main types of catalyses and their applications. But first, we need to cover some important definitions:
Stable catalyst: a catalyst which is a still life. Most catalyses are of this form. In this tutorial we will only focus on stable catalyses, as periodic catalyses are as of now quite underdeveloped.
Periodic catalyst: a catalyst which is an oscillator. These are generally considered inferior to stable catalyses because they usually only work in a fraction of the generations as a stable catalyst, since they usually only work in one phase of the periodic catalyse.
Recovery time: the time it takes for a catalyst to return back to its state before interacting. In the example above, the recovery time is 15.
Replacement catalyst: When a common catalyst fails, another, less common, catalyst must be used.
Rock: A catalyst which does not lose any of its cells, even temporarily, during the interaction process. The snake-type catalyses below are examples of rocks.
Let's explore the different types of catalysts now!
The marked white cells should always be on in a fishhook-type catalyst. Sometimes, the fishhook may engage in a more unusual manner, but these are rarer:
Recognizing when to use a fishhook catalysis is really a matter of experience. For practice, try placing an eater 1 somewhere within this blue field to eat the middleweight spaceship (click on the thumbnail below to draw cells in LifeViewer using the pencil icon):
They typically occur when there is a two cell leading edge, which is marked. Boat catalyses fail very often, so see the "Replacement catalysts" section below for potential replacement catalysts. As a general rule, boats are most likely to succeed when a cell adjacent to the leading edge (shown in blue) will turn on in the following generation.
Snake
Snake catalyses are examples of rocks, or catalysts which do not change even temporarily when interacting with an object. They must function this way due to the fact that the two cells in the center would die from overpopulation due to already having three live neighbors.
Three example snake catalysts are shown below. The white cells should always be on to trigger the important next generation, where the snake functions as a rock, and the blue cells should be born by the next generation in order to prevent a dot or domino spark from turning the snake into a blinker predecessor once the catalysis is done:
Conduit 1 is an example of a conduit using a snake catalyst. Try to catalyze the reaction below with a snake in the blue field (as a hint, the cell which should trigger the rock interaction is marked):
Tub catalyses can be tricky to find, but they are most likely to work when the frontend of the active region looks one of the examples below or two generations before the frontend looks like the example second from the left:
The reactions start differently but end up the same way through the same general mechanism: On generation one, one cell is born next to the tub. On generation two, two more cells are born next to the tub, causing overpopulation. This means that by generation three, the cells in the center have died, and there is a five-cell parent of the tub. The tub should turn into this five-cell parent in all catalyses.
Ship
Observe the following reaction:
x=5, y = 10, rule = B3/S23
2bo$2o2$2bo$2b2o3$2b2o$2bobo$3b2o!
[[ THEME 6 GRID THUMBNAIL THUMBSIZE 2 T 25 GPS 60 T 35 GPS 6 T 45 GPS 60 ]]
The ship rarely recovers by its own, but it's worth placing it because it has replacement catalysts that recover much more often.
Replacement catalyses
So, you've tried to catalyze a reaction, but the common catalyst fails! Luckily for you, this is not a complete disaster, since many replacement catalysts exist exactly to assist you when your fishhook or block doesn't just work out.
Fishhook replacements
The most common way for a fishhook to fail is something like this:
This was a bit cheating, however. Sometimes, we need a genuinely new catalyst. Here are two almost-catalyses where the fishhook almost recovers, but an extra spark destroys it or an extra cell gets attached to its recovering phase:
The most common replacement catalyst for a fishhook is the handy eater 2. An eater 2 recovers differently from an eater 1, making it more versatile. It can handle both of our problems:
Blocks are very hard to replace with a totally different catalyst, since there just aren't other catalysts which do the same thing. However, sometimes we can add extra objects next to the block to make a functioning catalyst. Take this, for example:
In this example, the white cell at generation 4 gets turned on when it shouldn't. We have a fix for this, as usual. Replace the boat with a tub with tail and block:
Note that this doesn't always work, since it requires a spark to recover properly.
Snake replacements
The only way to replace snake catalyses is by replacing them with a similar catalyst which does the same thing but might have better clearance. For example, if a snake is required to be in close proximity to a fishhook, the fishhook's interaction might destroy the snake (and subsequently the fishhook). In this case, replacing the snake with a higher-clearance second fishhook can prevent the interaction:
Sometimes, a single dot at the end of the tub parent is replaced with a line of three cells, causing the tub to turn into a block instead of recovering.
This is quite inconvenient for us because we can't use the loaf again (unless we flip it back around with another glider as shown in the viewer, which is unlikely), but a stable catalyst does exist to turn the loaf back around. Here is the eater 3:
This can also come in handy sometimes; for example the recently-discovered L122 conduit.
Finding new catalysts
Mike Playle's Bellman is a search program specifically designed to search for new catalysts, given a reaction to be perturbed. This part of the tutorial hasn't been written yet, so consult the discussion thread and original documentation.
An alternative is CatForce, which is a brute force placement program of certain given catalysts in a given area, and can also find transparent catalysts. [Somebody that's actually used CatForce before please add some more info]