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 change at all, 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.
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:
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):
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:
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:
The eater 3 has some other applications besides eating gliders, for example the rectifier.
Eater bridge eater
The eater bridge eater catalyst occurs when there is a one cell leading edge of an active region which "activates" the eater bridge, with the next generation consisting of a two cell edge for the catalyst to "slam" into. For example:
This seems pretty niche, but it is actually a fairly common application.
Finding new catalysts
Mike Payle'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.