So given a particular elementary cellular automaton one can then ask what other elementary cellular automata it can emulate using blocks up to a certain length.

The pictures on the facing page show a few examples.

The results are not particularly dramatic. No single rule is able to emulate many others—and the rules that are emulated tend to be rather simple. An example of a slight surprise is that rule 45 ends up being able to emulate rule 90. But at least with blocks up to length 25, rule 30 for example is not able to emulate any non-trivial rules at all.

From the proof of universality that we gave it follows that rule 110 must be able to emulate any other elementary cellular automaton with blocks of some size—but with the actual construction we discussed this size will be quite astronomical. And certainly in the picture on the facing page rule 110 does not seem to stand out.

But although it seems somewhat difficult to emulate the complete evolution of one cellular automaton with another, it turns out to be much easier to emulate fragments of evolution for limited numbers of steps. And as an example the picture below shows how rule 30 can be made to emulate the basic action of one step in rule 90.

The idea is to set up a configuration in rule 30 so that if one inserts input at particular positions the output from the underlying rule 30 evolution corresponds exactly to what one would get from a single step of rule 90 evolution. And in the particular case shown, this is achieved by having blocks 3 cells wide between each input position.

But as the picture on the next page indicates, by having appropriate blocks 5 cells wide rule 30 can actually be made to emulate

## Captions on this page:

Rule 30 set up to emulate a single Xor operation—as used in a step of rule 90 evolution. The initial conditions for rule 30 are fixed except at the two positions indicated, where input can effectively be given. The picture shows that for each possible combination of inputs, the result from the rule 30 evolution corresponds exactly to the output from the Xor.