Looking at the specific universal cellular automaton that we have discussed in this section, however, we would probably be led to assume that while the phenomenon of universality might be important in principle, it would rarely be relevant in practice. For the rules of the universal cellular automaton in this section are quite complicated—involving 19 possible colors for each cell, and next-nearest as well as nearest neighbors. And if such complication was indeed necessary in order to achieve universality, then one would not expect that universality would be common, for example, in the systems we see in nature.

But what we will discover later in this chapter is that such complication in underlying rules is in fact not needed. Indeed, in the end we will see that universality can actually occur in cellular automata with just two colors and nearest neighbors. The operation of such cellular automata is considerably more difficult to follow than the operation of the universal cellular automaton discussed in this section. But the existence of universal cellular automata with such simple underlying rules makes it clear that the basic results we have obtained in this section are potentially of very broad significance.

## Emulating Other Systems with Cellular Automata

The previous section showed that a particular universal cellular automaton could emulate any possible cellular automaton. But what about other types of systems? Can cellular automata also emulate these?

With their simple and rather specific underlying structure one might think that cellular automata would never be capable of emulating a very wide range of other systems. But what I will show in this section is that in fact this is not the case, and that in the end cellular automata can actually be made to emulate almost every single type of system that we have discussed in this book.

As a first example of this, the picture on the facing page shows how a cellular automaton can be made to emulate a mobile automaton.

The main difference between a mobile automaton and a cellular automaton is that in a mobile automaton there is a special active cell that moves around from one step to the next, while in a cellular