In his influential paper "University and Complexity in
Cellular Automata" [Physica D 10 (1984) 1-35; all page
numbers refer to the reprint in *Cellular Automata and
Complexity*, Addison-Wesley 1994, pp. 115-157],
Stephen Wolfram proposed a classification of cellular automaton
rules into four types, according to the results of evolving the
system from a "disordered" initial state:

- Evolution leads to a homogeneous state.
- Evolution leads to a set of separated simple stable or periodic structures.
- Evolution leads to a chaotic pattern.
- Evolution leads to complex localized structures, sometimes long-lived.

This classification was initially only for
one-dimensional cellular automata, but in "Two-Dimensional
Cellular Automata" [with N. H. Packard, J. Stat. Phys.
38 (1985) 901-946, reprinted in *Cellular Automata and
Complexity* pp. 211-249] he extended it to
two-dimensional automata.

Intuitively, according to this classification, one should only expect gliders, Turing-universality, and similar complex behavior in class 4 automata. Indeed, after discussing the existence of glider-like structures in a class 4 automaton, and their resemblance to structures in Conway's Life, Wolfram writes [p.151] "These analogies lead to the speculation that class 4 automata are characterized by the capability for universal computation."

However, there is a major flaw in this reasoning: because the classification is based on evolution from an unordered state, there is little reason for it to characterize behavior on initially ordered states. Our investigation has turned up gliders in systems that would likely be classified in each of the four classes: e.g. (in standard semitotalistic rule notation) B367/S3678 (class 1), B3/S256 or B3/S234 (both class 2 although having very different behavior), B345678/S12468 (class 3), and B3/S23 (class 4). This result throws the usefulness of this classification system into question.

The existence of a glider should automatically place a system into class 4, since such a complex localized structure would also arise from sufficiently large disordered initial conditions, however "sufficiently large" may be so huge that one can not in practice discover such structures by chance. Wolfram's classification may then be worse than useless: it may be ill-defined, since there is no quick test for distinguishing class 4 rules from other rules.

In "Two-Dimensional Cellular Automata", Wolfram mentions a number of particular rules. How do they stack up in terms of his classification and the existence of gliders?

Rule |
Ref. |
Classification |
Gliders? |

B236/S0468 | Figs. 7(a), 13(g) | Class 3 | No |

B257/S27 | Fig. 7(b) | Yes | |

B13456/S01356 | Fig. 7(d) | No | |

B1/S012345678 | Fig. 7(e) | No | |

B137/S45678 | Fig. 7(f) | No | |

B34/S03456 | Fig. 7(g) | Unknown | |

B0578/S12456 | Fig. 7(h) | No | |

B0578/S045 | Fig. 7(i) | No | |

B378/S012345678 | Fig. 9(a) | No | |

B3/S234 | Figs. 9(b), 13(b) | Class 2 | Yes |

B367/S2346 | Fig. 9(c) | Yes | |

B018/S018 | Fig. 13(c) | Class 2 | No |

B123567/S0238 | Fig. 13(f) | Class 3 | No |

B135/S135 | Fig. 13(h) | Class 3 | No |

B3/S23 | Fig. 13(i) | Class 4 | Yes |

(When I could find a classification in Wolfram's paper, I listed it above; otherwise the "Classification" column is blank.)

A simpler classification scheme explains the data better than Wolfram's:

**Contraction impossible.**If a rule includes B1, any pattern expands to infinity in all directions, no gliders can exist, and the question of whether a pattern lives or dies can not be universal. Similarly, if a rule (such as B378/S012345678 above) includes S01234, or includes B23/S0, the bounding box of a pattern can never shrink, and no pattern can die or move. However, as Tim Tyler has pointed out, universal computation could still occur in other ways; for instance the boundary of an expanding pattern could simulate the behavior of a 1d universal automaton.**Expansion impossible.**If a rule does not include B2 or B3, any pattern remains within its initial bounding box, no gliders can exist, and again the question of whether a pattern lives or dies is not universal (it can be solved in at most exponential time). Some rules with B3 (such as B345/S5, "longlife") appear to exhibit similar behavior of remaining inside a region bounded by the dimensions of the original pattern.**Both expansion and contraction possible.**Only in the remaining cases can gliders exist. Our investigations show that a large fraction of the remaining cases do indeed support gliders; much more work would be required to show that they are universal.