# Notes

## Section 10: The Sequencing of Events in the Universe

Sequential cellular automata

Ordinary cellular automata are set up so that every cell is updated in parallel at each step, based on the colors of neighboring cells on the previous step. But in analogy with generalized substitution systems, one can also consider sequential cellular automata, in which cells are updated sequentially rather than in parallel. The behavior of such systems is usually very different from that of corresponding ordinary cellular automata, mainly because in sequential cellular automata the new color of a particular cell can depend on new rather than old colors of neighboring cells.

The pictures below show the behavior of several sequential cellular automata with k = 2, r = 1 elementary rules. In the top picture of each pair every individual update is indicated by a black dot. In the bottom picture each line represents one complete step of evolution, including one update of each cell. Note that in this representation, effects can propagate all the way across the system in a single step. Size dependence.

Because effects can propagate all the way across the system in a single step, the overall size, as well as boundary conditions, for the system can be significant after just a few steps, as illustrated in the pictures of rule 60 below. Among elementary sequential cellular automata, those with additive rules turn out to yield some of the most complex behavior, as illustrated below. The top row shows evolution with the boundary forced to be white; the bottom row shows cyclic boundary conditions. Even though the basic rule is additive, there seems to be no simple traditional mathematical description of the results. Updating orders.

Somewhat different results are typically obtained if one allows different updating orders. For each complete update of a rule 90 sequential cellular automaton, the pictures below show results with (a) left-to-right scan, (b) random ordering of all cells, the same for each pass through the whole system, (c) random ordering of all cells, different for different passes, (d) completely random ordering, in which a particular cell can be updated twice before other cells have even been updated once. History.

Sequential cellular automata have a similar relationship to ordinary cellular automata as implicit updating schemes in finite difference methods have to explicit ones, or as infinite impulse response digital filters have to finite ones. There were several studies of sequential or asynchronous cellular automata done following my work on ordinary cellular automata in the early 1980s.

Implementation.

The following will update triples of cells in the specified order by using the function f:

OrderedUpdate[f_, a_, order_]:= Fold[ReplacePart[#1, f[Take[#1, {#2 - 1, #2 + 1}]], #2] &, a, order]

A random ordering of n cells corresponds to a random permutation of the form

Fold[Insert[#1, #2, Random[Integer, Length[#1]] + 1] &, {}, Range[n]]

From Stephen Wolfram: A New Kind of Science [citation]