The Cook Book
Recipe for the week of December 19  25
Soap Bubble clustering of a Plurality Vote Rule
 Some of the simplest selforganizing cellular automata can be
conceptualized as voter models. For instance, in the twocolor CA
known as majority vote, with each synchronous update the opinion
at x is replaced by the majority opinion within a neighborhood of
x. The special case p(1) = p(2) = 0 of last week's
nonlinear voter model is a nearestneighbor majority vote CA. As the
size of the neighbor set increases, such systems approximate a nonlinear
partial differential equation called motion by mean curvature
(mmc) that exhibits surfacetension clustering over time.
 This week's soup illustrates an intriguing multicolor variant of the
same idea that we call plurality vote. The process started from a
uniform random configuration of 16 pastels over the array of about 3/4
of a million sites. Think of an election, Italian say, with 16 political
parties instead of 2. Each individual changes allegiance in an attempt to
conform to the most popular local faction, adopting the plurality opinion
if there is a clear favorite. (In case of a tie, no change occurs). In
the present example the neighbor set for site x is a 9 by 9 box
centered at x.
 As time goes on, consensus forms on many droplets of increasing
size, while other small cohorts are swallowed by larger clusters that
surround them. The emergent dynamics are again driven by a digital
variant of motion by mean curvature, with surface tension effects
reminiscent of soap bubble interactions. Note in particular the
charactristic, nearly stable vertex angles of approximately 120 degrees.
One hopes that CA experiments such as this will shed light on the
nonlinear behavior of multitype mmc systems, which are poorly
understood at present.
