Growth of a finite pattern such that the population
tends to infinity, or at least is unbounded. Sometimes the term is
used for growth of something other than population (for example,
length), but here we will only consider infinite population growth.
The first known pattern with infinite growth in this sense was the
Gosper glider gun, created in a response to a $50 prize challenge
by John Conway. Martin Gardner's October 1970 article described the
challenge as "Conway conjectures that no pattern can grow without
limit", but Conway later explained that he had always expected that
this would be disproved. The original purpose in investigating CA
rules including B3/S23 was to show that a very simple two-state rule
could support a universal computer and/or universal constructor.
If all finite patterns could be proven to be bounded, neither of
these would be possible.
An interesting question is: What is the minimum population of a
pattern that exhibits infinite growth? In 1971 Charles Corderman
found that a switch engine could be stabilized by a pre-block in
a number of different ways, giving 11-cell patterns with infinite
growth. This record stood for more than quarter of a century until
Paul Callahan found, in November 1997, two 10-cell patterns with
infinite growth. The following month he found the one shown below,
which is much neater, being a single cluster. This produces a
stabilized switch engine of the block-laying type.
Nick Gotts and Paul Callahan showed in October 1997 that there is no
infinite growth pattern with fewer than 10 cells, so that question
has now been answered.
In October 2014, Michael Simkin discovered a three-glider collision
that produces a glider-producing stabilized switch engine and thus
produces infinite growth from the smallest possible number of gliders
(since all 71 2-glider collisions have a finite limit population).
Also of interest is the following pattern (again found by
Callahan), which is the only 5×5 pattern with infinite growth. This
too emits a block-laying switch engine.
Following a conjecture of Nick Gotts, Stephen Silver produced, in
May 1998, a pattern of width 1 which exhibits infinite growth. This
pattern was very large (12470×1 in the first version, reduced to
5447×1 the following day). In October 1998 Paul Callahan did an
exhaustive search, finding the smallest example, the 39×1 pattern
shown below. This produces two block-laying switch engines,
stability being achieved at generation 1483.
Larger patterns have since been constructed that display
quadratic growth.
Although the simplest infinite growth patterns grow at a rate that
is (asymptotically) linear, many other types of growth rate are
possible, quadratic growth (see also breeder) being the fastest.
Dean Hickerson has found many patterns with unusual growth rates,
such as sawtooths and a caber tosser. Another pattern with
superlinear but non-quadratic growth is Gotts dots.
See also Fermat prime calculator.