Darwin's Dangerous Idea - Evolution and the Meaning of Life

170 PRIMING DARWIN'S PUMP The Laws of the Game of Life 171
prey usually cannot. If the remainder of the prey dies out as with the glider, the
prey is consumed. [Poundstone 1985, p. 38.]
Notice that something curious happens to our "ontology"—our catalogue of
what exists—as we move between levels. At the physical level there is no
motion, just ON and OFF, and the only individual things that exist, cells, are
defined by their fixed spatial location. At the design level we suddenly have
the motion of persisting objects; it is one and the same glider (though
composed each generation of different cells) that has moved southeast in
figure 7.6, changing shape as it moves; and there is one less glider in the
world after the eater has eaten it in figure 7.8.
FIGURE 7.6
It is not a flasher; each generation, its two end ON cells die of isolation, and
there are no birth cells. The whole segment soon evaporates. In addition to
the configurations that never change—the still lifes—and those that evaporate
entirely—such as the diagonal line segment—there are configurations
with all manner of periodicity. The flasher, we saw, has a two-generation
period that continues ad infinitum, unless some other configuration encroaches.
Encroachment is what makes Life interesting: among the periodic
configurations are some that swim, amoebalike, across the plane. The simplest
is the glider, the five-pixel configuration shown taking a single stroke
to the southeast in figure 7.7.
Then there are the eaters, puffer trains, space rakes, and a host of other aptly
named denizens of the Life world that emerge as recognizable objects at a
new level. (This level is analogous to what in earlier work I have called the
design level.) This level has its own language, a transparent foreshortening of
the tedious descriptions one could give at the physical level. For instance:
An eater can eat a glider in four generations. Whatever is being consumed,
the basic process is the same. A bridge forms between the eater and its
prey. In the next generation, the bridge region dies from overpopulation,
taking a bite out of both eater and prey. The eater then repairs itself. The
Notice, too, that, whereas at the physical level there are absolutely no
exceptions to the general law, at this level our generalizations have to be
hedged: they require "usually" or "provided nothing encroaches" clauses.
Stray bits of debris from earlier events can "break" or "kill" one of the
objects in the ontology at this level. Their salience as real things is considerable,
but not guaranteed. To say that their salience is considerable is to
say that one can, with some small risk, ascend to this design level, adopt its
ontology, and proceed to predict—sketchily and riskily—the behavior of
larger configurations or systems of configurations, without bothering to
compute the physical level. For instance, one can set oneself the task of
designing some interesting supersystem out of the "parts" that the design
level makes available.
This is just what Conway and his students set out to do, and they succeeded
majestically. They designed, and proved the viability of the design
of, a self-reproducing entity composed entirely of Life cells that was also
(for good measure) a Universal Turing machine—it was a two-dimensional
computer that in principle can compute any computable function! What on
Earth inspired Conway and his students to create first this world and then
this amazing denizen of that world? They were trying to answer at a very
abstract level one of the central questions we have been considering in this
chapter: what is the minimal complexity required for a self-reproducing
thing? They were following up the brilliant early speculations of John von
Neumann, who had been working on the question at the time of his death