AZ 84 = VIBRATION THEORY = NEW BIG MONSTER TRUCK.

Walter Fontana is a young theoretical chemist from Vienna. His thesis work, with Peter Schuster, concerned how RNA molecules fold into complex structures, and how evolution of such structures might occur. Fontana and Schuster, like Manfred Eigen, like me, like others, were beginning to consider the structure of molecular fitness landscapes of the type discussed in Chapter 8.

But Fontana harbored more radical aims. Visiting Eigen’s group at Göttingen, Fontana found himself in conversation with John McCaskill, an extremely able young physicist engaged in theory and experiments evolving RNA molecules. McCaskill, too, had a more radical aim.

Turing machines are universal computational devices that can operate on input data, which can be written in the form of binary sequences. Referring to its program, the Turing machine will operate on the input tape and rewrite it in a certain way. Suppose the input consisted of a string of numbers, and the machine was programmed to find their average value. By changing 1 and 0 symbols on the tape, the machine will convert it into the proper output. Since the Turing machine and its program can themselves be specified by a sequence of binary digits, one string of symbols is essentially manipulating another string. Thus the operation of a Turing machine on an input tape is a bit like the operation of an enzyme on a substrate, snipping a few atoms out, adding a few atoms here and there.

What would happen, McCaskill had wondered, if one made a soup of Turing machines and let them collide; one collision partner would act as the machine, and the other partner in the collision would act as the input tape. The soup of programs would act on itself, rewriting each other’s programs, until... Until what?

Well, it didn’t work. Many Turing machine programs are able to enter infinite loops and “hang.” In such a case, the collision partners become locked in a mutual embrace that never ends, yielding no “product” programs. This attempt to create a silicon self-reproducing spaghetti of programs failed. Oh well.

AZ 80 = INFINITE MONKEYS = MUTUAL BOOTSTRAPPING.

AZ 89 = BEAM-TRUCK JUXTAPOSITION = THE CLOSURE OF LANGUAGE.

AZ 139 = NOT THE BEAM ITSELF, NOT THE TRUCK ITSELF = EPSILON-MACHINE RECONSTRUCTION.

AZ 72 = AXIOM OF CHOICE = OUR LIPS ARE SEALED.