Douglas Hofstadter - I Am a Strange Loop

CHAPTER 17

How We Live in Each Other

Universal Machines

WHEN I was around twelve, there were kits you could buy that allowed you to put together electronic circuitry that would carry out various interesting functions. You could build a radio, a circuit that would add two binary numbers, a device that could encode or decode a message using a substitution cipher, a “brain” that would play tic-tac-toe against you, and a few other devices like this. Each of these machines was dedicated : it could do just one kind of trick. This is the usual meaning of “machine” that we grow up with. We are accustomed to the idea of a refrigerator as a dedicated machine for keeping things cold, an alarm clock as a dedicated machine for waking us up, and so on. But more recently, we have started to get used to machines that transcend their original purposes.

Take cellular telephones, for instance. Nowadays, in order to be competitive, cell phones are marketed not so much (maybe even very little) on the basis of their original purpose as communication devices, but instead for the number of tunes they can hold, the number of games you can play on them, the quality of the photos they can take, and who knows what else! Cell phones once were, but no longer are, dedicated machines. And why is that? It is because their inner circuitry has surpassed a certain threshold of complexity, and that fact allows them to have a chameleon-like nature. You can use the hardware inside a cell phone to house a word processor, a Web browser, a gaggle of video games, and on and on. This, in essence, is what the computer revolution is all about: when a certain well-defined threshold — I’ll call it the “Gödel–Turing threshold” — is surpassed, then a computer can emulate any kind of machine.

This is the meaning of the term “universal machine”, introduced in 1936 by the English mathematician and computer pioneer Alan Turing, and today we are intimately familiar with the basic idea, although most people don’t know the technical term or concept. We routinely download virtual machines from the Web that can convert our universal laptops into temporarily specialized devices for watching movies, listening to music, playing games, making cheap international phone calls, who knows what. Machines of all sorts come to us through wires or even through the air, via software, via patterns, and they swarm into and inhabit our computational hardware. One single universal machine morphs into new functionalities at the drop of a hat, or, more precisely, at the double-click of a mouse. I bounce back and forth between my email program, my word processor, my Web browser, my photo displayer, and a dozen other “applications” that all live inside my computer. At any specific moment, most of these independent, dedicated machines are dormant, sleeping, waiting patiently (actually, unconsciously) to be awakened by my royal double-click and to jump obediently to life and do my bidding.

Inspired by Gödel’s mapping of PM into itself, Alan Turing realized that the critical threshold for this kind of computational universality comes at exactly that point where a machine is flexible enough to read and correctly interpret a set of data that describe its own structure. At this crucial juncture, a machine can, in principle, explicitly watch how it does any particular task, step by step. Turing realized that a machine that has this critical level of flexibility can imitate any another machine, no matter how complex the latter is. In other words, there is nothing more flexible than a universal machine. Universality is as far as you can go!

This is why my Macintosh can, if I happen to have fed it the proper software, act indistinguishably from my son’s more expensive and faster “Alienware” computer (running any specific program), and vice versa. The only difference is one of speed, because my Mac will always remain, deep in its guts, a Mac. It will therefore have to imitate the fast, alien hardware by constantly consulting tables of data that explicitly describe the hardware of the Alien, and doing all those lookups is very slow. This is like me trying to get you to sign my signature by writing out a long set of instructions telling you how to draw every tiny curve. In principle it’s possible, but it would be hugely slower than just signing with my own handware!

The Unexpectedness of Universality

There is a tight analogy linking universal machines of this sort with the universality I earlier spoke of (though I didn’t use that word) when I described the power of Principia Mathematica. What Bertrand Russell and Alfred North Whitehead did not suspect, but what Kurt Gödel realized, is that, simply by virtue of representing certain fundamental features of the positive integers (such basic facts as commutativity, distributivity, the law of mathematical induction), they had unwittingly made their formal system PM surpass a key threshold that made it “universal”, which is to say, capable of defining number-theoretical functions that imitate arbitrarily complex other patterns (or indeed, even capable of turning around and imitating itself — giving rise to Gödel’s black-belt maneuver).

Russell and Whitehead did not realize what they had wrought because it didn’t occur to them to use PM to “simulate” anything else. That idea was not on their radar screen (for that matter, radar itself wasn’t on anybody’s radar screen back then). Prime numbers, squares, sums of two squares, sums of two primes, Fibonacci numbers, and so forth were seen merely as beautiful mathematical patterns — and patterns consisting of numbers, though fabulously intricate and endlessly fascinating, were not thought of as being isomorphic to anything else, let alone as being stand-ins for, and thus standing for, anything else. After Gödel and Turing, though, such naïveté went down the drain in a flash.

By and large, the engineers who designed the earliest electronic computers were as unaware as Russell and Whitehead had been of the richness that they were unwittingly bringing into being. They thought they were building machines of very limited, and purely military, scopes — for instance, machines to calculate the trajectories of ballistic missiles, taking wind and air resistance into account, or machines to break very specific types of enemy codes. They envisioned their computers as being specialized, single-purpose machines — a little like wind-up music boxes that could play just one tune each.

But at some point, when Alan Turing’s abstract theory of computation, based in large part on Gödel’s 1931 paper, collided with the concrete engineering realities, some of the more perceptive people (Turing himself and John von Neumann especially) put two and two together and realized that their machines, incorporating the richness of integer arithmetic that Gödel had shown was so potent, were thereby universal. All at once, these machines were like music boxes that could read arbitrary paper scrolls with holes in them, and thus could play any tune. From then on, it was simply a matter of time until cell phones started being able to don many personas other than just the plain old cell-phone persona. All they had to do was surpass that threshold of complexity and memory size that limited them to a single “tune”, and then they could become anything.

The early computer engineers thought of their computers as number-crunching devices and did not see numbers as a universal medium. Today we (and by “we” I mean our culture as a whole, rather than specialists) do not see numbers that way either, but our lack of understanding is for an entirely different reason — in fact, for exactly the opposite reason. Today it is because all those numbers are so neatly hidden behind the screens of our laptops and desktops that we utterly forget they are there. We watch virtual football games unfolding on our screen between “dream teams” that exist only inside the central processing unit (which is carrying out arithmetical instructions, just as it was designed to do). Children build virtual towns inhabited by little people who virtually ride by on virtual bicycles, with leaves that virtually fall from trees and smoke that virtually dissipates into the virtual air. Cosmologists create virtual galaxies, let them loose, and watch what happens as they virtually collide. Biologists create virtual proteins and watch them fold up according to the complex virtual chemistry of their constituent virtual submolecules.