July 2001

Claude Shannon: Reluctant Father of the Digital Age

Continued from page 2

By M. Mitchell Waldrop

True or False?

Certainly his dissertation, "A Symbolic Analysis of Relay and Switching Circuits," makes for a compelling read-especially given what's happened in the 60-plus years since it was written. As an aside toward the end, for example, Shannon pointed out that the logical values true and false could equally well be denoted by the numerical digits 1 and 0. This reali-zation meant that the relays could perform the then arcane operations of binary arithmetic. Thus, Shannon wrote, "it is possible to perform complex mathematical operations by means of relay circuits." As an illustration, Shannon showed the design of a circuit that could add binary numbers.

Even more importantly, Shannon realized that such a circuit could also make comparisons. He saw the possibility of a device that could take alternative courses of action according to circumstances-as in, "if the number X equals the number Y, then do operation A." Shannon gave a simple illustration of this possibility in his thesis by showing how relay switches could be arranged to produce a lock that opened if and only if a series of buttons was pressed in the proper order.

The implications were profound: a switching circuit could decide-an ability that had once seemed unique to living beings. In the years to come, the prospect of decision-making machines would inspire the whole field of artificial intelligence, the attempt to model human thought via computer. And perhaps by no coincidence, that field would fascinate Claude Shannon for the rest of his life.

From a more immediate standpoint, though, a switching circuit's ability to decide was what would make the digital computers that emerged after World War II something fundamentally new. It wasn't their mathematical abilities per se that contemporaries found so startling (although the machines were certainly very fast); even in the 1940s, the world was full of electromechanical desktop calculators that could do simple additions and subtractions. The astonishing part was the new computers' ability to operate under the control of an internal program, deciding among various alternatives and executing complex sequences of commands on their own.

All of which is why "A Symbolic Analysis of Relay and Switching Circuits," published in 1938, has been called the most important master's thesis of the 20th century. In his early 20s, Claude Shannon had had the insight crucial for organizing the internal operations of a modern computer-almost a decade before such computers even existed. In the intervening years, switching tech-nology has progressed from electromechanical relays to microscopic transistors etched on silicon. But to this day, microchip designers still talk and think in terms of their chips' internal "logic"-a concept borne largely of Shannon's work.