John Doyle is worried about the Internet. In the next few years, millions more people will gain access to it, and existing users will place ever higher demands on our digital infrastructure, driven by applications like online movie services and Internet telephony. Doyle predicts that this skyrocketing traffic could cause the Internet to slow to a disastrous crawl, an endless digital gridlock stifling our economies. But Doyle, a professor of control and dynamic systems, electrical engineering, and bioengineering at Caltech, also believes the Internet can be saved. He and his colleagues have created a theory that has revealed some simple yet powerful ways to accelerate the flow of information. Vastly accelerate the flow: Doyle and his colleagues can now blast the entire text of all the books in the library of Congress across the United States in 15 minutes.
I travel to Pasadena to learn about Doyle’s work and am a bit flummoxed by his suggestion to meet not in his lab but in his gym. Doyle gets on a treadmill and begins to pound away. I turn on my machine and try to keep up. At 53, Doyle is long, lean, and hawk-faced. He is a championship athlete, and he works out furiously twice a day at least. It’s hard for me to catch enough air to ask Doyle questions, but he has no problem holding forth in response. As he talks I begin to realize his secret agenda in meeting me here. A treadmill is the perfect place to start to understand his ideas, because for Doyle, the world is filled with complex networks—and a body in the middle of a workout is a very good example of what those networks are all about.
A system of linked computers like the Internet is obviously a network, but so are jetliners, human bodies, and even bacterial cells. They’re all networks because they are made up of lots and lots of parts that work together. Robust networks have parts that continue to work together smoothly even if conditions fluctuate unpredictably. In the case of the Internet, a million people may try to send e-mail at once. In the case of Doyle’s body, here on his treadmill, its physiology holds steady even as he pushes himself to his limit. “Inside of you, everything’s going crazy,” Doyle says, “but it’s all keeping your body temperature steady and your body upright.”
Doyle knows, however, that networks that look perfectly sound can be headed for collapse with little warning. He has found that in order to achieve robustness, all systems must follow certain rules. Robustness doesn’t come cheap. As a system is tuned to become robust under one set of conditions, that tuning makes the system fragile under other, sometimes unexpected, conditions. Robustness and fragility go hand in hand. While Doyle pounds away on his treadmill, he offers his own body as exhibit number one. He has optimized his body to meet the grueling challenges of winning triathlons, but in doing so he has made his body vulnerable to problems that rarely plague a nonathlete. He has bad ankles, a groin injury, and other injuries earned over a lifetime of playing sports. In August, he almost died after falling down a rock face while hiking in Panama.
The treadmill slows to a stop. Doyle checks his pulse. “One of the reasons I’m so interested in robustness,” he said, “is that I’m so fragile.”
Doyle came to MIT in 1975 and fell in love with a science known as control theory. Control theorists, roughly speaking, try to understand how complicated things can run efficiently, quickly, and safely instead of crashing, exploding, or otherwise grinding to a halt. They analyze systems by modeling the variables that dictate how they will behave. But rather than checking through every possible combination of variables to see if, say, a plane will fly straight or stall when the wind picks up, control theorists look for underlying laws of control that can predict how something will behave using just a few key variables. “Control theory is at the center of modern technology,” Doyle explains.
As technology has become more complex over the past century, researchers have had to find new ways to control airplanes, factories, computers, and the like. Much of that progress has come by brute-force tinkering, but a lot of it has come from a growing understanding of the basic laws of control. Doyle began developing his own innovative ideas in control theory as an undergraduate. By 1976 he was consulting for Honeywell. By age 32 he had been hired with immediate tenure at Caltech.
Doyle made his mark by figuring out how to prove a system is robust. In the early 1980s, NASA asked him to look at the space shuttle. Several shuttles had already flown, but the agency wanted reassurance about their behavior during reentry. Using wind tunnels and computer simulations, NASA had come up with apparently stable designs, but there were too many variables to test everything. “You had this obscenely large space of possibilities,” Doyle says. “Somewhere lurking in there could be a crash, and you don’t know.”
Doyle looked at all the forces that might be exerted on a space shuttle due to atmospheric conditions, its velocity through the air, and so forth. NASA’s engineers had plotted these forces in a so-called multidimensional space—say, a pitching torque along one axis and a longitudinal acceleration along another. By developing new mathematical tools, Doyle proved that there was a volume of this multidimensional space, inside of which every combination of forces was certainly safe. Outside that region lurked disaster. The space shuttle design was lodged comfortably inside the safe region.
