“Simulations of dynamic systems are now indispensable throughout the physical and social sciences,” said John Sterman, the Jay W. Forrester professor of management at the M.I.T. Sloan School of Management. “Not just in management, but also, for example, in astrophysics, biology, chemistry and climate change.”
Professor Forrester expanded his approach in the late 1960s to consider social problems, including urban decay. In his 1971 book “World Dynamics,” he developed global modeling, which examines population growth and industrialization in a world with finite resources.
“Jay developed the first model that treated interactions of population, the economy, natural resources, food and pollution in the context of the world as a whole,” Professor Sterman said in an interview. “The work was counterintuitive and controversial, and it launched the field of global modeling.”
Professor Forrester also made significant contributions to engineering and computer science.
In the 1940s, he worked on servomechanisms, which use feedback loops to maintain stability in devices like gun turrets and automobile cruise-control systems. He was also one of the inventors of magnetic core memory, an efficient form of computer memory that dominated the computer industry for more than two decades.
System dynamics came to him shortly after he joined the faculty of the Sloan School, when he took on a project for General Electric. The company was grappling with big fluctuations in stock levels and work force numbers at an appliance plant in Kentucky.
Professor Forrester’s breakthrough came after he interviewed plant managers. He discovered that the fluctuations had been caused not by external factors, as the managers thought, but by a dynamic system of internal factors that included policies for inventory control and hiring. He then developed computer simulations of the G.E. case, planting the seeds for the field of system dynamics.
His training as an engineer was pivotal to his insights into industrial processes.
“Here you have a man who starts as an electrical engineer, so he understands current flowing down wires and charge accumulating in capacitors,” said David C. Lane, who teaches system dynamics at the Henley School of Business at the University of Reading in England. “Then he moves into servomechanisms and feedback control.
“Then he makes this remarkable imaginative leap,” Professor Lane continued, “which is that those ideas are a way of threading together parts of a system, all connected by feedback loops. And once you start thinking like that, you’ve created an entirely new way of thinking.”
Jay Wright Forrester was born on July 14, 1918, on his family’s cattle ranch, 18 miles from the nearest town, Anselmo, Neb. His parents, both of them teachers as well as ranchers, were among the first homesteaders in the area.
In an interview for this obituary in 2011, Professor Forrester said that living on an isolated ranch had made him practical-minded by necessity. He brought electricity to the ranch by building a 12-volt wind generator while he was still in high school.
“I was generally inclined to the mechanical and electrical things,” he said, “and since there was no one to fix things, you did it yourself.”
He took up the study of electrical engineering at the University of Nebraska after turning down a scholarship to enter its agricultural college. “I don’t think I ever really warmed up to taking care of cattle in a Nebraska blizzard,” Professor Forrester said.
He entered M.I.T. for graduate studies in electrical engineering in 1939. The next year, he went to work at M.I.T.’s new Servomechanisms Laboratory, doing research in servomechanism theory and feedback control systems.
The laboratory did much of its work for the military during World War II, developing servomechanisms for controlling radar antennas and gun mounts.
In late 1943, Professor Forrester developed a servomechanism device that would control a radar antenna for intercepting aircraft. He accompanied the device aboard the carrier Lexington and spent a month at sea as a civilian as it helped scan the horizon for torpedo bombers.
After the war, while in charge of the Digital Computer Division at M.I.T.’s Lincoln Laboratory, Professor Forrester embarked on the Whirlwind project, the first complete real-time computing system.
He perfected magnetic-core memory while working on Whirlwind, which was completed in 1951. Magnetic-core memory used tiny ferrite rings strung on wires. Each ring maintained a magnetic field traveling around the ring and holding its polarity until it received a signal from the wires to change direction. Every ring was a bit, magnetized one way for 0 and the other for 1. The earliest magnetic-core memory boards measured several square feet and resembled dense window screens.
The technology greatly increased computer memory capacity and remained the memory technology of choice until the early 1970s, when Intel Corporation began selling silicon-based solid-state dynamic random access memory, which could store more information in less space — and more quickly.
Professor Forrester abandoned digital computing in 1956, in part because he believed that the major innovations in the field had been made.
“I still stand by that,” he said in the 2011 interview. “More happened in percentage improvements in digital computers from 1946, when they didn’t exist, to 1956, when they came into the modern era. I might not have envisioned how much smaller and faster they’d be, but the fundamental logic hasn’t changed.”
His wife, the former Susan Swett, whom he married in 1946, died in 2010. Besides his son Nathan, he is survived by a daughter, Judith Forrester; another son, Ned; four grandchildren; and two great-grandchildren.
In the seven decades he was at M.I.T., Professor Forrester retained an engineer’s curiosity about how things work, and occasionally voiced dismay that his students were not always so inclined.
He recalled in 2011 that he once asked students in an engineering class if they understood how the feedback mechanism in a toilet’s water tank maintained the water level.
“I asked them, ‘How many of you have ever taken the lid off a toilet tank to see how it works?’” he recalled. “None of them had. How do you get to M.I.T. without having ever looked inside a toilet tank?”