Danny Cohen, a computer scientist whose work in the 1960s and ’70s on computer graphics and networks led to innovations in flight simulation, internet telephony, cloud computing and one of the first online dates — with him — died on Aug. 12 at his home in Palo Alto, Calif. He was 81.
His son, David, said the cause was Parkinson’s disease.
Dr. Cohen, an Israeli immigrant who started out as a mathematician, is credited with designing the first real-time computerized flight simulation system, providing the experience of piloting a plane without having to leave the ground. When he took on the project, he told Wired magazine in 2012, the challenge was not just to master flying as a skill — he later became an accomplished pilot — but also to represent it graphically on a computer.
His work in computer-network-based telephony began in the mid-1970s, when he was on the faculty of the University of Southern California. (He spent 20 years at its Information Sciences Institute in Marina del Rey.)
Managers at the Advanced Research Projects Agency, a research arm of the Department of Defense, had asked him to investigate sending voice calls over the Arpanet, the precursor to the internet.
“They wanted to know if we could apply some of our real-time graphics approach to voice over the early networks,” Dr. Cohen told Wired.
He set to work with a group of researchers from around the United States. They devised a forerunner of internet telephony and teleconferencing and conducted the first conference call over the Arpanet in 1978.
This early version of internet telephony — the technology that came to be known as voice over IP — helped pave the way for the delivery of voice, and eventually video, across the internet.
“Danny was largely responsible for our ability to receive streaming media,” said Leonard Kleinrock, a computer scientist at the University of California, Los Angeles, and a longtime colleague of Dr. Cohen’s.
Dr. Cohen, he said, “had the uncanny ability to employ his deep mathematical intellect and insight to real world challenges, with enormous impact.”
Dr. Cohen’s work in computing also changed his personal life. He met the woman who would become his second wife, Delia Heilig, over the Arpanet, kindling what may qualify as the first online romance.
Ms. Heilig said she was working the midnight shift at the Information Sciences Institute as a computer operator in 1973 when she came upon a puzzle in the abstract strategy game Gomoku that Dr. Cohen had posted on the Arpanet. She posted the solution.
Dr. Cohen was dazzled by Ms. Heilig’s ingenuity and set out to find her. In those days, with just a few dozen computers linked to the fledgling Arpanet, there was a decent chance that Dr. Cohen and Ms. Heilig were in the same building — and they were.
“He found me up in the machine room and asked if I wanted to go flying,” Ms. Heilig said in an interview.
Danny Cohen was born on Dec. 9, 1937, in Haifa, Israel, when it was part of Palestine under the British Mandate. His father, David, an immigrant from Russia, was a factory electrician. His mother, Dorit, from Hungary, ran the Haifa regional office of a support organization for soldiers. A younger brother, Gidi, died of a congenital heart disease as an infant.
Dr. Cohen became interested in computers in high school, when he read a newspaper article referring to them as “electronic brains.”
“The idea of having a machine that acts like a brain was very fascinating, and that got me interested in the area,” he said in an interview with the Computer History Museum in Mountain View, Calif., in 2011.
In Israel he belonged to the socialist Zionist youth movement and in the mid-1950s became a paratrooper in the Israeli Army. In Haifa he attended Technion — the Israel Institute of Technology, where he received a bachelor’s degree in applied mathematics in 1963. He began work on a master’s degree in math but by then had grown increasingly interested in the nascent field of computer science.
In 1961 he married Shoshana Brauner, a music teacher with whom he had attended high school. The marriage ended in divorce. His marriage to Ms. Heilig, in 1982, also ended in divorce, in 1996.
Dr. Cohen emigrated to the United States in 1965, when the Massachusetts Institute of Technology invited him to continue his studies there, in Cambridge, Mass. He started work on the visual flight simulator while he was at M.I.T.
Two years later he heard that Ivan Sutherland was teaching a graduate seminar on computer graphics at nearby Harvard University. Dr. Cohen, who had admired Dr. Sutherland’s pioneering work in the field, persuaded him to take him on as a graduate student. Transferring to Harvard, Dr. Cohen finished the flight simulator there and cemented his career as a computer scientist. He joined the Harvard faculty shortly after receiving his doctorate there in 1969.
Dr. Cohen went on to create an ultra-high-speed networking system, which made practical the first commodity computing clusters — groups of computers used for shared storage and computing that were the forerunners of today’s cloud computing systems, in which a network of remote servers, rather than a local one or a personal computer, is used to store, manage and process data.
He joined Sun Microsystems Laboratories as a distinguished engineer in 2001 and retired in 2012.
Along with his son, David, from his second marriage, Dr. Cohen’s survivors include two grandchildren. A daughter, Orly Kaplan, from his first marriage, died in 1993.
Long known to have a sly sense of humor, Dr. Cohen published technical papers over the years with a mysterious, imaginary co-author, Professor J. Finnegan, of Oceanview University in Oceanview, Kan.
In a 2013 profile of Dr. Cohen in The New York Times, Ron Ho, a colleague of Dr. Cohen’s, recalled him charging into his office one day, handing him a paper by Professor Finnegan and insisting that he read it.
“It wasn’t until I got to the very end where it said, ‘the more processors, the better the paper,’ that I realized it was a joke,” Dr. Ho said.
David Cohen said of his father: “He used humor to put people at ease, to skewer ideas he thought were wrong and, as a network engineer, to confirm transmission accuracy. He said he always began his talks with a joke to check whether people could understand his vowel-agnostic mix of Israeli accent and mild speech impediment. If they didn’t laugh, he knew they either couldn’t understand him or it wasn’t a good joke.”