Sidney Drell, 90, a Stanford University theoretical physicist who counseled government leaders for more than 50 years and who was an internationally prominent advocate of limits on nuclear weapons, died Wednesday at his home in Palo Alto, Calif. He had complications from pneumonia, said daughter, Joanna.

Dr. Drell, an Atlantic City native who received a prestigious MacArthur Foundation fellowship, made key advances in particle physics while occasionally stepping onto the public stage as a writer and an adviser to military and intelligence leaders at the highest levels.

His career, he wrote, was "divided between pursuing the dream of discovery and working to avoid the nightmare of a nuclear holocaust."

For many years, Dr. Drell was a key figure at the Stanford Linear Accelerator Center, a laboratory operated by the Energy Department. Now called the SLAC National Accelerator Laboratory, it is a leading research center and contains a particle accelerator in a building almost two miles long - the longest in the world.

Dr. Drell made several major advances in particle physics and quantum theory at SLAC. His specialties included quantum electrodynamics, which describes the interactions of matter and light, and quantum chromodynamics, a study of the behavior of subatomic particles.

He and a colleague, Tung-Mow Yan, developed the Drell-Yan Process, which describes the effects of a quark in one particle colliding with an antiquark in a second particle. It has become part of the framework of particle physics.

In addition to his scientific work at SLAC, Dr. Drell was co-director of the Stanford Center for International Security and Arms Control (now the Center for International Security and Cooperation). It followed an interdisciplinary approach in proposing solutions to worldwide problems through science, diplomacy, economics and other fields.

When Stanford refused to grant tenure to scholars in the program, Dr. Drell abruptly resigned in 1988, leading to the departure of several notable figures at the center, including the astronaut and physicist Sally Ride.

Since 1960, Dr. Drell was a fixture in an elite group of scientists known as "Jason" (after the leader of the Argonauts of Greek myth), who act as advisers to military and intelligence experts on sensitive issues of national security. He had been a consultant to the U.S. Arms Control and Disarmament Agency since 1961.

Long known as a proponent of arms control, Dr. Drell led a study that determined that the U.S. stockpile of nuclear weapons could be maintained without underground tests. He was a persuasive behind-the-scenes voice in high-level discussions that led to the 1996 signing of the Comprehensive Nuclear Test Ban Treaty, which prohibits global testing of nuclear devices in the Earth's atmosphere.

He received a MacArthur fellowship in 1984 and was elected to the National Academy of Sciences and the American Academy of Arts and Sciences. He was a past president of the American Physical Society. In 2013, he received the National Medal of Science at a White House ceremony.

In 2006, Dr. Drell teamed with former Secretary of State George P. Shultz to launch a program at Stanford's Hoover Institution dedicated to freeing the world of nuclear weapons. They published several books together.

Dr. Drell was a skilled violinist who often played in informal chamber groups - sometimes with fellow Stanford professor Condoleezza Rice, a pianist and former secretary of state.

"You have to meet them on the grounds where you are comfortable," he said in 1996, explaining his ability to steer his way through divisive policy debates. " 'These are the things I know. These are the things I don't.' " - AP