Yoichiro Nambu, a theoretical physicist who received the Nobel Prize for his groundbreaking work on subatomic particles, bringing about a fuller understanding of the behavior of matter at the most basic levels, died July 5 in Osaka, Japan. He was 94.

Yoichiro Nambu, a theoretical physicist who received the Nobel Prize for his groundbreaking work on subatomic particles, bringing about a fuller understanding of the behavior of matter at the most basic levels, died July 5 in Osaka, Japan. He was 94.

His death was announced by Osaka University, where he was an honorary professor. The cause was a heart attack.

Nambu, who had also been affiliated the University of Chicago since 1954, was a towering figure in physics. His research on the nature of atomic properties has influenced generations of scientists and has become part of the foundation of what physicists call the Standard Model, or a theoretical explanation of the fundamental structure of nature.

He shared the 2008 Nobel Prize in Physics with two Japanese researchers, Makoto Kobayashi and Toshihide Masakawa, who were recognized for their work on subatomic particles called quarks.

The three scientists, according to the Royal Swedish Academy of Sciences, which awards the Nobel, "give us a deeper understanding of what happens far inside the tiniest building blocks of matter."

Nambu had been rumored to be a candidate for the Nobel for at least 30 years. The honor came decades after his discoveries had become essential to the work of countless scientists in a variety of fields.

"Why did they wait so long, the Swedes?" James Cronin, Nambu’s University of Chicago colleague and a 1980 Nobel laureate in physics, wondered at the time. "His work preceded so much important work that other people did and is the cornerstone of the standard model of elementary particle physics."

Nambu’s most important discovery concerned a phenomenon known as spontaneous symmetry breaking. Since the late 1950s, he had been performing experiments in superconductivity, examining how electrical current could move with little resistance at extremely low temperatures.

According to the traditional laws of physics, interactions involving subatomic particles should be characterized by reversibility and symmetry. Outside forces should not produce more or fewer particles or alter their basic properties.

In his experiments, however, Nambu saw that materials sometimes moved from a symmetrical state to an asymmetrical one. He showed that, under certain conditions, particles could break down, resulting in changes in their mass and function.

Nambu often described the phenomenon in terms of a crowd of people standing in an open area, looking out in random directions. But if one person gazes intently in one direction, others will follow suit, acting on an unseen force.

When Nambu first defined spontaneous symmetry breaking in 1960, other physicists were skeptical, but it has become an underlying principle of much theoretical work in physics.

With another scientist, Moo-Young Han, Nambu developed a theory in 1965 to describe how protons and neutrons are bound together in the nucleus of an atom.

"He’s always been way ahead of his time," Cronin, his Chicago colleague, said in 1994, "to the point where what he found was thought to be discovered by other people."

The Standard Model of particle physics, which Nambu has strongly influenced, has been used to describe three of the four fundamental forces of nature: strong, weak and electromagnetic. The fourth force is gravity.

In addition to his other work, Nambu was considered a seminal thinker in the formulation of string theory. A still-developing field, string theory seeks to describe the composition of elementary materials in terms of a subatomic object known as a string.

Some experts believe advances in string theory may lead to a grand unification theory, explaining all four of the fundamental forces of nature.

Yoichiro Nambu was born Jan. 18, 1921, in Tokyo. His father was a teacher.

Nambu grew up in a rural part of Japan, and his childhood hero was inventor Thomas Edison. After receiving a master’s degree in physics from the University of Tokyo in 1942, he entered the Japanese army and worked at a radar laboratory during World War II. He survived the U.S. fire-bombing of Tokyo during the war.

While teaching at a university in Osaka, Nambu received a doctorate in physics from the University of Tokyo in 1952. That year, he was invited to the United States by Robert Oppenheimer, the physicist who ran the U.S. nuclear program that produced the atomic bombs dropped on Japan during World War II.

While in residence at the Institute for Advanced Study in Princeton, New Jersey, Nambu met Albert Einstein. In 1954, Nambu became a researcher at the University of Chicago and, four years later, a full professor.

He became a U.S. citizen in 1970 and was a member of the National Academy of Sciences and a fellow of the American Academy of Arts and Sciences. Among his many awards, he received the National Medal of Science in 1982.

Survivors include his wife since 1945, Chieko Hida; a son; and a sister.

When he won the Nobel Prize in 2008, Nambu was asked what advice he would offer students interested in science.

"Think independently, and think all the time," he said. "I like to tackle a problem first by myself, and then look up somebody’s answer, if there is one."