Val Logsdon Fitch, the Nebraska rancher’s son who shared the Nobel Prize for detecting a breakdown in the overarching symmetry of physical laws, thus helping explain how the universe evolved after the Big Bang, died Feb. 5 in Princeton, N.J. He was 91.

His death was confirmed by Princeton University, where he had been a longtime faculty member and led the physics department for several years.

Dr. Fitch and his Princeton colleague James Cronin received the Nobel Prize in physics in 1980 for high-energy experiments conducted in 1964 that overturned fundamental assumptions about symmetries and invariances that are characteristic of the laws of physics.

Their discovery stemmed from an experiment in which a certain fundamental particle decayed into other particles in quantities that proved as astonishing to them as to the entire world of physics. The experiment provoked reassessment of fundamental physical theory.

The initial particle in the experiment was the kaon; it decayed into pions. Most decays yielded three pions. Only 45 times out of about 22,000 did the experiment yield two pions. But in the understanding of physicists of the time, Dr. Fitch said, those 45 were “absolutely forbidden.” They should not have been there.

Val Logsdson Fitch, who won the Nobel Prize for physics, at Princeton University in 1980. (Kanthal/ASSOCIATED PRESS)

That was momentous. It was a sign that nature, on relatively rare occasions, violated what scientists called CP symmetry; in turn, such a breach indicated that there were flaws in the idea of total and complete symmetry between particles and their oppositely charged antiparticles, or more succinctly, between matter and antimatter.

Moreover, by upsetting the idea of CP symmetry, the experiment also undermined what physicists called CPT symmetry. In that shorthand, C stands for reversing the charge of particles, and P means the correspondence between events and their mirror image.

The third letter, the T in CPT stands for a particularly intriguing concept: time reversal. In essence, time-reversal symmetry means that turning around the one-way flow of the days, hours and minutes would cause objects to retrace the precise paths they took to reach the present.

These symmetries were cherished by physicists, undergirded their understanding of the universe, and were abandoned only with reluctance; indeed there were prominent and estimable figures who could never completely give them up.

Dr. Fitch’s work, done on the Alternating Gradient Synchrotron, a particle accelerator, at the Brookhaven National Laboratory in Upton, N.Y., was a great surprise to scientists. In fact, Dr. Fitch said, “I must confess we did not initially believe it ourselves.”

But Dr. Fitch was a brilliant experimenter, who was skilled in electronics and the construction of particle detectors and who had worked early in his career on the timing device for the world’s first nuclear detonation.

He checked all the possibilities for error in the experiment, spending months looking for alternative explanations for his and Cronin’s startling results. Finally, they became confident of the outcome, no matter how revolutionary it seemed.

A.J. Stewart Smith, a Princeton physics professor and science administrator, said the discovery “even 50 years later remains one of the profound mysteries of the early universe.”

Nothing foreshadowed it, no signs, suggestions or clever guesses. There were, Dr. Fitch said, “no precursive indications.” Moreover, the explanation — the “why” of it — remained mysterious. “It is a discovery for which after 16 years,” he said in his Nobel lecture, “there is no satisfactory accounting.”

Yet the physicists recognized the significance of their experiment. “We were acutely sensitive to the importance of the result,” Dr. Fitch said.

From recognizing the matter-antimatter asymmetry, however uncommon, scientists recognized that a door had opened to an understanding of the development of the universe.

If, as is thought, the Big Bang produced equal quantities of matter and antimatter, where is the antimatter? And why didn’t the antimatter and matter annihilate each other as physics says they would?

Scientists regard the work of Dr. Fitch and Cronin as lighting the path toward the answers.

A lecture he gave in connection with his receipt of the Nobel Prize in 1980 recognized the unexpected nature of his results and the unusual turns taken by his life. “It is highly improbable, a priori, to begin life on a cattle ranch and then appear in Stockholm” to receive the Nobel Prize, he said.

What mitigated the unlikelihood of it all, he said, was the help of his family, teachers, colleagues and students.

Val Logsdon Fitch was born March 10, 1923, on a cattle ranch in the remote, isolated sand hills region in the northwestern corner of Nebraska.

His mother was a teacher. A riding accident forced his father to move into town and open an insurance business.

From 1940 to 1943, Dr. Fitch attended Chadron State College in Nebraska before he entered the Army, where he wore fatigues to work at Los Alamos, N.M., on the Manhattan Project to build the atom bomb during World War II. His military experience gave him the equivalent of graduate training in electronics and circuit design.

In the democratic environment of Los Alamos, he rubbed shoulders with many of the world’s luminaries in physics.

As a specialist in the bomb’s timing circuits, he was one of the last survivors of those who witnessed the test of the plutonium bomb in the New Mexico desert in 1945. After the war, he was invited to become a research assistant at Cornell University, but he pointed out that he had not yet obtained his bachelor’s degree.

He enrolled at McGill University in Montreal and in 1948 graduated with a degree in electrical engineering.

His doctoral work was pursued at Columbia University and entailed pioneering work on mu-mesic atoms, those in which muons replace electrons. It led to improved measurements of the size of the atomic nucleus. After completing his degree at Columbia in 1954, he began at Princeton.

One of the things he liked about physics was the opportunity to be surprised. He said he recognized “the delights of unexpected results. And the challenge they present in understanding nature.”

The work that brought the Nobel Prize, he said, was the “ultimate in unexpected results.”

His first wife, Elise Cunningham, whom he married in 1949, died in 1972. They had two sons. In 1976, he married Daisy Harper Sharp and became father to her three children.

One episode from his early life demonstrates the difficulty of glancing around a busy room and spotting the young man who would win the Nobel Prize: On the morning of July 16, 1945, just before the test of the A-bomb at Alamogordo, N.M., he was among the small group in the control bunker. But, he once wrote, being anonymous in his enlisted man’s uniform, he “was largely ignored.”

He said it gave him a lifelong sense of kinship with those who like him, had witnessed history from just “outside the spotlight.”