Val Fitch, who shared the 1980 Nobel Prize in Physics for work that revealed a surprising imbalance in the laws of nature and helped explain why the collision of matter and antimatter has not destroyed everything in the universe, died on Thursday at his home in Princeton, N.J. He was 91.

Princeton University, where he was a longtime professor, announced his death.

One finding of modern physics is that every elementary particle on nature’s menu has an evil-twin antiparticle with equal and opposite charge. Upon contact the two annihilate. In experiments in 1964 at the Brookhaven National Laboratory on Long Island, Dr. Fitch and James Cronin, who shared the Nobel Prize with him and is now at the University of Chicago, found that matter and antimatter obeyed slightly different laws of physics.

One of the possible consequences of this, physicists say, is that if you could run the history of the universe — or any experiment — backward, like a movie in rewind, the laws of physics might not be quite the same. The finding contradicted a principle that had been a bedrock of science since Galileo.

Samuel Ting, a Nobel winner and professor at the Massachusetts Institute of Technology, called the work for which Dr. Fitch won his Nobel some of “the most important in the 20th century.” The experiment moreover suggested a way in which matter and antimatter could have avoided mutual destruction in the early universe, leaving a residue that could evolve into stars, galaxies and life.