"I had almost given up" on getting the Nobel, Nambu said.
Makoto Kobayashi and Toshihide Maskawa of Japan shared the other half for a 1972 theory that forecast the later discovery of a new family of subatomic particles.
The insights of the three scientists "give us a deeper understanding of what happens far inside the tiniest building blocks of matter," said the Royal Swedish Academy of Sciences, which presents the physics award.
Or, as physicist Phil Schewe, a spokesman for the American Institute of Physics, put it: "Nature works in strange ways, and these three physicists helped to explain that strangeness in an ingenious way."
They focused on a concept physicists call symmetry, and more specifically on occasions when that symmetry is violated.
In physics, the idea of symmetry means that a physical situation will be unaltered by certain changes. At the subatomic level, for example, things should happen the same way whether time is running forward or backward, so if you were watching a movie, you couldn't tell which way the movie was going. Similarly, symmetry could mean that you couldn't tell whether you were viewing action directly or through a mirror.
If one of those rules is violated, the symmetry is broken.
An important example of broken symmetry arose immediately after the big bang, when just a tiny bit more matter than antimatter was created. Because these two kinds of particles annihilate each other when they meet, that excess of matter was responsible for seeding the visible parts of the universe.
Nambu introduced his description of so-called spontaneous broken symmetry into particle physics in 1960. The Nobel citation said his theories now permeate the Standard Model of physics, which is the basic theory of how the universe operates. For example, spontaneous broken symmetry offers an explanation for how different particles acquired different masses.
In 1972, Kobayashi and Maskawa explained why an earlier experiment had found that some subatomic particles called kaons failed to follow the rules of symmetry. Their explanation correctly predicted the existence of a new family of quarks, which are a kind of subatomic particle.
Kobayashi and Maskawa also predicted that broken symmetry would arise for other particles called B-mesons. Nearly 30 years later, they were proven right.
The Japanese-born Nambu moved to the United States in 1952 and is a professor at the Enrico Fermi Institute at the University of Chicago, where he has worked for 40 years. He became a U.S. citizen in 1970.
Kobayashi, 64, works for the High Energy Accelerator Research Organization, or KEK, in Tsukuba, Japan. Maskawa, 68, is a physics professor at Kyoto Sangyo University in Japan's ancient capital of Kyoto. He also teaches at Nagoya University in his hometown in central Japan.
"I wasn't expecting the prize," Kobayashi said at a news conference. "I've been only pursuing my interest."
Maskawa told reporters that as a scientist he wasn't thrilled.
"The Nobel prize is a rather mundane thing," he said.
The prizes in chemistry, literature and the Nobel Peace Prize will be announced later this week. The economics award will be presented on Monday.
On the Net: http://nobelprize.org