STOCKHOLM — Takaaki Kajita of Japan and Arthur McDonald of Canada won the Nobel Prize in physics on Tuesday for discovering that tiny particles called neutrinos change identities as they whiz through the universe, proving that they have mass.
By uncovering the "chameleon-like" nature of neutrinos, the laureates had solved a long-standing puzzle in particle physics that could alter our grasp of the cosmos, the Royal Swedish Academy of Sciences said.
"The discovery has changed our understanding of the innermost workings of matter and can prove crucial to our view of the universe," the academy said.
Kajita, 56, is director of the Institute for Cosmic Ray Research and professor at the University of Tokyo. McDonald, 72, is a professor emeritus at Queen's University in Kingston, Canada.
"Neutrinos are among the fundamental particles (which) we do not know how to subdivide any further," McDonald told reporters in Stockholm by phone. "Therefore, their position within the models of physics at the most fundamental level is very important," he said.
"When you do not know whether they have mass, it's otherwise difficult to understand how to incorporate them into those theories that give us a more complete understanding of the world of physics at the most fundamental level. Discovering this property helps us tremendously in this regard."
Kajita seemed flummoxed at a news conference organized by his university. "My mind has gone completely blank. I don't know what to say," he said after taking the stage.
After getting his composure back, he stressed that many people had contributed to his work, and that there was much work still to do.
"The universe where we live in is still full of unknowns. A major discovery cannot be achieved in a day or two. It takes a lot of people and a long time. I would like to see young people try to join our pursuit of mystery solving," he said.
For decades the neutrino remained a hypothetical particle until American researchers proved that it was real in 1956.
There are three kinds, or flavors, of neutrinos and the laureates showed they oscillate from one flavor to another, dispelling the long-held notion that they were massless.
Kajita showed in 1998 that neutrinos captured at the Super-Kamiokande detector in Japan underwent a metamorphosis in the atmosphere, the academy said.
Three years later, while working at the Sudbury Neutrino Observatory in Canada, McDonald found that neutrinos coming from the sun also switched identities.
McDonald said that scientists would still like to know the actual masses of the various forms of neutrino. And experiments are looking at whether there are other types of neutrinos beyond the three clearly observed.
Neutrinos are the second most abundant particles in the universe after photons, "so any property of neutrinos can have dramatic repercussions on the life of the universe and on its evolution," said Antonio Ereditato, director of the Albert Einstein Center for Fundamental Physics at the University of Bern, Switzerland. "This is really one of the milestones in our understanding of nature."
Robert G.W. Brown, chief executive officer of the American Institute of Physics, said the laureates' work helps scientists understand how much mass exists in the universe. Brown said it's also a success for the theory known as quantum mechanics, which deals with fundamental particles and their interactions, because that's the only way to explain how neutrinos can change from one type to another.
"We are really down to understanding of the most fundamental aspects of science as we know them," he said.
The winners will split the 8 million Swedish kronor (about $960,000) prize money. Each winner also gets a diploma and a gold medal at the prize ceremony on Dec. 10.
On Monday the Nobel Prize in medicine went to scientists from Japan, the U.S. and China who discovered drugs that are now used to fight malaria and other tropical diseases.
The prize announcements continue with chemistry on Wednesday, literature on Thursday, the Nobel Peace Prize on Friday and the economics award next Monday.
Mari Yamaguchi in Tokyo, Frank Jordans in Berlin and AP Science Writer Malcolm Ritter in New York contributed to this report.