The Nobel Prize in physics was awarded Tuesday to Takaaki Kajita of the University of Tokyo and Arthur B. McDonald of Queen’s University in Canada. Kajita and McDonald are honored for their contributions to observations on the oscillations of neutrinos, which show that neutrinos -- previously thought to be massless -- indeed have mass.

"This year's prize is about changes in identity among some of the most abundant inhabitants in the universe," the committee said during a news conference.

"It's an absolutely fantastic choice. If anything, I think many physicists would say it's long overdue," Ray Jayawardhana, an astrophysicist at York University and author of "Neutrino Hunters," told The Post.

Neutrinos are some of the subatomic particles that make up our universe. They're everywhere, and trillions of them flow through your body every second. But because neutrinos move nearly at the speed of light and don't appear to interact with matter in most cases, they're little understood. Suggested in 1930 and confirmed in the 1950s, neutrinos were long thought to be totally massless.

This ended up posing a problem. When scientists calculated the number of neutrinos that should be created by the natural processes of the sun -- just one of the many processes, including those occurring in our own bodies, that emit the particles -- they found that up to two-thirds of them seemed to be missing.

One solution was that the neutrinos might change to different "flavors" on the way. In the late 1990s, Takaaki Kajita helped to confirm this at Super-Kamiokande, a research facility tucked deep into a zinc mine that can detect the subtle changes in neutrinos. According to Kajita's research, it was likely that neutrinos had three "flavors."

Around the same time, McDonald was observing the behavior of neutrinos from the sun at the Sudbury Neutrino Observatory. His team found evidence that the neutrinos were changing type along their way, and that the "missing" neutrinos were accounted for once all three flavors were numbered.

And if neutrinos change, they must have mass.

"These two different experiments provided independent confirmation of what's actually a very puzzling phenomenon," Jayawardhana said. "And it opened up the possibility of studying physics beyond what's called the standard model."

The discovery of neutrino oscillation represented the first scientific evidence that the standard model of particle physics, as developed in the 1970s, might need some revision.

"These experiments really opened up the floodgates," Jayawardhana said. "The field turned from this sleepy backwater into a thriving hub of activity."

There are still more questions about neutrinos than answers. But with this discovery, physicists changed one of our basic understandings of the way the universe works. The mass of neutrinos puts a kink in our model of the formation and evolution of the universe, forcing scientists to rethink the way the subatomic particles interact with the rest of the cosmos.

This post has been updated.

Correction: A previous version of this post stated that neutrinos don't interact with matter, which is over-simplification. They do interact with matter, but not in a way that's easily observed. They pass through most matter undetected. 

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