Physicists win Nobel for finding neutrinos, burdened with mass, aren't so fleeting after all

For decades, physicists thought the hard-to-detect fundamental particles were massless. Not so, Takaaki Kajita and Arthur McDonald discovered, opening new research into the universe's past and present.

Takaaki Kajita of Japan and Arthur B. McDonald of Canada won the 2015 Nobel Prize for Physics.
Takaaki Kajita of Japan and Arthur B. McDonald of Canada won the 2015 Nobel Prize for Physics. Royal Swedish Academy of Sciences

In 1960, physicists could agree with John Updike when he wrote in his poem "Cosmic Gall" that, "Neutrinos, they are very small / They have no size and have no mass." But on Tuesday, two physicists won the Nobel Prize for proving that poem -- and more than 50 years of scientific belief -- to be wrong.

Neutrinos, fleeting particles second in abundance in the universe only after the particles of light called photons, were thought for decades to have to have no mass. The Royal Swedish Academy of Sciences awarded the 2015 Nobel Prize in physics to Takaaki Kajita of Japan and Arthur B. McDonald of Canada for showing that the particles do indeed have mass, though at a millionth the mass of a single electron at most, they're very light indeed.

The fact that neutrinos have mass isn't immediately practical knowledge, unlike the invention of the blue-light LEDs that now permeate the computing industry and that drew the 2014 Nobel Prize in physics. But it does fulfill a deep desire many humans have: to understand how the universe actually works.

You do have a reason to be grateful for neutrinos. They are involved in the sun's fusion furnace that ultimately produces the heat and light that keep life on Earth going, Nobel Prize Committee member Olga Botner said during a press conference. And they're involved again during the colossal stellar explosions called supernovas that produce the chemical elements out of which we're made.

McDonald, too, shared a reason the neutrino mass is useful. The findings verified details about the inner workings of the sun that apply directly to the present effort to emulate that energy source in an earthbound machine called a fusion reactor.

The massive Super-Kamiokande neutrino detector in Japan is studded with thousands of light sensors. Along with a related facility in Canada, it was responsible for detecting neutrino behavior that meant the particles have mass. Super-Kamiokande

Clearly, though, the motivation behind the research is more about curiosity than practical spinoffs.

The two physicists led separate teams in Japan and Canada that in the 1990s built massive facilities to detect neutrinos produced from sources including radioactive decay of elements in the Earth and the fusion reaction at the heart of the sun.

Detecting neutrinos is difficult because they almost never react with other parts of the universe. Billions of neutrinos pass through each one of us each second, having no effect. Poet Updike was right when he said, "The Earth is just a silly ball / to them, through which they simply pass. ... They snub the most exquisite gas / Ignore the most substantial wall."

Physicists knew there were three varieties of neutrino: electron, muon and tau. The experiments found that one type could change into another, a process called oscillation.

"You can chalk up yet another success for quantum mechanics, because without it we would not be able to make sense of the experimental results that have led to this prize," said Robert Brown, chief executive of the American Institute of Physics, in a statement.

With the difficult-to-grasp but proven physics of quantum mechanics, particles also act like waves, traveling in a way analogous to ripples spreading in a pond. "Once again quantum mechanics and wave interference provided an explanation for oscillatory behavior -- this time with mass and previously with photons," Brown said.

"For particle physics this was a historic discovery," the Royal Swedish Academy of Sciences said. It upset the Standard Model that for 20 years had successfully described of the innermost workings of matter.

For further details, check the academy's information for a general audience or the advanced information with more technical depth on the neutrino mass discovery.

Despite the discovery, neutrinos remain cloaked in mystery.

"Nobody has ever seen a neutrino," Botner said. "What they are, no one really knows."