Explaining the Latest Nobel Prize in Physics

The 2015 Nobel Prize in Physics was awarded last week to the two leaders of research groups that experimentally solved a puzzle about neutrinos, which has important implications for our understanding of the basic laws of physics.

The experimental discoveries that earned this year’s Nobel Prize were made about 15 years ago, so it may seem anti-climactic to award the prize now. It took a while for the discoveries to be confirmed, and for the community of physicists to realize the consequences. “Waiting for the dust to settle” in physics takes time.

Really major physics experiments involving fundamental particles require huge facilities and equipment and are conducted by large research groups. The key published papers from 1998, 2001, and 2002 had long lists of authors, but it was understood that these were group efforts. The leaders of the research organizations were awarded the Nobel Prize: Arthur McDonald, for the Sudbury Neutrino Observatory; and Takaaki Kajita for the Super-Kamiokande detector.  Of course the Prize honors the accomplishments of both research groups.

Neutrinos are extremely tiny particles, very hard to detect. The first prediction of their existence came from Wolfgang Pauli in 1931, who hypothesized a tiny invisible neutral particle, in order to make sense of the process of beta-decay. There, an atomic nucleus changes from one element to another and an electron is given off. Without such a particle to carry away energy and momentum, the laws of Conservation of Momentum and Conservation of Energy would be violated -- and nobody would buy that!  Enrico Fermi coined the word “neutrino” -- in Italian, “little neutral one.” The new theory said that wherever electrons are being emitted, neutrinos are too, but neutrinos hardly ever interact with anything. That theory, although unproven, was fully plausible and saved the conservation laws.

Neutrinos were not actually detected until 25 years later -- 1956. That was accomplished inside a salt mine under Lake Erie, where 400 feet of dirt and lake water overhead took out every other kind of cosmic ray, leaving only neutrinos to get through. Measurements of neutrinos are very indirect -- flashes of light produced by intermediate particles. A lot of theory underlies any observation of a neutrino.  To this day, neutrino-detection experiments are done in deep abandoned mines, notably in Sudbury, Ontario, in South Dakota and in Japan.

By the later 1960s, it was realized that a huge stream of neutrinos flows constantly away from the core of the sun, where nuclear fusion takes place. To the best of our understanding, every second trillions pass through the earth (and through our bodies) totally unnoticed.

Separately, it was found that there are three members of the family of lightweight particles, or Leptons: electrons, mu mesons and tau mesons. While electrons are commonplace, the latter two are produced in synchrotrons (atom-smashers). For each of those Leptons, theory assigns a corresponding neutrino. The term “flavor” was whimsically used to distinguish the 3 different kinds of neutrinos.

Throughout the 20th century, neutrinos were believed to be massless particles. Advances in theory across that century produced what is termed “the Standard Model,” relating the forces of nature and the many known sub-atomic particles. That theory includes the notion of massless neutrinos.

In the meantime, further observations indicated that the number of neutrinos reaching us from the sun is way too low, by about 70%. The question “Where are the rest of the neutrinos?” became a major puzzle. One clue was the fact that we could only detect electron-neutrinos. (By then, we’d gotten better at detecting electron-neutrinos, but muon neutrinos are much harder to detect, and tau-neutrinos harder still.)

It was suggested that perhaps the various “flavors” of neutrinos were oscillating, that is, changing from one type to the other. It would certainly help explain why 2/3 of the expected neutrinos were missing if it could be shown that electron neutrinos had changed into muon or tau neutrinos during their brief time of flight from the sun to the earth.

Researchers set out to find a way to detect muon neutrinos. They took note of the fact that neutrinos showing up from underneath (having traveled completely through the earth) were delayed compared to neutrinos coming directly from above. Very small differences were detected with great precision. The experiments reported in 1998-2002 showed that a fair percentage of the original neutrinos (whether leaving the sun or produced in our atmosphere) did indeed oscillate or change flavor to become muon or tau neutrinos. That experimental proof is what earned the Nobel Prize.

You ask “So what?” Plenty. If neutrinos are capable of changing flavor, they must have mass; the change couldn’t happen otherwise. The concept of massless neutrinos had to be discarded. The best estimate of the neutrino mass puts it in the range of one millionth of an electron’s mass. We have no equipment today that can discern such a small mass.

But we do know that there are lots and lots of neutrinos -- every star in every galaxy gives off gigantic numbers of them. As lightweight as a single neutrino may be, their grand total mass is comparable to the visible mass of the universe. That connects to the topic of “dark matter,” which is another major puzzle in physics. About that, today we can only say “stay tuned.”

The Standard Model is beloved by physicists everywhere, but it must be corrected since neutrinos are not massless. Neutrinos travel very fast, but not quite at the speed of light. Theorists have been working on the necessary revisions for over a decade now. This is more than just a touch-up to the Standard Model.

For over a century, scientists have been tempted to say “now we know the full story for sure.” But then one correction after another comes along, some of which cause major changes in theory. Nothing in science is ever finally “settled.”

Thomas P. Sheahen Ph.D is an energy scientist who has B.S. and Ph.D. degrees in physics from the Massachusetts Institute of Technology.