Why a flipped neutrino just won the Nobel Prize

When scientists first started asking to build neutrino detectors, the big question was: why bother? Neutrinos were incredibly difficult to detect, they interact only weakly with regular matter, and they didn’t even seem to have any mass. These neutrino detectors were incredibly expensive and finicky rigs that had to be built deep underground, just for a hope of capturing the presence — let alone direction — of a neutrino bombarding the Earth from space. Yet now, researchers working in Japan and Canada have been awarded the 2015 Nobel Prize in Physics for their work on the neutrino. What did they find, and why was it so impressive?
First off, neutrinos are incredibly numerous. Though they don’t interact with the matter that makes up our bodies, there are many billions of neutrinos bombarding our bodies at any given second. Some originated during the Big Bang, while others arise from the impact of cosmic rays with the upper atmosphere and even radioactive decay of elements in the Earth’s crust. They come in three “flavors” referred to as tau, electron, and muon. Earlier experiments investigating the muon neutrino earned the Nobel Prize in Physics in 1988.
Yet, abundant as they are, physicists thought that they should be far more abundant than mankind’s new-fangled neutrino detectors observed them to be. Additionally, they were not always of the flavor that they ought to be — and, in 1998, a Japanese team under the leadership of Dr. Kajita found that muons can actually flip flavors on their own. Shortly after, a Canadian team proved that neutrinos from the Sun are undergoing a similar flavor-switch on their way to Earth.
Proving that this “oscillation” between flavors really does occur solved two problems at once: it showed that the roughly two thirds of neutrinos that were “missing” had in fact just switched away from their expected flavor, and it showed that neutrinos must have mass. The only problem was that both theory and observation said that the neutrino did not have mass.
For their work proving the neutrino cannot be massless, leading fairly quickly to separate observations of that mass, the two teams were awarded the 2015 Nobel Prize in Physics. The two recipients were Arthur McDonald of Canada’s Queens University, and Takaaki Fajita of Japan’s University of Tokyo.
Thanks to their work, we now know that the neutrino does in fact have some incredibly small amount of mass, probably about a millionth of the mass of an electron, a particle which was itself once believed to be massless. Yet, given the incredible number of neutrinos in the universe, these tiny masses could sum to weight more than all the stars in the universe.

McDonald and Kajita.

Neutrinos could potentially be a very powerful way of looking into the universe, since they can penetrate matter with so little trouble. A similar class of lepton called muons also penetrate deeply into dense matter and have been used to look inside the Fukushima Daichi reactor and the Great Pyramid of Giza — but muons can only bury a few kilometers into the Earth. Neutrinos, on the other hand, easily pass all the way through and come out the other side. They’ll need to be affected by core of the Earth in some way to be useful for deriving information, but it’s not inconceivable than neutrino detectors could one day let scientists look deep into the core of the Earth itself.
Or, crucially, inside stars. As mentioned, one major source of neutrinos are stars, and scientists have already used the neutrino emissions from the Sun to learn about the fusion reaction going on inside. With a better and better ability to read the language of neutrinos, there’s no telling what new insight astronomers might be able to gain about the universe and its components.