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.
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.
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