Nobel prize for physics 2015: how neutrinos saved the world
The prize goes to a discovery about the properties of neutrino particles
that has saved us from worrying that Earth might end in an icy death.
Astronomers called it the solar neutrino problem. It was much more than a
problem. Upon its discovery in the late 1960s, it meant that the sun could
be dying. And if the sun died, so would life on Earth. But thankfully the
latest winners of the Nobel prize for physics, Takaaki Kajita and Arthur B
McDonald, have been addressing such concerns to great success.
The sun was theorised to be powered by nuclear reactions in its core and
these produced neutrino particles. Theoretical models of the sun’s interior had predicted the number of neutrinos
that were being produced and by the mid-1960s, two American physicists had taken up the challenge of trying
to detect them: Raymond Davis Jr, and John Bahcall.
Neutrinos are incredibly unreactive particles. Davis and Bahcall calculated that they might catch one neutrino per
day out of the 10m bn that were expected to be coming from the sun.
Their detector was a cylindrical tank 20ft in diameter by 48ft long (6 metres by 14.6 metres). It contained 100,000
gallons (455,000 litres) of tetrachloroethylene, which was none other than dry cleaning fluid. A neutrino interacting
with a specific isotope of chlorine found in the cleaning fluid would transform it into an isotope of radioactive
argon.
They left the argon to build up for a month or two, then collected it by shooting jets of helium gas through the
fluid. Finally, they measured its radioactivity to determine how much had been created.
On one hand their experiment was a triumph: they did indeed detect neutrinos. There could be no doubt any
more that the sun was a vast nuclear reactor. But the number of captured neutrinos was only about one-third of
what they had expected, and that was a big worry.
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Although it takes many hundreds of thousands of years for the energy generated in the sun’s core to fight its way
to the surface and then fly to Earth, the neutrinos come straight out. So the surface brightness tells us about
the ferocity of the nuclear reactions that were taking place hundreds of thousands of years ago. The neutrinos
tell us about the reactions taking place today – and there was a clear difference in the two rates.
The most obvious solution was that the sun’s nuclear reactions had dropped to just one-third of their former
levels. This was not readily apparent as the older radiation was still percolating out but sometime in the next few
hundred thousand years, the brightness of the sun was going to drop indicated by the neutrinos, and that meant
Earth was going to suffer a premature, freezing death. The only ray of hope was if there was something wrong
with our understanding of the way neutrinos behave.
It had been assumed that neutrinos were essentially massless particles, like the photon particles that carry
light. However, if neutrinos did possess a small mass it was theoretically possible that they could change into
one of three guises, only one of which would be detectable by Bahcall and Davis’s experiment. Could this be
the explanation for the mysterious one-third smaller detection rate, that the neutrinos leaving the sun were
“oscillating” between these three different states?
Yes it could, and this is what Kajita and McDonald have just been awarded the 2015 Nobel prize in physics for
confirming, by using advanced neutrino detectors in Japan and Canada. And by proving that there was nothing
wrong with the nuclear reactions inside the sun, they saved the world from an icy death (sort of).
http://www.theguardian.com/science/across-the-universe
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