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This year’s Nobel prizewinning physics discovery is passing through your body right now. Takaaki Kajita and Arthur McDonald share the prize “for the discovery of neutrino oscillations, which shows that neutrinos have mass”. But just what are these ghostly particles, which barely interact with the real world?

Neutrinos were predicted in 1930 by Wolfgang Pauli, who said they were emitted during radioactive decay within atomic nuclei. They were assumed to be massless, and since they also have no electric charge, they can pass through normal matter almost completely undetected.


That made spotting them pretty tricky, and their first detection in 1956 was itself worthy of the 1995 Nobel prize in physics.

But there was a problem: nuclear reactions within the sun didn’t seem to be producing enough neutrinos. When carefully shielded detectors on Earth picked up the stream of solar neutrinos, only a third of the predicted numbers were arriving.

Flavours beget mass

As it turns out, neutrinos come in three “flavours”, electron, muon and tau, suggesting a way to make the numbers match theory. The sun was thought to produce electron neutrinos only, and if these particles were somehow morphing into the other two flavours as they travelled through space, it could explain the anomaly. But it would also mean that neutrinos were not massless – instead they would have an almost immeasurably tiny mass.

Takaaki Kajita heads up the Super-Kamiokande neutrino detector near Hida, Japan, and in 1998 he announced the discovery that muon neutrinos, created by cosmic rays in the atmosphere, seemed to be transforming into taus.

But what about the electron neutrinos? That came a few years later in 2001, when Arthur McDonald of the Sudbury Neutrino Detector in Ontario, Canada, announced that electron neutrinos could also change into the two other types. In fact, neutrinos seem to oscillate between all three flavours as they travel.

Discovering that neutrinos have mass was a major change for physics. “When you do not know whether they have mass, it’s otherwise difficult to understand how to incorporate them into those theories that give us a more fundamental understanding of the world of physics,” McDonald said during a press conference announcing the prize, which he accepted on behalf of his whole team.

“We are very satisfied that we have been able to add to the world’s knowledge at a very fundamental level. This recognition is a tremendous accolade for our group,” he said.

But just what masses neutrinos can have remains a mystery. Experiments around the world are still attempting to pin the neutrino down, and the field as a whole is blossoming.

Since neutrinos aren’t held up by matter, we can use them to peer beneath the Earth’s surface, deep out into space , and even within nuclear reactors, potentially to monitor nuclear proliferation agreements – not bad for a particle that is barely there.

Yesterday, the 2015 Nobel prize for medicine was awarded to scientists who developed new drugs for roundworm parasites and malaria. Click here to read about the three scientists who will share the prize and their work.