We've pushed the limits of engineering, chemistry... and patience, in order to push the limits of physics

Dr Steve Biller, Oxford University

New observations made by a giant underground neutrino detector in Canada show that the solution lies not with the Sun, but with the neutrinos, which change as they travel from the core of the Sun to the Earth.

The finding raises new questions about the so-called Standard Model of Particle Physics, which seeks to explain the basic building blocks of matter.

'High confidence'

The research was carried out at the Sudbury Neutrino Observatory (SNO), Ontario, in collaboration with Oxford University, UK.

Light detectors around the water tank detect the neutrinos

"We now have high confidence that the discrepancy is not caused by problems with the models of the Sun but by changes in the neutrinos themselves as they travel from the core of the Sun to the Earth," says Dr Art McDonald, SNO project director and professor of physics at Queen's University in Kingston, Ontario, Canada.

"It's taken longer than we thought, but it's all been well worthwhile," says Dr Steve Biller, of Oxford University. "We've pushed the limits of engineering, chemistry... and patience, in order to push the limits of physics."

Fundamental particles

Neutrinos are fundamental particles of matter. They are often called 'ghostly' because they interact so weakly with other forms of matter.

They come in three types: the electron-neutrino, the muon-neutrino and the tau-neutrino. Electron-neutrinos are emitted in vast numbers by the nuclear reactions that power the Sun.

Since the early 1970s, several experiments have detected neutrinos arriving on Earth, but they have found only a fraction of the number expected.

This meant there was something wrong with either the theories of the Sun, or the understanding of neutrinos.

It was to solve this puzzle that the SNO experiment was conceived nearly 15 years ago.

The detector is located 2 kilometres (1.4 miles) below ground in a nickel mine near Sudbury, Ontario. It is unique in its use of 1000 tonnes of heavy water to trap different types of neutrino interactions.

Installing the giant underground tank

..there were times that we weren't sure we were going to make it

Professor Nick Jelley, Oxford University

Dr McDonald points out that earlier measurements had been unable to provide definitive results showing that this transformation from solar electron neutrinos to other types occurs.

"The new results from SNO, combined with previous work, now reveal this transformation clearly, and show that the total number of electron neutrinos produced in the Sun are just as predicted by detailed solar models," he says.

Expansion of the Universe

The confirmation that neutrinos change has implications for the future of the Universe.

"Even though there is an enormous number of neutrinos in the Universe, the mass limits show that neutrinos make up only a small fraction of the total mass and energy content of the Universe," says Dr Hamish Robertson, Professor of Physics at the University of Washington in Seattle.

This means that neutrinos cannot contribute enough mass to the Universe to halt its expansion.

Under this scenario, the Universe will expand forever, eventually becoming starless and just diffuse gas.