A neutrino breakthrough sheds light on a huge cosmic mystery

A neutrino observatory on the South Pole joined forces with telescopes around the world to track a cosmic neutrino to its source four billion light-years away

And then there was one. A single high-energy neutrino from deep space detected deep below the Antarctic ice sheet last September prompted astronomers around the world to swing telescopes into position. Their coordinated observations have paid off, tracking the cosmic traveller back to its source – for the first time ever.

But the find is more than the story of this single particle with hardly any weight or charge – which gave it the moniker “ghost particle”. Tracing the path of the neutrino is the start of solving a centuries-old mystery and launching a new type of astronomy – one that will allow us to probe areas of deep space that are completely off-limits to optical telescopes.

Two papers published in the journal Science describe how the IceCube Neutrino Observatory on the South Pole detected and traced a high-energy neutrino. The IceCube scientists had support from teams at 18 other observatories, who followed up on the discovery within minutes. Working among others with the Fermi and MAGIC gamma ray observatories, they pinpointed the neutrino’s source as the TXS 0506+056 blazar, an active supermassive black hole in a galaxy approximately four billion light years away.

“A blazar is typically like a lighthouse in the sky for us,” says Elisa Resconi, physicist at Technical University Munich, explaining that we’re looking directly into the jet of high energy particles spouting from the blazar.

The find is also spectacular, because for the first time, scientists may have tracked down (or at least have come closer to tracking down) the origin of high-energy cosmic rays – a mystery that has puzzled astrophysicists since the first cosmic rays were observed in 1912.

“We live in this little Earth bubble and we get hit by things from space,” says Cliff Burgess, theoretical physicist at the Perimeter Institute unaffiliated with the research. “Mostly that’s good, because a lot of that is sunlight from the sun, but we also get hit by energetic particles from space and we have no idea why and where they came from.”

Among those particles are neutrinos, nearly massless and electrically neutral high-energy particles that barely interact with anything, usually passing through Earth and all of us unnoticed and unimpeded. Despite detecting hundreds of lower energy solar neutrinos every day, we only spot a handful of these ‘ghost particles’ from far-away parts of the universe a year.

The neutrino that bumped into IceCube on September 22, 2017 measured at 300 terra-electron-volts (TeV). That’s far higher energy than we can ever hope to produce in any particle accelerator on Earth. The Large Hadron Collider at CERN, for example, bumps protons to just 6.5 TeV.

The blazar, which is the possible source of the neutrino, is like a gigantic accelerator, says Resconi. Tracing the neutrino back to it and looking at the light or radio waves arriving from the stars helps researchers understand the strange processes taking place in the very heart of galaxies, the supermassive black holes or “big monsters in the universe,” she adds.

Successfully identifying the source of a high-energy neutrino can also help researchers understand where cosmic rays come from like the protons with extremely high energies that scientists spot from time to time, says Darren Grant, a physicist at the University of Alberta and spokesperson for the IceCube Neutrino Observatory.

That’s because neutrinos are a by-product of cosmic rays – and unlike their parent particles, neutrinos travel in a straight line. “Neutrinos are snobs,” says Roopesh Ojha, astrophysicist at Nasa Goddard Space Flight Centre. “They hardly ever interact with anything else.”

This is both good and bad, he explains, as this failure to interact makes them very difficult to detect, yet also means they can reach Earth without being affected by any matter such as stars, planets, gas or dust.

Light (photons) on the other hand is. “For some of the most interesting physical problems of the universe – what’s going on in the centre of black holes, near the gamma ray bursts, all of these things – light has a pretty tough time escaping and making it to us,” Ojha explains.

Neutrinos can come right through, though. If both a photon and a neutrino are born at the same time in the centre of the Sun, says Ojha, it’ll take a photon twenty to over a hundred thousand years to bump its way to the surface of the sun, while a neutrino can travel the same distance in under three seconds.

Astronomy started with just light, although in a variety of wavelengths. “Every time you look with a different wavelength, you learn different things,” says Burgess. What we can’t see with optical telescopes suddenly becomes visible in microwave radiation, or radio, or UV – as well as with gravitational waves detectors such as LIGO, and – now – neutrinos.

So the emerging field of neutrino astronomy is a new sense, in a way, says Ojha. “It’s going to bring us information that has not been available to us from any type of light. Being able to detect astrophysical neutrinos is an entirely new way for humanity to look at the cosmos.”

Researchers hope that in the future, as they trace more and more neutrinos to their origins, they will be able to get a much more complete understanding of the universe, especially about the extremely powerful objects and events in the sky.

Over a thousand coauthors are listed on the two papers, which Gregory Sivakoff, physicist at the University of Alberta describes as a “collaboration of collaborations.” Unlike just a few years ago, IceCube now issues (near) real-time alerts that allow scientists around the globe to work together to get the clearest possible picture across a broad range of wavelengths, which helps them to better understand specific events deep in the universe.

The latest observations are considered a revelation – but there’s still a slim chance it’s just a fluke, says Ignacio Taboada, physicist at University of Pennsylvania. Ojha, though, is convinced that they have found the neutrino’s source, because they can narrow its path down to a relatively tiny patch of sky, and TXS 0506+056 is the only strong gamma ray blazar in the region. “I’m not really a betting person, but I would probably make a reasonable bet that this is going to be borne out,” he says.

“Scientifically, it’s really hard to prove that something is wrong because these things don’t repeat,” says Taboada. “It may repeat, but in a different part of the sky. How would you know that the first interpretation was wrong?”

After the recent observations, researchers went back to their data archives, to see if this was a one-time event – and found several more neutrinos that arrived from the same region. While any single event might not be statistically significant, it helps to form a more accurate picture. “As you build that evidence together, your case becomes stronger and stronger,” says Grant.

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A neutrino breakthrough sheds light on a huge cosmic mystery

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