Traditionally, astronomy has used light to study distant celestial objects. But photons are not the only particles that reach us. Neutrinos are also powerful tools for studying the universe, especially at its extremes, because they don’t get deflected or hindered.
They’re tiny, nearly massless, and absolutely everywhere – they’re constantly being created by the Sun’s nuclear fusion processes, for example. If you hold up your hand, about 60 billion neutrinos will pass through your thumbnail every second, like miniscule ghosts.
But these are not the most interesting kinds of neutrinos, according to astrophysicists. Instead, they want to study super-fast, super-energetic neutrinos that have come from far, far away. And these turn out to be incredibly difficult to spot.
That’s why scientists want to build a neutrino telescope more than two kilometres beneath the surface of the Pacific Ocean.
“Neutrino telescopes are a technological marvel,” says project leader Elisa Resconi, an astrophysicist at the Technical University of Munich, Germany.
“In a neutrino telescope we measure the dimmest light possible, with the shortest time synchronisations imaginable, using the lowest power consumption and producing the highest data. All of this from the most remote place on Earth.”
She says that a prototype of the telescope – called the Pacific Ocean Neutrino Experiment, or P-ONE – will be built by end of next year.
So here’s the plan: Just off the coast of British Columbia in the north-east Pacific Ocean, seventy one-kilometre-long strings will be sunk deep into the darkness. The strings – each studded with sensitive light detectors – will be attached to floats so they stand upright like kelp.
Then, they’ll wait and watch for flashes of radiation that occur when the nearly massless subatomic particles interact in the water.
Specifically, the telescope will look for high-energy neutrinos that have come from far across the universe, produced in exotic events like supernovae, gamma ray bursts or colliding stars.
“With neutrinos,” Resconi says, “we can study mechanisms and regions which are obscured in photons – we can study beyond any dense cloud or accretion disk.”
Using these messengers from the stars, astrophysicists can investigate places like the centre of our Milky Way Galaxy, which is shrouded in dust, or search for hints of cosmic ray production or even dark matter.
“And of course, we could also discover something we can’t imagine today,” Resconi adds. “We understand only a very small fraction of the universe.”
The only telescope currently sensitive enough to detect these high-energy neutrinos is the IceCube Neutrino Observatory. It’s been operating in Antarctica since 2011, with detectors drilled into a cubic kilometre of pure ice.
IceCube spotted the very first high-energy astrophysical neutrinos, and even found evidence that some of these particles come from blazars – a type of highly active galaxy, powered by a massive black hole.
But in total, it’s only spotted a handful of astrophysical neutrinos.
“While this progress is exciting,” says Resconi, “it also reveals that with a telescope only of the size of the order of a cubic kilometre, the neutrino sample collected from the cosmos is still too limited to advance on this promising, rich path of fundamental discoveries in astro- and particle physics.”
With a total of 1400 detectors spanning three cubic kilometres, P-ONE will maximise its chance of spotting these massively powerful particles. Since it’s larger than IceCube, it will hopefully be able to spot detect rarer and more high-energy events.
This is not the first or only attempt to build a neutrino telescope in water. In fact, others are under construction, including the Cubic Kilometre Neutrino Telescope (KM3NeT) in the Mediterranean and the Baikal Deep Underwater Neutrino Telescope (Baikal-GVD) in a frozen lake in Siberia.
According to Clancy James, an astronomer who is part of the KM3NeT collaboration, the more observatories looking for neutrinos, the better.
“Neutrino astronomy is still in its infancy,” explains James, who works at the Curtin Institute of Radio Astronomy in WA. “The more groups trying to detect these elusive particles, the more new ideas, better engineering solutions, and raw statistical power we will have for understanding them.”
Why choose the ocean?
Sure, there are 60 billion neutrinos passing through your thumbnail every second, but they rarely actually interact with anything. Throughout your lifetime, only a single neutrino will interact with your body.
According to James, to see neutrinos you need “a huge volume of optically transparent material in a place where it is very dark – so at a basic level, both the ocean and Antarctic ice work well”.
The location needs to be dark because the detectors look for flashes of Cherenkov radiation: light that neutrinos produce when they interact with a water or ice molecule. From that flash, the detectors can accurately trace the path of that neutrino to figure out where it came from, how much energy it had, and even how it might have formed.
But, James says, there are some key differences between building a detector in water and ice.
“For instance, water scatters light less, meaning you can get a more accurate idea of where neutrinos come from – but it absorbs light more, so that you have less light to work with,” he says.
P-ONE has another a pretty major advantage over ice: it will take use existing oceanographic infrastructure off the coast of Vancouver Island. Built and operated by Oceans Networks Canada (ONC), an institute at the University of Victoria in Canada, this network is the biggest one on Earth.
Resconi explains that the infrastructure is composed of an 800-kilometre loop of fibre optical cable, 2.6 kilometres below the surface. This is already connected to shore to supply power and ferry data to and from seafloor instruments.
She adds that no other attempts to build telescopes in water have existing infrastructure, resulting in much larger overheads to get started.
“I believe we have identified new ideal conditions for a neutrino telescope.”
But the deep ocean has its own challenges.
“The currents are of course an issue, making the kilometre-tall instrumented lines move,” Resconi says.
Building such long lines rigidly would be costly and logistically tricky, so the solution is to let them drift freely – but monitor them scrupulously.
“We are addressing this challenge [by] modifying the buoys on top and installing an optical monitoring system, which constantly will detect where the modules are relative to each other,” she says. “With these two methods we believe we will have the geometry of the telescope under control at the level of 10–20cm.”
This accuracy is needed, because astronomers want to reconstruct the path of a neutrino back to its astrophysical origin – and if you don’t know where your detector is, it could mess up this extrapolation.
Before you even get to that point, even deploying the telescope is tricky – it’s basically tipped off the side of a ship.
James explains how KM3NeT is being installed: “You roll each detector line into a ball, weight it down with concrete, and drop it off a ship – in very calm weather only! It gets to the bottom with plus or minus a few metres accuracy.
“Then an acoustic signal is sent, which releases a clip. The buoyancy of the detectors themselves (the glass spheres containing the optical sensors also contain vacuum) causes them to unravel upwards.
“Later, you need an underwater unmanned robotic sub to connect the power and electronic cables from the base of the line to an underwater hub.”
Watch the detection units being deployed for KM3NeT:
Location, location, location
Being located in the Pacific Ocean will also be a boon for neutrino astronomers, according to Resconi.
“The neutrino sky that we can explore from the north-east Pacific will be complementary to the neutrino sky covered by IceCube,” she says. “A neutrino telescope looks through the Earth, so the sky covered corresponds to the opposite hemisphere.”
This is because neutrinos barely interact with anything, so they’re the only known particles that make it all the way through the Earth.
“IceCube monitors the northern sky and a neutrino telescope in the north will study the southern sky,” Resconi says.
This is good news, because the southern hemisphere sky is the best place to see the centre of our galaxy and study potential sources of neutrinos there.
KM3NeT and GVD are also being built in the northern hemisphere. While their data will be similar to that of P-ONE, the telescopes are not in direct competition.
“Two or three telescopes at different location like Siberia, Europe and Pacific Ocean will also provide a constant coverage of the sky during the day so that also rapid transients of neutrino signals could be covered,” Resconi explains.
James says that data from P-ONE could help the KM3NeT team achieve one of their main science goals: to discover the sources of cosmic rays in our galaxy.
“Cosmic rays are high-energy particles bombarding the Earth from space, with energies up to ten million times what can be produced on Earth in the Large Hadron Collider,” he explains. “But we don’t know where they come from.”
They could have been jettisoned out of supernova explosions, forming neutrinos when they collide with interstellar gas.
“The problem,” James says, “is that these neutrinos interact very rarely, and detecting them from the relatively weak sources in our galaxy will be much more difficult than detecting them from the supermassive black holes spewing out huge jets of gas in more distant galaxies. So using data from P-ONE together with data from KM3NeT and GVD will be a big plus.”
So what comes next?
The P-ONE team is now entering their prototype phase, focused on building and deploying three lines of detectors out of the proposed 70.
“The prototype phase is very interesting: there are a lot of learned lessons in the community from IceCube and KM3NeT,” Resconi says. “Technology has also evolved a lot. We have identified good partners in industry and we are also looking forward to push for synergies with oceanographers and try to help us on the study of the changing ocean as much as possible.”
The next stage will be mass production. The best-case scenario is to have a telescope equivalent to IceCube by the end of the decade – and then perhaps expand from there.
And there is no maximum size for an ocean-based neutrino detector – the size is only limited by funding.
“We aim to really go big, and we have the infrastructure available,” Resconi says.