A tiny Australian mining town might hold the key to solving one of the universe's biggest mysteries – and to a local economic boom. What do scientists hope to find in a cave 1km underground?
By Lisa Clausen
The public lookout point for the Stawell gold mine in western Victoria is an unremarkable spot; a few faded information boards and pieces of old equipment, rusting among crooked gums beside the mine’s high perimeter fence.
Beyond the wire, near the slurry-coloured mine machinery which roars into the chill air, a dirt road descends steadily, through a rocky cutting, into the mine’s black mouth. It’s noisy, muddy and industrious – much like any number of working mines on a weekday – but not the sort of place where you’d imagine science might finally answer one of the great questions of our universe.
And yet it could be. For more than 40 years, the mystery of dark matter has defied the world’s best physicists. These invisible particles are thought to be everywhere – constantly passing through each of us and our planet. In fact, we can only observe five per cent of the whole universe; the rest is dark matter and dark energy.
Scientists have found compelling indirect evidence of dark matter’s existence, called gravitational lensing – where dark matter bends the visible light we see coming from distant galaxies. Yet this “stuff”, thought to shape galaxies and be the universe’s missing mass, remains frustratingly elusive. Directly detecting dark matter will be one of the greatest prizes of modern physics.
“Dark matter holds galaxies together,” says University of Melbourne particle physicist, Professor Elisabetta Barberio. “If we understand it, we will understand how the universe evolved from the Big Bang to now, and how it might continue to evolve.”
Barberio is the project leader of an ambitious experiment set to happen in Australia. Until now, efforts to find dark matter have all taken place in the northern hemisphere, with plenty of funding and facilities. Now, thanks to the Stawell Underground Physics Laboratory (SUPL), the southern hemisphere will join the global hunt. Its SABRE dark matter experiment will happen 1km underground in a country town of just over 6,000 people, best known for gold, farming, and a famous annual footrace – the Stawell Gift.
Stawell's 160 years of gold mining history have left a network of tunnels under its streets, and disused shafts which occasionally open up in people’s gardens. It's very much a mining town, but faced disastrous news when, in late 2012, the mine’s then-owners came to town council warning of the mine’s potential closure. After all, at its peak the mine employed about 400 people, while hundreds of others in local businesses benefited from its success.
A panel of councillors, council staff, locals and mine management was convened to brainstorm ideas for what might come next. The list of community proposals quickly grew: should they start growing mushrooms? Or open a subterranean hotel?
Scientists wrote to several mines across the country, with an unusual request for a spare underground cavern.
That same year, three hours down the highway in Melbourne, a group of physicists was wondering where they could stage a dark matter detection experiment. Swinburne University of Technology astrophysicist Jeremy Mould wrote to several mines across the country, outlining the group’s unusual request for a spare underground cavern. One of those letters went to Stawell’s council.
In a neat coincidence, at around the same time, Melbourne University astrophysicist Katie Mack was contacted by an adviser working on alternative uses for the Stawell mine. He’d read one of her articles on underground dark matter detection. When Mack put the adviser in touch with Mould, suddenly Stawell was very much in the picture.
Probing the nature of the universe hadn’t been on Stawell’s short list – yet. But what physicists needed was an underground site deep enough and in the right sort of landscape to block out the highly radioactive cosmic rays which relentlessly pelt the Earth’s surface. To the experiment’s incredibly sensitive detector, these rays are like a raucous radio station. The best way to turn down the volume is to head underground.
Stawell’s mine, in places dug almost 2km deep through dense volcanic basalt, looked promising. Because it’s a mine with ramp access, rather than a vertical shaft, people and equipment could be driven in. And because it was in operation, power, ventilation and internet access were already in place.
Most importantly, initial background radiation readings inside the mine were encouragingly low. Talks with council began and, in 2014, 60 scientists from around the world arrived to inspect the proposed site of the southern hemisphere’s first underground physics lab.
“The penny dropped then that there was really something in this,” says Northern Grampians Shire Mayor Murray Emerson.
“Things you would do on the surface without thinking have a significant difficulty when they’re underground.”
Now, with $3.5m from the Victorian and federal governments, construction is due to begin later this year. The rock above SUPL will be a radiation shield equivalent to 3km of water. Even so, maintaining the lowest radiation levels possible means a complex build.
It means everything from concrete to rock bolts must be tested before it can be used. By May this year, 26 component samples from sources as widespread as Adelaide to Gladstone had been tested, but only two found suitable. Quarry materials such as sand and aggregate must travel by road or train for analysis at the Australian Nuclear Science and Technology Organisation in Sydney – air travel gives off too much radiation. Materials such as the special concrete spray coating for the rock walls will have to be mixed on-site in specific containers, and stored away from mine materials.
“It’s certainly an unusual challenge,” says site project engineer Allan Ralph. “Things that you would do on the surface without thinking about them have a significant difficulty to them when they’re underground and forming part of a world-class physics laboratory.”
The safety, insurance and logistical demands of such a project are just as immense. Though it has abandoned deep ore mining, the mine has remained open. Running a construction site in a working mine, which is still blasting twice daily, brings a host of challenges. Builders and other contractors will have to be escorted to the site by trained personnel in approved vehicles, along the rough 13km road which spirals down into the humid darkness.
“If someone forgets their nail bag, they can’t just rush back up and get it,” says Ralph. Their first job will be to blast out an enormous cavern to accommodate the 12m high, 30m long main experimental hall – along with a second, smaller area with room for other experiments. Underground services and change rooms are needed, while both areas will need air purification systems and an emergency shelter pod.
If building the lab seems a demanding task, the design of the dark matter detector at its core is a more dazzling feat. This sophisticated instrument will rely on seven ultra-pure crystals, together weighing 150kg, and made from sodium iodide (a type of salt), each in its own cylinder of extremely pure copper – all of which will be housed in a larger cylinder made of low-radioactive iron. This will be inside a cube of lead bricks, weighing more than 100 tons, using lead sourced from Roman archaeological sites in Italy.
“The small amount of radioactivity contained in lead decreases with time,” says Professor Barberio, who was also part of the international team which discovered the Higgs boson in 2012, and a chief investigator at the ARC Centre of Excellence for Particle Physics at the Terascale (CoEPP). “So the older the lead is, the less radioactive it is.”
Having so many shields around the crystals is essential. As Barberio points out, even “a banana or our body will be too radioactive to be close to the detector.”
Once SABRE gets underway at the end of 2017, the hope is that dark matter particles streaming through the mine will occasionally collide with the highly-sensitive crystal target. Though dark matter is thought to rarely interact with ‘normal’ matter, when it does smash into the nuclei of a crystal atom, those atoms should recoil like a billiard ball that’s been hit – a reaction that will be emitted by the detector as visible light.
The experiment is already lined up to work with the world’s largest underground physics laboratory – the INFN Gran Sasso National Laboratory north-east of Rome – to help determine whether seasonal fluctuations recorded in light signals from Gran Sasso’s own detector are due to our planet’s movement through a sea of dark matter as it orbits the sun.
Astrophysicist Jeremy Mould, also chief investigator at the ARC Centre of Excellence for All-sky Astrophysics (CAASTRO), says SABRE embodies “top science”.
“Brian Schmidt got the Nobel Prize for his work on the acceleration of the universe,” Mould says, “and there are more Nobel Prizes to be had for dark matter.”
“We have the hole and they don’t.”
Despite its new place on the global scientific map, Stawell remains the sort of place where Mayor Murray Emerson heads out for a few drinks every Friday night to hear what’s on people’s minds. He is a retired police officer who drives the local school bus in between civic duties. When the lab was first proposed, Emerson says he was asked a lot of questions, along the lines of, “What’s this bloody dark matter thing?”
Now, with several town meetings about the project having drawn large crowds, he says the mood has changed. “Now people are asking, ‘Where’s it up to?’.”
Emerson is happy to admit he doesn’t fully understand himself how the experiment will work. But he says there’s no missing the envy coming his way when he attends regional local government meetings – after all, as he cheerfully puts it, “We have the hole and they don’t.”
With projections of around $18m in annual income for the local economy, and an estimated 80 new jobs in associated industries like tourism, many in Stawell share the mayor’s optimism that the spin-offs can be enormous. Thousands of school students, international researchers and curious tourists tour Gran Sasso’s facility every year. While SUPL will be smaller, tours and a visitor’s centre are on the cards. There are no brochures yet in the town’s Information Centre, and when SBS visits, the staff member at the desk jokes that even if anyone asked, she has no idea what dark matter is.
At their Sportspower sports store up the road, locals Robyn and Shane Young hope the lab will bring people who need somewhere to eat, sleep and meet. “The science is above most people’s heads, but it will be good for us if people are talking about Stawell,” says Robyn, standing in their shop – which occasionally shakes from the mining detonations deep beneath their building. Like many, they wonder what it could mean for the community they love.
“It could be,” says Shane, “like winning Tattslotto.”
Others hope that the dazzling expertise driving SABRE may rub off on local kids, expanding their own ambitions in new directions. “Maybe kids will aspire to do something along those lines. It could change a child’s life,” says Janita Perry, owner of Perry’s Footwear, a third-generation business on the town’s historic main street.
“One of our dreams is that one day, one of our students will be a professor of physics working in the lab.”
Murray Emerson agrees: “One of our dreams is that one day, one of our students will be a professor of physics and working in the lab.”
As a child growing up in Stawell, 19-year-old Lauren Knights says the mine was embedded in everyday life. Now a second-year civil engineering student at Melbourne’s Monash University, Knights believes having such high-level science in their backyard will energise other young people.
“It makes the opportunities within science more realistic and a little bit more attainable," she says. "And it will help Stawell find a new identity; it’s always been known as a mining town, now it could be known as a dark matter town.”
The chances of that are good – already more than 25 projects are queuing up to use this unique facility in coming years. Among them are Australian researchers looking at the effect of low-radiation environments on cancer cells, and meteorite experts who have had to travel to the northern hemisphere for previous experiments.
Council’s major projects manager Amanda Western says an extra $3m in government funding will be sought, to help meet the demand: “I don’t think we really comprehend the size of the opportunity yet.” For local industries who can grasp that chance, the rewards could be substantial; Elisabetta Barberio points out that at least one small family business in the regional area of Gran Sasso is now a multinational company, after leaping to the challenge of making specific components for that facility.
When John Anselmi, now 81, moved to Stawell almost 40 years ago to teach science at the local high school, the town was full of vibrant local industries, from woollen mills to stonemasons. Since then, he says he’s watched it slowly decline. However, the mine’s reopening in bushland on the edge of town in the 1980s “perked things up enormously”. He remembers locals gathering at the lookout to watch the new tunnel being built.
Perhaps there’ll be crowds among the dark-skinned gums at the lookout again, when work soon begins on bringing together Roman lead, a group of crystals, and extraordinary ingenuity, for a project which might not just change an Australian country town, but the universe as we know it.