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Thu 18 Aug 2022 19.00 EDT
From deep inside a gold mine in Stawell, Victoria, researchers are hunting for the invisible substance thought to make up 85% of the matter in the universe
The office in question is the only one of its kind in the southern hemisphere: the Stawell Underground Physics Laboratory (SUPL), where researchers will hunt for dark matter, the invisible substance thought to comprise about 85% of the matter in the universe.
The lab is officially open, and Guardian Australia recently visited with Prof Elisabetta Barberio, the director of the ARC Centre of Excellence for Dark Matter Particle Physics.
It is a half-hour descent via four-wheel drive, deep in the bowels of a working gold mine. John Bourne, the mine’s underground operational superintendent, steers us down a narrow tunnel, past pools of grey dripping wastewater, whirring fans that circulate air, and the remains of a historical shaft from the 19th century, with collapsed wooden beams and rusted metal. (The town of Stawell was founded in 1853, during the Victorian gold rush.) The miners are on lunch break – or, as they call it, crib – so there is less traffic underground than usual.
Numbers scrawled onto the walls mark how deep we have descended. Bourne, who has been at Stawell since 1995, says the mine bottoms out at 1,640 metres, but is flooded at depths beneath the physics lab. “The lowest we work down to [for gold] is probably 400 to 500 metres,” he says.
Prof Elisabetta Barberio with a mock-up of a Sabre, a sodium iodide core that will detect dark matter. Photograph: Penny Stephens/The Guardian
Halfway to the lab, Bourne stops the car and turns all the lights off. Our eyes try and fail to adjust to the darkness – it is overwhelming, almost solid, and it is difficult not to think about the hundreds of metres of rock above separating us from clear sky.
This dark place is a fitting location to hunt for dark matter. Even though we can’t see it, “we are surrounded by dark matter all the time,” says Barberio, a physicist at the University of Melbourne.
What exactly is it? Scientists are not entirely sure. They know dark matter is something that has mass, but does not absorb, emit or reflect light. They think it must hardly interact with ordinary matter – the visible stuff that we, Earth, stars and galaxies are made of. But if it does, Barberio and her colleagues want to see it. They are setting up a dark matter detector deep underground, where it will be shielded by rock against cosmic rays – high-energy particles from outside our solar system that rain down on Earth.
Prof Elisabetta Barberio near the laboratory entrance. Photograph: Penny Stephens/The Guardian
John Bourne drives through the Stawell gold mine to the lab. Photograph: Penny Stephens/The Guardian
When we arrive at the lab’s entrance, the tunnel opens out into a high-ceilinged cavern. Above ground, it is a sunny, cloudless winter day – a brisk 11C. Below, it is balmy and the air is thick with humidity. The deeper underground you go, the hotter it gets – heat flows from the Earth’s interior toward its surface.
Outside the lab, the ground is uneven and rocky, and our boots kick up dust with every step. Inside the lab, there are clean, bright cement floors and, surprisingly, air conditioning. We enter into a large rectangular room with ceilings around 12 metres high. It has all the makings of a regular laboratory: gas and electricity lines, a drainage area and shower for chemical spills. On the far side of the room, Barberio points out a nine-tonne overhead crane under which the detector will be mounted.
The detector will consist of seven cylinders of sodium iodide crystal, encapsulated in copper tubes. These will be submerged in a liquid called benzene, and encased in 200 tonnes of steel.
The aim is to detect a theoretical particle of dark matter known as a wimp – a weakly interacting massive particle. The idea is that a collision between a wimp – theorised to be around the size of an atom – and ordinary matter would result in energy detectable as light. The sodium iodide crystal should, in theory, flash if dark matter collides with it.
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