Australian physicists create water-based 'tractor beam'
Image: Stuart Hay, ANU

In a world-first, researchers from the Australian National University in Canberra have used simple wave generators to drag floating objects around in a body of water, even getting them to move against the direction of the waves.

The team demonstrated the phenomenon using a ping-pong ball floating in a wave tank. They created three-dimensional waves in the tank, and by adjusting their size and frequency, were able to create patterns in the water to keep the ball in one place, or move it towards or away from the wave generator, just like a 'tractor beam' straight out of science fiction. Advanced particle tracking tools were used to visualise how the waves were generating currents and flow patterns on the surface of the water so the team could more precisely manipulate the ball's movements.

"We found that above a certain height, these complex three-dimensional waves generate flow patterns on the surface of the water," said the lead researcher and physicist Michael Shats in a press release. "The tractor beam is just one of the patterns, they can be inward flows, outward flows or vortices." 

It sounds so simple - in fact, you can recreate the experiment at home - but even the scientists who achieved the technique can’t explain the mathematics behind it. "It's one of the great unresolved problems, yet anyone in the bathtub can reproduce it," said one of the team, physicist Horst Punzmann. "We were very surprised no one had described it before."

The research has been published in the journal Nature Physics, and it’s hoped that if it can be scaled up, the technique will provide solutions to oil spills and maritime disasters, and could help scientists to better understand the behaviour of rip currents - fast-moving narrow currents of water that can pull a swimmer out to sea even if the waves are heading inland. 

"One can imagine the technique being deployed in the open sea using special wave-generating floats, basically creating a hydraulic vacuum cleaner,” says Michael Byrne at Motherboard. "In the case of an oil spill, typically contained nowadays using floating barriers and somewhat sketchy chemicals that break down oil molecules, you can see the potential."

Watch how it works: