Researchers from the Australian National University in Canberra have used simple artificial wave generators to move floating objects around in a body of water, sometimes even making them swim against the direction of 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’.
‘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 simple – in fact, you can recreate the experiment at home – but the mathematics, say the scientists, are complex, almost inexplicable. ‘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.’
If this research can be scaled up, it will provide solutions to oil spills and maritime disasters, and could help scientists better understand the behaviour of rip currents, which are fast-moving narrow currents of water that can pull a swimmer out to sea even if the waves are heading inland.

Brinda Rajkumar Shah

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