Tractor beams have long been a science fiction fantasy—a magical force that attracts objects from a distance, like an alien’s spaceship sucking up its earthling specimens. We’re not quite that advanced, but physicists at The Australian National University have created a “tractor beam on water” that makes floating objects move wherever they are directed.
The group, led by Professor Michael Shats, started with a ping pong ball. Placing it in a small pool, they created waves using a thin, cylindrical device with suction cups on the bottom. When the device vibrates up and down, these cups come in contact with the water, creating waves. Different wave frequencies and sizes, along with different cup shapes, create different kinds of currents. Some pulled the ball inward, some pushed it outward.
Dr. Horst Punzmann, from the Research School of Physics and Engineering, said the team even found a way of “creating waves that can force a floating object to move against the direction of the wave.”
This doesn’t sound like rocket science. It seems quite obvious that waves make floating objects move, right? But actually, what propels floating objects aren’t the waves themselves, it’s the currents they create. Never before have we identified the precise wave patterns that make an object go one way or another, or how to replicate them. “It’s one of the great unresolved problems, yet anyone in the bathtub can reproduce it,” Punzmann said. Interestingly, there’s still no mathematical theory to explain why these experiments work. That said, this new knowledge could help us clean up oil spills and ocean trash, or retrieve drifting boats.
"The applications could be numerous," Shats said. A paper documenting the findings is featured in the journal Nature Physics.