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Physics + MathPhysics & Math

Weird Wave Patterns Draw Tractor Beams Closer to Reality

Two techniques, published within two weeks of each other, detail researchers’ recent—and successful—attempts to manipulate objects without touching them.

ByTim De ChantNOVA NextNOVA Next
Using nothing more than waves, researchers were able to move this ping-pong ball around a tank of water.

Once the figment of science fiction authors’ imaginations, tractor beams are inching closer to reality. Two techniques, published within two weeks of each other, detail researchers’ recent—and successful—attempts to manipulate objects without touching them. Both appear to use the same general principles, and the results are nothing short of surreal.

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The first group, composed of researchers from four U.K. universities, spent nine months perfecting their tractor beam, which used sonic beams to guide a small, half-inch prism. Sound waves, like other waves, produce pressure, which can move an object away from the source of the sound. It’s how we hear—our eardrums are pushed away by the minute pulses of air that sound is composed of. Those vibrations are then reconstructed as sound by our brains.

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But a real tractor beam can’t just push objects, it has to move them toward the source. To do that, the researchers focused ultrasonic waves just past the object. Laura Parker, reporting for the New Yorker :

“It’s a relatively simple concept, but it’s just obscured by complex math,” [co-author Christine Démoré] told me. “By shaping a beam of energy so that it goes around an object in some way, hitting it in the back, it’s possible to then pull the object instead of push it.”

The second group, this one from the Australian National University, used a similar approach. Rather than using sound waves, though, they produced waves in a tank of water. When the waves were small, they exerted positive pressure on a ping-pong ball floating in the tank. Like splashing at a beach ball in a pool, an outward flowing current carried the ping-pong ball away from the wave source.

But when the waves got bigger, the wave pattern started getting weird. Rather than successive rows of waves forming in front of the wave maker, an array of peaks started appearing. The current directly in front of the device, which had been flowing outward when the waves were small, started flowing in. By changing the amplitude of the waves, the researchers could move the ping-pong ball at will.

Jonathan Webb, reporting for BBC News :

“We can engineer surface flows of practically any shape,” said Prof Michael Shats, the paper’s senior author. “These could be vortices, these could be outward and inward jets – it’s a variety of different flow configurations.”

It will, of course, be several years before large ships are able to retrieve their dinghies via tractor beam, but the Australian team is confident that the technique could be used on open water, not just in a tank in a lab.

After that, who knows? Between early photon-powered tractor beams and these latest sonic and aquatic versions, tractor beams are seeming less fictional every day.

Photo credit: Australian National University

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