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A tractor beam reels in objects with sound

Ultrasound machine pulls on a target that is large enough to see

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3:18pm, March 17, 2014

BEAM ME IN  At left, waves (yellow dashed lines) from a regular beam of light or sound tend to bounce off an object (blue arrows) and deliver a subtle push. Waves from a new tractor beam, illustrated at right, bounce off the sides of an object and rebound upward. As a result, the object gets pulled toward the source of the beam.

Tractor beams have hit the big time. A newly constructed device generates a beam of concentrated sound that, for the first time, exerts a continuous, perceptible tug on objects large enough to see. The researchers didn’t actually reel in an object, but they demonstrated that an ultrasound tractor beam could do the job.

Using a tractor beam to haul a damaged spaceship may look simple on Star Trek, but nature makes it very difficult to pull objects from a distance. Waves of light or sound fired at an object tend to bounce off its surface like raindrops on an umbrella, collectively exerting a subtle nudge called radiation pressure that pushes the object away.

Christine Démoré, an engineering physicist at the University of Dundee in Scotland, and colleagues set out to reverse the direction of that radiation pressure, so that an object would get pulled rather than pushed. Their goal was to sculpt a beam of sound waves to bounce off the target and then scatter

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