Science fiction is littered with them—devices that hover effortlessly above the surface of a planet, spinning and turning without the aid of airfoils or other banal requirements of flight. Or tractor beams that push, pull, and move objects without touching them. Either way, some kind of forcefield is doing the work, but how that forcefield works is, well, fiction.
Well, maybe it was fiction. Using an array of ultrasonic speakers, a team of researchers at the Public University of Navarre in Spain showed that they can manipulate small beads of polystyrene, levitating them, pushing them left and right, and—when the array is hung upside down—pulling the beads toward it just like a tractor beam.
Scientists have been using ultrasound to move small objects for years, but this is the first time they’ve been able to manipulate them in three-dimensions using only one array. Previously, if you wanted complete control of an object, you had to suspend it within a square of four arrays. Cool, but not quite what sci-fi authors had in mind.
The new array creates a hologram that essentially forms a claw of ultrasound waves that wraps around objects. The square of 400 10-millimeter speakers pumps out finely controlled 40 kHz sound waves that combine to form an ultrasonic structure. Here’s Evan Ackerman, writing for IEEE Spectrum, with more detail:
Each structure consists of two elements: a holographic lens that’s generated by making all of the emitted sound waves coincide in phase at the focal point of the structure, and a second element that defines the type of structure around the focal point. To create a structure, the transducer array emits a holographic “signature” of sound waves that, combined with the holographic lens, yields a specific pattern of constructive and destructive sound waves that can “trap” small polystyrene particles up to 3 mm in diameter.
In other words, by adjusting the waves’ phase and controlling which speakers are active, scientists were able to achieve an impressive level of control. Not as impressive as when the Death Star reels in the Millennium Falcon in Star Wars , but it’s a start.
