Researchers at the University of Tokyo have published a paper and video describing a technique that is explicitly not an anti-gravity system, and doesn’t pretend to be, but looks very much like one.
“The essence of levitation is the countervailing of gravity,” according to the provocative opening of a paper published Dec. 14 on the Cornell University science-publishing site arXiv.org that describes a way to not only raise an object into the air, but maneuver it in three dimensions using only standing waves of ultrasound. (Full paper as PDF available here.)
Since the mid-1970s, researchers have been able to levitate small objects using focused beams of high-frequency sound that bounce off a flat surface and create a wave of pressure that pushes the object into the air. But they couldn’t cause an object to float, and they couldn’t move it around in any direction other than up or down.
The University of Tokyo team led by Yoichi Ochiai built a system that could raise small particles, water droplets and even “small creatures” off a flat surface and zoom them around within an open, cubical area about 21 inches on each side.
The system uses four sets of phased arrays – speakers producing focused beams of sound at around 40kHz – to create waves of ultrasonic force on every side of the object rather than just one.
The force produced by each of the four ultrasound sources can be changed – and the force on the object manipulated – using the same techniques utilized by older systems.
Coordinating the frequencies and force of ultrasound arrays on four sides, however, creates a consistent focal point for the force from each. By keeping frequency changes in sync, the system creates a “bubble” within which the force from all four sources is consistent no matter where within the target area the focus is directed.
Once raised off the floor of the target zone, anything wrapped in that bubble of balanced forces has no choice but to move wherever the focal point goes.
The system does not cancel out gravity or do anything to make a levitated object less susceptible to it, any more than the cables attached to the harness of a Cirque du Soleil acrobat counteracts gravity, inertia or any other force. The beams of ultrasound simply take the place of the cables, pushing on every side with a force at least equal to the weight of the object, making it weightless within that bubble of lift.
In a video choreographed to Strauss’ Blue Danube, objects (including bits of dry ice, match heads, water droplets and bits of circuit board) waltz individually and in formation until a researcher’s hand pushes them out of the bubble of force and back under the predominantly one-directional influence of gravity, where they fall in a spray to the table’s surface.
Later versions of the system could use lower frequencies, or lift heavier items, but the approach doesn’t appear to have the promise of more powerful systems, such as magnetic levitation that lifts commuter trains off a single rail and resists g-forces during at speeds approaching 300 miles per hour.
Coordinated ultrasound levitation is relatively compact and has little impact on systems or objects around it, possibly making it appropriate to move objects under microgravity in a space station or similar environments, the paper suggests.
Image: Yoichi Ochia et al/Univ. Tokyo