You've seen holograms before: they're images that seem to jump out of a flat surface, full of depth that you can experience through perspective changes and parallax cues. The three-dimensional effect that a hologram creates comes from the three dimensional light field that's created when photons diffract through the interference pattern on a holographic plate. It's essentially a structure made of light that gets projected out into space when the seemingly random pattern of features on the plate interact with each other.
Light isn't the only wave that can be manipulated to create structures in space; the same thing goes for sound waves. The structures generated by constructively and destructively interfering with ultrasonic waves are tangible things that can exert force on objects. Researchers at the Public University of Navarre in Spain have used ultrasonic acoustic holograms to manipulate things just like the tractor beam used by the crew of the USS Enterprise on the TV show Star Trek.
Acoustic levitation is usually accomplished with a pair of ultrasound emitter arrays, or with one array aimed at a reflector. A standing wave of ultrasound is formed between these two elements, and small particles can be suspended in the nodes of the wave. Varying the phase of the wave moves the nodes, causing particles to be transported along a single axis. This is cool stuff, but it only works if you have the thing you want to push around completely inside your device. That’s obviously less useful than it could be, and nothing like the Star Trek technology, which is, after all, the driving force behind all research everywhere.
Acoustic holograms offer a way to generate acoustic structures that are essentially 3-D objects made of sound. They're created by a single 20 by 20 ultrasonic phased array of 10 millimeter transducers generating 40 kilohertz ultrasonic sound waves with programmable relative phase modulation. Each structure consists of two elements: a holographic lens that's generated by making all of the emitted sound waves coincide in
Images: Asier Marzo, Bruce Drinkwater and Sriram Subramanian/Nature
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. The image [right] illustrates the phase modulations and holographic signatures of a "twin" structure, a "vortex" structure, and a "bottle" structure.
And the array of images [left] offer various visualizations of the vortex structure:
But the really cool bit is the system in action; note that the orientation of objects can be controlled as well as their position, and that multiple objects can be controlled at once:
The fact that you can create 3-D acoustic structures that can manipulate objects with just a single array is what's cool here, because not needing hardware on the other side of the object opens up all kinds of intriguing possibilities. According to the researchers, acoustic tractor beams “could be applied directly onto the skin with the manipulation taking place inside the body; similar to an ultrasound scanner but for manipulating particles (that is, drug capsules, kidney stones or micro-surgical instruments).” Or, they could be used to create "tangible displays."
At the moment, the size of the object that can be manipulated depends on the power of the transducers, so moving around (and perhaps even assembling) bigger and heavier stuff is certainly possible. We're not saying that it'll ever be possible to levitate people, because that would be ridiculous. But we're thinking it, because that would be awesome.