By Jeff Eckert
Sure, it would be great to have a USS Enterprise-style tractor beam so you could drag alien spaceships out of the sky or even pull your Jeep out of the mud. Sorry, Grasshopper, but we’re not there yet. However, you actually can build your own single-sided acoustic tractor beam, i.e., a device that is capable of trapping and pulling an (extremely small) object using sound waves. We’re not talking about conventional sonic levitation which involves using sound waves to push things away. These tractor beams — based on decades-old fundamentals of physics used to create optical traps — can actually pull objects toward the source. Well, sort of.
According to the inventor, Asier Marzo (a research assistant at the University of Bristol; www.bris.ac.uk), “When you move the tractor beam, the particle moves, but otherwise the trap is static. It can levitate small plastics; it can also levitate a fly and small biological samples. It’s quite handy.”
He continued, “We can modulate a simple wave using what’s called a metamaterial, which is basically a piece of matter with lots of tubes of different lengths. The sound passes through these tubes, and when it exits the metamaterial, it has the correct phases to create a tractor beam.”
The beam can be built from readily accessible components, such as those available from Arduino (www.arduino.cc) for about $70. There are three designs of the device, each with trapping profiles suitable for different object sizes relative to the wavelength of sound used.
For any practical frequencies, the size of trappable objects is limited to a few millimeters.