http://www.popularmechanics.com/science/a17955/scientists-invent-a-tractor-beam-made-of-sound/Scientists have long been able to levitate small objects, like drops of water or plastic pellets, using only the power of sound. By sandwiching an object between two sets of speakers—or a speaker and a reflector—you can send precise, energetic blasts of ultrasonic noise to trap objects midair. But today, a fascinating new breakthrough can now accomplish this odd feat of levitation with a single array of speakers on just one side. It's like using sonic tongs with just one side instead of two. A team of researchers led by Asier Marzo, a software engineer at the Public University of Navarre in Pamplona, Spain, has just created a working tractor beam with sound. The scientists discovered a way to trap, move, twist, and rotate small objects with only a single set of speakers that are silent to the human ear and even work upside down. As they report in the journal Nature Communications, the invention could be used in developing tangible, 3D holograms made of tiny particles, or could find a future in acoustic surgery that requires no incision.
