With the tap of a keyboard, Ryuji Hirayama brings a listless foam bead to life. The white speck jumps up and hovers with perfect stillness in space. Another tap, and the dot transforms into a luminous butterfly-like shape, which flaps its wings as it circles inside a black box. Diego Martinez Plasencia, Hirayama’s colleague at the University of Sussex in Brighton, UK, reaches into the box to show that there are no strings. The effect seems to be pure magic. “I showed it first to my daughters. They were like — ‘wow’,” Martinez Plasencia says, his eyes widening in child-like delight.

Behind the mid-air metamorphosis is a relatively simple set-up. Two slender arrays of 256 tiny speakers above and below the bead move it by generating ultrasound waves. The object darts so fast that all the eye sees is a continuously evolving 3D image a few centimetres across, drawn in the air as if by a high-speed Etch a Sketch. The same ultrasound speakers that create the image can also generate audio and tactile sensations. Reach towards the butterfly, and your finger might feel a flutter. In another case, a smiley face appears, accompanied by the strains of Queen’s ‘We Will Rock You’. Remarkably, most of the components used to create these effects are off-the-shelf.

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