For Star Wars super-fans, spring is a special time of year. The fourth of May is Star Wars Day, aka “May the fourth be with you”, a play on the franchise’s well-known phrase “may the Force be with you”. The Force is a mysterious entity within the Star Wars Universe that Jedi harness to give them special powers, including the ability to move and levitate objects from a distance. But apart from being the answer to the pub quiz question “What links Star Wars, Matilda and Stranger Things?”, telekinetic powers only exist in science fiction.
And yet, we have recently mastered the art of manipulating objects without touching them. How? By using light and sound waves. When these waves bounce from the surface of an object, a little bit of the momentum of the wave is transferred to the object. This change in momentum, occurring over a short time-period, manifests as a force. And if the force is larger than any other forces acting on the object, like gravity or friction for example, then the object will move in the direction of the momentum shift.
This effect is easily achieved for tiny objects similar in size to the wavelength of the wave being used – millimetres for sound and nanometers for light. Any bigger than that and the effect is a mere tickle on the surface. That is, unless you do something special to the surface that guides and concentrates the momentum shift in a particular direction so that the force becomes controllable and significant.
A group from the University of Minnesota has done just that, by using metamaterials that have very specific, sub-millimetre patterns etched into the surface to manipulate and steer the sound waves as they interact. Their experiment, published recently in Nature Communications, involved a 7cm-long piece of 3D-printed plastic, designed to incorporate a pattern of shallow grooves along one edge. Sure enough, the team was able to move the object sideways and, remarkably, pull it back towards the source of the sound.
By optimising and scaling up this approach, the team is hoping to apply it to contactless control in robotics.
This piece is a preview from the New Humanist spring 2023 edition.
