One day in 1945, the crew of the aircraft carrier Intrepid gathered on deck for a group picture with what looks like a model airplane, not too much larger than the radio-controlled planes that hobbyists had been flying since the 1930s.
Robots clean rivers and sort waste, monitor ecosystems, and inspect renewable-energy infrastructure. But even the greenest robot has an environmental footprint.
EPFL engineers have designed acoustic cavities that convert sound waves into thrust, propelling small robots and ultralight aerial vehicles without on-board actuators or electronics.