In our everyday world, waves are stubbornly democratic. Whether it’s the sound of a conversation, the glow of a lightbulb, or the undulations of the ocean, waves tend to flow equally in both directions. You speak, and your voice travels to your friend standing across from you — just as theirs reaches back to you. We like it this way. But what if we needed waves to move in only one direction, free from interference, like cars on a one-way street? That’s the kind of control a team of researchers at ETH Zurich has just achieved. After years of effort, they’ve figured out how to direct sound waves…