Imagine a world that’s filled with invisible graffiti. Open an app, point your phone at a wall, and blank brick or cement becomes a canvas. Create art with digital spraypaint and stencils, and an augmented reality system will permanently store its location and placement, creating the illusion of real street art. If friends or social media followers have the app, they can find your painting on a map and come see it. You might scrawl an in-joke across the door of a friend’s apartment, or paint a gorgeous mural on the side of a local store.
Now imagine a darker world. Members of hate groups gleefully swap pictures of racist tags on civil rights monuments. Students bully each other by spreading vicious rumors on the walls of a target’s…