Physicists have long understood waves — including water waves, sound waves, and waves on a string — as some type of energetic signal that travels, or propagates, through a medium. In the 1800s, physicists demonstrated that light is a wave, too, but the search for a medium that light propagates through came up empty: the supposed “luminiferous aether” doesn’t need to exist. Light, gravitational waves, and all other quantum phenomena propagate just fine, requiring no detectable medium to travel through. It’s a puzzle as curious as a Cheshire Cat.