Light acts like a wave, but unlike sound waves, light isn’t a material that’s moving back-and-forth. At yet, the air around you doesn’t noticeably move anywhere when sound moves through it. Sound is waves in air, ripples are waves in water, earthquakes are waves in the ground; every wave seems to need some material to do the waving. So Young’s experiment led to the completely natural, titular question of this post: if light is a wave, then what’s doing the waving? Physicists, never shy to name things they’re pretty sure exists, declared that the material light moves through is the Luminiferous Aether (“luminous” for “light” and “aether” for “I don’t know what it is either”).