Original: Measuring aether Physics theories of the late 19th century postulated that, just as water waves must have a medium to move across (water), and audible sound waves require a medium to move through (such as air or water), so also light waves require a medium, the "luminiferous aether".
Simplified: Since waves in water need something to move in (water) and sound waves do as well (air), it was believed that light also needed something to move in.