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 .