What is Aether? The term aether (or ether) originates from the ancient Greeks. It meant the upper regions of space, or the heavens; or the substance that filled these upper regions. Hundreds of years later the term was adopted by physicists to describe a substance or medium that fills all of space, and through which certain waves or forces can propagate. For example, in order to explain the seemingly instantaneous and distant force of gravity, the existence of a substance that mimics the action of gravity - a "gravitational aether" - was proposed (most notably by Le Sage in the 18th century). Probably the most well known and academically accepted aether theory was the Luminiferous Aether - the medium through which light propagates. To understand why this medium was needed to explain how light propagates we must first understand the nature of light.