The idea of ether (or "aether") dates back to ancient Greek philosophy, where it was considered the substance filling the universe, the medium through which light and celestial bodies moved. By the 19th century, ether was believed to be a luminiferous medium — an invisible, all-pervading substance that filled space and was necessary for the propagation of light waves. Scientists thought that just as sound waves require air, light waves needed ether to travel through the vacuum of space.

In the 1860s, James Clerk Maxwell developed his famous equations that described how electric and magnetic fields propagate as waves. Although Maxwell's equations didn't explicitly require an ether, the prevailing belief was that these electromagnetic waves still needed a medium to travel through — hence, the ether. As scientists began to experiment with and understand radio waves, they initially believed these waves, like light, traveled through ether. The ether was thought to be the medium that allowed radio waves to propagate over long distances.