As people in the nineteenth century came to understand that light has wave properties, they thought in terms of a medium that they supposed was necessary to transmit light through the emptiness of space, much as air transmits sound here on the surface of the Earth.

The luminiferous aether was thought to be a mysterious substance with amazing properties. It needed to be both rigid and fluid. It needed to be rigid to transmit transverse light waves, but it needed to be fluid so that the planets could pass through it. Thought to be stationary with respect to the "fixed stars," the planets should experience an "aether wind" as circulate about the sun. If our sun is moving about our galaxy, its motion should add to the aether wind.

Many wonderful experiments were done to measure the aether wind. The most famous is the Michelson-Morley experiment of 1887. In fact, this is one of the most famous experiments in all of physics, and it was a failure! The Michelson-Morley experiment led to many other ingenious attempts to measure the aether wind. They too failed. When Einstein cited the experimental evidence against the aether, the Michelson-Morley experiment was far down on his list. By then, it was old hat.

These days, we like to think of the Michelson-Morley experiment as leading directly to the special theory of relativity, but that is because it so beautifully illustrates the need for length contraction and time dilation in a moving frame of reference.