The Michelson-Morley experiment

In the nineteenth century, light was known to behave like a wave. One of the fundamental properties of waves is that they require a medium through which to travel:

Light waves, therefore, must also travel through something. Physicists coined the term luminiferous ether, which means "light-bearing ether," to describe the material.

One big question about this mysterious substance was: how does it interact with matter? Does it remain fixed as stars and planets move through it? Or does it become "caught up" with ordinary matter where they meet? It might be possible to use this ether as a universal reference frame, against which all motions could be measured. Many scientists tried to detect the ether, but none succeeded. In the eighteen-eighties, Albert A. Michelson and Edward W. Morley set out to find the ether indirectly, by measuring very precisely the properties of light waves passing through it....