The MichelsonMorley Experiment
In the winter of 1880-81, the American physicist Albert Michelson worked in the laboratory of the noted German physicist Hermann Helmholtz in Berlin. During that year he got the idea for his most important innovation in scientific apparatus, the so-called Michelson interferometer. To understand what this invention has got to do with relativity, we must back up a little.
Not long before Michelson's trip to Europe, the Scottish physicist James Clerk Maxwell had taken an interest in the question of how to measure the Earth's speed through the ether. The Earth travels once around the Sun in a year, in a nearly circular orbit. Maxwell argued that there was no reason to believe that the Earth dragged the ether along with it during this orbit. It just plowed through it like a frictionless ship through the water. Hence the Earth should have a speed through the ether that is at least as great as the speed with which it orbits the Sun. This speed is easy to estimate. The distance from here to the Sun is known to be about