One of the greatest topics for debate in the late nineteenth century was the nature of light: classical physicists believed that light was an undulatory wave--that it moved through a medium, just as sound does through air and tides through water. They proposed the existence of drifting ether, an invisible substance that filled the universe, and that this ether-wind carried light along with it. A new school of thought disputed the ether-drift theory, claiming that light acted as a particle and required no medium through which to travel. The American physicist Albert Abraham Michelson was a staunch subscriber to the ether theory, and in 1881 he conducted a series of experiments to prove its existence.

Michelson hypothesized that the ether-wind was strong enough to alter the speed of light--a beam of light moving with the current would move faster than a beam moving against it. He built a device...