By the late 1800s, scientists had already established that light travels in wave form at a speed of 186,000 miles -- or 300,000 kilometers -- per second. Most scientists also believed that light waves had to move through some sort of substance that filled space, just as ocean waves move through water and sound waves move through air. They called this invisible and weightless substance "luminiferous ether, " though no instrument could detect its existence.

In 1887, two American scientists named Michelson and Morley set out to prove that this ether did, in fact, exist. They supposed that as the Earth orbited the Sun, it moved through the ether. And just as a person riding a bike generated a breeze, so too did Earth speeding through the ether generate an "ether wind. " Using a half-silvered mirror, the surface of which reflects half the light that strikes it and lets the other half pass through, Michelson and Morley split a beam of light into two parts, angling the mirror in such a way that one part traveled in the direction of Earth's motion in orbit -- or against the ether wind -- and the other part perpendicular to this. Then they measured the time it took each part to reflect off a distant mirror and return. They predicted that the beam directed into the ether wind would move more slowly -- like a boat moving against a current would. However, the two parts of the beam returned at precisely the same time. Because their results never varied with repeated testing, they were forced to conclude that there was no ether wind.

Albert Einstein used the Michelson-Morley results in his special theory of relativity, published in 1905. Einstein argued that light travels at a speed of 186,000 miles per second in all directions at all times and for all observers -- even if one observer is moving relative to another observer. That the speed of light does not vary defied the fundamental laws of physics passed down from Galileo and Newton. This and other assertions in the special theory of relativity completely changed the way scientists thought of time and space.