In the late 19th century, scientists believed that every type of wave traveled through some kind of substance or medium. As sound waves traveled through air and ocean waves through water, it stood to reason that light waves should travel through something, too. When light wasn’t traveling through air, water, or glass, it must still be traveling through something. However, it was just very difficult to observe this substance, which scientists called “ether.” In the 1880s, two American scientists set out to prove its existence.

Michelson and Morley reasoned that as Earth orbited the Sun, it moved through the ether and that this, in turn, generated a wind. In their experiment, they would measure the effect of this wind on a light beam. They predicted that a beam split so that one part was directed into the ether wind and another part directed perpendicular to it would travel at different velocities—thus proving the existence of the ether. However, when they measured how long it took each part to reflect off mirrors placed equally far apart, they observed that the two beams returned at precisely the same time. Because their results never varied with repeated testing, regardless of which direction they set up the mirrors, they had to conclude that there was no ether wind and thus no ether.

In trying to make sense of the universe, scientists use observation and experimentation and ultimately develop theories and laws. The results of Michelson and Morley’s experiment showed that the prevailing theory of ether was not supported by the facts. Their failure to prove a theory actually led to a much more important truth: Albert Einstein used the results in his special theory of relativity to support his assertion that, in a vacuum, light travels at a constant 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. From the moment it was published in 1905, Einstein’s theory changed the way scientists thought of time and space.

Interestingly, interferometers, the instrument Michelson and Morley used to determine light wave velocities based on interference patterns, have been adapted for modern astronomy. An Earth-based interferometer, located high atop the “Big Island” of Hawaii, combines beams of light from two of the world’s largest telescopes used for optical and infrared astronomy. With the power of these two closely spaced telescopes operating together, this “binocular” interferometer can make out dust disks present around stars, which are associated with planet formation.

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