The Doppler effect refers to the way sound and light waves behave, depending on the motion of the transmitting and receiving objects. As objects move away from each other, waves become stretched out; and as objects move toward each other, waves become compressed. When it comes to sound waves, the result is that one hears a higher-pitched sound if an object making noise comes closer; if that object is moving away, the result is a lower-pitched sound. When it comes to light waves, compressed visible light is shifted toward the blue spectrum, while it is red-shifted if objects are moving away. Astronomer Edwin Hubble (1889–1953) used this information about light waves to form his theory of an expanding universe.

The Doppler effect gets its name from Austrian physicist and mathematician Johann Christian Doppler (1803–1853), a figure in history whose real name has caused considerable confusion. His baptismal certificate indicates that he was actually born Christian Andreas Doppler, but on his gravesite his name is presented as Christian Johann Doppler. Depending on sources, he has also been cited as Christian Andreas Doppler and Johann Christian Andreas Doppler! Let’s just go with his last name. Interestingly, the Doppler effect was not confirmed by its theorizer. Rather, it was a Dutch scientist named Christoph Hendrik Diederik Buys Ballot (1817–1890) who proved it by conducting an experiment in which he was trying to refute Doppler’s theory. Buys Ballot set up a series of trials by a railroad track in Utrecht, the Netherlands. An accomplished horn player road a train while a second highly trained musician stood by the tracks, listening to the first musician play a note on the horn. After two trials called on weather, a third trial confirmed that Doppler was correct about how sound waves change with motion.