The Doppler Effect is physics that is so ingrained in our brain that even the movies get it right. Any time some character is walking down a street and a car or train rushes past, the sound of the horn drops down a few notes as it passes by. Anything else would be jarring. We all hear it, all the time. But how was it first proved, all the way back in the early 1800s, when none of the instruments now used to measure it existed? Hint — it involved two different brass bands.
Sound waves travel at a set speed through the air. That speed can't be changed. If an object is coming towards us fast, the sound waves bunch together in front of it, like ripples piling up at the front of a ship. Behind the object, the sounds waves are lag behind and spread out.
The pitch, or note, of a sound depends on the frequency of the sound waves — the number of "peaks" that pass by a certain area in a certain period of time. The more peaks, the higher the note. Because the waves in front of a moving object crowd together, they sound a higher note. Behind the object, where the waves are further apart, they sound a lower note. As the object moves past us, we hear the transition from higher to lower.
The Doppler Effect has been a way to measure everything from the movement of submarines to the movement of stars (though it's expressed slightly differently in light waves). It's an incredibly useful tool, and we have many instruments that are calibrated to measure the Doppler Shift in frequency in various situations.
But back in 1842, none of those instruments existed. In fact, there were few things that even went fast enough to let people hear Doppler Shift. Christian Doppler, a frail academic, theorized the shift was much more than the quirk of a train whistle. He believed that it could be used to measure the motion of anything, even the stars (although he got the exact application of Doppler Shift in the stars wrong). In 1845, Christophe Ballot, a Dutch meteorologist, conducted a public demonstration to prove it. Ballot lived beside a train line, and he noticed the change in pitch in whistles and sounds as the train passed him by. He pulled a few strings and got a train to pull along an open-air cart filled with trumpet players. Meanwhile, more trumpet players stood in a station. Ballot checked the pitch of their instruments, and then got each section to play the same note as the train hurtled — at an astonishing forty miles per hour — past the station. Although the players in the station stayed right on the note, the players in the train by seemed to be pitched a little too high, and dropped a little too low as the train passed the station. Everyone in the station could hear the changing notes as the trained passed, and one of the most useful tools in all of physics was finally recognized.
Image: Guillaume Piolle