1848

Doppler Shift of Light

Christian Doppler (1803–1853), Armand Hippolyte Fizeau (1819–1896), Vesto Slipher (1875–1969), Edwin Hubble (1889–1953)

Most of us are familiar with the dramatic change in the sound of an ambulance or a train whistle or a race car as it approaches us and then speeds past. As the vehicle recedes, its sound changes to a distinctly lower pitch or frequency than when it was approaching. This change is known as the Doppler effect, named after the Austrian physicist Christian Doppler, who first proposed in 1842 the idea that the observed frequency of any kind of wave should depend on the relative difference in speeds between the wave’s source and the observer.

In 1845 Doppler’s hypothesis was verified for sound waves in some clever experiments by the Dutch meteorologist C. H. D. Buys Ballot, who hired musicians to play notes on a moving train and then had stationary observers report the pitches that they heard as the musicians approached and receded. In 1848, the French physicist Armand Hippolyte Fizeau showed how Doppler’s hypothesis applied to light waves by noting slight changes in frequency or shifts of absorption lines in the spectra of stars.

Astronomers call these frequency changes Doppler shifts, and the size and direction of the frequency change can be used to determine the speed at which astronomical bodies are approaching or receding from each other. Objects approaching us have their spectra shifted to higher frequencies, or shorter (bluer) wavelengths; conversely, objects receding from us have spectra that are red-shifted. In the 1860s the first accurate relative stellar velocities were measured, and in the 1870s it became possible to detect the Doppler shift of the stars caused by the Earth’s annual motion around the Sun. In the early twentieth century, the American astronomer Vesto Slipher made observations showing that most of the known nebulae (such as those in Messier’s list) were red-shifted—or moving away from us. Soon after, Edwin Hubble, another American, showed that many of these nebulae were actually other galaxies, enormously far from the Milky Way. Hubble’s work led directly to the concept of an expanding universe and the Big Bang theory.

A graphical representation of the Doppler effect: waves are being emitted by a source moving here from right to left. To the observer, waves in front of the source are compressed to a higher frequency (shorter wavelength, or bluer); waves behind the source have a longer wavelength (redder).