Waves - The Doppler Effect and Redshift

He used two groups of musicians, one group on a platform at a station and the other group on an open wagon on a train. Both groups played the same note. As the train steamed towards the station at full speed (about 40 miles per hour), the observers on the platform could hear that the note played by the musicians on the train was slightly higher pitched than the musicians on the platform and lower as it steamed away. The two notes matched each other as the train passed the observers.

It is easy to spot this effect for yourself these days - listen to the sound of a car going past, it's quite noticeable when you are at a motorway services. Before the British police and emergency services adopted American style sirens, emergency vehicles were fitted with two-tone sirens (and before that they had bells). It was always possible to hear from which direction they were approaching as the pitch of the sirens was higher or lower than normal. It is much more difficult to tell with modern sirens as they use a continually changing pitch so one second they sound like they are approaching and the next second it seems like they are moving away. The police use the Doppler effect to catch speeding motorists - their radar equipment measures the wavelength change of the returning reflections of the radar waves and calculates the speed of the targeted vehicle.

Doppler was more interested in light from the stars than in sound waves. He believed that his effect could be used to measure the movements of stars. He knew that the colour that you see depends on the wavelength of the light hitting your retina. If the star was approaching, he said the wavelength would be shorter and vice versa. Doppler made a few mistakes but that is no surprise, this was new science. He believed that all stars where the same colour (white or a pale yellow like the Sun) and if star was approaching it would appear redder. We now know that neither of these things are true but he was on the right track.

When you split starlight into a spectrum, it has a number of dark lines that correspond to the elements in the star or galaxy. We know from studying spectra in a laboratory where these should be. At the beginning of the twentieth century, astronomers studying light from distant stars and galaxies noticed that for more distant objects, these lines were displaced. This is called redshift if the lines are displaced towards the red end of the spectrum and blueshift if they are more towards the blue end. Redshift indicates the star or galaxy is receding (moving away from us) and blueshift indicates it is approaching. American astronomer Edwin Hubble is credited with the discovery that the more distant objects are, the faster they are receding from us and realised that we live in an expanding universe. This had in fact been worked out from Einstein's theory of relativity by a Belgian, Georges Lemaître 2 years before Hubble but for a variety of reasons, it was Hubble who took the glory!