This week in Science History saw, in 1853, the death of Christian Doppler, the Austrian physicist.
Although he published over 50 articles in his life, his most notable work, published in 1842 when he was 39, was his discovery of what came to be known after his death as the Doppler effect.
He suggested that the frequency of a light wave was dependent on the relative movement of the source of the light and the observer.
This is the same effect that makes the sound of a police siren go up in pitch as it comes towards you, then decrease as it moves away. Basically what happens is that the sound waves get squashed as they come towards you, fitting them into a smaller space – making the frequency, or pitch, go up, then they stretch out again after they pass you, making the frequency/pitch go down again. The application of the Doppler effect to sound waves was proven by the Dutch scientist Buys in 1845, and the first observation of the shift to occur in light was described by the Frenchman Fizeau in 1848.
We now know that in the case of light, it’s a bit more complicated than Doppler’s explanation as light is not in fact made up of waves like sound – it is made up of particles called photons that show wave-like properties. However, for the sake of argument, the wave explanation used for the Doppler effect in sound is a useful analogy for how it works for light as well.
In 1929, the theory was used by Edwin Hubble to formulate Hubble’s Law – that you can tell how far away a star is by how ‘red-shifted’ the light coming from it is. ‘Red shift’ is what happens to light when the source (i.e. the star in this case) is moving away from the observer (i.e. us) – like when the police siren goes past and away from you and the pitch gets lower. Red light has a longer wavelength than red light, so when something is moving away, the wavelength stretches out and the light is moved closer to the red end of the spectrum.
All stars have a particular signature – the combination of gases and other compounds in them give off a particular pattern of wavelengths of light.
This is still some of the most important evidence for the Expanding Universe and Big Bang theories – because all stars appear to be moving away from each other, the only explanation is that the whole universe must be expanding – a bit like drawing dots on a balloon and then blowing it up – all the dots will get further and further apart.
So although now we know that the mechanism behind the ‘red shift’ of light is not as Doppler first suggested, this was only after over 50 years of research and technological advance since Doppler’s time. His theory for the behaviour of waves relative to the observer have played an important part in forming some of the most important astronomical theories of the past 100 years.