In this clip from Genius by Stephen Hawking watch the volunteers apply the Doppler effect to the expansion of the universe. By attaching a siren to a fast drag race car, the volunteers witness the Doppler effect in action. They learn that the frequency and wavelength of soundwaves changes depending on where the observer is positioned as the race car (source) goes past. In 1929, Edwin Hubble discovered that almost all the galaxies in the universe were moving away from us. He also discovered the further away a galaxy is the quicker it is moving. By applying the Doppler theory of sound to this concept, the volunteers learn how they can establish a frequency change in light. High frequency light appears blue and low frequency light appears red. So it is possible to work out the speed and direction a galaxy is travelling in by observing its colour.