The Doppler Effect

Have you ever noticed how the sound of an ambulance seems to have a high pitch when it’s moving towards you, but then the pitch gets lower and lower as it moves away? Well, that’s because of phenomenon called the Doppler Effect… and it’s also how we figured out the universe is expanding (but more on that later).

Imagine an object producing a wave, whether it be a light wave or a sound wave. When the object is stationary, the is a certain length between each crest (the wavelength) and it is the same as wavelength seen by the observer. It looks something like this…

But, when the source of the wave is moving towards the observer, each newly produced crest ‘catches up’ with the one in front of it. So the wavelength appears to be shorter. A shorter wavelength implies a higher frequency (because the speed is constant). So if it’s a sound, to the observer it appears higher pitched. If it was a light wave, to the observer it becomes blue-shifted (because blue light is on the higher frequency end of the visible light spectrum).

The opposite happens when the source moves away from the observer: each new crest produces appears further from the last one than it should be, so the wavelength increases. This corresponds to a lower frequency. To an observer, sound waves appears lower pitched, while light waves appear red-shifted.

This is what scientists noticed when they observed the light coming from other galaxies, which appeared almost always red-shifted. This indicated that the galaxies were moving away from us (and from each-other), which is how we came to the baffling fact that the universe is expanding!

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I’ll end on a terrible (quite terrible) joke that I made up: what did the light wave say to the guy it was moving away from?

…Are you alright? I think you’re seeing red.