When an ambulance rushes towards you the pitch of its sirens is different then when its speeding away. Ever wonder why that is? It’s called Doppler shift. Let’s find out what that is.
Waves
Sound is a type of wave. Vibrations travel through the air (or another medium) in the form of a wave. There are many types of waves.
There’s actual waves in the sea. Light is a type of wave as are radio signals. And then there are more exotic things like gravity waves.
If you look at a one-dimensional wave, they are usually depicted as a sine, like the diagram at the top of this post.
The reason for this is that waves have a frequency and an amplitude. A diagram like the one above is great for showing both. Imagine the orange dot as the source of the signal and the orange line as the two directions it’s sending the wave.
There is a lot you can say about waves, but for now I’ll leave that be. Let’s only look at what happens to waves when they are emitted from a source that is moving relative to an observer, the so-called Doppler shift.
Doppler shift
When the source of a wave starts moving, things get interesting. Take a look at the following diagram.
When the source of a wave moves in a direction, the waves going in the same direction gets squashed together (left side of the diagram). The waves going the other way are pulled further apart (right side of the diagram).
The above diagrams are 1-dimensional. For 2 dimensions things work almost the same, as shown below.
You can fill in three dimensions yourself.
The effect of this movement on a wave means the frequency goes up or down for the observer. Which is why an ambulance siren will change frequency as it passes by. It’s good to note here that it’s the relative speed in respect to an observer that matters. Driving next to the ambulance will not cause the same effect.
Some oddities
Sound moves at a speed of about 343 meters per second. That’s fast, but not as fast as the top speed of a jet fighter. So, what happens to the sound when the jet fighter goes faster than sound? The sound waves bunch up behind the plane in a so-called mach-cone. That’s why you hear a sonic boom when you’re on the ground. It the sound ‘bow wave’ rolling over you.
Interestingly enough, a pilot can still hear things inside the cockpit, as the air inside is moving with the plane. Remember, Doppler shift is for relative speed in respect to the observer.
Light can also be considered to be a wave – and a particle, of course, but let’s not open that can of worms. When a source of light is moving toward you, the frequency will indeed shift. Of course, light travels way faster than sound. As a result, the effect isn’t noticeable at low speeds. However, stars tend to have much greater speed differences. And indeed, this leads to light from other stars being ‘blue-shifted’ or ‘red-shifted’. The light of those stars has a Doppler shifted frequency, putting it more ‘toward’ red or blue. If you were to allow FTL travel in your scifi, you might invent an FTL-cone of infrared light. Although how this would work in relation to the fact that light always travels at the same speed regardless of the observer…
Since you can define even gravity as a wave, gravity can also be Doppler shifted, but I can’t really imagine what that means. If you do, you can try to explain it to me over a beer some time. Or in the comments.
Conclusion
Doppler shift is somewhat weird, but also very interesting. Think about it the next time you see an ambulance.
