It’s making the news in oceans both Indian and Saturnian, tracking the movements of space probes and missing Malaysian airliners. And yet you encounter it every day, when you hear a car passing you on the road change from high to low pitch. So what exactly is the Doppler effect, and how does it work?
(Q: What sound does a cat make when it goes past at high speed? A: Meeeeeeeeeeeeeeeowwwww.)
As you might expect, the Doppler effect was named after the Christian Doppler, an Austrian physicist—although he only became a physicist because he was too frail to enter his father’s stonemason business—who proposed it in Prague in 1842.
It happens whenever there is movement relative to a source that’s emitting waves, whether they’re light, sound, water or something else. In the case of the moving car, think of its soundwaves as a series of peaks and troughs. The car emits one wave, i.e. one peak, and then another about 1 millisecond later.
But in that millisecond the car has moved closer to you, so the second peak has less distance to travel. It therefore reaches you less than 1 millisecond after the first peak does. This means that for you each peak is separated by less than a millisecond, so you hear the sound at a higher frequency.
OK, that maybe a little hard to picture, so try it visually instead. Imagine the waves as concentric rings being emitted by the source, they bunch up at the front and stretch out behind it. Or don’t imagine it: look at the picture below.
However you imagine it, the frequency change due to the Doppler effect makes a very convenient way to measure velocity, so it has many applications. Talking about moving cars, well it’s the Doppler effect that the police radar uses to tell whether you’re speeding (see the NSW Police Radar Manual [PDF 4.3 MB]).
It’s also famously what we use to measure the expansion of the universe. When a light source like a star or a galaxy is moving away from us, the electromagnetic waves it emits go to the low frequency or red end of the spectrum, so we say it’s red-shifted. If it’s coming towards us, it’s blue-shifted. By measuring the redshift of galaxies depending on how far away they are from us, we can calculate how fast the universe is expanding (due to the expansion of the universe, the further something is, the faster it is moving away).
But if understanding the history of the universe isn’t enough, the Doppler effect still makes the news; specifically, in the hunt for missing Malaysian Airlines flight MH370.
Using what the BBC called “cutting-edge methods”, the British satellite firm Inmarsat received radio pings from the missing plane, and by comparing how the frequency of the signal differed from what it’s supposed to be when it’s transmitted, they could work out how the plane was moving. That’s how they determined it flew to the Southern Indian Ocean, where the search is currently focussed.
The other bit of recent Doppler effect news was the discovery of an ocean under the icy surface of Enceladus, a moon of Saturn. Again, the scientists used changes in the frequency of radio signals, this time from the spacecraft Cassini, which was flying past it (Iess L, Stevenson DJ, Parisi M, Hemingway D, Jacobson RA, Lunine JI, Nimmo F, Armstrong JW, Asmar SW, Ducci M & Tortora P 2014, “The gravity field and interior structure of Enceladus”, Science, vol. 344, no. 6179, pp. 78–80, DOI: 10.1126/science.1250551).
By looking at how Cassini’s speed changed as it flew past Enceladus, they could determine the forces of gravity acting on it, which in turn allowed them to calculate the distribution of mass inside the moon. These were changes in speed of mere millimetres per second, but allowed them to figure out there was liquid water—which is denser than ice—and a relatively light rocky core.
So it may be commonplace, everyday science, but it’s good to see the Doppler effect is still making waves after all these years.