Your Train Keeps Changing Colour

Nov 07 2013

“Eureka!” has been the classic phrase associated with scientific discovery for over 2000 years. It is easy to see why – it entered the vocabulary thanks to an entertaining story of political intrigue, profound scientific insight, skilled detective work and shameless public nudity. Archimedes: the archetype of the mad scientist. But the image is not quite right. The real phrase uttered by scientists just before they discover something new is actually something closer to “Hmmm. That’s funny…”

Have you ever noticed that you can tell whether an unseen emergency vehicle is approaching or moving away from you only from the sound of the siren? Not just because the siren is getting louder or quieter, but because the actual sound of the siren in the two situations is different? That’s funny. How does that work?

The answer lies in something known as the Doppler effect. It was proposed in the 1840s by an Austrian physicist named Christian Doppler when he was trying to explain why certain stars might look different to others, which certainly appeared funny to him and others at the time. The idea is simple: if an object is emitting a regular signal – they could be sound waves, or radio waves, or light waves – but the emitting object is moving relative to a receiver somewhere else, then the received signal is changed.  When the emitter and receiver are moving closer together, the signal gets bunched up, and oncoming sirens appear to have a higher pitch than usual. Similarly, receding signals seem more spread out and so a siren moving away has a lower pitch.  And it’s not just sirens. Try to make the noise of a car passing by at speed. You’ll instinctively change from a higher pitch to a lower one to indicate the moment the car swooshes past.

This effect gets used a lot, from monitoring blood flow, to monitoring traffic, to monitoring the universe. Using the light gathered from distant galaxies and comparing it to light from our sun, we were able to observe a Doppler shift in the observed colours and deduce that the galaxies are moving away from us at very high speeds. In fact, the further away the galaxy, the faster it seems to be moving. That’s funny. What does it mean? If you’ve heard of the phrase ‘the expanding universe’, these Doppler observations are the foundations of that idea. Sometimes, a little observation can provide profound insight.

So if the Doppler effect works for the sound of moving vehicles and the colours of moving stars, can we combine the two? Can we see a Doppler shift in the colours of moving vehicles? Does a moving DART change colour? Yes and no. It does, because the Doppler shift is described by some pretty simple mathematics and so we can calculate the shift in frequency associated with the green of the DART train as it approaches a station. It will be a tiny bit more blue-green than a DART leaving the station, which in turn will be a tiny bit more red-green than one which is stopped in front of us. But then again while the shift is real, it’s all but undetectable. The human eye and brain can differentiate between millions of colours, but most of this is to do with noting changes in relative mixtures of different frequencies and intensities of light, not just a tiny change in the frequency of green. The change will be real, but too subtle to notice. To get an observable – barely! – Doppler shift, we’d need the train to be travelling at hundreds of kilometres per second.

Speaking of which, one group of people who would notice the subtle Doppler shift: traffic police. Those handheld speed guns work using this principle. They emit radar waves – part of the same radiation family as light, but with a different set of associated frequencies – and note whether the waves that reflect off a vehicle and return to the gun have been Doppler shifted due to being bounced off an object that was moving toward or away from the gun, and by how much. Computer software analyses the change and calculates an associated speed, and identifies drivers breaking the speed limit. That’s not so funny.

This guest post was written by Dr David Rickard, a lecturer in the Dublin Institute of Technology teaching science on the Access Foundation Programme.

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