Observatory · the siren
The Doppler effect
An ambulance comes towards you on a high note and leaves on a low one. The siren never changed: each wave crest it sends out leaves from where the ambulance is at that moment, so in front of it the crests are squeezed together and behind it they are stretched apart. Light does exactly the same, and that one effect lets astronomers measure the speed of everything they can see, from planets tugging their stars to galaxies at the edge of the universe.
The siren
Fig. I. Wave crests from a moving source, squeezed ahead and stretched behind.
- Source
- Arriving at you
- Shifted
- What happens
Every circle is one crest, spreading from the spot where the source was when it sent it. Press listen and you hear what arrives where you stand, worked out crest by crest: high while it comes, a slide down as it passes, low as it goes. The pitch does not change smoothly at a constant rate; it drops fastest when the source is closest, which is why the drop is so sudden when a car passes close by and so slow when it passes far away. Push the speed past the speed of sound and the source outruns its own crests: they pile up into a cone, and the cone sweeping over you is the sonic boom; light in water does the same, the blue Cherenkov glow of the speed limit.
Switch to light and the same squeeze turns colour: green light from a source coming at you arrives blue, from one going away arrives red, the motion half of the spectroscope. Near the speed of light a second thing shows: the light is also thrown forwards, so the front gets much brighter and the back much fainter, the beaming that makes one side of the disk in the lens blaze.
Who worked it out
Where it already matters
Trumpets on a train. Christian Doppler predicted the effect in 1842, for the colours of double stars. In 1845 the Dutch meteorologist Buys Ballot put it to the test near Utrecht: musicians on an open railway carriage held a note while others on the platform, with perfect pitch, noted what they heard as the train went by. The note went up coming and down going, as predicted.
Speed traps and rain. A police radar gun sends out radio waves and measures how much the echo off a car comes back shifted. Weather radar does it with raindrops, which is how it sees the wind inside a storm, and a hospital's Doppler ultrasound does it with blood, to see how fast it flows.
A star that wobbles. A planet pulls its star round in a small circle, so the star's light shifts blue and red, back and forth, with the planet's year. In 1995 the first planet round a Sun-like star, 51 Pegasi b, was found this way, the Nobel prize of 2019; today's spectrographs see stars wobble by less than a metre per second, a walking pace, from light years away.
The universe. The light of almost every distant galaxy arrives red. At small distances that is a Doppler shift; further out it is space itself stretching the light on its way, the story of the loophole.