HOW COME?: Why the engine siren sound changes

Fire engine sirens are like train whistles: As the train comes into view on the tracks, its warning whistle sounding, its sound shifts, second by second. As the train approaches, its whistle becomes not just louder, but also higher-pitched. Then, as the train engine roars through the crossing and off into the distance, the whistle's sound drops in pitch, becoming lower and lower as the train recedes.

It's the same story with fire engine sirens: As we stand still and a (loud) moving object passes us, its sound shifts -- and then shifts again. But to the engineer driving the train, the whistle blares with a steady pitch.

We can thank the Doppler effect for the built-in sound effects of noisy objects on the move. An Austrian physicist and mathematician named Christian Doppler was one of the first to do experiments on the strange effect. In 1845, Doppler coaxed a group of trumpet players to board an open train car, to show the effect in action. As the musicians began to play, the train engine moved the car back and forth along the track. Meanwhile, a second group of musicians stood by the track, carefully noting the trumpets' changing pitch.

Here's how it works: As, say, a whistling train rushes toward us, the sound waves coming from its whistle are pushed together. This creates waves of higher frequency, so we hear a higher-pitched sound. But as the train passes, the sound waves spread out behind it, like smoke from a coal-fired train's smokestack. Result: We hear a lower-frequency, deeper sound.

But the whistle, the train, and the people aboard it are all moving together. So to the train crew and passengers, the whistle's pitch remains the same.

And the Doppler effect doesn't just apply to sound waves. It also applies to waves of electromagnetic radiation -- for example, visible light. White light comes in a hidden rainbow of colors: red, orange yellow, green, blue indigo, violet. Thanks to the Doppler effect, light waves bouncing off an approaching object become bunched up and bluer as the object comes nearer, and spread-out and redder as it moves away.

Our human eyes aren't sensitive enough to perceive the color shift. But by the 1920s, astronomers like Edwin Hubble had realized light from distant galaxies is shifted toward the red end of the spectrum. By measuring the Doppler light shift with sensitive instruments, Hubble and others verified the universe is expanding. Galaxies are rushing away from each other, their light spreading out like the sound waves of receding trains.