Doppler Effect

The Doppler Effect

The discovery of the Big Bang and the recession of the galaxies came from a commonplace of nature called the Doppler effect. We are used to it in the physics of sound. An automobile driver speeding by us blows his horn. Inside the car, the driver hears a steady blare at a fixed pitch. To us, the sound of the horn elides from high frequencies to low. A racing car traveling at 200 kilometers per hour (120 miles) per hour) is going almost one-fifth the speed of sound. Sound is a succession of waves in air, a crest and a trough, a crest and a trough.

The Dopler effect: a stationary source of sound or light emits a set of spherical waves. If the source is the motion from right to left, it emits spherical waves progressively. The Dopler effect is the key to cosmology.

The object itself might be any color, even blue. Image © Elena

Unlike sound, light travels perfectly well through a vacuum. The Doppler effect works here as well. If instead of sound the automobile were for some reason emitting, front and back, a beam of pure yellow light, the frequency of the light would increase slightly as the car approached and decrease slightly as the car receded. At ordinary speeds the effect would be imperceptible. If, however, the car were somehow traveling at a good fraction of the speed of light, we would be able to observe the color of the light changing toward higher frequency, that is, toward blue, as the car approached us; and toward lower frequencies, that is, toward red, as the car recede from us. An object approaching us at very high velocities is perceived to have the color of its spectral lines blue-shifted. This red shift, observed in the spectral lines of distant galaxies and interpreted as a Doppler effect, is the key to cosmology.

The object itself might be any color, even blue. The red shift means only that each spectral line appears at longer wavelengths than when the object is at rest; the amount of the red shift is proportional both to the velocity and to the wavelength of the spectral line when the object is at rest.