Light, therefore, plays by similar rules as sound.īut, in the case of light, we perceive changes in wave frequency as changes in color, not changes in pitch. Like sound, light is also a wave traveling at a fixed speed: 186,000 miles (300,000 km) per second, or some one billion kilometers per hour. Our brains interpret changes in the frequency of sound waves as changes in pitch.

As an ambulance races forward and blares its siren, the sound waves in front of the ambulance get squished together. Meanwhile, the sound waves behind the ambulance get spread out. This means the frequency of the sound waves is higher ahead of the ambulance ( more sound waves will strike a listener’s ear, over a set amount of time) and lower behind it ( fewer sound waves will strike a listener’s ear, over a set amount of time). Sound can only move so fast through the air sound travels at about 750 miles (1,200 kilometers) per hour. It’s a good comparison because both sound and light travel in waves, which are affected by their movement through air and space. That change in the sound of an ambulance is due to what’s called the Doppler effect. What is a redshift? It’s often compared to the high-pitched whine of an ambulance siren coming at you, which drops in pitch as the ambulance moves past you and then away from you. Astronomers use redshifts to measure how the universe is expanding, and thus to determine the distance to our universe’s most distant (and therefore oldest) objects.

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