In 1905, Einstein used a well-known acoustic phenomenon to help substantiate the reasoning behind his theory of general relativity. Doppler effect theory argues that the wavelength of sound appears to change from its initial frequency if the distance between the sound source and the person hearing the sound is changing. Einstein posited that since light—like sound—consists of waveforms, this same effect can be perceived when a light source is traveling away from an observer.

How Does the Doppler Effect Work?

Imagine you are standing by the side of a road, observing the approach of a motor vehicle with a dysfunctional muffler. When the vehicle is a few hundred feet away, the sound it emits may seem high-pitched. As it draws closer and passes by you, however, the drone of the motor will sound much lower. That’s because the sound waves your ears are receiving as the car is moving toward you are more compressed that the sound waves you hear once the car has gone by. As the car begins to travel past you, the waveforms lengthen.

Redshift and Blueshift

Einstein was the among the first physicists to describe electromagnetic radiation or light as a type of wave that shared properties with other types of waves like, for example, sound. Just as sound waves lengthen as they recede from an observer, so do light waves lengthen as they escape from gravity in their travels across the vast universe.

Since the speed of light is a constant, the shift must be in the frequency rather than the speed of its waves. As light waves travel farther away from an observer, there is an apparent shift in their color to the red portion of the electromagnetic spectrum. This shift is known as the “gravitational redshift.” Conversely, as light waves approach an observer, there is an apparent shift to the bluer portions of the electromagnetic spectrum. This particular phenomenon is known as “gravitational blueshift.” Light waves travel so much faster than sound waves, however, that we don’t notice a Doppler light effect on a daily basis.