The Color of Speed

9 November 2012

Doppler effect for a source moving at 70% light speed. Wikipedia
Doppler effect for a source moving at 70% light speed.

In astronomy and astrophysics, much of the information we gather comes to us in the form of light. Visible light, radio waves, x-rays and the like. We gather information not only from the type light emitted from an object, but also how that light appears to us as an observer. The two are not often the same.

One way in which light is effected is known as the Doppler effect. In our everyday lives, we’re familiar with the Doppler effect as it applies to sound. You might notice when a car or train passes you, its sound shifts downward as it passes. This is because the sound waves from an object are bunched together as it moves toward you, and stretched apart as it moves away from you. The bunched together waves sound higher, and the stretched waves sound lower, hence a shift in tone known as the Doppler shift.

For light a similar thing occurs. Waves of light coming from an object are compressed as an object moves toward us, making them look bluer (blue shifted). They’re stretched as the object moves away from us and look more red (red shifted). We don’t notice this color shift in our daily lives because nothing moves very fast compared to light, however stars and galaxies move fast enough for us to measure their redshift or blueshift. This tells us how fast they are moving relative to us.

Redshift and blueshift can also be caused by gravity. If you were to shine a beam of light into space, the light has to escape Earth’s gravity. As it does this it loses a little bit of energy, making it slightly more red than it was initially. If you were to shine a light down to Earth from space, the light would gain a bit of energy, making it slightly bluer as it reaches Earth. This means that we can use the Doppler shift of light to measure both relative motion and gravitational strength.

Of course there is another way that light is redshifted, and that is due to the expansion of the universe. But I’ll talk about that next time.