Redshifting 101

RedShift: Say what?

To talk about "redshift", let's first explain what Doppler Effect is. Have you ever been watching traffic as you stand on the sidewalk and listened to the passing cars as they whizzed by? Imagine the sound of a car as it races by you. Vaaaaa (car coming toward you) rooooom (car moving away from you)! Now lets examine that sound.
  1. Short wavelength
    • higher frequency
    • higher pitch
  2. Long wavelength
    • lower frequency
    • lower pitch
When a moving object comes towards you, the sound waves it sends towards you are compressed, and their wavelengths become shorter. When the wavelength of the sound wave shortens, it has a higher frequency because more cycles of the wave can go by in a given interval of time. A higher frequency means a higher pitch, so you probably noticed that the sound of the car is higher as it moves toward you than when it moves away. As the car drives away, however, just the opposite happens. The wavelength of the sound wave becomes longer as the car moves away. A longer wavelength means that not as many wave cycles can pass in a given time interval, so the frequency is lower, and thus the pitch of the sound is much lower.

Relating Doppler Effect to Light and Redshift

You might be asking yourself now, does this Doppler Effect stuff work with light waves? Yes, but in a slightly different way. We cannot "listen" to a radiating object to tell if it is moving away, but we can observe its radiation.

  • An object that is a certain distance away from us is radiating light (represented by the green light wave)
  • If the object starts towards us, the wavelength of the light it emits will get shorter, which means the light will be more energetic and have a higher frequency (represented by the blue light wave)
  • Conversely, if the object begins to move away from us, the wavelength of the light it emits will become longer and less energetic, and have a lower frequency (represented by the red light wave)

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