You hear the terms “redshift” and “blue shift” in astronomy talking about the velocity and direction of travel of celestial bodies.  This isn’t a difficult basic concept to understand, but it’s a little bulky to explain.  Also, like everything else in science, it gets more difficult as you go along.  The least difficult is the Doppler redshift.

You know about the Doppler shift in sound as an object approaches you or moves away from you.  The pitch of a train whistle seems to change as it approaches and passes you.  You know the sound doesn’t actually change in pitch at the source (the train), but due to the movement of the train the sound waves are compressed or lengthened, changing the pitch you hear.  In Doppler redshift, the visible light waves are compressed or lengthened depending on the velocity and direction of the source relative to you.  I say “relative to you” because if the source is moving away from you, the visible light shifts to red.  If it’s moving toward you, the visible light shifts to blue.

Doppler redshift - Image Ales Tosovsky, All Rights Reserved

That, in a nutshell, is Doppler redshift.

From there it gets more complicated and bulkier to explain.  For example, you have cosmological redshift (or Hubble redshift) which deals with the expansion of the universe; the relativistic Doppler effect, which deals with time dilation of objects traveling at near light speed; the gravitational redshift (or Einstein redshift) which handles redshift in a gravitational well (i.e., near a black hole).  Each “complication” tells you something new, exciting, and different.  For example, the Hubble redshift applies to objects far, far away – in the neighborhood of 13 billion light years away.  That’s creeping up on the time of the Big Bang.  Anyway, scientists know these objects are that far away because the Hubble redshift tells us that the further away an object is, the larger the redshift.

Red and Blue Shifting - Image, WikiPedia user Anynobody, All Rights Reserved

Redshifting tells us how far away an object is, how fast it’s moving, in what direction it’s moving.  It gives us ideas about what the object is and how old it is.  We get information about black holes, exoplanets, and the nature of the universe itself.

Pretty great, right?  Also, when you get caught speeding on radar… you can blame Doppler redshift.