Expert: Philip Stahl - 10/3/2010

Question
If a star or galaxy is moving away from us or moving toward us, I can understand the change in the frequency of the light emitted. However, If galaxy A is moving in a straight line with galaxy B, and at a greater speed, what is the effect seen from galaxy A? As a complete novice, I would assume that there would be no change in frequency. If that was to be the case, then how would it be possible to know that galaxy B is moving slower than galaxy A?

Kind regards,
Peter Allchin

Answer
Hello,

Actually, if such a case were to occur (dynamically doubtful when one examines the kinetics of galaxies in clusters), the same principle applies.  Thus if one has:


---> ( A)  ----> (B)  
 

and v(A) > v(B)

then by virtue of v(B) < v(A) the light waves are foreshortened in galaxy A's direction, so that a *blue shift* is detected.  Since both are putatively moving in the same direction (and this can be verified!) then one can deduce from the foreshortening of light waves from B that it must be moving *slower* than A.

Of course, since a blue shift is observed there is also a change in frequency consistent with that - so in this case the foreshortening of light waves in galaxy A's direction implies the frequency is in the direction of higher f (compatible with shorter lambda (wavelength))



v(A)