Question 6

  An astronomer measures the Doppler change in frequency for the light reaching the earth from a distant star. From this measurement, can the astronomer tell whether the star is moving away from the earth or whether the earth is moving away from the star? Explain.

SOLUTION: The same effect arises for electromagnetic (EM) waves whether the source or observer moves. Only the relative motion of the source and observer matters (this is one of Relativity's fundamental postulates). Therefore, the astronomer has no idea whether the star is moving away or the earth is moving away; actually, neither makes sense since there is no absolute reference frame (no ether) relative to which either is moving. (You might try the following gedanken (thought) experiment: imagine you are the only object in the universe. Are you moving? Now imagine only you and a ball exist. Throw the ball. Are you moving, or is the ball, or does it make sense to say either?)