THE MOST CRUCIAL QUESTION IN RELATIVITY

A light source emits a series of pulses the distance between which is d (e.g. d=300000km). A stationary observer/receiver measures the frequency of the pulses to be f=c/d:

http://www.einstein-online.info/imag...ler_static.gif

An observer/receiver moving with speed v (let v be small so that the relativistic corrections can be ignored) towards the light source measures the frequency of the pulses to be f'=(c+v)/d:

http://www.einstein-online.info/imag...ector_blue.gif

The most crucial question:

Why does the frequency shift from f=c/d to f'=(c+v)/d ?

Answer 1 (fatal for relativity): Because the speed of the pulses relative to the observer/receiver shifts from c to c'=c+v.

Answer 2 (saving relativity): Because...

I know of no reasonable statement that could become Answer 2.

Pentcho Valev