Einstein said in his 1916 book: ”If an observer sitting in the position M1 in the train did not possess this velocity, then he would remain permanently at M, and the light rays emitted by the flashes of lightning A and B would reach him simultaneously, i.e. they would meet just where he is situated. Now in reality (considered with reference to the railway embankment) he is hastening towards the beam of light coming from B, whilst he is riding on ahead of the beam of light coming from A. Hence the observer will see the beam of light emitted from B earlier than he will see that emitted from A.” What Einstein means by “he is hastening towards the beam of light coming from B” is that the light beam from the flash at B is converging on the observer that is moving toward B at the relative velocity of c + v, with v being the velocity of the observer toward B. What he means by “he is riding on ahead of the beam of light coming from A” means the light from the A flash is converging on the observer at the velocity of c – v. This is very simple. The observer-relative speed of the light, relative to the moving observer is NOT “c”, it is NOT constant. It is c + v in one direction and c – v in the other direction. This is why the earth sees a blueshift in the light of the star the earth is moving toward in its revolution around the sun and this is why the earth sees a redshift in the light of a star the earth is moving away from during its revolution around the sun. This is caused by the Second Cause of the Doppler effects that I told you about earlier. This is a Doppler Law of Physics. It can not be revoked.